diff --git a/pyproject.toml b/pyproject.toml
index 6673ca5e4..254f2a2c8 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -90,6 +90,7 @@ dependencies = [
     "rich >= 12.6, < 14.0",
     "tomli >= 2.0, < 3.0; python_version <= '3.10'",
     "tomli_w >= 1.0, < 2.0",
+    "watchdog >= 3.0.0, < 5.0",
 ]
 
 [project.optional-dependencies]
diff --git a/src/briefcase/__main__.py b/src/briefcase/__main__.py
index 94544c278..fb0883092 100644
--- a/src/briefcase/__main__.py
+++ b/src/briefcase/__main__.py
@@ -15,48 +15,49 @@
 def main():
     result = 0
     command = None
+
     printer = Printer()
     console = Console(printer=printer)
     logger = Log(printer=printer)
-    try:
-        Command, extra_cmdline = parse_cmdline(sys.argv[1:], console=console)
-        command = Command(logger=logger, console=console)
-        options, overrides = command.parse_options(extra=extra_cmdline)
-        command.parse_config(
-            Path.cwd() / "pyproject.toml",
-            overrides=overrides,
-        )
-        command(**options)
-    except HelpText as e:
-        logger.info()
-        logger.info(str(e))
-        result = e.error_code
-    except BriefcaseWarning as w:
-        # The case of something that hasn't gone right, but in an
-        # acceptable way.
-        logger.warning(str(w))
-        result = w.error_code
-    except BriefcaseTestSuiteFailure as e:
-        # Test suite status is logged when the test is executed.
-        # Set the return code, but don't log anything else.
-        result = e.error_code
-    except BriefcaseError as e:
-        logger.error()
-        logger.error(str(e))
-        result = e.error_code
-        logger.capture_stacktrace()
-    except Exception:
-        logger.capture_stacktrace()
-        raise
-    except KeyboardInterrupt:
-        logger.warning()
-        logger.warning("Aborted by user.")
-        logger.warning()
-        result = -42
-        if logger.save_log:
+
+    with suppress(KeyboardInterrupt):
+        try:
+            Command, extra_cmdline = parse_cmdline(sys.argv[1:], console=console)
+            command = Command(logger=logger, console=console)
+            options, overrides = command.parse_options(extra=extra_cmdline)
+            command.parse_config(Path.cwd() / "pyproject.toml", overrides=overrides)
+            command(**options)
+        except HelpText as e:
+            logger.info()
+            logger.info(str(e))
+            result = e.error_code
+        except BriefcaseWarning as w:
+            # The case of something that hasn't gone right, but in an
+            # acceptable way.
+            logger.warning(str(w))
+            result = w.error_code
+        except BriefcaseTestSuiteFailure as e:
+            # Test suite status is logged when the test is executed.
+            # Set the return code, but don't log anything else.
+            result = e.error_code
+        except BriefcaseError as e:
+            logger.error()
+            logger.error(str(e))
+            result = e.error_code
+            logger.capture_stacktrace()
+        except Exception:
             logger.capture_stacktrace()
-    finally:
-        with suppress(KeyboardInterrupt):
+            raise
+        except KeyboardInterrupt:
+            logger.warning()
+            logger.warning("Aborted by user.")
+            logger.warning()
+            result = -42
+            if logger.save_log:
+                logger.capture_stacktrace()
+        finally:
+            if command is not None:
+                command.tracking_save()
             logger.save_log_to_file(command)
 
     return result
diff --git a/src/briefcase/commands/base.py b/src/briefcase/commands/base.py
index 160108308..e29a8608d 100644
--- a/src/briefcase/commands/base.py
+++ b/src/briefcase/commands/base.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import argparse
+import hashlib
 import importlib
 import importlib.metadata
 import inspect
@@ -8,22 +9,26 @@
 import platform
 import subprocess
 import sys
+import time
 from abc import ABC, abstractmethod
 from argparse import RawDescriptionHelpFormatter
+from collections.abc import Iterable
+from functools import lru_cache
 from pathlib import Path
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
+import tomli_w
 from cookiecutter import exceptions as cookiecutter_exceptions
 from cookiecutter.repository import is_repo_url
 from packaging.version import Version
 from platformdirs import PlatformDirs
+from watchdog.utils.dirsnapshot import DirectorySnapshot
 
 if sys.version_info >= (3, 11):  # pragma: no-cover-if-lt-py311
     import tomllib
 else:  # pragma: no-cover-if-gte-py311
     import tomli as tomllib
 
-import briefcase
 from briefcase import __version__
 from briefcase.config import AppConfig, GlobalConfig, parse_config
 from briefcase.console import MAX_TEXT_WIDTH, Console, Log
@@ -41,6 +46,27 @@
 from briefcase.integrations.subprocess import Subprocess
 from briefcase.platforms import get_output_formats, get_platforms
 
+if TYPE_CHECKING:
+    from briefcase.commands import (
+        BuildCommand,
+        CreateCommand,
+        PackageCommand,
+        PublishCommand,
+        RunCommand,
+        UpdateCommand,
+    )
+
+
+def timeit(func):  # TODO:PR: remove
+    def wrapper(*a, **kw):
+        start_time = time.time()
+        try:
+            return func(*a, **kw)
+        finally:
+            Log().warning(f"{func.__name__}: {round(time.time() - start_time, 3)}s")
+
+    return wrapper
+
 
 def create_config(klass, config, msg):
     try:
@@ -153,11 +179,12 @@ def __init__(
         self,
         logger: Log,
         console: Console,
-        tools: ToolCache = None,
-        apps: dict = None,
-        base_path: Path = None,
-        data_path: Path = None,
+        tools: ToolCache | None = None,
+        apps: dict[str, AppConfig] | None = None,
+        base_path: Path | None = None,
+        data_path: Path | None = None,
         is_clone: bool = False,
+        tracking: dict[AppConfig, dict[str, ...]] = None,
     ):
         """Base for all Commands.
 
@@ -171,10 +198,7 @@ def __init__(
             Command; for instance, RunCommand can invoke UpdateCommand and/or
             BuildCommand.
         """
-        if base_path is None:
-            self.base_path = Path.cwd()
-        else:
-            self.base_path = base_path
+        self.base_path = Path.cwd() if base_path is None else base_path
         self.data_path = self.validate_data_path(data_path)
         self.apps = {} if apps is None else apps
         self.is_clone = is_clone
@@ -194,6 +218,9 @@ def __init__(
 
         self.global_config = None
         self._briefcase_toml: dict[AppConfig, dict[str, ...]] = {}
+        self._tracking: dict[AppConfig, dict[str, ...]] = (
+            {} if tracking is None else tracking
+        )
 
     @property
     def logger(self):
@@ -319,40 +346,56 @@ def _command_factory(self, command_name: str):
             console=self.input,
             tools=self.tools,
             is_clone=True,
+            tracking=self._tracking,
         )
         command.clone_options(self)
         return command
 
     @property
-    def create_command(self):
+    def create_command(self) -> CreateCommand:
         """Create Command factory for the same platform and format."""
         return self._command_factory("create")
 
     @property
-    def update_command(self):
+    def update_command(self) -> UpdateCommand:
         """Update Command factory for the same platform and format."""
         return self._command_factory("update")
 
     @property
-    def build_command(self):
+    def build_command(self) -> BuildCommand:
         """Build Command factory for the same platform and format."""
         return self._command_factory("build")
 
     @property
-    def run_command(self):
+    def run_command(self) -> RunCommand:
         """Run Command factory for the same platform and format."""
         return self._command_factory("run")
 
     @property
-    def package_command(self):
+    def package_command(self) -> PackageCommand:
         """Package Command factory for the same platform and format."""
         return self._command_factory("package")
 
     @property
-    def publish_command(self):
+    def publish_command(self) -> PublishCommand:
         """Publish Command factory for the same platform and format."""
         return self._command_factory("publish")
 
+    @property
+    @lru_cache
+    def briefcase_version(self) -> Version:
+        """Parsed Briefcase version."""
+        return Version(__version__)
+
+    @property
+    @lru_cache
+    def briefcase_project_cache_path(self) -> Path:
+        """The path for project-specific information cache."""
+        path = self.base_path / ".briefcase"
+        # TODO:PR: should we go through the trouble to mark hidden on Windows?
+        path.mkdir(exist_ok=True)
+        return path
+
     def build_path(self, app) -> Path:
         """The path in which all platform artefacts for the app will be built.
 
@@ -389,6 +432,10 @@ def binary_path(self, app) -> Path:
         :param app: The app config
         """
 
+    def briefcase_toml_path(self, app: AppConfig) -> Path:
+        """Path to ``briefcase.toml`` for output format bundle."""
+        return self.bundle_path(app) / "briefcase.toml"
+
     def briefcase_toml(self, app: AppConfig) -> dict[str, ...]:
         """Load the ``briefcase.toml`` file provided by the app template.
 
@@ -399,11 +446,11 @@ def briefcase_toml(self, app: AppConfig) -> dict[str, ...]:
             return self._briefcase_toml[app]
         except KeyError:
             try:
-                with (self.bundle_path(app) / "briefcase.toml").open("rb") as f:
-                    self._briefcase_toml[app] = tomllib.load(f)
+                toml = self.briefcase_toml_path(app).read_text(encoding="utf-8")
             except OSError as e:
                 raise MissingAppMetadata(self.bundle_path(app)) from e
             else:
+                self._briefcase_toml[app] = tomllib.loads(toml)
                 return self._briefcase_toml[app]
 
     def path_index(self, app: AppConfig, path_name: str) -> str | dict | list:
@@ -487,20 +534,20 @@ def app_module_path(self, app: AppConfig) -> Path:
         """Find the path for the application module for an app.
 
         :param app: The config object for the app
-        :returns: The Path to the dist-info folder.
+        :returns: The Path to the app module
         """
         app_home = [
             path.split("/")
-            for path in app.sources
+            for path in app.sources()
             if path.rsplit("/", 1)[-1] == app.module_name
         ]
 
-        if len(app_home) == 0:
+        if len(app_home) == 1:
+            path = Path(self.base_path, *app_home[0])
+        elif len(app_home) == 0:
             raise BriefcaseCommandError(
                 f"Unable to find code for application {app.app_name!r}"
             )
-        elif len(app_home) == 1:
-            path = Path(str(self.base_path), *app_home[0])
         else:
             raise BriefcaseCommandError(
                 f"Multiple paths in sources found for application {app.app_name!r}"
@@ -508,6 +555,10 @@ def app_module_path(self, app: AppConfig) -> Path:
 
         return path
 
+    def dist_info_path(self, app: AppConfig) -> Path:
+        """Path to dist-info for the app in the output format build."""
+        return self.app_path(app) / f"{app.module_name}-{app.version}.dist-info"
+
     @property
     def briefcase_required_python_version(self):
         """The major.minor of the minimum Python version required by Briefcase itself.
@@ -754,12 +805,7 @@ def add_default_options(self, parser):
             help="Save a detailed log to file. By default, this log file is only created for critical errors",
         )
 
-    def _add_update_options(
-        self,
-        parser,
-        context_label="",
-        update=True,
-    ):
+    def _add_update_options(self, parser, context_label="", update=True):
         """Internal utility method for adding common update options.
 
         :param parser: The parser to which options should be added.
@@ -992,9 +1038,8 @@ def generate_template(
     ) -> None:
         # If a branch wasn't supplied through the --template-branch argument,
         # use the branch derived from the Briefcase version
-        version = Version(briefcase.__version__)
         if branch is None:
-            template_branch = f"v{version.base_version}"
+            template_branch = f"v{self.briefcase_version.base_version}"
         else:
             template_branch = branch
 
@@ -1005,7 +1050,7 @@ def generate_template(
             {
                 "template_source": template,
                 "template_branch": template_branch,
-                "briefcase_version": str(version),
+                "briefcase_version": str(self.briefcase_version.base_version),
             }
         )
 
@@ -1023,7 +1068,7 @@ def generate_template(
         except TemplateUnsupportedVersion:
             # Only use the main template if we're on a development branch of briefcase
             # and the user didn't explicitly specify which branch to use.
-            if version.dev is None or branch is not None:
+            if self.briefcase_version.dev is None or branch is not None:
                 raise
 
             # Development branches can use the main template.
@@ -1038,3 +1083,233 @@ def generate_template(
                 output_path=output_path,
                 extra_context=extra_context,
             )
+
+    # ------------------------------
+    # Tracking
+    # ------------------------------
+    def tracking_database_path(self, app: AppConfig) -> Path:
+        """Path to tracking database for the app.
+
+        For most commands, the database lives in the bundle directory for the output
+        format. Certain commands, such as DevCommand, will store the database elsewhere
+        since a relevant build directory will not be available.
+        """
+        return self.bundle_path(app) / "tracking.toml"
+
+    def tracking(self, app: AppConfig) -> dict[str, ...]:
+        """Load the tracking database for the app."""
+        try:
+            return self._tracking[app]["briefcase"]["app"][app.app_name]
+        except KeyError:
+            try:
+                toml = self.tracking_database_path(app).read_text(encoding="utf-8")
+            except (OSError, AttributeError):
+                toml = ""
+
+            self._tracking[app] = tomllib.loads(toml)
+            self._tracking[app].setdefault("briefcase", {})
+            self._tracking[app]["briefcase"].setdefault("app", {})
+            self._tracking[app]["briefcase"]["app"].setdefault(app.app_name, {})
+            return self._tracking[app]["briefcase"]["app"][app.app_name]
+
+    def tracking_save(self) -> None:
+        """Update the persistent tracking database for each app."""
+        for app in self.apps.values():
+            try:
+                if not self.tracking_database_path(app).parent.exists():
+                    continue
+            except AttributeError:
+                continue
+            try:
+                toml = tomli_w.dumps(self._tracking[app])
+            except KeyError:
+                pass  # skip saving tracking info for apps that never loaded it
+            else:
+                try:
+                    self.tracking_database_path(app).write_text(toml, encoding="utf-8")
+                except OSError as e:
+                    self.logger.warning(
+                        f"Failed to update build tracking for {app.app_name!r}: "
+                        f"{type(e).__name__}: {e}"
+                    )
+
+    def tracking_set(self, app: AppConfig, key: str, value: object) -> None:
+        """Set a key/value pair in the tracking cache."""
+        self.tracking(app)[key] = value
+
+    def tracking_add_briefcase_version(self, app: AppConfig) -> None:
+        """Track the version of Briefcase that created an app bundle."""
+        self.tracking_set(
+            app, key="briefcase-version", value=self.briefcase_version.base_version
+        )
+
+    def tracking_is_briefcase_version_updated(self, app: AppConfig) -> bool:
+        """Has the version of Briefcase changed since the app was created?"""
+        try:
+            tracked_briefcase_version = self.tracking(app)["briefcase-version"]
+        except KeyError:
+            return True
+        else:
+            return tracked_briefcase_version != self.briefcase_version.base_version
+
+    def tracking_add_python_env(self, app: AppConfig) -> None:
+        """Track the Python environment used for the app."""
+        self.tracking_set(
+            app,
+            key="python-exe-mtime",
+            value=self.tools.os.stat(sys.executable, follow_symlinks=False).st_mtime,
+        )
+        self.tracking_set(app, key="python-version", value=self.python_version_tag)
+
+    def tracking_is_python_env_updated(self, app: AppConfig) -> bool:
+        """Has the Python environment changed for the app?"""
+        try:
+            tracked_python_mtime = self.tracking(app)["python-exe-mtime"]
+            tracked_python_version = self.tracking(app)["python-version"]
+        except KeyError:
+            return True
+        else:
+            curr_python_mtime = self.tools.os.stat(
+                sys.executable, follow_symlinks=False
+            ).st_mtime
+            return (
+                tracked_python_mtime != curr_python_mtime
+                or tracked_python_version != self.python_version_tag
+            )
+
+    def tracking_add_created_instant(self, app: AppConfig) -> None:
+        """Track the instant when an app bundle was created."""
+        self.tracking_set(app, key="created", value=time.time())
+
+    def tracking_is_created(self, app: AppConfig) -> bool:
+        """Has the app bundle been created?"""
+        return self.tracking(app).get("created", None) is not None
+
+    @timeit
+    def tracking_add_requirements(
+        self,
+        app: AppConfig,
+        requires: Iterable[str],
+    ) -> None:
+        """Track the requirements installed for the app."""
+        self.tracking_set(app, key="requires", value=requires)
+        self.tracking_set(
+            app,
+            key="requires-files-hash",
+            value=self._tracking_fs_hash(filter(is_local_requirement, requires)),
+        )
+
+    @timeit
+    def tracking_is_requirements_updated(
+        self,
+        app: AppConfig,
+        requires: Iterable[str],
+    ) -> bool:
+        """Have the app's requirements changed since last run?"""
+        try:
+            tracked_requires = self.tracking(app)["requires"]
+        except KeyError:
+            is_requires_changed = True
+        else:
+            is_requires_changed = tracked_requires != requires
+
+        try:
+            tracked_requires_hash = self.tracking(app)["requires-files-hash"]
+        except KeyError:
+            tracked_requires_hash = ""
+
+        is_hash_changed = tracked_requires_hash != self._tracking_fs_hash(
+            filter(is_local_requirement, requires)
+        )
+
+        return is_requires_changed or is_hash_changed
+
+    def _tracking_fs_hash(self, directories: Iterable[str | os.PathLike]) -> str:
+        """Return a hash representing the current state of the directories."""
+        if not (directories := list(directories)):
+            return ""
+
+        h = hashlib.new("md5", usedforsecurity=False)
+        for directory in map(os.fsdecode, directories):
+            snapshot = DirectorySnapshot(path=directory, recursive=True)
+            # the paths must be added in the same order each time so the same
+            # hash is produced for the same set of files/dirs
+            for snapshot_path in sorted(snapshot.paths):
+                h.update(repr(snapshot.stat_info(snapshot_path)).encode())
+        return h.hexdigest()
+
+    @timeit
+    def tracking_add_sources(
+        self,
+        app: AppConfig,
+        sources: Iterable[str | os.PathLike],
+    ) -> None:
+        """Track the sources installed for the app."""
+        self.tracking_set(
+            app, key="sources-files-hash", value=self._tracking_fs_hash(sources)
+        )
+
+    @timeit
+    def tracking_is_source_modified(
+        self,
+        app: AppConfig,
+        sources: Iterable[str | os.PathLike],
+    ) -> bool:
+        """Has the app's source been modified since last run?"""
+        try:
+            tracked_hash = self.tracking(app)["sources-files-hash"]
+        except KeyError:
+            return True
+        else:
+            return tracked_hash != self._tracking_fs_hash(sources)
+
+    @timeit
+    def tracking_add_support_package(self, app: AppConfig, support_url: str) -> None:
+        """Track the support package installed for the app."""
+        self.tracking_set(app, key="support-package-url", value=support_url)
+
+    @timeit
+    def tracking_is_support_package_updated(
+        self, app: AppConfig, support_url: str
+    ) -> bool:
+        try:
+            tracked_support_url = self.tracking(app)["support-package-url"]
+        except KeyError:
+            return True
+        else:
+            return tracked_support_url != support_url
+
+
+def _has_url(requirement: str) -> bool:
+    """Determine if the requirement is defined as a URL.
+
+    Detects any of the URL schemes supported by pip
+    (https://pip.pypa.io/en/stable/topics/vcs-support/).
+
+    :param requirement: The requirement to check
+    :returns: True if the requirement is a URL supported by pip.
+    """
+    return any(
+        f"{scheme}:" in requirement
+        for scheme in (
+            ["http", "https", "file", "ftp"]
+            + ["git+file", "git+https", "git+ssh", "git+http", "git+git", "git"]
+            + ["hg+file", "hg+http", "hg+https", "hg+ssh", "hg+static-http"]
+            + ["svn", "svn+svn", "svn+http", "svn+https", "svn+ssh"]
+            + ["bzr+http", "bzr+https", "bzr+ssh", "bzr+sftp", "bzr+ftp", "bzr+lp"]
+        )
+    )
+
+
+def is_local_requirement(requirement: str) -> bool:
+    """Determine if the requirement is a local file path.
+
+    :param requirement: The requirement to check
+    :returns: True if the requirement is a local file path
+    """
+    # Windows allows both / and \ as a path separator in requirements.
+    separators = [os.sep]
+    if os.altsep:
+        separators.append(os.altsep)
+
+    return any(sep in requirement for sep in separators) and (not _has_url(requirement))
diff --git a/src/briefcase/commands/build.py b/src/briefcase/commands/build.py
index 9c3c7ddc4..a579f2a56 100644
--- a/src/briefcase/commands/build.py
+++ b/src/briefcase/commands/build.py
@@ -14,16 +14,68 @@ def add_options(self, parser):
         self._add_update_options(parser, context_label=" before building")
         self._add_test_options(parser, context_label="Build")
 
-    def build_app(self, app: AppConfig, **options):
+    def build_app(self, app: AppConfig, test_mode: bool, **options):
         """Build an application.
 
         :param app: The application to build
+        :param test_mode: Is the app being build in test mode?
         """
         # Default implementation; nothing to build.
 
+    def check_for_recreate(self, app: AppConfig) -> bool:
+        """Should the app be re-created because the environment changed?"""
+        do_create = False
+
+        if self.tracking_is_briefcase_version_updated(app):
+            self.logger.info("Environment changed", prefix=app.app_name)
+            self.logger.info(
+                self.input.textwrap(
+                    "The version of Briefcase has changed since the app's bundle was "
+                    "originally created.\n"
+                    "\n"
+                    "It is recommended to re-create your app after changing the Briefcase "
+                    "version. This will overwrite any manual updates to the files in the "
+                    "app build directory."
+                )
+            )
+            self.input.prompt()
+            do_create = self.input.boolean_input("Would you like to do this now")
+
+        if not do_create and self.tracking_is_python_env_updated(app):
+            self.logger.info("Environment changes detected", prefix=app.app_name)
+            self.logger.info(
+                self.input.textwrap(
+                    "The version of Python has changed since the app's bundle was "
+                    "originally created.\n"
+                    "\n"
+                    "It is recommended to re-create your app after changing the Python "
+                    "version. This will overwrite any manual updates to the files in the "
+                    "app build directory."
+                )
+            )
+            self.input.prompt()
+            do_create = self.input.boolean_input("Would you like to do this now")
+
+        return do_create
+
+    def update_tracking(self, app: AppConfig, test_mode: bool):
+        """Updates the tracking database for a successful build."""
+        # if an app build uses a requirements file, then the app's requirements are
+        # updated during the build; therefore, a successful build means the requirements
+        # were successfully reinstalled and need to be updated in the tracking database
+        try:
+            self.app_requirements_path(app)
+        except KeyError:
+            pass
+        else:
+            self.tracking_add_requirements(
+                app, requires=app.requires(test_mode=test_mode)
+            )
+
     def _build_app(
         self,
         app: AppConfig,
+        build: bool,
         update: bool,
         update_requirements: bool,
         update_resources: bool,
@@ -31,12 +83,13 @@ def _build_app(
         no_update: bool,
         test_mode: bool,
         **options,
-    ) -> dict | None:
+    ) -> dict:
         """Internal method to invoke a build on a single app. Ensures the app exists,
         and has been updated (if requested) before attempting to issue the actual build
         command.
 
         :param app: The application to build
+        :param build: Should the application be built irrespective?
         :param update: Should the application be updated before building?
         :param update_requirements: Should the application requirements be updated
             before building?
@@ -46,19 +99,22 @@ def _build_app(
         :param no_update: Should automated updates be disabled?
         :param test_mode: Is the app being build in test mode?
         """
-        if not self.bundle_path(app).exists():
-            state = self.create_command(app, test_mode=test_mode, **options)
-        elif (
-            update  # An explicit update has been requested
-            or update_requirements  # An explicit update of requirements has been requested
-            or update_resources  # An explicit update of resources has been requested
-            or update_support  # An explicit update of app support has been requested
-            or (
-                test_mode and not no_update
-            )  # Test mode, but updates have not been disabled
-        ):
+        bundle_exists = self.bundle_path(app).exists()
+
+        force_recreate = bundle_exists and self.check_for_recreate(app)
+
+        if not bundle_exists or force_recreate:
+            state = self.create_command(
+                app,
+                test_mode=test_mode,
+                force=force_recreate,
+                **options,
+            )
+            build = True  # always build after creating the app
+        elif not no_update:
             state = self.update_command(
                 app,
+                update_app=update,
                 update_requirements=update_requirements,
                 update_resources=update_resources,
                 update_support=update_support,
@@ -66,22 +122,28 @@ def _build_app(
                 **options,
             )
         else:
-            state = None
+            state = {}
+
+        if build or (state and state.pop("is_app_updated", False)):
+            self.verify_app(app)
 
-        self.verify_app(app)
+            state = self.build_app(
+                app, test_mode=test_mode, **full_options(state, options)
+            )
+            self.update_tracking(app, test_mode=test_mode)
 
-        state = self.build_app(app, test_mode=test_mode, **full_options(state, options))
+            qualifier = " (test mode)" if test_mode else ""
+            self.logger.info(
+                f"Built {self.binary_path(app).relative_to(self.base_path)}{qualifier}",
+                prefix=app.app_name,
+            )
 
-        qualifier = " (test mode)" if test_mode else ""
-        self.logger.info(
-            f"Built {self.binary_path(app).relative_to(self.base_path)}{qualifier}",
-            prefix=app.app_name,
-        )
         return state
 
     def __call__(
         self,
         app: AppConfig | None = None,
+        build: bool = True,
         update: bool = False,
         update_requirements: bool = False,
         update_resources: bool = False,
@@ -110,13 +172,13 @@ def __call__(
                     "Cannot specify both --update-support and --no-update"
                 )
 
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         if app:
             state = self._build_app(
                 app,
+                build=build,
                 update=update,
                 update_requirements=update_requirements,
                 update_resources=update_resources,
@@ -130,6 +192,7 @@ def __call__(
             for app_name, app in sorted(self.apps.items()):
                 state = self._build_app(
                     app,
+                    build=build,
                     update=update,
                     update_requirements=update_requirements,
                     update_resources=update_resources,
diff --git a/src/briefcase/commands/create.py b/src/briefcase/commands/create.py
index 3f62a9c4c..fb65dc427 100644
--- a/src/briefcase/commands/create.py
+++ b/src/briefcase/commands/create.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import contextlib
 import hashlib
 import os
 import platform
@@ -10,6 +11,7 @@
 from pathlib import Path
 
 import briefcase
+from briefcase.commands.base import is_local_requirement
 from briefcase.config import AppConfig
 from briefcase.exceptions import (
     BriefcaseCommandError,
@@ -210,9 +212,6 @@ def generate_app_template(self, app: AppConfig):
 
         :param app: The config object for the app
         """
-        # If the app config doesn't explicitly define a template,
-        # use a default template.
-
         # Construct a template context from the app configuration.
         extra_context = {
             key: value
@@ -224,7 +223,7 @@ def generate_app_template(self, app: AppConfig):
         extra_context.pop("template")
         extra_context.pop("template_branch")
 
-        # Augment with some extra fields.
+        # Augment with some extra fields
         extra_context.update(
             {
                 # Ensure the output format is in the case we expect
@@ -248,7 +247,7 @@ def generate_app_template(self, app: AppConfig):
         # Add in any extra template context to support permissions
         extra_context.update(self.permissions_context(app, self._x_permissions(app)))
 
-        # Add in any extra template context required by the output format.
+        # Add in any extra template context required by the output format
         extra_context.update(self.output_format_template_context(app))
 
         # Create the platform directory (if it doesn't already exist)
@@ -262,7 +261,7 @@ def generate_app_template(self, app: AppConfig):
             extra_context=extra_context,
         )
 
-    def _unpack_support_package(self, support_file_path, support_path):
+    def _unpack_support_package(self, support_file_path: Path, support_path: Path):
         """Unpack a support package into a specific location.
 
         :param support_file_path: The path to the support file to be unpacked.
@@ -271,7 +270,7 @@ def _unpack_support_package(self, support_file_path, support_path):
         # Additional protections for unpacking tar files were introduced in Python 3.12.
         # This enables the behavior that will be the default in Python 3.14.
         # However, the protections can only be enabled for tar files...not zip files.
-        is_zip = support_file_path.name.endswith("zip")
+        is_zip = support_file_path.suffix == ".zip"
         if sys.version_info >= (3, 12) and not is_zip:  # pragma: no-cover-if-lt-py312
             tarfile_kwargs = {"filter": "data"}
         else:
@@ -318,23 +317,51 @@ def install_app_support_package(self, app: AppConfig):
 
         :param app: The config object for the app
         """
-        try:
-            support_path = self.support_path(app)
-        except KeyError:
+        support_path, support_url, custom = self._app_support_package(app)
+
+        if support_path is None:
             self.logger.info("No support package required.")
+            self.tracking_add_support_package(app, support_url="")
         else:
-            support_file_path = self._download_support_package(app)
+            self.logger.info(
+                f"Using{' custom' if custom else ''} support package {support_url}"
+            )
+            support_file_path = self._resolve_support_package_url(support_url, custom)
             self._unpack_support_package(support_file_path, support_path)
+            self.tracking_add_support_package(app, support_url=support_url)
+
+    def _app_support_package(
+        self,
+        app: AppConfig,
+        warn_user: bool = True,
+    ) -> tuple[Path | None, str, bool]:
+        """Derive support package download and install locations.
+
+        Raises MissingSupportPackage if app does not define a support package.
 
-    def _download_support_package(self, app: AppConfig):
+        :param app: The config object for the app
+        :param warn_user: Disable warnings for ignored app support configuration
+        :returns: A tuple of the:
+            [0] ``Path`` where the support package should be installed; will be ``None``
+                if the app does not require a support package
+            [1] string for filesystem path or URL for support package archive
+            [2] ``True``/``False`` for whether the support package is custom and not
+                the Briefcase-provided package
+        """
+        # If the app does not define a filesystem location to install the support
+        # package in to the app, one is
+        # app doesn't require a support package if an install location
+        # is not defined in the template
         try:
-            # Work out if the app defines a custom override for
-            # the support package URL.
-            try:
-                support_package_url = app.support_package
-                custom_support_package = True
-                self.logger.info(f"Using custom support package {support_package_url}")
-                try:
+            support_path = self.support_path(app)
+        except KeyError:
+            return None, "", False
+
+        try:
+            support_url = app.support_package
+            custom_support = True
+            if warn_user:
+                with contextlib.suppress(AttributeError):
                     # If the app has a custom support package *and* a support revision,
                     # that's an error.
                     app.support_revision
@@ -342,66 +369,76 @@ def _download_support_package(self, app: AppConfig):
                         "App specifies both a support package and a support revision; "
                         "support revision will be ignored."
                     )
-                except AttributeError:
-                    pass
+        except AttributeError:
+            # If the app specifies a support revision, use it;
+            # otherwise, use the support revision named by the template
+            try:
+                support_revision = app.support_revision
             except AttributeError:
-                # If the app specifies a support revision, use it;
-                # otherwise, use the support revision named by the template
+                # No support revision specified; use the template-specified version
                 try:
-                    support_revision = app.support_revision
-                except AttributeError:
-                    # No support revision specified; use the template-specified version
-                    try:
-                        support_revision = self.support_revision(app)
-                    except KeyError:
-                        # No template-specified support revision
-                        raise MissingSupportPackage(
-                            python_version_tag=self.python_version_tag,
-                            platform=self.platform,
-                            host_arch=self.tools.host_arch,
-                            is_32bit=self.tools.is_32bit_python,
-                        )
-
-                support_package_url = self.support_package_url(support_revision)
-                custom_support_package = False
-                self.logger.info(f"Using support package {support_package_url}")
-
-            if support_package_url.startswith(("https://", "http://")):
-                if custom_support_package:
-                    # If the support package is custom, cache it using a hash of
-                    # the download URL. This is needed to differentiate to support
-                    # packages with the same filename, served at different URLs.
-                    # (or a custom package that collides with an official package name)
-                    download_path = (
-                        self.data_path
-                        / "support"
-                        / hashlib.sha256(
-                            support_package_url.encode("utf-8")
-                        ).hexdigest()
+                    support_revision = self.support_revision(app)
+                except KeyError:
+                    # No template-specified support revision
+                    raise MissingSupportPackage(
+                        python_version_tag=self.python_version_tag,
+                        platform=self.platform,
+                        host_arch=self.tools.host_arch,
+                        is_32bit=self.tools.is_32bit_python,
                     )
-                else:
-                    download_path = self.data_path / "support"
 
+            support_url = self.support_package_url(support_revision)
+            custom_support = False
+
+        return support_path, support_url, custom_support
+
+    def _resolve_support_package_url(self, support_url: str, custom: bool) -> Path:
+        """Resolve a filesystem location for the support package.
+
+        The support package for an app can be either a remote HTTP resource or a local
+        filesystem path. If the support package is a remote address, the archive is
+        downloaded and cached in the Briefcase data directory.
+
+        :param support_url: URL or filepath to support package
+        :param custom: ``True`` if user is not using a Briefcase-provided support package
+        :returns: ``Path`` for archive of support package to install in to the app
+        """
+        if support_url.startswith(("https://", "http://")):
+            if custom:
+                # If the support package is custom, cache it using a hash of
+                # the download URL. This is needed to differentiate to support
+                # packages with the same filename, served at different URLs.
+                # (or a custom package that collides with an official package name)
+                download_path = (
+                    self.data_path
+                    / "support"
+                    / hashlib.sha256(support_url.encode("utf-8")).hexdigest()
+                )
+            else:
+                download_path = self.data_path / "support"
+
+            try:
                 # Download the support file, caching the result
                 # in the user's briefcase support cache directory.
                 return self.tools.download.file(
-                    url=support_package_url,
+                    url=support_url,
                     download_path=download_path,
                     role="support package",
                 )
-            else:
-                return Path(support_package_url)
-        except MissingNetworkResourceError as e:
-            # If there is a custom support package, report the missing resource as-is.
-            if custom_support_package:
-                raise
-            else:
-                raise MissingSupportPackage(
-                    python_version_tag=self.python_version_tag,
-                    platform=self.platform,
-                    host_arch=self.tools.host_arch,
-                    is_32bit=self.tools.is_32bit_python,
-                ) from e
+            except MissingNetworkResourceError as e:
+                # If there is a custom support package, report the missing resource as-is.
+                if custom:
+                    raise
+                else:
+                    raise MissingSupportPackage(
+                        python_version_tag=self.python_version_tag,
+                        platform=self.platform,
+                        host_arch=self.tools.host_arch,
+                        is_32bit=self.tools.is_32bit_python,
+                    ) from e
+
+        else:
+            return Path(support_url)
 
     def _write_requirements_file(
         self,
@@ -424,7 +461,7 @@ def _write_requirements_file(
                         # If the requirement is a local path, convert it to
                         # absolute, because Flatpak moves the requirements file
                         # to a different place before using it.
-                        if _is_local_requirement(requirement):
+                        if is_local_requirement(requirement):
                             # We use os.path.abspath() rather than Path.resolve()
                             # because we *don't* want Path's symlink resolving behavior.
                             requirement = os.path.abspath(self.base_path / requirement)
@@ -509,7 +546,7 @@ def _install_app_requirements(
         :param requires: The list of requirements to install
         :param app_packages_path: The full path of the app_packages folder into which
             requirements should be installed.
-        :param progress_message: The waitbar progress message to display to the user.
+        :param progress_message: The Wait Bar progress message to display to the user.
         :param pip_kwargs: Any additional keyword arguments to pass to the subprocess
             when invoking pip.
         """
@@ -547,23 +584,34 @@ def install_app_requirements(self, app: AppConfig, test_mode: bool):
         :param app: The config object for the app
         :param test_mode: Should the test requirements be installed?
         """
-        requires = app.requires.copy() if app.requires else []
-        if test_mode and app.test_requires:
-            requires.extend(app.test_requires)
+        requires = app.requires(test_mode=test_mode)
 
+        requirements_path = app_packages_path = None
         try:
             requirements_path = self.app_requirements_path(app)
-            self._write_requirements_file(app, requires, requirements_path)
         except KeyError:
             try:
                 app_packages_path = self.app_packages_path(app)
-                self._install_app_requirements(app, requires, app_packages_path)
             except KeyError as e:
                 raise BriefcaseCommandError(
                     "Application path index file does not define "
                     "`app_requirements_path` or `app_packages_path`"
                 ) from e
 
+        if requirements_path:
+            self._write_requirements_file(app, requires, requirements_path)
+        else:
+            try:
+                self._install_app_requirements(app, requires, app_packages_path)
+            except BaseException:
+                # Installing the app's requirements will delete any currently installed
+                # requirements; so, if anything goes wrong, clear the tracking info to
+                # ensure the requirements are installed on the next run.
+                self.tracking_add_requirements(app, requires=[])
+                raise
+            else:
+                self.tracking_add_requirements(app, requires=requires)
+
     def install_app_code(self, app: AppConfig, test_mode: bool):
         """Install the application code into the bundle.
 
@@ -576,9 +624,7 @@ def install_app_code(self, app: AppConfig, test_mode: bool):
             self.tools.shutil.rmtree(app_path)
         self.tools.os.mkdir(app_path)
 
-        sources = app.sources.copy() if app.sources else []
-        if test_mode and app.test_sources:
-            sources.extend(app.test_sources)
+        sources = app.sources(test_mode=test_mode)
 
         # Install app code.
         if sources:
@@ -597,12 +643,10 @@ def install_app_code(self, app: AppConfig, test_mode: bool):
         else:
             self.logger.info(f"No sources defined for {app.app_name}.")
 
-        # Write the dist-info folder for the application.
-        write_dist_info(
-            app=app,
-            dist_info_path=self.app_path(app)
-            / f"{app.module_name}-{app.version}.dist-info",
-        )
+        self.tracking_add_sources(app, sources=sources)
+
+        # Write the dist-info folder for the application
+        write_dist_info(app=app, dist_info_path=self.dist_info_path(app))
 
     def install_image(self, role, variant, size, source, target):
         """Install an icon/image of the requested size at a target location, using the
@@ -772,26 +816,41 @@ def cleanup_app_content(self, app: AppConfig):
                         self.logger.verbose(f"Removing {relative_path}")
                         path.unlink()
 
-    def create_app(self, app: AppConfig, test_mode: bool = False, **options):
+    def update_tracking(self, app: AppConfig):
+        """Updates the tracking database when an app is successfully created."""
+        self.tracking_add_created_instant(app)
+        self.tracking_add_briefcase_version(app)
+        self.tracking_add_python_env(app)
+
+    def create_app(
+        self,
+        app: AppConfig,
+        test_mode: bool = False,
+        force: bool = False,
+        **options,
+    ):
         """Create an application bundle.
 
         :param app: The config object for the app
         :param test_mode: Should the app be updated in test mode? (default: False)
+        :param force: Should the app be created if it already exists? (default: False)
         """
         if not app.supported:
             raise UnsupportedPlatform(self.platform)
 
         bundle_path = self.bundle_path(app)
+
         if bundle_path.exists():
-            self.logger.info()
-            confirm = self.input.boolean_input(
-                f"Application {app.app_name!r} already exists; overwrite", default=False
-            )
-            if not confirm:
-                self.logger.error(
-                    f"Aborting creation of app {app.app_name!r}; existing application will not be overwritten."
-                )
-                return
+            if not force:
+                self.logger.info()
+                if not self.input.boolean_input(
+                    f"Application {app.app_name!r} already exists; overwrite",
+                    default=False,
+                ):
+                    raise BriefcaseCommandError(
+                        f"Aborting re-creation of app {app.app_name!r}",
+                        skip_logfile=True,
+                    )
             self.logger.info("Removing old application bundle...", prefix=app.app_name)
             self.tools.shutil.rmtree(bundle_path)
 
@@ -817,6 +876,8 @@ def create_app(self, app: AppConfig, test_mode: bool = False, **options):
         self.logger.info("Removing unneeded app content...", prefix=app.app_name)
         self.cleanup_app_content(app=app)
 
+        self.update_tracking(app=app)
+
         self.logger.info(
             f"Created {bundle_path.relative_to(self.base_path)}",
             prefix=app.app_name,
@@ -840,8 +901,7 @@ def __call__(
         app: AppConfig | None = None,
         **options,
     ) -> dict | None:
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         if app:
@@ -852,38 +912,3 @@ def __call__(
                 state = self.create_app(app, **full_options(state, options))
 
         return state
-
-
-def _has_url(requirement):
-    """Determine if the requirement is defined as a URL.
-
-    Detects any of the URL schemes supported by pip
-    (https://pip.pypa.io/en/stable/topics/vcs-support/).
-
-    :param requirement: The requirement to check
-    :returns: True if the requirement is a URL supported by pip.
-    """
-    return any(
-        f"{scheme}:" in requirement
-        for scheme in (
-            ["http", "https", "file", "ftp"]
-            + ["git+file", "git+https", "git+ssh", "git+http", "git+git", "git"]
-            + ["hg+file", "hg+http", "hg+https", "hg+ssh", "hg+static-http"]
-            + ["svn", "svn+svn", "svn+http", "svn+https", "svn+ssh"]
-            + ["bzr+http", "bzr+https", "bzr+ssh", "bzr+sftp", "bzr+ftp", "bzr+lp"]
-        )
-    )
-
-
-def _is_local_requirement(requirement):
-    """Determine if the requirement is a local file path.
-
-    :param requirement: The requirement to check
-    :returns: True if the requirement is a local file path
-    """
-    # Windows allows both / and \ as a path separator in requirements.
-    separators = [os.sep]
-    if os.altsep:
-        separators.append(os.altsep)
-
-    return any(sep in requirement for sep in separators) and (not _has_url(requirement))
diff --git a/src/briefcase/commands/dev.py b/src/briefcase/commands/dev.py
index f37e36ec5..fe5f7f25b 100644
--- a/src/briefcase/commands/dev.py
+++ b/src/briefcase/commands/dev.py
@@ -74,16 +74,26 @@ def add_options(self, parser):
             help="Run the app in test mode",
         )
 
-    def install_dev_requirements(self, app: AppConfig, **options):
+    def dist_info_path(self, app: AppConfig) -> Path:
+        """Path to dist-info for the app where the app source lives."""
+        return self.app_module_path(app).parent / f"{app.module_name}.dist-info"
+
+    def tracking_database_path(self, app: AppConfig) -> Path:
+        """Path to tracking database when running in dev mode."""
+        return self.briefcase_project_cache_path / "tracking.toml"
+
+    def update_tracking(self, app: AppConfig) -> None:
+        self.tracking_add_python_env(app)
+
+    def install_dev_requirements(self, app: AppConfig, test_mode: bool, **options):
         """Install the requirements for the app dev.
 
         This will always include test requirements, if specified.
 
         :param app: The config object for the app
+        :param test_mode: Whether the test suite is being run, rather than the app?
         """
-        requires = app.requires if app.requires else []
-        if app.test_requires:
-            requires.extend(app.test_requires)
+        requires = app.requires(test_mode=test_mode)
 
         if requires:
             with self.input.wait_bar("Installing dev requirements..."):
@@ -106,6 +116,8 @@ def install_dev_requirements(self, app: AppConfig, **options):
                     )
                 except subprocess.CalledProcessError as e:
                     raise RequirementsInstallError() from e
+                else:
+                    self.tracking_add_requirements(app, requires=requires)
         else:
             self.logger.info("No application requirements.")
 
@@ -157,6 +169,8 @@ def run_dev_app(
             clean_output=False,
         )
 
+        self.update_tracking(app)
+
     def get_environment(self, app, test_mode: bool):
         # Create a shell environment where PYTHONPATH points to the source
         # directories described by the app config.
@@ -195,31 +209,35 @@ def __call__(
                 raise BriefcaseCommandError(
                     f"Project doesn't define an application named '{appname}'"
                 ) from e
-
         else:
             raise BriefcaseCommandError(
                 "Project specifies more than one application; use --app to specify which one to start."
             )
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         self.verify_app(app)
 
-        # Look for the existence of a dist-info file.
-        # If one exists, assume that the requirements have already been
-        # installed. If a dependency update has been manually requested,
-        # do it regardless.
-        dist_info_path = (
-            self.app_module_path(app).parent / f"{app.module_name}.dist-info"
-        )
-        if not run_app:
-            # If we are not running the app, it means we should update requirements.
-            update_requirements = True
-        if update_requirements or not dist_info_path.exists():
+        # If we are not running the app, it means we should update requirements.
+        update_requirements |= not run_app
+
+        if not update_requirements:
+            update_requirements = self.tracking_is_python_env_updated(app)
+            if update_requirements:  # TODO:PR: delete
+                self.logger.warning("Python environment change detected")
+
+        if not update_requirements:
+            update_requirements = self.tracking_is_requirements_updated(
+                app, requires=app.requires(test_mode=test_mode)
+            )
+            if update_requirements:  # TODO:PR: delete
+                self.logger.warning("Requirements change detected")
+
+        if update_requirements:
             self.logger.info("Installing requirements...", prefix=app.app_name)
-            self.install_dev_requirements(app, **options)
-            write_dist_info(app, dist_info_path)
+            self.install_dev_requirements(app, test_mode, **options)
+            write_dist_info(app, self.dist_info_path(app))
 
         if run_app:
             if test_mode:
@@ -228,10 +246,10 @@ def __call__(
                 )
             else:
                 self.logger.info("Starting in dev mode...", prefix=app.app_name)
-            env = self.get_environment(app, test_mode=test_mode)
+
             return self.run_dev_app(
                 app,
-                env,
+                env=self.get_environment(app, test_mode=test_mode),
                 test_mode=test_mode,
                 passthrough=[] if passthrough is None else passthrough,
                 **options,
diff --git a/src/briefcase/commands/open.py b/src/briefcase/commands/open.py
index 5d7b34629..a869bcdab 100644
--- a/src/briefcase/commands/open.py
+++ b/src/briefcase/commands/open.py
@@ -49,8 +49,7 @@ def __call__(
         app: AppConfig | None = None,
         **options,
     ):
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         if app:
diff --git a/src/briefcase/commands/package.py b/src/briefcase/commands/package.py
index 403481907..114d70971 100644
--- a/src/briefcase/commands/package.py
+++ b/src/briefcase/commands/package.py
@@ -58,28 +58,16 @@ def _package_app(
         :param update: Should the application be updated (and rebuilt) first?
         :param packaging_format: The format of the packaging artefact to create.
         """
-
-        template_file = self.bundle_path(app)
-        binary_file = self.binary_path(app)
-        if not template_file.exists():
-            state = self.create_command(app, **options)
-            state = self.build_command(app, **full_options(state, options))
-        elif update:
-            # If we're updating for packaging, update everything.
-            # This ensures everything in the packaged artefact is up to date,
-            # and is in a production state
-            state = self.update_command(
-                app,
-                update_resources=True,
-                update_requirements=True,
-                update_support=True,
-                **options,
-            )
-            state = self.build_command(app, **full_options(state, options))
-        elif not binary_file.exists():
-            state = self.build_command(app, **options)
-        else:
-            state = None
+        # Update and build the app if necessary
+        state = self.build_command(
+            app,
+            build=not self.binary_path(app).exists(),
+            update=update,
+            update_resources=update,
+            update_requirements=update,
+            update_support=update,
+            **options,
+        )
 
         # Annotate the packaging format onto the app
         app.packaging_format = packaging_format
@@ -100,6 +88,7 @@ def _package_app(
 
         filename = self.distribution_path(app).relative_to(self.base_path)
         self.logger.info(f"Packaged {filename}", prefix=app.app_name)
+
         return state
 
     def add_options(self, parser):
@@ -139,8 +128,7 @@ def __call__(
         update: bool = False,
         **options,
     ) -> dict | None:
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         if app:
diff --git a/src/briefcase/commands/run.py b/src/briefcase/commands/run.py
index 4977b3419..9dd41cbf0 100644
--- a/src/briefcase/commands/run.py
+++ b/src/briefcase/commands/run.py
@@ -279,25 +279,13 @@ def __call__(
                 "Project specifies more than one application; use --app to specify which one to start."
             )
 
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
-        template_file = self.bundle_path(app)
-        binary_file = self.binary_path(app)
-        if (
-            (not template_file.exists())  # App hasn't been created
-            or update  # An explicit update has been requested
-            or update_requirements  # An explicit update of requirements has been requested
-            or update_resources  # An explicit update of resources has been requested
-            or update_support  # An explicit update of support files has been requested
-            or (not binary_file.exists())  # Binary doesn't exist yet
-            or (
-                test_mode and not no_update
-            )  # Test mode, but updates have not been disabled
-        ):
+        if not no_update:
             state = self.build_command(
                 app,
+                build=not self.binary_path(app).exists(),
                 update=update,
                 update_requirements=update_requirements,
                 update_resources=update_resources,
diff --git a/src/briefcase/commands/update.py b/src/briefcase/commands/update.py
index 4b3809fcc..f754771b0 100644
--- a/src/briefcase/commands/update.py
+++ b/src/briefcase/commands/update.py
@@ -1,6 +1,9 @@
 from __future__ import annotations
 
+from contextlib import suppress
+
 from briefcase.config import AppConfig
+from briefcase.exceptions import MissingNetworkResourceError, MissingSupportPackage
 
 from .base import full_options
 from .create import CreateCommand
@@ -17,31 +20,63 @@ def add_options(self, parser):
     def update_app(
         self,
         app: AppConfig,
+        update_app: bool,
         update_requirements: bool,
         update_resources: bool,
         update_support: bool,
         test_mode: bool,
         **options,
-    ) -> dict | None:
+    ) -> dict:
         """Update an existing application bundle.
 
         :param app: The config object for the app
+        :param update_app: Should the app sources be updated?
         :param update_requirements: Should requirements be updated?
         :param update_resources: Should extra resources be updated?
         :param update_support: Should app support be updated?
-        :param test_mode: Should the app be updated in test mode?
+        :param test_mode: Should the app be updated for test mode?
         """
-
         if not self.bundle_path(app).exists():
             self.logger.error(
                 "Application does not exist; call create first!", prefix=app.app_name
             )
-            return
+            return {}
+
+        if not update_app:
+            update_app = self.tracking_is_source_modified(
+                app, sources=app.sources(test_mode=test_mode)
+            )
+            if update_app:  # TODO:PR: delete
+                self.logger.warning("App source change detected")
 
-        self.verify_app(app)
+        if not update_requirements:
+            update_requirements = self.tracking_is_requirements_updated(
+                app, requires=app.requires(test_mode=test_mode)
+            )
+            if update_requirements:  # TODO:PR: delete
+                self.logger.warning("Requirements change detected")
 
-        self.logger.info("Updating application code...", prefix=app.app_name)
-        self.install_app_code(app=app, test_mode=test_mode)
+        if not update_resources:
+            update_resources = update_app
+
+        if not update_support:
+            with suppress(MissingSupportPackage, MissingNetworkResourceError):
+                update_support = self.tracking_is_support_package_updated(
+                    app, support_url=self._app_support_package(app, warn_user=False)[1]
+                )
+            if update_support:  # TODO:PR: delete
+                self.logger.warning("Support package change detected")
+
+        is_app_being_updated = (
+            update_app or update_requirements or update_resources or update_support
+        )
+
+        if is_app_being_updated:
+            self.verify_app(app)
+
+        if update_app:
+            self.logger.info("Updating application code...", prefix=app.app_name)
+            self.install_app_code(app=app, test_mode=test_mode)
 
         if update_requirements:
             self.logger.info("Updating requirements...", prefix=app.app_name)
@@ -56,27 +91,32 @@ def update_app(
             self.cleanup_app_support_package(app=app)
             self.install_app_support_package(app=app)
 
-        self.logger.info("Removing unneeded app content...", prefix=app.app_name)
-        self.cleanup_app_content(app=app)
+        if is_app_being_updated:
+            self.logger.info("Removing unneeded app content...", prefix=app.app_name)
+            self.cleanup_app_content(app=app)
+
+            self.logger.info("Application updated.", prefix=app.app_name)
 
-        self.logger.info("Application updated.", prefix=app.app_name)
+        # TODO:PR: expand to track each event type
+        return {"is_app_updated": is_app_being_updated}
 
     def __call__(
         self,
         app: AppConfig | None = None,
+        update_app: bool = True,
         update_requirements: bool = False,
         update_resources: bool = False,
         update_support: bool = False,
         test_mode: bool = False,
         **options,
-    ) -> dict | None:
-        # Confirm host compatibility, that all required tools are available,
-        # and that the app configuration is finalized.
+    ) -> dict:
+        # Finish preparing the AppConfig and run final checks required to for command
         self.finalize(app)
 
         if app:
             state = self.update_app(
                 app,
+                update_app=update_app,
                 update_requirements=update_requirements,
                 update_resources=update_resources,
                 update_support=update_support,
@@ -88,6 +128,7 @@ def __call__(
             for app_name, app in sorted(self.apps.items()):
                 state = self.update_app(
                     app,
+                    update_app=update_app,
                     update_requirements=update_requirements,
                     update_resources=update_resources,
                     update_support=update_support,
diff --git a/src/briefcase/config.py b/src/briefcase/config.py
index a86f8f2ac..6c4806b55 100644
--- a/src/briefcase/config.py
+++ b/src/briefcase/config.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import copy
 import keyword
 import re
@@ -210,12 +212,12 @@ def __init__(
         self.bundle = bundle
         # Description can only be a single line. Ignore everything else.
         self.description = description.split("\n")[0]
-        self.sources = sources
+        self._sources = sources
         self.formal_name = app_name if formal_name is None else formal_name
         self.url = url
         self.author = author
         self.author_email = author_email
-        self.requires = requires
+        self._requires = requires
         self.icon = icon
         self.splash = splash
         self.document_types = {} if document_type is None else document_type
@@ -253,8 +255,8 @@ def __init__(
             )
 
         # Sources list doesn't include any duplicates
-        source_modules = {source.rsplit("/", 1)[-1] for source in self.sources}
-        if len(self.sources) != len(source_modules):
+        source_modules = {source.rsplit("/", 1)[-1] for source in self._sources}
+        if len(self._sources) != len(source_modules):
             raise BriefcaseConfigError(
                 f"The `sources` list for {self.app_name!r} contains duplicated "
                 "package names."
@@ -267,11 +269,11 @@ def __init__(
                 f"package named {self.module_name!r}."
             )
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<{self.bundle_identifier} v{self.version} AppConfig>"
 
     @property
-    def module_name(self):
+    def module_name(self) -> str:
         """The module name for the app.
 
         This is derived from the name, but:
@@ -280,7 +282,7 @@ def module_name(self):
         return self.app_name.replace("-", "_")
 
     @property
-    def bundle_name(self):
+    def bundle_name(self) -> str:
         """The bundle name for the app.
 
         This is derived from the app name, but:
@@ -289,7 +291,7 @@ def bundle_name(self):
         return self.app_name.replace("_", "-")
 
     @property
-    def bundle_identifier(self):
+    def bundle_identifier(self) -> str:
         """The bundle identifier for the app.
 
         This is derived from the bundle and the bundle name, joined by a `.`.
@@ -297,7 +299,7 @@ def bundle_identifier(self):
         return f"{self.bundle}.{self.bundle_name}"
 
     @property
-    def class_name(self):
+    def class_name(self) -> str:
         """The class name for the app.
 
         This is derived from the formal name for the app.
@@ -305,19 +307,19 @@ def class_name(self):
         return make_class_name(self.formal_name)
 
     @property
-    def package_name(self):
+    def package_name(self) -> str:
         """The bundle name of the app, with `-` replaced with `_` to create something
         that can be used a namespace identifier on Python or Java, similar to
         `module_name`."""
         return self.bundle.replace("-", "_")
 
-    def PYTHONPATH(self, test_mode):
+    def PYTHONPATH(self, test_mode: bool) -> list[str]:
         """The PYTHONPATH modifications needed to run this app.
 
         :param test_mode: Should test_mode sources be included?
         """
         paths = []
-        sources = self.sources
+        sources = self._sources
         if test_mode and self.test_sources:
             sources.extend(self.test_sources)
 
@@ -327,7 +329,7 @@ def PYTHONPATH(self, test_mode):
                 paths.append(path)
         return paths
 
-    def main_module(self, test_mode: bool):
+    def main_module(self, test_mode: bool) -> str:
         """The path to the main module for the app.
 
         In normal operation, this is ``app.module_name``; however,
@@ -340,6 +342,20 @@ def main_module(self, test_mode: bool):
         else:
             return self.module_name
 
+    def requires(self, test_mode: bool = False) -> list[str]:
+        """App requirements incorporating whether test mode is active."""
+        requires = self._requires.copy() if self._requires else []
+        if test_mode and self.test_requires:
+            requires.extend(self.test_requires)
+        return requires
+
+    def sources(self, test_mode: bool = False) -> list[str]:
+        """App sources incorporating whether test mode is active."""
+        sources = self._sources.copy() if self._sources else []
+        if test_mode and self.test_sources:
+            sources.extend(self.test_sources)
+        return sources
+
 
 def merge_config(config, data):
     """Merge a new set of configuration requirements into a base configuration.
@@ -464,9 +480,11 @@ def parse_config(config_file, platform, output_format):
 
     # Merge the PEP621 configuration (if it exists)
     try:
-        merge_pep621_config(global_config, pyproject["project"])
+        pep612_config = pyproject["project"]
     except KeyError:
         pass
+    else:
+        merge_pep621_config(global_config, pep612_config)
 
     # For consistent results, sort the platforms and formats
     all_platforms = sorted(get_platforms().keys())
diff --git a/src/briefcase/integrations/docker.py b/src/briefcase/integrations/docker.py
index e7660aaf1..cd853f7a5 100644
--- a/src/briefcase/integrations/docker.py
+++ b/src/briefcase/integrations/docker.py
@@ -924,7 +924,7 @@ def prepare(
                             f"HOST_GID={self.tools.os.getgid()}",
                             Path(
                                 self.app_base_path,
-                                *self.app.sources[0].split("/")[:-1],
+                                *self.app.sources()[0].split("/")[:-1],
                             ),
                         ]
                         + (extra_build_args if extra_build_args is not None else []),
diff --git a/src/briefcase/platforms/linux/__init__.py b/src/briefcase/platforms/linux/__init__.py
index 63ec625db..6166151b8 100644
--- a/src/briefcase/platforms/linux/__init__.py
+++ b/src/briefcase/platforms/linux/__init__.py
@@ -5,7 +5,7 @@
 from pathlib import Path
 from typing import List
 
-from briefcase.commands.create import _is_local_requirement
+from briefcase.commands.base import is_local_requirement
 from briefcase.commands.open import OpenCommand
 from briefcase.config import AppConfig
 from briefcase.exceptions import BriefcaseCommandError, ParseError
@@ -156,7 +156,7 @@ def _install_app_requirements(
 
         # Iterate over every requirement, looking for local references
         for requirement in requires:
-            if _is_local_requirement(requirement):
+            if is_local_requirement(requirement):
                 if Path(requirement).is_dir():
                     # Requirement is a filesystem reference
                     # Build an sdist for the local requirement
@@ -210,7 +210,7 @@ def _pip_requires(self, app: AppConfig, requires: List[str]):
         final = [
             requirement
             for requirement in super()._pip_requires(app, requires)
-            if not _is_local_requirement(requirement)
+            if not is_local_requirement(requirement)
         ]
 
         # Add in any local packages.
diff --git a/src/briefcase/platforms/linux/flatpak.py b/src/briefcase/platforms/linux/flatpak.py
index 94ccc27b3..da4f21f01 100644
--- a/src/briefcase/platforms/linux/flatpak.py
+++ b/src/briefcase/platforms/linux/flatpak.py
@@ -160,10 +160,11 @@ class LinuxFlatpakOpenCommand(LinuxFlatpakMixin, OpenCommand):
 class LinuxFlatpakBuildCommand(LinuxFlatpakMixin, BuildCommand):
     description = "Build a Linux Flatpak."
 
-    def build_app(self, app: AppConfig, **kwargs):
+    def build_app(self, app: AppConfig, test_mode: bool, **kwargs):
         """Build an application.
 
         :param app: The application to build
+        :param test_mode: Is the app being build in test mode?
         """
         self.logger.info(
             "Ensuring Flatpak runtime for the app is available...",
diff --git a/tests/commands/base/test_app_module_path.py b/tests/commands/base/test_app_module_path.py
index bce0edf05..784faa4f3 100644
--- a/tests/commands/base/test_app_module_path.py
+++ b/tests/commands/base/test_app_module_path.py
@@ -6,7 +6,7 @@
 def test_single_source(base_command, my_app):
     """If an app provides a single source location and it matches, it is selected as the
     dist-info location."""
-    my_app.sources = ["src/my_app"]
+    my_app._sources = ["src/my_app"]
 
     assert base_command.app_module_path(my_app) == base_command.base_path / "src/my_app"
 
@@ -14,7 +14,7 @@ def test_single_source(base_command, my_app):
 def test_no_prefix(base_command, my_app):
     """If an app provides a source location without a prefix and it matches, it is
     selected as the dist-info location."""
-    my_app.sources = ["my_app"]
+    my_app._sources = ["my_app"]
 
     assert base_command.app_module_path(my_app) == base_command.base_path / "my_app"
 
@@ -22,7 +22,7 @@ def test_no_prefix(base_command, my_app):
 def test_long_prefix(base_command, my_app):
     """If an app provides a source location with a long prefix and it matches, it is
     selected as the dist-info location."""
-    my_app.sources = ["path/to/src/my_app"]
+    my_app._sources = ["path/to/src/my_app"]
 
     assert (
         base_command.app_module_path(my_app)
@@ -33,14 +33,14 @@ def test_long_prefix(base_command, my_app):
 def test_matching_source(base_command, my_app):
     """If an app provides a single matching source location, it is selected as the dist-
     info location."""
-    my_app.sources = ["src/other", "src/my_app", "src/extra"]
+    my_app._sources = ["src/other", "src/my_app", "src/extra"]
 
     assert base_command.app_module_path(my_app) == base_command.base_path / "src/my_app"
 
 
 def test_multiple_match(base_command, my_app):
     """If an app provides multiple matching source location, an error is raised."""
-    my_app.sources = ["src/my_app", "extra/my_app"]
+    my_app._sources = ["src/my_app", "extra/my_app"]
 
     with pytest.raises(
         BriefcaseCommandError,
@@ -52,7 +52,7 @@ def test_multiple_match(base_command, my_app):
 def test_hyphen_source(base_command, my_app):
     """If an app provides a single source location with a hyphen, an error is raised."""
     # The source directory must be a valid module, so hyphens aren't legal.
-    my_app.sources = ["src/my-app"]
+    my_app._sources = ["src/my-app"]
 
     with pytest.raises(
         BriefcaseCommandError,
@@ -64,7 +64,7 @@ def test_hyphen_source(base_command, my_app):
 def test_no_match(base_command, my_app):
     """If an app provides a multiple locations, none of which match, an error is
     raised."""
-    my_app.sources = ["src/pork", "src/spam"]
+    my_app._sources = ["src/pork", "src/spam"]
 
     with pytest.raises(
         BriefcaseCommandError,
@@ -75,7 +75,7 @@ def test_no_match(base_command, my_app):
 
 def test_no_source(base_command, my_app):
     """If an app provides no source locations, an error is raised."""
-    my_app.sources = []
+    my_app._sources = []
 
     with pytest.raises(
         BriefcaseCommandError,
diff --git a/tests/commands/create/test_install_app_code.py b/tests/commands/create/test_install_app_code.py
index ab0a754c5..a398001e2 100644
--- a/tests/commands/create/test_install_app_code.py
+++ b/tests/commands/create/test_install_app_code.py
@@ -50,7 +50,7 @@ def test_no_code(
     create_command.tools.shutil = mock.MagicMock(spec_set=shutil)
     create_command.tools.os = mock.MagicMock(spec_set=os)
 
-    myapp.sources = None
+    myapp._sources = None
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -76,7 +76,7 @@ def test_empty_code(
     create_command.tools.shutil = mock.MagicMock(spec_set=shutil)
     create_command.tools.os = mock.MagicMock(spec_set=os)
 
-    myapp.sources = []
+    myapp._sources = []
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -98,7 +98,7 @@ def test_source_missing(
 ):
     """If an app defines sources that are missing, an error is raised."""
     # Set the app definition to point at sources that don't exist
-    myapp.sources = ["missing"]
+    myapp._sources = ["missing"]
 
     with pytest.raises(MissingAppSources):
         create_command.install_app_code(myapp, test_mode=False)
@@ -138,7 +138,7 @@ def test_source_dir(
     )
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/first", "src/second"]
+    myapp._sources = ["src/first", "src/second"]
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -156,7 +156,7 @@ def test_source_dir(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/first", "src/second"]
+    assert myapp._sources == ["src/first", "src/second"]
     assert myapp.test_sources is None
 
 
@@ -182,7 +182,7 @@ def test_source_file(
     )
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/demo.py", "other.py"]
+    myapp._sources = ["src/demo.py", "other.py"]
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -194,7 +194,7 @@ def test_source_file(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/demo.py", "other.py"]
+    assert myapp._sources == ["src/demo.py", "other.py"]
     assert myapp.test_sources is None
 
 
@@ -231,7 +231,7 @@ def test_no_existing_app_folder(
     shutil.rmtree(app_path)
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/first/demo.py", "src/second"]
+    myapp._sources = ["src/first/demo.py", "src/second"]
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -260,7 +260,7 @@ def test_no_existing_app_folder(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/first/demo.py", "src/second"]
+    assert myapp._sources == ["src/first/demo.py", "src/second"]
     assert myapp.test_sources is None
 
 
@@ -334,7 +334,7 @@ def test_replace_sources(
     old_dist_info_dir.mkdir()
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/first/demo.py", "src/second"]
+    myapp._sources = ["src/first/demo.py", "src/second"]
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -363,7 +363,7 @@ def test_replace_sources(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/first/demo.py", "src/second"]
+    assert myapp._sources == ["src/first/demo.py", "src/second"]
     assert myapp.test_sources is None
 
 
@@ -385,7 +385,7 @@ def test_non_latin_metadata(
     create_command.tools.shutil = mock.MagicMock(spec_set=shutil)
     create_command.tools.os = mock.MagicMock(spec_set=os)
 
-    myapp.sources = []
+    myapp._sources = []
 
     create_command.install_app_code(myapp, test_mode=False)
 
@@ -471,7 +471,7 @@ def test_test_sources(
     )
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/first", "src/second"]
+    myapp._sources = ["src/first", "src/second"]
     myapp.test_sources = ["tests", "othertests"]
 
     create_command.install_app_code(myapp, test_mode=False)
@@ -491,7 +491,7 @@ def test_test_sources(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/first", "src/second"]
+    assert myapp._sources == ["src/first", "src/second"]
     assert myapp.test_sources == ["tests", "othertests"]
 
 
@@ -543,7 +543,7 @@ def test_test_sources_test_mode(
     )
 
     # Set the app definition, and install sources
-    myapp.sources = ["src/first", "src/second"]
+    myapp._sources = ["src/first", "src/second"]
     myapp.test_sources = ["tests", "othertests"]
 
     create_command.install_app_code(myapp, test_mode=True)
@@ -566,7 +566,7 @@ def test_test_sources_test_mode(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources == ["src/first", "src/second"]
+    assert myapp._sources == ["src/first", "src/second"]
     assert myapp.test_sources == ["tests", "othertests"]
 
 
@@ -614,7 +614,7 @@ def test_only_test_sources_test_mode(
     )
 
     # Set the app definition, and install sources
-    myapp.sources = None
+    myapp._sources = None
     myapp.test_sources = ["tests", "othertests"]
 
     create_command.install_app_code(myapp, test_mode=True)
@@ -634,5 +634,5 @@ def test_only_test_sources_test_mode(
     assert_dist_info(app_path)
 
     # Original app definitions haven't changed
-    assert myapp.sources is None
+    assert myapp._sources is None
     assert myapp.test_sources == ["tests", "othertests"]
diff --git a/tests/commands/create/test_install_app_requirements.py b/tests/commands/create/test_install_app_requirements.py
index 59dda9dc4..6e2b2f836 100644
--- a/tests/commands/create/test_install_app_requirements.py
+++ b/tests/commands/create/test_install_app_requirements.py
@@ -6,7 +6,7 @@
 import pytest
 import tomli_w
 
-from briefcase.commands.create import _is_local_requirement
+from briefcase.commands.create import is_local_requirement
 from briefcase.console import LogLevel
 from briefcase.exceptions import BriefcaseCommandError, RequirementsInstallError
 from briefcase.integrations.subprocess import Subprocess
@@ -56,7 +56,7 @@ def test_bad_path_index(create_command, myapp, bundle_path, app_requirements_pat
         tomli_w.dump(index, f)
 
     # Set up requirements for the app
-    myapp.requires = ["first", "second", "third"]
+    myapp._requires = ["first", "second", "third"]
 
     # Install requirements
     with pytest.raises(
@@ -72,7 +72,7 @@ def test_bad_path_index(create_command, myapp, bundle_path, app_requirements_pat
     assert not app_requirements_path.exists()
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second", "third"]
+    assert myapp._requires == ["first", "second", "third"]
     assert myapp.test_requires is None
 
 
@@ -83,7 +83,7 @@ def test_app_packages_no_requires(
     app_packages_path_index,
 ):
     """If an app has no requirements, install_app_requirements is a no-op."""
-    myapp.requires = None
+    myapp._requires = None
 
     create_command.install_app_requirements(myapp, test_mode=False)
 
@@ -98,7 +98,7 @@ def test_app_packages_empty_requires(
     app_packages_path_index,
 ):
     """If an app has an empty requirements list, install_app_requirements is a no-op."""
-    myapp.requires = []
+    myapp._requires = []
 
     create_command.install_app_requirements(myapp, test_mode=False)
 
@@ -113,7 +113,7 @@ def test_app_packages_valid_requires(
     app_packages_path_index,
 ):
     """If an app has a valid list of requirements, pip is invoked."""
-    myapp.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    myapp._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     create_command.install_app_requirements(myapp, test_mode=False)
 
@@ -141,7 +141,7 @@ def test_app_packages_valid_requires(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second==1.2.3", "third>=3.2.1"]
+    assert myapp._requires == ["first", "second==1.2.3", "third>=3.2.1"]
     assert myapp.test_requires is None
 
 
@@ -153,7 +153,7 @@ def test_app_packages_valid_requires_no_support_package(
 ):
     """If the template doesn't specify a support package, the cross-platform site isn't
     specified."""
-    myapp.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    myapp._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     # Override the cache of paths to specify an app packages path, but no support package path
     create_command._briefcase_toml[myapp] = {
@@ -186,7 +186,7 @@ def test_app_packages_valid_requires_no_support_package(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second==1.2.3", "third>=3.2.1"]
+    assert myapp._requires == ["first", "second==1.2.3", "third>=3.2.1"]
     assert myapp.test_requires is None
 
 
@@ -197,7 +197,7 @@ def test_app_packages_invalid_requires(
     app_packages_path_index,
 ):
     """If an app has a valid list of requirements, pip is invoked."""
-    myapp.requires = ["does-not-exist"]
+    myapp._requires = ["does-not-exist"]
 
     # Unfortunately, no way to tell the difference between "offline" and
     # "your requirements are invalid"; pip returns status code 1 for all
@@ -233,7 +233,7 @@ def test_app_packages_invalid_requires(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["does-not-exist"]
+    assert myapp._requires == ["does-not-exist"]
     assert myapp.test_requires is None
 
 
@@ -244,7 +244,7 @@ def test_app_packages_offline(
     app_packages_path_index,
 ):
     """If user is offline, pip fails."""
-    myapp.requires = ["first", "second", "third"]
+    myapp._requires = ["first", "second", "third"]
 
     # Unfortunately, no way to tell the difference between "offline" and
     # "your requirements are invalid"; pip returns status code 1 for all
@@ -282,7 +282,7 @@ def test_app_packages_offline(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second", "third"]
+    assert myapp._requires == ["first", "second", "third"]
     assert myapp.test_requires is None
 
 
@@ -299,11 +299,11 @@ def test_app_packages_install_requirements(
     create_command.logger.verbosity = logging_level
 
     # Set up the app requirements
-    myapp.requires = ["first", "second", "third"]
+    myapp._requires = ["first", "second", "third"]
 
     # The side effect of calling pip is creating installation artefacts
     create_command.tools[myapp].app_context.run.side_effect = (
-        create_installation_artefacts(app_packages_path, myapp.requires)
+        create_installation_artefacts(app_packages_path, myapp._requires)
     )
 
     # Install the requirements
@@ -340,7 +340,7 @@ def test_app_packages_install_requirements(
     assert (app_packages_path / "third/__main__.py").exists()
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second", "third"]
+    assert myapp._requires == ["first", "second", "third"]
     assert myapp.test_requires is None
 
 
@@ -355,11 +355,11 @@ def test_app_packages_replace_existing_requirements(
     create_installation_artefacts(app_packages_path, ["old", "ancient"])()
 
     # Set up the app requirements
-    myapp.requires = ["first", "second", "third"]
+    myapp._requires = ["first", "second", "third"]
 
     # The side effect of calling pip is creating installation artefacts
     create_command.tools[myapp].app_context.run.side_effect = (
-        create_installation_artefacts(app_packages_path, myapp.requires)
+        create_installation_artefacts(app_packages_path, myapp._requires)
     )
 
     # Install the requirements
@@ -401,7 +401,7 @@ def test_app_packages_replace_existing_requirements(
     assert not (app_packages_path / "ancient").exists()
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second", "third"]
+    assert myapp._requires == ["first", "second", "third"]
     assert myapp.test_requires is None
 
 
@@ -412,7 +412,7 @@ def test_app_requirements_no_requires(
     app_requirements_path_index,
 ):
     """If an app has no requirements, a requirements file is still written."""
-    myapp.requires = None
+    myapp._requires = None
 
     # Install requirements into the bundle
     create_command.install_app_requirements(myapp, test_mode=False)
@@ -423,7 +423,7 @@ def test_app_requirements_no_requires(
         assert f.read() == ""
 
     # Original app definitions haven't changed
-    assert myapp.requires is None
+    assert myapp._requires is None
     assert myapp.test_requires is None
 
 
@@ -435,7 +435,7 @@ def test_app_requirements_empty_requires(
 ):
     """If an app has an empty requirements list, a requirements file is still
     written."""
-    myapp.requires = []
+    myapp._requires = []
 
     # Install requirements into the bundle
     create_command.install_app_requirements(myapp, test_mode=False)
@@ -446,7 +446,7 @@ def test_app_requirements_empty_requires(
         assert f.read() == ""
 
     # Original app definitions haven't changed
-    assert myapp.requires == []
+    assert myapp._requires == []
     assert myapp.test_requires is None
 
 
@@ -458,7 +458,7 @@ def test_app_requirements_requires(
 ):
     """If an app has an empty requirements list, a requirements file is still
     written."""
-    myapp.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    myapp._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     # Install requirements into the bundle
     create_command.install_app_requirements(myapp, test_mode=False)
@@ -469,7 +469,7 @@ def test_app_requirements_requires(
         assert f.read() == "first\nsecond==1.2.3\nthird>=3.2.1\n"
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second==1.2.3", "third>=3.2.1"]
+    assert myapp._requires == ["first", "second==1.2.3", "third>=3.2.1"]
     assert myapp.test_requires is None
 
 
@@ -483,7 +483,7 @@ def test_app_requirements_requires(
         (">", "asdf+xcvb", False),
     ],
 )
-def test__is_local_requirement_altsep_respected(
+def test_is_local_requirement_altsep_respected(
     altsep,
     requirement,
     expected,
@@ -492,7 +492,7 @@ def test__is_local_requirement_altsep_respected(
     """``os.altsep`` is included as a separator when available."""
     monkeypatch.setattr(os, "sep", "/")
     monkeypatch.setattr(os, "altsep", altsep)
-    assert _is_local_requirement(requirement) is expected
+    assert is_local_requirement(requirement) is expected
 
 
 def _test_app_requirements_paths(
@@ -508,7 +508,7 @@ def _test_app_requirements_paths(
         requirement, converted = requirement
     else:
         converted = requirement
-    myapp.requires = ["first", requirement, "third"]
+    myapp._requires = ["first", requirement, "third"]
 
     create_command.install_app_requirements(myapp, test_mode=False)
     with app_requirements_path.open(encoding="utf-8") as f:
@@ -524,7 +524,7 @@ def _test_app_requirements_paths(
         )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", requirement, "third"]
+    assert myapp._requires == ["first", requirement, "third"]
     assert myapp.test_requires is None
 
 
@@ -645,7 +645,7 @@ def test_app_packages_test_requires(
 ):
     """If an app has test requirements, they're not included unless we are in test
     mode."""
-    myapp.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    myapp._requires = ["first", "second==1.2.3", "third>=3.2.1"]
     myapp.test_requires = ["pytest", "pytest-tldr"]
 
     create_command.install_app_requirements(myapp, test_mode=False)
@@ -674,7 +674,7 @@ def test_app_packages_test_requires(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second==1.2.3", "third>=3.2.1"]
+    assert myapp._requires == ["first", "second==1.2.3", "third>=3.2.1"]
     assert myapp.test_requires == ["pytest", "pytest-tldr"]
 
 
@@ -685,7 +685,7 @@ def test_app_packages_test_requires_test_mode(
     app_packages_path_index,
 ):
     """If an app has test requirements and we're in test mode, they are installed."""
-    myapp.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    myapp._requires = ["first", "second==1.2.3", "third>=3.2.1"]
     myapp.test_requires = ["pytest", "pytest-tldr"]
 
     create_command.install_app_requirements(myapp, test_mode=True)
@@ -716,7 +716,7 @@ def test_app_packages_test_requires_test_mode(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires == ["first", "second==1.2.3", "third>=3.2.1"]
+    assert myapp._requires == ["first", "second==1.2.3", "third>=3.2.1"]
     assert myapp.test_requires == ["pytest", "pytest-tldr"]
 
 
@@ -728,7 +728,7 @@ def test_app_packages_only_test_requires_test_mode(
 ):
     """If an app only has test requirements and we're in test mode, they are
     installed."""
-    myapp.requires = None
+    myapp._requires = None
     myapp.test_requires = ["pytest", "pytest-tldr"]
 
     create_command.install_app_requirements(myapp, test_mode=True)
@@ -756,5 +756,5 @@ def test_app_packages_only_test_requires_test_mode(
     )
 
     # Original app definitions haven't changed
-    assert myapp.requires is None
+    assert myapp._requires is None
     assert myapp.test_requires == ["pytest", "pytest-tldr"]
diff --git a/tests/commands/dev/test_install_dev_requirements.py b/tests/commands/dev/test_install_dev_requirements.py
index a813a4cdc..c3f421bae 100644
--- a/tests/commands/dev/test_install_dev_requirements.py
+++ b/tests/commands/dev/test_install_dev_requirements.py
@@ -13,7 +13,7 @@ def test_install_requirements_no_error(dev_command, first_app, logging_level):
     # Configure logging level
     dev_command.logger.verbosity = logging_level
 
-    first_app.requires = ["package-one", "package_two", "packagethree"]
+    first_app._requires = ["package-one", "package_two", "packagethree"]
 
     dev_command.install_dev_requirements(app=first_app)
 
@@ -37,7 +37,7 @@ def test_install_requirements_no_error(dev_command, first_app, logging_level):
 
 def test_install_requirements_error(dev_command, first_app):
     """Ensure RequirementsInstallError exception is raised for install errors."""
-    first_app.requires = ["package-one", "package_two", "packagethree"]
+    first_app._requires = ["package-one", "package_two", "packagethree"]
 
     dev_command.tools.subprocess.run.side_effect = CalledProcessError(
         returncode=-1, cmd="pip"
@@ -70,7 +70,7 @@ def test_install_requirements_error(dev_command, first_app):
 
 def test_no_requirements(dev_command, first_app):
     """Ensure dependency installation is not attempted when nothing to install."""
-    first_app.requires = []
+    first_app._requires = []
 
     dev_command.install_dev_requirements(app=first_app)
 
@@ -79,7 +79,7 @@ def test_no_requirements(dev_command, first_app):
 
 def test_install_requirements_test_mode(dev_command, first_app):
     """If an app has test requirements, they are also installed."""
-    first_app.requires = ["package-one", "package_two", "packagethree"]
+    first_app._requires = ["package-one", "package_two", "packagethree"]
     first_app.test_requires = ["test-one", "test_two"]
 
     dev_command.install_dev_requirements(app=first_app)
@@ -107,7 +107,7 @@ def test_install_requirements_test_mode(dev_command, first_app):
 
 def test_only_test_requirements(dev_command, first_app):
     """If an app only has test requirements, they're installed correctly."""
-    first_app.requires = None
+    first_app._requires = None
     first_app.test_requires = ["test-one", "test_two"]
 
     dev_command.install_dev_requirements(app=first_app)
diff --git a/tests/config/test_AppConfig.py b/tests/config/test_AppConfig.py
index 8d3cb6b5c..a016b5083 100644
--- a/tests/config/test_AppConfig.py
+++ b/tests/config/test_AppConfig.py
@@ -19,7 +19,7 @@ def test_minimal_AppConfig():
     assert config.version == "1.2.3"
     assert config.bundle == "org.beeware"
     assert config.description == "A simple app"
-    assert config.requires is None
+    assert config._requires is None
 
     # Derived properties have been set.
     assert config.bundle_name == "myapp"
@@ -70,7 +70,7 @@ def test_extra_attrs():
     assert config.description == "A simple app"
     assert config.long_description == "A longer description\nof the app"
     assert config.template == "/path/to/template"
-    assert config.requires == ["first", "second", "third"]
+    assert config._requires == ["first", "second", "third"]
 
     # Properties that are derived by default have been set explicitly
     assert config.formal_name == "My App!"
diff --git a/tests/platforms/iOS/xcode/test_create.py b/tests/platforms/iOS/xcode/test_create.py
index 6e9ba9d9a..8ec6abf91 100644
--- a/tests/platforms/iOS/xcode/test_create.py
+++ b/tests/platforms/iOS/xcode/test_create.py
@@ -37,7 +37,7 @@ def test_extra_pip_args(create_command, first_app_generated, tmp_path):
     # requirements for the current platform.
     create_command.tools.host_arch = "wonky"
 
-    first_app_generated.requires = ["something==1.2.3", "other>=2.3.4"]
+    first_app_generated._requires = ["something==1.2.3", "other>=2.3.4"]
 
     create_command.tools[first_app_generated].app_context = MagicMock(
         spec_set=Subprocess
diff --git a/tests/platforms/iOS/xcode/test_update.py b/tests/platforms/iOS/xcode/test_update.py
index 9f2300151..cb5627a27 100644
--- a/tests/platforms/iOS/xcode/test_update.py
+++ b/tests/platforms/iOS/xcode/test_update.py
@@ -24,7 +24,7 @@ def test_extra_pip_args(update_command, first_app_generated, tmp_path):
     # requirements for the current platform.
     update_command.tools.host_arch = "wonky"
 
-    first_app_generated.requires = ["something==1.2.3", "other>=2.3.4"]
+    first_app_generated._requires = ["something==1.2.3", "other>=2.3.4"]
 
     update_command.tools[first_app_generated].app_context = MagicMock(
         spec_set=Subprocess
diff --git a/tests/platforms/linux/test_LocalRequirementsMixin.py b/tests/platforms/linux/test_LocalRequirementsMixin.py
index db3d8002c..6c9317f60 100644
--- a/tests/platforms/linux/test_LocalRequirementsMixin.py
+++ b/tests/platforms/linux/test_LocalRequirementsMixin.py
@@ -267,7 +267,7 @@ def test_install_app_requirements_with_locals(
     """If the app has local requirements, they are compiled into sdists for
     installation."""
     # Add local requirements
-    first_app_config.requires.extend([first_package, second_package, third_package])
+    first_app_config._requires.extend([first_package, second_package, third_package])
 
     # Mock the side effect of building an sdist
     def build_sdist(*args, **kwargs):
@@ -373,7 +373,7 @@ def test_install_app_requirements_with_bad_local(
 ):
     """If the app has local requirement that can't be built, an error is raised."""
     # Add a local requirement
-    first_app_config.requires.append(first_package)
+    first_app_config._requires.append(first_package)
 
     # Mock the building an sdist raising an error
     create_command.tools.subprocess.check_output.side_effect = (
@@ -425,7 +425,7 @@ def test_install_app_requirements_with_missing_local_build(
     """If the app references a requirement that needs to be built, but is missing, an
     error is raised."""
     # Define a local requirement, but don't create the files it points at
-    first_app_config.requires.append(str(tmp_path / "local/first"))
+    first_app_config._requires.append(str(tmp_path / "local/first"))
 
     # Install requirements
     with pytest.raises(
@@ -457,7 +457,7 @@ def test_install_app_requirements_with_bad_local_file(
     """If the app references a local requirement file that doesn't exist, an error is
     raised."""
     # Add a local requirement that doesn't exist
-    first_app_config.requires.append(str(tmp_path / "local/missing-2.3.4.tar.gz"))
+    first_app_config._requires.append(str(tmp_path / "local/missing-2.3.4.tar.gz"))
 
     # Install requirements
     with pytest.raises(
diff --git a/tests/platforms/macOS/app/test_create.py b/tests/platforms/macOS/app/test_create.py
index b7e286741..4b66ccfd2 100644
--- a/tests/platforms/macOS/app/test_create.py
+++ b/tests/platforms/macOS/app/test_create.py
@@ -303,7 +303,7 @@ def test_install_app_packages(
     bundle_path = tmp_path / "base_path/build/first-app/macos/app"
 
     create_command.tools.host_arch = host_arch
-    first_app_templated.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    first_app_templated._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     # Mock the result of finding the binary packages - 2 of the packages are binary;
     # the version on the loosely specified package doesn't match the lower bound.
@@ -428,7 +428,7 @@ def test_install_app_packages_no_binary(
     create_installed_package(bundle_path / f"app_packages.{other_arch}", "legacy")
 
     create_command.tools.host_arch = host_arch
-    first_app_templated.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    first_app_templated._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     # Mock the result of finding no binary packages.
     create_command.find_binary_packages = mock.Mock(return_value=[])
@@ -504,7 +504,7 @@ def test_install_app_packages_failure(create_command, first_app_templated, tmp_p
     create_installed_package(bundle_path / "app_packages.x86_64", "legacy")
 
     create_command.tools.host_arch = "arm64"
-    first_app_templated.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    first_app_templated._requires = ["first", "second==1.2.3", "third>=3.2.1"]
 
     # Mock the result of finding the binary packages - 2 of the packages are binary;
     # the version on the loosely specified package doesn't match the lower bound.
@@ -627,7 +627,7 @@ def test_install_app_packages_non_universal(
     bundle_path = tmp_path / "base_path/build/first-app/macos/app"
 
     create_command.tools.host_arch = host_arch
-    first_app_templated.requires = ["first", "second==1.2.3", "third>=3.2.1"]
+    first_app_templated._requires = ["first", "second==1.2.3", "third>=3.2.1"]
     first_app_templated.universal_build = False
 
     # Mock the find_binary_packages command so we can confirm it wasn't invoked.