fix: write errors/warnings to stderr rather than stdout

error.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"os"
 )
 
 // ErrorDetailer returns error details for responses & debugging. This enables
@@ -264,7 +265,7 @@ func WriteErr(api API, ctx Context, status int, msg string, errs ...error) error
 	writeErr := writeResponse(api, ctx, status, "", err)
 	if writeErr != nil {
 		// If we can't write the error, log it so we know what happened.
-		fmt.Printf("could not write error: %s\n", writeErr)
+		fmt.Fprintf(os.Stderr, "could not write error: %v\n", writeErr)
 	}
 	return writeErr
 }

humacli/humacli.go

@@ -165,7 +165,7 @@ func (c *cli[O]) setupOptions(t reflect.Type, path []int) {
 		if !field.IsExported() {
 			// This isn't a public field, so we cannot use reflect.Value.Set with
 			// it. This is usually a struct field with a lowercase name.
-			fmt.Println("warning: ignoring unexported options field", field.Name)
+			fmt.Fprintln(os.Stderr, "warning: ignoring unexported options field", field.Name)
 			continue
 		}
 
@@ -298,7 +298,7 @@ func New[O any](onParsed func(Hooks, *O)) CLI {
 			// Server is done, just exit.
 		case <-quit:
 			if c.stop != nil {
-				fmt.Println("Gracefully shutting down the server...")
+				fmt.Fprintln(os.Stderr, "Gracefully shutting down the server...")
 				c.stop()
 			}
 		}

sse/sse.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"os"
 	"reflect"
 	"runtime/debug"
 	"time"
@@ -160,10 +161,10 @@ func Register[I any](api huma.API, op huma.Operation, eventTypeMap map[string]an
 				send := func(msg Message) error {
 					if deadliner != nil {
 						if err := deadliner.SetWriteDeadline(time.Now().Add(WriteTimeout)); err != nil {
-							fmt.Println("warning: unable to set write deadline: " + err.Error())
+							fmt.Fprintf(os.Stderr, "warning: unable to set write deadline: %v\n", err)
 						}
 					} else {
-						fmt.Println("warning: unable to set write deadline")
+						fmt.Fprintln(os.Stderr, "write deadline not supported by underlying writer")
 					}
 
 					// Write optional fields
@@ -176,7 +177,7 @@ func Register[I any](api huma.API, op huma.Operation, eventTypeMap map[string]an
 
 					event, ok := typeToEvent[deref(reflect.TypeOf(msg.Data))]
 					if !ok {
-						fmt.Println("error: unknown event type", reflect.TypeOf(msg.Data))
+						fmt.Fprintf(os.Stderr, "error: unknown event type %v\n", reflect.TypeOf(msg.Data))
 						debug.PrintStack()
 					}
 					if event != "" && event != "message" {
@@ -198,7 +199,7 @@ func Register[I any](api huma.API, op huma.Operation, eventTypeMap map[string]an
 					if flusher != nil {
 						flusher.Flush()
 					} else {
-						fmt.Println("error: unable to flush")
+						fmt.Fprintln(os.Stderr, "error: unable to flush")
 						return fmt.Errorf("unable to flush: %w", http.ErrNotSupported)
 					}
 					return nil

transforms.go

@@ -3,6 +3,7 @@ package huma
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"path"
 	"reflect"
 )
@@ -132,7 +133,7 @@ func (t *SchemaLinkTransformer) OnAddOperation(oapi *OpenAPI, op *Operation) {
 						// Catch some scenarios that just aren't supported in Go at the
 						// moment. Logs an error so people know what's going on.
 						// https://github.com/danielgtaylor/huma/issues/371
-						fmt.Println("Warning: unable to create schema link for type", typ, ":", r)
+						fmt.Fprintln(os.Stderr, "Warning: unable to create schema link for type", typ, ":", r)
 					}
 				}()
 				newType := reflect.StructOf(fields)