diff --git a/NEWS.md b/NEWS.md
index d64a3d22..e24e6b55 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,6 +1,9 @@
 
 # callr development version
 
+* `callr::r_session` now handles large messages from the subprocess
+  well (#168).
+
 # callr 3.5.0
 
 * callr can now pass the environment of the function to the subprocess,
diff --git a/R/r-session.R b/R/r-session.R
index 58a0c2eb..f517af23 100644
--- a/R/r-session.R
+++ b/R/r-session.R
@@ -227,6 +227,12 @@ r_session <- R6::R6Class(
     func_file = NULL,
     res_file = NULL,
 
+    buffer = NULL,
+    read_buffer = function()
+      rs__read_buffer(self, private),
+    read_message = function()
+      rs__read_message(self, private),
+
     get_result_and_output = function(std = FALSE)
       rs__get_result_and_output(self, private, std),
     report_back = function(code, text = "")
@@ -305,22 +311,98 @@ rs_init <- function(self, private, super, options, wait, wait_timeout) {
 }
 
 rs_read <- function(self, private) {
-  out <- processx::processx_conn_read_lines(private$pipe, 1)
+  if (!is.null(private$buffer)) {
+    # There is a partial message in the buffer, try to finish it.
+    out <- private$read_buffer()
+  } else {
+    # A new message.
+    out <- private$read_message()
+  }
   if (!length(out)) {
     if (processx::processx_conn_is_incomplete(private$pipe)) return()
     if (self$is_alive()) {
       # We do this in on.exit(), because parse_msg still reads the streams
       on.exit(self$kill(), add = TRUE)
-      out <- "502 R session closed the process connection, killed"
+      out <- list(header = list(
+        code = 502, length = 0,
+        rest = "R session closed the process connection, killed"
+      ))
     } else if (identical(es <- self$get_exit_status(), 0L)) {
-      out <- "500 R session finished cleanly"
+      out <- list(header = list(
+        code = 500, length = 0,
+        rest = "R session finished cleanly"
+      ))
     } else {
-      out <- paste0("501 R session crashed with exit code ", es)
+      out <- list(header = list(
+        code = 501, length = 0,
+        rest = paste0("R session crashed with exit code ", es)
+      ))
     }
   }
   if (length(out)) private$parse_msg(out)
 }
 
+rs__read_buffer <- function(self, private) {
+  # There is a partial message in the buffer already, we need to
+  # read some more
+  need <- private$buffer$header$length - private$buffer$got
+  chunk <- processx::processx_conn_read_chars(private$pipe, need)
+  got <- nchar(chunk)
+  if (got == 0) {
+    # make this special case fast
+    NULL
+  } else if (got == need) {
+    msg <- list(
+      header = private$buffer$header,
+      body = paste(c(private$buffer$chunks, list(chunk)), collapse = "")
+    )
+    private$buffer <- NULL
+    msg
+  } else {
+    private$buffer$got <- private$buffer$got + got
+    private$buffer$chunks <- c(private$buffer$chunks, list(chunk))
+    NULL
+  }
+}
+
+rs__read_message <- function(self, private) {
+  # A new message, we can surely read the first line
+  out <- processx::processx_conn_read_lines(private$pipe, 1)
+  if (length(out) == 0) return(NULL)
+
+  header <- rs__parse_header(out)
+  body <- ""
+  if (header$length > 0) {
+    body <- processx::processx_conn_read_chars(
+      private$pipe,
+      header$length
+    )
+  }
+  got <- nchar(body)
+  if (got < header$length) {
+    # Partial message
+    private$buffer <- list(
+      header = header,
+      got = got,
+      chunks = list(body)
+    )
+    NULL
+  } else {
+    list(header = header, body = body)
+  }
+}
+
+rs__parse_header <- function(line) {
+  parts <- strsplit(line, " ", fixed = TRUE)[[1]]
+  parts2 <- suppressWarnings(as.integer(parts[1:2]))
+  rest <- paste(parts[-(1:2)], collapse = " ")
+  header <- list(code = parts2[1], length = parts2[2], rest = rest)
+  if (is.na(header$code) || is.na(header$length)) {
+    stop("Internal callr error, invalid message header")
+  }
+  header
+}
+
 rs_close <- function(self, private, grace) {
   processx::processx_conn_close(self$get_input_connection())
   self$poll_process(grace)
@@ -571,7 +653,10 @@ rs__attach_wait <- function(self, private) {
 }
 
 rs__report_back <- function(self, private, code, text) {
-  cmd <- paste0(deparse(rs__status_expr(code, text, fd = 3)), "\n")
+  cmd <- paste0(
+    deparse(rs__status_expr(code, text, fd = 3)),
+    "\n"
+  )
   private$write_for_sure(cmd)
 }
 
@@ -584,19 +669,20 @@ rs__write_for_sure <- function(self, private, text) {
 }
 
 rs__parse_msg <- function(self, private, msg) {
-  s <- strsplit(msg, " ", fixed = TRUE)[[1]]
-  code <- as.integer(s[1])
-  message <- paste(s[-1], collapse = " ")
-  if (substr(message, 1, 8) == "base64::") {
+  code <- as.character(msg$header$code)
+  message <- msg$body
+  if (length(message) && substr(message, 1, 8) == "base64::") {
     message <- substr(message, 9, nchar(message))
     message <- unserialize(processx::base64_decode(message))
+  } else {
+    message <- msg$header$rest
   }
 
-  if (! s[1] %in% names(rs__parse_msg_funcs)) {
-    throw(new_error("Unknown message code: `", s[1], "`"))
+  if (! code %in% names(rs__parse_msg_funcs)) {
+    throw(new_error("Unknown message code: `", code, "`"))
   }
   structure(
-    rs__parse_msg_funcs[[ s[1] ]](self, private, code, message),
+    rs__parse_msg_funcs[[code]](self, private, msg$header$code, message),
     class = "callr_session_result")
 }
 
@@ -652,7 +738,7 @@ rs__status_expr <- function(code, text = "", fd = 3L) {
     local({
       pxlib <- as.environment("tools:callr")$`__callr_data__`$pxlib
       code_ <- code; fd_ <- fd; text_ <- text
-      data <- paste0(code_, " ", text_, "\n")
+      data <- paste0(code_, " 0 ", text_, "\n")
       pxlib$write_fd(as.integer(fd), data)
     }),
     list(code = code, fd = fd, text = text)
diff --git a/R/script.R b/R/script.R
index d3eddaec..e9cccd65 100644
--- a/R/script.R
+++ b/R/script.R
@@ -68,7 +68,7 @@ make_vanilla_script_expr <- function(expr_file, res, error,
         pxlib <- as.environment("tools:callr")$`__callr_data__`$pxlib
         if (is.null(e$code)) e$code <- "301"
         msg <- paste0("base64::", pxlib$base64_encode(serialize(e, NULL)))
-        data <- paste(e$code, msg, "\n")
+        data <- paste0(e$code, " ", nchar(msg), "\n", msg)
         pxlib$write_fd(3L, data)
 
         if (inherits(e, "cli_message") &&
diff --git a/tests/testthat/test-r-session-messages.R b/tests/testthat/test-r-session-messages.R
index d802da32..0f2fb9fa 100644
--- a/tests/testthat/test-r-session-messages.R
+++ b/tests/testthat/test-r-session-messages.R
@@ -58,3 +58,36 @@ test_that("message handlers", {
 
   rs$close()
 })
+
+test_that("large messages", {
+  rs <- r_session$new()
+  on.exit(rs$close(), add = TRUE)
+
+  do <- function() {
+    msg <- structure(list(message = paste(1:150000, sep = " ")),
+                     class = c("myclass", "callr_message", "condition"))
+    signalCondition(msg)
+    for (i in 1:5) {
+      msg <- structure(list(message = paste("message", i)),
+                       class = c("myclass", "callr_message", "condition"))
+      signalCondition(msg)
+    }
+  }
+
+  cond <- list()
+  withr::with_options(
+    list(callr.condition_handler_myclass = function(x) {
+      cond <<- c(cond, list(x))
+    }),
+    rs$run(do)
+  )
+
+  expect_equal(length(cond), 6)
+  expect_s3_class(cond[[1]], "myclass")
+  expect_equal(cond[[1]]$message, paste(1:150000, sep = " "))
+  for (i in 1:5) {
+    expect_equal(cond[[i + 1]]$message, paste("message", i))
+  }
+
+  rs$close()
+})