What causes "codex app-server line exceeds max size (%d bytes): %w" in chenhg5/cc-connect?

Increase maxLineSize (e.g. to 10–64 MB) to tolerate large JSON payloads from codex

codex app-server line exceeds max size (%d bytes): %w

Error message

codex app-server line exceeds max size (%d bytes): %w

What it means

The app-server stdout read loop uses a bufio.Scanner bounded by maxLineSize; when a single JSON-RPC line exceeds that limit the scanner fails with bufio.ErrTooLong and the library emits this error. The oversized line is dropped — it cannot be parsed — and the session marks itself not alive and rejects pending requests.

Source

Thrown at agent/codex/appserver_session.go:1026

			s.handleServerRequest(probe)

		default:
			// Notification (no id).
			var notif rpcNotificationEnvelope
			if err := json.Unmarshal(data, &notif); err != nil {
				slog.Debug("codex app-server: bad notification envelope", "error", err)
				continue
			}
			s.handleNotification(notif.Method, notif.Params)
		}
	}

	err := scanner.Err()
	if err != nil {
		if s.ctx.Err() == nil && !errors.Is(err, io.EOF) {
			slog.Warn("codex app-server read failed", "error", err)
			if errors.Is(err, bufio.ErrTooLong) {
				s.emitError(fmt.Errorf("codex app-server line exceeds max size (%d bytes): %w", maxLineSize, err))
			} else {
				s.emitError(fmt.Errorf("codex app-server connection closed: %w", err))
			}
		}
		s.alive.Store(false)
		s.rejectPending(err)
		s.rejectPendingApprovals(err)
		return
	}

	s.alive.Store(false)
	s.rejectPending(io.EOF)
	s.rejectPendingApprovals(io.EOF)
}

func (s *appServerSession) stderrLoop(r io.Reader) {
	defer s.wg.Done()
	scanner := bufio.NewScanner(r)

View on GitHub (pinned to 4000b2338a)

Solutions

Increase maxLineSize (e.g. to 10–64 MB) to tolerate large JSON payloads from codex
Switch the read loop to bufio.Reader.ReadSlice/ReadString with manual buffering so arbitrarily long lines are handled instead of aborted
Reduce payload size on the codex side (truncate file contents passed to the agent, avoid pasting huge outputs)
If this recurs, replace Scanner with json.Decoder reading length-unbounded values from the same stream

Example fix

// before
scanner := bufio.NewScanner(r)
scanner.Buffer(make([]byte, 0, 64*1024), maxLineSize) // e.g. 1MB, too small
// after
const maxLineSize = 64 << 20 // 64MB — large diffs/outputs no longer abort the reader
scanner.Buffer(make([]byte, 0, 1024*1024), maxLineSize)

Defensive patterns

Strategy: try-catch

Try / catch

err := sess.Send(ctx, prompt, nil)
var fatal bool
if err != nil && !sess.IsAlive() {
    // session dead (possibly ErrTooLong) → recreate session, optionally resend
    sess, err = agent.StartSession(ctx, opts)
}

Prevention

Size maxLineSize generously (tens of MB) for repo-scale payloads
Prefer bufio.Reader over bufio.Scanner for unbounded protocol lines
Truncate large file contents before handing them to the agent
Alert on this error — it means a whole turn's events were dropped

When it happens

Trigger: The codex app-server writes a single line larger than maxLineSize — typically a turn event embedding a huge file diff, a very long tool output, or a giant base64 payload — causing scanner.Scan() to fail with bufio.ErrTooLong.

Common situations: Agent reads or echoes an entire large file into a turn notification; codex streams compacted JSON with no line breaks for very large payloads; maxLineSize left at a small default while working with large repos.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

AI-assisted analysis of chenhg5/cc-connect@4000b2338a (2026-09-06). Data as JSON: /api/errors/4bc2789fa4224734. Report an issue: GitHub.