Currently there is no bound on the length of a user message submitted in
the TUI or through the app server interface. That means users can paste
many megabytes of text, which can lead to bad performance, hangs, and
crashes. In extreme cases, it can lead to a [kernel
panic](https://github.com/openai/codex/issues/12323).
This PR limits the length of a user input to 2**20 (about 1M)
characters. This value was chosen because it fills the entire context
window on the latest models, so accepting longer inputs wouldn't make
sense anyway.
Summary
- add a shared `MAX_USER_INPUT_TEXT_CHARS` constant in codex-protocol
and surface it in TUI and app server code
- block oversized submissions in the TUI submit flow and emit error
history cells when validation fails
- reject heavy app-server requests with JSON-RPC `-32602` and structured
`input_too_large` data, plus document the behavior
Testing
- ran the IDE extension with this change and verified that when I
attempt to paste a user message that's several MB long, it correctly
reports an error instead of crashing or making my computer hot.
## Why
Several tests intentionally exercise behavior while a turn is still
active. The cleanup sequence for those tests (`turn/interrupt` + waiting
for `codex/event/turn_aborted`) was duplicated across files, which made
the rationale easy to lose and the pattern easy to apply inconsistently.
This change centralizes that cleanup in one place with a single
explanatory doc comment.
## What Changed
### Added shared helper
In `codex-rs/app-server/tests/common/mcp_process.rs`:
- Added `McpProcess::interrupt_turn_and_wait_for_aborted(...)`.
- Added a doc comment explaining why explicit interrupt + terminal wait
is required for tests that intentionally leave a turn in-flight.
### Migrated call sites
Replaced duplicated interrupt/aborted blocks with the helper in:
- `codex-rs/app-server/tests/suite/v2/thread_resume.rs`
- `thread_resume_rejects_history_when_thread_is_running`
- `thread_resume_rejects_mismatched_path_when_thread_is_running`
- `codex-rs/app-server/tests/suite/v2/turn_start_zsh_fork.rs`
- `turn_start_shell_zsh_fork_executes_command_v2`
-
`turn_start_shell_zsh_fork_subcommand_decline_marks_parent_declined_v2`
- `codex-rs/app-server/tests/suite/v2/turn_steer.rs`
- `turn_steer_returns_active_turn_id`
### Existing cleanup retained
In `codex-rs/app-server/tests/suite/v2/turn_start.rs`:
- `turn_start_accepts_local_image_input` continues to explicitly wait
for `turn/completed` so the turn lifecycle is fully drained before test
exit.
## Verification
- `cargo test -p codex-app-server`
This PR adds a dedicated `turn/steer` API for appending user input to an
in-flight turn.
## Motivation
Currently, steering in the app is implemented by just calling
`turn/start` while a turn is running. This has some really weird quirks:
- Client gets back a new `turn.id`, even though streamed
events/approvals remained tied to the original active turn ID.
- All the various turn-level override params on `turn/start` do not
apply to the "steer", and would only apply to the next real turn.
- There can also be a race condition where the client thinks the turn is
active but the server has already completed it, so there might be bugs
if the client has baked in some client-specific behavior thinking it's a
steer when in fact the server kicked off a new turn. This is
particularly possible when running a client against a remote app-server.
Having a dedicated `turn/steer` API eliminates all those quirks.
`turn/steer` behavior:
- Requires an active turn on threadId. Returns a JSON-RPC error if there
is no active turn.
- If expectedTurnId is provided, it must match the active turn (more
useful when connecting to a remote app-server).
- Does not emit `turn/started`.
- Does not accept turn overrides (`cwd`, `model`, `sandbox`, etc.) or
`outputSchema` to accurately reflect that these are not applied when
steering.
Eric Traut ·
Michael Bolin ·
Owen Lin ·