Continuation of breaking up this PR
https://github.com/openai/codex/pull/9116
## Summary
- Thread user text element ranges through TUI/TUI2 input, submission,
queueing, and history so placeholders survive resume/edit flows.
- Preserve local image attachments alongside text elements and rehydrate
placeholders when restoring drafts.
- Keep model-facing content shapes clean by attaching UI metadata only
to user input/events (no API content changes).
## Key Changes
- TUI/TUI2 composer now captures text element ranges, trims them with
text edits, and restores them when submission is suppressed.
- User history cells render styled spans for text elements and keep
local image paths for future rehydration.
- Initial chat widget bootstraps accept empty `initial_text_elements` to
keep initialization uniform.
- Protocol/core helpers updated to tolerate the new InputText field
shape without changing payloads sent to the API.
- `tui/` and `tui2/` submit `Op::UserTurn` and own full turn context
(cwd/approval/sandbox/model/etc.).
- `Op::UserInput` is documented as legacy in `codex-protocol` (doc-only;
no `#[deprecated]` to avoid `-D warnings` fallout).
- Remove obsolete `#[allow(deprecated)]` and the unused `ConversationId`
alias/re-export.
Fixes#9058
## Summary
When the transcript backtrack preview is armed (press `Esc`), allow
navigating to newer user messages with the `→` arrow, in addition to
navigating backwards with `Esc`/`←`, before confirming with `Enter`.
## Changes
- Backtrack preview navigation: `Esc`/`←` steps to older user messages,
`→` steps to newer ones, `Enter` edits the selected message (clamped at
bounds, no wrap-around).
- Transcript overlay footer hints updated to advertise `esc/←`, `→`, and
`enter` when a message is highlighted.
## Related
- WSL shortcut-overlay snapshot determinism: #9359
## Testing
- `just fmt`
- `just fix -p codex-tui`
- `just fix -p codex-tui2`
- `cargo test -p codex-tui app_backtrack::`
- `cargo test -p codex-tui pager_overlay::`
- `cargo test -p codex-tui2 app_backtrack::`
- `cargo test -p codex-tui2 pager_overlay::`
---------
Co-authored-by: Josh McKinney <joshka@openai.com>
### Description
- Remove the now-unused `instructions` field from the session metadata
to simplify SessionMeta and stop propagating transient instruction text
through the rollout recorder API. This was only saving
user_instructions, and was never being read.
- Stop passing user instructions into the rollout writer at session
creation so the rollout header only contains canonical session metadata.
### Testing
- Ran `just fmt` which completed successfully.
- Ran `just fix -p codex-protocol`, `just fix -p codex-core`, `just fix
-p codex-app-server`, `just fix -p codex-tui`, and `just fix -p
codex-tui2` which completed (Clippy fixes applied) as part of
verification.
- Ran `cargo test -p codex-protocol` which passed (28 tests).
- Ran `cargo test -p codex-core` which showed failures in a small set of
tests (not caused by the protocol type change directly):
`default_client::tests::test_create_client_sets_default_headers`,
several `models_manager::manager::tests::refresh_available_models_*`,
and `shell_snapshot::tests::linux_sh_snapshot_includes_sections` (these
tests failed in this CI run).
- Ran `cargo test -p codex-app-server` which reported several failing
integration tests (including
`suite::codex_message_processor_flow::test_codex_jsonrpc_conversation_flow`,
`suite::output_schema::send_user_turn_*`, and
`suite::user_agent::get_user_agent_returns_current_codex_user_agent`).
- `cargo test -p codex-tui` and `cargo test -p codex-tui2` were
attempted but aborted due to disk space exhaustion (`No space left on
device`).
------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_696bd8ce632483228d298cf07c7eb41c)
Document the backtrack/rollback state machine and invariants between the
transcript overlay, in-flight “live tail”, and core thread state (tui + tui2).
Also adjust behavior for correctness:
- Track a single pending rollback and block additional rollbacks until core responds.
- Defer trimming transcript cells until ThreadRolledBack for the active session.
- Clear the guard on ThreadRollbackFailed so the user can retry.
- After a confirmed trim, schedule a one-shot scrollback refresh on the next draw.
- Clear stale pending rollback state when switching sessions.
---------
Co-authored-by: Josh McKinney <joshka@openai.com>
Summary:
- Add forked_from to SessionMeta/SessionConfiguredEvent and persist it
for forked sessions.
- Surface forked_from in /status for tui + tui2 and add snapshots.
Add support for returning threads by either `created_at` OR `updated_at`
descending. Previously core always returned threads ordered by
`created_at`.
This PR:
- updates core to be able to list threads by `updated_at` OR
`created_at` descending based on what the caller wants
- also update `thread/list` in app-server to expose this (default to
`created_at` if not specified)
All existing codepaths (app-server, TUI) still default to `created_at`,
so no behavior change is expected with this PR.
**Implementation**
To sort by `updated_at` is a bit nontrivial (whereas `created_at` is
easy due to the way we structure the folders and filenames on disk,
which are all based on `created_at`).
The most naive way to do this without introducing a cache file or sqlite
DB (which we have to implement/maintain) is to scan files in reverse
`created_at` order on disk, and look at the file's mtime (last modified
timestamp according to the filesystem) until we reach `MAX_SCAN_FILES`
(currently set to 10,000). Then, we can return the most recent N
threads.
Based on some quick and dirty benchmarking on my machine with ~1000
rollout files, calling `thread/list` with limit 50, the `updated_at`
path is slower as expected due to all the I/O:
- updated-at: average 103.10 ms
- created-at: average 41.10 ms
Those absolute numbers aren't a big deal IMO, but we can certainly
optimize this in a followup if needed by introducing more state stored
on disk.
**Caveat**
There's also a limitation in that any files older than `MAX_SCAN_FILES`
will be excluded, which means if a user continues a REALLY old thread,
it's possible to not be included. In practice that should not be too big
of an issue.
If a user makes...
- 1000 rollouts/day → threads older than 10 days won't show up
- 100 rollouts/day → ~100 days
If this becomes a problem for some reason, even more motivation to
implement an updated_at cache.
Implemented /fork to fork the current session directly (no picker),
handling it via a new ForkCurrentSession app event in both tui and tui2.
Updated slash command descriptions/tooltips and adjusted the fork tests
accordingly. Removed the unused in-session fork picker event.
Fixes#7919.
This PR addresses a TUI display bug where the "Worked for" separator
would appear prematurely during the planning stage.
**Changes:**
- Added `had_work_activity` flag to `ChatWidget` to track if actual work
(exec commands, MCP tool calls, patches) was performed in the current
turn.
- Updated `handle_streaming_delta` to only display the
`FinalMessageSeparator` if both `needs_final_message_separator` AND
`had_work_activity` are true.
- Updated `handle_exec_end_now`, `handle_patch_apply_end_now`, and
`handle_mcp_end_now` to set `had_work_activity = true`.
**Verification:**
- Ran `cargo test -p codex-tui` to ensure no regressions.
- Manual verification confirms the separator now only appears after
actual work is completed.
---------
Co-authored-by: Josh McKinney <joshka@openai.com>
We’re introducing a new SKILL.toml to hold skill metadata so Codex can
deliver a richer Skills experience.
Initial focus is the interface block:
```
[interface]
display_name = "Optional user-facing name"
short_description = "Optional user-facing description"
icon_small = "./assets/small-400px.png"
icon_large = "./assets/large-logo.svg"
brand_color = "#3B82F6"
default_prompt = "Optional surrounding prompt to use the skill with"
```
All fields are exposed via the app server API.
display_name and short_description are consumed by the TUI.
A recent change in commit ccba737d26 modified the styling of the
placeholder text (e.g. "Implement {feature}") in the input box of the
CLI, changing it from non-italic to italic. I think this was likely
unintentional. It results in a bad display appearance on some terminal
emulators, and several users have complained about it.
This change switches back to non-italic styling, restoring the older
behavior.
It addresses #9262
This PR changes `codex resume --last` to work consistently with `codex
resume`. Namely, it filters based on the cwd when selecting the last
session. It also supports the `--all` modifier as an override.
This addresses #8700
### What
Add `WebSearchMode` enum (disabled, cached live, defaults to cached) to
config + V2 protocol. This enum takes precedence over legacy flags:
`web_search_cached`, `web_search_request`, and `tools.web_search`.
Keep `--search` as live.
### Tests
Added tests
Disables the default Ctrl+C/Ctrl+D double-press quit UX (keeps the code
path behind a const) while we rethink the quit/interrupt flow.
Tests:
- just fmt
- cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs
-p codex-tui
- cargo test -p codex-tui --lib
- Remove legacy Ctrl+K queuing in tui2; Tab is the queue key.
- Make Enter queue when Steer is disabled and submit immediately when
Steer is enabled.
- Add Steer keybinding docs on both tui and tui2 chat composers.
- Don't try to precompute model unless you know it from `config`
- Block `/model` on session configured
- Queue messages until session configured
- show "loading" in status until session configured
Emit the following events around the collab tools. On the `app-server`
this will be under `item/started` and `item/completed`
```
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabAgentSpawnBeginEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Initial prompt sent to the agent. Can be empty to prevent CoT leaking at the
/// beginning.
pub prompt: String,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabAgentSpawnEndEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the newly spawned agent, if it was created.
pub new_thread_id: Option<ThreadId>,
/// Initial prompt sent to the agent. Can be empty to prevent CoT leaking at the
/// beginning.
pub prompt: String,
/// Last known status of the new agent reported to the sender agent.
pub status: AgentStatus,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabAgentInteractionBeginEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
/// Prompt sent from the sender to the receiver. Can be empty to prevent CoT
/// leaking at the beginning.
pub prompt: String,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabAgentInteractionEndEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
/// Prompt sent from the sender to the receiver. Can be empty to prevent CoT
/// leaking at the beginning.
pub prompt: String,
/// Last known status of the receiver agent reported to the sender agent.
pub status: AgentStatus,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabWaitingBeginEvent {
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
/// ID of the waiting call.
pub call_id: String,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabWaitingEndEvent {
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
/// ID of the waiting call.
pub call_id: String,
/// Last known status of the receiver agent reported to the sender agent.
pub status: AgentStatus,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabCloseBeginEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
}
#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
pub struct CollabCloseEndEvent {
/// Identifier for the collab tool call.
pub call_id: String,
/// Thread ID of the sender.
pub sender_thread_id: ThreadId,
/// Thread ID of the receiver.
pub receiver_thread_id: ThreadId,
/// Last known status of the receiver agent reported to the sender agent before
/// the close.
pub status: AgentStatus,
}
```
## Problem
Codex’s TUI quit behavior has historically been easy to trigger
accidentally and hard to reason
about.
- `Ctrl+C`/`Ctrl+D` could terminate the UI immediately, which is a
common key to press while trying
to dismiss a modal, cancel a command, or recover from a stuck state.
- “Quit” and “shutdown” were not consistently separated, so some exit
paths could bypass the
shutdown/cleanup work that should run before the process terminates.
This PR makes quitting both safer (harder to do by accident) and more
uniform across quit
gestures, while keeping the shutdown-first semantics explicit.
## Mental model
After this change, the system treats quitting as a UI request that is
coordinated by the app
layer.
- The UI requests exit via `AppEvent::Exit(ExitMode)`.
- `ExitMode::ShutdownFirst` is the normal user path: the app triggers
`Op::Shutdown`, continues
rendering while shutdown runs, and only ends the UI loop once shutdown
has completed.
- `ExitMode::Immediate` exists as an escape hatch (and as the
post-shutdown “now actually exit”
signal); it bypasses cleanup and should not be the default for
user-triggered quits.
User-facing quit gestures are intentionally “two-step” for safety:
- `Ctrl+C` and `Ctrl+D` no longer exit immediately.
- The first press arms a 1-second window and shows a footer hint (“ctrl
+ <key> again to quit”).
- Pressing the same key again within the window requests a
shutdown-first quit; otherwise the
hint expires and the next press starts a fresh window.
Key routing remains modal-first:
- A modal/popup gets first chance to consume `Ctrl+C`.
- If a modal handles `Ctrl+C`, any armed quit shortcut is cleared so
dismissing a modal cannot
prime a subsequent `Ctrl+C` to quit.
- `Ctrl+D` only participates in quitting when the composer is empty and
no modal/popup is active.
The design doc `docs/exit-confirmation-prompt-design.md` captures the
intended routing and the
invariants the UI should maintain.
## Non-goals
- This does not attempt to redesign modal UX or make modals uniformly
dismissible via `Ctrl+C`.
It only ensures modals get priority and that quit arming does not leak
across modal handling.
- This does not introduce a persistent confirmation prompt/menu for
quitting; the goal is to keep
the exit gesture lightweight and consistent.
- This does not change the semantics of core shutdown itself; it changes
how the UI requests and
sequences it.
## Tradeoffs
- Quitting via `Ctrl+C`/`Ctrl+D` now requires a deliberate second
keypress, which adds friction for
users who relied on the old “instant quit” behavior.
- The UI now maintains a small time-bounded state machine for the armed
shortcut, which increases
complexity and introduces timing-dependent behavior.
This design was chosen over alternatives (a modal confirmation prompt or
a long-lived “are you
sure” state) because it provides an explicit safety barrier while
keeping the flow fast and
keyboard-native.
## Architecture
- `ChatWidget` owns the quit-shortcut state machine and decides when a
quit gesture is allowed
(idle vs cancellable work, composer state, etc.).
- `BottomPane` owns rendering and local input routing for modals/popups.
It is responsible for
consuming cancellation keys when a view is active and for
showing/expiring the footer hint.
- `App` owns shutdown sequencing: translating
`AppEvent::Exit(ShutdownFirst)` into `Op::Shutdown`
and only terminating the UI loop when exit is safe.
This keeps “what should happen” decisions (quit vs interrupt vs ignore)
in the chat/widget layer,
while keeping “how it looks and which view gets the key” in the
bottom-pane layer.
## Observability
You can tell this is working by running the TUIs and exercising the quit
gestures:
- While idle: pressing `Ctrl+C` (or `Ctrl+D` with an empty composer and
no modal) shows a footer
hint for ~1 second; pressing again within that window exits via
shutdown-first.
- While streaming/tools/review are active: `Ctrl+C` interrupts work
rather than quitting.
- With a modal/popup open: `Ctrl+C` dismisses/handles the modal (if it
chooses to) and does not
arm a quit shortcut; a subsequent quick `Ctrl+C` should not quit unless
the user re-arms it.
Failure modes are visible as:
- Quits that happen immediately (no hint window) from `Ctrl+C`/`Ctrl+D`.
- Quits that occur while a modal is open and consuming `Ctrl+C`.
- UI termination before shutdown completes (cleanup skipped).
## Tests
- Updated/added unit and snapshot coverage in `codex-tui` and
`codex-tui2` to validate:
- The quit hint appears and expires on the expected key.
- Double-press within the window triggers a shutdown-first quit request.
- Modal-first routing prevents quit bypass and clears any armed shortcut
when a modal consumes
`Ctrl+C`.
These tests focus on the UI-level invariants and rendered output; they
do not attempt to validate
real terminal key-repeat timing or end-to-end process shutdown behavior.
---
Screenshot:
<img width="912" height="740" alt="Screenshot 2026-01-13 at 1 05 28 PM"
src="https://github.com/user-attachments/assets/18f3d22e-2557-47f2-a369-ae7a9531f29f"
/>
When an invalid config.toml key or value is detected, the CLI currently
just quits. This leaves the VSCE in a dead state.
This PR changes the behavior to not quit and bubble up the config error
to users to make it actionable. It also surfaces errors related to
"rules" parsing.
This allows us to surface these errors to users in the VSCE, like this:
<img width="342" height="129" alt="Screenshot 2026-01-13 at 4 29 22 PM"
src="https://github.com/user-attachments/assets/a79ffbe7-7604-400c-a304-c5165b6eebc4"
/>
<img width="346" height="244" alt="Screenshot 2026-01-13 at 4 45 06 PM"
src="https://github.com/user-attachments/assets/de874f7c-16a2-4a95-8c6d-15f10482e67b"
/>
User-facing symptom: On terminals that deliver pastes as rapid
KeyCode::Char/Enter streams (notably Windows), paste-burst transient
state
can leak into the next input. Users can see Enter insert a newline when
they meant to submit, or see characters appear late / handled through
the
wrong path.
System problem: PasteBurst is time-based. Clearing only the
classification window (e.g. via clear_window_after_non_char()) can erase
last_plain_char_time without emitting buffered text. If a buffer is
still
non-empty after that, flush_if_due() no longer has a timeout clock to
flush against, so the buffer can get "stuck" until another plain char
arrives.
This was surfaced while adding deterministic regression tests for
paste-burst behavior.
Fix: when disabling burst detection, defuse any in-flight burst state:
flush held/buffered text through handle_paste() (so it follows normal
paste integration), then clear timing and Enter suppression.
Document the rationale inline and update docs/tui-chat-composer.md so
"disable_paste_burst" matches the actual behavior.
Have only the following Methods:
- `list_models`: getting current available models
- `try_list_models`: sync version no refresh for tui use
- `get_default_model`: get the default model (should be tightened to
core and received on session configuration)
- `get_model_info`: get `ModelInfo` for a specific model (should be
tightened to core but used in tests)
- `refresh_if_new_etag`: trigger refresh on different etags
Also move the cache to its own struct
Adds an integration test for the new behavior introduced in
https://github.com/openai/codex/pull/9011. The work to create the test
setup was substantial enough that I thought it merited a separate PR.
This integration test spawns `codex` in TUI mode, which requires
spawning a PTY to run successfully, so I had to introduce quite a bit of
scaffolding in `run_codex_cli()`. I was surprised to discover that we
have not done this in our codebase before, so perhaps this should get
moved to a common location so it can be reused.
The test itself verifies that a malformed `rules` in `$CODEX_HOME`
prints a human-readable error message and exits nonzero.
The underlying issue is that when we encountered an error starting a
conversation (any sort of error, though making `$CODEX_HOME/rules` a
file rather than folder was the example in #8803), then we were writing
the message to stderr, but this could be printed over by our UI
framework so the user would not see it. In general, we disallow the use
of `eprintln!()` in this part of the code for exactly this reason,
though this was suppressed by an `#[allow(clippy::print_stderr)]`.
This attempts to clean things up by changing `handle_event()` and
`handle_tui_event()` to return a `Result<AppRunControl>` instead of a
`Result<bool>`, which is a new type introduced in this PR (and depends
on `ExitReason`, also a new type):
```rust
#[derive(Debug)]
pub(crate) enum AppRunControl {
Continue,
Exit(ExitReason),
}
#[derive(Debug, Clone)]
pub enum ExitReason {
UserRequested,
Fatal(String),
}
```
This makes it possible to exit the primary control flow of the TUI with
richer information. This PR adds `ExitReason` to the existing
`AppExitInfo` struct and updates `handle_app_exit()` to print the error
and exit code `1` in the event of `ExitReason::Fatal`.
I tried to create an integration test for this, but it was a bit
involved, so I published it as a separate PR:
https://github.com/openai/codex/pull/9166. For this PR, please have
faith in my manual testing!
Fixes https://github.com/openai/codex/issues/8803.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/9011).
* #9166
* __->__ #9011
## **Problem**
Codex’s TUI uses a single “task running” indicator (spinner + Esc interrupt hint)
to communicate “the UI is busy”. In practice, “busy” can mean two different
things: an agent turn is running, or MCP servers are still starting up. Without a
clear contract, those lifecycles can interfere: startup completion can clear the
spinner while a turn is still in progress, or the UI can appear idle while MCP is
still booting. This is user-visible confusion during the most important moments
(startup and the first turn), so it was worth making the contract explicit and
guarding it.
## **Mental model**
`ChatWidget` is the UI-side adapter for the `codex_core::protocol` event stream.
It receives `EventMsg` events and updates two major UI surfaces: the transcript
(history/streaming cells) and the bottom pane (composer + status indicator).
The key concept after this change is that the bottom pane’s “task running”
indicator is treated as **derived UI-busy state**, not “agent is running”. It is
considered active while either:
- an agent turn is in progress (`TurnStarted` → completion/abort), or
- MCP startup is in progress (`McpStartupUpdate` → `McpStartupComplete`).
Those lifecycles are tracked independently, and the bottom-pane indicator is
defined as their union.
## **Non-goals**
- This does not introduce separate UI indicators for “turn busy” vs “MCP busy”.
- This does not change MCP startup behavior, ordering guarantees, or core
protocol semantics.
- This does not rework unrelated status/header rendering or transcript layout.
## **Tradeoffs**
- The “one flag represents multiple lifecycles” approach remains lossy: it
preserves correct “busy vs idle” semantics but cannot express *which* kind of
busy is happening without further UI changes.
- The design keeps complexity low by keeping a single derived boolean, rather
than adding a more expressive bottom-pane state machine. That’s chosen because
it matches existing UX and minimizes churn while fixing the confusion.
## **Architecture**
- `codex-core` owns the actual lifecycles and emits `codex_core::protocol`
events.
- `ChatWidget` owns the UI interpretation of those lifecycles. It is responsible
for keeping the bottom pane’s derived “busy” state consistent with the event
stream, and for updating the status header when MCP progress updates arrive.
- The bottom pane remains a dumb renderer of the single “task running” flag; it
does not learn about MCP or agent turns directly.
## **Observability**
- When working: the spinner/Esc hint stays visible during MCP startup and does
not disappear mid-turn when `McpStartupComplete` arrives; startup status
headers can update without clearing “busy” for an active turn.
- When broken: you’ll see the spinner/hint flicker off while output is still
streaming, or the UI appears idle while MCP startup status is still changing.
## **Tests**
- Adds/strengthens a regression test that asserts MCP startup completion does
not clear the “task running” indicator for an active turn (in both `tui` and
`tui2` variants).
- These tests prove the **contract** (“busy is the union of turn + startup”) at
the UI boundary; they do not attempt to validate MCP startup ordering,
real-world startup timing, or backend integration behavior.
Fixes#7017
Signed-off-by: 2mawi2 <2mawi2@users.noreply.github.com>
Co-authored-by: 2mawi2 <2mawi2@users.noreply.github.com>
Co-authored-by: Josh McKinney <joshka@openai.com>
Replace the old timing-dependent non-ASCII paste test with deterministic
coverage by forcing an active `PasteBurst` and asserting the exact flush
payload.
Add focused unit tests for `PasteBurst` transitions, and add short
"Behavior:" rustdoc notes on chat composer tests to make the state
machine contracts explicit.
Add a narrative doc and inline rustdoc explaining how `ChatComposer`
and `PasteBurst` compose into a single state machine on terminals that
lack reliable bracketed paste (notably Windows).
This documents the key states, invariants, and integration points
(`handle_input_basic`, `handle_non_ascii_char`, tick-driven flush) so
future changes are easier to reason about.
