## Why
Goal steering prompts have grown into long inline Rust strings, which
makes the authored prompt text hard to review and easy to damage while
changing the surrounding plumbing. Moving those prompts into embedded
Markdown templates keeps the policy text in the shape reviewers actually
read, while preserving the existing runtime substitution and objective
escaping behavior.
## What changed
- Added `ext/goal/templates/goals/continuation.md`, `budget_limit.md`,
and `objective_updated.md` for the three goal steering prompts.
- Updated `ext/goal/src/steering.rs` to parse those embedded templates
once with `codex-utils-template` and render the existing goal values
into them.
- Kept user objectives XML-escaped before rendering and converted budget
counters into template variables.
- Added the template directory to `ext/goal/BUILD.bazel` `compile_data`
so Bazel has the same embedded prompt inputs as Cargo.
## Testing
- Not run locally.
## Why
The goal extension needs a way to resume an active goal after the thread
becomes idle, but the old core goal runtime should not be refactored as
part of this step. The missing piece is a small core-owned turn-start
primitive: let an extension ask for a normal model turn only when the
thread is idle, and otherwise fail without injecting into whatever is
currently active.
## What Changed
- Adds `CodexThread::try_start_turn_if_idle(...)` as the narrow
extension-facing primitive for synthetic idle work.
- Implements the session side so it refuses to start when:
- the provided input is empty,
- the session is in plan mode,
- a turn is already active, or
- trigger-turn mailbox work is pending.
- Gives trigger-turn mailbox work priority if it appears while the idle
turn is being prepared.
- Wires `GoalExtension::on_thread_idle` to read the active persisted
goal and submit the continuation prompt through this idle-only
primitive.
- Keeps the legacy core goal continuation implementation in place
instead of folding it into this PR.
## Behavior
This is intentionally best-effort. If `try_start_turn_if_idle` observes
that the thread is not idle, or that higher-priority mailbox work should
run first, it returns the input to the caller. The goal extension drops
that continuation prompt and waits for a future idle opportunity instead
of injecting stale synthetic goal text into an active turn.
## Validation
- `just test -p codex-core
try_start_turn_if_idle_rejects_active_turn_without_injecting`
- `just test -p codex-goal-extension`
## Why
This PR is stacked on #24918, which moves goal steering onto
source-labeled internal model context fragments. Active-turn goal
steering should use the same running-turn injection path as other
runtime steering, so those fragments enter the pending input queue as
`ResponseItem`s through the existing
[`Session::inject_if_running`](https://github.com/openai/codex/blob/8d6f6cdf69b055c27682e7cdea9caf72a3e2ee7f/codex-rs/core/src/session/inject.rs#L12-L27)
behavior instead of through a goal-specific conversion wrapper.
## What Changed
- Exposes a narrow `CodexThread::inject_if_running` bridge for callers
that only hold a thread handle.
- Changes `ext/goal` active-turn steering to pass `ResponseItem`s
directly.
- Builds goal steering prompts as contextual internal model context
`ResponseItem`s before injecting them into the running turn.
## Testing
Not run locally; PR metadata update only.
## Why
Goal steering is one form of runtime-owned model context, but the old
`<goal_context>` wrapper made the contextual-fragment hiding path
goal-specific. Using a source-labeled internal context fragment gives
core and extensions a shared shape for hidden model steering while
keeping those prompts out of visible turn history.
The change also keeps legacy `<goal_context>` messages recognized as
hidden contextual input so existing stored history does not start
rendering old goal-steering prompts as user-visible turn items.
## What Changed
- Replaces `GoalContext` with `InternalModelContextFragment` plus a
validated `InternalContextSource`.
- Renders goal steering as `<codex_internal_context
source="goal">...</codex_internal_context>`.
- Updates core goal steering and `ext/goal` steering to inject the new
internal-context fragment.
- Updates contextual-fragment, event-mapping, goal, and session tests
for the new wrapper.
## Test Coverage
- Adds coverage for detecting the new internal model context fragment.
- Preserves coverage for hiding legacy `<goal_context>` fragments.
- Verifies invalid internal context sources are rejected and arbitrary
context tags are not hidden.
- Updates goal steering/session assertions to expect the new
`source="goal"` wrapper.
## Summary
Adds experimental `additionalContext` support to `turn/start` and
`turn/steer` so clients can provide ephemeral external context, such as
browser or automation state, without turning that plumbing into a
visible user prompt or triggering user-prompt lifecycle behavior.
## API Shape
The parameter shape is:
```ts
additionalContext?: Record<string, {
value: string
kind: "untrusted" | "application"
}> | null
```
Example:
```json
{
"additionalContext": {
"browser_info": {
"value": "Active tab is CI failures.",
"kind": "untrusted"
},
"automation_info": {
"value": "CI rerun is in progress.",
"kind": "application"
}
}
}
```
The keys are opaque and caller-defined.
## Context Injection
When provided, accepted entries are inserted into model context as
hidden contextual message items, not as visible thread user-message
items.
`kind: "untrusted"` entries are inserted with role `user`:
```text
<external_${key}>${value}</external_${key}>
```
`kind: "application"` entries are inserted with role `developer`:
```text
<${key}>${value}</${key}>
```
Values are not escaped. Each value is truncated to 1k approximate tokens
before wrapping.
For `turn/start`, accepted additional context is inserted before normal
user input. For `turn/steer`, additional context is merged only when the
steer includes non-empty user input; context-only steers still reject as
empty input.
## Dedupe Strategy
`AdditionalContextStore` lives on session state and stores the latest
complete additional-context map.
Each `turn/start` or non-empty `turn/steer` treats its
`additionalContext` as the current complete set of values. Entries are
injected only when the key is new or the exact entry for that key
changed, including `value` or `kind`. After merging, the store is
replaced with the provided map, so omitted keys are removed from the
retained set and can be injected again later if reintroduced.
Omitting `additionalContext`, passing `null`, or passing an empty object
resets the store to empty and injects nothing.
## What Changed
- Threads experimental v2 `additionalContext` through app-server into
core turn start and steer handling.
- Adds separate contextual fragment types for untrusted user-role
context and application developer-role context.
- Uses pending response input items so additional context can be
combined with normal user input without treating it as prompt text.
- Adds integration coverage for start/steer flow, role routing,
dedupe/reset behavior, deletion/re-add behavior, hook-blocked input
behavior, empty context-only steer rejection, external-fragment marker
matching, and truncation.
## What
- Add a small extension capability for injecting model-visible response
items into the active turn
- Have the goal extension inject hidden goal-context steering when
tool-finish accounting reaches `BudgetLimited`
- Cover the extension backend path with an assertion on the injected
steering item
## Why
PR #23696 persists and emits the budget-limited goal update from
tool-finish accounting, but it leaves the model unaware of that
transition. The existing core runtime steers the model to wrap up in
this case; the extension path should do the same through an explicit
host capability.
## Testing
- `just fmt`
- `cargo test -p codex-goal-extension`
- `cargo test -p codex-extension-api`
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