fix(server): newer Codex models get T3 Code's instructions again

[t3dotgg]

Codex agents on newer models stopped getting T3 Code's instructions. They did not know they ran in T3 Code, did not follow the PR linking rules, and did not get the browser and device tool guidance.

T3 Code sent all of this inside collaborationMode.settings.developer_instructions. When the Codex model catalog has its own text for a collaboration mode, Codex uses the catalog text and ignores ours. Newer models ship that text for Default mode, so everything we added was dropped. Plan mode was not affected.

Now the collaboration mode carries only the mode prompt. The runtime info, PR linking, and tool guidance go through turn/start.additionalContext as application entries, which the catalog cannot override:

• Two entries. t3_code_runtime and t3_code_tools each stay under Codex's 1,000-token limit per entry. A test fails if either one grows too close to that limit.
• Compaction. Codex drops these messages when it compacts a thread, and it only sends an entry again when its value changes. When the root thread reports a contextCompaction item, we put the last context back with thread/inject_items.
• Model name. runtime_info now gives the display name and the slug, for example "GPT-5.4 (model slug: gpt-5.4)". The name comes from the provider's model list.

Verified on the real Codex 0.156.1 CLI:

• Real model: with a model whose catalog overrides Default mode, the model correctly reported T3 Code, the Codex harness, its model, and its effort.
• Fake Responses endpoint (no credentials): the context reached the model on the first turn and again after a manual compaction, exactly once. Without the restore, it was missing after compaction.

Created with Claude Opus 5.5 in Claude Code.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements
  • Codex sessions now keep collaboration-mode instructions separate from runtime and available browser or device guidance.
  • Model and reasoning settings are applied to each turn, with defaults used when settings are not specified.
  • Tool guidance is included only when relevant tools are available.
  • Runtime guidance identifies the selected model by its display name and identifier when they differ.
  • Runtime and tool guidance is restored after context compaction.

[macroscopeapp]

Approvability

Verdict: Approved at 13b0ac2

Macroscope's review found this PR approvable — This is a contained Codex bug fix that restores existing T3 Code instructions for newer models and after compaction, with targeted unit and integration coverage. It does not alter product defaults, schemas, deployment behavior, or sensitive functionality.

Notes:

• No code objects were reviewed. Approvability was decided on eligibility alone.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	−18 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+6 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	−24 B (−0.4%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.3 KiB	56.3 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	−4 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+7 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.4 KiB	−11 B (−0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 568c9bc · PR result: 13b0ac2 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 114.0 KiB
• Claude decoded thread snapshot: 114.7 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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• apps/server/src/provider/Layers/CodexCollabRuntime.integration.test.ts

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📝 Walkthrough

Walkthrough

Codex collaboration-mode prompts now contain mode instructions only. Turn-start requests carry runtime and available tool guidance in separate additional-context entries. The runtime uses model display names in runtime guidance and restores saved context after root-thread compaction.

Changes

Codex turn context

Layer / File(s)	Summary
Separate mode prompts from runtime and tool context apps/server/src/provider/CodexDeveloperInstructions.ts, apps/server/src/provider/RuntimeInstructions.ts, apps/server/src/provider/RuntimeInstructions.test.ts, apps/server/src/provider/Layers/CodexSessionRuntime.test.ts	Mode prompts are static and omit runtime and tool guidance. Runtime and available tool guidance are generated as separate context entries. Runtime instructions include both model display name and slug when they differ.
Build and pass additional context at turn start apps/server/src/provider/Layers/CodexSessionRuntime.ts, apps/server/src/provider/Drivers/CodexDriver.ts, apps/server/src/provider/Layers/CodexAdapter.ts, apps/server/src/provider/Layers/CodexSessionRuntime.test.ts	Turn-start requests include collaboration settings and additional context when an interaction mode is set. The driver and adapter supply model metadata to the runtime, which selects the display name for the normalized model.
Restore context after root-thread compaction apps/server/src/provider/Layers/CodexSessionRuntime.ts, apps/server/src/provider/Layers/CodexCollabRuntime.integration.test.ts, apps/server/src/provider/testFixtures/codexCollabMockPeer.mjs	The runtime saves additional context and injects it as developer messages after root-thread compaction. The integration test checks the injected context, and the mock peer records injection requests.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant CodexSessionRuntime
  participant buildCodexAdditionalContext
  participant CodexTurnStart
  participant CodexThread
  CodexSessionRuntime->>buildCodexAdditionalContext: model, effort, and tool availability
  buildCodexAdditionalContext-->>CodexSessionRuntime: runtime and available-tool entries
  CodexSessionRuntime->>CodexTurnStart: collaboration settings and additional context
  CodexThread-->>CodexSessionRuntime: root-thread contextCompaction completion
  CodexSessionRuntime->>CodexThread: thread/inject_items with saved context

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Suggested reviewers: juliusmarminge

Merge Risk: 🟡 Moderate · up to 13b0a

A queued follow-up with different model settings can cause root-thread compaction to restore the wrong guidance for the active turn. Fix the turn-to-context association before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 13b0a

Guidance is now restored after a session’s history is shortened, but a failed restoration can leave it absent, and a later restoration may use guidance from a turn that did not start successfully. The review found no new authorization capability.

Retained concerns

• Medium · reliability · observed: After root-thread compaction, a failed context injection is treated as a warning. Because the guidance was removed from the mode instructions and the saved context is not retried here, subsequent work may proceed without the application’s runtime and tool guidance.
• Low · reliability · inferred: A turn replaces the session-wide restoration value before turn/start succeeds. If a newer turn fails after changing its context, a later root compaction can restore that attempted turn’s guidance rather than the last accepted turn’s guidance. The available evidence does not establish whether higher-level scheduling prevents overlapping turns.

Security review details

Security Blast Radius

• inferred — The new restoration state is held per runtime session and injection is gated on the current root-thread ID. The observed child-compaction path does not inject into the root; the identified exposure is guidance continuity within an affected session, not demonstrated cross-thread authority.

Trust Boundaries and Controls

• observed — Application context crosses from the server into the Codex turn request; restoration reintroduces it as developer messages. Root-thread identity is checked before restoration, while approval and sandbox policy remain independent request fields. No evidence establishes an attacker-controlled route to the restored context or a gain in tool privileges.

Resilience and Maintainability Implications

• inferred — The absence of retry or accepted-turn correlation can let operational guidance drift after a failed injection or failed newer turn. The evidence does not show that this changes enforced tool access.

Hardening Proposals

• proposed — Associate saved context with an accepted turn and make restoration failure observable for recovery; establish whether repeated compaction notifications require idempotent injection.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 9 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary change: restoring T3 Code instructions for newer Codex models.
Description check	✅ Passed	The description clearly explains the problem, the implementation, the compaction behavior, model-name handling, and verification results. It does not use the template headings or include the checklist…

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Actionable comments posted: 2

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🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@apps/server/src/provider/Layers/CodexSessionRuntime.ts`:
- Line 2585: Update the additional-context storage and restoreAdditionalContext
flow to associate context with its provider turn ID instead of overwriting a
single lastAdditionalContextRef value. When handling contextCompaction, select
and restore only the context keyed by the notification’s turnId so queued-turn
settings cannot affect the active turn.

In `@apps/server/src/provider/RuntimeInstructions.ts`:
- Around line 19-20: Escape the user-authored model and modelName values before
modelLabel inserts them into runtime_info, so markup-like text cannot break out
of the block. Preserve the existing whitespace normalization while escaping
markup delimiters in both values.

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📥 Commits

Reviewing files that changed from the base of the PR and between 355ca7d and 1a4d442.

📒 Files selected for processing (9)

• apps/server/src/provider/CodexDeveloperInstructions.ts
• apps/server/src/provider/Drivers/CodexDriver.ts
• apps/server/src/provider/Layers/CodexAdapter.ts
• apps/server/src/provider/Layers/CodexCollabRuntime.integration.test.ts
• apps/server/src/provider/Layers/CodexSessionRuntime.test.ts
• apps/server/src/provider/Layers/CodexSessionRuntime.ts
• apps/server/src/provider/RuntimeInstructions.test.ts
• apps/server/src/provider/RuntimeInstructions.ts
• apps/server/src/provider/testFixtures/codexCollabMockPeer.mjs

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