docs(structure): scope canonical Fast injection to the bridged Chat path

[lidge-jun]

Summary

Campaign wp9 docs-drift fix (the release-readiness follow-up item). structure/04's native Chat passthrough paragraph claimed canonical Fast follows the resolved Fast policy without chatServiceTier. The code does not do that on the native path: chat-native.ts has no tier wiring — tier resolution lives only in the Responses pipeline feeding adapter buildRequest, which the passthrough bypasses. On the native path every caller service_tier is forwarded raw and only under chatServiceTier: true, and fastMode injects nothing (openai-chat.ts:121, chat-native.ts:54-72; adversarial verification recorded in devlog/_plan/260818_bug_pr_resolution/040).

Docs-only: one paragraph in structure/04_transports-and-sidecars.md.

Verification

• git show --stat: 1 file changed (structure/04_transports-and-sidecars.md), docs-only
• Claim verified by adversarial plan-audit subagent with file:line citations (r6/F2)

Checklist

• Docs-only, no runtime change
• Matches code behavior with citations
• No dependencies

Summary by CodeRabbit

• Bug Fixes
  • Native Chat Completions requests now preserve caller-supplied service tier values when supported.
  • Fast mode no longer alters native Chat Completions passthrough requests.
  • Automatic Fast-policy handling remains limited to the Chat-to-Responses-to-Chat conversion flow.

[github-actions]

✅ Deterministic PR hygiene checks passed.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: e799acd8-ffde-483a-a735-b162a3e77df3

📥 Commits

Reviewing files that changed from the base of the PR and between e446607 and 383d959.

📒 Files selected for processing (1)

• structure/04_transports-and-sidecars.md

Included review availability: Your plan includes up to 10 reviews per rolling hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

The transport documentation updates native Chat passthrough behavior. Caller-supplied service_tier values pass through unchanged when chatServiceTier is enabled. fastMode has no effect on this path. Fast-policy injection remains on the bridge path.

Changes

Chat service tier policy

Layer / File(s)	Summary
Native Chat passthrough behavior structure/04_transports-and-sidecars.md	Lines 663–666 document raw forwarding of caller-supplied tiers under chatServiceTier: true. The native path no longer injects canonical Fast tiers or performs wire mapping. The bridge path retains resolved Fast-policy injection.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Merge Risk: ⚪ Minimal · up to 383d9

This documentation-only change corrects the description of Fast injection behavior without changing runtime code; no actionable merge-blocking risk remains.

Suggested reviewers: ingwannu

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the documentation change that limits canonical Fast injection to the bridged Chat path.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/wp9-structure04-drift

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