[benbrandt]

Summary

Closes #285.

Codex still normalizes configured fast and priority service tiers to priority in 0.166.0. Recognize both values when initializing Fast mode so the adapter does not mark it off and clear the configured tier on the first prompt.

• Share the tier check across session creation, resume, history load, and fork, for both ACP v1 and v2.
• Preserve the app-server string tier in session metadata instead of casting it to the narrower outbound ServiceTier enum. Continue sending the supported fast value for Fast-mode turns.
• Cover fast, priority, flex, null, and unknown tiers; verify first-prompt behavior in v1 and lifecycle response snapshots over the v2 router, including resume with and without history replay.

Refreshed against current main by merging it into this branch without rewriting the original commit.

Validation

• npm run typecheck — passed.
• npm test — 2,504 passed; 39 skipped (159 test files passed, 7 skipped).
• npm run build — passed.
• Focused Fast-mode and v2 lifecycle suites — 75 passed.
• Regression check: temporarily restoring the old fast-only predicate makes all five priority cases fail; restoring the fix makes both focused suites pass.
• Live Codex 0.166.0 smoke test: configured priority, observed priority in the app-server response, verified Fast mode enabled for new/resume/load/fork, and completed a real prompt sent with serviceTier: "fast".

The dedicated integration and E2E suites were not run; the live smoke test above was run separately.

[Copilot]

Pull request overview

Fixes an ACP fast-mode regression where Codex normalizes the configured fast tier to the canonical priority tier, causing codex-acp to incorrectly initialize Fast mode as disabled and then clear the session tier on first prompt.

Changes:

• Treat serviceTier: "priority" as Fast mode enabled when initializing/resuming a session (same as "fast").
• Introduce a small helper (serviceTierEnablesFast) and a widened tier type (SupportedServiceTier) to represent Codex’s canonical tier value.
• Add a Vitest case ensuring a priority session tier results in serviceTier: "fast" being sent on the first turn.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/FastModeConfig.ts	Adds SupportedServiceTier and serviceTierEnablesFast() to treat "priority" as enabling Fast mode.
src/CodexAcpServer.ts	Uses serviceTierEnablesFast() to initialize fastModeEnabled for new/resumed sessions.
src/CodexAcpClient.ts	Stores response.serviceTier as SupportedServiceTier in SessionMetadata to preserve "priority".
src/tests/CodexACPAgent/fast-mode-config.test.ts	Adds coverage verifying "priority" initializes Fast mode on and triggers serviceTier: "fast" on prompt.

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