fix(anthropic): fail over on proven pre-output account 403 refusals

[lidge-jun]

Summary

The Anthropic OAuth account pool rotated to the next account only on HTTP 429. When one pooled account's Claude subscription had expired, the upstream answered 403 before any output, and the request failed even though other accounts in the pool were healthy.

A pre-output 403 now fails over to the next account only when it proves an account-level refusal. That requires complete, bounded JSON, a recognized error type, and exact account-entitlement or billing wording, and the bearer and credential generation that actually sent the request must still match. A recognized refusal clears affinity and cools that account, using Retry-After or ten minutes by default, then the request moves to the next account under the existing rotation cap and send budget. Generic permission, policy, malformed, overridden-credential and stale-credential 403s stay terminal. No 403 failover happens after output has begun. Main dispatch, continuations, sidecars and web search follow the same rule. Docs and translations that described 429-only rotation are updated.

Closes #6340

Verification

• bun test tests/adapters/anthropic/anthropic-quota-dispatch.test.ts: red 18 pass / 9 fail before the fix, green 40 pass / 0 fail after. It covers entitlement recovery, negative classifications, credential replacement, header overrides, exhausted pools, strict routes, continuations and sidecars.
• Rebased on current dev: the changed tests plus test-layout and file-size guards pass (67 pass / 0 fail); bun x tsc --noEmit, structure:check and privacy:scan pass.
• Security review (account-pool credential surface): PASS by an independent reviewer. The classifier is bounded and fails closed on unknown wording. Attribution binds only when the outgoing headers prove the selected bearer. New diagnostics carry no tokens, account identifiers, emails or upstream bodies. There is no new credential destination.
• Not tested against a live expired Claude account. Cooldowns are process-local, as for 429.
• Cross-platform CI for this head is deferred by the owner's instruction to one run on the final dev tip before release.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

[chatgpt-codex-connector]

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📝 Code Review	✅ Completed 2026-10-01T02:39:05.044077Z	b948392	PR opened

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📒 Files selected for processing (26)

• docs-site/src/content/docs/fr/guides/claude-code.md
• docs-site/src/content/docs/fr/reference/configuration/providers.md
• docs-site/src/content/docs/guides/claude-code.md
• docs-site/src/content/docs/ja/reference/configuration/providers.md
• docs-site/src/content/docs/ko/reference/configuration/providers.md
• docs-site/src/content/docs/reference/configuration/providers.md
• docs-site/src/content/docs/ru/reference/configuration/providers.md
• docs-site/src/content/docs/tr/guides/claude-code.md
• docs-site/src/content/docs/tr/reference/configuration/providers.md
• docs-site/src/content/docs/zh-cn/reference/configuration/providers.md
• docs-site/src/content/docs/zh-tw/guides/claude-code.md
• docs-site/src/content/docs/zh-tw/reference/configuration/providers.md
• src/images/loop.ts
• src/oauth/anthropic-account-refusal.ts
• src/oauth/anthropic-routing.ts
• src/server/responses/adapter-continuation.ts
• src/server/responses/adapter-delivery.ts
• src/server/responses/adapter-dispatch.ts
• src/server/responses/request-transport.ts
• src/server/responses/sidecar-execution.ts
• src/web-search/loop.ts
• structure/providers-and-adapters.md
• structure/providers/anthropic-account-pool.md
• structure/transports/inventory.md
• structure/transports/responses-failover.md
• tests/adapters/anthropic/anthropic-quota-dispatch.test.ts

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[lidge-jun]

Maintainer integration into dev by @lidge-jun. Security review (account-pool credential surface): PASS by an independent reviewer; details and red-green evidence are in the description. Rebased on current dev; changed tests and guards 67 pass, typecheck, structure and privacy checks pass. Cross-platform CI for this head is deferred by the owner's instruction to one run on the final dev tip before release; scripts/ci/assert-mergeable-review.sh --maintainer-integration OK at this head.

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Reviewed commit: b94839264e

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[chatgpt-codex-connector]

Preserve 429 cooldown recording after cancellation

When an upstream 429 has already resolved but the client aborts before this helper runs, this early return skips both rotation and recordAnthropicAccountRefusal. Previously the 429 path recorded the sender's cooldown even when no retry could be made, so the next request now may immediately select an account already proven rate-limited. Keep cancellation from authorizing a resend, but still record a replayable 429—for example, by applying the abort guard only to 403 body classification and passing canRetry: false for an aborted 429.

AGENTS.md reference: src/AGENTS.md:L17-L19

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