[Bug]: AgentRouter 400 content-blocked still affects openai-chat adapter (AGR-OAI) — apply language framing on Chat/Responses

[rrmlima]

Summary

#2074 fixed the non-English content-blocked 400 for the anthropic adapter only. The openai-chat adapter (AGR-OAI provider with adapter: "openai-chat") still 400s on non-English first user messages on every wire — /v1/responses, /v1/chat/completions, and even a raw Anthropic-format call to the gateway.

Affected: gpt-5.6-sol (and any OpenAI-format model routed through AgentRouter) used by Codex CLI (wire_api = "responses") and chat-style clients. The model is listed in subagentModels, so subagent turns in PT fail hard.

Reproduction (verified against agentrouter.org with a real key)

All requests use model: "gpt-5.6-sol" (endpoint support: ["openai"] only per /v1/models), base https://agentrouter.org:

#	Wire	First message	Result
1	/v1/chat/completions (via proxy)	"What is the capital of Brazil?" (EN)	200
2	/v1/chat/completions (via proxy)	"Qual a capital do Brasil?" (PT)	400 content-blocked
3	/v1/responses (via proxy)	"Qual a capital do Brasil?" (PT)	400 content-blocked
4	Anthropic /v1/messages direct with claude-cli headers	PT, no framing	400 content-blocked
5	Anthropic /v1/messages direct with claude-cli headers	PT + [Instruction: ...] framing	200
6	/v1/chat/completions direct with all claude-cli/X-Stainless headers	PT, no framing	400 content-blocked (headers do NOT bypass the filter)
7	/v1/responses via proxy + local framing (patch)	PT	200

So: the gateway validates the opening user turn language on all wires, regardless of adapter, endpoint, or client headers. See #1804 for the original correlation research.

Note on the "model is openai-only" fact

GET /v1/models (with recognized User-Agent) reports:

{"id":"gpt-5.6-sol","model":"gpt-5.6-sol","supported_endpoint_types":["openai"]}

A raw Anthropic-format call does work for it today (test #4/#5), but that is undocumented/unofficial behavior — the supported surface is OpenAI-compatible. So the fix must live on the openai-chat path, not by re-routing users to the anthropic adapter.

Proposed fix

Extend the framing from #2074 to the openai-chat adapter. Mirror the existing applyAgentRouterLanguageFraming (src/adapters/anthropic.ts) with:

• Host exact match on agentrouter.org / *.agentrouter.org (never substring — same rationale as fix(anthropic): frame the opening turn so AgentRouter stops blocking non-English #2162's comment about notagentrouter.example).
• Idempotent: keyed on the leading block/content being exactly the marker, not includes() (a user quoting the marker later in their prompt must not suppress the frame, and retries/replays must not stack frames).
• Frame as its own text block (or prefix for string content) — the user's original text survives byte-for-byte.
• Hook points: messagesToChatFormat() result in buildRequest() for the translated Chat path, and buildOpenAIChatPassthroughRequest() for /v1/chat/completions passthrough — both are needed; patching only one leaves the other wire 400ing.
• Only the first user role message is framed (the gateway inspects the opening turn; system/developer prefixes do not help — verified: a system EN + user PT request still 400s).

Test matrix

Case	Expected
AGR-OAI PT via /v1/responses and /v1/chat/completions	200, answer in PT
AGR-OAI EN (no frame needed)	200, no double frame
AGR-CLA (anthropic)	unchanged (#2074 already handles it)
Non-AGR openai-chat providers (e.g. OpenAI, command-code, Z.AI)	byte-for-byte unchanged — no framing
Retry/replay of a framed turn	single marker, no stacking
Subsequent non-English messages after the first	unchanged (only first user turn framed)
String content AND content-part arrays (Claude Code style)	covered

Alternative considered

Shipping a provider-level flag (e.g. languagePreamble: true) instead of host-derived detection. Rejected for scope: the transform is safe, idempotent, and host-scoped, matching the #2074 decision; a flag adds config surface for no behavioral benefit.

References: #2074 (closed, anthropic-only), #2162 (merged PR), #1804 (correlation research), #1689 (closed by research).

[coderabbitai]

🔗 Related PRs

#2082 - [WRONG BRANCH] fix(adapters): frame AgentRouter first messages to pass the language filter [open]

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