[Bug]: Anthropic adaptive thinking is requested with default display (omitted), so long Opus max-effort thinking reaches Chat clients as minutes of heartbeat-only silence #5824

Client or integration

OMO / senpi (openai-completions provider pointed at the ocx loopback /v1/chat/completions)

Area

Anthropic adapter (adaptive thinking request shape)

Summary

For Claude Opus 5.x adaptive-thinking models the Anthropic adapter sends thinking: { type: "adaptive" } without display. On Opus 4.7+ the API default is display: "omitted", so the upstream stream contains empty, signature-only thinking blocks and no thinking text. At effort: max on a large context (~220k tokens) Opus routinely thinks for 2-5 minutes before the first text or tool call. During that time a Chat Completions client gets only the role chunk and : opencodex heartbeat SSE comments (#5806), with no semantic event.

Clients whose stream-start or idle watchdog counts semantic events, not transport bytes, therefore see a 5-minute silence and cancel. In OMO this surfaces as Provider stream start timed out after 300000ms. #5806's ADR says this outcome is intentional ("a client's separate semantic-progress watchdog may still expire"). The fix needs real reasoning deltas.

Requesting display: "summarized" would stream summarized thinking and relay it as reasoning deltas on the Chat surface. Long-thinking turns would then look alive to every client. It is also what the Anthropic SDK-based harnesses request by default.

Reproduction

1. Messages surface, with an explicit display in the request:

   curl -sN http://127.0.0.1:10100/v1/messages -H 'content-type: application/json' -H 'anthropic-version: 2023-06-01' -H 'x-api-key: x' \
     -d '{"model":"anthropic/claude-opus-5-5","max_tokens":4000,"stream":true,"thinking":{"type":"adaptive","display":"summarized"},"output_config":{"effort":"high"},"messages":[{"role":"user","content":"Think carefully: how many primes are below 200?"}]}'
   

   The thinking block arrives as {"type":"thinking","thinking":"","signature":""} followed only by signature_delta. There is no thinking_delta text, even though the caller explicitly asked for summarized.
2. Chat surface at reasoning_effort: max: 53 s of : opencodex heartbeat every 2 s, then the answer. completion_tokens was 3397, reasoning_tokens was 0, and no reasoning delta was ever emitted.

Expected: summarized thinking text is streamed, either always or at least when the caller requests display: "summarized". The Chat bridge would then emit reasoning deltas during the thinking phase.

Version

opencodex 2.65.0 (includes #5806)

Operating system

Linux x64

Provider and model

anthropic (OAuth) / claude-opus-5-5, effort max

Logs or error output

usage.jsonl on the failing session. There were six 502s on one conversation, each upstreamError: "The connection was closed.". Four ended at ~300 s, when the client gave up, with failureStage: protocol-prelude and semanticBytes: 0. Successful turns in the same conversation took 120-295 s.

Checks

• I searched existing issues and documentation. Related: Anthropic adapter drops ping events, so long pre-output thinking on Opus 5.5 hits upstream_stall_timeout at 300 s #5707 (ping → heartbeat) and [Bug]: Chat Completions bridge drops heartbeat frames after the initial role chunk #5805/fix(chat): relay translated heartbeats #5806 (heartbeat relay as SSE comment). Neither gives the client semantic progress.
• I removed secrets, tokens, account details, request credentials, and personal data.