Thinking block not generated despite thinking mode being enabled

Description

Thinking blocks are sometimes not generated by the model even when thinking mode is explicitly enabled in Claude Code. This results in responses without any thinking block in the conversation JSON, and no thinking visible in the UI (Ctrl+O transcript).

Evidence

Full conversation log: https://gist.github.com/alberduris/c845a662faf0a4316b80a39764ee895c

In the same session with thinking mode enabled, we observed:

The first user message (line 2) has no thinkingMetadata field, and the corresponding assistant response (line 13) has no thinking block:

"content":[{"type":"text","text":"No, con `caffeinate -dims`..."}]

The second user message (line 32) does have thinkingMetadata:

"thinkingMetadata":{"level":"high","disabled":false,"triggers":[]}

And its corresponding assistant response (lines 33-34) does have a thinking block:

"content":[{"type":"thinking","thinking":"Jaja, el usuario tiene razón...","signature":"..."}]

Thinking mode was enabled from the start of the session. Note that this bug has also been observed in long conversations at advanced turns, not just the first message. The evidence above is from the case we were able to capture.

Reproduction

This bug is intermittent. We do not have reliable reproduction steps.

What we know:

• Thinking mode was confirmed ON at the time of the request
• The question was a simple factual query about macOS caffeinate command behavior
• The response quality felt degraded + no UI indicator (no inline "thinking" and no "Thinking block" on Ctrl+O)
• Verified in .jsonl file: no "type": "thinking" exists in the first response
• This is NOT a UI/display bug - the thinking block is completely absent from the stored API response

Expected Behavior

When thinking mode is enabled, every assistant response should include a thinking block, and every user message should have thinkingMetadata recorded.

Actual Behavior

Some messages lack thinkingMetadata and their responses have no thinking block, even though thinking mode was enabled. This has been observed both at session start and in later turns of long conversations.

Environment

• Claude Code version: 2.1.7
• Model: claude-opus-4-5-20251101
• OS: macOS
• Thinking mode: Enabled
• Feedback ID: 83372005-76d1-4f3d-924a-9d555b49aa41