Greptile Summary

This PR fixes a bug where multi-turn conversations with Anthropic's extended thinking feature would fail on replay because four sanitization functions in the session history pipeline were mutating thinking and redacted_thinking content blocks — which the API requires to be returned byte-for-byte identical.

The fix introduces a shared isImmutableThinkingBlock() guard in a new thinking-guard.ts module and applies it consistently across all four mutation sites: stripThoughtSignatures, sanitizeToolResult, pruneProcessedHistoryImages, and dropThinkingBlocks. The approach is clean and well-structured.

Key points:

• The new guard is well-documented with the exact API error message and is correctly implemented for both thinking and redacted_thinking types.
• All four mutation sites are correctly protected: spread-and-delete in stripThoughtSignatures, .data field deletion in sanitizeToolResult, in-place replacement in pruneProcessedHistoryImages, and the type filter in dropThinkingBlocks.
• dropThinkingBlocks now also drops redacted_thinking blocks, which was a pre-existing gap.
• The JSDoc comment on dropThinkingBlocks (line 15 of thinking.ts) still only mentions "thinking" and should be updated to include "redacted_thinking" now that the function handles both types.
• Test coverage is solid: 5 unit tests for the new guard and a new integration-style test for dropThinkingBlocks with mixed block types.

Confidence Score: 5/5

• This PR is safe to merge — the changes are well-targeted, non-breaking, and correctly fix a documented API requirement.
• The fix is minimal and surgical: a new two-line guard function applied at four clearly identified mutation sites. All changes are additive (early-returns / continue statements), meaning no existing logic paths are altered for non-thinking blocks. The new tests confirm the guard's behavior across edge cases. The only finding is a trivial outdated doc comment.
• No files require special attention. The single style note is the outdated JSDoc on dropThinkingBlocks in thinking.ts.

This is a comment left during a code review.
Path: src/agents/pi-embedded-runner/thinking.ts
Line: 15

Comment:
**Outdated JSDoc comment**

The function now drops both `thinking` and `redacted_thinking` blocks, but the doc comment on line 15 still only mentions `type: "thinking"`. This could mislead future readers into thinking `redacted_thinking` blocks are only dropped incidentally.

```suggestion
 * Strip all `type: "thinking"` and `type: "redacted_thinking"` content blocks
 * from assistant messages.
```

How can I resolve this? If you propose a fix, please make it concise.

_{Last reviewed commit: b9acea5}