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fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms#23755

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fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms#23755
bbartels wants to merge 15 commits intoanomalyco:devfrom
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@bbartels bbartels commented Apr 21, 2026

Issue for this PR

Closes #14332

Type of change

  • Bug fix
  • New feature
  • Refactor / code improvement
  • Documentation

What does this PR do?

Fixes the Anthropic API error: messages.X.content.Y: 'thinking' or 'redacted_thinking' blocks in the latest assistant message cannot be modified.
The Anthropic API requires that thinking and redacted_thinking blocks in assistant messages are sent back byte-for-byte identical in subsequent requests. Three code paths in normalizeMessages / applyCaching in src/provider/transform.ts were inadvertently modifying these blocks:

  1. Empty content filtering removed redacted_thinking blocks. The AI SDK represents these as { type: "reasoning", text: "", providerOptions: { anthropic: { redactedData: "..." } } } — the empty text caused them to be filtered out. Fix: only filter reasoning parts that have no providerOptions.
  2. Tool-use reordering split assistant messages containing [thinking, tool_call, text] into two separate messages, changing which message the thinking blocks belong to. Fix: skip the reorder when reasoning blocks are present.
  3. Cache control hints could be applied to a reasoning block if it was the last content block in the message. Fix: find the last non-reasoning block for cache annotations instead.

How did you verify your code works?

Added tests covering all three fixes to test/provider/transform.test.ts. All 143 tests pass (130 existing + 13 new).
bun test test/provider/transform.test.ts
143 pass, 0 fail

Screenshots / recordings

N/A - not a UI change.

Checklist

  • I have tested my changes locally
  • I have not included unrelated changes in this PR

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github-actions Bot commented Apr 21, 2026

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate/Related PRs Found:

  1. PR fix(provider): skip empty-text filtering for assistant messages in normalizeMessages (#16748) #16750 - fix(provider): skip empty-text filtering for assistant messages in normalizeMessages

    • Directly related to the empty content filtering issue addressed in the current PR

  2. PR fix(provider): preserve assistant message content when reasoning blocks present #21370 - fix(provider): preserve assistant message content when reasoning blocks present

    • Addresses similar issue of preserving assistant messages when reasoning blocks are present

  3. PR fix(provider): preserve redacted_thinking blocks and fix signature validation #12131 - fix(provider): preserve redacted_thinking blocks and fix signature validation

    • Related to preserving redacted_thinking block integrity

  4. PR fix: replace empty text in reasoning messages to preserve thinking block positions #21860 - fix: replace empty text in reasoning messages to preserve thinking block positions

    • Similar fix for preserving thinking block positions with empty text handling

These PRs appear to be addressing overlapping issues with preserving thinking/reasoning blocks in Anthropic message transforms. The current PR (23755) consolidates fixes for three specific code paths (empty content filtering, tool-use reordering, and cache control hints) that were inadvertently modifying these blocks.

@bbartels
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bbartels commented Apr 22, 2026

@rekram1-node Any chance you could have a look? ~20 LOC of actual changes, rest is tests

@bbartels bbartels mentioned this pull request Apr 23, 2026
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@bainos bainos mentioned this pull request Apr 23, 2026
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@bbartels bbartels mentioned this pull request May 5, 2026
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