Replay assistant reasoning as Anthropic thinking blocks in buildMessageParam

[PratikDhanave (PratikDhanave)]

What

buildMessageParam in provider/anthropicprovider/agent.go switches over the outbound msg.Contents but had no case for *message.TextReasoningContent. It handled TextContent, FunctionCallContent, FunctionResultContent and DataContent only.

The inbound path buildBlock produces exactly that type from Anthropic responses:

• anthropic.ThinkingBlock -> TextReasoningContent{ProtectedData: Signature, Text: Thinking}
• anthropic.RedactedThinkingBlock -> TextReasoningContent{ProtectedData: Data}

So when a prior assistant turn's history was sent back (e.g. multi-turn tool loops with extended thinking), the reasoning block and its cryptographic signature were silently dropped.

Fix

Add a *message.TextReasoningContent case that replays the block, appended in iteration order (Anthropic emits reasoning before the rest of the turn):

• signature + text present -> anthropic.NewThinkingBlock(ProtectedData, Text)
• data only (text empty) -> anthropic.NewRedactedThinkingBlock(ProtectedData)
• both empty -> skipped, so streamed partials that never received a signature don't produce an invalid unsigned thinking block the API would reject.

Why

Preserving thinking-block signatures across turns is required for Anthropic extended-thinking with tool use; the signature must round-trip or the API rejects the replayed assistant turn. This matches the .NET/Python SDKs, which reserialize reasoning content back into thinking / redacted_thinking blocks rather than dropping them, keeping cross-SDK conversation replay behavior aligned.

Tests

Added black-box tests in the canonical agent_test.go, driven through the exported Run().Collect() path with the existing request-capturing httptest harness:

• TestAssistantReasoningReplayedAsThinkingBlock — asserts a thinking block carrying signature sig123 is emitted first, ahead of the tool_use block.
• TestAssistantRedactedReasoningReplayedAsRedactedThinkingBlock — asserts a redacted_thinking block with its data is emitted.
• TestAssistantUnsignedReasoningIsSkipped — asserts an unsigned partial is dropped.

All three fail before the fix and pass after. go build ./..., go vet and go test on the package are green.

[Copilot]

Pull request overview

This PR fixes Anthropic conversation replay for extended-thinking/tool-use by ensuring prior assistant reasoning (TextReasoningContent) is serialized back into Anthropic thinking / redacted_thinking content blocks (including the required signature), instead of being silently dropped.

Changes:

• Add *message.TextReasoningContent handling in buildMessageParam to emit thinking / redacted_thinking blocks and skip unsigned partial reasoning.
• Add black-box request-capture tests validating correct replay behavior, ordering, and skipping of unsigned partials.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
provider/anthropicprovider/agent.go	Re-serializes TextReasoningContent into Anthropic thinking blocks so signatures round-trip across turns.
provider/anthropicprovider/agent_test.go	Adds request-body assertions to verify thinking/redacted_thinking replay and skipping unsigned partial reasoning.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Quim Muntal (qmuntal)]

Needs rebase.

[github-actions]

Parity Review: ✅ No issues found

This PR fixes a bug in the unexported buildMessageParam function in provider/anthropicprovider/agent.go where *message.TextReasoningContent was silently dropped when replaying prior assistant turns with extended thinking.

No exported Go APIs changed. The public-api-change label is not warranted.

Cross-SDK parity check: The upstream Python implementation (python/packages/anthropic/agent_framework_anthropic/_chat_client.py) applies the same round-trip logic — text_reasoning content with protected_data + text → {type: "thinking", thinking: ..., signature: ...}; with only protected_data → {type: "redacted_thinking", ...}; unsigned partials dropped. The Go fix matches this behavior exactly.

The parity-approved label already present on this PR remains correct.

Generated by Go API Consistency Review Agent · sonnet46 · 26.4 AIC · ⌖ 5.67 AIC · ⊞ 6K · ◷