feat: add reasoning/thinking support to Anthropic /v1/messages endpoint

[timon0305]

Summary

Adds support for extended thinking / reasoning output in the Anthropic Messages API (/v1/messages), resolving a feature gap where reasoning tokens from models like QwQ, DeepSeek-R1, and other thinking-capable models were not exposed through the Anthropic-compatible endpoint.

Changes:

• protocol.py: Added AnthropicThinkingConfig model for the thinking request parameter (matching Anthropic's {"type": "enabled", "budget_tokens": N} format), added "thinking" content block type to AnthropicContentBlock, and added "thinking_delta" delta type to AnthropicDelta
• serving.py:
  • Request conversion: Maps thinking.type == "enabled" to include_reasoning=True on the OpenAI request; handles incoming thinking content blocks from prior assistant turns by converting them to text for the model
  • Non-streaming: Extracts message.reasoning from the OpenAI response and prepends a thinking content block before the text content block
  • Streaming: Handles delta.reasoning by emitting proper content_block_start/content_block_delta/content_block_stop events with thinking type blocks, correctly transitioning between thinking → text → tool_use block types
• test_anthropic_reasoning.py: Unit tests covering protocol validation, request conversion, non-streaming response conversion, streaming response conversion with reasoning→text transitions, and serialization round-trips

Protocol compatibility:
The implementation follows the Anthropic API spec for extended thinking:

• Request: {"thinking": {"type": "enabled", "budget_tokens": 4096}}
• Response content blocks: [{"type": "thinking", "thinking": "..."}, {"type": "text", "text": "..."}]
• Streaming: thinking_delta events with thinking field

Closes #29915

Test plan

• Unit tests for AnthropicThinkingConfig validation (enabled requires budget_tokens)
• Unit tests for request conversion (include_reasoning flag propagation)
• Unit tests for non-streaming response with/without reasoning
• Unit tests for streaming response with reasoning→text block transitions
• Unit tests for serialization round-trips of thinking content blocks
• Ruff lint and format checks pass

[dosubot]

Related Documentation

Checked 0 published document(s) in 1 knowledge base(s). No updates required.

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[gemini-code-assist]

Code Review

The pull request successfully implements reasoning/thinking support for the Anthropic Messages API. The protocol models are correctly updated to include the thinking content type and configuration, and the serving logic handles both streaming and non-streaming responses. However, there are several issues in the streaming converter logic where continue and elif statements could lead to data loss if multiple types of deltas (reasoning, content, tool calls) are present in a single chunk from the engine. These should be addressed to ensure robustness.

[gemini-code-assist]

Similar to the reasoning block, using continue here prevents the processing of tool_calls if they are packed into the same chunk as a content delta. Removing the continue and adjusting the logic to use a non-skipping check for empty content ensures that all parts of the delta are handled.

Suggested change

	if delta.content == "":
	continue
	chunk = AnthropicStreamEvent(
	index=content_block_index,
	type="content_block_delta",
	delta=AnthropicDelta(
	type="text_delta",
	text=origin_chunk.choices[0].delta.content,
	text=delta.content,
	),
	)
	data = chunk.model_dump_json(exclude_unset=True)
	yield wrap_data_with_event(data, "content_block_delta")
	continue
	if delta.content != "":
	chunk = AnthropicStreamEvent(
	index=content_block_index,
	type="content_block_delta",
	delta=AnthropicDelta(
	type="text_delta",
	text=delta.content,
	),
	)
	data = chunk.model_dump_json(exclude_unset=True)
	yield wrap_data_with_event(data, "content_block_delta")

[gemini-code-assist]

Using elif here means that tool calls will be ignored if delta.content was also present in the same chunk (and the continue above was removed). Changing this to an if allows the converter to sequentially close the text block and open a tool block within the same iteration if the engine emits them together.

Suggested change

	elif len(delta.tool_calls) > 0:
	if len(delta.tool_calls) > 0:

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @timon0305.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[chaunceyjiang]

Thanks~ @timon0305 see #33671