[Bugfix][Frontend] Anthropic API: honor the `thinking` request parameter by jryberg · Pull Request #53058 · vllm-project/vllm

jryberg · 2026-08-20T06:14:23Z

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Claude Code, Opus 5 was part of this change including writing this PR, overseen and validated with human.

Purpose

AnthropicMessagesRequest never declared a thinking field, so the documented Anthropic request
parameter was silently discarded — the model sets no Pydantic extra policy, so the default
extra="ignore" drops it without an error. Clients asking for disabled thinking, a token budget, or
omitted reasoning display were answered as though they had asked for nothing, at whatever
output_config.effort implied.

This is user-visible: Claude Code sends {"type": "adaptive", "display": "omitted"} on every
request. Both instructions were dropped, so every request reasoned at the effort ceiling and
streamed the reasoning back. On a DeepSeek-V4-Flash-0731 deployment an ordinary coding question
produced 2237 reasoning events over 151 seconds without a single visible output token — the client
gave up before the model finished thinking.

This adds AnthropicThinkingConfig and maps it onto reasoning controls ChatCompletionRequest
already exposes:

`thinking`	maps to
`{"type": "disabled"}`	`reasoning_effort = "none"`
`{"type": "enabled", "budget_tokens": N}`	`thinking_token_budget = N`
`{"display": "omitted"}`	`include_reasoning = False`
`{"type": "adaptive"}`	nothing pinned — the model chooses depth

Two design points worth reviewer attention:

adaptive deliberately pins nothing. Per Anthropic's API, output_config.effort is the
ceiling and adaptive lets the model choose depth beneath it. No frontend-side action makes a
model adaptive, so the correct behaviour is to avoid forcing a static value.
display maps to include_reasoning, not to any depth control. It governs visibility only —
reasoning still runs and is still billed — so it must not be conflated with effort.

_handle_thinking runs after _handle_output_config so an explicit thinking overrides the
effort-derived default rather than being overwritten by it.

Test Plan

Frontend-only change, so the Python-only dev install is enough — no CUDA rebuild, no GPU:

uv venv .venv --python 3.12
VLLM_USE_PRECOMPILED=1 uv pip install -e .
uv pip install pytest pytest-asyncio httpx tblib   # tblib is required by tests/conftest.py

# New unit tests
.venv/bin/python -m pytest -q \
  tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -k TestThinkingConfig

# Affected files in full, as a regression check
.venv/bin/python -m pytest -q \
  tests/entrypoints/anthropic/test_anthropic_messages_conversion.py \
  tests/entrypoints/anthropic/test_protocol_exports.py

TestThinkingConfig covers nine conversion cases:

Case	Asserts
`thinking` absent	`reasoning_effort is None`, `thinking_token_budget is None`, `include_reasoning is True` (regression guard)
`{"type": "disabled"}`	`reasoning_effort == "none"`
`disabled` + `output_config.effort="high"`	`reasoning_effort == "none"` — ordering: `thinking` beats the ceiling
`{"type": "enabled", "budget_tokens": 2048}`	`thinking_token_budget == 2048`
`{"type": "enabled"}` without a budget	nothing pinned
`{"type": "adaptive"}` + `effort="low"`	`reasoning_effort == "low"`, budget unset
`{"display": "omitted"}`	`include_reasoning is False`, depth untouched
`{"display": "summarized"}`	`include_reasoning is True`
`effort="high"` + `adaptive` + `omitted`	the exact payload Claude Code sends

End-to-end against a served model with a reasoning parser
(--reasoning-parser deepseek_v4 --tool-call-parser deepseek_v4 --enable-auto-tool-choice):

curl -s localhost:8000/v1/messages -H 'content-type: application/json' -d '{
  "model": "'"$MODEL"'", "max_tokens": 800,
  "thinking": {"type": "disabled"},
  "messages": [{"role": "user", "content": "Explain why sourcing an untrusted config file is risky."}]
}' | jq '[.content[] | select(.type=="thinking") | .thinking | length]'
# expected after this change: [] (or [0]); before: a non-empty reasoning block

Test Result

Unit tests:

$ pytest -q tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -k TestThinkingConfig
.........                                                                [100%]
9 passed, 55 deselected in 2.97s

$ pytest -q tests/entrypoints/anthropic/test_anthropic_messages_conversion.py \
          tests/entrypoints/anthropic/test_protocol_exports.py
..................................................................       [100%]
66 passed in 13.66s

The remaining tests under tests/entrypoints/anthropic/ (test_messages.py, 7 tests) error at
fixture setup on this machine with DeviceConfig unable to detect a device — they boot a real model
server and need a GPU. Unrelated to this change; they fail before any of it executes.

Lint: ruff-check, ruff-format, check-spdx-header and typos all pass via pre-commit run.

End-to-end on DeepSeek-V4-Flash-0731, TP=2, deepseek_v4 reasoning parser, reasoning-block length
in the response:

Request	Before	After
no `thinking` field	149 chars	149 — unchanged
`{"type": "disabled"}`	3058 (silently ignored)	0, content returned
`{"type": "adaptive", "display": "omitted"}`	reasoning streamed	0 returned, `stop_reason: end_turn`
`{"type": "adaptive"}`	pinned to the ceiling	641 chars, nothing pinned

thinking_token_budget is rejected by the V2 model runner on current builds ("not yet supported by
the V2 model runner"), so the budget_tokens branch is covered at the conversion layer rather than
end-to-end.

Impact

No behaviour change for requests that omit thinking: the field defaults to None and the handler
returns immediately, so output_config.effort remains the only input to reasoning configuration for
existing callers. The thinking-absent unit test guards this.

`AnthropicMessagesRequest` never declared a `thinking` field, so the documented Anthropic parameter was silently discarded -- Pydantic's default `extra="ignore"` drops it without an error. Clients that ask for disabled thinking, a token budget, or omitted reasoning display were answered as though they had asked for nothing, at whatever `output_config.effort` implied. Add `AnthropicThinkingConfig` and map it onto the reasoning controls that `ChatCompletionRequest` already exposes: {"type": "disabled"} -> reasoning_effort = "none" {"type": "enabled", budget_tokens: N} -> thinking_token_budget = N {"display": "omitted"} -> include_reasoning = False `{"type": "adaptive"}` deliberately pins nothing: the model chooses depth and `output_config.effort` stays the ceiling. `_handle_thinking` runs after `_handle_output_config` so an explicit `thinking` overrides the effort-derived default. `display` controls visibility only -- reasoning still runs and is still billed -- so it maps to `include_reasoning` rather than to any depth control. Verified against a DeepSeek-V4-Flash-0731 deployment: `reasoning_effort="none" takes reasoning from 1356 chars to 0, and `include_reasoning=False` suppresses it from the response. `thinking_token_budget` is rejected by the V2 model runner on current builds, so that branch is covered at the conversion layer. Signed-off-by: jryberg <johan.ryberg@security.ntt>

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github-actions · 2026-08-20T06:14:33Z

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chaunceyjiang · 2026-08-20T06:37:33Z

+    billed under every setting.
+    """
+
+    type: Literal["enabled", "disabled", "adaptive"] = "enabled"


The reason it hasn’t been introduced yet is that, if I remember correctly, these fields are going to be deprecated.

Deprecated in Claude Code? Is it not beneficial to be able to control reasoning. It's a huge difference using different harness like OpenCode (OpenAI api) vs. Claude Code (Anthropic API) since vLLM handles reasoning well via OpenAI API but not in Anthropic.

cf. https://platform.claude.com/doc/en/build-with-claude/extended-thinking

Although I do not reckon that the whole thinking field is deprecated, thinking.budget_tokens is no longer supported since 4.7

@chaunceyjiang I think this is a good addition since it's not going to be deprecated. The newly released DeepSeek Harness does support it as one example: https://github.com/earendil-works/pi/blob/5cd93f688aaab89dbb6dfa4aca535f21796ae185/packages/ai/src/api/anthropic-messages.ts#L1069 This is what DeepSeek Harness are using, dynamic thinking / effort for each prompt.

Okay, I’ll test it locally.

cjackal · 2026-08-21T15:34:42Z

+    budget_tokens: int | None = None
+    display: Literal["summarized", "omitted"] | None = None
+


Per Anthropic Messages API spec, budget_tokens is a required field when type is enabled (BetaThinkingConfigEnabled) and unsupported otherwise (BetaThinkingConfigDisabled, BetaThinkingConfigAdaptive). It might be better to add schema validation for consistency I think.

Suggested change

budget_tokens: int | None = None

display: Literal["summarized", "omitted"] | None = None

budget_tokens: int | None = None

display: Literal["summarized", "omitted"] | None = None

@model_validator(mode="after")

def validate_budget_tokens(self) -> "AnthropicThinkingConfig":

if self.type == "enabled" and self.budget_tokens is None:

raise ValueError("thinking.budget_tokens is required when thinking.type is 'enabled'.")

elif self.budget_tokens is not None:

raise ValueError(

f"thinking.budget_tokens must not be set when thinking.type is '{self.type}'."

)

return self

But the goal are just to be able to emulate Anthropics API to support Claude Code harness. vLLM will not be able to handle budget_tokens anyway? I don't think it really matters so I let the maintainer decide if vLLM need that kind of consistency

jryberg requested review from AndreasKaratzas, DarkLight1337, NickLucche, aarnphm, mgoin and robertgshaw2-redhat as code owners August 20, 2026 06:14

claude Bot reviewed Aug 20, 2026

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mergify Bot added frontend bug Something isn't working labels Aug 20, 2026

chaunceyjiang reviewed Aug 20, 2026

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chaunceyjiang self-assigned this Aug 21, 2026

cjackal reviewed Aug 21, 2026

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mimeding mentioned this pull request Sep 13, 2026

[Bugfix] Honor DeepSeek-V4 enable_thinking precedence #56708

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[Bugfix][Frontend] Anthropic API: honor the `thinking` request parameter - #53058

[Bugfix][Frontend] Anthropic API: honor the `thinking` request parameter#53058
jryberg wants to merge 1 commit into
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NTTS-Innovation:fix/anthropic-thinking-param

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		budget_tokens: int \| None = None
		display: Literal["summarized", "omitted"] \| None = None

-    budget_tokens: int | None = None
-    display: Literal["summarized", "omitted"] | None = None
+    budget_tokens: int | None = None
+    display: Literal["summarized", "omitted"] | None = None
+    @model_validator(mode="after")
+    def validate_budget_tokens(self) -> "AnthropicThinkingConfig":
+        if self.type == "enabled" and self.budget_tokens is None:
+            raise ValueError("thinking.budget_tokens is required when thinking.type is 'enabled'.")
+        elif self.budget_tokens is not None:
+            raise ValueError(
+                f"thinking.budget_tokens must not be set when thinking.type is '{self.type}'."
+            )
+        return self

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jryberg commented Aug 20, 2026

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