From d64f277c54d444284bdbcd16e7fd1073b0a3b50e Mon Sep 17 00:00:00 2001
From: Robert Shaw <114415538+robertgshaw2-redhat@users.noreply.github.com>
Date: Sat, 26 Sep 2026 09:50:57 -0400
Subject: [PATCH] [Bugfix] Fix Anthropic Thinking Disabled with P/D (#58786)

Signed-off-by: Robert Shaw <robshaw@redhat.com>
Co-authored-by: Robert Shaw <robshaw@redhat.com>
---
 .../test_anthropic_messages_conversion.py     | 22 +++++++++++++++++++
 vllm/entrypoints/anthropic/protocol.py        |  4 ++++
 2 files changed, 26 insertions(+)

diff --git a/tests/entrypoints/anthropic/test_anthropic_messages_conversion.py b/tests/entrypoints/anthropic/test_anthropic_messages_conversion.py
index 5790b9b082..a6cfb15d60 100644
--- a/tests/entrypoints/anthropic/test_anthropic_messages_conversion.py
+++ b/tests/entrypoints/anthropic/test_anthropic_messages_conversion.py
@@ -1804,6 +1804,28 @@ class TestThinkingConfig:
                 thinking=thinking,
             )
 
+    def test_pd_prefill_leg_skips_budget_check(self):
+        """P/D sidecars resend the request to the prefill node with
+        max_tokens=1; the budget is enforced on the decode leg instead."""
+        thinking = {"type": "enabled", "budget_tokens": 1024}
+        request = AnthropicMessagesRequest(
+            model="test-model",
+            max_tokens=1,
+            messages=[{"role": "user", "content": "Hello"}],
+            thinking=thinking,
+            kv_transfer_params={"do_remote_decode": True},
+        )
+
+        assert _convert(request).thinking_token_budget == 1024
+        with pytest.raises(ValidationError):
+            AnthropicMessagesRequest(
+                model="test-model",
+                max_tokens=1,
+                messages=[{"role": "user", "content": "Hello"}],
+                thinking=thinking,
+                kv_transfer_params={"do_remote_decode": False},
+            )
+
     def test_adaptive_pins_nothing_and_keeps_effort_ceiling(self):
         """`adaptive` lets the model choose depth, so only the ceiling from
         output_config.effort should survive."""
diff --git a/vllm/entrypoints/anthropic/protocol.py b/vllm/entrypoints/anthropic/protocol.py
index 48202831fe..9bd40a9161 100644
--- a/vllm/entrypoints/anthropic/protocol.py
+++ b/vllm/entrypoints/anthropic/protocol.py
@@ -233,6 +233,10 @@ class AnthropicMessagesRequest(BaseModel):
 
     @model_validator(mode="after")
     def validate_thinking_budget(self) -> "AnthropicMessagesRequest":
+        # P/D prefill legs are sent with max_tokens=1 and never decode; the
+        # decode leg carries the client's max_tokens and is still checked.
+        if self.kv_transfer_params and self.kv_transfer_params.get("do_remote_decode"):
+            return self
         if (
             isinstance(self.thinking, AnthropicThinkingConfigEnabled)
             and self.thinking.budget_tokens >= self.max_tokens