fix(streaming): stop stream_chunk_builder duplicating Anthropic thinking text

Anthropic re-sends a thinking block's whole text alongside its signature.
`ModelResponseIterator._handle_content_block_delta` builds that signature chunk
by joining every prior thinking delta, so `get_combined_thinking_content`
appended the full text on top of the parts it had already accumulated and the
reassembled reasoning came back twice.

Skip the append when the block carries a signature and its text is exactly what
has been accumulated so far. A provider that sends only the last increment
alongside the signature still has that increment appended.

Rebased onto litellm_internal_staging: the branch had drifted far enough that it
was carrying twelve commits belonging to other PRs, so the diff showed six
unrelated e2e files. Rebuilt on the current tip so it is the two files it should
have been.

litellm/litellm_core_utils/streaming_chunk_builder_utils.py

@@ -631,9 +631,12 @@ class ChunkProcessor:
                                 )
                         else:
                             thinking_text = thinking_block.get("thinking", None)
-                            if thinking_text:
-                                current_thinking_text_parts.append(thinking_text)
                             signature = thinking_block.get("signature", None)
+                            already_accumulated = bool(signature) and thinking_text == "".join(
+                                current_thinking_text_parts
+                            )
+                            if thinking_text and not already_accumulated:
+                                current_thinking_text_parts.append(thinking_text)
                             if signature:
                                 current_signature = signature
                                 _flush_thinking_block()

tests/test_litellm/litellm_core_utils/test_streaming_chunk_builder_utils.py

@@ -155,6 +155,99 @@ def test_get_combined_tool_content():
     ]
 
 
+def test_get_combined_thinking_content_does_not_duplicate_resent_thinking():
+    """Anthropic re-sends a block's whole thinking text alongside its signature.
+
+    ``ModelResponseIterator._handle_content_block_delta`` builds the signature
+    chunk by joining every prior thinking delta, so appending it on top of the
+    parts already accumulated emitted the reasoning twice.
+    """
+    base_chunk = {
+        "id": "chatcmpl-123",
+        "object": "chat.completion.chunk",
+        "created": 1234567890,
+        "model": "claude-sonnet-4-20250514",
+    }
+
+    def make_chunk(**delta_kwargs):
+        return ModelResponseStream(
+            **base_chunk,
+            choices=[
+                StreamingChoices(
+                    index=0,
+                    delta=Delta(**delta_kwargs),
+                    finish_reason=None,
+                )
+            ],
+        )
+
+    parts = ["Let me ", "work through ", "this step by step."]
+    chunks = [
+        make_chunk(role="assistant", content=None),
+        *[make_chunk(thinking_blocks=[{"type": "thinking", "thinking": part}]) for part in parts],
+        make_chunk(
+            thinking_blocks=[
+                {
+                    "type": "thinking",
+                    "thinking": "".join(parts),
+                    "signature": "sig_block1",
+                }
+            ]
+        ),
+    ]
+    thinking_chunks = [
+        chunk for chunk in chunks if chunk["choices"][0]["delta"].get("thinking_blocks")
+    ]
+    processor = ChunkProcessor(chunks=chunks)
+
+    result = processor.get_combined_thinking_content(thinking_chunks)
+
+    assert result is not None
+    assert len(result) == 1
+    assert result[0]["thinking"] == "".join(parts)
+    assert result[0]["signature"] == "sig_block1"
+
+
+def test_get_combined_thinking_content_keeps_a_genuine_final_increment():
+    """A provider that sends only the last increment with the signature must
+    still have that increment appended, not swallowed by the de-duplication.
+    """
+    base_chunk = {
+        "id": "chatcmpl-123",
+        "object": "chat.completion.chunk",
+        "created": 1234567890,
+        "model": "claude-sonnet-4-20250514",
+    }
+
+    def make_chunk(**delta_kwargs):
+        return ModelResponseStream(
+            **base_chunk,
+            choices=[
+                StreamingChoices(
+                    index=0,
+                    delta=Delta(**delta_kwargs),
+                    finish_reason=None,
+                )
+            ],
+        )
+
+    chunks = [
+        make_chunk(thinking_blocks=[{"type": "thinking", "thinking": "Let me "}]),
+        make_chunk(
+            thinking_blocks=[
+                {"type": "thinking", "thinking": "think.", "signature": "sig_block1"}
+            ]
+        ),
+    ]
+    processor = ChunkProcessor(chunks=chunks)
+
+    result = processor.get_combined_thinking_content(chunks)
+
+    assert result is not None
+    assert len(result) == 1
+    assert result[0]["thinking"] == "Let me think."
+
+
 def test_get_combined_thinking_content_preserves_interleaved_blocks():
     base_chunk = {
         "id": "chatcmpl-123",