fix(bedrock): normalise streaming choice index=0 for extended-thinkin…

tests/test_litellm/llms/bedrock/chat/test_streaming_choice_index.py

@@ -6,14 +6,22 @@
 text=0, toolUse=1), NOT parallel completions. Since Bedrock doesn't support
 n > 1, all chunks must use choice index 0.
 
+This includes extended-thinking responses where the reasoning block arrives on
+contentBlockIndex=0 and the text block on contentBlockIndex=1 — both must be
+normalised to choices[0].index == 0 for OpenAI-SDK compatibility.
+(Regression for https://github.com/BerriAI/litellm/issues/23178)
+
 References:
 - Bedrock InferenceConfiguration (no n parameter):
   https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_InferenceConfiguration.html
 - OpenAI choice.index (for n > 1):
   https://platform.openai.com/docs/api-reference/chat/object
 """
 
-from litellm.llms.bedrock.chat.invoke_handler import AWSEventStreamDecoder
+from litellm.llms.bedrock.chat.invoke_handler import (
+    AWSEventStreamDecoder,
+    AmazonAnthropicClaudeStreamDecoder,
+)
 
 
 class TestBedrockStreamingChoiceIndex:
@@ -112,3 +120,129 @@ def test_mixed_content_blocks_all_use_choice_index_zero(self):
         }
         result4 = handler.converse_chunk_parser(finish_chunk)
         assert result4.choices[0].index == 0, "Finish reason should have index=0"
+
+    # ------------------------------------------------------------------
+    # Extended-thinking / reasoning tests (issue #23178)
+    # ------------------------------------------------------------------
+
+    def test_thinking_block_chunks_use_choice_index_zero(self):
+        """
+        Regression for https://github.com/BerriAI/litellm/issues/23178.
+
+        When Claude extended-thinking is enabled the Bedrock converse API emits:
+          contentBlockIndex=0  ->  reasoning / thinking block
+          contentBlockIndex=1  ->  text block
+
+        LiteLLM must normalise both to choices[0].index == 0 so that
+        OpenAI-compatible clients (e.g. openai.ChatCompletionAccumulator) do not
+        crash when they see an unexpected index switch mid-stream.
+        """
+        handler = AWSEventStreamDecoder(
+            model="anthropic.claude-sonnet-4-5-20250929-v1:0"
+        )
+
+        # thinking block start (contentBlockIndex=0)
+        r = handler.converse_chunk_parser(
+            {"start": {"reasoningContent": {}}, "contentBlockIndex": 0}
+        )
+        assert r.choices[0].index == 0, "thinking start should have index=0"
+
+        # thinking block delta (contentBlockIndex=0)
+        r = handler.converse_chunk_parser(
+            {
+                "delta": {"reasoningContent": {"text": "Let me think step by step..."}},
+                "contentBlockIndex": 0,
+            }
+        )
+        assert r.choices[0].index == 0, "thinking delta should have index=0"
+        assert r.choices[0].delta.reasoning_content == "Let me think step by step..."
+
+        # thinking block stop (contentBlockIndex=0)
+        r = handler.converse_chunk_parser({"contentBlockIndex": 0})
+        assert r.choices[0].index == 0, "thinking stop should have index=0"
+
+        # text block start (contentBlockIndex=1) — this is the previously broken case
+        r = handler.converse_chunk_parser(
+            {"start": {}, "contentBlockIndex": 1}
+        )
+        assert r.choices[0].index == 0, (
+            "text start after thinking block should have index=0, not contentBlockIndex=1"
+        )
+
+        # text block delta (contentBlockIndex=1)
+        r = handler.converse_chunk_parser(
+            {"delta": {"text": "The answer is 42."}, "contentBlockIndex": 1}
+        )
+        assert r.choices[0].index == 0, (
+            "text delta after thinking block should have index=0, not contentBlockIndex=1"
+        )
+        assert r.choices[0].delta.content == "The answer is 42."
+
+        # text block stop (contentBlockIndex=1)
+        r = handler.converse_chunk_parser({"contentBlockIndex": 1})
+        assert r.choices[0].index == 0, "text stop should have index=0"
+
+        # message stop
+        r = handler.converse_chunk_parser({"stopReason": "end_turn"})
+        assert r.choices[0].index == 0, "message stop should have index=0"
+
+    def test_thinking_signature_chunk_uses_choice_index_zero(self):
+        """The reasoning block ends with a cryptographic signature chunk; it must also carry index=0."""
+        handler = AWSEventStreamDecoder(
+            model="anthropic.claude-sonnet-4-5-20250929-v1:0"
+        )
+        r = handler.converse_chunk_parser(
+            {
+                "delta": {"reasoningContent": {"signature": "abc123signatureXYZ"}},
+                "contentBlockIndex": 0,
+            }
+        )
+        assert r.choices[0].index == 0, "signature delta should have index=0"
+        assert r.choices[0].delta.thinking_blocks is not None
+        assert len(r.choices[0].delta.thinking_blocks) > 0
+
+    def test_anthropic_invoke_decoder_thinking_uses_choice_index_zero(self):
+        """
+        When Claude models are invoked via the Bedrock /invoke endpoint
+        (bedrock_invoke_provider='anthropic'), AmazonAnthropicClaudeStreamDecoder
+        is used.  It must normalise the raw Anthropic index=1 on text blocks to
+        choices[0].index == 0, matching the behaviour of the direct Anthropic provider.
+        """
+        handler = AmazonAnthropicClaudeStreamDecoder(
+            model="anthropic.claude-sonnet-4-5-20250929-v1:0",
+            sync_stream=True,
+        )
+
+        anthropic_chunks = [
+            # thinking block — raw Anthropic index=0
+            {
+                "type": "content_block_start",
+                "index": 0,
+                "content_block": {"type": "thinking", "thinking": ""},
+            },
+            {
+                "type": "content_block_delta",
+                "index": 0,
+                "delta": {"type": "thinking_delta", "thinking": "I should multiply."},
+            },
+            {"type": "content_block_stop", "index": 0},
+            # text block — raw Anthropic index=1 (the problematic case)
+            {
+                "type": "content_block_start",
+                "index": 1,
+                "content_block": {"type": "text", "text": ""},
+            },
+            {
+                "type": "content_block_delta",
+                "index": 1,
+                "delta": {"type": "text_delta", "text": "27 x 453 = 12231"},
+            },
+            {"type": "content_block_stop", "index": 1},
+        ]
+
+        for chunk in anthropic_chunks:
+            result = handler._chunk_parser(chunk)
+            assert result.choices[0].index == 0, (
+                f"chunk type={chunk.get('type')} index={chunk.get('index')} "
+                f"produced choices[0].index={result.choices[0].index}, expected 0"
+            )