From a40b9983c55039485e65258fb6b75004a3c67816 Mon Sep 17 00:00:00 2001
From: zhanghanduo <hans@apodex.com>
Date: Sun, 16 Aug 2026 21:25:36 +0800
Subject: [PATCH] fix(anthropic): reopen a content block when a resumed item id
 returns

_get_or_start_block trusted the item_id -> block index map without checking
whether that block was still open, so a provider that reuses one item id for
a whole run and interleaves channels got a delta addressed to a stopped
block. Replaying reasoning, text, reasoning, text produced
content_block_delta index=0 after content_block_stop index=0, which is not a
valid Anthropic stream.

Treat the mapping as valid only while it points at the open block, and
rebind the item to a fresh block otherwise. Items registered through
response.output_item.added still reuse their block, since that block is the
open one while its deltas arrive.

Apodex Deep Research is what surfaced this: it labels every reasoning delta
of a run rs_<response_id> and every answer delta msg_<response_id>, so any
interleaving hits the stale mapping.
---
 .../responses_adapters/streaming_iterator.py  |  5 ++-
 ...t_responses_adapters_streaming_iterator.py | 37 +++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py b/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
index 8b9a3ed6764..3e6e6df4565 100644
--- a/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
+++ b/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
@@ -101,8 +101,11 @@ class AnthropicResponsesStreamWrapper:
         content_block: Mapping[str, object],
     ) -> int:
         mapped_index: Final = self._item_id_to_block_index.get(item_id) if item_id else None
-        if mapped_index is not None:
+        if mapped_index is not None and mapped_index == self._open_block_index:
             return mapped_index
+        # A resumed item whose block already closed needs a fresh one: Anthropic rejects
+        # a delta addressed to a stopped block. Providers that reuse one item id for a
+        # whole run, then interleave channels, land here.
         if item_id is None and self._open_block_index is not None and self._open_block_type == block_type:
             return self._open_block_index
 
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py
index f8c34c9bf7d..81b207bb259 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py
@@ -181,6 +181,43 @@ class TestProcessEventReasoningDeltaWithoutOutputItemAdded:
         ]
         assert chunks[3]["content_block"] == {"type": "text", "text": ""}
 
+    def test_resumed_item_id_opens_a_fresh_block(self):
+        """A provider that reuses one item id per run, then interleaves channels, would
+        otherwise address a delta to a block that has already stopped."""
+        response = SimpleNamespace(status="completed", output=[], usage=None)
+        chunks = _process_all(
+            [
+                {"type": "response.reasoning_summary_text.delta", "item_id": "rs_1", "delta": "Think A"},
+                {"type": "response.output_text.delta", "item_id": "msg_1", "delta": "Answer A"},
+                {"type": "response.reasoning_summary_text.delta", "item_id": "rs_1", "delta": "Think B"},
+                {"type": "response.output_text.delta", "item_id": "msg_1", "delta": "Answer B"},
+                {"type": "response.completed", "response": response},
+            ]
+        )
+
+        assert [(chunk["type"], chunk.get("index")) for chunk in chunks] == [
+            ("content_block_start", 0),
+            ("content_block_delta", 0),
+            ("content_block_stop", 0),
+            ("content_block_start", 1),
+            ("content_block_delta", 1),
+            ("content_block_stop", 1),
+            ("content_block_start", 2),
+            ("content_block_delta", 2),
+            ("content_block_stop", 2),
+            ("content_block_start", 3),
+            ("content_block_delta", 3),
+            ("content_block_stop", 3),
+            ("message_delta", None),
+            ("message_stop", None),
+        ]
+        assert [chunk["content_block"]["type"] for chunk in chunks if chunk["type"] == "content_block_start"] == [
+            "thinking",
+            "text",
+            "thinking",
+            "text",
+        ]
+
 
 class TestResponseCompletedUsage:
     """The Anthropic ``message_delta`` usage must report cache reads/writes and