fix(router): drop bridge-tagged reasoning blocks whole when the minting deployment is not in the routed group

The cross-group branch of EncryptedContentAffinityCheck removed only the signature from
Anthropic-shaped thinking blocks, which left unsigned thinking blocks that Anthropic and
Bedrock reject (thinking.signature: Field required). Drop the whole block, the way #40280
drops undecryptable Responses input items, so the routed request carries the conversation
text with no reasoning item for those turns

litellm/litellm_core_utils/prompt_templates/common_utils.py

@@ -1890,8 +1890,13 @@ def is_encrypted_reasoning_block(block: object) -> bool:
 
 
 def strip_encrypted_reasoning_from_messages(messages: object) -> None:
-    """Drop the encrypted reasoning a routed deployment cannot decrypt from Anthropic-shaped
-    history, keeping the readable thinking text.
+    """Drop the bridge-tagged reasoning blocks a routed deployment cannot decrypt from
+    Anthropic-shaped history.
+
+    The whole block goes, the way #40280 drops undecryptable Responses ``input`` items: a
+    provider that did not mint the block rejects it signed (a foreign signature) and unsigned
+    (a missing signature) alike, so keeping its text as an unsigned thinking block only moves
+    the 400 from the router to the provider.
 
     Mutates the content lists in place: the router's fallback snapshot shares these
     message objects, so a rebound list would replay the stripped blocks on the fallback hop.
@@ -1914,20 +1919,8 @@ def _anthropic_content_lists(messages: Sequence[object]) -> Iterator[object]:
 
 def _strip_encrypted_reasoning_from_blocks(content: object) -> None:
     blocks: Final = cast(list[object], content)  # cast-ok: narrowed by the caller's isinstance
-    stripped: Final = tuple(_without_encrypted_reasoning_block(block) for block in blocks)
-    blocks[:] = (block for block in stripped if block is not None)  # rebind-ok: list shared with fallback snapshot
-
-
-def _without_encrypted_reasoning_block(block: object) -> object | None:
-    if not is_encrypted_reasoning_block(block):
-        return block
-    mapping: Final = cast(Mapping[str, object], block)  # cast-ok: narrowed by is_encrypted_reasoning_block
-    if mapping.get("type") != "thinking" or not mapping.get("thinking"):
-        return None
-    kept: Final[dict[str, object]] = {  # mutable-ok: thinking block rebuilt without the undecryptable signature
-        key: value for key, value in mapping.items() if key != "signature"
-    }
-    return kept
+    kept: Final = tuple(block for block in blocks if not is_encrypted_reasoning_block(block))
+    blocks[:] = kept  # rebind-ok: shared with fallback snapshot
 
 
 def _reasoning_replay_group_key(indexed_block: tuple[int, Mapping[str, object]]) -> str:

tests/test_litellm/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_common_utils.py

@@ -1633,7 +1633,7 @@ class TestEncryptedReasoningReplay:
     def test_is_encrypted_reasoning_block(self, block, expected):
         assert is_encrypted_reasoning_block(block) is expected
 
-    def test_strip_keeps_the_readable_thinking_and_drops_the_undecryptable_bytes(self):
+    def test_strip_drops_every_bridge_tagged_block_and_leaves_no_unsigned_thinking_behind(self):
         assistant_content = [
             {"type": "thinking", "thinking": "minted by Anthropic", "signature": "ErcBCkgIValid"},
             {"type": "thinking", "thinking": "packed by the bridge", "signature": encrypted_reasoning_signature("g1")},
@@ -1652,9 +1652,9 @@ class TestEncryptedReasoningReplay:
         assert messages[1]["content"] is assistant_content
         assert assistant_content == [
             {"type": "thinking", "thinking": "minted by Anthropic", "signature": "ErcBCkgIValid"},
-            {"type": "thinking", "thinking": "packed by the bridge"},
             {"type": "text", "text": "answer"},
         ]
+        assert all(block["signature"] for block in assistant_content if block["type"] == "thinking")
         assert messages[0] == {"role": "user", "content": "question"}
         assert messages[2] == {"role": "user", "content": [{"type": "text", "text": "follow-up"}]}
 

tests/test_litellm/router_utils/pre_call_checks/test_encrypted_content_affinity_check.py

@@ -1635,8 +1635,10 @@ def _bridge_replayed_anthropic_messages(minted_by: str) -> list:
 async def test_encrypted_content_affinity_strips_bridge_reasoning_from_messages_routed_to_another_group():
     """
     The /v1/messages twin of the tier-change case: the routed group holds no deployment
-    of the org that minted the reasoning, so the bridge-tagged blocks are stripped down
-    to their readable thinking text and the request dispatches to the routed pool.
+    of the org that minted the reasoning, so the bridge-tagged blocks are dropped whole
+    and the request dispatches to the routed pool. No unsigned thinking block may be left
+    behind: Anthropic and Bedrock reject a thinking block with a missing signature the
+    same way they reject a foreign one.
     """
     originating = _make_originating_mock(None, "key-a", model_name="gpt-reasoning-tier")
     mock_router = _make_router_mock_with_cooldown(
@@ -1660,9 +1662,9 @@ async def test_encrypted_content_affinity_strips_bridge_reasoning_from_messages_
     assert messages[1]["content"] is assistant_content
     assert assistant_content == [
         {"type": "thinking", "thinking": "Anthropic minted this one", "signature": "ErcCCpIBCBEYAipA"},
-        {"type": "thinking", "thinking": "The bridge packed this one"},
         {"type": "text", "text": "The zebra owner lives in the green house."},
     ]
+    assert all(block["signature"] for block in assistant_content if block["type"] == "thinking")
 
 
 class TestStripEncryptedReasoningFromInput: