fix(bedrock): forward the compact-2026-09-04 compaction param on Invoke /v1/messages

The compact-2026-09-04 beta is a top-level `compaction` request parameter
({"type": "summarize"}), not a context_management edit. The Bedrock Invoke
allowlist is built from BedrockInvokeAnthropicMessagesRequest, which had no
`compaction` field, so the field was dropped and the request reached Bedrock
as an ordinary turn. The bedrock beta map also dropped compact-2026-09-04, so
adopting requests lost the beta.

- add `compaction` to BedrockInvokeAnthropicMessagesRequest
- add compact-2026-09-04 to the bedrock beta map (Invoke only, not Converse)
- add the beta automatically when a `compaction` param is present

Ref: https://docs.aws.amazon.com/bedrock/latest/userguide/claude-messages-compaction.html

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

litellm/anthropic_beta_headers_config.json

@@ -200,6 +200,7 @@
     "bash_20250124": null,
     "code-execution-2025-08-25": null,
     "compact-2026-01-12": "compact-2026-01-12",
+    "compact-2026-09-04": "compact-2026-09-04",
     "computer-use-2025-01-24": "computer-use-2025-01-24",
     "computer-use-2025-11-24": "computer-use-2025-11-24",
     "context-1m-2025-08-07": "context-1m-2025-08-07",

litellm/llms/bedrock/messages/invoke_transformations/anthropic_claude_transformation.py

@@ -550,6 +550,19 @@ class AmazonAnthropicClaudeMessagesConfig(
             "edits": retained_edits,
         }
 
+    @staticmethod
+    def _add_compaction_beta_for_bedrock_invoke(anthropic_messages_request: dict, beta_set: set) -> None:
+        """
+        A top-level ``compaction`` request needs the ``compact-2026-09-04``
+        anthropic-beta; without it Bedrock answers 400 "compaction: Extra inputs
+        are not permitted". Later requests that adopt the returned block need the
+        same beta, which the caller sends as a header.
+
+        Ref: https://docs.aws.amazon.com/bedrock/latest/userguide/claude-messages-compaction.html
+        """
+        if isinstance(anthropic_messages_request.get("compaction"), dict):
+            beta_set.add("compact-2026-09-04")
+
     def strips_beta_gated_fields_from_final_headers(self) -> bool:
         return False
 
@@ -617,6 +630,7 @@ class AmazonAnthropicClaudeMessagesConfig(
             anthropic_messages_request=anthropic_messages_request,
             beta_set=beta_set,
         )
+        self._add_compaction_beta_for_bedrock_invoke(anthropic_messages_request, beta_set)
 
         self._get_tool_search_beta_header_for_bedrock(
             model=model,

litellm/types/llms/bedrock.py

@@ -1257,6 +1257,13 @@ class BedrockInvokeAnthropicMessagesRequest(TypedDict, total=False):
     # other edit types (e.g. `clear_thinking_20251015`) never reach Bedrock.
     context_management: dict
 
+    # `compaction` ({"type": "summarize", "instructions"?: str}) is the
+    # `compact-2026-09-04` beta: one summarization call that returns a single
+    # signed compaction block. Bedrock rejects it together with
+    # `context_management`. Ref:
+    # https://docs.aws.amazon.com/bedrock/latest/userguide/claude-messages-compaction.html
+    compaction: dict
+
 
 class BedrockBatchRecordKind(Enum):
     """

tests/unit/llms/bedrock/messages/invoke_transformations/test_anthropic_claude_transformation.py

@@ -1547,6 +1547,62 @@ def test_bedrock_messages_preserves_compact_context_management_and_adds_beta():
     assert result["max_tokens"] == 4096
 
 
+def test_bedrock_messages_keeps_compaction_param_and_adds_beta():
+    """
+    The ``compact-2026-09-04`` beta uses a top-level ``compaction`` parameter, not a
+    ``context_management`` edit. Bedrock InvokeModel accepts it with that beta and
+    returns one signed compaction block. Before this fix the top-level allowlist
+    dropped ``compaction``, so the request reached Bedrock as an ordinary turn.
+
+    Ref: https://docs.aws.amazon.com/bedrock/latest/userguide/claude-messages-compaction.html
+    """
+    from litellm.types.router import GenericLiteLLMParams
+
+    cfg = AmazonAnthropicClaudeMessagesConfig()
+    messages = [
+        {"role": "user", "content": [{"type": "text", "text": "Is CHG-TR-0042 Type IB in TR-TITCK?"}]},
+        {"role": "assistant", "content": [{"type": "text", "text": "Type IB."}]},
+        {"role": "user", "content": [{"type": "text", "text": "Now KR-MFDS."}]},
+    ]
+    optional_params = {"max_tokens": 1024, "compaction": {"type": "summarize", "instructions": "Keep rule ids."}}
+
+    result = cfg.transform_anthropic_messages_request(
+        model="anthropic.claude-opus-5",
+        messages=messages,
+        anthropic_messages_optional_request_params=optional_params,
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+
+    assert result["compaction"] == {"type": "summarize", "instructions": "Keep rule ids."}
+    assert "compact-2026-09-04" in result["anthropic_beta"]
+    assert "context_management" not in result
+    assert set(result).issubset(cfg.BEDROCK_INVOKE_ALLOWED_TOP_LEVEL_FIELDS)
+
+
+def test_bedrock_messages_keeps_compact_2026_09_04_beta_for_adopting_request():
+    """
+    A request that adopts the signed block sends it first in ``messages`` and must
+    carry the ``compact-2026-09-04`` beta again. The bedrock beta map has to keep it.
+    """
+    from litellm.types.router import GenericLiteLLMParams
+
+    cfg = AmazonAnthropicClaudeMessagesConfig()
+    block = {"type": "compaction", "content": "<summary>TR-TITCK: Type IB.</summary>", "signature": "sig"}
+    messages = [{"role": "user", "content": [block, {"type": "text", "text": "Now KR-MFDS."}]}]
+
+    result = cfg.transform_anthropic_messages_request(
+        model="anthropic.claude-opus-5",
+        messages=messages,
+        anthropic_messages_optional_request_params={"max_tokens": 512},
+        litellm_params=GenericLiteLLMParams(),
+        headers={"anthropic-beta": "compact-2026-09-04"},
+    )
+
+    assert "compact-2026-09-04" in result["anthropic_beta"]
+    assert result["messages"][0]["content"][0] == block
+
+
 def test_bedrock_messages_filters_unsupported_context_management_edits():
     """
     Mixed edit lists must drop the LiteLLM-internal ``clear_thinking_20251015``

tests/unit/test_anthropic_beta_headers_filtering.py

@@ -99,7 +99,9 @@ class TestAnthropicBetaHeadersFiltering:
         filtered = filter_and_transform_beta_headers(
             beta_headers=all_headers, provider=provider
         )
-        assert ("compact-2026-09-04" in filtered) is (provider in ("anthropic", "azure_ai", "vertex_ai"))
+        # Bedrock InvokeModel serves the compact-2026-09-04 `compaction` parameter; Converse does not.
+        # https://docs.aws.amazon.com/bedrock/latest/userguide/claude-messages-compaction.html
+        assert ("compact-2026-09-04" in filtered) is (provider in ("anthropic", "azure_ai", "vertex_ai", "bedrock"))
         assert ("mid-conversation-tool-changes-2026-07-01" in filtered) is (provider != "bedrock_converse")
 
         for header in unsupported_headers: