fix(compact_20260112): use beta-header constant; require type discriminator; skip sync bridge when idle

- bedrock: replace hardcoded "compact-2026-01-12" beta string with
  ANTHROPIC_BETA_HEADER_VALUES.COMPACT_2026_01_12.value in both
  Converse (_filter_context_management_for_bedrock_converse) and Invoke
  (anthropic_claude3) compact-edit handlers.
- types: mark the "type" discriminator as Required[...] on the new
  CompactionBlock and UsageIteration TypedDicts so the discriminator
  is not silently optional under total=False.
- adapters/handler: short-circuit the sync /v1/messages adapter path
  before spawning the run_async_function worker-thread event loop when
  the request has no context_management spec and no client-sent
  compaction block in the message history.

Test plan:
- uv run pytest tests/test_litellm/llms/anthropic/experimental_pass_through/     tests/test_litellm/llms/bedrock/test_converse_context_management.py -q
  (370 + 10 = 380 passed)
- uv run pytest tests/test_litellm/llms/azure_ai/claude/test_azure_anthropic_transformation.py     tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_transformation.py     -k compact (3 passed)

litellm/llms/anthropic/experimental_pass_through/adapters/handler.py

@@ -40,6 +40,18 @@ if TYPE_CHECKING:
 ANTHROPIC_ONLY_REQUEST_KEYS: frozenset[str] = frozenset({"output_config"})
 
 
+def _messages_have_compaction_block(messages: List[Dict]) -> bool:
+    """Return True when any message carries a ``compaction`` content block."""
+    for msg in messages:
+        content = msg.get("content")
+        if not isinstance(content, list):
+            continue
+        for block in content:
+            if isinstance(block, dict) and block.get("type") == "compaction":
+                return True
+    return False
+
+
 async def _prepare_context_managed_request(
     *,
     model: str,
@@ -677,17 +689,26 @@ class LiteLLMMessagesToCompletionTransformationHandler:
         # unaffected because it ``await``s within the original event loop.
         litellm_router = kwargs.pop("litellm_router", None)
 
-        polyfill_result = run_async_function(
-            _prepare_context_managed_request,
-            model=model,
-            messages=messages,
-            tools=tools,
-            system=system,
-            context_management_spec=context_management,
-            metadata=metadata,
-            drop_params=drop_params,
-            llm_router=litellm_router,
-        )
+        # Skip the async bridge entirely when there is nothing for either the
+        # polyfill or the client-history slice-only fallback to do. The vast
+        # majority of sync ``litellm.messages.create()`` requests carry no
+        # ``context_management`` spec and no client-sent ``compaction`` block,
+        # and bridging through a worker-thread event loop just to discover
+        # there is no work is pure overhead.
+        if context_management is None and not _messages_have_compaction_block(messages):
+            polyfill_result: Optional[PolyfillResult] = None
+        else:
+            polyfill_result = run_async_function(
+                _prepare_context_managed_request,
+                model=model,
+                messages=messages,
+                tools=tools,
+                system=system,
+                context_management_spec=context_management,
+                metadata=metadata,
+                drop_params=drop_params,
+                llm_router=litellm_router,
+            )
 
         effective_messages = (
             polyfill_result.messages if polyfill_result is not None else messages

litellm/llms/bedrock/chat/converse_transformation.py

@@ -1477,6 +1477,8 @@ class AmazonConverseConfig(BaseConfig):
         anthropic_beta_list: list,
     ) -> None:
         """Keep only compact_20260112 edits for Bedrock; add beta header or drop field."""
+        from litellm.types.llms.anthropic import ANTHROPIC_BETA_HEADER_VALUES
+
         cm = additional_request_params.get("context_management")
         if not isinstance(cm, dict):
             additional_request_params.pop("context_management", None)
@@ -1492,8 +1494,9 @@ class AmazonConverseConfig(BaseConfig):
             if isinstance(e, dict) and e.get("type") == "compact_20260112"
         ]
         if compact_edits:
-            if "compact-2026-01-12" not in anthropic_beta_list:
-                anthropic_beta_list.append("compact-2026-01-12")
+            compact_beta = ANTHROPIC_BETA_HEADER_VALUES.COMPACT_2026_01_12.value
+            if compact_beta not in anthropic_beta_list:
+                anthropic_beta_list.append(compact_beta)
             additional_request_params["context_management"] = {
                 **cm,
                 "edits": compact_edits,

litellm/llms/bedrock/messages/invoke_transformations/anthropic_claude3_transformation.py

@@ -38,7 +38,10 @@ from litellm.llms.bedrock.common_utils import (
     normalize_tool_input_schema_types_for_bedrock_invoke,
     remove_custom_field_from_tools,
 )
-from litellm.types.llms.anthropic import ANTHROPIC_TOOL_SEARCH_BETA_HEADER
+from litellm.types.llms.anthropic import (
+    ANTHROPIC_BETA_HEADER_VALUES,
+    ANTHROPIC_TOOL_SEARCH_BETA_HEADER,
+)
 from litellm.types.llms.bedrock import BedrockInvokeAnthropicMessagesRequest
 from litellm.types.llms.openai import AllMessageValues
 from litellm.types.router import GenericLiteLLMParams
@@ -442,7 +445,7 @@ class AmazonAnthropicClaudeMessagesConfig(
             if isinstance(e, dict) and e.get("type") == "compact_20260112"
         ]
         if compact_edits:
-            beta_set.add("compact-2026-01-12")
+            beta_set.add(ANTHROPIC_BETA_HEADER_VALUES.COMPACT_2026_01_12.value)
             anthropic_messages_request["context_management"] = {
                 **cm,
                 "edits": compact_edits,

litellm/types/llms/anthropic.py

@@ -537,14 +537,14 @@ class ContextManagementResponse(TypedDict, total=False):
 class CompactionBlock(TypedDict, total=False):
     """Synthesized ``compaction`` content block (compact_20260112)."""
 
-    type: Literal["compaction"]
+    type: Required[Literal["compaction"]]
     content: Optional[str]
 
 
 class UsageIteration(TypedDict, total=False):
     """One sampling iteration's token usage (compact_20260112)."""
 
-    type: Literal["compaction", "message"]
+    type: Required[Literal["compaction", "message"]]
     input_tokens: int
     output_tokens: int