feat(anthropic): auto-inject context-1m beta header for [1m] model suffix

A model name ending in [1m] (case-insensitive) now adds the
context-1m-2025-08-07 anthropic-beta value and is sent upstream without the
suffix, on both the chat completions and the /v1/messages paths

Ported onto the current default branch: the pass-through module moved from
experimental_pass_through to pass_through and the tests moved to tests/unit

Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/llms/anthropic/chat/transformation.py

@@ -99,10 +99,12 @@ from litellm.utils import (
 from ..common_utils import (
     AnthropicError,
     AnthropicModelInfo,
+    context_1m_requested,
     eager_input_streaming_flag,
     process_anthropic_headers,
     requires_native_compaction_beta,
     strip_advisor_blocks_from_messages,
+    strip_context_1m_suffix,
 )
 
 if TYPE_CHECKING:
@@ -1779,6 +1781,17 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             )
         return tools
 
+    def _maybe_add_context_1m_beta(
+        self,
+        headers: dict,
+        *,
+        model: str,
+        optional_params: dict,
+        litellm_params: Mapping[str, object] | None,
+    ) -> None:
+        if context_1m_requested(model=model, optional_params=optional_params, litellm_params=litellm_params):
+            self._ensure_beta_header(headers, "context-1m-2025-08-07")
+
     def _ensure_beta_header(self, headers: dict[str, str], beta_value: str) -> None:
         """
         Ensure a beta header value is present in the anthropic-beta header.
@@ -1837,7 +1850,12 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             )
 
     def update_headers_with_optional_anthropic_beta(
-        self, headers: dict, optional_params: dict, messages: Sequence[object] = ()
+        self,
+        headers: dict,
+        optional_params: dict,
+        messages: Sequence[object] = (),
+        litellm_params: Mapping[str, object] | None = None,
+        model: str = "",
     ) -> dict:
         """Update headers with optional anthropic beta."""
 
@@ -1850,6 +1868,10 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
         if requires_native_compaction_beta(self._resolved_provider, optional_params, messages):
             self._ensure_beta_header(headers, ANTHROPIC_BETA_HEADER_VALUES.COMPACT_2026_09_04.value)
 
+        self._maybe_add_context_1m_beta(
+            headers, model=model, optional_params=optional_params, litellm_params=litellm_params
+        )
+
         _tools: Final = optional_params.get("tools", [])
         for tool in _tools:
             if tool.get("type", None) and tool.get("type").startswith(ANTHROPIC_HOSTED_TOOLS.WEB_FETCH.value):
@@ -2004,7 +2026,11 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             )  # don't use verbose_logger.exception, if exception is raised
 
         self.update_headers_with_optional_anthropic_beta(
-            headers=headers, optional_params=optional_params, messages=anthropic_messages
+            headers=headers,
+            optional_params=optional_params,
+            messages=anthropic_messages,
+            litellm_params=litellm_params,
+            model=model,
         )
 
         ## Auto-strip advisor blocks from history if advisor tool is absent.
@@ -2072,7 +2098,7 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             )
 
         data: Final = {
-            "model": model,
+            "model": strip_context_1m_suffix(model),
             "messages": anthropic_messages,
             **optional_params,
         }

litellm/llms/anthropic/common_utils.py

@@ -71,6 +71,46 @@ _BEDROCK_VERSION_SUFFIX_RE: Final = re.compile(r"-v\d+(?::\d+)?$")
 _INFERENCE_PROFILE_MINOR_RE: Final = re.compile(r":\d+$")
 _DATED_RELEASE_SUFFIX_RE: Final = re.compile(r"-\d{8}$")
 _DOTTED_VERSION_RE: Final = re.compile(r"(\d)\.(\d)")
+_CONTEXT_1M_SUFFIX: Final = re.compile(r"\[1m\]$", flags=re.IGNORECASE)
+
+
+def model_has_context_1m_suffix(model: object) -> bool:
+    return isinstance(model, str) and _CONTEXT_1M_SUFFIX.search(model) is not None
+
+
+def strip_context_1m_suffix(model: str) -> str:
+    return _CONTEXT_1M_SUFFIX.sub("", model)
+
+
+def _original_model_from(params: object) -> object:
+    if not isinstance(params, Mapping):
+        return None
+    return params.get("_original_model")
+
+
+def context_1m_requested(
+    *,
+    model: object = "",
+    optional_params: object = None,
+    litellm_params: object = None,
+) -> bool:
+    return any(
+        model_has_context_1m_suffix(candidate)
+        for candidate in (
+            model,
+            _original_model_from(optional_params),
+            _original_model_from(litellm_params),
+        )
+    )
+
+
+_CONTEXT_1M_BETA: Final = "context-1m-2025-08-07"
+
+
+def context_1m_beta_values(supported: bool) -> frozenset[str]:
+    return frozenset((_CONTEXT_1M_BETA,)) if supported else frozenset()
+
+
 _CLAUDE_CODE_BILLING_HEADER_PREFIX: Final = "x-anthropic-billing-header:"
 _CLAUDE_CODE_OBJECT_MAPPING_ADAPTER: Final = TypeAdapter(dict[object, object])
 _CLAUDE_CODE_OBJECT_LIST_ADAPTER: Final = TypeAdapter(list[object])
@@ -915,8 +955,9 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         container_with_skills_used: bool = False,
         api_base: str | None = None,
         use_bearer_for_custom_base: bool = False,
+        context_1m_supported: bool = False,
     ) -> dict:
-        betas: Final = set()
+        betas: Final = set(context_1m_beta_values(context_1m_supported))
         # Anthropic no longer requires the prompt-caching beta header
         # Prompt caching now works automatically when cache_control is used in messages
         # Reference: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching
@@ -1025,8 +1066,14 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         user_anthropic_beta_headers: Final = self._get_user_anthropic_beta_headers(
             anthropic_beta_header=headers.get("anthropic-beta")
         )
+        context_1m_supported: Final = context_1m_requested(
+            model=model,
+            optional_params=optional_params,
+            litellm_params=litellm_params,
+        )
         anthropic_headers: Final = self.get_anthropic_headers(
             computer_tool_used=computer_tool_used,
+            context_1m_supported=context_1m_supported,
             prompt_caching_set=prompt_caching_set,
             pdf_used=pdf_used,
             api_key=api_key,

litellm/llms/anthropic/pass_through/messages/transformation.py

@@ -23,9 +23,12 @@ from litellm.types.router import GenericLiteLLMParams
 from ...common_utils import (
     AnthropicError,
     AnthropicModelInfo,
+    context_1m_beta_values,
+    context_1m_requested,
     optionally_handle_anthropic_oauth,
     requires_native_compaction_beta,
     strip_advisor_blocks_from_messages,
+    strip_context_1m_suffix,
     strip_encrypted_reasoning_blocks_from_anthropic_messages,
 )
 from .mid_conversation_system import (
@@ -277,6 +280,8 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
             headers=headers,
             optional_params=optional_params,
             messages=messages,
+            model=model,
+            litellm_params=litellm_params,
         )
 
         return headers, api_base
@@ -562,7 +567,7 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         anthropic_messages_request: Final[AnthropicMessagesRequest] = AnthropicMessagesRequest(
             messages=strip_encrypted_reasoning_blocks_from_anthropic_messages(messages),
             max_tokens=max_tokens,
-            model=model,
+            model=strip_context_1m_suffix(model),
             **anthropic_messages_optional_request_params,
         )
         return dict(anthropic_messages_request)
@@ -611,6 +616,8 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         optional_params: dict,
         custom_llm_provider: str = "anthropic",
         messages: Sequence[object] = (),
+        model: str = "",
+        litellm_params: object = None,
     ) -> dict:
         """
         Auto-inject anthropic-beta headers based on features used.
@@ -621,6 +628,7 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         - output_format: adds 'structured-outputs-2025-11-13'
         - speed: adds 'fast-mode-2026-02-01'
         - a message carrying output_config: adds 'per-turn-control-2026-07-01'
+        - [1m] suffix: adds 'context-1m-2025-08-07'
 
         Args:
             headers: Request headers dict
@@ -642,6 +650,12 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         if requires_native_compaction_beta(custom_llm_provider, optional_params, messages):
             beta_values.add(ANTHROPIC_BETA_HEADER_VALUES.COMPACT_2026_09_04.value)
 
+        beta_values.update(
+            context_1m_beta_values(
+                context_1m_requested(model=model, optional_params=optional_params, litellm_params=litellm_params)
+            )
+        )
+
         # Check for context management
         context_management_param: Final = optional_params.get("context_management")
         if context_management_param is not None:

tests/unit/llms/anthropic/test_context_1m_header.py

@@ -0,0 +1,144 @@
+from litellm.llms.anthropic.chat.transformation import AnthropicConfig
+from litellm.llms.anthropic.common_utils import (
+    context_1m_requested,
+    strip_context_1m_suffix,
+)
+from litellm.llms.anthropic.pass_through.messages.transformation import (
+    AnthropicMessagesConfig,
+)
+
+
+class TestAnthropicContext1mBetaHeader:
+    def test_strip_context_1m_suffix_is_pure(self):
+        assert strip_context_1m_suffix("claude-sonnet-4-6[1m]") == "claude-sonnet-4-6"
+        assert strip_context_1m_suffix("claude-sonnet-4-6[1M]") == "claude-sonnet-4-6"
+        assert strip_context_1m_suffix("claude-sonnet-4-6") == "claude-sonnet-4-6"
+        assert strip_context_1m_suffix("gpt-4o[1m]") == "gpt-4o"
+
+    def test_context_1m_requested_from_model_or_stashed_original(self):
+        assert context_1m_requested(model="claude-sonnet-4-6[1m]")
+        assert context_1m_requested(
+            model="claude-sonnet-4-6",
+            litellm_params={"_original_model": "claude-sonnet-4-6[1M]"},
+        )
+        assert not context_1m_requested(model="claude-sonnet-4-6")
+        assert not context_1m_requested(model="gpt-4o")
+
+    def test_get_anthropic_headers_with_context_1m(self):
+        headers = AnthropicConfig().get_anthropic_headers(api_key="sk-test", context_1m_supported=True)
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_get_anthropic_headers_without_context_1m(self):
+        headers = AnthropicConfig().get_anthropic_headers(api_key="sk-test", context_1m_supported=False)
+        assert "context-1m-2025-08-07" not in headers.get("anthropic-beta", "")
+
+    def test_validate_environment_adds_context_1m_for_suffixed_model(self):
+        headers = AnthropicConfig().validate_environment(
+            headers={},
+            model="claude-sonnet-4-6[1m]",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={},
+            litellm_params={},
+            api_key="sk-test",
+        )
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_validate_environment_no_context_1m_without_suffix(self):
+        headers = AnthropicConfig().validate_environment(
+            headers={},
+            model="claude-sonnet-4-6",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={},
+            litellm_params={},
+            api_key="sk-test",
+        )
+        assert "context-1m-2025-08-07" not in headers.get("anthropic-beta", "")
+
+    def test_validate_environment_uses_original_model_from_litellm_params(self):
+        headers = AnthropicConfig().validate_environment(
+            headers={},
+            model="claude-sonnet-4-6",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={},
+            litellm_params={"_original_model": "claude-sonnet-4-6[1M]"},
+            api_key="sk-test",
+        )
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_update_headers_adds_context_1m_with_original_model(self):
+        headers = AnthropicConfig().update_headers_with_optional_anthropic_beta(
+            headers={},
+            optional_params={"_original_model": "claude-sonnet-4-6[1m]"},
+        )
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_update_headers_no_context_1m_without_original_model(self):
+        headers = AnthropicConfig().update_headers_with_optional_anthropic_beta(
+            headers={},
+            optional_params={},
+        )
+        assert "context-1m-2025-08-07" not in headers.get("anthropic-beta", "")
+
+    def test_pass_through_adds_context_1m(self):
+        headers = AnthropicMessagesConfig._update_headers_with_anthropic_beta(
+            headers={},
+            optional_params={"_original_model": "claude-sonnet-4-6[1m]"},
+        )
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_pass_through_no_context_1m_without_suffix(self):
+        headers = AnthropicMessagesConfig._update_headers_with_anthropic_beta(
+            headers={},
+            optional_params={},
+        )
+        assert "context-1m-2025-08-07" not in headers.get("anthropic-beta", "")
+
+    def test_pass_through_adds_context_1m_from_model_without_original_model(self):
+        headers, _ = AnthropicMessagesConfig().validate_anthropic_messages_environment(
+            headers={},
+            model="claude-sonnet-4-6[1m]",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={},
+            litellm_params={},
+            api_key="sk-test",
+        )
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_pass_through_transform_strips_1m_from_upstream_model(self):
+        result = AnthropicMessagesConfig().transform_anthropic_messages_request(
+            model="claude-sonnet-4-6[1m]",
+            messages=[{"role": "user", "content": "hi"}],
+            anthropic_messages_optional_request_params={"max_tokens": 16},
+            litellm_params={},
+            headers={},
+        )
+        assert result["model"] == "claude-sonnet-4-6"
+
+    def test_chat_transform_request_strips_1m_and_injects_header(self):
+        headers = {}
+        result = AnthropicConfig().transform_request(
+            model="claude-sonnet-4-6[1m]",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={"max_tokens": 16},
+            litellm_params={},
+            headers=headers,
+        )
+        assert result["model"] == "claude-sonnet-4-6"
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_chat_transform_request_uppercase_suffix_strips_and_injects_header(self):
+        headers = {}
+        result = AnthropicConfig().transform_request(
+            model="claude-sonnet-4-6[1M]",
+            messages=[{"role": "user", "content": "hi"}],
+            optional_params={"max_tokens": 16},
+            litellm_params={},
+            headers=headers,
+        )
+        assert result["model"] == "claude-sonnet-4-6"
+        assert "context-1m-2025-08-07" in headers.get("anthropic-beta", "")
+
+    def test_proxy_does_not_globally_strip_non_anthropic_suffix(self):
+        from litellm.proxy import route_llm_request
+
+        assert not hasattr(route_llm_request, "stash_and_strip_context_1m_model_suffix")