From b9d54d15bc8e95be2050284a1a9ed501e0ced31f Mon Sep 17 00:00:00 2001
From: ujangm <ujangm@xlsmart.co.id>
Date: Wed, 14 Oct 2026 20:01:52 +0700
Subject: [PATCH] fix(anthropic): default max_tokens from the deployment's
 max_output_tokens

A chat-completions request without max_tokens gets its default from get_config(), which looks the
model up by its provider-stripped name. A proxy deployment such as `anthropic/<custom-model>` is
registered under the prefixed name, so the lookup misses and the request goes upstream capped at
DEFAULT_ANTHROPIC_CHAT_MAX_TOKENS (4096) even though model_info sets max_output_tokens.

transform_request now uses litellm_params["model_info"]["max_output_tokens"] when the request has no
max_tokens and no class-level default is set. An explicit request value and AnthropicConfig(max_tokens=...)
still win; deployments without the field keep the existing lookup and 4096 fallback.
---
 litellm/llms/anthropic/chat/transformation.py | 11 +++
 .../test_anthropic_chat_transformation.py     | 70 +++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/litellm/llms/anthropic/chat/transformation.py b/litellm/llms/anthropic/chat/transformation.py
index c43e5f9b05b..44cc5151edb 100644
--- a/litellm/llms/anthropic/chat/transformation.py
+++ b/litellm/llms/anthropic/chat/transformation.py
@@ -2101,6 +2101,17 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
         if len(tools) > 1:
             optional_params["tools"] = tools
 
+        ## Deployment max output (proxy model_info) beats get_config(), which only sees the provider-stripped
+        ## model name and so falls back to 4096 for custom deployments; a request or class default still wins
+        if "max_tokens" not in optional_params and litellm.AnthropicConfig.max_tokens is None:
+            match litellm_params:
+                case {"model_info": {"max_output_tokens": int(deployment_max_tokens)}} if (
+                    deployment_max_tokens > 0 and not isinstance(deployment_max_tokens, bool)
+                ):
+                    optional_params["max_tokens"] = deployment_max_tokens  # rebind-ok: out-param store
+                case object():
+                    pass  # no deployment limit: get_config() below fills the default
+
         ## Load Config
         config: Final = litellm.AnthropicConfig.get_config(model=model)
         for k, v in config.items():
diff --git a/tests/unit/llms/anthropic/chat/test_anthropic_chat_transformation.py b/tests/unit/llms/anthropic/chat/test_anthropic_chat_transformation.py
index b682d8661f3..25ce191c166 100644
--- a/tests/unit/llms/anthropic/chat/test_anthropic_chat_transformation.py
+++ b/tests/unit/llms/anthropic/chat/test_anthropic_chat_transformation.py
@@ -2646,6 +2646,76 @@ def test_transform_request_respects_user_max_tokens():
     assert result["max_tokens"] == 1000
 
 
+@pytest.mark.parametrize(
+    ("optional_params", "litellm_params", "expected"),
+    [
+        # custom deployment: the configured output limit replaces the 4096 fallback
+        ({}, {"model_info": {"max_output_tokens": 131072}}, 131072),
+        # an explicit request value still wins over the deployment limit
+        ({"max_tokens": 1000}, {"model_info": {"max_output_tokens": 131072}}, 1000),
+        # no deployment limit: unknown models keep the 4096 fallback
+        ({}, {}, 4096),
+        ({}, {"model_info": {"max_output_tokens": None}}, 4096),
+    ],
+)
+def test_transform_request_default_max_tokens_from_deployment(
+    optional_params: dict[str, object], litellm_params: dict[str, object], expected: int
+) -> None:
+    result = AnthropicConfig().transform_request(
+        model="glm-5.3-flash",
+        messages=[{"role": "user", "content": "Hello"}],
+        optional_params=optional_params,
+        litellm_params=litellm_params,
+        headers={},
+    )
+
+    assert result["max_tokens"] == expected, f"model_info={litellm_params!r} request={optional_params!r}"
+
+
+async def test_router_sends_deployment_max_output_tokens_when_request_omits_max_tokens(
+    monkeypatch: pytest.MonkeyPatch,
+    respx_mock: respx.MockRouter,
+) -> None:
+    """A proxy deployment `anthropic/<custom>` with model_info.max_output_tokens must not go upstream capped at 4096."""
+    monkeypatch.setenv("DISABLE_AIOHTTP_TRANSPORT", "True")
+    sent: Final[list[dict]] = []
+
+    def respond(request: httpx.Request) -> httpx.Response:
+        sent.append(json.loads(request.content))
+        return httpx.Response(
+            200,
+            json={
+                "id": "msg_1",
+                "type": "message",
+                "role": "assistant",
+                "model": "glm-5.3-flash",
+                "content": [{"type": "text", "text": "OK"}],
+                "stop_reason": "end_turn",
+                "usage": {"input_tokens": 3, "output_tokens": 1},
+            },
+        )
+
+    respx_mock.post("https://anthropic-compatible.test/v1/messages").mock(side_effect=respond)
+    router: Final = litellm.Router(
+        model_list=[
+            {
+                "model_name": "claude-glm-5.3-flash",
+                "litellm_params": {
+                    "model": "anthropic/glm-5.3-flash",
+                    "api_base": "https://anthropic-compatible.test",
+                    "api_key": "test-key",
+                },
+                "model_info": {"max_output_tokens": 131072},
+            }
+        ]
+    )
+
+    await router.acompletion(model="claude-glm-5.3-flash", messages=[{"role": "user", "content": "hi"}])
+    await router.acompletion(model="claude-glm-5.3-flash", messages=[{"role": "user", "content": "hi"}], max_tokens=512)
+
+    assert [body["max_tokens"] for body in sent] == [131072, 512], "first call omits max_tokens, second sends 512"
+
+
 def test_calculate_usage_completion_tokens_details_always_populated():
     """
     Test that completion_tokens_details is always populated in Usage object,