fix(anthropic): coerce tool input_schema.type to "object" on /v1/messages pass-through

Anthropic's /v1/messages requires every tool input_schema to have
type == "object" and rejects the request otherwise with:

    tools.N.custom.input_schema.type: Field required

Tools emitted by Claude Code / MCP clients sometimes omit `type`, declaring a
JSON-schema "$schema" draft reference instead. The beta /v1/messages
pass-through forwarded them verbatim, so these requests 400. ($schema itself is
tolerated by Anthropic when type is present — the missing type is the cause.)
The /v1/chat/completions path already normalizes this in
AnthropicConfig._map_tool_helper; the pass-through path had no equivalent.

Add _normalize_tool_input_schemas() and call it in
transform_anthropic_messages_request: coerce type -> "object" (ensuring
properties), and drop the "$schema"/"$id" reference fields via the shared
_remove_json_schema_refs helper (as the Mistral chat transformation does).
Inheriting Azure/Vertex/DeepSeek/Bedrock configs benefit via super().

Verified end-to-end against the live Anthropic API: a tool whose input_schema
omits `type` returns HTTP 400 before this change and HTTP 200 after.

Fixes #24121

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

@@ -253,6 +253,38 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         existing_output_config.setdefault("effort", effort)
         optional_params["output_config"] = existing_output_config
 
+    @staticmethod
+    def _normalize_tool_input_schemas(tools: Optional[list[dict]]) -> None:
+        """
+        Normalize each tool's input_schema for Anthropic /v1/messages.
+
+        Anthropic requires ``input_schema.type == "object"`` and rejects the
+        request otherwise with:
+            tools.N.custom.input_schema.type: Field required
+
+        Tools emitted by Claude Code / MCP-backed clients sometimes omit ``type``
+        (declaring a JSON-schema ``$schema`` draft reference instead). The
+        /v1/chat/completions path already handles this in
+        ``AnthropicConfig._map_tool_helper``; the pass-through path had no
+        equivalent. Mirror that behavior: coerce ``type`` to ``"object"`` (and
+        ensure ``properties`` exists), and drop the ``$schema``/``$id`` reference
+        fields via the shared ``_remove_json_schema_refs`` helper (the same helper
+        the Mistral chat transformation uses). Mutates input_schema in place.
+        """
+        if not tools:
+            return
+        from litellm.constants import DEFAULT_MAX_RECURSE_DEPTH
+        from litellm.utils import _remove_json_schema_refs
+
+        for tool in tools:
+            input_schema = tool.get("input_schema")
+            if not isinstance(input_schema, dict):
+                continue
+            _remove_json_schema_refs(input_schema, max_depth=DEFAULT_MAX_RECURSE_DEPTH)
+            if input_schema.get("type") != "object":
+                input_schema["type"] = "object"
+                input_schema.setdefault("properties", {})
+
     def transform_anthropic_messages_request(
         self,
         model: str,
@@ -309,6 +341,11 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         # request body on the hot path). Defer it to when DEBUG is enabled.
         verbose_logger.debug("TRANSFORMATION DEBUG - Messages: %s", messages)
 
+        # Normalize tool input_schema (coerce type->"object", drop $schema/$id).
+        # Anthropic rejects a missing/non-object type; the chat path already does
+        # this in _map_tool_helper, but the pass-through path did not.
+        self._normalize_tool_input_schemas(anthropic_messages_optional_request_params.get("tools"))
+
         # Auto-strip advisor blocks from history if advisor tool is absent.
         # Prevents Anthropic 400: advisor_tool_result in history requires advisor tool.
         _tools = anthropic_messages_optional_request_params.get("tools") or []

tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_tool_schema_sanitization.py

@@ -0,0 +1,113 @@
+"""Tests for tool ``input_schema`` normalization on the Anthropic /v1/messages route.
+
+Regression coverage for #24121: tools whose ``input_schema`` omits ``type`` (as Claude
+Code / MCP clients emit, declaring a JSON-schema ``$schema`` draft reference instead) are
+forwarded to Anthropic unchanged on the beta ``/v1/messages`` pass-through and rejected
+with::
+
+    tools.0.custom.input_schema.type: Input should be 'object'
+
+The root cause is the missing ``type``, not the ``$schema`` field (Anthropic tolerates
+``$schema`` when ``type: object`` is present). The ``/v1/chat/completions`` path already
+coerces ``type`` -> ``"object"`` in ``_map_tool_helper``; these tests assert the
+pass-through path now does the same (and also drops ``$schema``/``$id`` for parity).
+"""
+
+from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+    AnthropicMessagesConfig,
+)
+
+
+def _transform(tools):
+    return AnthropicMessagesConfig().transform_anthropic_messages_request(
+        model="claude-sonnet-4-5",
+        messages=[{"role": "user", "content": "Hello"}],
+        anthropic_messages_optional_request_params={"max_tokens": 1024, "tools": tools},
+        litellm_params={},
+        headers={},
+    )
+
+
+def test_missing_type_is_coerced_to_object():
+    """THE fix: input_schema without ``type`` must get ``type: "object"`` injected.
+
+    This is exactly the shape that Anthropic rejects with HTTP 400 before the fix.
+    """
+    tool = {
+        "name": "get_weather",
+        "description": "Get weather",
+        "input_schema": {
+            "$schema": "http://json-schema.org/draft-07/schema#",
+            "properties": {"location": {"type": "string"}},
+            "required": ["location"],
+        },
+    }
+    input_schema = _transform([tool])["tools"][0]["input_schema"]
+    assert input_schema["type"] == "object"
+    # $schema/$id are also stripped for parity with the chat path.
+    assert "$schema" not in input_schema
+    # Legitimate fields preserved.
+    assert input_schema["properties"]["location"]["type"] == "string"
+    assert input_schema["required"] == ["location"]
+
+
+def test_non_object_type_is_coerced_and_properties_ensured():
+    """A root-level non-object ``type`` is coerced, and ``properties`` is injected."""
+    tool = {
+        "name": "echo",
+        "description": "echo",
+        "input_schema": {"type": "string"},
+    }
+    input_schema = _transform([tool])["tools"][0]["input_schema"]
+    assert input_schema["type"] == "object"
+    assert input_schema["properties"] == {}
+
+
+def test_valid_object_schema_is_preserved():
+    """A well-formed object schema is left intact (aside from $schema/$id removal)."""
+    tool = {
+        "name": "ok",
+        "description": "ok",
+        "input_schema": {
+            "type": "object",
+            "properties": {"x": {"type": "string"}},
+            "required": ["x"],
+        },
+    }
+    input_schema = _transform([tool])["tools"][0]["input_schema"]
+    assert input_schema["type"] == "object"
+    assert input_schema["properties"] == {"x": {"type": "string"}}
+    assert input_schema["required"] == ["x"]
+
+
+def test_schema_refs_stripped_recursively():
+    """Nested ``$schema``/``$id`` inside properties are also removed."""
+    tool = {
+        "name": "nested",
+        "description": "nested",
+        "input_schema": {
+            "type": "object",
+            "properties": {
+                "inner": {
+                    "$id": "https://example.com/x.json",
+                    "type": "object",
+                    "properties": {"x": {"type": "string"}},
+                }
+            },
+        },
+    }
+    inner = _transform([tool])["tools"][0]["input_schema"]["properties"]["inner"]
+    assert "$id" not in inner
+    assert inner["properties"]["x"]["type"] == "string"
+
+
+def test_no_tools_is_a_noop():
+    """Requests without tools are unaffected."""
+    result = AnthropicMessagesConfig().transform_anthropic_messages_request(
+        model="claude-sonnet-4-5",
+        messages=[{"role": "user", "content": "Hello"}],
+        anthropic_messages_optional_request_params={"max_tokens": 1024},
+        litellm_params={},
+        headers={},
+    )
+    assert "tools" not in result or result.get("tools") is None