fix(fetch): use OpenAPI 3.0 compatible schema keywords for Gemini support

The Fetch model's JSON schema used keywords incompatible with LLM
providers that only support OpenAPI 3.0 (e.g. Google Gemini 2.5 Pro),
causing 400 INVALID_ARGUMENT errors:

1. max_length field used gt=0/lt=1000000 (Pydantic Field constraints),
   which generated exclusiveMinimum/exclusiveMaximum — not recognized
   by Gemini. Changed to ge=1/le=999999 (identical semantics for
   integers), which emits the supported minimum/maximum keywords.

2. url field used AnyUrl, which generated format: "uri" and minLength: 1
   — Gemini only supports "enum" and "date-time" for string format.
   Added WithJsonSchema override to emit a plain string schema while
   preserving AnyUrl runtime validation.

Fixes #1624

Made-with: Cursor

Author: dhshah13

src/fetch/src/mcp_server_fetch/server.py

@@ -18,7 +18,7 @@
     INTERNAL_ERROR,
 )
 from protego import Protego
-from pydantic import BaseModel, Field, AnyUrl
+from pydantic import AnyUrl, BaseModel, Field, WithJsonSchema
 
 DEFAULT_USER_AGENT_AUTONOMOUS = "ModelContextProtocol/1.0 (Autonomous; +https://github.com/modelcontextprotocol/servers)"
 DEFAULT_USER_AGENT_MANUAL = "ModelContextProtocol/1.0 (User-Specified; +https://github.com/modelcontextprotocol/servers)"
@@ -151,14 +151,18 @@ async def fetch_url(
 class Fetch(BaseModel):
     """Parameters for fetching a URL."""
 
-    url: Annotated[AnyUrl, Field(description="URL to fetch")]
+    url: Annotated[
+        AnyUrl,
+        Field(description="URL to fetch"),
+        WithJsonSchema({"type": "string", "description": "URL to fetch"}),
+    ]
     max_length: Annotated[
         int,
         Field(
             default=5000,
             description="Maximum number of characters to return.",
-            gt=0,
-            lt=1000000,
+            ge=1,
+            le=999999,
         ),
     ]
     start_index: Annotated[

src/fetch/tests/test_server.py

@@ -5,6 +5,7 @@
 from mcp.shared.exceptions import McpError
 
 from mcp_server_fetch.server import (
+    Fetch,
     extract_content_from_html,
     get_robots_txt_url,
     check_may_autonomously_fetch_url,
@@ -13,6 +14,38 @@
 )
 
 
+class TestFetchToolSchema:
+    """Tests for the Fetch model JSON schema compatibility."""
+
+    def test_schema_uses_inclusive_bounds(self):
+        """Ensure the schema uses minimum/maximum instead of exclusiveMinimum/exclusiveMaximum.
+
+        Some LLM providers (e.g. Google Gemini) only support OpenAPI 3.0
+        schema keywords and reject exclusiveMinimum/exclusiveMaximum.
+        """
+        schema = Fetch.model_json_schema()
+        max_length_schema = schema["properties"]["max_length"]
+
+        assert "exclusiveMinimum" not in max_length_schema
+        assert "exclusiveMaximum" not in max_length_schema
+        assert max_length_schema["minimum"] == 1
+        assert max_length_schema["maximum"] == 999999
+
+    def test_url_schema_omits_unsupported_keywords(self):
+        """Ensure the url field schema avoids format/minLength keywords.
+
+        Some LLM providers (e.g. Google Gemini) only support a limited set
+        of keywords for string types and reject unsupported ones like
+        format: "uri" or minLength.
+        """
+        schema = Fetch.model_json_schema()
+        url_schema = schema["properties"]["url"]
+
+        assert "format" not in url_schema
+        assert "minLength" not in url_schema
+        assert url_schema["type"] == "string"
+
+
 class TestGetRobotsTxtUrl:
     """Tests for get_robots_txt_url function."""