From 067766b576b7f8cf98f3e071a31fe48baaf7aea2 Mon Sep 17 00:00:00 2001
From: Dharit Shah <dhshah@redhat.com>
Date: Fri, 3 Apr 2026 23:10:49 -0400
Subject: [PATCH] fix(fetch): use OpenAPI 3.0 compatible schema keywords for
 Gemini support
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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
---
 src/fetch/src/mcp_server_fetch/server.py | 12 ++++++---
 src/fetch/tests/test_server.py           | 33 ++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/src/fetch/src/mcp_server_fetch/server.py b/src/fetch/src/mcp_server_fetch/server.py
index b42c7b1f6b..1f1c0cc530 100644
--- a/src/fetch/src/mcp_server_fetch/server.py
+++ b/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[
diff --git a/src/fetch/tests/test_server.py b/src/fetch/tests/test_server.py
index 96c1cb38c7..ff87b34d34 100644
--- a/src/fetch/tests/test_server.py
+++ b/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."""