From ddb6b61de2b70e88741ecdd3e6516eda676c1f42 Mon Sep 17 00:00:00 2001
From: Rohit Ghumare <ghumare64@gmail.com>
Date: Sat, 18 Apr 2026 20:06:25 +0100
Subject: [PATCH] fix(fetch): accept null for optional parameters

Closes #2035.

Clients like LibreChat send explicit `null` for optional params instead
of omitting them. The Fetch model declared `max_length: int`,
`start_index: int`, and `raw: bool`, so Pydantic rejected `null` at the
type-coercion step before field defaults could apply.

Adds a `mode="before"` field_validator that maps `None` back to the
field's declared default. Keeps field types as `int`/`bool`, preserves
the `gt=0`/`lt=1_000_000`/`ge=0` validators on resolved values, and
does not rely on falsy-coalescing (which would have replaced valid
`start_index=0` or explicit `raw=false`).

Adds 9 tests covering null handling, default preservation, explicit
zero/false preservation, and validator behavior.
---
 src/fetch/src/mcp_server_fetch/server.py |  9 +++-
 src/fetch/tests/test_server.py           | 62 ++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/src/fetch/src/mcp_server_fetch/server.py b/src/fetch/src/mcp_server_fetch/server.py
index b42c7b1f6b..aeca59bd32 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 BaseModel, Field, AnyUrl, field_validator
 
 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)"
@@ -177,6 +177,13 @@ class Fetch(BaseModel):
         ),
     ]
 
+    @field_validator("max_length", "start_index", "raw", mode="before")
+    @classmethod
+    def _null_to_default(cls, value, info):
+        if value is None:
+            return cls.model_fields[info.field_name].default
+        return value
+
 
 async def serve(
     custom_user_agent: str | None = None,
diff --git a/src/fetch/tests/test_server.py b/src/fetch/tests/test_server.py
index 96c1cb38c7..aac450cdab 100644
--- a/src/fetch/tests/test_server.py
+++ b/src/fetch/tests/test_server.py
@@ -9,10 +9,72 @@
     get_robots_txt_url,
     check_may_autonomously_fetch_url,
     fetch_url,
+    Fetch,
     DEFAULT_USER_AGENT_AUTONOMOUS,
 )
 
 
+class TestFetchParamsNullHandling:
+    """Issue #2035: clients like LibreChat pass explicit null for optional params."""
+
+    def test_all_optional_params_omitted(self):
+        args = Fetch(url="https://example.com")
+        assert args.max_length == 5000
+        assert args.start_index == 0
+        assert args.raw is False
+
+    def test_explicit_null_max_length(self):
+        args = Fetch(url="https://example.com", max_length=None)
+        assert args.max_length == 5000
+
+    def test_explicit_null_start_index(self):
+        args = Fetch(url="https://example.com", start_index=None)
+        assert args.start_index == 0
+
+    def test_explicit_null_raw(self):
+        args = Fetch(url="https://example.com", raw=None)
+        assert args.raw is False
+
+    def test_all_nulls_together(self):
+        args = Fetch(
+            url="https://example.com",
+            max_length=None,
+            start_index=None,
+            raw=None,
+        )
+        assert args.max_length == 5000
+        assert args.start_index == 0
+        assert args.raw is False
+
+    def test_explicit_values_are_preserved(self):
+        args = Fetch(
+            url="https://example.com",
+            max_length=100,
+            start_index=50,
+            raw=True,
+        )
+        assert args.max_length == 100
+        assert args.start_index == 50
+        assert args.raw is True
+
+    def test_start_index_zero_preserved(self):
+        """Regression: `or`-coalescing would incorrectly fall through 0."""
+        args = Fetch(url="https://example.com", start_index=0)
+        assert args.start_index == 0
+
+    def test_raw_false_preserved(self):
+        """Regression: explicit False must stay False."""
+        args = Fetch(url="https://example.com", raw=False)
+        assert args.raw is False
+
+    def test_validators_still_apply(self):
+        """gt=0 on max_length must still reject invalid values after null handling."""
+        with pytest.raises(ValueError):
+            Fetch(url="https://example.com", max_length=0)
+        with pytest.raises(ValueError):
+            Fetch(url="https://example.com", start_index=-1)
+
+
 class TestGetRobotsTxtUrl:
     """Tests for get_robots_txt_url function."""