From d69a86dccfe2a21615d020985ddba0654e4dcf13 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 7 Apr 2026 17:47:27 -0400
Subject: [PATCH] fix: enforce false boolean schemas as unsatisfiable

`true` returns Any (any value valid), but `false` now returns an
unsatisfiable type that rejects all values during Pydantic validation.
---
 src/fastmcp/utilities/json_schema_type.py     | 37 +++++++++++++------
 .../json_schema_type/test_json_schema_type.py |  7 +++-
 2 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/src/fastmcp/utilities/json_schema_type.py b/src/fastmcp/utilities/json_schema_type.py
index a975f8e90..8b50ff568 100644
--- a/src/fastmcp/utilities/json_schema_type.py
+++ b/src/fastmcp/utilities/json_schema_type.py
@@ -53,6 +53,7 @@ from typing import (
 from pydantic import (
     AnyUrl,
     BaseModel,
+    BeforeValidator,
     ConfigDict,
     EmailStr,
     Field,
@@ -65,6 +66,15 @@ from typing_extensions import NotRequired, TypedDict
 __all__ = ["JSONSchema", "json_schema_to_type"]
 
 
+def _reject_all(v: Any) -> Any:
+    """Validator that rejects every value, implementing JSON Schema `false`."""
+    raise ValueError("No value is valid against a false schema")
+
+
+# JSON Schema `false` means no value is valid. This type rejects everything
+# during Pydantic validation.
+_UnsatisfiableType = Annotated[Any, BeforeValidator(_reject_all)]
+
 FORMAT_TYPES: dict[str, Any] = {
     "date-time": datetime,
     "email": EmailStr,
@@ -319,9 +329,11 @@ def _schema_to_type(
     """Convert schema to appropriate Python type."""
     # Boolean schemas are valid in JSON Schema draft-06+:
     # true means "any value is valid" (equivalent to {}),
-    # false means "no value is valid".
-    if isinstance(schema, bool):
+    # false means "no value is valid" (unsatisfiable).
+    if schema is True:
         return Any
+    if schema is False:
+        return _UnsatisfiableType  # type: ignore[return-value]  # ty:ignore[invalid-return-type]
 
     if not schema:
         return object
@@ -481,11 +493,13 @@ def _create_pydantic_model(
     defaults = {}
 
     for prop_name, prop_schema in properties.items():
-        # Normalize boolean schemas (JSON Schema draft-06+)
+        # Boolean schemas (JSON Schema draft-06+): resolve type directly,
+        # then use an empty dict for .get() calls below.
         if isinstance(prop_schema, bool):
+            field_type = _schema_to_type(prop_schema, schemas or {})
             prop_schema = {}
-
-        field_type = _schema_to_type(prop_schema, schemas or {})
+        else:
+            field_type = _schema_to_type(prop_schema, schemas or {})
 
         # Handle defaults
         default_value = prop_schema.get("default", MISSING)
@@ -548,14 +562,15 @@ def _create_dataclass(
 
     fields: list[tuple[Any, ...]] = []
     for prop_name, prop_schema in properties.items():
-        # Normalize boolean schemas (JSON Schema draft-06+)
-        if isinstance(prop_schema, bool):
-            prop_schema = {}
-
         field_name = _sanitize_name(prop_name)
 
-        # Check for self-reference in property
-        if prop_schema.get("$ref") == "#":
+        # Boolean schemas (JSON Schema draft-06+): resolve type directly,
+        # then use an empty dict for .get() calls below.
+        if isinstance(prop_schema, bool):
+            field_type = _schema_to_type(prop_schema, schemas or {})
+            prop_schema = {}
+        elif prop_schema.get("$ref") == "#":
+            # Check for self-reference in property
             field_type = ForwardRef(sanitized_name)
         else:
             field_type = _schema_to_type(prop_schema, schemas or {})
diff --git a/tests/utilities/json_schema_type/test_json_schema_type.py b/tests/utilities/json_schema_type/test_json_schema_type.py
index 6905eed14..ec83f8b88 100644
--- a/tests/utilities/json_schema_type/test_json_schema_type.py
+++ b/tests/utilities/json_schema_type/test_json_schema_type.py
@@ -127,8 +127,8 @@ class TestBooleanSchemas:
         assert obj.name == "test"  # type: ignore[attr-defined]  # ty:ignore[unresolved-attribute]
         assert obj.anything == 42  # type: ignore[attr-defined]  # ty:ignore[unresolved-attribute]
 
-    def test_false_property_schema_does_not_crash(self):
-        """A property with schema `false` should not crash parsing."""
+    def test_false_property_schema_rejects_values(self):
+        """A property with schema `false` should reject any provided value."""
         schema = {
             "type": "object",
             "properties": {"name": {"type": "string"}, "never": False},
@@ -139,6 +139,9 @@ class TestBooleanSchemas:
         obj = validator.validate_python({"name": "test"})
         assert obj.name == "test"  # type: ignore[attr-defined]  # ty:ignore[unresolved-attribute]
 
+        with pytest.raises(ValidationError):
+            validator.validate_python({"name": "test", "never": "anything"})
+
     def test_boolean_schema_in_object_with_additional_properties(self):
         """Boolean property schemas work alongside additionalProperties=True."""
         schema = {