diff --git a/mcp/tools.go b/mcp/tools.go
index 80ae50911..42e888d52 100644
--- a/mcp/tools.go
+++ b/mcp/tools.go
@@ -653,6 +653,31 @@ func (tis ToolArgumentsSchema) MarshalJSON() ([]byte, error) {
 	return json.Marshal(m)
 }
 
+// UnmarshalJSON implements the json.Unmarshaler interface for ToolArgumentsSchema.
+// It handles both "$defs" (JSON Schema 2019-09+) and "definitions" (JSON Schema draft-07)
+// by reading either field and storing it in the Defs field.
+func (tis *ToolArgumentsSchema) UnmarshalJSON(data []byte) error {
+	// Use a temporary type to avoid infinite recursion
+	type Alias ToolArgumentsSchema
+	aux := &struct {
+		Definitions map[string]any `json:"definitions,omitempty"`
+		*Alias
+	}{
+		Alias: (*Alias)(tis),
+	}
+
+	if err := json.Unmarshal(data, aux); err != nil {
+		return err
+	}
+
+	// If $defs wasn't provided but definitions was, use definitions
+	if tis.Defs == nil && aux.Definitions != nil {
+		tis.Defs = aux.Definitions
+	}
+
+	return nil
+}
+
 type ToolAnnotation struct {
 	// Human-readable title for the tool
 	Title string `json:"title,omitempty"`
diff --git a/mcp/tools_test.go b/mcp/tools_test.go
index 16aeb5dfc..ef472e477 100644
--- a/mcp/tools_test.go
+++ b/mcp/tools_test.go
@@ -1549,3 +1549,136 @@ func TestToolMetaMarshalingOmitsWhenNil(t *testing.T) {
 	// Check that _meta field is not present
 	assert.NotContains(t, result, "_meta", "Tool without Meta should not include _meta field")
 }
+
+func TestToolArgumentsSchema_UnmarshalWithDefinitions(t *testing.T) {
+	// Test that "definitions" (JSON Schema draft-07) is properly unmarshaled into Defs field
+	jsonData := `{
+		"type": "object",
+		"properties": {
+			"operation": {
+				"$ref": "#/definitions/operation_type"
+			}
+		},
+		"required": ["operation"],
+		"definitions": {
+			"operation_type": {
+				"type": "string",
+				"enum": ["create", "read", "update", "delete"]
+			}
+		}
+	}`
+
+	var schema ToolArgumentsSchema
+	err := json.Unmarshal([]byte(jsonData), &schema)
+	assert.NoError(t, err)
+
+	// Verify the schema was properly unmarshaled
+	assert.Equal(t, "object", schema.Type)
+	assert.Contains(t, schema.Properties, "operation")
+	assert.Equal(t, []string{"operation"}, schema.Required)
+
+	// Most importantly: verify that "definitions" was read into Defs field
+	assert.NotNil(t, schema.Defs)
+	assert.Contains(t, schema.Defs, "operation_type")
+
+	operationType, ok := schema.Defs["operation_type"].(map[string]any)
+	assert.True(t, ok)
+	assert.Equal(t, "string", operationType["type"])
+	assert.NotNil(t, operationType["enum"])
+}
+
+func TestToolArgumentsSchema_UnmarshalWithDefs(t *testing.T) {
+	// Test that "$defs" (JSON Schema 2019-09+) is properly unmarshaled into Defs field
+	jsonData := `{
+		"type": "object",
+		"properties": {
+			"operation": {
+				"$ref": "#/$defs/operation_type"
+			}
+		},
+		"required": ["operation"],
+		"$defs": {
+			"operation_type": {
+				"type": "string",
+				"enum": ["create", "read", "update", "delete"]
+			}
+		}
+	}`
+
+	var schema ToolArgumentsSchema
+	err := json.Unmarshal([]byte(jsonData), &schema)
+	assert.NoError(t, err)
+
+	// Verify the schema was properly unmarshaled
+	assert.Equal(t, "object", schema.Type)
+	assert.Contains(t, schema.Properties, "operation")
+	assert.Equal(t, []string{"operation"}, schema.Required)
+
+	// Verify that "$defs" was read into Defs field
+	assert.NotNil(t, schema.Defs)
+	assert.Contains(t, schema.Defs, "operation_type")
+
+	operationType, ok := schema.Defs["operation_type"].(map[string]any)
+	assert.True(t, ok)
+	assert.Equal(t, "string", operationType["type"])
+	assert.NotNil(t, operationType["enum"])
+}
+
+func TestToolArgumentsSchema_UnmarshalPrefersDefs(t *testing.T) {
+	// Test that if both "$defs" and "definitions" are present, "$defs" takes precedence
+	jsonData := `{
+		"type": "object",
+		"$defs": {
+			"from_defs": {
+				"type": "string"
+			}
+		},
+		"definitions": {
+			"from_definitions": {
+				"type": "integer"
+			}
+		}
+	}`
+
+	var schema ToolArgumentsSchema
+	err := json.Unmarshal([]byte(jsonData), &schema)
+	assert.NoError(t, err)
+
+	// $defs should take precedence
+	assert.Contains(t, schema.Defs, "from_defs")
+	assert.NotContains(t, schema.Defs, "from_definitions")
+}
+
+func TestToolArgumentsSchema_MarshalRoundTrip(t *testing.T) {
+	// Test that marshaling and unmarshaling preserves definitions
+	original := ToolArgumentsSchema{
+		Type: "object",
+		Properties: map[string]any{
+			"field": map[string]any{
+				"$ref": "#/$defs/my_type",
+			},
+		},
+		Required: []string{"field"},
+		Defs: map[string]any{
+			"my_type": map[string]any{
+				"type": "string",
+				"enum": []string{"a", "b", "c"},
+			},
+		},
+	}
+
+	// Marshal
+	data, err := json.Marshal(original)
+	assert.NoError(t, err)
+
+	// Unmarshal
+	var unmarshaled ToolArgumentsSchema
+	err = json.Unmarshal(data, &unmarshaled)
+	assert.NoError(t, err)
+
+	// Verify round-trip
+	assert.Equal(t, original.Type, unmarshaled.Type)
+	assert.Equal(t, original.Required, unmarshaled.Required)
+	assert.NotNil(t, unmarshaled.Defs)
+	assert.Contains(t, unmarshaled.Defs, "my_type")
+}