Add AdditionalProperties to ToolInputSchema (#678)

- Add AdditionalProperties field to ToolArgumentsSchema struct
- Update MarshalJSON to include additionalProperties in output
- Add WithSchemaAdditionalProperties ToolOption function
- Add comprehensive tests for marshal/unmarshal and ToolOption

Fixes #672

Author: mohit-gupta-glean

mcp/tools.go

@@ -635,10 +635,11 @@ func (t Tool) MarshalJSON() ([]byte, error) {
 
 // ToolArgumentsSchema represents a JSON Schema for tool arguments.
 type ToolArgumentsSchema struct {
-	Defs       map[string]any `json:"$defs,omitempty"`
-	Type       string         `json:"type"`
-	Properties map[string]any `json:"properties,omitempty"`
-	Required   []string       `json:"required,omitempty"`
+	Defs                 map[string]any `json:"$defs,omitempty"`
+	Type                 string         `json:"type"`
+	Properties           map[string]any `json:"properties,omitempty"`
+	Required             []string       `json:"required,omitempty"`
+	AdditionalProperties any            `json:"additionalProperties,omitempty"`
 }
 
 type ToolInputSchema ToolArgumentsSchema // For retro-compatibility
@@ -662,6 +663,10 @@ func (tis ToolArgumentsSchema) MarshalJSON() ([]byte, error) {
 		m["required"] = tis.Required
 	}
 
+	if tis.AdditionalProperties != nil {
+		m["additionalProperties"] = tis.AdditionalProperties
+	}
+
 	return json.Marshal(m)
 }
 
@@ -919,6 +924,15 @@ func WithOpenWorldHintAnnotation(value bool) ToolOption {
 	}
 }
 
+// WithSchemaAdditionalProperties sets the additionalProperties field on the tool's input schema.
+// It accepts false (disallow extra properties), true (allow any), or a schema map
+// to validate additional properties against.
+func WithSchemaAdditionalProperties(schema any) ToolOption {
+	return func(t *Tool) {
+		t.InputSchema.AdditionalProperties = schema
+	}
+}
+
 //
 // Common Property Options
 //

mcp/tools_test.go

@@ -1693,3 +1693,75 @@ func TestWithToolIcons(t *testing.T) {
 
 	assert.Equal(t, icons, tool.Icons)
 }
+
+func TestToolArgumentsSchema_UnmarshalWithAdditionalPropertiesFalse(t *testing.T) {
+	jsonData := `{
+		"type": "object",
+		"properties": {
+			"name": {"type": "string"}
+		},
+		"additionalProperties": false
+	}`
+
+	var schema ToolArgumentsSchema
+	err := json.Unmarshal([]byte(jsonData), &schema)
+	assert.NoError(t, err)
+
+	assert.Equal(t, "object", schema.Type)
+	assert.Contains(t, schema.Properties, "name")
+	assert.Equal(t, false, schema.AdditionalProperties)
+}
+
+func TestToolArgumentsSchema_UnmarshalWithAdditionalPropertiesSchema(t *testing.T) {
+	jsonData := `{
+		"type": "object",
+		"properties": {
+			"name": {"type": "string"}
+		},
+		"additionalProperties": {"type": "string"}
+	}`
+
+	var schema ToolArgumentsSchema
+	err := json.Unmarshal([]byte(jsonData), &schema)
+	assert.NoError(t, err)
+
+	assert.Equal(t, "object", schema.Type)
+	additionalProps, ok := schema.AdditionalProperties.(map[string]any)
+	assert.True(t, ok)
+	assert.Equal(t, "string", additionalProps["type"])
+}
+
+func TestToolArgumentsSchema_MarshalWithAdditionalProperties(t *testing.T) {
+	schema := ToolArgumentsSchema{
+		Type:                 "object",
+		AdditionalProperties: false,
+	}
+
+	data, err := json.Marshal(schema)
+	assert.NoError(t, err)
+	assert.Contains(t, string(data), `"additionalProperties":false`)
+}
+
+func TestToolArgumentsSchema_MarshalOmitsNilAdditionalProperties(t *testing.T) {
+	schema := ToolArgumentsSchema{
+		Type:                 "object",
+		AdditionalProperties: nil,
+	}
+
+	data, err := json.Marshal(schema)
+	assert.NoError(t, err)
+	assert.NotContains(t, string(data), "additionalProperties")
+}
+
+func TestWithSchemaAdditionalProperties(t *testing.T) {
+	tool := NewTool(
+		"strict-tool",
+		WithSchemaAdditionalProperties(false),
+	)
+
+	assert.Equal(t, false, tool.InputSchema.AdditionalProperties)
+
+	data, err := json.Marshal(tool)
+	assert.NoError(t, err)
+	assert.Contains(t, string(data), `"additionalProperties":false`)
+}