feat: support creating tools using go-struct-style input schema

Author: Grivn

examples/structured_output/README.md renamed to examples/structured_input_and_output/README.md

@@ -6,6 +6,15 @@ Defined in the MCP spec here: https://modelcontextprotocol.io/specification/2025
 
 ## Usage
 
+Define a struct for your input:
+
+```go
+type WeatherRequest struct {
+    Location string `json:"location,required" jsonschema_description:"City or location"`
+    Units    string `json:"units,omitempty" jsonschema_description:"celsius or fahrenheit" jsonschema:"enum=celsius,enum=fahrenheit"`
+}
+```
+
 Define a struct for your output:
 
 ```go
@@ -21,8 +30,8 @@ Add it to your tool:
 ```go
 tool := mcp.NewTool("get_weather",
     mcp.WithDescription("Get weather information"),
+    mcp.WithInputSchema[WeatherRequest](),
     mcp.WithOutputSchema[WeatherResponse](),
-    mcp.WithString("location", mcp.Required()),
 )
 ```
 

examples/structured_output/main.go renamed to examples/structured_input_and_output/main.go

@@ -12,8 +12,8 @@ import (
 // Note: The jsonschema_description tag is added to the JSON schema as description
 // Ideally use better descriptions, this is just an example
 type WeatherRequest struct {
-	Location string `json:"location" jsonschema_description:"City or location"`
-	Units    string `json:"units,omitempty" jsonschema_description:"celsius or fahrenheit"`
+	Location string `json:"location" jsonschema_description:"City or location" jsonschema:"required"`
+	Units    string `json:"units,omitempty" jsonschema_description:"celsius or fahrenheit" jsonschema:"enum=celsius,enum=fahrenheit"`
 }
 
 type WeatherResponse struct {
@@ -32,7 +32,7 @@ type UserProfile struct {
 }
 
 type UserRequest struct {
-	UserID string `json:"userId" jsonschema_description:"User ID"`
+	UserID string `json:"userId" jsonschema_description:"User ID" jsonschema:"required"`
 }
 
 type Asset struct {
@@ -43,46 +43,45 @@ type Asset struct {
 }
 
 type AssetListRequest struct {
-	Limit int `json:"limit,omitempty" jsonschema_description:"Number of assets to return"`
+	Limit int `json:"limit,omitempty" jsonschema_description:"Number of assets to return" jsonschema:"minimum=1,maximum=100,default=10"`
 }
 
 func main() {
 	s := server.NewMCPServer(
-		"Structured Output Example",
+		"Structured Input/Output Example",
 		"1.0.0",
 		server.WithToolCapabilities(false),
 	)
 
 	// Example 1: Auto-generated schema from struct
 	weatherTool := mcp.NewTool("get_weather",
 		mcp.WithDescription("Get weather with structured output"),
+		mcp.WithInputSchema[WeatherRequest](),
 		mcp.WithOutputSchema[WeatherResponse](),
-		mcp.WithString("location", mcp.Required()),
-		mcp.WithString("units", mcp.Enum("celsius", "fahrenheit"), mcp.DefaultString("celsius")),
 	)
 	s.AddTool(weatherTool, mcp.NewStructuredToolHandler(getWeatherHandler))
 
 	// Example 2: Nested struct schema
 	userTool := mcp.NewTool("get_user_profile",
 		mcp.WithDescription("Get user profile"),
+		mcp.WithInputSchema[UserRequest](),
 		mcp.WithOutputSchema[UserProfile](),
-		mcp.WithString("userId", mcp.Required()),
 	)
 	s.AddTool(userTool, mcp.NewStructuredToolHandler(getUserProfileHandler))
 
 	// Example 3: Array output - direct array of objects
 	assetsTool := mcp.NewTool("get_assets",
 		mcp.WithDescription("Get list of assets as array"),
+		mcp.WithInputSchema[AssetListRequest](),
 		mcp.WithOutputSchema[[]Asset](),
-		mcp.WithNumber("limit", mcp.Min(1), mcp.Max(100), mcp.DefaultNumber(10)),
 	)
 	s.AddTool(assetsTool, mcp.NewStructuredToolHandler(getAssetsHandler))
 
 	// Example 4: Manual result creation
 	manualTool := mcp.NewTool("manual_structured",
 		mcp.WithDescription("Manual structured result"),
+		mcp.WithInputSchema[WeatherRequest](),
 		mcp.WithOutputSchema[WeatherResponse](),
-		mcp.WithString("location", mcp.Required()),
 	)
 	s.AddTool(manualTool, mcp.NewTypedToolHandler(manualWeatherHandler))
 

mcp/tools.go

@@ -704,6 +704,47 @@ func WithDescription(description string) ToolOption {
 	}
 }
 
+// WithInputSchema creates a ToolOption that sets the input schema for a tool.
+// It accepts any Go type, usually a struct, and automatically generates a JSON schema from it.
+func WithInputSchema[T any]() ToolOption {
+	return func(t *Tool) {
+		var zero T
+
+		// Generate schema using invopop/jsonschema library
+		// Configure reflector to generate clean, MCP-compatible schemas
+		reflector := jsonschema.Reflector{
+			DoNotReference:            true, // Removes $defs map, outputs entire structure inline
+			Anonymous:                 true, // Hides auto-generated Schema IDs
+			AllowAdditionalProperties: true, // Removes additionalProperties: false
+		}
+		schema := reflector.Reflect(zero)
+
+		// Clean up schema for MCP compliance
+		schema.Version = "" // Remove $schema field
+
+		// Convert to raw JSON for MCP
+		mcpSchema, err := json.Marshal(schema)
+		if err != nil {
+			// Skip and maintain backward compatibility
+			return
+		}
+
+		t.InputSchema.Type = ""
+		t.RawInputSchema = json.RawMessage(mcpSchema)
+	}
+}
+
+// WithRawInputSchema sets a raw JSON schema for the tool's input.
+// Use this when you need full control over the schema or when working with
+// complex schemas that can't be generated from Go types. The jsonschema library
+// can handle complex schemas and provides nice extension points, so be sure to
+// check that out before using this.
+func WithRawInputSchema(schema json.RawMessage) ToolOption {
+	return func(t *Tool) {
+		t.RawInputSchema = schema
+	}
+}
+
 // WithOutputSchema creates a ToolOption that sets the output schema for a tool.
 // It accepts any Go type, usually a struct, and automatically generates a JSON schema from it.
 func WithOutputSchema[T any]() ToolOption {

mcp/tools_test.go

@@ -529,6 +529,55 @@ func TestFlexibleArgumentsJSONMarshalUnmarshal(t *testing.T) {
 	assert.Equal(t, float64(123), args["key2"]) // JSON numbers are unmarshaled as float64
 }
 
+// TestToolWithInputSchema tests that the WithInputSchema function
+// generates an MCP-compatible JSON output schema for a tool
+func TestToolWithInputSchema(t *testing.T) {
+	type TestInput struct {
+		Name  string `json:"name" jsonschema_description:"Person's name" jsonschema:"required"`
+		Age   int    `json:"age" jsonschema_description:"Person's age"`
+		Email string `json:"email,omitempty" jsonschema_description:"Email address" jsonschema:"required"`
+	}
+
+	tool := NewTool("test_tool",
+		WithDescription("Test tool with output schema"),
+		WithInputSchema[TestInput](),
+	)
+
+	// Check that RawOutputSchema was set
+	assert.NotNil(t, tool.RawInputSchema)
+
+	// Marshal and verify structure
+	data, err := json.Marshal(tool)
+	assert.NoError(t, err)
+
+	var toolData map[string]any
+	err = json.Unmarshal(data, &toolData)
+	assert.NoError(t, err)
+
+	// Verify inputSchema exists
+	inputSchema, exists := toolData["inputSchema"]
+	assert.True(t, exists)
+	assert.NotNil(t, inputSchema)
+
+	// Verify required list exists
+	schemaMap, ok := inputSchema.(map[string]interface{})
+	assert.True(t, ok)
+	requiredList, exists := schemaMap["required"]
+	assert.True(t, exists)
+	assert.NotNil(t, requiredList)
+
+	// Verify properties exist
+	properties, exists := schemaMap["properties"]
+	assert.True(t, exists)
+	propertiesMap, ok := properties.(map[string]interface{})
+	assert.True(t, ok)
+
+	// Verify specific properties
+	assert.Contains(t, propertiesMap, "name")
+	assert.Contains(t, propertiesMap, "age")
+	assert.Contains(t, propertiesMap, "email")
+}
+
 // TestToolWithOutputSchema tests that the WithOutputSchema function
 // generates an MCP-compatible JSON output schema for a tool
 func TestToolWithOutputSchema(t *testing.T) {