Add Callbacks Feature

MCPServer is upgraded with the capacity to notify before and/or
after requests run.  Developers can choose Before / After events,
and can submit callbacks that are notified during EVERY request
lifecycle, or else specific request methods.  The specific request
method callbacks receive typed message and/or result objects.

A special OnError callback is also provided.

To accomplish this with a minimal margin for error, a bit of a
refactoring was done:

- The `MCPServer.HandleMessage` method was moved from server.go
  into its own file.
- The individual request handlers' prototypes were changed.
  rather than returning `mcp.JSONRPCMessage`, they return a tuple
  of their specific result type and error.
- A code generation schema was introduced, to facilitate adding
  functionality to each clause in the switch statement within
  `HandleMessage()`.
- Now that HandleMessage() now receives a typed result object
  and possible error, it is able to pass those results to any
  configured callbacks.

I considered several options including a strategy pattern to make
HandleMessage() more DRY, but opted to stick with the minimal
reorganization of the code, and to keep the algorithm as flat
and obvious as possible.  I think the codegen mitigates the
repetitiveness of that section of code.

Author: tylergannon

README.md

@@ -509,6 +509,18 @@ Prompts can include:
 
 For examples, see the `examples/` directory.
 
+## Extras
+
+### Request Callbacks
+
+Hook into the request lifecycle by creating a `Callbacks` object with your
+selection among the possible callbacks.  This enables telemetry across all
+functionality, and observability of various facts, for example the ability
+to count improperly-formatted requests, or to log the agent identity during
+initialization.
+
+Add the `Callbacks` to the server at the time of creation using the
+`server.WithCallbacks` option.
 
 ## Contributing
 

examples/everything/main.go

@@ -29,12 +29,38 @@ const (
 )
 
 func NewMCPServer() *server.MCPServer {
+
+	callbacks := &server.Callbacks{}
+
+	callbacks.AddBeforeAny(func(id any, method mcp.MCPMethod, message any) {
+		fmt.Printf("beforeAny: %s, %v, %v\n", method, id, message)
+	})
+	callbacks.AddAfterAny(func(id any, method mcp.MCPMethod, message any, result any) {
+		fmt.Printf("afterAny: %s, %v, %v, %v\n", method, id, message, result)
+	})
+	callbacks.AddOnError(func(id any, method mcp.MCPMethod, message any, err error) {
+		fmt.Printf("onError: %s, %v, %v, %v\n", method, id, message, err)
+	})
+	callbacks.AddBeforeInitialize(func(id any, message *mcp.InitializeRequest) {
+		fmt.Printf("beforeInitialize: %v, %v\n", id, message)
+	})
+	callbacks.AddAfterInitialize(func(id any, message *mcp.InitializeRequest, result *mcp.InitializeResult) {
+		fmt.Printf("afterInitialize: %v, %v, %v\n", id, message, result)
+	})
+	callbacks.AddAfterCallTool(func(id any, message *mcp.CallToolRequest, result *mcp.CallToolResult) {
+		fmt.Printf("afterCallTool: %v, %v, %v\n", id, message, result)
+	})
+	callbacks.AddBeforeCallTool(func(id any, message *mcp.CallToolRequest) {
+		fmt.Printf("beforeCallTool: %v, %v\n", id, message)
+	})
+
 	mcpServer := server.NewMCPServer(
 		"example-servers/everything",
 		"1.0.0",
 		server.WithResourceCapabilities(true, true),
 		server.WithPromptCapabilities(true),
 		server.WithLogging(),
+		server.WithCallbacks(callbacks),
 	)
 
 	mcpServer.AddResource(mcp.NewResource("test://static/resource",

go.mod

@@ -5,11 +5,11 @@ go 1.23
 require (
 	github.com/google/uuid v1.6.0
 	github.com/stretchr/testify v1.9.0
+	github.com/yosida95/uritemplate/v3 v3.0.2
 )
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

mcp/types.go

@@ -8,6 +8,20 @@ import (
 	"github.com/yosida95/uritemplate/v3"
 )
 
+type MCPMethod string
+
+const (
+	MethodInitialize             MCPMethod = "initialize"
+	MethodPing                   MCPMethod = "ping"
+	MethodResourcesList          MCPMethod = "resources/list"
+	MethodResourcesTemplatesList MCPMethod = "resources/templates/list"
+	MethodResourcesRead          MCPMethod = "resources/read"
+	MethodPromptsList            MCPMethod = "prompts/list"
+	MethodPromptsGet             MCPMethod = "prompts/get"
+	MethodToolsList              MCPMethod = "tools/list"
+	MethodToolsCall              MCPMethod = "tools/call"
+)
+
 type URITemplate struct {
 	*uritemplate.Template
 }