feat(server/sse): Add support for dynamic base paths

This change introduces the ability to mount SSE endpoints at dynamic
paths with variable segments (e.g., `/api/{tenant}/sse`) by adding a new
`WithDynamicBasePath` option and related functionality. This enables
advanced use cases such as multi-tenant architectures or integration
with routers that support path parameters.

Key Features:

* DynamicBasePathFunc: New function type and option
  (WithDynamicBasePath) to generate the SSE server's base path
  dynamically per request/session.
* Flexible Routing: New SSEHandler() and MessageHandler() methods allow
  mounting handlers at arbitrary or dynamic paths using any router
  (e.g., net/http, chi, gorilla/mux).
* Endpoint Generation: GetMessageEndpointForClient now supports both
  static and dynamic path modes, and correctly generates full URLs when
  configured.
* Example: Added examples/dynamic_path/main.go demonstrating dynamic
  path mounting and usage.

```go
mcpServer := mcp.NewMCPServer("dynamic-path-example", "1.0.0")
sseServer := mcp.NewSSEServer(
  mcpServer,
  mcp.WithDynamicBasePath(func(r *http.Request, sessionID string) string {
    tenant := r.PathValue("tenant")
    return "/api/" + tenant
  }),
  mcp.WithBaseURL("http://localhost:8080"),
)

mux := http.NewServeMux()
mux.Handle("/api/{tenant}/sse", sseServer.SSEHandler())
mux.Handle("/api/{tenant}/message", sseServer.MessageHandler())
```

Author: rwjblue-glean

examples/dynamic_path/main.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"net/http"
+
+	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/mark3labs/mcp-go/server"
+)
+
+func main() {
+	var addr string
+	flag.StringVar(&addr, "addr", ":8080", "address to listen on")
+	flag.Parse()
+
+	mcpServer := server.NewMCPServer("dynamic-path-example", "1.0.0")
+
+	// Add a trivial tool for demonstration
+	mcpServer.AddTool(mcp.NewTool("echo"), func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+		return mcp.NewToolResultText(fmt.Sprintf("Echo: %v", req.Params.Arguments["message"])), nil
+	})
+
+	// Use a dynamic base path based on a path parameter (Go 1.22+)
+	sseServer := server.NewSSEServer(
+		mcpServer,
+		server.WithDynamicBasePath(func(r *http.Request, sessionID string) string {
+			tenant := r.PathValue("tenant")
+			return "/api/" + tenant
+		}),
+		server.WithBaseURL("http://localhost:8080"),
+		server.WithUseFullURLForMessageEndpoint(true),
+	)
+
+	mux := http.NewServeMux()
+	mux.Handle("/api/{tenant}/sse", sseServer.SSEHandler())
+	mux.Handle("/api/{tenant}/message", sseServer.MessageHandler())
+
+	log.Printf("Dynamic SSE server listening on %s", addr)
+	if err := http.ListenAndServe(addr, mux); err != nil {
+		log.Fatalf("Server error: %v", err)
+	}
+} 

server/sse.go

@@ -33,6 +33,13 @@ type sseSession struct {
 // content. This can be used to inject context values from headers, for example.
 type SSEContextFunc func(ctx context.Context, r *http.Request) context.Context
 
+// DynamicBasePathFunc allows the user to provide a function to generate the
+// base path for a given request and sessionID. This is useful for cases where
+// the base path is not known at the time of SSE server creation, such as when
+// using a reverse proxy or when the base path is dynamically generated. The
+// function should return the base path (e.g., "/mcp/tenant123").
+type DynamicBasePathFunc func(r *http.Request, sessionID string) string
+
 func (s *sseSession) SessionID() string {
 	return s.sessionID
 }
@@ -68,6 +75,9 @@ type SSEServer struct {
 	keepAliveInterval time.Duration
 
 	mu sync.RWMutex
+
+	// user-provided function for determining the dynamic base path
+	dynamicBasePathFunc DynamicBasePathFunc
 }
 
 // SSEOption defines a function type for configuring SSEServer
@@ -96,7 +106,7 @@ func WithBaseURL(baseURL string) SSEOption {
 	}
 }
 
-// Add a new option for setting base path
+// Add a new option for setting a static base path
 func WithBasePath(basePath string) SSEOption {
 	return func(s *SSEServer) {
 		// Ensure the path starts with / and doesn't end with /
@@ -107,6 +117,16 @@ func WithBasePath(basePath string) SSEOption {
 	}
 }
 
+// WithDynamicBasePath accepts a function for generating the base path. This is
+// useful for cases where the base path is not known at the time of SSE server
+// creation, such as when using a reverse proxy or when the server is mounted
+// at a dynamic path.
+func WithDynamicBasePath(fn DynamicBasePathFunc) SSEOption {
+	return func(s *SSEServer) {
+		s.dynamicBasePathFunc = fn
+	}
+}
+
 // WithMessageEndpoint sets the message endpoint path
 func WithMessageEndpoint(endpoint string) SSEOption {
 	return func(s *SSEServer) {
@@ -308,7 +328,8 @@ func (s *SSEServer) handleSSE(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Send the initial endpoint event
-	fmt.Fprintf(w, "event: endpoint\ndata: %s\r\n\r\n", s.GetMessageEndpointForClient(sessionID))
+	endpoint := s.GetMessageEndpointForClient(r, sessionID)
+	fmt.Fprintf(w, "event: endpoint\ndata: %s\r\n\r\n", endpoint)
 	flusher.Flush()
 
 	// Main event loop - this runs in the HTTP handler goroutine
@@ -328,13 +349,20 @@ func (s *SSEServer) handleSSE(w http.ResponseWriter, r *http.Request) {
 }
 
 // GetMessageEndpointForClient returns the appropriate message endpoint URL with session ID
-// based on the useFullURLForMessageEndpoint configuration.
-func (s *SSEServer) GetMessageEndpointForClient(sessionID string) string {
-	messageEndpoint := s.messageEndpoint
-	if s.useFullURLForMessageEndpoint {
-		messageEndpoint = s.CompleteMessageEndpoint()
+// for the given request. This is the canonical way to compute the message endpoint for a client.
+// It handles both dynamic and static path modes, and honors the WithUseFullURLForMessageEndpoint flag.
+func (s *SSEServer) GetMessageEndpointForClient(r *http.Request, sessionID string) string {
+	basePath := s.basePath
+	if s.dynamicBasePathFunc != nil {
+		basePath = s.dynamicBasePathFunc(r, sessionID)
+	}
+
+	endpointPath := basePath + s.messageEndpoint
+	if s.useFullURLForMessageEndpoint && s.baseURL != "" {
+		endpointPath = s.baseURL + endpointPath
 	}
-	return fmt.Sprintf("%s?sessionId=%s", messageEndpoint, sessionID)
+
+	return fmt.Sprintf("%s?sessionId=%s", endpointPath, sessionID)
 }
 
 // handleMessage processes incoming JSON-RPC messages from clients and sends responses
@@ -447,6 +475,9 @@ func (s *SSEServer) GetUrlPath(input string) (string, error) {
 }
 
 func (s *SSEServer) CompleteSseEndpoint() string {
+	if s.dynamicBasePathFunc != nil {
+		panic("CompleteSseEndpoint cannot be used with WithDynamicBasePath. Use dynamic path logic in your router.")
+	}
 	return s.baseURL + s.basePath + s.sseEndpoint
 }
 
@@ -459,6 +490,9 @@ func (s *SSEServer) CompleteSsePath() string {
 }
 
 func (s *SSEServer) CompleteMessageEndpoint() string {
+	if s.dynamicBasePathFunc != nil {
+		panic("CompleteMessageEndpoint cannot be used with WithDynamicBasePath. Use dynamic path logic in your router.")
+	}
 	return s.baseURL + s.basePath + s.messageEndpoint
 }
 
@@ -470,8 +504,69 @@ func (s *SSEServer) CompleteMessagePath() string {
 	return path
 }
 
+// SSEHandler returns an http.Handler for the SSE endpoint.
+//
+// This method allows you to mount the SSE handler at any arbitrary path
+// using your own router (e.g. net/http, gorilla/mux, chi, etc.). It is
+// intended for advanced scenarios where you want to control the routing or
+// support dynamic segments.
+//
+// IMPORTANT: When using this handler in advanced/dynamic mounting scenarios,
+// you must use the WithDynamicBasePath option to ensure the correct base path
+// is communicated to clients.
+//
+// Example usage:
+//
+//	// Advanced/dynamic:
+//	sseServer := NewSSEServer(mcpServer,
+//		WithDynamicBasePath(func(r *http.Request, sessionID string) string {
+//			tenant := r.PathValue("tenant")
+//			return "/mcp/" + tenant
+//		}),
+//		WithBaseURL("http://localhost:8080")
+//	)
+//	mux.Handle("/mcp/{tenant}/sse", sseServer.SSEHandler())
+//	mux.Handle("/mcp/{tenant}/message", sseServer.MessageHandler())
+//
+// For non-dynamic cases, use ServeHTTP method instead.
+func (s *SSEServer) SSEHandler() http.Handler {
+	return http.HandlerFunc(s.handleSSE)
+}
+
+// MessageHandler returns an http.Handler for the message endpoint.
+//
+// This method allows you to mount the message handler at any arbitrary path
+// using your own router (e.g. net/http, gorilla/mux, chi, etc.). It is
+// intended for advanced scenarios where you want to control the routing or
+// support dynamic segments.
+//
+// IMPORTANT: When using this handler in advanced/dynamic mounting scenarios,
+// you must use the WithDynamicBasePath option to ensure the correct base path
+// is communicated to clients.
+//
+// Example usage:
+//
+//	// Advanced/dynamic:
+//	sseServer := NewSSEServer(mcpServer,
+//		WithDynamicBasePath(func(r *http.Request, sessionID string) string {
+//			tenant := r.PathValue("tenant")
+//			return "/mcp/" + tenant
+//		}),
+//		WithBaseURL("http://localhost:8080")
+//	)
+//	mux.Handle("/mcp/{tenant}/sse", sseServer.SSEHandler())
+//	mux.Handle("/mcp/{tenant}/message", sseServer.MessageHandler())
+//
+// For non-dynamic cases, use ServeHTTP method instead.
+func (s *SSEServer) MessageHandler() http.Handler {
+	return http.HandlerFunc(s.handleMessage)
+}
+
 // ServeHTTP implements the http.Handler interface.
 func (s *SSEServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if s.dynamicBasePathFunc != nil {
+		panic("ServeHTTP cannot be used with WithDynamicBasePath. Use SSEHandler/MessageHandler and mount them with your router.")
+	}
 	path := r.URL.Path
 	// Use exact path matching rather than Contains
 	ssePath := s.CompleteSsePath()