mcp: add syntax and scheme validation to AddResourceTemplate (#253)

Add template validation and scheme check in `AddResourceTemplate`.

mcp/server.go

@@ -22,6 +22,7 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/internal/util"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
+	"github.com/yosida95/uritemplate/v3"
 )
 
 const DefaultPageSize = 1000
@@ -229,7 +230,19 @@ func (s *Server) RemoveResources(uris ...string) {
 func (s *Server) AddResourceTemplate(t *ResourceTemplate, h ResourceHandler) {
 	s.changeAndNotify(notificationResourceListChanged, &ResourceListChangedParams{},
 		func() bool {
-			// TODO: check template validity.
+			// Validate the URI template syntax
+			_, err := uritemplate.New(t.URITemplate)
+			if err != nil {
+				panic(fmt.Errorf("URI template %q is invalid: %w", t.URITemplate, err))
+			}
+			// Ensure the URI template has a valid scheme
+			u, err := url.Parse(t.URITemplate)
+			if err != nil {
+				panic(err) // url.Parse includes the URI in the error
+			}
+			if !u.IsAbs() {
+				panic(fmt.Errorf("URI template %q needs a scheme", t.URITemplate))
+			}
 			s.resourceTemplates.add(&serverResourceTemplate{t, h})
 			return true
 		})

mcp/server_test.go

@@ -373,6 +373,42 @@ func TestServerCapabilities(t *testing.T) {
 	}
 }
 
+func TestServerAddResourceTemplate(t *testing.T) {
+	tests := []struct {
+		name        string
+		template    string
+		expectPanic bool
+	}{
+		{"ValidFileTemplate", "file:///{a}/{b}", false},
+		{"ValidCustomScheme", "myproto:///{a}", false},
+		{"MissingScheme1", "://example.com/{path}", true},
+		{"MissingScheme2", "/api/v1/users/{id}", true},
+		{"EmptyVariable", "file:///{}/{b}", true},
+		{"UnclosedVariable", "file:///{a", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rt := ResourceTemplate{URITemplate: tt.template}
+
+			defer func() {
+				if r := recover(); r != nil {
+					if !tt.expectPanic {
+						t.Errorf("%s: unexpected panic: %v", tt.name, r)
+					}
+				} else {
+					if tt.expectPanic {
+						t.Errorf("%s: expected panic but did not panic", tt.name)
+					}
+				}
+			}()
+
+			s := NewServer(testImpl, nil)
+			s.AddResourceTemplate(&rt, nil)
+		})
+	}
+}
+
 // TestServerSessionkeepaliveCancelOverwritten is to verify that `ServerSession.keepaliveCancel` is assigned exactly once,
 // ensuring that only a single goroutine is responsible for the session's keepalive ping mechanism.
 func TestServerSessionkeepaliveCancelOverwritten(t *testing.T) {