mcp: make the streamable client transport less strict (#524)

In practice, the streamable client transport was having trouble
connecting to various backends, because they are nonconformant in
various ways.

While it would be nice if all servers conformed to the spec, in practice
there are certain spec violations that are recoverable, and we can and
should recover them. Specifically:

- tolerate 404 instead of 405 for the hanging GET (#393)
- tolerate (=ignore) spurious response body for notifications and
responses, since we know none are expected

Additionally, fix a bug that we weren't parsing Content-Type correctly.

Fixes #521

examples/client/listfeatures/main.go

@@ -27,30 +27,51 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
+var (
+	endpoint = flag.String("http", "", "if set, connect to this streamable endpoint rather than running a stdio server")
+)
+
 func main() {
 	flag.Parse()
 	args := flag.Args()
-	if len(args) == 0 {
+	if len(args) == 0 && *endpoint == "" {
 		fmt.Fprintln(os.Stderr, "Usage: listfeatures <command> [<args>]")
+		fmt.Fprintln(os.Stderr, "Usage: listfeatures --http=\"https://example.com/server/mcp\"")
 		fmt.Fprintln(os.Stderr, "List all features for a stdio MCP server")
 		fmt.Fprintln(os.Stderr)
 		fmt.Fprintln(os.Stderr, "Example:\n\tlistfeatures npx @modelcontextprotocol/server-everything")
 		os.Exit(2)
 	}
 
-	ctx := context.Background()
-	cmd := exec.Command(args[0], args[1:]...)
+	var (
+		ctx       = context.Background()
+		transport mcp.Transport
+	)
+	if *endpoint != "" {
+		transport = &mcp.StreamableClientTransport{
+			Endpoint: *endpoint,
+		}
+	} else {
+		cmd := exec.Command(args[0], args[1:]...)
+		transport = &mcp.CommandTransport{Command: cmd}
+	}
 	client := mcp.NewClient(&mcp.Implementation{Name: "mcp-client", Version: "v1.0.0"}, nil)
-	cs, err := client.Connect(ctx, &mcp.CommandTransport{Command: cmd}, nil)
+	cs, err := client.Connect(ctx, transport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer cs.Close()
 
-	printSection("tools", cs.Tools(ctx, nil), func(t *mcp.Tool) string { return t.Name })
-	printSection("resources", cs.Resources(ctx, nil), func(r *mcp.Resource) string { return r.Name })
-	printSection("resource templates", cs.ResourceTemplates(ctx, nil), func(r *mcp.ResourceTemplate) string { return r.Name })
-	printSection("prompts", cs.Prompts(ctx, nil), func(p *mcp.Prompt) string { return p.Name })
+	if cs.InitializeResult().Capabilities.Tools != nil {
+		printSection("tools", cs.Tools(ctx, nil), func(t *mcp.Tool) string { return t.Name })
+	}
+	if cs.InitializeResult().Capabilities.Resources != nil {
+		printSection("resources", cs.Resources(ctx, nil), func(r *mcp.Resource) string { return r.Name })
+		printSection("resource templates", cs.ResourceTemplates(ctx, nil), func(r *mcp.ResourceTemplate) string { return r.Name })
+	}
+	if cs.InitializeResult().Capabilities.Prompts != nil {
+		printSection("prompts", cs.Prompts(ctx, nil), func(p *mcp.Prompt) string { return p.Name })
+	}
 }
 
 func printSection[T any](name string, features iter.Seq2[T, error], featName func(T) string) {