feat(mcp): optional bearer-token auth via SUPERVISOR_MCP_TOKEN

Enables exposing the supervisor MCP via Tailscale Funnel for claude.ai
custom-connector tests. Auth is opt-in: empty SUPERVISOR_MCP_TOKEN
preserves the existing unauthenticated behavior for tailnet-internal
callers and local dev.

When the token is set, every request must carry
"Authorization: Bearer <token>" or it is rejected with HTTP 401 and a
JSON-RPC -32001 error. Comparison uses crypto/subtle.ConstantTimeCompare;
the token value and the supplied header are never logged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/mcp/server.go

@@ -2,8 +2,11 @@ package mcp
 
 import (
 	"context"
+	"crypto/subtle"
 	"encoding/json"
+	"log/slog"
 	"net/http"
+	"strings"
 
 	"github.com/mathiasbq/supervisor/internal/registry"
 )
@@ -29,14 +32,22 @@ type rpcError struct {
 
 // Server is an HTTP handler implementing the MCP JSON-RPC protocol.
 type Server struct {
-	reg *registry.Registry
+	reg   *registry.Registry
+	token string
 }
 
-func NewServer(reg *registry.Registry) *Server {
-	return &Server{reg: reg}
+// NewServer constructs an MCP HTTP handler. If token is non-empty, every
+// request must carry "Authorization: Bearer <token>" or it is rejected with
+// HTTP 401 and JSON-RPC error -32001. Empty token disables auth (default).
+func NewServer(reg *registry.Registry, token string) *Server {
+	return &Server{reg: reg, token: token}
 }
 
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if !s.checkAuth(w, r) {
+		return
+	}
+
 	var req request
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, nil, -32700, "parse error")
@@ -93,6 +104,29 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// checkAuth verifies the bearer token when one is configured. Returns true if
+// the request may proceed, false if it has been rejected (401 already written).
+func (s *Server) checkAuth(w http.ResponseWriter, r *http.Request) bool {
+	if s.token == "" {
+		return true
+	}
+
+	const prefix = "Bearer "
+	hdr := r.Header.Get("Authorization")
+	if !strings.HasPrefix(hdr, prefix) ||
+		subtle.ConstantTimeCompare([]byte(hdr[len(prefix):]), []byte(s.token)) != 1 {
+		slog.Warn("mcp auth rejected", "remote", r.RemoteAddr, "method", r.Method)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusUnauthorized)
+		_ = json.NewEncoder(w).Encode(response{
+			JSONRPC: "2.0",
+			Error:   &rpcError{Code: -32001, Message: "unauthorized"},
+		})
+		return false
+	}
+	return true
+}
+
 func writeError(w http.ResponseWriter, id any, code int, msg string) {
 	w.Header().Set("Content-Type", "application/json")
 	_ = json.NewEncoder(w).Encode(response{

internal/mcp/server_test.go

@@ -23,7 +23,7 @@ func jsonBody(t *testing.T, v any) *bytes.Buffer {
 
 func TestMCPInitialize(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg)
+	srv := mcp.NewServer(reg, "")
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0",
@@ -45,7 +45,7 @@ func TestMCPInitialize(t *testing.T) {
 
 func TestMCPToolsList(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg)
+	srv := mcp.NewServer(reg, "")
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0", "id": 2, "method": "tools/list", "params": map[string]any{},
@@ -63,7 +63,7 @@ func TestMCPToolsList(t *testing.T) {
 
 func TestMCPUnknownMethod(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg)
+	srv := mcp.NewServer(reg, "")
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0", "id": 3, "method": "unknown/method", "params": map[string]any{},
@@ -80,7 +80,7 @@ func TestMCPUnknownMethod(t *testing.T) {
 
 func TestMCPNotificationKnownMethodGetsNoResponseBody(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg)
+	srv := mcp.NewServer(reg, "")
 
 	// JSON-RPC 2.0 notification: "id" field absent. Per spec, server MUST NOT
 	// reply. notifications/initialized is part of the standard MCP handshake.
@@ -97,9 +97,52 @@ func TestMCPNotificationKnownMethodGetsNoResponseBody(t *testing.T) {
 		"notifications must not receive a response body")
 }
 
+func TestMCPAuth(t *testing.T) {
+	const token = "s3cr3t"
+
+	cases := []struct {
+		name       string
+		token      string
+		authHeader string
+		wantStatus int
+	}{
+		{"no token configured passes without header", "", "", http.StatusOK},
+		{"correct bearer passes", token, "Bearer " + token, http.StatusOK},
+		{"wrong bearer rejected", token, "Bearer wrong", http.StatusUnauthorized},
+		{"missing header rejected", token, "", http.StatusUnauthorized},
+		{"wrong scheme rejected", token, "Basic " + token, http.StatusUnauthorized},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			reg := registry.New()
+			srv := mcp.NewServer(reg, tc.token)
+
+			req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
+				"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": map[string]any{},
+			}))
+			req.Header.Set("Content-Type", "application/json")
+			if tc.authHeader != "" {
+				req.Header.Set("Authorization", tc.authHeader)
+			}
+			rr := httptest.NewRecorder()
+			srv.ServeHTTP(rr, req)
+
+			assert.Equal(t, tc.wantStatus, rr.Code)
+			if tc.wantStatus == http.StatusUnauthorized {
+				var resp map[string]any
+				require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+				rpcErr, ok := resp["error"].(map[string]any)
+				require.True(t, ok, "expected error object in response")
+				assert.Equal(t, float64(-32001), rpcErr["code"])
+			}
+		})
+	}
+}
+
 func TestMCPNotificationUnknownMethodGetsNoResponseBody(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg)
+	srv := mcp.NewServer(reg, "")
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0",