MCP Auth OAUTH

MCP Auth — OAuth 2.1

Remote MCP servers use OAuth 2.1 + PKCE with Dynamic Client Registration. Any spec-compliant MCP client — Claude Desktop, Claude Code, Cursor, ChatGPT Apps — can authenticate users without hardcoded client IDs.

When to Use

• Building a remote MCP server that serves multiple users
• Migrating a static-bearer-token server to per-user auth
• Debugging "401 Unauthorized" or "failed to authenticate" in an MCP client
• Adding scope-based permission to an MCP server

The Flow (RFC 8414 + RFC 7591 + PKCE)

1. Client connects to MCP server → server returns 401 with WWW-Authenticate pointing to the authorization server metadata URL
2. Client fetches /.well-known/oauth-authorization-server → gets authorization_endpoint, token_endpoint, registration_endpoint
3. Client POSTs to registration_endpoint (RFC 7591) → receives a client_id dynamically
4. Client opens browser to authorization_endpoint with PKCE challenge → user approves
5. Client exchanges code at token_endpoint → receives access + refresh tokens
6. Client retries MCP request with Authorization: Bearer <token>

No pre-shared client ID. No manual app registration. The user just clicks "Allow" in a browser.

Server-Side Requirements

1. Return discovery metadata

// GET /.well-known/oauth-authorization-server
app.get("/.well-known/oauth-authorization-server", (req, res) => {
  res.json({
    issuer: "https://mcp.example.com",
    authorization_endpoint: "https://mcp.example.com/oauth/authorize",
    token_endpoint: "https://mcp.example.com/oauth/token",
    registration_endpoint: "https://mcp.example.com/oauth/register",
    scopes_supported: ["read:issues", "write:issues"],
    response_types_supported: ["code"],
    grant_types_supported: ["authorization_code", "refresh_token"],
    code_challenge_methods_supported: ["S256"],
    token_endpoint_auth_methods_supported: ["none"], // public clients
  });
});

2. 401 with WWW-Authenticate

app.use("/mcp", (req, res, next) => {
  const token = req.headers.authorization?.replace("Bearer ", "");
  if (!token || !verifyJwt(token)) {
    res.setHeader(
      "WWW-Authenticate",
      `Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource"`,
    );
    return res.status(401).json({ error: "unauthorized" });
  }
  req.user = decodeJwt(token);
  next();
});

3. Dynamic Client Registration (RFC 7591)

app.post("/oauth/register", async (req, res) => {
  const { redirect_uris, client_name } = req.body;
  const clientId = `mcp_${crypto.randomUUID()}`;
  await db.clients.insert({ clientId, redirectUris: redirect_uris, name: client_name });
  res.json({
    client_id: clientId,
    redirect_uris,
    token_endpoint_auth_method: "none",
    grant_types: ["authorization_code", "refresh_token"],
  });
});

4. Authorization endpoint (PKCE S256)

app.get("/oauth/authorize", async (req, res) => {
  const { client_id, redirect_uri, code_challenge, code_challenge_method, state, scope } = req.query;
  if (code_challenge_method !== "S256") return res.status(400).send("S256 required");
  // render consent UI; on approval:
  const code = crypto.randomUUID();
  await db.codes.insert({ code, clientId: client_id, userId: req.session.userId, codeChallenge: code_challenge, scope });
  res.redirect(`${redirect_uri}?code=${code}&state=${state}`);
});

5. Token endpoint (PKCE verify)

app.post("/oauth/token", async (req, res) => {
  const { grant_type, code, code_verifier, refresh_token } = req.body;
  if (grant_type === "authorization_code") {
    const row = await db.codes.findAndDelete({ code });
    const hash = crypto.createHash("sha256").update(code_verifier).digest("base64url");
    if (hash !== row.codeChallenge) return res.status(400).json({ error: "invalid_grant" });
    return res.json(issueTokens(row.userId, row.scope));
  }
  if (grant_type === "refresh_token") {
    const userId = verifyRefresh(refresh_token);
    return res.json(issueTokens(userId));
  }
});

Scope Design

Group by resource + action, not by tool:

read:issues        → list/get issues
write:issues       → create/update/close issues
read:repos         → list/get repos
admin:webhooks     → create/delete webhooks

Don't use full_access — clients can't reason about what they're granting.

Map scopes to MCP tools in your handler:

server.tool("create_issue", "...", schema, async (args, { authInfo }) => {
  if (!authInfo.scopes.includes("write:issues")) {
    throw new Error("Missing scope: write:issues. Reconnect with this scope enabled.");
  }
  // ...
});

Token Lifetimes

• Access token: 1 hour (JWT, stateless)
• Refresh token: 30 days, rotating (one-time use, issue new one on each refresh)
• Authorization code: 60 seconds, one-time

Client-Side (Reference Only)

Claude Desktop, Claude Code, and the Claude Agent SDK handle all of this automatically when you specify an url server without headers:

{
  "mcpServers": {
    "linear": { "url": "https://mcp.linear.app/sse" }
  }
}

Debugging

1. Browser never opens: check WWW-Authenticate header is present on 401
2. "Client registration failed": your /oauth/register must accept application/json and not require auth
3. "Invalid grant": PKCE verifier mismatch — the code was issued with a different challenge than the token request is using
4. Scope escalation: client asks for scopes you don't advertise in discovery → reject with invalid_scope
5. Token expires mid-session: client should refresh on 401; if your server returns 403 for expired tokens, fix it to return 401

Best Practices

1. Always require PKCE S256 — reject plain and missing challenges
2. Rotate refresh tokens on every use; revoke entire chain on reuse detection
3. Bind tokens to the client_id that issued them — block cross-client token use
4. Expose a /.well-known/oauth-protected-resource metadata doc per MCP 2025 spec
5. Log every token issuance with user, client, scope, IP for audit
6. Never log tokens themselves; log JTI (JWT ID) instead