MCP OAuth Remote Gateway

MCP OAuth on a Remote Hermes Gateway

Overview

Hermes' built-in MCP OAuth client runs a one-shot HTTP listener on 127.0.0.1:<port> inside the Hermes process and registers that loopback address as the OAuth redirect_uri. That works perfectly for a local CLI on the user's own machine. It breaks completely when Hermes runs as a remote gateway (container, VPS, messaging bot), because the user's browser resolves 127.0.0.1 to the user's own laptop, not the remote container — so the authorization code never reaches Hermes.

This skill does the OAuth dance by hand and writes the resulting tokens into the exact files Hermes' token storage expects, so a subsequent /reload-mcp finds cached tokens and skips the browser flow entirely.

When to Use

Use this skill when all of the following are true:

1. The user wants to add a remote HTTP MCP server that requires OAuth (not a static Bearer token).
2. Hermes is running as a remote gateway (container, VPS, Docker, managed service) — NOT a local CLI on the user's laptop.
3. The server supports OAuth 2.1 with PKCE and RFC 7591 Dynamic Client Registration (most modern MCP servers do — Better Stack, Linear, Cloudflare, Datadog, etc.). If it doesn't support DCR (GitHub is the notable exception), this skill does not apply — use a pre-registered OAuth App or a Personal Access Token instead.

Do NOT use this for:

• Local CLI Hermes — just set auth: oauth in mcp_servers.<name> and /reload-mcp. The built-in flow opens a browser and captures the callback on localhost. Works perfectly.
• Servers that accept a static Bearer token (API key) — always prefer headers.Authorization: "Bearer <token>" when the user is willing. Simpler, no refresh dance.
• GitHub Copilot MCP (api.githubcopilot.com/mcp/) — GitHub does not expose DCR. Use a PAT or a pre-registered OAuth App (see pitfall 12).

Why the Built-in OAuth Flow Fails on a Remote Gateway

Hermes' native MCP OAuth client (tools/mcp_oauth.py):

1. Picks a free local port P.
2. Registers a dynamic OAuth client with the AS, sending redirect_uri = http://127.0.0.1:P/callback.
3. Starts an HTTP server on 127.0.0.1:P inside the Hermes process.
4. Prints the authorize URL and waits for the code at its local endpoint.

When Hermes runs remotely, the 127.0.0.1 in the redirect_uri is the remote container's loopback, not the user's. After authorizing, the user's browser 302s to http://127.0.0.1:P/callback?code=..., which resolves to the user's own laptop and fails to connect. The callback never reaches the Hermes process, the flow times out, and /reload-mcp returns "No MCP tools available" with no detail.

Symptoms to recognize: [xdg-open] <defunct> processes under the hermes user, an empty or missing tokens directory ($HERMES_HOME/mcp-tokens/), and a reload that responds without any "Added/Reconnected: X" line in change_detail.

Cheap First Fallbacks: the Built-in Flow's Own Escape Hatches

Before any manual token surgery, check whether the built-in flow's fallbacks already cover the deployment. When Hermes detects a remote session it prints two options alongside the authorize URL (tools/mcp_oauth.py):

1. Paste-back — on an interactive TTY, a stdin reader races the HTTP listener. The user authorizes, the browser fails to connect to 127.0.0.1:<port>, and they paste the full address-bar URL (?code=...&state=...) back at the prompt. Works for SSH'd-in CLI sessions.
2. SSH port-forward — ssh -N -L <port>:127.0.0.1:<port> <user>@<host> makes the redirect reach the remote listener normally.

Both require an interactive terminal to the Hermes host. The rest of this skill is for when there is NO interactive TTY — Hermes running purely as a messaging gateway/bot where /reload-mcp triggers the flow with nobody at a prompt.

Preferred Front Door: the Hermes Dashboard (try this BEFORE manual token surgery)

A remote Hermes gateway often also runs the dashboard web UI as a SEPARATE process (e.g. hermes dashboard --host 0.0.0.0 --port <port>; check with ps aux | grep 'hermes dashboard'). It exposes a connector/MCP console — endpoints like /api/mcp/servers, /api/mcp/status, and /connectors (all login-gated; a cookieless curl returning 401/302 confirms they exist).

Why the dashboard solves the core problem: when the user drives OAuth from the dashboard in their own browser, the redirect lands in a context the dashboard can capture — sidestepping the 127.0.0.1-callback failure that breaks the CLI/manual flow. So the correct escalation order for "add or re-auth an OAuth MCP server on a remote gateway" is:

1. Dashboard, in the user's browser — the intended front door. Add servers, run OAuth, reload, all authenticated as the user. No copy-paste-callback dance, no hand-writing token files.
2. Manual token surgery (the rest of this skill) — the FALLBACK for when there's no browser session to the dashboard (pure-chat/headless context).

Finding the dashboard's PUBLIC URL. The dashboard binds internally to 0.0.0.0:<port>, but the user needs the externally-reachable URL. Most deploy platforms inject it into the environment — grep for it rather than making the user hunt:

env | grep -iE "HERMES_DASHBOARD_PUBLIC_URL|RAILWAY_PUBLIC_DOMAIN|RAILWAY_STATIC_URL|RAILWAY_SERVICE_.*_URL|PUBLIC_URL|BASE_URL|DOMAIN" \
  | sed -E 's/(TOKEN|SECRET|KEY|PASSWORD)=.*/\1=***REDACTED***/I'

HERMES_DASHBOARD_PUBLIC_URL is authoritative when present. On Railway also check RAILWAY_PUBLIC_DOMAIN / RAILWAY_STATIC_URL (the *.up.railway.app host) and RAILWAY_SERVICE_*_URL vars, which sometimes carry a friendlier custom domain. Hand the user the full https:// URL and point them at the Connectors/MCP section. ALWAYS pipe through the sed redaction above — these env greps sit next to *_TOKEN/*_SECRET vars.

What the dashboard does NOT fix (still host-side / shell): stdio servers that need shell auth state (a CLI login command whose credentials may not persist across restarts) and anything reading credentials from $HERMES_HOME/.env. Those are out of the dashboard's scope regardless.

The Workaround

Do the OAuth dance manually, then write the resulting tokens into the exact files Hermes' HermesTokenStorage would have written, so on /reload-mcp Hermes finds cached tokens and skips the browser flow entirely.

Run the shell commands below through the terminal tool on the gateway host and do the Python steps (PKCE generation, token exchange, file writes) via execute_code or a terminal python3 invocation — file writes must happen in the SAME code block as the token exchange (see pitfall 16).

1. Confirm it's a remote gateway

env | grep -iE "HERMES|RAILWAY|CONTAINER"
echo "$DISPLAY $WAYLAND_DISPLAY $SSH_CLIENT"

No display + a remote indicator = remote gateway. tools/mcp_oauth.py::_can_open_browser() uses these same env vars, so if Hermes' own auto-detect says "headless", the built-in flow won't work.

2. Find HERMES_HOME and the config path

HERMES_HOME=$(python3 -c 'from hermes_constants import get_hermes_home; print(get_hermes_home())')
echo "config: $HERMES_HOME/config.yaml"
echo "tokens: $HERMES_HOME/mcp-tokens/"

3. Discover OAuth metadata from the MCP server

MCP servers advertise their OAuth setup via RFC 9728 (OAuth 2.0 Protected Resource Metadata). The WWW-Authenticate header on a 401 tells you where to look:

curl -sI https://mcp.example.com | grep -i www-authenticate
# → Bearer realm="mcp", resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource"

Not every server returns WWW-Authenticate. Some return a bare {"errors":["Unauthorized"]} 401 with no auth-discovery hint. When that happens, probe well-known paths directly:

for p in \
  /.well-known/oauth-protected-resource \
  /.well-known/oauth-authorization-server \
  /.well-known/openid-configuration ; do
  echo "=== $p ==="
  curl -s -A "python-httpx/0.27" "https://mcp.example.com$p" | head -c 400; echo
done

Fetch the resource metadata to get authorization_servers, then fetch the AS's /.well-known/oauth-authorization-server to get authorization_endpoint, token_endpoint, and registration_endpoint.

Pitfall: many servers sit behind Cloudflare and 403 bare urllib user agents. Always set User-Agent: python-httpx/0.27 (or similar) on requests in this flow.

4. Dynamic Client Registration (RFC 7591)

POST to the registration_endpoint with:

{
  "client_name": "Hermes Agent (manual OAuth)",
  "redirect_uris": ["http://127.0.0.1:8765/callback"],
  "grant_types": ["authorization_code", "refresh_token"],
  "response_types": ["code"],
  "token_endpoint_auth_method": "none",
  "scope": "<scopes_from_resource_metadata>"
}

Omit scope entirely if the AS's scopes_supported is empty — see step 5 pitfall. Use port 8765 (or any port — nothing will listen). token_endpoint_auth_method: none marks this as a public PKCE client. Save the returned client_id.

5. Build the authorize URL with PKCE

Generate:

• code_verifier: secrets.token_urlsafe(64)[:128]
• code_challenge: base64url(sha256(code_verifier)) (no padding)
• state: secrets.token_urlsafe(24)

Query params: response_type=code, client_id, redirect_uri, code_challenge, code_challenge_method=S256, state, plus resource=<mcp_server_url> (RFC 8707 — many servers require this to bind the token to the specific MCP resource). Include scope=<space-separated> ONLY if the AS metadata's scopes_supported is a non-empty array AND/OR the resource metadata declares specific scopes. If scopes_supported: [], omit the scope parameter — the server grants its full default set on its own. Fabricating scope strings against an empty scopes_supported can cause invalid_scope errors on some ASes.

Stash code_verifier and state to disk (e.g. /tmp/.mcp-oauth-work/<server>.json, 0600 perms). You need them for step 7, possibly across multiple chat turns.

6. Give the user the authorize URL

Open this URL in your browser:
<authorize_url>

After approving, your browser will try to load http://127.0.0.1:8765/callback
and fail to connect — THAT'S EXPECTED. Just copy the entire URL from the
address bar (it will contain ?code=...&state=...) and paste it back here.

7. Exchange the code for tokens

When the user pastes the callback URL:

1. Parse code and state from the query string.
2. Verify state matches the stashed value (CSRF check — do not skip).
3. POST application/x-www-form-urlencoded to the token_endpoint:
   • grant_type=authorization_code
   • code=<from callback>
   • redirect_uri=<same as step 4>
   • client_id=<from step 4>
   • code_verifier=<stashed>
   • resource=<mcp_server_url> (if the AS required it in step 5, include here too)
4. Response contains access_token, refresh_token, token_type, expires_in, scope.

8. Write tokens in Hermes' exact schema

tools/mcp_oauth.py::HermesTokenStorage expects two files under $HERMES_HOME/mcp-tokens/ (create dir with 0o700, files with 0o600):

<server_name>.json — the OAuthToken pydantic model:

{
  "access_token": "...",
  "token_type": "Bearer",
  "expires_in": 7200,
  "refresh_token": "...",
  "scope": "read write"
}

<server_name>.client.json — the OAuthClientInformationFull model:

{
  "client_id": "...",
  "redirect_uris": ["http://127.0.0.1:8765/callback"],
  "grant_types": ["authorization_code", "refresh_token"],
  "response_types": ["code"],
  "token_endpoint_auth_method": "none",
  "scope": "read write",
  "client_name": "..."
}

Write each file via json.dumps(..., indent=2). Sanitize the filename with re.sub(r'[^\w\-]', '_', server_name)[:128] — this matches _safe_filename() in Hermes' token storage.

9. Add the server to config.yaml

mcp_servers:
  <name>:
    url: "https://mcp.example.com"
    auth: oauth
    timeout: 180
    connect_timeout: 60

10. Smoke-test the token BEFORE asking the user to reload

Manually POST an MCP initialize request to confirm the token works end-to-end — this catches scope misconfigurations, wrong resource values, and CF blocks before the user is confused by another "No MCP tools available" reload:

body = json.dumps({
    "jsonrpc": "2.0", "id": 1, "method": "initialize",
    "params": {
        "protocolVersion": "2025-06-18",
        "capabilities": {},
        "clientInfo": {"name": "hermes-debug", "version": "1.0"},
    },
}).encode()
# POST to the MCP URL with:
#   Authorization: Bearer <access_token>
#   Accept: application/json, text/event-stream
#   Content-Type: application/json
#   MCP-Protocol-Version: 2025-06-18
#   User-Agent: python-httpx/0.27

Expect HTTP 200 with Content-Type: text/event-stream and a JSON-RPC result containing serverInfo and capabilities. Do not use urllib with its default UA — Cloudflare will 403 you even though Hermes (which uses httpx) will succeed. scripts/diagnose-oauth-mcp.py automates this smoke test.

11. Tell the user to run /reload-mcp

On reload, Hermes sees auth: oauth, calls HermesTokenStorage.get_tokens(), finds your cached tokens, skips the browser flow, and registers mcp_<name>_* tools. Refresh happens automatically before expires_in elapses.