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.

Pitfalls & Lessons Learned

1. Do not assume "headless" means "OAuth impossible." The built-in flow works fine for local CLI; the issue is strictly remote deployments where the user's browser and the Hermes process are on different machines. Check the execution environment before claiming OAuth isn't an option.

2. Read the source, not just the skill docs. tools/mcp_oauth.py and the MCP config reference in website/docs/ are the authoritative references. Grep the tree before telling the user a feature "doesn't exist."

3. Cloudflare UA filter. Many MCP/OAuth providers front their infra with Cloudflare, which 403s python-urllib/* user agents on metadata endpoints even though those endpoints are public. Set User-Agent: python-httpx/0.27 (or any browser-like string) on every request in this flow. Hermes itself uses httpx, so this is never a problem in the real connection path.

4. Include resource in both authorize and token requests. RFC 8707 resource indicators are not optional for most modern MCP servers — they bind the issued token to the specific MCP resource URL. Leaving it out sometimes still works but may yield a token that later fails at the MCP server with a scope/audience error.

5. Trailing slash matters. Some servers advertise the resource as https://mcp.example.com/ with a trailing slash and reject tokens issued against the no-slash variant. Copy the resource value verbatim from the .well-known/oauth-protected-resource response.

6. /reload-mcp is silent on failure. If the reload shows "No MCP tools available" with no change_detail line, a server is in config but failed to connect and no error bubbled up. Tail the error log, smoke-test the token directly with a manual initialize POST, and — if everything looks good — ask for a full process restart.

7. Circuit breaker can survive /reload-mcp. tools/mcp_tool.py keeps a module-level error-count dict with a small threshold. Once tripped (e.g. after token expiry produces several consecutive failures), the tool handler can short-circuit before calling the server, so no successful call resets the counter. Symptom: reload says "Reconnected: X" but subsequent calls still fail with "server unreachable" in the same conversation. Recovery order: try /reload-mcp FIRST (cheap, no chat-process blip) — on current builds it can clear the counter; only escalate to a full gateway process restart if a live call STILL short-circuits after reload. Do not lead with "you must restart."

8. Refresh on an expired access_token + a tripped breaker is a deadlock. The auto-refresh logic runs inside the MCP call path, which the breaker short-circuits once tripped. Manually refreshing the token on disk does not help by itself — pair a manual token refresh with a full restart, not a /reload-mcp.

9. invalid_grant on a manual refresh means the refresh token is DEAD — re-auth is the only fix, do not loop. When the access_token has been expired long enough, the refresh_token can also be revoked/expired server-side. A grant_type=refresh_token POST then returns HTTP 400 {"error":"invalid_grant",...} (wording varies: "Grant not found", "Token expired", "refresh token is invalid"). There is NO recovery from the gateway side. Hand back to the user with two options: (a) re-run the full manual OAuth dance (steps 3–10), or (b) if the provider offers a static personal API key, switch to that — no refresh/expiry cycle, more durable for an unattended remote gateway. Detect early: before any create/update operation against an OAuth MCP, check expires_at vs time.time(); if already expired, attempt the refresh first and surface invalid_grant immediately rather than failing mid-task.

10. A successful refresh that STILL yields a rejected token = server-side SESSION revocation; only a fresh authorization_code flow fixes it. Distinct from pitfall 9. The stored token file can look healthy (expires_at well out, refresh_token present), yet a live initialize POST returns 401 invalid_token with a JSON-RPC body like {"error":{"code":-32002,"message":"Session expired. Please re-authenticate."}}. The grant_type=refresh_token POST may succeed (HTTP 200, new access_token) — yet the brand-new token gets the SAME -32002. The provider revoked the underlying MCP session server-side; the OAuth refresh chain re-mints credentials but cannot re-establish a revoked session. Decision rule when an OAuth MCP reports "not connected": (1) smoke-test the stored access_token with a manual initialize POST; (2) if 401 invalid_token, attempt a refresh and smoke-test the NEW token; (3a) new token works → write it + restart to clear the breaker; (3b) new token STILL gets -32002/"Session expired" → stop, this is session revocation, hand the user the authorize URL for a full re-auth. scripts/diagnose-oauth-mcp.py automates steps 1–2 and prints which branch you're in. For an unattended gateway whose session keeps getting revoked, prefer a static Personal API key. See references/stripe-mcp-oauth-revocation.md for a worked example of a provider that revokes weekly.

11. Client info file is NOT optional. Hermes needs <server>.client.json to know the client_id for refresh grants. Skipping it means the first refresh fails and the user has to re-auth — writing both files is the whole point of this skill.

12. Never hand-type the redirect URL for the user to open. Generate the authorize URL programmatically with urllib.parse.urlencode(). Spaces in scopes and special chars in state break string-concatenated URLs.

13. Security: the stash file contains the code_verifier. Delete /tmp/.mcp-oauth-work/<server>.json immediately after successful token exchange. There's no reason to keep a proof-of-identity secret around once it's consumed.

14. Write what the token endpoint actually returned. The AS may grant a narrower (or wider) scope than requested. Write the scope from the token-exchange response to <server>.json, not what you asked for in step 5. When scopes_supported: [], the explicit scope list you send IS authoritative both ways: some servers grant exactly what you list (pass narrow scopes for least-privilege, or enumerate the full set if the user needs everything), and some won't echo the granted scope back at registration time — only the token-exchange response is authoritative.

15. OAuth tokens often double as Bearer tokens against the provider's public REST API. The access_token in <server>.json is frequently not "MCP-only" — Authorization: Bearer <token> against the provider's documented REST API succeeds whenever the corresponding resource scope was granted. This is the OAuth 2.0 spec, not a provider quirk. When the MCP server is read-only but you need a write operation, check whether the OAuth token can hit the provider's REST API directly before suggesting a separate API key.

16. Secret redaction can mask tokens in tool output. If secret redaction is enabled, tokens and long opaque strings render as *** in tool-result output, so you cannot print(response) to keep the access_token visible across turns. Combined with single-use code values from authorization_code grants: if you print the token-exchange response, you may lose the token AND consume the code, forcing a restart with a fresh authorize URL. Always write the access_token directly to its final destination file in the SAME code block that performs the token exchange. If you must print for debugging, print only len(access_token), token_type, scope, expires_in — never the secret.

17. GitHub MCP (api.githubcopilot.com/mcp/) uses a pre-registered confidential OAuth App, not DCR + PKCE-public. Its client info ships with a real client_secret and token_endpoint_auth_method: client_secret_post. The token-exchange POST to https://github.com/login/oauth/access_token must include client_secret as a form field alongside client_id, code, code_verifier, and redirect_uri (PKCE is still honored on top of the secret). The redirect URI is fixed in the OAuth App config — you cannot change it, so the manual listener-port trick doesn't apply; the user just lets the browser fail to connect on that port and pastes the address-bar URL back.

What NOT to do

• Don't use mcp-remote as a fallback. It runs an npx subprocess whose OAuth callback server ALSO sits on the remote container's localhost — same problem. mcp-remote only helps when the MCP client doesn't speak remote HTTP at all (Hermes does natively).
• Don't push "paste your API token and I'll add headers" if the user explicitly asked for OAuth. Offer the static-token shortcut only after explaining why the native OAuth flow fails in remote deployments. Respect the user's choice to do the extra legwork for rotation-free, scope-limited access.
• Don't claim Hermes doesn't support a feature without reading the source. Grep the source tree before making capability claims.

Quick Reference Files

• scripts/diagnose-oauth-mcp.py — re-runnable, read-only-by-default diagnostic. Given a server name, it smoke-tests the stored access_token, attempts a refresh, smoke-tests the new token, and prints exactly which recovery branch you're in (TOKEN_OK = breaker/restart, REFRESH_FIXED = persist+restart, SESSION_REVOKED = full re-auth, REFRESH_DEAD = full re-auth/API key). Pass --write to persist a working refreshed token atomically. Never prints secret values. Run this FIRST when an OAuth MCP server reports "not connected" — it encodes the pitfall 7/9/10 decision tree.
• references/stripe-mcp-oauth-revocation.md — a worked example (Stripe) of a provider that revokes its OAuth session on a recurring basis, and the durable fix: switch to a static restricted API key.

Related

• native-mcp — general guide to configuring MCP in Hermes. Authoritative config reference lives there.
• mcporter — the external CLI bridge, for ad-hoc MCP calls outside of Hermes' config.