Capmus MCP

supost-mcp

Remote MCP server for SUpost, the marketplace for Stanford — and, via BRAND=capmus, for Capmus (capmus.com), which serves the same public API from the same supost-web codebase. One repo, two Vercel projects:

BRAND selects the server name, tool titles/descriptions, and default base URL; SUPOST_BASE_URL still overrides the base URL for previews.

Lets AI agents search active listings, fetch listing details, read verified market statistics, message posters, and create draft listings.

This is doc 190 workstream E3 (see supost-web/docs/dev/190-ai-agent-discovery-implementation-plan.md). It is a pure client of SUpost's public surfaces — the read-only listings API (E2), public listing pages, and /stats.md. It has no privileged database access and holds no secrets; the only configuration is the public base URL.

Tools

No personal information is ever returned. send_message is the supported way to contact a poster; the listing's url also carries the on-site message form. API terms: https://supost.com/api/public/openapi.json.

Bouncing addresses

A message is only ever delivered after the human clicks the confirmation link, so an address the link cannot reach is a message that never arrives (supost-web docs/dev/20260917-2040-guest-reply-bounce-feedback-handoff.md). send_message therefore never answers a plain "pending" for a dead address:

• An address whose confirmation email hard-bounced in the last 30 days is refused by the API with 422 email_undeliverable before anything is stored. The tool returns an error result with {status: "email_undeliverable", reason, email, message} and tells the agent to ask for a different address rather than retry.
• Otherwise the API answers 202 with a status_key, and the tool polls the delivery status of the confirmation email for up to ~8 s (four checks, two seconds apart — a suppressed address fails in under a second, a fresh hard bounce in 1–16 s). A failed verdict is reported the same way as the 422; sent / queued come back in the result as confirmation_email, with the status_key for a later check_message_status.

The poll is the same anon-callable PostgREST RPC the web form uses, get_guest_verification_status(status_key), reached with the marketplace's publishable key (the value every browser already gets). It returns only queued | sent | failed and a failure classification (mailbox_unknown | no_mx | suppressed | other) — never the redemption token, the message, or the address. Env: SUPOST_STATUS_URL / SUPOST_STATUS_KEY override the defaults (production project); SUPOST_STATUS_KEY="" disables polling (tools report unknown); SUPOST_STATUS_POLL_ATTEMPTS / SUPOST_STATUS_POLL_INTERVAL_MS tune the in-call wait.

Hosting

Deployed on Vercel as a stateless streamable-HTTP MCP endpoint (mcp-handler + @modelcontextprotocol/sdk):

https://mcp.supost.com/mcp        (rewritten to /api/mcp)

https://mcp.supost.com/health (rewritten to /api/health)

No sessions, no Redis, no auth — every request is independently served and all upstream data is public and CDN-cached.

Which commit is live

The initialize response's serverInfo.version is + on Vercel (e.g. 0.3.0+835efb3, from VERCEL_GIT_COMMIT_SHA; plain 0.3.0 locally — see src/version.ts), so any MCP client, or GET /health, shows the deployed commit. Compare it with git rev-parse --short origin/master when something looks stale.

Health check and uptime monitoring

GET /health answers {"ok": true, "brand": "supost", "version": "…"} without calling upstream (api/health.ts): it says the function is serving, not that supost.com is up. Point an UptimeRobot HTTP monitor at each brand (manual step, both needed):

• https://mcp.supost.com/health
• https://mcp.capmus.com/health

Alerting on a non-200 (keyword "ok":true optional) catches a broken deploy or a dropped domain; the version in the body shows which commit answered.

Deploy

vercel deploy          # preview

vercel deploy --prod # production

Optional env var: SUPOST_BASE_URL (default https://supost.com; set to https://preview.supost.com on preview deployments to point at the dev stack).

Production deploys come from the GitHub integration on master. Avoid vercel deploy --prod from a local checkout unless it is at origin/master: a stale CLI deploy takes the production alias over the git build (this happened on 2026-09-17; fixed with vercel promote).

Usage analytics

Every tool call emits one PostHog mcp_tool_called event (sanitized args, no PII — see src/analytics.ts) and, when TOOL_LOG_URL / TOOL_LOG_KEY are set, a full-argument row in the marketplace's private ops.mcp_tool_call table (src/toollog.ts).

Failed calls carry error_code: the public API's error code (rate_limited, not_found, email_undeliverable, invalid_request, …) or unknown for anything else (network failure, unexpected response shape); null on success. The error *message* is never sent, since it can quote user content — break down by error_code to tell a rate-limit spike from an upstream outage.

The PostHog distinct_id is a salted SHA-256 of the caller's IP + User-Agent (src/client.ts), so uniq(distinct_id) counts distinct agents without storing the IP. Events also carry client_ua (the MCP client's User-Agent, which names the harness) and client_country (Vercel's geo header). Set CLIENT_ID_SALT in the Vercel project env to make the hash unguessable; the in-repo default keeps local dev working. Calls that arrive without IP or User-Agent fall back to the per-brand id mcp. .com.

Connect a client

claude mcp add --transport http supost https://mcp.supost.com/mcp

or in any MCP client that supports remote servers, add the URL above as a streamable-HTTP server.

Rate limiting

The public API enforces ~60 requests/minute/IP and serves 5-minute CDN caching. The client in src/http.ts respects this: on a 429 it honors Retry-After (capped at 5 s), retries once, and otherwise surfaces a structured rate_limited error instructing the agent to back off — it never retries in a loop. All requests carry a supost-mcp/… User-Agent.

Every marketplace call (never the status RPC) also carries the trusted-agent headers: x-supost-api-key (SUPOST_API_KEY in the deployment env, never in git) and, when the calling agent's IP is known, x-supost-agent-client-ip with it (first hop of the MCP's own inbound x-forwarded-for). The API trusts that IP only when the key verifies (supost-web docs/dev/469-mcp-agent-api-hardening-handoff.md), so the per-IP limit can apply per agent rather than to the single Vercel egress IP all agents share. Plain X-Forwarded-For would not work: supost.com runs on Vercel, which overwrites it at the edge.

Development

npm install

npm run check # typecheck + tests SUPOST_BASE_URL=https://preview.supost.com npx tsx scripts/smoke.ts # live end-to-end

Tests (vitest) cover each tool's request/response mapping, error mapping, JSON-LD extraction, and the rate-limit contract (Retry-After honored, capped, single retry, structured failure).

Follow-ups (manual steps)

• Publish to the MCP registry — server.json is ready. Make the repo public first (gh repo edit Capmus-Team/supost-mcp --visibility public), then: sh brew install mcp-publisher mcp-publisher login github # device flow, needs a browser mcp-publisher publish # from the repo root For the Capmus entry, publish server.capmus.json the same way (cp server.capmus.json server.json in a scratch checkout, or mcp-publisher publish --file server.capmus.json if supported).
• `/help/mcp` docs page in supost-web — PR #1242 (guide registry entry + llms.txt MCP line); live once merged to dev → master.
• PR/citation announcement — draft and story beats in docs/announcement-draft.md; post after the registry listing and /help/mcp are live (doc 190 workstream F).
• E2 is live on production (supost.com, 2026-07-09); smoke script verified against production, and the server is deployed at https://mcp.supost.com/mcp.