Tollbooth OAuth2 Collector

An unauthenticated "dumb mailbox" FastMCP server that holds OAuth2 authorization codes for retrieval by the originating MCP server. A companion serverless function (Val Town) receives the browser redirect from the OAuth provider and hands the code to this collector's store_code MCP tool. This two-part design works around Horizon proxying only POST traffic on the /mcp/ path — browser GET redirects from OAuth providers cannot reach @mcp.custom_route endpoints, so a serverless callback bridges the gap.

How It Works

┌───────────────┐   1. redirect_uri → serverless    ┌──────────────────────┐
│ OAuth Provider│ ─────────────────────────────────→│ Serverless callback  │
│ (e.g. Schwab) │   GET ?code=AUTH_CODE&state=TOKEN  │ (Val Town)           │
└───────────────┘                                    └──────────────────────┘
                                                               │
                                        2. POST store_code     │
                                           (JSON-RPC /mcp/)     ▼
                                                     ┌──────────────────────┐
                                                     │ OAuth2 Collector     │
                                                     │ store_code tool      │
                                                     │ → encrypts + Postgres│
                                                     └──────────────────────┘
                                                               │
                                                               │ 3. code stored
                                                               ▼
┌──────────────┐   4. retrieve_code(state=TOKEN)     ┌──────────────────────┐
│ MCP Server   │ ─────────────────────────────────→  │ OAuth2 Collector     │
│ (e.g.        │        (JSON-RPC /mcp/)             │ retrieve_code tool   │
│  schwab-mcp) │ ←───────────────────────────────── │ → returns code once  │
└──────────────┘   {"found": true, "code": ...}      │ → deletes from DB    │
                                                     └──────────────────────┘

1. MCP server starts an OAuth flow, setting redirect_uri to the serverless callback function's URL

2. User authorizes in the browser; the OAuth provider redirects to the serverless callback with ?code=...&state=...

3. Serverless callback (Val Town) POSTs the code to the collector's store_code MCP tool over /mcp/

4. Collector encrypts the code (AES-256-GCM keyed on SHA-256 of the state) and stores it in Postgres (600s TTL)

5. MCP server calls the retrieve_code(state=...) MCP tool to pick up the code (one-time read, auto-deleted)

6. MCP server decrypts and exchanges the code for a token using its own credentials

The collector also exposes collector_status (pending-code count and TTL) and service_status (deployed build, incl. git sha) as free MCP tools. The serverless callback source lives in val/oauth_callback.js.

Related MCP server: MCP Credentials Broker

Deployment

Deploy to Horizon:

fastmcp deploy server.py

Set the NEON_DATABASE_URL environment variable in Horizon to point to your Neon Postgres instance.

DPYC Advocate Identity

This collector is registered as an Advocate in the DPYC Social Contract. Consuming MCP servers discover its URL automatically via the DPYC registry:

from tollbooth.registry import resolve_service_by_name

svc = await resolve_service_by_name("tollbooth-oauth2-collector")
collector_url = svc["url"]  # e.g., "https://tollbooth-oauth2-collector.fastmcp.app"

No OAUTH_COLLECTOR_URL env var needed — peer discovery is handled by the registry.

Register the serverless callback function's URL (not the collector's) in your OAuth provider's developer portal. It is registered separately in the DPYC registry under the name tollbooth-oauth2-callback:

callback = await resolve_service_by_name("tollbooth-oauth2-callback")
redirect_uri = callback["url"]  # e.g., "https://tollbooth-dpyc-oauth.val.run"

Security Model

• Auth codes are useless alone — exchanging a code requires client_id + client_secret, held only by the consuming MCP server

• Encrypted at rest — codes are stored AES-256-GCM-encrypted (keyed on SHA-256 of the state) via the SDK's encrypt_collector_code; the consuming MCP server decrypts with the same state

• HMAC-signed state tokens — the originating MCP server generates tamper-proof state tokens

• One-time read — codes are deleted immediately after retrieval (prevents replay)

• Short TTL — expired codes are automatically cleaned up (600s)

• No secrets stored — the collector never sees client credentials or tokens

Related Repositories

License

Copyright 2026 Lonnie VanZandt. Licensed under the Apache License, Version 2.0.