Underclass: An OpenAI-compatible pooling proxy that pins sessions to one account

Developer Geoffrey Huntley released underclass, an OpenAI-compatible local proxy that pools multiple ChatGPT/Codex and GitHub Copilot subscriptions behind a single endpoint at /v1/responses, /v1/chat/completions, and /v1/models. The proxy pins sessions to one subscription for 24 hours to keep upstream prompt caches warm, moves quota-exhausted accounts (429 or usage-limit responses) into a cooling state until retry-after, and returns 429 with the earliest reset time when all accounts for a requested model are cooling. It is written in Rust and run via `cargo run -- serve`, with accounts added through a web UI using ChatGPT/Codex OAuth device flow or GitHub Copilot device flow.

A local proxy that pools multiple ChatGPT/Codex and GitHub Copilot subscriptions behind one OpenAI-compatible endpoint. ┌────────────────────────────────────┐ │ underclass │ │ │ opencode ────────────►│ /v1/responses │ any OpenAI-compatible│ /v1/chat/completions │────► chatgpt.com client │ /v1/models │ N Codex subs, │ │ OAuth device flow web UI ◄─────────────►│ sticky sessions · health pool │ accounts, catalog, │ fail-fast saturation · tracing │────► api.githubcopilot.com live request feed │ │ M Copilot subs, └────────────────────────────────────┘ GitHub device flow One subscription runs out of quota? It leaves rotation until its window resets — and comes back on its own. Sessions stay pinned to one subscription so upstream prompt caches stay warm. When everything is exhausted, the proxy fails fast with the earliest reset time instead of hanging. Subscription-based model access has a per-account quota. One account is a ceiling; twenty accounts are a pool. underclass turns a pile of personal subscriptions into a single durable endpoint that behaves like one well-provisioned provider: - No client changes — the surface is plain /v1/ ; point opencode or anything OpenAI-compatible at it. - No quota whiplash — exhausted accounts cool down and recover automatically; clients never see account churn. - No cache waste — sessions are sticky, so the upstream prompt cache keeps working across turns. UNDERCLASS PROXY KEY="$ openssl rand -hex 32 " \ UNDERCLASS UI TOKEN="$ openssl rand -hex 32 " \ cargo run -- serve underclass listening on http://127.0.0.1:8080 web ui: http://127.0.0.1:8080/ Keep the generated values in a password manager or runtime secret file; underclass never writes them to diagnostics. Open the web UI and paste the value supplied as UNDERCLASS UI TOKEN . 1. Open the web UI and paste the configured admin token. 2. Click Add account → pick ChatGPT / Codex or GitHub Copilot → enter the device code at the shown URL. The account is labeled automatically with the account's email or username. 3. Repeat for every subscription you want in the pool. 4. Point opencode at the pool: cargo run -- connect opencode --provider underclass --model underclass/gpt-5.5 underclass connect writes the provider block and credentials into your global opencode config ~/.config/opencode/opencode.json{,c} + auth.json , idempotently and with backups. Re-run it any time; --remove undoes it. - Sticky sessions. Requests carrying prompt cache key / promptCacheKey opencode sends the session ID when configured with setCacheKey: true always land on the same subscription. Bindings live for 24h, survive restarts, and rebind preferentially within the same backend when an account cools. - Health pool. A quota response 429 /usage-limit bodies moves an account to cooling until retry-after or a per-backend default . 401s trigger one token refresh + retry, then the account needs re-login. Cooling accounts stay configured and return to rotation automatically. - Fail fast. If every account eligible for the requested model is cooling, the proxy answers 429 + Retry-After = earliest reset. No queuing. - Flat pool. Codex and Copilot accounts compete by least-in-flight, filtered by per-backend model catalogs. Unknown model IDs pass through to Codex so new models work without proxy changes. - Pre-first-byte failover only. Once a stream starts, upstream errors pass through — no silent re-send of half-finished turns. Design decisions and their trade-offs live in docs/adr/ https://github.com/ghuntley/underclass/blob/main/docs/adr — start with ADR 0010 https://github.com/ghuntley/underclass/blob/main/docs/adr/0010-unprefixed-routing-flat-pool.md for the routing model. Optional ~/.config/underclass/config.toml : | key | default | meaning | |---|---|---| | bind | 127.0.0.1:8080 | listen address | | proxy key | minted on first run | bearer key clients must send to /v1/ | | ui token | minted on first run | admin token for the web UI + /admin/api/ | | codex cooldown secs | 1800 | cooling window when upstream omits retry-after | | copilot cooldown secs | 1800 | same, for Copilot | Environment overrides: UNDERCLASS BIND , UNDERCLASS PROXY KEY , UNDERCLASS UI TOKEN . For testing against a mock upstream: UNDERCLASS CODEX UPSTREAM , UNDERCLASS COPILOT UPSTREAM default to the real endpoints . State credentials, sticky bindings, model catalog, minted keys lives in ~/.local/share/underclass/pool.db . Delete it to start fresh. underclass serve --bind ADDR underclass connect --base-url URL --api-key KEY --model MODEL --project --no-default-model --dry-run --remove connect targets the global opencode config by default; --project writes ./.opencode/opencode.json instead. --dry-run prints the merged documents without writing. | route | auth | purpose | |---|---|---| | POST /v1/responses | proxy key | Responses API, streamed through to the pool | | POST /v1/chat/completions | proxy key | Chat Completions, same | | GET /v1/models | proxy key | union catalog with merged limits | | GET / | none | web UI | | GET /admin/api/state | admin token | accounts, catalog, last 200 requests | | POST /admin/api/flows | admin token | start a device-flow onboarding | | GET /admin/api/flows/{id} | admin token | poll an onboarding flow | | POST /admin/api/accounts/{id}/enable | disable | relogin | | DELETE /admin/api/accounts/{id} | admin token | remove from pool | | GET | PUT /admin/api/catalog/{backend} | admin token | | GET /admin/api/client-key | admin token | retrieve the proxy key for connect | Every response carries x-request-id ; logs are JSON RUST LOG filters, --log-format json|pretty and each request logs the account label that served it. The catalog is data, not code: seeded with the Codex families gpt-5.4 , gpt-5.4-mini , gpt-5.3-codex-spark , gpt-5.5 , gpt-5.6-sol , gpt-5.6-terra , gpt-5.6-luna , gpt-6-astra and Copilot's live /models list. Edit it in the UI or via the admin API; routing eligibility and the opencode model block follow it. See ADR 0007 https://github.com/ghuntley/underclass/blob/main/docs/adr/0007-config-driven-model-catalog.md . The repository is a Nix flake: it exposes the CLI as a package/app and the devenv development shell as devShells.default . Run the proxy without installing: nix run github:ghuntley/underclass -- serve Install it into your profile: nix profile install github:ghuntley/underclass Use the devenv shell Rust toolchain, cargo for development: nix develop --no-pure-eval cargo test --no-pure-eval is required for the devenv shell devenv inspects the working directory; this matches devenv's own flake template . The package and app outputs are pure — nix run and nix profile install need no flags. Long-term devenv users can keep using devenv shell / devenv test directly — nix develop and devenv shell activate the same devenv.nix . Add underclass to your flake inputs and import its module: { inputs.underclass.url = "github:ghuntley/underclass"; outputs = { nixpkgs, underclass, ... }: { nixosConfigurations.my-host = nixpkgs.lib.nixosSystem { system = "x86 64-linux"; modules = underclass.nixosModules.default { services.underclass = { enable = true; bindAddress = "127.0.0.1:8080"; environmentFile = "/run/secrets/underclass.env"; settings = { codex cooldown secs = 1800; copilot cooldown secs = 1800; }; }; } ; }; }; } The runtime environment file can provide credentials without placing them in the Nix store: UNDERCLASS PROXY KEY=sk-underclass-... UNDERCLASS UI TOKEN=... The service uses a dynamic user, persists its database in /var/lib/underclass , binds to localhost by default, and leaves the firewall closed. Set services.underclass.openFirewall = true only when intentionally binding beyond localhost. The flake also exports overlays.default . Validate the module and its QEMU machine test with: nix build . checks.x86 64-linux.underclass-module nix build . checks.x86 64-linux.underclass-vm Notes: - The package builds from the committed Cargo.lock ; dependency versions are pinned there. - nix build skips cargo test because the property tests compile the Hegel engine as a build step, which needs network access that the Nix sandbox denies. CI runs the full suite via devenv see .github/workflows/ci.yml . - Access/refresh tokens, authorization headers, and prompt bodies are never logged. - Account labels email/username appear in logs and the UI by design; raw UUID account IDs are truncated. - auth.json written by connect uses 0600 . Never commit pool.db or .bak . - The proxy binds to localhost by default; put it behind a tunnel only if you understand the exposure. - OAuth tokens rotate: every Codex refresh persists the new refresh token immediately. cargo build cargo test 43 unit tests + 8 Hegel property tests + e2e suite Testing is two-tier: plain unit tests for exact behavior headers, merges, redaction , and Hegel https://hegel.dev property tests over the pure pool core — stickiness stability, health-state invariants, saturation minimums, TTL/cap bounds. The core src/pool.rs , src/health.rs is synchronous with an injected clock; async lives only at the edges. New backends implement the provider::Backend trait and register — nothing else changes. Agent conventions and the ADR policy are in AGENTS.md https://github.com/ghuntley/underclass/blob/main/AGENTS.md . Architecture decision records: docs/adr/ https://github.com/ghuntley/underclass/blob/main/docs/adr . src/ main.rs bootstrap, router, background tasks config.rs TOML + env config models.rs domain types Account, status, catalog, log entries store.rs SQLite persistence accounts, bindings, catalog, config pool.rs pure pool core: stickiness, eligibility, selection health.rs quota classification, retry-after parsing provider.rs Backend trait codex.rs ChatGPT/Codex backend device flow, refresh, headers copilot.rs GitHub Copilot backend device flow, catalog, headers tokens.rs single-flight token refresh proxy.rs /v1/ handlers, failover, streaming logging.rs structured logs, correlation IDs, redaction ui.rs admin API ui.html embedded single-page web UI cli.rs connect verb, JSONC-safe opencode config merge tests/ properties.rs Hegel property tests over the pure core e2e.rs full-pool story against a mock upstream