SubTrack

Track recurring subscriptions from an LLM agent — a thin MCP server over a Go backend.

SubTrack is a small monorepo: a Go HTTP backend (api/) that owns all subscription data and business logic, and a Python FastMCP server (mcp/) that exposes a curated set of tools to MCP clients such as Claude Desktop. The MCP layer holds no business logic of its own — it only speaks REST to the backend, so it's language-agnostic and the same backend could sit behind any other client.

Architecture

flowchart TD
A["Claude Desktop / MCP client"] -->|"MCP · stdio"| B["mcp<br/>FastMCP · 7 curated tools"]
B -->|"REST · X-API-Key"| C["api<br/>Go · net/http"]
C -->|pgxpool| D[("PostgreSQL 16")]

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Tools

Tool	Kind	Backend call
list_subscriptions	read	GET /v1/subscriptions
get_subscription	read	GET /v1/subscriptions/{id}
spending_summary	read	GET /v1/subscriptions/summary
upcoming_charges	read	GET /v1/subscriptions/summary (filtered client-side)
add_subscription	write	POST /v1/subscriptions
update_subscription	write	PATCH /v1/subscriptions/{id}
cancel_subscription	write	POST /v1/subscriptions/{id}/cancel

The surface is curated, not auto-generated from the REST API: tools are task-shaped (upcoming_charges, spending_summary) rather than a 1:1 endpoint mirror, which keeps the agent's choices small and unambiguous.

Quickstart

Requires Docker, Go (see api/go.mod), and uv.

One command demo

make demo

This starts Postgres 16 (5432) and the API (8080) in Docker, applies migrations, seeds 3 sample subscriptions, and prints the spending summary. Run make down afterwards to stop the stack and remove its data volume.

Manual setup

# 1. Start the backend: Postgres 16 (5432) and the API (8080).cd api
cp .env.example .env # optional — compose ships working dev defaults
docker compose up -d
DATABASE_URL="postgres://subtrack:devpassword@localhost:5432/subtrack?sslmode=disable" \
API_KEY=dev-local-key make migrate-up
# 2. Sanity check.
curl http://localhost:8080/healthz
curl -H "X-API-Key: dev-local-key" http://localhost:8080/v1/subscriptions
# 3. Run the MCP server.cd ../mcp
uv sync
export SUBTRACK_API_URL=http://localhost:8080
export SUBTRACK_API_KEY=dev-local-key # matches api/.env.example
uv run subtrack-mcp

Note: make migrate-up runs on the host via go run, so it needs DATABASE_URL (or POSTGRES_*) and API_KEY set in your shell — .env is only read by docker compose, not by the Go binaries.

SUBTRACK_API_URL and SUBTRACK_API_KEY are mandatory — the MCP server fails fast at startup if either is missing. Point your MCP client at uv run subtrack-mcp with the same two variables set, and it connects over stdio.

Key design decisions

• Logic in the backend, not the MCP. The MCP server is stateless and thin; all validation, spending-total and next-charge calculations live in Go.
• Curated tool surface. Seven task-shaped tools instead of an auto-exposed endpoint mirror.
• Money as integer cents. No floats anywhere in the wire shapes — enforced by a CHECK (cost_cents > 0) constraint in the schema.
• Schema only in migrations. The database is defined in api/migrations/, never created from application code.
• Constant-time API-key auth. Every /v1/* route requires a shared secret via X-API-Key, compared with crypto/subtle.

Repo structure

.
├── api/ # Go backend: handler -> service -> repository -> model, Postgres, migrations
├── mcp/ # Python FastMCP server: thin REST client, curated tools
└── docs/ # demo.gif for this README

• api/README.md — backend layering, data model, endpoints, auth, migrations.
• mcp/README.md — MCP tools, auth against the backend, dev gates.

License

MIT.