Caudal

Ask, don't guess.

Caudal (Spanish: both water flow and wealth) — the finance OS for a bootstrapped SaaS.

Excel wasn't going to hold up as the finances of the business grew, and a nicer spreadsheet wasn't the fix — that's just the same manual entry with better colors. Caudal was built from day one as something else: a navigation map for Vertex and whatever it ships next, not a chore that demands data entry as the price of admission. So it lives inside the tools already in use — Hermes Agent, Vertex's day-to-day operating agent, for quick capture and checks, and Claude as the interface for decisions that go beyond what a daily agent should carry (scenario modeling, quarterly reviews, what's next). Concretely, that's two doors into one source of truth: a server-side web interface, and an MCP server exposing the same capabilities as tools.

What this is

• Money is stored as integer minor units (never floats), every write is idempotent and audit-logged.

• Single-currency ledger by construction: USD amounts are converted to COP at the official daily TRM (Banco de la República, via datos.gov.co) at record time (core/fx.py) — aggregates never mix currencies.

• Beyond recording what happened, the system computes forward-looking projections (cash-flow, runway, MRR) and runs a proactive alert engine (budget overruns, runway thresholds) on a schedule.

• Ships with CI, structured logging/tracing/metrics, backups, and a tested restore path — production hygiene, not a script.

Domain background (SaaS accounting fundamentals, category taxonomy, metric formulas) lives in finanzas-saas.md.

Why MCP, not A2A: Hermes has no Google A2A support but has first-class MCP client support, and MCP's elicitation capability (elicitation/create) lets a tool call pause and ask a clarifying question mid-conversation — exactly the "don't guess, ask" behavior this needs.

Related MCP server: Expense Tracker MCP Server

Architecture

flowchart LR
    subgraph Chat
        H[Hermes Agent<br/>Telegram / Slack / CLI]
    end

    subgraph Caudal
        M[MCP server<br/>stdio]
        C[core/<br/>validation · repository<br/>projections · alerts]
        S[scheduler<br/>proactive digests & alerts]
        W[Internal web UI<br/>FastAPI]
    end

    PG[(Postgres)]
    LF[Langfuse Cloud<br/>optional, via OpenRouter Broadcast]

    H <--MCP tools--> M
    M --> C
    S --> C
    W --> C
    C --> PG
    M -. traces .-> LF
    W -. traces .-> LF

Both entry points — MCP tools and the internal UI — go through the same core/ validation and storage layer, so a transaction created by chat and one entered by hand follow identical rules.

Tech stack

Repository layout

caudal/
  core/          # domain layer: repository, validation, reporting, projections, alerts, fx (TRM), logging/tracing
  mcp_server/    # MCP tool definitions (thin wrappers over core/)
  web/           # FastAPI app, Jinja templates, static design system, server-side SVG charts
  scheduler/     # proactive digest/alert runner (no-Hermes fallback)
  config.py      # environment-driven settings, fail-fast on missing values
tests/
alembic/         # database migrations
planning/        # roadmap — one doc per phase, statuses tracked there
docker/          # compose profile support files (hermes-dev config, etc.)
.github/workflows/
Dockerfile
docker-compose.yml

How to run

make all     # core app + pre-build the caudal venv Hermes needs
make chat    # interactive Hermes chat session (set OPENROUTER_API_KEY in .env first)
make help    # every target: up / hermes-warm / chat / ps / logs / down / down-all / restore-drill

Or directly with Docker Compose (just the core app):

cp .env.example .env
docker compose up -d
curl http://localhost:8000/healthz

This brings up Postgres, runs migrations, the web UI (:8000), the proactive scheduler, and a daily-backup service (docker/backups/, RETENTION_DAYS default 14). Restore a backup with scripts/restore.sh <dump> or make restore-drill.

Local dev without Docker:

uv sync --all-groups
cp .env.example .env
uv run pytest --cov=caudal --cov-report=term-missing   # 85% coverage gate, real Postgres via testcontainers
uv run pre-commit install

CI (.github/workflows/ci.yml) runs lint → type-check → SAST → dependency audit → tests on every push, plus a separate gitleaks job.

Core concepts

• core/ is the single implementation both MCP tools and the UI call into: validation.py (pure input validation), repository.py (idempotent CRUD + audit log), reporting.py/projections.py (SQL aggregates, deterministic forecast/runway math — no LLM), alerts.py (deduplicated proactive rules).

• MCP tools (caudal/mcp_server/server.py, 8 tools): record_transaction, update_transaction, list_transactions, get_totals, list_categories, get_projections, get_digest, check_alerts. record_transaction tries MCP elicitation for missing/invalid fields before falling back to a structured clarification_needed result. Try it without Hermes via uv run mcp dev src/caudal/mcp_server/server.py.

• Internal UI (caudal/web/, uv run uvicorn caudal.web.app:app --reload): dashboard (net cash flow hero, runway meter, expense/infra breakdowns), transaction/budget CRUD, alert history with human-readable payloads, projections (forecast + assumptions), reports (net by month, MoM deltas, category breakdowns with date filters), /healthz, /metrics. Light/dark, mobile bottom-tab navigation. No auth in v1 — single-user, localhost/private-network use only (see planning/03-remote-access.md).

• Scheduler (uv run caudal-scheduler): daily alert check + weekly digest, delivered via webhook (NOTIFIER_WEBHOOK_URL) or logged if unset. This is the fallback path — once Hermes is available, its own cron calling get_digest/check_alerts is the primary delivery path (see below).

Connecting to Hermes

On the machine where Hermes actually runs:

hermes mcp add caudal --command "/app/.venv/bin/caudal-mcp"

or the equivalent in config.yaml:

mcp_servers:
  caudal:
    command: "/app/.venv/bin/caudal-mcp"
    env:
      DATABASE_URL: "postgresql+psycopg://finance:finance@<host>:5432/finance"
    tools:
      include:
        [record_transaction, update_transaction, list_transactions, get_totals,
         list_categories, get_projections, get_digest, check_alerts]

Then set up Hermes cron for proactive delivery: "Every Monday at 9am, call the caudal get_digest tool and post the result to Telegram."

Optional: local Hermes chat (--profile hermes-dev)

docker-compose.hermes-dev.yml runs a local Hermes instance for testing the chat → MCP flow without Telegram/Slack, routed directly to OpenRouter (docker/hermes/config.yaml). Set OPENROUTER_API_KEY in .env; budget cap and LLM tracing are OpenRouter dashboard settings (per-key spending cap, "Broadcast to Langfuse") — see docs/observability.md.

Verified end-to-end: a chat message correctly triggers record_transaction, the elicitation flow corrects an invalid field, and the row lands in Postgres. Two notes if you're touching this profile:

• Hermes writes its own config.yaml into its data volume on first launch — scripts/patch_hermes_config.py (run automatically by make chat) merges this repo's provider/MCP config into it.

• A freshly-registered MCP server's tools aren't auto-enabled for the cli platform — also handled by make chat via hermes tools enable.

Engineering log

• CHANGELOG.md — what changed, per release (Keep a Changelog).

• docs/adr/ — why it changed: Architecture Decision Records for every decision that shapes the data model, an invariant, or the architecture — including the bugs that earned one (e.g. ADR-0008, a forecast double-count found with real data).

• planning/ — where it's going, one doc per phase.

Status & roadmap

Core platform: complete. Bootstrap, data model, core layer, MCP tools + elicitation, internal UI (redesigned: shadcn-style design system, SVG charts, mobile nav), automatic USD→COP conversion at the daily TRM, proactive scheduler, observability, CI, containerization, Hermes dev integration.

Where it's going next, in plain terms: registering receipts and invoices without typing them in by hand, keeping track of what clients owe and chasing that down automatically, and getting flagged the moment a spend spike or an unexpected charge shows up — before it's just a number buried in next month's report. Full detail per phase in planning/:

License

MIT — see LICENSE.