Ledger

An accounting-ops agent that books what is provably safe and hands a human everything else. The policy gate lives in code, not in the prompt.

What it is

Ledger is an AI agent that closes the daily books for a small e-commerce shop. Given a batch of incoming bank payments, it reconciles each one against the open orders and invoices through a set of typed tools, books only the payments that pass a deterministic policy gate, and routes everything else to a human queue with the reason attached. Every write it makes is appended to an audit log.

The design point is the boundary, not the automation. The model drives the workflow, but it cannot book a payment, send a document, or move money by talking its way past the rules: the decision to book is made by a single function in code that the agent can only call, never override.

Related MCP server: sigmodx-mcp

What is real and what is fiction

Being explicit about this, because a bookkeeping demo lives or dies on trust:

Real (working code you can run):

• The MCP server and its ten typed tools.

• The policy engine in store.py::evaluate_booking: one choke point, deterministic branches, one verdict per payment.

• The append-only audit log and the human approval queue.

• The end-to-end test suite that pins every policy branch.

Fiction (nothing here touches a real ledger):

• The company (Meridian Supply Co., an outdoor-gear shop), its customers, orders, invoices, and the whole payment batch are generated synthetically by generate_data.py. Dates are stamped relative to today, so "close yesterday's books" resolves on any day.

• No bank, no accounting system, and no money are connected. The worst the agent can do is mislabel an internal JSON record, which the audit log captures and a human reverses.

This is a sanitized, synthetic-data demo of a bookkeeping automation I run in production against my own company's books. It runs on invented data (Meridian Supply Co.) so the design and the guardrails are fully on display with nothing real exposed. It is deliberately not framed as banking or regulated finance: it is bookkeeping-ops with a hard capability boundary.

Demos

Two short clips, recorded live against the running system. No mockups.

https://github.com/user-attachments/assets/b6d84570-0d60-4636-b906-fbd26feffc5a

What to watch: one command closes the day. The agent works the batch of eight payments, auto-books the four that are safe (three exact EUR matches plus one USD payment that reconciles only after a visible daily-rate conversion), refuses the four that are not, and prints its full audit trail: every action, every decision, every refusal with a reason.

https://github.com/user-attachments/assets/ee688cb3-f16b-4245-92c9-6f3d1621c593

What to watch: the approval queue. A partial payment (250 of 495 EUR due) is refused, not squeezed through. A second payment against an already-paid invoice is flagged as a likely duplicate with a refund proposed, never executed. An unreadable reference is escalated for research. The agent ends on the point that every open item needs a human, because no tool in the system can send a document or move money.

How it works

The architecture is a short list of design decisions, each aimed at a specific failure mode.

The policy gate is code, not prompt. Whether a payment may auto-book is decided by evaluate_booking(), a single deterministic function. It is the only path to a booking. The agent chooses which payment and invoice to test; the function returns the verdict, and the tool reports it verbatim. A cleverly worded request cannot change the outcome, because the outcome is not the model's to make.

Auto-book has a narrow, explicit definition. A payment books automatically only when the reference names the invoice or its order and the amount matches exactly (tolerance 0.01 EUR). Non-EUR payments convert at a fixed daily rate and auto-book only within 0.5 percent of the amount due, with the conversion written into the audit note. Everything outside that window (partial payments, overpayments, duplicates of paid invoices, unmatchable references, negative amounts) is refused and queued.

The write surface is a capability boundary. There are exactly four write tools, and none of them sends a document, emails a customer, or transfers funds. A refund is a proposal (flag_refund, executed: false). A new invoice is a draft (create_draft_invoice, status: draft, and a draft cannot be booked against). Booking only flips internal record state after the gate passes. The system has no tool that could move money, so no prompt can make it.

Every write is audited and reconciled. Each booking, refusal, draft, refund proposal, and escalation appends an entry to an audit log with the actor, the decision code, the reason, and the references touched. The daily close reports it back in full, so the trail is a first-class output, not a hidden side effect.

Ambiguity is a routing decision, not a guess. When the agent cannot safely book, the answer is not a best-effort booking. It is an item in the human queue with a machine-readable reason code and a plain-language explanation.

flowchart TB
    BATCH["Incoming payment batch"] --> AGENT["Ledger agent<br/>(LLM + persona)"]
    AGENT -->|"4 read-only lookups"| READ["list unmatched · find match<br/>get_order · get_invoice"]
    READ --> SOR[("Orders · invoices · payments")]
    AGENT -->|"book_payment"| GATE{"evaluate_booking<br/>policy gate, in code"}
    GATE -->|"exact amount + reference match"| BOOK["Mark payment matched,<br/>invoice paid"]
    GATE -->|"partial · overpay · duplicate<br/>bad reference · refund"| QUEUE["Human approval queue"]
    AGENT -->|"create_draft_invoice"| DRAFT["Draft invoice<br/>(never sent)"] --> QUEUE
    AGENT -->|"flag_refund"| PROP["Refund proposal<br/>(never executed)"] --> QUEUE
    BOOK --> AUDIT["Append-only audit log"]
    QUEUE --> AUDIT
    QUEUE --> HUMAN["Human decides"]

Stack

• Python for the whole system.

• FastMCP server (mcp_server.py) exposing ten typed tools over streamable-http: six read-only lookups (list_unmatched_payments, find_matching_invoice, get_order, get_invoice, list_pending_approvals, get_audit_log) and four policy-gated writes (book_payment, create_draft_invoice, flag_refund, escalate_payment).

• Policy engine (store.py) holding evaluate_booking, the single booking choke point, plus the audit log and approval queue as append-only JSON state.

• Synthetic data generator (generate_data.py) that rebuilds a pristine, date-relative fixture, so any run starts clean and "yesterday" always resolves.

• Agent persona / system prompt (system_prompt.md) that instructs the model to reconcile, book what the server allows, and escalate the rest with reasons. It has no authority over the policy gate; at most it wastes a tool call.

• Recording face: a rebranded open-source LibreChat front end, used only to film the demo. It is disposable. The durable engineering is the server, the gate, and the audit trail, and the agent runs over any MCP-capable client.

Correctness

Because the policy gate is deterministic, correctness here means pinning every hazard branch, not measuring a distribution. test_flow.py is that check: it isolates each hazard in a throwaway temp directory (so a run never touches the demo state/ fixture), rebuilds fresh state, runs the exact flow the agent runs on camera, asserts one outcome per hazard, and exits non-zero on any drift.

Latent branches the engine also enforces and the suite guards against by construction: overpayment (OVERPAYMENT), a currency with no configured rate (NO_FX_RATE), and any negative amount (REFUND_REQUIRES_HUMAN). The value is that each of these is a named, tested branch rather than a behavior that happens to hold today.

How it is built

I work AI-first: I direct tools like Claude Code to generate and refactor the implementation, and I own the parts that decide whether it is safe to ship, which here are the policy boundary, the capability limits, and the failure modes each guardrail is meant to catch.

Status and contact

Demo. Synthetic data, real engineering. It is one instance of a pattern I reuse across systems: an agent gets typed, allowlisted tools; a guardrail in code (not the prompt) gates every state change; and anything consequential waits for a human.

• More systems and the shared architecture: github.com/janvrsinsky

• LinkedIn: linkedin.com/in/janvrsinsky