> ## Documentation Index
> Fetch the complete documentation index at: https://docs.nika.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# Invoice chaser

> T2 chain · finance — overdue reminders drafted, and nothing goes out until a human says yes.

> **T2 chain · finance / freelance** — the human-in-the-loop example.
> `nika:prompt` blocks the workflow until someone answers; the
> drafting is automated, the **sending decision never is**.

## The job

Friday: open the ledger, find who's late, write polite-but-firm
reminders, don't send the awkward one to the client who paid this
morning. This file filters the CSV deterministically (jq, not an LLM
guessing at numbers), drafts every reminder, then stops and asks you.

## The shape

```mermaid theme={"system"}
flowchart LR
  ledger["ledger · nika:read"]:::invoke
  rows["rows · nika:convert"]:::invoke
  overdue["overdue · nika:jq"]:::invoke
  drafts["drafts"]:::infer
  approve["approve · nika:prompt"]:::invoke
  save["save · nika:write"]:::invoke
  ledger --> rows
  rows --> overdue
  overdue -.-> drafts
  drafts -.-> approve
  approve -.-> save
  drafts -.-> save
  classDef infer fill:#5b8cff22,stroke:#5b8cff,color:#5b8cff
  classDef invoke fill:#22d3ee22,stroke:#22d3ee,color:#22d3ee
```

## The file

```yaml t2-invoice-chaser.nika.yaml theme={"system"}
nika: v1
workflow: invoice-chaser
description: "Ledger CSV → overdue filter → drafted reminders → human gate → drafts file"

model: mock/echo            # swap for groq/llama-3.3-70b — drafting is a fast-model job

vars:
  ledger_csv: "./finance/invoices.csv"

tasks:
  - id: ledger
    invoke:
      tool: "nika:read"
      args: { path: "${{ vars.ledger_csv }}" }

  - id: rows
    depends_on: [ledger]
    invoke:
      tool: "nika:convert"
      args:
        input: "${{ tasks.ledger.output }}"
        from: csv
        to: json
        has_header: true

  - id: overdue
    depends_on: [rows]
    invoke:
      tool: "nika:jq"
      args:
        input: "${{ tasks.rows.output }}"
        expression: 'map(select(.status == "overdue"))'

  - id: drafts
    depends_on: [overdue]
    when: ${{ size(tasks.overdue.output) > 0 }}
    infer:
      prompt: |
        Draft one short, polite payment reminder per overdue invoice ·
        ${{ tasks.overdue.output }}
        Markdown · one section per client · firm but warm.

  - id: approve
    depends_on: [drafts]
    when: ${{ tasks.drafts.output != null }}   # nothing drafted · nothing to approve
    invoke:
      tool: "nika:prompt"
      args:
        message: "Save the reminder drafts for sending?"
        default: false

  - id: save
    depends_on: [approve, drafts]
    when: ${{ tasks.drafts.output != null && tasks.approve.output == true }}   # zero overdue → drafts skipped (null) · don't write nothing
    invoke:
      tool: "nika:write"
      args:
        path: "./finance/reminders-to-send.md"
        content: "${{ tasks.drafts.output }}"

outputs:
  overdue:
    value: ${{ tasks.overdue.output }}
    type: array
    description: "The overdue rows the reminders were drafted for"
```

## How it works

<Steps>
  <Step title="Data work is jq work">
    CSV → JSON via `nika:convert`, then `map(select(.status == "overdue"))`
    in `nika:jq`. No model touches the numbers — deterministic, free,
    auditable.
  </Step>

  <Step title="size() gates the whole tail">
    `when: ${{ size(tasks.overdue.output) > 0 }}` — the canonical
    empty-check. No overdue invoices → the rest of the file skips.
  </Step>

  <Step title="The approval gate">
    `nika:prompt` blocks until answered (CI can use `default:`). The
    save runs only `when: ${{ tasks.approve.output == true }}`.
  </Step>
</Steps>

## Constructs you just used

| Construct                | Where         | Reference                        |
| ------------------------ | ------------- | -------------------------------- |
| `nika:convert` from/to   | `rows`        | [Builtins](/reference/builtins)  |
| `nika:jq` filtering      | `overdue`     | [Builtins](/reference/builtins)  |
| CEL `size()`             | `drafts.when` | [Workflows](/concepts/workflows) |
| `nika:prompt` human gate | `approve`     | [Builtins](/reference/builtins)  |

## Make it yours

* Per-client emails instead of one file: `for_each` over the overdue rows ([fan-out tier](/examples/competitor-radar)).
* Escalate 60-days-late differently: a second jq filter + a sterner prompt.
* Wire the approved drafts to your mail API with `nika:fetch method: POST`.

<Card title="Next · Support triage" icon="headset" href="/examples/support-triage">
  Classify a whole queue in one schema-typed call, then let jq slice the
  urgent ones out.
</Card>