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Tech Showcase Code published

Three LLMs, one dunning run

A Python script reads 40 open invoices, assesses each case, enforces clean JSON and drafts the reminder email — using the right model for each step.

Not a client project: a runnable teaching piece with synthetic data, real code and real model outputs. Script and dataset are available for download below.

The question almost everyone asks

“We can chat with AI now. But how do we use LLMs in daily operations — without someone typing prompts all day?”

The answer is unspectacular: take away the chat window. An LLM becomes a working tool when it sits inside a loop — a script feeds it one data row at a time, machine-checks every answer, and writes the result to wherever the work continues.

That is exactly what this showcase demonstrates, on a process every SME knows: the dunning run.

The scenario

The fictional Muster Handwerk AG has 40 open invoices. The receivables list looks the way such lists really look: bookkeeping notes in German, French and English, half-sentences, capital letters, contradictions. Four examples:

InvoiceCustomerAmountDays overdueBookkeeping note
2026-1002 Café Bellevue GmbH CHF 1'350.00 34 hat angerufen, zahlt Ende Monat. tel. bestätigt 08.07.
2026-1003 Menuiserie Perrin Sàrl CHF 8'920.00 41 attend le décompte final avant de payer – à clarifier avec chef de projet
2026-1009 Elektro Wyss AG CHF 6'540.00 88 Gerücht vom Lieferanten: Zahlungsschwierigkeiten?? nichts konkretes. VORSICHT mit Ton
2026-1031 Tech Solutions Ltd CHF 21'430.00 42 invoice stuck in their approval workflow, says 'processed next run' – 2nd time they say this

All 40 rows are synthetic — companies, amounts and notes are invented. The full dataset is in the download.

The pipeline: three steps, three models

Every row passes through three steps. A different model handles each one — always the one that is sufficient for the job:

offene_posten.csv 40 rows Pseudonymization “Jurablick AG” → KUNDE_04 1 Assess Claude Opus 4.8 ANTHROPIC 2 Structure GPT-5 mini OPENAI 3 Write Gemini 2.5 Flash GOOGLE Drafts + results CSV nothing gets sent 🧑‍💼 Edge cases → a human decides zurueckstellen · chef_fragen
1 · Assess

Claude Opus 4.8

Anthropic · $5 / $25 per 1M Tokens

Reads the messy note and weighs the case: payment promise? dispute? partial payment? The strongest model sits where judgement is required.

2 · Structure

GPT-5 mini

OpenAI · $0.25 / $2 per 1M Tokens

Presses the assessment into strict JSON that software can process. Formatting work — a small, cheap model is enough.

3 · Write

Gemini 2.5 Flash

Google · $0.30 / $2.50 per 1M Tokens

Drafts the email in the customer's language — German, French or English, in the tone step 1 prescribed.

Why not one model for everything?

That would work too — the split is deliberately didactic. It shows two things: providers can be swapped per step (no lock-in), and the expensive model only works where it is needed. With 40 invoices that hardly matters; with 40,000 it does.

The code

The full script is a good 200 lines, runs with three API keys as environment variables and sends nothing — emails end up as drafts on disk. The three decisive parts (code comments in German — it is written for Swiss SMEs):

The step-1 prompt — business rules in plain language

The company's collection rules live directly in the prompt. This is the part you can adjust yourself without programming knowledge:

PROMPT_BEURTEILEN = """Du bist erfahrene:r Debitoren-Sachbearbeiter:in der Muster Handwerk AG,
einem Schweizer KMU. Beurteile diesen offenen Posten (Stichtag {stichtag}):

Rechnung {rechnung_nr} an {kunde_pseudo} über CHF {betrag_chf},
fällig seit {tage_ueberfaellig} Tagen, bisherige Mahnungen: {mahnstufe}.
Notiz aus der Buchhaltung (Originalton, evtl. DE/FR/EN): «{notizen}»

Empfiehl in 3–5 Sätzen das weitere Vorgehen. Regeln des Betriebs:
- Zahlungszusagen, Ratenvereinbarungen, Streitfälle und Teilzahlungen NICHT stur weitermahnen.
- Unklare Fälle (Insolvenzgerüchte, unzustellbare Post, Rechts-/Verrechnungsfragen) gehören zum Chef.
- Bei wichtigen Kunden bestimmt, aber beziehungsschonend formulieren.
- Erkenne die Sprache des Kunden (de/fr/en) aus Notiz und Firmenname."""

Three providers, three official SDKs

Each step is a short function call following the official SDK documentation of the respective provider — including token counts for the cost meter:

def beurteilen(client, prompt: str):
    """Schritt 1 — Claude: liest den Fall und empfiehlt das Vorgehen."""
    msg = client.messages.create(
        model=MODELL_BEURTEILEN,
        max_tokens=600,
        thinking={"type": "adaptive"},
        messages=[{"role": "user", "content": prompt}],
    )
    text = "".join(b.text for b in msg.content if b.type == "text")
    return text, msg.usage.input_tokens, msg.usage.output_tokens


def strukturieren(client, prompt: str):
    """Schritt 2 — GPT-5 mini: erzwingt sauberes JSON."""
    r = client.chat.completions.create(
        model=MODELL_STRUKTUR,
        response_format={"type": "json_object"},
        messages=[{"role": "user", "content": prompt}],
    )
    return r.choices[0].message.content, r.usage.prompt_tokens, r.usage.completion_tokens


def schreiben(client, prompt: str):
    """Schritt 3 — Gemini: formuliert den E-Mail-Entwurf."""
    r = client.models.generate_content(model=MODELL_TEXT, contents=prompt)
    u = r.usage_metadata
    return r.text, u.prompt_token_count, u.candidates_token_count

The loop — validate, retry, log

The JSON from step 2 is machine-validated (allowed actions, deadlines, languages). If it is invalid, the model gets exactly one retry with the error message — after that the run aborts rather than producing garbage:

    for p in posten:
        # Schritt 1: beurteilen
        beurteilung, t_in, t_out = beurteilen(c_claude, PROMPT_BEURTEILEN.format(**p))
        kosten_usd += (t_in * PREISE[MODELL_BEURTEILEN][0] + t_out * PREISE[MODELL_BEURTEILEN][1]) / 1e6

        # Schritt 2: strukturieren (1 Wiederholung bei ungültigem JSON)
        prompt2 = PROMPT_STRUKTUR.format(beurteilung=beurteilung, **p)
        for versuch in (1, 2):
            roh, t_in, t_out = strukturieren(c_openai, prompt2)
            kosten_usd += (t_in * PREISE[MODELL_STRUKTUR][0] + t_out * PREISE[MODELL_STRUKTUR][1]) / 1e6
            try:
                entscheid = json_pruefen(roh)
                break
            except (ValueError, json.JSONDecodeError) as e:
                if versuch == 2:
                    raise SystemExit(f"{p['rechnung_nr']}: JSON nach 2 Versuchen ungültig — {e}")
                prompt2 += f"\n\nDeine letzte Antwort war ungültig ({e}). Korrigiere sie."

        # Schritt 3: schreiben — nur wenn wirklich eine E-Mail raus soll
        if entscheid["email_senden"] and entscheid["aktion"] not in ("zurueckstellen", "chef_fragen"):
            entwurf, t_in, t_out = schreiben(c_gemini, PROMPT_TEXT.format(**entscheid, **p))
            kosten_usd += (t_in * PREISE[MODELL_TEXT][0] + t_out * PREISE[MODELL_TEXT][1]) / 1e6
            # Pseudonym erst jetzt, lokal, wieder durch den Klarnamen ersetzen
            entwurf = entwurf.replace(p["kunde_pseudo"], p["kunde"])
            with open(f"entwuerfe/{p['rechnung_nr']}.txt", "w", encoding="utf-8") as f:
                f.write(entwurf)

        ergebnisse.append({"rechnung_nr": p["rechnung_nr"], "kunde": p["kunde"],
                           "betrag_chf": p["betrag_chf"], "tage": p["tage_ueberfaellig"],
                           **{k: entscheid[k] for k in ("aktion", "sprache", "ton",
                                                        "frist_tage", "risiko", "grund")}})
        print(f"{p['rechnung_nr']}  {entscheid['aktion']:<16} {entscheid['risiko']:<7} {entscheid['grund']}")

The test run: real outputs

We ran the pipeline over all 40 invoices. Result: 18× friendly reminder, 3× reminder, 1× final reminder — and 18 cases the system deliberately did NOT dun, but put on hold or escalated to the boss. Those cases are the whole point: a rigid dunning program would have sent a reminder to the customer with a written instalment agreement.

CaseNote (original)DecisionModel's reasoning
2026-1002
CHF 1'350.00 · 34 days overdue
hat angerufen, zahlt Ende Monat. tel. bestätigt 08.07. On hold
risk: medium
Telefonische Zahlungszusage vom 08.07. liegt vor; Zusage abwarten, Wiedervorlage Anfang August.
2026-1004
CHF 23'760.00 · 67 days overdue
2. Mahnung per Post 12.06. KEINE Reaktion. grosser Folgeauftrag offen!! Escalate to boss
risk: high
67 Tage überfällig, zwei Mahnungen unbeantwortet, grosser Folgeauftrag blockiert. Direkte telefonische Klärung nötig, bei Misserfolg Chef übergeben.
2026-1007
CHF 4'870.00 · 51 days overdue
Teilzahlung CHF 2'000 am 30.06. eingegangen, Rest offen. war ok für ihn per mail Friendly reminder
risk: low
Teilzahlung und Zahlungszusage liegen vor; freundliche Erinnerung an Restbetrag von CHF 2'870.00.
2026-1009
CHF 6'540.00 · 88 days overdue
Gerücht vom Lieferanten: Zahlungsschwierigkeiten?? nichts konkretes. VORSICHT mit Ton Escalate to boss
risk: high
Rechnung CHF 6'540.00 seit 88 Tagen überfällig (2. Mahnung). Gerücht über Zahlungsschwierigkeiten – Fall zur Klärung an GL weiterleiten.
2026-1006
CHF 15'300.00 · 28 days overdue
says PO number missing on invoice – resend with PO 4711, then payment 'immediately' On hold
risk: low
Kunde beanstandet fehlende Bestellnummer, bittet um korrigierte Rechnung mit PO 4711 und sagt sofortige Zahlung zu.
2026-1023
CHF 430.00 · 63 days overdue
Kleinbetrag. 2 Mahnungen raus. lohnt sich Betreibung überhaupt? CHEF FRAGEN Escalate to boss
risk: medium
Kleinbetrag von CHF 430, zwei Mahnungen erfolglos. Chef entscheiden, ob Betreibung lohnt.

22 email drafts, three languages

Two examples from the run — unedited, exactly as the model filed them:

How these outputs were produced

Our development environment holds no Anthropic, OpenAI or Google keys. The test run therefore used the exact same prompts and validation through a single OpenAI-compatible endpoint (DeepSeek — the same model that powers the chat on this website). The published code calls the three providers per their official SDKs; the decisions and drafts shown are real, unedited model outputs from that test run.

What does a run cost?

Projection for the 40 invoices: text volumes measured in the test run, converted to tokens (~3.6 characters/token) and multiplied by the three providers' list prices (as of July 2026, USD per 1M tokens):

StepPrice in / outTokens in / out (40 invoices)Cost
Assess
Claude Opus 4.8
5.00 / 25.00 ~8'342 / ~7'661 ~$0.23
Structure
GPT-5 mini
0.25 / 2.00 ~14'061 / ~2'702 ~$0.01
Write
Gemini 2.5 Flash
0.30 / 2.50 ~4'098 / ~3'206 ~$0.01

Total per run: ~$0.25 — with a weekly dunning run, roughly $1.09 per month.

This is a projection, not a bill: token counts are estimated from character volumes; real provider invoices may differ. The order of magnitude is the point — the most expensive item in dunning is not the AI, it is the hour of manual work.

Security, privacy, limits

Real names stay in-house

Before every API call, customer names are replaced by pseudonyms (KUNDE_07) and only re-inserted locally in the finished draft. Providers see amounts and notes, but no names.

Nothing goes out automatically

The script sends not a single email — it writes drafts. And it flagged 18 of 40 cases as boss-matters or holds on its own, instead of dunning.

Keys do not belong in code

API keys come from environment variables. There is not a single key in the published code — that is how it should be.

Limits

An LLM can misread a note. Hence: drafts instead of sending, strict JSON validation, edge cases to a human. Automation here means: machine-prepare the 80% routine, leave the 20% judgement with people.

Transparency

  • Not a client project — a teaching piece, built for this website.
  • All data synthetic: companies, amounts and notes are invented.
  • The code is runnable, provided you supply your own API keys for the three providers.
  • The outputs shown come from a test run through a single DeepSeek endpoint (see above) — not from runs at Anthropic, OpenAI or Google.
  • The cost table is a projection from estimated token counts × list prices.

Try it yourself

Script, dataset and instructions as a ZIP — MIT licence, free to use. And if you want to point it at your real receivables list: that is exactly what we help with.

Download: script + dataset (ZIP)

Which of your processes has a “notes column”?

Dunning, quoting, order handling, the support inbox — wherever text piles up, this pattern works. In a free first call we find the process with the best effort-to-impact ratio.

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