AI Agents in Practice: Expense Report Auditor & Reimbursement Agent (Prompts + Code)

Agent Loop (proposal → verification → execution → receipts)

# agent_expense.py
import uuid, json
from datetime import date
from typing import Any, Dict, List
from pydantic import BaseModel, ValidationError
from tools import *
from adapters import *

ALLOWED_TOOLS = {"CheckDuplicates","CheckPolicy","CreateReimbursement"}

def new_idem() -> str:
    return f"idem-{uuid.uuid4()}"

def verify_proposal(p: Dict[str, Any]) -> str:
    required = {"name","args","preconditions","idempotency_key"}
    if not required.issubset(p): return "Missing proposal fields"
    if p["name"] not in ALLOWED_TOOLS: return "Tool not allowed"
    if p["name"] == "CreateReimbursement" and p["args"].get("amount_cents", 0) <= 0:
        return "Amount must be positive"
    return ""

def execute(p: Dict[str, Any]) -> ToolReceipt:
    n, a = p["name"], p["args"]
    if n == "CheckDuplicates":
        a["expense_date"] = date.fromisoformat(a["expense_date"])
        return check_duplicates(CheckDuplicatesArgs(**a))
    if n == "CheckPolicy":
        return check_policy(CheckPolicyArgs(**a))
    if n == "CreateReimbursement":
        return create_reimbursement(CreateReimbursementArgs(**a))
    return ToolReceipt(tool=n, ok=False, ref="none", message="Unknown tool")

# ---- Pretend model call that returns a plan matching the contract ----
def call_model(contract_yaml: str, claims: List[Dict[str,Any]], expense: Dict[str,Any]) -> Dict[str,Any]:
    needs_adjust = expense["category"]=="meal" and expense["amount_cents"]>7500*expense.get("attendees",1)
    approved_amount = min(expense["amount_cents"], 7500*expense.get("attendees",1))
    decision = "approve_with_adjustment" if needs_adjust else "approve"
    return {
      "summary": "Expense audited; eligible with cap applied.",
      "findings": ["Receipt present","No duplicate found","Meal cap applied if needed"],
      "decision": decision,
      "citations": ["policy:meal:cap_usd","policy:receipt_required_over_25","policy:no_alcohol_reimbursed"],
      "tool_proposals": [
        {"name":"CheckDuplicates",
         "args":{"user_id":expense["user_id"],"merchant":expense["merchant"],
                 "amount_cents":expense["amount_cents"],"expense_date":expense["date"]},
         "preconditions":"Same user+merchant+date+amount not submitted.",
         "idempotency_key": new_idem()},
        {"name":"CheckPolicy",
         "args":{"category":expense["category"],"amount_cents":expense["amount_cents"],
                 "attendees":expense.get("attendees",1),
                 "has_receipt": bool(expense.get("receipt_url")),
                 "contains_alcohol": expense.get("contains_alcohol", False)},
         "preconditions":"Receipt present if >$25; meal under cap; no alcohol.",
         "idempotency_key": new_idem()},
        {"name":"CreateReimbursement",
         "args":{"user_id":expense["user_id"],
                 "amount_cents": approved_amount if decision.startswith("approve") else 0,
                 "expense_id": expense["expense_id"],
                 "memo": f"{expense['category']} at {expense['merchant']}"},
         "preconditions":"Policy ok and no duplicate.",
         "idempotency_key": new_idem()}
      ]
    }

def render_response(model_json: Dict[str,Any], receipts: List[ToolReceipt]) -> str:
    idx = {r.tool:r for r in receipts}
    out = []
    out.append(model_json["summary"])
    out.append("")
    out.append("Findings:")
    for f in model_json["findings"]:
        out.append(f"• {f}")
    out.append(f"\nDecision: {model_json['decision']}")
    if "CreateReimbursement" in idx and idx["CreateReimbursement"].ok:
        amt = idx["CreateReimbursement"].data["amount_cents"] / 100
        out.append(f"Reimbursement created: ${amt:.2f} (ref {idx['CreateReimbursement'].ref})")
    elif "CreateReimbursement" in idx and not idx["CreateReimbursement"].ok:
        out.append(f"Reimbursement not created: {idx['CreateReimbursement'].message}")
    out.append("\nCitations: " + ", ".join(model_json["citations"]))
    return "\n".join(out)

def handle(expense: Dict[str,Any]) -> str:
    contract = open("contracts/expense_auditor_v1.yaml").read()
    claims: List[Dict[str,Any]] = []  # plug real claim loader (policy + user history)
    plan = call_model(contract, claims, expense)

    receipts: List[ToolReceipt] = []
    for prop in plan["tool_proposals"]:
        reason = verify_proposal(prop)
        if reason:
            receipts.append(ToolReceipt(tool=prop["name"], ok=False, ref="blocked", message=reason)); continue
        rec = execute(prop)
        receipts.append(rec)
        if not rec.ok and prop["name"] in {"CreateReimbursement"}:
            break
    return render_response(plan, receipts)

if __name__ == "__main__":
    example_expense = {
      "expense_id":"exp-999",
      "user_id":"U007",
      "merchant":"MERCH123",
      "category":"meal",
      "amount_cents": 9800,
      "date":"2025-10-18",
      "attendees":1,
      "receipt_url":"https://receipts/r1.png",
      "contains_alcohol": False
    }
    print(handle(example_expense))

The Prompt You’d Send to the Model (concise and testable)

System:
You are ExpenseAuditor. Follow the contract:
- Audit the expense; ask once if required fields are missing.
- Cite 1–2 claim_ids per factual sentence using provided claims.
- Propose tools; never assert success without a receipt.
- Output JSON with keys: summary, findings[], decision, citations[], tool_proposals[].

Claims (eligible only):
[ ... JSON array of claim records like above ... ]

User:
Please audit this expense and reimburse if eligible:
{"expense_id":"exp-999","user_id":"U007","merchant":"MERCH123","category":"meal",
 "amount_cents":9800,"date":"2025-10-18","attendees":1,
 "receipt_url":"https://receipts/r1.png","contains_alcohol":false}

How to adapt fast

Swap the mock adapters with your finance APIs, enforce idempotency on payouts, and load claims from your policy docs and expense history with freshness windows. Add a validator step before execution to enforce schema, lexicon, locale, citation coverage, and “no implied writes.” Ship the contract, policy bundle, and decoder settings behind a flag with canary + rollback.