Simulator

A local FedNow Service simulator: POST a pacs.008 customer credit transfer, receive the pacs.002 advice the FedNow Service would send — under scenarios you control, including the one that hurts in production (no answer at all).

v0 speaks HTTP (dev mode). An MQ-compatible interface is planned; the scenario engine is shared.

Quickstart

cargo run -p fednow-sim
# in another shell: generate a valid pacs.008 and send it
cargo run -p fednow-core --example build_pacs008 > /tmp/pacs008.xml
curl -s -X POST --data-binary @/tmp/pacs008.xml \
  -H "content-type: application/xml" http://localhost:8080/fednow/messages

The response is a pacs.002 ACSC advice (accepted, settlement completed) that fednow-core itself validates as direction-clean.

Docker:

docker build -f simulator/Dockerfile -t fednow-sim .
docker run -p 8080:8080 fednow-sim

Scenarios

Priority: config file → amount trigger → default.

Trigger	Scenario	Advice
default	settle	ACSC + acceptance time + effective settlement date
amount ends .11	reject	RJCT reason AC04
amount ends .22	accept without posting	ACWP
amount ends .33	timeout	none — HTTP 202, no pacs.002
amount ends .44	delayed settle	ACSC after 2 s
amount ends .55	reject by the service	RJCT proprietary reason E990 (vs .11’s participant reject)
amount ends .66	ACWP + follow-up	ACWP now; ACCC follow-up queued, retrieved with a pacs.028
profile-invalid message	always rejected	RJCT proprietary reason SIMV, violated rule codes in AddtlInf

Each trigger maps to an official Customer Testing Program scenario — see the zero-to-CTP chapter.

Config file (fednow-sim.toml or FEDNOW_SIM_CONFIG), keyed by creditor-agent routing number — see fednow-sim.toml.example. Actions: settle, accept-without-posting, reject (+ reason), timeout, delay (+ delay_ms). The real service’s technical error codes will replace SIMV once the Technical Specifications are available (issue #14).

Timeout reconciliation — the point of this simulator

A timed-out payment is unresolved, not failed: internally it still settled; your sender just never heard it. The simulator keeps an advice ledger for every processed payment, and a payment status request (pacs.028) — posted to the same endpoint, like on the real MQ channel — returns the withheld advice:

# 1. Send a pacs.008 with an amount ending in .33 → HTTP 202, no advice.
# 2. Ask what happened:
curl -s -X POST --data-binary @pacs028.xml \
  -H "content-type: application/xml" http://localhost:8080/fednow/messages
# → the ACSC advice: it settled all along. A blind resend would have paid twice.

Unknown original message id → HTTP 404. The full walkthrough lives in the FedNow Integration Handbook chapter on timeout reconciliation (docs/handbook/).

MQ mode — the real connection’s semantics

The production FedNow connection is an IBM MQ queue pair, and it is asynchronous: a send returns nothing; advices arrive later on your receive queue, wrapped in the FedNow technical envelope with a Business Application Header. MQ mode mirrors exactly that:

# PUT: fire-and-forget send of a FedNowIncoming envelope → 202, empty body
curl -s -X POST --data-binary @envelope.xml \
  -H "content-type: application/xml" \
  http://localhost:8080/mq/participants/991000009/send

# GET: next FedNowOutgoing envelope from your receive queue (204 when empty)
curl -s http://localhost:8080/mq/participants/991000009/receive

Differences from the HTTP dev mode, all deliberate:

• Every outcome is asynchronous — even profile-validation rejections (RJCT/SIMV) arrive as queued advices, not HTTP errors.
• The post-ACWP follow-up status (.66 trigger) is pushed to the queue ~500 ms later; no pacs.028 needed (the HTTP mode cannot push).
• A timeout (.33) leaves the queue empty — poll all you want, nothing comes until a pacs.028 (sent through the same /send) replays the withheld advice.

Scenario triggers, config file and the advice ledger are shared between modes.

Endpoints

Method	Path	Purpose
POST	/fednow/messages	HTTP dev mode: pacs.008 → pacs.002 advice; pacs.028 → stored advice replay
POST	/mq/participants/{rtn}/send	MQ mode: fire-and-forget FedNowIncoming (pacs.008, pacs.028)
GET	/mq/participants/{rtn}/receive	MQ mode: destructive get of the next FedNowOutgoing
GET	/healthz	liveness

State is in-memory and per-process (v0): restart forgets past payments and queued advices.