Payment routing orchestrator

Evidence in the hot path, AI at the edges.

Given a card authorization about to be sent, it decides which payment service provider should receive it — on empirical approval evidence, at a fee tolerance the operator states in real units. If the attempt declines, a state machine keyed on the decline's error class decides what happens next: same PSP later, failover now, a different channel, or stop. The routing decision itself is deterministic and auditable; no language model runs inside it.

Built by Bryan Rodríguez Abarca — a payments product manager who decides where a model earns its place, and where it doesn't. More at https://vryahn.com/work/routing

Built and shipped inside Claude Code: the engine, the AI edge layer, the eval harness, the web UI, the API and this README were produced in agentic sessions — one orchestrating session delegating to subagents (backend, UI, publish, case study) — and are gated by the tests and evals you can run yourself. The engineering discipline is the same one described in nutri.: conventions the agent must load, structural guardrails, and a machine — not a promise — as the definition of done. The guardrails caught what would otherwise have shipped: a rewrite that dropped every API path (found by post-deploy verification), a UI mislabeling valid issuers as unseen, and two wrong assumptions of the author's (a page-count rule, a DNS setup) that subagents refused to act on.

Live demo https://orchestrator.vryahn.com · Case study https://vryahn.com/work/routing · API api/README.md · MCP MCP.md

Out-of-sample replay on 84,011 TEST transactions (days 22–31, tables trained on days 1–21): expected approval 72.02% at cost_bias=0 against 66.13% actually observed — +5.89 pp, directional, not an A/B result. See Limitations.

Run it

Python 3.11. requirements.txt is the runtime — fastapi plus the standard library, and the only thing Vercel installs. requirements-dev.txt adds the offline stack (duckdb, pandas, numpy, pyarrow, mcp, uvicorn, httpx) needed to regenerate data, run the backtest, serve MCP, or test locally.

python3.11 -m venv .venv && .venv/bin/pip install -r requirements-dev.txt
.venv/bin/python cli.py --txn-file demo_transactions.json   # 8 decision-boundary cases
.venv/bin/uvicorn api.index:app --reload --port 8000        # API on /api/*, UI from public/

The tables the engine reads (routing_tables.json, routing_meta.json) are committed, so a fresh clone routes — and deploys — without the offline pipeline. Regenerate them only if the data model changes:

.venv/bin/python synth_attempts.py         # seeded ~300k attempts -> attempts.parquet
.venv/bin/python build_routing_tables.py   # -> routing_tables.json, routing_meta.json
.venv/bin/python backtest.py --json        # -> backtest_summary.json
.venv/bin/python tests.py                  # engine
.venv/bin/python tests_ai.py               # AI edges + HTTP contract
.venv/bin/python evals/decline_eval.py     # normalizer against the golden set

Architecture

flowchart LR
    subgraph offline["OFFLINE — batch, once per rebuild"]
        G["synth_attempts.py<br/>seeded generator"] --> A[("attempts.parquet<br/>1 row = 1 attempt, ~300k")]
        A -->|"build_routing_tables.py"| T[("routing_tables.json + routing_meta.json<br/>segment x PSP: n, approvals, p_hat, Wilson LB<br/>4-level hierarchy")]
        A -->|"backtest.py — train d1-21, test d22-31"| B["backtest_summary.json<br/>out-of-sample lift"]
    end

    subgraph edgein["EDGE IN — language to enum"]
        RAW["raw PSP decline<br/>ISO 8583 / decline_code / refusalReason / bank prose"] --> N{"decline_normalizer.py<br/>table -> LLM -> safe fallback"}
        EV["evals/ — 48 golden declines<br/>accuracy by route, hallucination gate"] -.->|"scores"| N
    end

    subgraph online["ONLINE — pure engine, never touches raw data"]
        X["txn: amount, bin6/issuer, funding,<br/>channel, attempt #, error history"] --> D{"decide(txn, config)"}
        T --> D
        N -->|"error_class"| D
        D --> S1["1. resolve segment per PSP<br/>walk L0 to L3 until n >= min_support"]
        S1 --> S2["2. score = Wilson LB x amount x (1 - fee)<br/>= expected net collected"]
        S2 --> S3["3. pick PSP — cost_bias 0..1 maps to<br/>fee tolerance 0..10pp; psps_down excluded"]
        S3 --> S4["4. retry state machine<br/>keyed on last error_class"]
        S4 --> R["Decision: route_psp, eligible_psps with scores,<br/>retry policy, reasoning lines"]
    end

    R --> OPS["ops.py — route, explain, simulate,<br/>evidence, normalize, backtest"]
    B --> OPS
    OPS --> CLI["cli.py"]
    OPS --> API["api/index.py — FastAPI on Vercel<br/>+ public/ web UI, same origin"]
    OPS --> MCP["mcp_server.py — 6 MCP tools"]

Why this design

• Offline/online split. decide(txn, config) -> Decision is pure. It loads pre-materialized tables once and never reads raw attempts, so the decision is microseconds, testable without a database, and auditable after the fact. It is the same boundary you would draw in production between a reconciliation layer and a routing layer.
• Segment hierarchy with fallback. L0 is gateway_group × funding × issuer_bucket × amount_band; L3 is gateway_group alone. Support is resolved per PSP, one dimension at a time (amount band → issuer → funding), until a cell clears min_support (default 200), and the level used is reported with the decision. The channel is first-class and never dropped: user-present and off-session are different worlds.
• Wilson lower bound, not the raw rate. A segment with 3/3 approvals is not a 100% segment. The bound shrinks toward zero as support thins, so a well-evidenced 78% beats a lucky 100% without a separate confidence rule bolted on.
• cost_bias as an explicit knob, in real units. The trade-off is stated as "percentage points of approval I will give up for a cheaper PSP" — tolerance = cost_bias × 10pp, and the cheapest PSP within that tolerance of the best approver wins. A blended score would let a fractions-of-a-point fee difference silently outvote a double-digit approval gap; a tolerance filter cannot. The backtest prices the knob: 72.02% / +5.89 pp approval at cost_bias=0, 71.56% / +5.43 pp at 0.5, 69.57% / +3.44 pp at 1.0.
• Retry by error class, not by a blind counter. insufficient_funds is an account problem and retries the same PSP on the next billing window; bank_auth_required off-session cannot be satisfied without the customer, so it reschedules to a user-present channel instead of burning attempts; fraud_risk stops the chain permanently; generic_decline fails over to the next PSP by score. An unrecognized class degrades to the generic failover policy and says so.

Where AI belongs — and where it does not

There is no LLM inside decide(). Money should not move on a sampled token. The language model is confined to the two edges where natural language actually is the problem.

In — decline_normalizer.py. Each PSP declines in its own dialect: ISO 8583 numerics, a Stripe-like decline_code, an Adyen-like refusalReason, or raw bank prose. The retry state machine keys on one enum, so the dialects have to collapse before the engine sees them. A deterministic table handles the codes that carry volume — confidence 1.0, no latency, no cost. Only a table miss reaches the model chain (Gemini, then Mistral), which answers under a constrained enum schema. Anything outside the enum, or below 0.6 confidence, is discarded in favour of generic_decline, which is the retry policy's own safe default. The repo runs green with no API keys set.

Measured, not trusted — evals/. 48 golden declines: ~60% table hits, ~40% deliberately off-table (misspellings, verbose bank text, unusual codes) plus a few genuinely ambiguous ones where generic_decline is the correct answer. The evals/baseline.json records two baselines. Table-only (no keys): 32/48 = 66.67%, i.e. 100% on the 28 table-route cases and the safe generic_decline default on the 20 that fall through. LLM (keys configured, run against the deployed API with --remote): 48/48 = 100% — 28 table, 19 answered by gemini-3.6-flash, 1 by the low-confidence fallback where generic_decline was the expected answer. The runner reports accuracy by route and a per-class confusion table, asserts zero hallucinations, and fails the build if accuracy drops more than 2 pp below the matching baseline.

Out — mcp_server.py. Six MCP tools — route_transaction, explain_decision, simulate, segment_evidence, normalize_decline, backtest_summary — let an agent operate the engine in English. The agent can interrogate every decision and change none of them. See MCP.md.

Limitations

• Data is synthetic. The structure is designed to make routing decisions non-trivial, not to reproduce any real portfolio.
• No live PSP connectors: the engine decides, it does not send.
• No fraud scoring, 3DS orchestration, network tokens, or scheme retry-rule enforcement.
• The backtest is directional. Historical routing was not randomized, capacity limits are not modeled, and "expected approval" is a TRAIN-period Wilson-LB rate applied to TEST volume — not a live A/B result.
• Tables pool all attempts while the backtest trains and replays on first attempts only; first-attempt-only production tables would be the next fix.
• The LLM eval is 48 cases and one run; 100% on a golden set this small is a gate against regressions, not a claim about the long tail in production.

File map

file	purpose
synth_attempts.py	seeded generator for attempts.parquet; channel mix, PSP fees, approval model, error mix and retry behavior documented at the top
build_routing_tables.py	builds routing_tables.json (segment × PSP: n, approvals, p_hat, wilson_lb, all 4 levels) and routing_meta.json (amount-band edges, issuer whitelist, bin6 → issuer map, PSP fees, error-class enum)
orchestrator.py	the engine: decide(txn, config) -> Decision, Config dataclass. Stdlib only
decline_normalizer.py	PSP decline dialects → the engine's error_class enum: table first, LLM on a miss, safe fallback
ops.py	the operator functions (route, explain, simulate, evidence, normalize, backtest) shared by the API and MCP
api/index.py	FastAPI on Vercel; contract in api/README.md
mcp_server.py	MCP stdio server, six tools; see MCP.md
cli.py	CLI front end: one transaction via flags, or a batch via --txn-file
public/	static web UI, served by Vercel from the same origin as the API
demo_transactions.json	8 decision-boundary transactions with why_interesting notes
backtest.py	TRAIN days 1–21 / TEST days 22–31 replay at cost_bias 0 / 0.5 / 1.0; --json rewrites backtest_summary.json
evals/	48 golden declines, the scoring runner, and the recorded baseline
tests.py / tests_ai.py	assert-based checks: the engine, then the AI edges and the HTTP contract

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Bryan Rodríguez Abarca · vryahn.com · Started from a technical exercise, generalized as a personal project. Synthetic data.