# Deduplicate a signed Flask webhook inside its state transaction

A provider may deliver an authenticated event several times. Signature verification establishes its origin; a unique receipt and a transaction establish whether this application already processed it. Neither check replaces the other.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

## Implement the Flask boundary

Save this illustrative integration module as `webhooks.py`.

```python
from sqlalchemy import text

def apply_paid_event(engine, event_id, payment_session_id):
    with engine.begin() as connection:
        receipt = connection.execute(text(
            "INSERT INTO webhook_receipts (event_id) VALUES (:event_id) "
            "ON CONFLICT (event_id) DO NOTHING RETURNING event_id"
        ), {"event_id": event_id}).scalar_one_or_none()
        if receipt is None:
            return "duplicate"
        changed = connection.execute(text(
            "UPDATE orders SET status = 'paid' "
            "WHERE payment_session_id = :session AND status = 'pending' "
            "RETURNING id"
        ), {"session": payment_session_id}).scalar_one_or_none()
        if changed is None:
            raise ValueError("Expected pending payment session")
        return "applied"
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/checkout.py). This Flask implementation is an illustrative adaptation.

## Integrate with your application

At the Flask route boundary, verify the signature against request.get_data(cache=True) using the provider SDK before passing event_id or payment_session_id to this service. Use PostgreSQL tables with unique event_id and unique payment_session_id. Return success for a known duplicate; surface transient failures so the provider retries. Put any downstream notification into an outbox in this same transaction.



## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| First signed paid event | Receipt and paid transition commit |
| Same event delivered again | Duplicate result, no new state change |
| Invalid signature | Route rejects before service call |
| Order mutation fails | Receipt rolls back so a retry can run |

Add regression tests for these cases in your existing Flask test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

```sh
python -m pytest tests/test_paid_webhook.py
```

The scenarios above specify tests to implement. A completed Perch source check below is separate from running these application integration tests.

## Introduce the specific agent regression

In a throwaway copy, replace this source span:

```python
if receipt is None:
            return "duplicate"
```

with:

```python
// Remove the source span shown above.
```

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

## Preserve the requirement as a Perch rule

Save this in `.perch/rules/perch-guide-flask-webhook-deduplication.yaml` at the repository root, keeping your existing `perch.yaml`:

```yaml
- name: webhook-receipt-gates-side-effect
  where: "webhooks.py"
  ensure: >-
    If the unique webhook receipt insert returns no row, apply_paid_event must return before changing any order.
```

Install and authenticate the CLI once, then check the current working file:

```sh
npm install -g @lakeday/perch
perch login
perch check webhooks.py --rules webhook-receipt-gates-side-effect --json
```

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

## Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](/webhook-deduplication/capture.json) · [Exact sources and mutation](/webhook-deduplication/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](/webhook-deduplication/rule-pack.zip) · [Download the YAML rules](/webhook-deduplication/rules.yaml) · [Agent task](/webhook-deduplication/agent-task.txt). The pack contains the rule, its example paths and the repair/recheck instructions. Review the `where` selector before using it in an existing project. Keep your root `perch.yaml`; install this rule as a separate file under `.perch/rules/`.

1. Install Perch using Node 22 or newer and sign in once:

```sh
npm install -g @lakeday/perch
perch login
```

2. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

```sh
curl -fL 'https://reporules.fyi/flask/webhook-deduplication/rule-pack.zip' -o 'perch-guide-flask-webhook-deduplication.zip'
unzip -n 'perch-guide-flask-webhook-deduplication.zip'
```

The installed rule is `.perch/rules/perch-guide-flask-webhook-deduplication.yaml`. Its selector matches `webhooks.py` in this example; adapt the selector to your application file before checking an existing project.

3. Configure the coding agent you use. Run one of these commands:

Claude Code:

```sh
perch setup claude-code
```

Codex:

```sh
perch setup codex
```

Cursor:

```sh
perch setup cursor
```

Pi:

```sh
perch setup pi
```

4. Ask the agent to edit the implementation and check the saved requirement:

> Use the installed Perch skill while editing webhooks.py. Preserve webhook-receipt-gates-side-effect. Run perch check webhooks.py --rules webhook-receipt-gates-side-effect --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

5. After repairing a finding, run the same check again and inspect the result:

```sh
perch check webhooks.py --rules webhook-receipt-gates-side-effect --json
```

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use `perch scan --since origin/main --filter type=lint` in CI.

## Framework references and verification scope

- [flask.palletsprojects.com reference](https://flask.palletsprojects.com/en/stable/api/#flask.Request.get_data)
- [docs.sqlalchemy.org reference](https://docs.sqlalchemy.org/en/21/core/connections.html#using-transactions)

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.