expense-approval-bot/README.md

# Expense Approval Bot

Slack reaction-driven expense approval router. Replaces the Pipedream `expenses_pipeline` workflow.

## Flow

A user reacts ✅ to a message in an expense channel. The bot:

1. Maps the source channel to the next stage's channel.
2. Copies the message text (stripped of any prior "react to advance" hint) into the next channel, with a permalink back to the original on the first advance.
3. Replies in-thread on the original message with `➡️ Advanced to {Stage}`.

Channel chain: Submitted → Processed → Authorized → Matched.

## Architecture

- **Receiver Lambda** (`src/receiver/`) — fronted by API Gateway HTTP API. Verifies the Slack request signature, handles the URL-verification handshake, and async-invokes the processor. Returns 200 within Slack's 3-second window to prevent retries.
- **Processor Lambda** (`src/processor/`) — does the 5-step sequence of Slack API calls. No inbound HTTP; invoked only by the receiver.
- **Secrets** — Slack bot token and signing secret each stored as separate AWS Secrets Manager secrets. Each Lambda only has IAM access to the secret it needs.

Runtime: Python 3.12 on arm64, no third-party dependencies (stdlib `urllib` for HTTP, `boto3` from the Lambda runtime).

## Setup

1. Store the two Slack credentials in Secrets Manager:
   ```bash
   aws secretsmanager create-secret \
     --name expense-bot-slack-token \
     --secret-string "xoxb-..."

   aws secretsmanager create-secret \
     --name expense-bot-slack-signing-secret \
     --secret-string "..."
   ```

2. Copy the sample SAM config and fill in real values:
   ```bash
   cp samconfig.toml.example samconfig.toml
   ```
   - `SlackBotTokenSecretArn` — ARN of the bot-token secret
   - `SlackSigningSecretArn` — ARN of the signing-secret secret

3. Build and deploy:
   ```bash
   sam build && sam deploy
   ```

4. After the first deploy, take the `SlackEventsUrl` output and paste it into the Slack app's **Event Subscriptions → Request URL**. Subscribe the bot to the `reaction_added` event.

## Configuration

The channel → stage mapping lives in `src/processor/app.py` (`STAGES`). Update there if channels change. The "on first advance, include a permalink" check is keyed on the Processed-stage channel ID — update that too if the first-stage channel changes.

## Manual testing

Invoke the processor directly with a synthetic Slack reaction event:

```bash
aws lambda invoke \
  --function-name expense-approval-processor \
  --payload '{"reaction":"white_check_mark","item":{"channel":"C0AQ2AWLNEN","ts":"1700000000.000000"}}' \
  /dev/stdout
```