Edit Issue - .issue.db

Title *

Description

## Problem
Database state is atomic, but external side effects (e.g., tools.http.request) execute *before* the transaction commits. If a worker crashes post-execution but pre-commit, the retry will duplicate the side effect.

## Impact
External systems (APIs, emails) may receive duplicate requests during failure scenarios. This is an at-least-once guarantee instead of exactly-once.

## Technical Details
- Current flow: Execute HTTP request → Commit transaction
- Failure scenario: HTTP succeeds → Worker crashes before commit → Retry sends duplicate request
- Affected tools: tools.http.request, potentially tools.email.send

## Proposed Solution
Enhance tools.http.request to automatically inject a deterministic Idempotency-Key header derived from:
- ctx.workflow_run_id
- step_name
- attempt_number (optional, for retry tracking)

Example: Idempotency-Key: {workflow_run_id}:{step_name}:{attempt}

## Implementation Steps
1. Add idempotency_key generation in tools/http_request.py
2. Auto-inject header unless explicitly disabled via skip_idempotency_key=True
3. Document the pattern for external API integrations
4. Add integration test verifying idempotency on retry

## Acceptance Criteria
- [ ] HTTP requests include deterministic Idempotency-Key header by default
- [ ] Key derivation is documented
- [ ] Integration test covers crash-recovery scenario
- [ ] Email tool also implements idempotency pattern

## Tags: atomicity, reliability, http, side-effects

Due Date (YYYY-MM-DD)

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Edit Issue #261