fix: allow null content_id in inbound email attachments

[sweetmantech]

Summary

• Resend sends content_id: null for regular file attachments (mp3s, etc.) — only inline attachments (embedded images) have a string content_id
• The Zod schema in validateInboundEmailEvent was rejecting these payloads with a 400 error, preventing the webhook from processing emails with file attachments
• Changed content_id: z.string() to content_id: z.string().nullable() to accept null values

Test plan

• Added validateInboundEmailEvent.test.ts with 4 tests covering: no attachments, string content_id, null content_id, and invalid payload
• Verified null content_id test fails before fix (RED)
• Verified all tests pass after fix (GREEN)
• Sidney to re-send test email with mp3 attachments to confirm end-to-end

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved email attachment handling to support attachments without content identifiers, enhancing compatibility with various email formats.

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Project	Deployment	Actions	Updated (UTC)
recoup-api	Ready	Preview	Mar 3, 2026 9:01pm

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 7fb08b0 and d35dde9.

⛔ Files ignored due to path filters (1)

• lib/emails/__tests__/validateInboundEmailEvent.test.ts is excluded by !**/*.test.*, !**/__tests__/** and included by lib/**

📒 Files selected for processing (1)

• lib/emails/validateInboundEmailEvent.ts

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📝 Walkthrough

Walkthrough

A schema validation change to allow null values for the content_id field in email attachments. The resendAttachmentSchema now accepts string | null instead of strictly string, updating the inferred ResendEmailData type accordingly.

Changes

Cohort / File(s)	Summary
Email Attachment Schema lib/emails/validateInboundEmailEvent.ts	Modified resendAttachmentSchema to make content_id nullable. The field now accepts z.string().nullable() instead of z.string(), updating the ResendEmailData type signature to reflect attachments[].content_id: string | null.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A field grows flexible, now it can be null,
Attachments whisper "maybe" down the hall,
No strings attached, just possibilities,
Content IDs dance—or skip—with ease. 🎭✨

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Solid & Clean Code	✅ Passed	Pull request exemplifies SOLID principles and clean code practices with minimal, focused schema change that maintains single responsibility and respects composition patterns.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch sweetmantech/myc-4395-api-inbound-email-content_id-error

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