Cross-grant message dedup in v3: how do you avoid duplicate storage when multiple connected accounts receive the same email?

Question and Answers
1

I’m building a multi-account integration where one user connects multiple grants (e.g., a personal Gmail and a work Outlook), and I need to handle the case where both inboxes receive the same email (CC, BCC, distribution lists). Today my code stores one row per Nylas message.id, which means the same physical email shows up as two rows, once per grant.

The natural dedup key would be the RFC822 Message-ID: header, which is set by the sending MTA and stable across all recipients of the same message. But v3’s Message object doesn’t seem to expose it. I verified against both the list and find endpoints:

Keys returned by both list and find:
[:attachments, :bcc, :body, :cc, :date, :folders, :from, :grant_id, :id,
 :object, :reply_to, :snippet, :starred, :subject, :thread_id, :to, :unread]

No internet_message_id, no headers, no rfc822_message_id.

What I’ve considered:

  1. Use thread_id as the dedup key: but thread_id identifies a conversation, not a message. Multiple distinct messages in the same thread share it. So this would collapse a 5-message conversation into one row.

  2. Pivot to thread-level canonical: keep one row per thread_id (canonical thread), and per-grant rows for individual messages. Works if thread_id is stable across grants for the same conversation, but I’m not sure Nylas normalizes this across providers (Gmail’s threadId vs Microsoft’s ConversationId).

  3. Synthetic dedup key: hash of (date + from + subject). Fragile against forwarded emails, subject edits, etc.

Questions for the community:

  1. Is there a documented or undocumented way to get the RFC822 Message-ID header (or any per-message-stable identifier) from a v3 Message?

  2. For those running multi-grant integrations: is thread_id reliably stable across grants for the same conversation, including cross-provider (Gmail ↔ Outlook on the same thread)?

  3. How are you handling cross-grant dedup in v3 today?

2

Hello,

You can get the RFC822 Message-ID — it’s just not in the default response. Use the fields query param:

GET /v3/grants/{grant_id}/messages/{id}?fields=include_headers

This returns a headers array on the message object. Pull Message-ID from there — that’s your cross-grant dedup key.

Re: thread_id for dedup — don’t. It’s conversation-level, not message-level, and it’s not guaranteed to match across providers (Gmail’s threadId vs Outlook’s conversationId are normalized differently).

Recommended pattern:

  1. Use Message-ID (from headers) as the canonical dedup key

  2. Store message.id + grant_id as per-account instances

  3. Fall back to a synthetic hash (date + from + subject) only if Message-ID is missing (rare, but some calendar invites omit it)

Tip: If you’re processing messages via webhooks instead of polling, you can set include_headers in your webhook’s field selection so the notification payload already contains the headers — no extra API call needed.

Many thanks,
Samuel R.
Support Engineer, Nylas