LLM Email Reply Pitfalls: Gmail Threading Requires Matching Subjects
beeblebrox381 · reddit · 2026-08-06
A developer shared engineering hurdles when building an LLM-based email auto-reply agent, specifically regarding Gmail's conversation threading. They discovered that standard RFC headers like References and In-Reply-To are insufficient for Gmail; the subject line must also match exactly.
Furthermore, various email clients add localized prefixes (e.g., AW:, SV:, Re[2]:). Developers must use regex to strip and normalize these prefixes before comparison. The author ultimately restricted the LLM to generating only the email body, handling subject normalization via deterministic code, and asked the community for other hidden provider quirks.
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