fix(gmail): send emails as multipart/alternative so they render full-width

[waleedlatif1]

Summary

• Gmail send/draft tools were emitting bare text/plain, which Gmail renders in a narrow hard-wrapped column — LLM-generated bodies looked cropped width-wise compared to hand-composed emails.
• Switched to multipart/alternative (text/plain first, text/html last, per RFC 2046) so HTML clients render flowed paragraphs while plain-text-only clients keep the original body as the fallback. Attachments now nest as multipart/mixed → multipart/alternative + parts, the canonical Gmail API structure.
• Backwards compatible: same tool params, same response shape, same threading/RFC 2047 subject encoding; only the wire MIME structure changes. Verified by decoding the actual base64url payload.

Type of Change

• Bug fix

Testing

Added 11 unit tests for the new helpers and both build functions (15/15 pass). Decoded real wire output to confirm MIME structure. bun run check:api-validation:strict passes.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Skipped		May 15, 2026 2:16am

[cursor]

PR Summary

Medium Risk
Changes the wire-format MIME structure and content-transfer encoding for all outgoing Gmail messages, which could affect rendering or client compatibility if boundaries/encodings are wrong. Covered by new unit tests, but still impacts a core integration path.

Overview
Switches Gmail message generation from single-part text/plain/text/html to multipart/alternative (text/plain + text/html) so messages render full-width in Gmail while preserving a plain-text fallback.

Adds helpers to safely convert between plain text and HTML (escapeHtml, plainTextToHtml, htmlToPlainText, buildBodyAlternatives) and changes body transfer encoding to base64 with RFC2045 line wrapping for UTF-8 safety.

Updates attachment handling to use multipart/mixed → multipart/alternative + attachments and expands unit coverage to validate MIME structure, threading headers, and UTF-8 round-trips.

^{Reviewed by Cursor Bugbot for commit 01ca2dd. Configure here.}

[waleedlatif1]

@greptile

[waleedlatif1]

@cursor review

[greptile-apps]

Greptile Summary

This PR fixes the narrow hard-wrapped rendering issue in Gmail by switching both the send and draft tools from bare text/plain to multipart/alternative, emitting a proper text/plain + text/html pair for every message. When attachments are present, the alternative block is nested inside a multipart/mixed envelope following the canonical Gmail API structure.

• MIME structure change: buildSimpleEmailMessage now emits multipart/alternative directly; buildMimeMessage wraps it in multipart/mixed when attachments exist, using two independent boundaries.
• Encoding fix: Both body parts now use Content-Transfer-Encoding: base64 (76-char line-wrapped) instead of 7bit, allowing safe transport of emoji, accented characters, and other multi-byte UTF-8 content.
• New helpers: escapeHtml, plainTextToHtml, htmlToPlainText, and buildBodyAlternatives cover the text↔HTML conversion path; all are exported and covered by tests that decode real base64 wire output.

Confidence Score: 5/5

Safe to merge — the change is backwards-compatible (same tool params, same response shape), the MIME structure follows RFC 2046, and the new encoding path is well-covered by tests that decode real wire output.

The MIME restructuring is straightforward and localized to two builder functions. Both the attachment and non-attachment paths produce valid RFC 2046 MIME. Base64 encoding correctly handles arbitrary UTF-8. The duplicate MIME-Version header that existed in the old else branch of buildMimeMessage is silently cleaned up as a side effect. No pre-existing callers are broken since the public API surface is unchanged.

No files require special attention.

Important Files Changed

Filename	Overview
apps/sim/tools/gmail/utils.ts	Adds multipart/alternative MIME structure for both send and draft paths; introduces escapeHtml, plainTextToHtml, htmlToPlainText, buildBodyAlternatives, and renderAlternativeParts helpers; switches body encoding from 7bit to base64 to handle arbitrary UTF-8
apps/sim/tools/gmail/utils.test.ts	Adds 11 unit tests covering the new helper functions and both message-build paths; decodePart helper validates MIME structure by decoding actual base64 payloads

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[buildSimpleEmailMessage / buildMimeMessage] --> B{Has attachments?}
B -- No --> C[multipart/alternative]
B -- Yes --> D[multipart/mixed]
D --> E[multipart/alternative nested part]
D --> F[attachment parts base64 encoded]
C --> G[text/plain base64 + UTF-8]
C --> H[text/html base64 + UTF-8]
E --> G
E --> H
I[contentType = text or undefined] --> J[plainTextToHtml blank lines to p newlines to br]
I --> K[plain = body as-is]
L[contentType = html] --> M[htmlToPlainText strip tags decode entities]
L --> N[html = body as-is]
J --> H
K --> G
M --> G
N --> H

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[waleedlatif1]

@greptile

[waleedlatif1]

@cursor review

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✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 02b88f9. Configure here.}

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit 01ca2dd. Configure here.}