Fix draft_gmail_message: reply-threading crash, threadId not attaching, signature line breaks

[erikholz]

Three related bugs in draft_gmail_message's reply-threading path, all reproduced against a live Gmail account.

1. Crash when thread_id is set without explicit in_reply_to/references. _fetch_thread_reply_context did an unguarded message_contexts[-1] to pick a fallback target message. If every message in the thread carries the TRASH label, the earlier skip-trash filter empties message_contexts while the thread itself still has messages, so this raises IndexError: list index out of range. Fixed by only taking the fallback when the list is non-empty; a missing target already degrades gracefully everywhere it's consumed.

2. threadId not forwarded to the Gmail API — already fixed on main via #889, included here for completeness/testing, no additional change.

3. Signature line breaks lost when appended to a plain-text body. The general _html_to_text/_HTMLTextExtractor (built for reading arbitrary message bodies as flowing text) inserts no separator between block elements and collapses all whitespace to single spaces, so adjacent signature lines (e.g. one <p> per line) get concatenated with zero whitespace between them -- no amount of post-hoc whitespace collapsing can fix that since there was never a separating character to begin with. Added a dedicated _HTMLSignatureExtractor that treats <br> and block boundaries as real newlines, used only for signature-to-text conversion so the general message-reading path (and its test coverage) is untouched.

Reproduction/verification harness included at scripts/repro_bugs.py. 143 existing tests pass.

Summary by CodeRabbit

• Bug Fixes
  • Gmail signatures now preserve readable line breaks and formatting when included in outgoing messages.
  • Improved plain-text conversion for HTML signatures.
  • Prevented errors when replying to threads with no available message context.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 016831d5-8367-4c7c-861b-a766cd37e325

📥 Commits

Reviewing files that changed from the base of the PR and between 30d5438 and 73b1477.

📒 Files selected for processing (2)

• gmail/gmail_helpers.py
• gmail/gmail_tools.py

---

📝 Walkthrough

Walkthrough

Gmail signature HTML now uses a dedicated parser-based conversion path in outgoing HTML and plain-text composition. Reply-context selection also avoids indexing an empty context list when the requested message is unavailable.

Changes

Gmail draft defect handling

Layer / File(s)	Summary
Signature HTML conversion gmail/gmail_helpers.py	Adds a specialized HTML parser that preserves block and <br> line breaks, skips script/style content, normalizes whitespace, and falls back to the original HTML on conversion errors.
Signature composition integration gmail/gmail_tools.py	Uses the signature-specific converter in outgoing HTML and plain-text signature paths.
Reply-context fallback handling gmail/gmail_tools.py	Prevents an empty message_contexts list from causing an invalid last-item lookup.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• taylorwilsdon/google_workspace_mcp#530: Adds Gmail signature fetching and appending paths touched by this conversion refinement.
• taylorwilsdon/google_workspace_mcp#728: Adjusts the Gmail thread context handling that this PR makes safe for empty contexts.
• taylorwilsdon/google_workspace_mcp#902: Also modifies Gmail HTML-to-text handling for <br> behavior.

Suggested reviewers: taylorwilsdon, jcwatson11

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description explains the changes, but it does not follow the required template sections or include the checklist and testing fields.	Rewrite the PR description using the repository template: fill Description, Type of Change, Testing, Checklist, and Additional Notes.
Docstring Coverage	⚠️ Warning	Docstring coverage is 53.85% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main fixes: reply-threading crash and signature line-break handling, though it also mentions threadId forwarding that wasn't changed here.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[taylorwilsdon]

Please move helpers to gmail_helpers.py and drop the bug test script thing and I will get this merged thanks!