fix(gmail): bring +forward behavior in line with Gmail web UI

[malob]

Description

The +forward helper (added in #105) sends forwards as new threads, which doesn't match Gmail's web UI behavior. When you forward from Gmail, the forward stays in the sender's original conversation thread. This PR fixes that and cleans up the forwarded message formatting.

Threading fix:

• Pass the original threadId to send_raw_email (matching what +reply already does)
• Add In-Reply-To and References headers for RFC 5322 compliance — required by the Gmail API for thread placement alongside threadId

Formatting fixes:

• Add blank line between the forwarded message metadata block and body (was missing, causing the body to run into the To:/Cc: line)
• Remove spurious closing ---------- delimiter that Gmail doesn't produce

SKILL.md:

• Remove tip that said forwards are sent as new messages

Related: #88

Dry Run Output:

$ gws gmail +forward --message-id 18f1a2b3c4d --to dave@example.com --body 'FYI see below' --dry-run

{
  "body": {
    "raw": "VG86IGRhdmVAZXhhbXBsZS5jb20NClN1YmplY3Q6...<truncated>",
    "threadId": "thread-18f1a2b3c4d"
  },
  "dry_run": true,
  "is_multipart_upload": false,
  "method": "POST",
  "query_params": {},
  "url": "https://gmail.googleapis.com/gmail/v1/users/me/messages/send"
}

Decoded raw (base64 → plaintext):

To: dave@example.com
Subject: Fwd: Original subject
In-Reply-To: <18f1a2b3c4d@example.com>
References: <18f1a2b3c4d@example.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8

FYI see below

---------- Forwarded message ---------
From: sender@example.com
Date: Thu, 1 Jan 2026 00:00:00 +0000
Subject: Original subject
To: you@example.com

Original message body

Checklist:

• My code follows the AGENTS.md guidelines (no generated google-* crates).
• I have run cargo fmt --all to format the code perfectly.
• I have run cargo clippy -- -D warnings and resolved all warnings.
• I have added tests that prove my fix is effective or that my feature works.
• I have provided a Changeset file (e.g. via pnpx changeset) to document my changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: 11e3901

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the +forward command to mirror Gmail's native forwarding behavior. The primary goal is to ensure that forwarded emails remain within their original conversation threads and are formatted consistently with the web UI, enhancing user experience and message organization.

Highlights

• Threading Alignment: The +forward functionality now aligns with Gmail's web UI by keeping forwarded messages within the sender's original conversation thread, rather than creating new ones.
• RFC 5322 Compliance: Implemented In-Reply-To and References headers in forwarded emails, which are crucial for correct thread placement by the Gmail API.
• Formatting Improvements: Cleaned up forwarded message formatting by adding a blank line between the forwarded message metadata block and its body, and by removing a spurious closing delimiter.
• Documentation Update: Updated the SKILL.md documentation to remove an outdated tip regarding forwards being sent as new messages.

Changelog

• .changeset/fix-forward-threading.md
  • Added a new changeset file documenting the +forward behavior fix.
• skills/gws-gmail-forward/SKILL.md
  • Removed the tip indicating that forwards are sent as new messages, reflecting the updated behavior.
• src/helpers/gmail/forward.rs
  • Modified handle_forward to pass the original message's threadId to send_raw_email.
  • Updated create_forward_raw_message to include In-Reply-To and References headers for RFC 5322 compliance, ensuring proper threading.
  • Adjusted format_forwarded_message to add a blank line between the forwarded message metadata and its body.
  • Removed the unnecessary closing ---------- delimiter from forwarded messages.
  • Expanded test cases to verify the presence of new headers and the corrected formatting, including a new test for References header chaining.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

The pull request updates the Gmail +forward functionality to align with Gmail's web UI by ensuring forwarded messages remain in the original thread, adding a blank line between the forwarded message metadata and body, and removing the closing delimiter. However, a security vulnerability was identified in the create_forward_raw_message function, where unsanitized user inputs and message metadata are directly used in email headers, leading to potential Email Header Injection and an invalid MIME-Version header due to incorrect formatting.

[codecov]

Codecov Report

❌ Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.77%. Comparing base (cfa0984) to head (11e3901).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/helpers/gmail/forward.rs	79.48%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
+ Coverage   61.72%   61.77%   +0.04%     
==========================================
  Files          38       38              
  Lines       14603    14640      +37     
==========================================
+ Hits         9014     9044      +30     
- Misses       5589     5596       +7     

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