Is the Sendmux Attachments skill free?

Yes. The skill itself is free from Sendmux/skills. SkillProof publishes the install command and an independent test verdict at no cost.

What is the Sendmux Attachments SkillProof Score?

9.6/10 — installs cleanly 5/5, triggers reliably 5/5, output vs. baseline 9/10, docs & honesty 5/5.

How do I install Sendmux Attachments?

Copy the install command from this page, run it in your terminal, and restart Claude Code. Skills live in ~/.claude/skills/ (global) or .claude/skills/ inside a project.

Can I use Sendmux Attachments with Cursor, Copilot, Gemini CLI, Codex or other AI tools?

The SKILL.md format is native to Claude (Claude Code, Desktop, claude.ai). The instructions inside adapt to other assistants: Cursor rules, GitHub Copilot instructions, Windsurf rules, Custom GPTs, AGENTS.md for OpenAI Codex, and GEMINI.md for Google Gemini CLI — our conversion guides cover each, and the free converter on the tools page does the wrapping for you.

Sendmux Attachments

Picks the cheapest Sendmux attachment path (file_path or presigned URL) instead of burning context on base64.

by Sendmux · Sendmux/skills

Tested · Works

Install — copy the command Download source ↗

Test report

Verdict: Tested · Works
Score: 9.6/10
Tested: Jul 14, 2026
Environment: Claude Code 2.x (agent harness)
Upstream re-checked: Aug 10, 2026 · b394073

For a 5MB local PDF, an unguided agent reaches for base64 (roughly 2M+ tokens, over the 32KiB inline cap by ~150x); the skill correctly routes to a local file_path upload instead, near-zero token cost.

Scored on four weighted criteria — install, triggering, output vs. baseline, docs. How scoring works

Installs cleanly 5/5
Triggers reliably 5/5
Output vs. baseline 9/10
Docs & honesty 5/5

What Sendmux Attachments does

Reference skill for Sendmux MCP/CLI/SDK attachment workflows: tells the agent when to use a local file_path, a presigned upload URL, CLI --attach, SDK file helpers, or inline base64 for uploading, downloading, or forwarding email attachments. Documents exact size caps (7.5MB mailbox, 18MiB send upload, 32KiB inline base64) and token-cost math so an agent doesn't accidentally pipe megabytes of base64 through the context window. Use whenever an agent is about to touch attachment bytes through Sendmux, in MCP, CLI, SDK, or direct HTTP form.

How to install Sendmux Attachments

git clone https://github.com/Sendmux/skills
cd skills
mkdir -p ~/.claude/skills
cp -r plugins/sendmux/skills/sendmux-attachments ~/.claude/skills/sendmux-attachments

Skills live in ~/.claude/skills/ (global) or .claude/skills/ (per-project). Restart Claude Code after installing.

Commands — how to trigger Sendmux Attachments

/sendmux-attachments Picks the cheapest Sendmux attachment path (file_path or presigned URL) instead of burning context on base64.

It also activates on plain-language prompts like these:

What's the cheapest way to send this 5MB PDF via Sendmux?
Should I use file_path or base64 for this Sendmux upload?
Forward this email attachment through Sendmux without wasting tokens

Frequently asked questions

Is the Sendmux Attachments skill free?: Yes. The skill itself is free from Sendmux/skills. SkillProof publishes the install command and an independent test verdict at no cost.
Does Sendmux Attachments work with Claude Code?: We tested it with Claude Code 2.x (agent harness) on Jul 14, 2026. Verdict: Tested · Works. For a 5MB local PDF, an unguided agent reaches for base64 (roughly 2M+ tokens, over the 32KiB inline cap by ~150x); the skill correctly routes to a local file_path upload instead, near-zero token cost.
What is the Sendmux Attachments SkillProof Score?: 9.6/10 — installs cleanly 5/5, triggers reliably 5/5, output vs. baseline 9/10, docs & honesty 5/5.
How do I install Sendmux Attachments?: Copy the install command from this page, run it in your terminal, and restart Claude Code. Skills live in ~/.claude/skills/ (global) or .claude/skills/ inside a project.
Can I use Sendmux Attachments with Cursor, Copilot, Gemini CLI, Codex or other AI tools?: The SKILL.md format is native to Claude (Claude Code, Desktop, claude.ai). The instructions inside adapt to other assistants: Cursor rules, GitHub Copilot instructions, Windsurf rules, Custom GPTs, AGENTS.md for OpenAI Codex, and GEMINI.md for Google Gemini CLI — our conversion guides cover each, and the free converter on the tools page does the wrapping for you.