Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/letta-ai/letta-code/creating-modsnpx skills add letta-ai/letta-code --skill creating-modsgit clone --depth 1 https://github.com/letta-ai/letta-codeWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00090 | $0.02149 |
| Opus 5 | $0.00045 | $0.01074 |
| Sonnet 5 | $0.00018 | $0.00430 |
| Haiku 4.5 | $0.00009 | $0.00215 |
Grade A, and why
creating-mods scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Runs shell commandslowCapability
Expected in a hook, worth knowing in a rule or an instructions file.
- Prefer Node/Bun standard APIs (`node:child_process`, `node:fs`, etc.) for local work. How it starts
The opening of the file, as written. The whole thing — 162 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Creating Mods
Use this skill to create or update trusted Letta Code mod files. Mods are trusted local code that add small composable capabilities through mod APIs, not by importing app internals. Dynamic agent/conversation/workspace/model state is passed as ctx to tool, command, event, and permission callbacks (panels receive live agent/model in their render context); do not read mutable global context for model-callable behavior. Prefer scoped handles (ctx.conversation, ctx.cwd, ctx.agent) and guard optional UI with letta.capabilities.
Capabilities vary by surface — not every surface loads every capability. The TUI/headless host can load tools, commands, events, UI, and providers; the desktop listener loads tools, commands, providers, and tool/turn events, but not panel UI. Always guard each registration on the capabilities its behavior needs.
Choose where the mod file lives
Default to a single mod file unless the user asks for something larger.
| Location | Use when |
|---|---|
~/.letta/mods/foo.ts |
The behavior should apply to local sessions on this machine. Use this by default. |
$MEMORY_DIR/mods/foo.ts |
The behavior should travel with one agent's MemFS/memory. |
Do not create project mods.
Packaging is an upgrade path, not the default authoring path. If the user asks to share, publish, distribute, or use third-party package dependencies, first build a working mod file, then use letta mods package <mod-file> --name <package-name>. Package install/update/download/publish details belong outside this skill.
Choose the right capability
| User wants | Build |
|---|---|
| Agent/model should autonomously call a local capability | Mod tool |
User wants /foo to send a prompt or run local UI logic |
Mod command |
| Slash command represents a reusable agent workflow | Skill + thin mod command |
| Command should work while the main agent is busy | Command with runWhenBusy: true, handled, panel/status, and usually ctx.conversation.fork() |
| Show transient output above input | Panel, usually from a command |
| Show small persistent state | Status value |
| React to app/session lifecycle or transform outbound turns | Event |
| Enforce dynamic allow/ask/deny policy for tool calls | Permission overlay |
| Add a custom model/API provider for local agents | Provider mod (local agents only) |
| Change the bottom statusline appearance | Use customizing-statusline, not this skill |
What ships with it
9 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- yesterday First seen · 162 lines · 90 tokens per session scan A 0e2521f612b8
creating-mods is a skill published in the GitHub repository letta-ai/letta-code (3,156 stars, last pushed 2d ago), licensed Apache-2.0. It adds 90 tokens to every session and 2,149 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 1 finding (runs shell commands). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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