AI agent skill · by @yvgude

Lean Ctx

Pick the right lean-ctx tool for reading, searching, understanding and running code with fewer tokens and fewer round trips. Use when lean-ctx MCP tools (ctx_*) are available or when a command output, file read or search would be large.

Archived 2026-10-23 · Source updated 2026-10-11 · License: Apache-2.0. Check the original repository for the latest version.

SKILL.md instructions

# LeanCTX — Context Gateway for AI Systems

Control what your AI can see. LeanCTX Engine provides local context selection,
supported access and content controls, and evidence of observed context operations
for existing agents: Select → Control → Prove. Recovery depends on source or
archive availability and access permissions. LeanCTX SDK is the separate
integration surface for applications.

lean-ctx shapes context before it reaches you: cached reads, compressed shell
output, focused search, recovery to exact source. Two rules beat every detail:

1. **Fewer round trips beat smaller outputs.** One call that answers the
   question is cheaper than three small ones. Never add a call just to save a
   few hundred tokens.
2. **Never wait by polling.** Do not loop `sleep` + status checks (see below).

## Intent → tool

| You want to… | Call |
|---|---|
| Understand how something works (unknown location) | `ctx_compose(task)` — ranked files + source in one call; use it FIRST instead of search→read→read |
| Find the exact definition of a symbol | `ctx_search(action="symbol", name=…)` |
| Find code by meaning | `ctx_search(action="semantic", query=…)` |
| Find a text/regex pattern | `ctx_search(pattern=…, path=…)` |
| Who calls X / what calls X makes | `ctx_callgraph(action="callers"|"callees", symbol=…)` |
| Read a file you will edit | `ctx_read(path, mode="anchored")` → edit with `ctx_patch`, or the host's native edit tool |
| Read a file only for context | `ctx_read(path, mode="signatures"|"map")`; a range: `mode="lines:N-M"` |
| Re-check a file after editing | `ctx_read(path, mode="diff")` |
| Run a command (build, test, git, …) | `ctx_shell(command)` — output is compressed |
| List files / directory shape | `ctx_glob(pattern)` / `ctx_tree(path, depth)` |
| Remember across sessions | `ctx_session(action="finding"|"decision"|"task", value=…)`; durable project facts: `ctx_knowledge(action="remember", …)` |

Do not read or search through `ctx_shell` (`cat`, `sed -n`, `grep`, `rg`):
that bypasses the read cache and symbol index. Use `ctx_read` / `ctx_search`.

## Deferred tools (ToolSearch)

If the host lists lean-ctx tools by name only, load what the task needs in
**one** call, e.g. `select:mcp__lean-ctx__ctx_compose,mcp__lean-ctx__ctx_search,mcp__lean-ctx__ctx_read`.
Do not fall back to `ctx_shell` for everything because it happens to be loaded.
A tool named here that your tool list lacks (the default profile shows a
small core) is still callable: `ctx_call(name="ctx_knowledge", arguments={…})`.
`ctx_call(name="ctx_discover_tools", arguments={"query": "…"})` lists all of them.

## Long-running commands

- Prefer the host's own background execution that notifies on completion
  (Claude Code: `Bash` with `run_in_background`, or `Monitor`). Start it, then
  continue other work or end the turn — you will be told when it finishes.
- `ctx_shell(run_in_background=true)` jobs do not notify by themselves. Check
  a job at most once when you actually need its result; never `sleep` in a loop.

## Truncated output

Compressed or archived output ends with a recovery handle. Fetch only the
slice you need: `ctx_expand(id, search=… | head=N | json_path=…)`, or
`ctx_read(path, raw=true)` for exact bytes. Do not re-run the command.

## Without MCP (CLI)

```bash
lean-ctx read <file> -m signatures    # or map | diff | lines:N-M
lean-ctx grep <pattern> [path]
lean-ctx -c "<command>"               # only where the shell is not already wrapped
lean-ctx raw "<command>"              # exact, uncompressed output
```

## Setup

```bash
which lean-ctx || curl -fsSL https://raw.githubusercontent.com/yvgude/lean-ctx/main/skills/lean-ctx/scripts/install.sh | bash
lean-ctx setup        # editor wiring, hooks, rules, this skill
lean-ctx doctor       # verify
```

Docs: https://leanctx.com/docs