ypi

ypi — a recursive coding agent built on Pi.

Named after the Y combinator from lambda calculus — the fixed-point combinator that enables recursion. ypi is Pi that can call itself. (rpi already has another connotation.)

Inspired by Recursive Language Models (RLM), which showed that an LLM with a code REPL and a llm_query() function can recursively decompose problems, analyze massive contexts, and write code — all through self-delegation.

The Idea

Pi already has an extension system and a bash REPL. ypi's core is a Pi extension that registers one native tool — rlm_query — and teaches Pi to use it recursively. The ypi launcher and shell-compatible rlm_query command are convenience layers around that extension. jj workspace isolation is used when available, but it is not required for the minimal path.

┌──────────────────────────────────────────┐
│  ypi (depth 0)                           │
│  Tools: native rlm_query, bash           │
│  Workspace: default                      │
│                                          │
│  > grep -n "bug" src/*.py                │
│  > sed -n '50,80p' src/app.py \          │
│      | rlm_query "Fix this bug"          │
│            │                             │
│            ▼                             │
│    ┌────────────────────────────┐        │
│    │  ypi (depth 1)            │        │
│    │  Workspace: jj if present │        │
│    │  Edits files safely       │        │
│    │  Returns: patch on stdout │        │
│    └────────────────────────────┘        │
│                                          │
│  > jj squash --from <child-change>       │
│  # absorb the fix into our working copy  │
└──────────────────────────────────────────┘

---

Using ypi

Install

# bun (global)
bun install -g ypi

# or npm (global)
npm install -g ypi

# or run without installing
bunx ypi "What does this repo do?"

# or curl
curl -fsSL https://raw.githubusercontent.com/rawwerks/ypi/master/install.sh | bash

# or manual
git clone https://github.com/rawwerks/ypi.git && cd ypi
git submodule update --init --depth 1
export PATH="$PWD:$PATH"

Run

# Interactive
ypi

# One-shot
ypi "Refactor the error handling in this repo"

# Different model
ypi --provider anthropic --model claude-sonnet-4-5-20250929 "What does this codebase do?"

Use As A Pi Extension

The minimal path is the pi-recursive package — a pure Pi extension. It gives Pi the native recursive rlm_query tool without the ypi launcher, shell helper, or jj requirement:

# Try for one run
pi -e npm:pi-recursive "Use rlm_query to ask a child what 2 + 2 is."

# Install globally for normal pi sessions
pi install npm:pi-recursive
pi

# Install project-locally into .pi/settings.json
pi install -l npm:pi-recursive

The npm package has a Pi manifest that exposes only ./extensions/recursive.ts. The ypi binary remains available for users who want the wrapper defaults and shell-compatible helper commands.

How It Works

Three pieces (same architecture as Python RLM):

Piece	Python RLM	ypi
System prompt	RLM_SYSTEM_PROMPT	SYSTEM_PROMPT.md
Context / REPL	Python context variable	$CONTEXT file + bash
Sub-call function	llm_query("prompt")	native Pi tool rlm_query; optional shell command rlm_query "prompt"
Recursion: the extensions/recursive.ts extension registers a native rlm_query tool that spawns a child Pi process with the same extension and tools. The child can call rlm_query too:

Depth 0 (root)    -> full Pi with native rlm_query + bash
  Depth 1 (child) -> full Pi with native rlm_query + bash
    Depth 2 (leaf) -> full Pi with bash, but no rlm_query (max depth)

File isolation with jj: When jj is available and RLM_JJ is not 0, recursive children use jj workspaces for isolation. Without jj, the minimal extension still works, but children default to read-only tools in the current checkout. Set RLM_UNSAFE_NO_JJ_WRITE=1 only when you intentionally want writable no-jj child agents.

Why It Works

The design has three properties that compound:

1. Self-similarity — Every depth runs the same prompt, same tools, same agent. No specialized "scout" or "planner" roles. The intelligence is in decomposition, not specialization. The system prompt teaches one pattern — size-first → search → chunk → delegate → combine — and it works at every scale.

2. Self-hosting — The extension is the canonical recursion machinery. When the shell helper is enabled (the ypi wrapper, or any load with YPI_SHELL_HELPER=1), the prompt also includes its source for inspection and modification. A bare pi -e / npm extension install uses the native tool only and does not require that shell file.

3. Bounded recursion — Five concentric guardrails (depth limit, PATH scrubbing, call count, budget, timeout) guarantee termination. The system prompt also installs cognitive pressure: deeper agents are told to be more conservative, preferring direct action over spawning more children.

4. Symbolic access — Anything the agent needs to manipulate precisely is a file, not just tokens in context. $CONTEXT holds the data, $RLM_PROMPT_FILE holds the original prompt, and hashline provides line-addressed edits. Agents grep/sed/cat instead of copying tokens from memory.

Guardrails

Feature	Env var	What it does
Budget	RLM_BUDGET=0.50	Max dollar spend for entire recursive tree; native extension mode requires JSON output so child cost can be measured
Timeout	RLM_TIMEOUT=60	Wall-clock limit for entire recursive tree
Call limit	RLM_MAX_CALLS=20	Max total rlm_query invocations
Model routing	RLM_CHILD_MODEL=haiku	Use cheaper model for sub-calls
Depth limit	RLM_MAX_DEPTH=3	How deep recursion can go
jj disable	RLM_JJ=0	Skip workspace isolation; child agents are read-only unless RLM_UNSAFE_NO_JJ_WRITE=1
Plain text	RLM_JSON=0	Disable JSON mode (no cost tracking)
Tracing	PI_TRACE_FILE=$HOME/scratch/trace.log	Log all calls with timing + cost

The agent can check spend at any time:

rlm_cost          # "$0.042381"
rlm_cost --json   # {"cost": 0.042381, "tokens": 12450, "calls": 3}

Pi Compatibility

ypi is a thin layer on top of Pi. We strive not to break or duplicate what Pi already does:

Pi feature	ypi behavior	Tests
Session history	Uses Pi's native session manager when a parent session exists. Child sessions go in the same dir with trace-encoded filenames. RLM_SHARED_SESSIONS=0 uses --no-session and clears child session env. No separate session store.	G24–G30
Extensions	Child processes disable ambient extension discovery and explicitly reload the ypi extension. RLM_EXTENSIONS=0 disables recursion extension loading; RLM_CHILD_EXTENSIONS=0 disables it for child depths.	G34–G38, N8
Native recursion	The canonical extensions/recursive.ts extension registers a native Pi rlm_query tool. Minimal mode works with only Pi plus extension files: no ypi launcher, no shell helper, no jj.	extension smoke, pure-extension E2E
System prompt	The extension injects SYSTEM_PROMPT.md when present and falls back to a minimal built-in prompt when it is not. If the shell rlm_query file exists, its source is appended as optional compatibility context. Standalone shell rlm_query falls back to Pi's --system-prompt.	T8–T9, parity E2E
-p mode	All child Pi calls run non-interactive (-p). ypi never fakes a terminal.	T3–T4
--session flag	Used when session sharing is enabled and Pi has a session dir; --no-session otherwise. Never both.	G24, G28
Provider/model	Never hardcoded. ypi and rlm_query use Pi's defaults unless the user sets RLM_PROVIDER/RLM_MODEL.	T14, T14c

If Pi changes how sessions or extensions work, our guardrail tests should catch it.

Package Boundary

There are two published packages, built from one canonical source:

Package	Audience	Entry point	Includes
pi-recursive	Pi users who want recursion inside plain pi	pi install npm:pi-recursive or pi -e npm:pi-recursive	The native rlm_query tool, prompt injection, depth/status/env handling. No bin; host pi is a peer dependency.
ypi	Users who want a preconfigured recursive CLI	npm install -g ypi / bun install -g ypi	The same extension plus launcher defaults, the shell-compatible rlm_query (pipes/async), cost/session helpers. Bundles pi so the CLI runs without a separate global install.

Both ship the same extensions/ source. pi-recursive is the extension-only publish view, staged from the repo root by scripts/build-pi-recursive; ypi ships the extension plus its launcher and shell helpers. The shell helper is opt-in (YPI_SHELL_HELPER=1, set by the ypi wrapper), so installing pi-recursive gives you the native tool only.

---

Contributing

Project Structure

ypi/
├── ypi                    # Thin launcher: sets env, loads extensions/recursive.ts
├── rlm_query              # Optional shell-compatible recursive sub-call command
├── extensions/
│   ├── recursive.ts       # Canonical ypi Pi extension
│   ├── ypi/               # Native tool, env, prompt, status modules
│   └── ypi.ts             # Compatibility alias for recursive.ts
├── SYSTEM_PROMPT.md       # Teaches the LLM to be recursive + edit code
├── AGENTS.md              # Meta-instructions for the agent (read by ypi itself)
├── Makefile               # test targets
├── tests/
│   ├── test_unit.sh       # Mock pi, test bash logic (no LLM, fast)
│   ├── test_guardrails.sh # Test guardrails (no LLM, fast)
│   └── test_e2e.sh        # Real LLM calls (slow, costs ~$0.05)
├── pi-mono/               # Git submodule: upstream Pi coding agent
└── README.md

Version Control

This repo strongly prefers jj for version control. Git remains the remote-facing substrate.

jj status                    # What's changed
jj describe -m "message"     # Describe current change
jj new                       # Start a new change
jj bookmark set master       # Point master at current change
jj git push                  # Push to GitHub

Prefer jj for local changes, especially recursive agent work. Use the repo's safe push/land helpers for remote-facing operations.

Testing

make test-fast         # unit + guardrails
make test-extensions   # latest Pi + extension compatibility, including minimal mode
make pre-push-checks   # shared local/CI gate (recommended before push)
make test-e2e          # real LLM calls, costs money
make test-recursion-e2e # focused live proof that ypi invokes rlm_query
make test-extension-recursion-e2e # direct pi -e native tool recursion proof
make test-parity-e2e   # wrapper vs direct-extension parity proof
make test              # all of the above

Install hooks once per clone to run checks automatically on git push:

make install-hooks

Release/update helper:

make release-preflight   # same checks + upstream dry-run in one command
make land                # deterministic-ish landing helper

Before any change to rlm_query: run make test-fast. After: run it again. rlm_query is a live dependency of the agent's own execution — breaking it breaks the agent.

CI helper commands:

make ci-status N=15      # recent workflow runs
make ci-last-failure     # dump latest failing workflow log

History

ypi went through five approaches before landing on the current design:

1. Tool-use REPL (exp 010/012) — Pi's completeWithTools(), ReAct loop. 77.6% on LongMemEval.
2. Python bridge — HTTP server between Pi and Python RLM. Too complex.
3. Pi extension — Custom provider with search tools. Not true recursion.
4. Bash RLM (rlm_query + SYSTEM_PROMPT.md) — True recursion via bash.
5. Pi-native extension RLM — extensions/recursive.ts registers native recursion; ypi and shell rlm_query are compatibility/ergonomics layers. Current approach.

The key insight: Pi's extension API can expose recursion as a first-class tool, while Pi's bash tool remains the REPL for command-line composition. No bridge needed.

---

See Also

• Pi coding agent — the underlying agent
• Recursive Language Models — the library that inspired this
• rlm-cli — Python RLM CLI (budget, timeout, model routing)