pi-devin-fusion

Devin Fusion "sidekick" pattern for pi.

Two models work together: the active planner (you pick it) plans and reviews, and a cheaper executor (auto-selected) implements. The planner delegates to the executor via the sidekick tool. Mutating executor tools require consent and run serialized to prevent clobbered writes.

Why

From Cognition's blog post:

the main agent should take minimal actions, and only read what is absolutely necessary. By default it should delegate and monitor, while making the significant decisions: the plan, the interpretation of ambiguity, the final review.

This extension makes that pattern work in pi — not as a suggestion, but as a mechanical tool wrapper. The planner model is always in control; the executor model runs a controlled tool loop with bounded turns.

Installation

Install directly in your pi project:

pi install ./

Or install from npm:

pi install npm:pi-devin-fusion

You can also install from a local path:

pi install ./path/to/pi-devin-fusion

Usage

Commands

Command	Description
/devin on / forced / force	Force every user message through the planner/sidekick split
/devin available / auto	Let the model decide when to use the sidekick (default)
/devin off / disable / disabled	Disable the sidekick tool for this session
/devin <prompt>	Send a prompt through the forced planner prefix once; blocked while mode is off
/devin-setup	Interactive picker to choose mode, executor model, tools, and config (session-scoped)
/devin-init	Create a .pi/devin.json template to set the executor model and tool selection
/devin-status	Show current Devin mode, executor, tool selection, and consent state

Tools

The extension registers one tool:

• sidekick — Delegate a task to the executor model. Accepts a prompt string, optional context_mode ("none" / "recent"), and optional context_turns (1–10, default 4 for "recent" mode).

Workflow

Interactive setup (recommended for first use)

1. Run /devin-setup to pick mode, executor model, and session config interactively.
2. Ask your question. In available mode the planner decides when to use the sidekick; in forced mode every normal prompt is routed through the planner/sidekick split.

Config-file setup

1. (Optional) Run /devin-init to create a .pi/devin.json template.
2. Turn Devin on with /devin on.
3. Ask your question.

The planner delegates to the sidekick executor for exploration and implementation; it reviews the result before responding. Use /devin <prompt> for a one-off delegation without changing the mode.

Session vs file configuration

Settings from /devin-setup take precedence over .pi/devin.json. Session state persists within the conversation and is restored on session restore. Use /devin-status to inspect the effective configuration.

Configuration

.pi/devin.json in your project root:

{
  "$schema": "devin-fusion-config",
  "executor": "openai/gpt-4.1-mini",
  "executorTools": "all",
  "executorToolsConsent": false,
  "maxExecutorOutputTokens": 4096,
  "temperature": 0.2,
  "maxToolCalls": 16,
  "footerDisplay": "full"
}

Key	Default	Description
executor	auto-selected	Model identifier for the executor (e.g. "openai/gpt-4.1-mini"). Auto-selects the first non-current text model if unset.
executorTools	"all"	Tool selection: "none", "readonly", "all", or an array like ["read", "grep", "write"].
executorToolsConsent	false	When true, skips the consent prompt for mutating tools in trusted projects. Untrusted projects always block mutating tools.
maxExecutorOutputTokens	4096	Max output tokens per executor call (validated and capped).
temperature	0.2	Temperature for the executor model (0..2).
maxToolCalls	16	Max tool calls per executor run (clamped to 1–100).
footerDisplay	"full"	Footer verbosity: "full", "compact", or "off".

Invalid config values are ignored or clamped before use. Project-local .pi/devin.json is read only when the project is trusted; global devin.json remains the fallback.

Session precedence: /devin-setup selection overrides .pi/devin.json. Session state persists in the conversation and is restored on session restore.

Files

File	Purpose
src/index.ts	Extension entry: sidekick tool, /devin commands, session state, footers
src/config.ts	Config loading, defaults, template generation
src/context.ts	Context normalization and recent-history builder
src/executor.ts	Executor pipeline: model resolution, consent gate, serialized mutating runs
src/executor_policy.ts	Pure consent-policy check (testable without models)
src/llm.ts	Low-level LLM calls, tool loop, circuit breakers, output truncation
src/models.ts	Model resolution helpers, executor auto-selection with auth check
src/prompts.ts	Planner prefix prompt and sidekick system prompt
src/tools.ts	Tool-definition factory, selection normalization, mutating detection
src/types.ts	Shared DevinConfig, SidekickOptions, ToolSelection, FooterDisplay types
src/ui.ts	Interactive session setup picker via pi TUI components
src/utils.ts	Concurrency-limited map, byte truncation, JSON extraction

Credit

Inspired by Devin Fusion by Cognition.

License

MIT