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