co-mono/packages/coding-agent/DEVELOPMENT.md

coding-agent Development Guide

This document describes the architecture and development workflow for the coding-agent package.

Architecture Overview

The coding-agent is structured into distinct layers:

┌─────────────────────────────────────────────────────────────┐
│                         CLI Layer                           │
│  cli.ts → main.ts → cli/args.ts, cli/file-processor.ts     │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                       Mode Layer                            │
│  modes/interactive/   modes/print-mode.ts   modes/rpc/     │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                       Core Layer                            │
│  core/agent-session.ts (central abstraction)               │
│  core/session-manager.ts, core/model-config.ts, etc.       │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                   External Dependencies                     │
│  @mariozechner/pi-agent (Agent, tools)                     │
│  @mariozechner/pi-ai (models, providers)                   │
│  @mariozechner/pi-tui (TUI components)                     │
└─────────────────────────────────────────────────────────────┘

Directory Structure

src/
├── cli.ts                    # CLI entry point (shebang, calls main)
├── main.ts                   # Main orchestration, argument handling, mode routing
├── index.ts                  # Public API exports
├── config.ts                 # APP_NAME, VERSION, paths (getAgentDir, etc.)

├── cli/                      # CLI-specific utilities
│   ├── args.ts               # parseArgs(), printHelp(), Args interface
│   ├── file-processor.ts     # processFileArguments() for @file args
│   ├── list-models.ts        # --list-models implementation
│   └── session-picker.ts     # selectSession() TUI for --resume

├── core/                     # Core business logic (mode-agnostic)
│   ├── index.ts              # Core exports
│   ├── agent-session.ts      # AgentSession class - THE central abstraction
│   ├── bash-executor.ts      # executeBash() with streaming, abort
│   ├── compaction.ts         # Context compaction logic
│   ├── export-html.ts        # exportSession(), exportFromFile()
│   ├── messages.ts           # BashExecutionMessage, messageTransformer
│   ├── model-config.ts       # findModel(), getAvailableModels(), getApiKeyForModel()
│   ├── model-resolver.ts     # resolveModelScope(), restoreModelFromSession()
│   ├── session-manager.ts    # SessionManager class - JSONL persistence
│   ├── settings-manager.ts   # SettingsManager class - user preferences
│   ├── skills.ts             # loadSkills(), skill discovery from multiple locations
│   ├── slash-commands.ts     # loadSlashCommands() from ~/.pi/agent/commands/
│   ├── system-prompt.ts      # buildSystemPrompt(), loadProjectContextFiles()
│   │
│   ├── oauth/                # OAuth authentication
│   │   ├── index.ts          # OAuth exports
│   │   ├── anthropic.ts      # Anthropic OAuth (Claude Pro/Max)
│   │   ├── github-copilot.ts # GitHub Copilot OAuth
│   │   └── storage.ts        # Token persistence
│   │
│   ├── hooks/                # Hook system for extending behavior
│   │   ├── index.ts          # Hook exports
│   │   ├── types.ts          # HookAPI, HookContext, event types
│   │   ├── loader.ts         # loadHooks() from multiple locations
│   │   ├── runner.ts         # runHook() event dispatch
│   │   └── tool-wrapper.ts   # wrapToolsWithHooks() for tool_call events
│   │
│   ├── custom-tools/         # Custom tool loading system
│   │   ├── index.ts          # Custom tool exports
│   │   ├── types.ts          # CustomToolFactory, CustomToolDefinition
│   │   └── loader.ts         # loadCustomTools() from multiple locations
│   │
│   └── tools/                # Built-in tool implementations
│       ├── index.ts          # Tool exports, allTools, codingTools
│       ├── bash.ts           # Bash command execution
│       ├── edit.ts           # Surgical file editing
│       ├── find.ts           # File search by glob
│       ├── grep.ts           # Content search (regex/literal)
│       ├── ls.ts             # Directory listing
│       ├── read.ts           # File reading (text and images)
│       ├── write.ts          # File writing
│       ├── path-utils.ts     # Path resolution utilities
│       └── truncate.ts       # Output truncation utilities

├── modes/                    # Run mode implementations
│   ├── index.ts              # Re-exports InteractiveMode, runPrintMode, runRpcMode, RpcClient
│   ├── print-mode.ts         # Non-interactive: process messages, print output, exit
│   │
│   ├── rpc/                  # RPC mode for programmatic control
│   │   ├── rpc-mode.ts       # runRpcMode() - JSON stdin/stdout protocol
│   │   ├── rpc-types.ts      # RpcCommand, RpcResponse, RpcSessionState types
│   │   └── rpc-client.ts     # RpcClient class for spawning/controlling agent
│   │
│   └── interactive/          # Interactive TUI mode
│       ├── interactive-mode.ts   # InteractiveMode class
│       │
│       ├── components/           # TUI components
│       │   ├── assistant-message.ts    # Agent response rendering
│       │   ├── bash-execution.ts       # Bash output display
│       │   ├── compaction.ts           # Compaction status display
│       │   ├── custom-editor.ts        # Multi-line input editor
│       │   ├── dynamic-border.ts       # Adaptive border rendering
│       │   ├── footer.ts               # Status bar / footer
│       │   ├── hook-input.ts           # Hook input dialog
│       │   ├── hook-selector.ts        # Hook selection UI
│       │   ├── model-selector.ts       # Model picker
│       │   ├── oauth-selector.ts       # OAuth provider picker
│       │   ├── queue-mode-selector.ts  # Message queue mode picker
│       │   ├── session-selector.ts     # Session browser for --resume
│       │   ├── show-images-selector.ts # Image display toggle
│       │   ├── theme-selector.ts       # Theme picker
│       │   ├── thinking-selector.ts    # Thinking level picker
│       │   ├── tool-execution.ts       # Tool call/result rendering
│       │   ├── user-message-selector.ts # Message selector for /branch
│       │   └── user-message.ts         # User message rendering
│       │
│       └── theme/
│           ├── theme.ts      # Theme loading, getEditorTheme(), etc.
│           ├── dark.json
│           ├── light.json
│           └── theme-schema.json

└── utils/                    # Generic utilities
    ├── changelog.ts          # parseChangelog(), getNewEntries()
    ├── clipboard.ts          # copyToClipboard()
    ├── fuzzy.ts              # Fuzzy string matching
    ├── mime.ts               # MIME type detection
    ├── shell.ts              # getShellConfig()
    └── tools-manager.ts      # ensureTool() - download fd, etc.

Key Abstractions

AgentSession (core/agent-session.ts)

The central abstraction that wraps the low-level Agent with:

• Session persistence (via SessionManager)
• Settings persistence (via SettingsManager)
• Model cycling with scoped models
• Context compaction
• Bash command execution
• Message queuing
• Hook integration
• Custom tool loading

All three modes (interactive, print, rpc) use AgentSession.

InteractiveMode (modes/interactive/interactive-mode.ts)

Handles TUI rendering and user interaction:

• Subscribes to AgentSession events
• Renders messages, tool executions, streaming
• Manages editor, selectors, key handlers
• Delegates all business logic to AgentSession

RPC Mode (modes/rpc/)

Headless operation via JSON protocol over stdin/stdout:

• rpc-mode.ts: runRpcMode() function that listens for JSON commands on stdin and emits responses/events on stdout
• rpc-types.ts: Typed protocol definitions (RpcCommand, RpcResponse, RpcSessionState)
• rpc-client.ts: RpcClient class for spawning the agent as a subprocess and controlling it programmatically

The RPC mode exposes the full AgentSession API via JSON commands. See docs/rpc.md for protocol documentation.

SessionManager (core/session-manager.ts)

Handles session persistence:

• JSONL format for append-only writes
• Session file location management
• Message loading/saving
• Model/thinking level persistence

SettingsManager (core/settings-manager.ts)

Handles user preferences:

• Default model/provider
• Theme selection
• Queue mode
• Thinking block visibility
• Compaction settings
• Hook/custom tool paths

Hook System (core/hooks/)

Extensibility layer for intercepting agent behavior:

• loader.ts: Discovers and loads hooks from ~/.pi/agent/hooks/, .pi/hooks/, and CLI
• runner.ts: Dispatches events to registered hooks
• tool-wrapper.ts: Wraps tools to emit tool_call and tool_result events
• types.ts: Event types (session, tool_call, tool_result, message, error)

See docs/hooks.md for full documentation.

Custom Tools (core/custom-tools/)

System for adding LLM-callable tools:

• loader.ts: Discovers and loads tools from ~/.pi/agent/tools/, .pi/tools/, and CLI
• types.ts: CustomToolFactory, CustomToolDefinition, CustomToolResult

See docs/custom-tools.md for full documentation.

Skills (core/skills.ts)

On-demand capability packages:

• Discovers SKILL.md files from multiple locations
• Provides specialized workflows and instructions
• Loaded when task matches description

See docs/skills.md for full documentation.

Development Workflow

Running in Development

Start the watch build in the monorepo root to continuously rebuild all packages:

# Terminal 1: Watch build (from monorepo root)
npm run dev

Then run the CLI with tsx in a separate terminal:

# Terminal 2: Run CLI (from monorepo root)
npx tsx packages/coding-agent/src/cli.ts

# With arguments
npx tsx packages/coding-agent/src/cli.ts --help
npx tsx packages/coding-agent/src/cli.ts -p "Hello"

# RPC mode
npx tsx packages/coding-agent/src/cli.ts --mode rpc --no-session

The watch build ensures changes to dependent packages (pi-agent, pi-ai, pi-tui) are automatically rebuilt.

Type Checking

# From monorepo root
npm run check

Building

# Build all packages
npm run build

# Build standalone binary
cd packages/coding-agent
npm run build:binary

Adding New Features

Adding a New Slash Command

1. If it's a UI-only command (e.g., /theme), add handler in interactive-mode.ts setupEditorSubmitHandler()
2. If it needs session logic, add method to AgentSession and call from mode

Adding a New Tool

1. Create tool in core/tools/ following existing patterns
2. Export from core/tools/index.ts
3. Add to allTools and optionally codingTools
4. Add description to toolDescriptions in core/system-prompt.ts

Adding a New Hook Event

1. Add event type to HookEvent union in core/hooks/types.ts
2. Add emission point in relevant code (AgentSession, tool wrapper, etc.)
3. Document in docs/hooks.md

Adding a New RPC Command

1. Add command type to RpcCommand union in rpc-types.ts
2. Add response type to RpcResponse union in rpc-types.ts
3. Add handler case in handleCommand() switch in rpc-mode.ts
4. Add client method in RpcClient class in rpc-client.ts
5. Document in docs/rpc.md

Adding a New Selector

1. Create component in modes/interactive/components/
2. Use showSelector() helper in interactive-mode.ts:

private showMySelector(): void {
    this.showSelector((done) => {
        const selector = new MySelectorComponent(
            // ... params
            (result) => {
                // Handle selection
                done();
                this.showStatus(`Selected: ${result}`);
            },
            () => {
                done();
                this.ui.requestRender();
            },
        );
        return { component: selector, focus: selector.getSelectList() };
    });
}

Testing

The package uses E2E tests only (no unit tests by design). Tests are in test/:

# Run all tests
npm test

# Run specific test pattern
npm test -- --testNamePattern="RPC"

# Run RPC example interactively
npx tsx test/rpc-example.ts

Code Style

• No any types unless absolutely necessary
• No inline dynamic imports
• Use showStatus() for dim status messages
• Use showError() / showWarning() for errors/warnings
• Keep InteractiveMode focused on UI, delegate logic to AgentSession