g3/docs/architecture.md

g3 Architecture

Last updated: January 2025
Source of truth: Crate structure in crates/, Cargo.toml, DESIGN.md

Purpose

This document describes the internal architecture of g3, a modular AI coding agent built in Rust. It is intended for developers who want to understand, extend, or maintain the codebase.

High-Level Overview

g3 follows a tool-first philosophy: instead of just providing advice, it actively uses tools to read files, write code, execute commands, and complete tasks autonomously.

┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│   g3-cli        │    │   g3-core       │    │ g3-providers    │
│                 │    │                 │    │                 │
│ • CLI parsing   │◄──►│ • Agent engine  │◄──►│ • Anthropic     │
│ • Interactive   │    │ • Context mgmt  │    │ • Databricks    │
│ • Streaming MD  │    │ • Tool system   │    │ • OpenAI        │
│ • Autonomous    │    │ • Streaming     │    │ • Embedded      │
│   mode          │    │ • Task exec     │    │   (llama.cpp)   │
│                 │    │ • TODO mgmt     │    │ • OAuth flow    │
└─────────────────┘    └─────────────────┘    └─────────────────┘
         │                       │                       │
         └───────────────────────┼───────────────────────┘
                                 │
         ┌───────────────────────┼───────────────────────┐
         │                       │                       │
┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│ g3-execution    │    │   g3-config     │    │  g3-planner     │
│                 │    │                 │    │                 │
│ • Code exec     │    │ • TOML config   │    │ • Requirements  │
│ • Shell cmds    │    │ • Env overrides │    │ • Git ops       │
│ • Streaming     │    │ • Provider      │    │ • Planning      │
│ • Error hdlg    │    │   settings      │    │   workflow      │
└─────────────────┘    └─────────────────┘    └─────────────────┘
         │                       │                       │
         │              ┌─────────────────┐              │
         │              │ g3-computer-    │              │
         └─────────────►│   control       │◄─────────────┘
                        │ • Mouse/kbd     │
                        │ • Screenshots   │
                        │ • OCR/Vision    │
                        │ • WebDriver     │
                        │ • macOS Ax API  │
                        └─────────────────┘
                                 │
         ┌───────────────────────┼───────────────────────┐
         │                       │                       │
         ┌─────────────────┐
         │     studio      │
         │ • Worktree mgmt │
         │ • Session mgmt  │
         └─────────────────┘

Workspace Structure

g3 is organized as a Rust workspace with 9 crates:

g3/
├── src/main.rs                   # Entry point (delegates to g3-cli)
├── crates/
│   ├── g3-cli/                   # Command-line interface and TUI
│   ├── g3-core/                  # Core agent engine and tools
│   ├── g3-providers/             # LLM provider abstractions
│   ├── g3-config/                # Configuration management
│   ├── g3-execution/             # Code execution engine
│   ├── g3-computer-control/      # Computer automation
│   ├── g3-planner/               # Planning mode workflow
│   └── studio/                   # Multi-agent workspace manager
├── agents/                       # Agent persona definitions
├── logs/                         # Session logs (auto-created)
└── g3-plan/                      # Planning artifacts

Crate Responsibilities

g3-core (Central Hub)

Location: crates/g3-core/
Purpose: Core agent engine, tool system, and orchestration logic

Key modules:

• lib.rs - Main Agent struct and orchestration (~3400 lines)
• context_window.rs - Token tracking and context management
• streaming_parser.rs - Real-time LLM response parsing
• tool_definitions.rs - JSON schema definitions for all tools
• tool_dispatch.rs - Routes tool calls to implementations
• tools/ - Tool implementations (file ops, shell, vision, webdriver, etc.)
• error_handling.rs - Error classification and recovery
• retry.rs - Retry logic with exponential backoff
• prompts.rs - System prompt generation
• code_search/ - Tree-sitter based code search

Key types:

• Agent<W: UiWriter> - Main agent struct, generic over UI output
• ContextWindow - Manages conversation history and token limits
• StreamingToolParser - Parses streaming LLM responses for tool calls
• ToolCall - Represents a tool invocation

g3-providers (LLM Abstraction)

Location: crates/g3-providers/
Purpose: Unified interface for multiple LLM backends

Key modules:

• lib.rs - LLMProvider trait and ProviderRegistry
• anthropic.rs - Anthropic Claude API (~51k chars)
• databricks.rs - Databricks Foundation Models (~58k chars)
• openai.rs - OpenAI and compatible APIs (~18k chars)
• embedded.rs - Local models via llama.cpp (~34k chars)
• oauth.rs - OAuth authentication flow

Key traits:

#[async_trait]
pub trait LLMProvider: Send + Sync {
    async fn complete(&self, request: CompletionRequest) -> Result<CompletionResponse>;
    async fn stream(&self, request: CompletionRequest) -> Result<CompletionStream>;
    fn name(&self) -> &str;
    fn model(&self) -> &str;
    fn has_native_tool_calling(&self) -> bool;
    fn supports_cache_control(&self) -> bool;
    fn max_tokens(&self) -> u32;
    fn temperature(&self) -> f32;
}

g3-cli (User Interface)

Location: crates/g3-cli/
Purpose: Command-line interface, TUI, and execution modes

Key modules:

• lib.rs - Main CLI entry point and mode dispatch
• interactive.rs - Interactive REPL mode
• autonomous.rs - Autonomous coach-player mode
• accumulative.rs - Accumulative autonomous mode
• agent_mode.rs - Specialized agent execution
• filter_json.rs - JSON tool call filtering for display
• ui_writer_impl.rs - Console output implementation
• streaming_markdown.rs - Real-time markdown formatting

Execution modes:

1. Single-shot: g3 "task description" - Execute one task and exit
2. Interactive: g3 - REPL-style conversation (default)
3. Autonomous: g3 --autonomous - Coach-player feedback loop
4. Accumulative: Default interactive mode with autonomous runs
5. Planning: g3 --planning - Requirements-driven development
6. Agent Mode: g3 --agent <name> - Run specialized agent personas

g3-config (Configuration)

Location: crates/g3-config/
Purpose: TOML-based configuration management

Key structures:

• Config - Root configuration
• ProvidersConfig - Provider settings with named configs
• AgentConfig - Agent behavior settings
• WebDriverConfig - Browser automation settings
• MacAxConfig - macOS Accessibility API settings

Configuration hierarchy (highest priority last):

1. Default configuration
2. ~/.config/g3/config.toml
3. ./g3.toml
4. Environment variables (G3_*)
5. CLI arguments

g3-execution (Code Execution)

Location: crates/g3-execution/
Purpose: Safe execution of shell commands and scripts

Features:

• Streaming output capture
• Exit code tracking
• Async execution via Tokio
• Error handling and formatting

g3-computer-control (Automation)

Location: crates/g3-computer-control/
Purpose: Cross-platform computer control and automation

Key modules:

• platform/ - Platform-specific implementations (macOS, Linux, Windows)
• webdriver/ - Safari and Chrome WebDriver integration
• ocr/ - Text extraction (Tesseract, Apple Vision)

Platform support:

• macOS: Core Graphics, Cocoa, screencapture, Vision framework
• Linux: X11/Xtest for input
• Windows: Win32 APIs

g3-planner (Planning Mode)

Location: crates/g3-planner/
Purpose: Requirements-driven development workflow

Key modules:

• planner.rs - Main planning state machine (~40k chars)
• state.rs - Planning state management
• git.rs - Git operations
• code_explore.rs - Codebase exploration
• llm.rs - LLM interactions for planning
• history.rs - Planning history tracking

Workflow:

1. Write requirements in <codepath>/g3-plan/new_requirements.md
2. LLM refines requirements
3. Requirements renamed to current_requirements.md
4. Coach/player loop implements
5. Files archived with timestamps
6. Git commit with LLM-generated message

studio (Multi-Agent Workspace Manager)

Location: crates/studio/
Purpose: Manage multiple g3 agent sessions using git worktrees

Key modules:

• main.rs - CLI commands (run, exec, list, status, accept, discard)
• git.rs - Git worktree management
• session.rs - Session metadata and status tracking

Studio enables isolated agent sessions by creating git worktrees, allowing multiple agents to work on the same codebase without conflicts.

Data Flow

Request Flow

User Input
    │
    ▼
┌─────────────┐
│  g3-cli     │  Parse input, determine mode
└─────────────┘
    │
    ▼
┌─────────────┐
│  g3-core    │  Add to context window
│  Agent      │  Build completion request
└─────────────┘
    │
    ▼
┌─────────────┐
│ g3-providers│  Send to LLM provider
│ Registry    │  Stream response
└─────────────┘
    │
    ▼
┌─────────────┐
│  g3-core    │  Parse streaming response
│  Parser     │  Detect tool calls
└─────────────┘
    │
    ▼
┌─────────────┐
│  g3-core    │  Execute tools
│  Tools      │  Return results
└─────────────┘
    │
    ▼
┌─────────────┐
│  g3-core    │  Add results to context
│  Agent      │  Continue or complete
└─────────────┘

Context Window Management

The ContextWindow struct manages conversation history with intelligent token tracking:

1. Token Tracking: Monitors usage as percentage of provider's context limit
2. Context Thinning: At 50%, 60%, 70%, 80% thresholds, replaces large tool results with file references
3. Auto-Compaction: At 80% capacity, triggers conversation compaction
4. Provider Adaptation: Adjusts to different model context windows (4k to 200k+ tokens)

Error Handling

g3 implements comprehensive error handling:

1. Error Classification: Distinguishes recoverable vs non-recoverable errors
2. Automatic Retry: Exponential backoff with jitter for:
   • Rate limits (HTTP 429)
   • Network errors
   • Server errors (HTTP 5xx)
   • Timeouts
3. Error Logging: Detailed logs saved to logs/errors/
4. Graceful Degradation: Continues when possible, fails gracefully when not

Session Management

Sessions are tracked in .g3/sessions/<session_id>/:

• session.json - Full conversation history and metadata
• todo.g3.md - Session-scoped TODO list
• Context summaries and thinned content

Legacy logs are stored in logs/g3_session_*.json.

Extension Points

Adding a New Tool

1. Add tool definition in g3-core/src/tool_definitions.rs
2. Implement handler in g3-core/src/tools/
3. Add dispatch case in g3-core/src/tool_dispatch.rs
4. Update system prompt if needed in g3-core/src/prompts.rs

Adding a New Provider

1. Implement LLMProvider trait in g3-providers/src/
2. Add configuration struct in g3-config/src/lib.rs
3. Register provider in g3-core/src/lib.rs (in new_with_mode_and_readme)
4. Update documentation

Adding a New Execution Mode

1. Add CLI arguments in g3-cli/src/lib.rs
2. Implement mode logic in the CLI
3. May require new agent methods in g3-core

Key Files for Understanding

Start reading here:

1. src/main.rs - Entry point (trivial, delegates to g3-cli)
2. crates/g3-cli/src/lib.rs - CLI and execution modes
3. crates/g3-core/src/lib.rs - Agent implementation
4. crates/g3-providers/src/lib.rs - Provider trait and registry
5. crates/g3-core/src/tool_definitions.rs - Available tools
6. crates/g3-config/src/lib.rs - Configuration structures
7. DESIGN.md - Original design document

Dependencies

Key external dependencies:

• tokio: Async runtime
• reqwest: HTTP client for API calls
• serde/serde_json: Serialization
• clap: CLI argument parsing
• tree-sitter: Syntax-aware code search
• llama_cpp: Local model inference (with Metal acceleration)
• fantoccini: WebDriver client