LetCode

letcode is a terminal Agent written in Rust.

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It provides an opencode-style TUI based on Ratatui, and also keeps a REPL CLI mode.

Build and run

cargo build
cargo test
cargo fmt --check

Run the default TUI:

cargo run

Run the line-based CLI:

cargo run -- --cli

CLI mode can also be selected with cli or repl. TUI can be selected explicitly with --tui or tui.

Configuration

letcode loads configuration from:

~/.config/letcode/letcode.toml

Minimal example:

active_provider = "openai"

[global]
# Optional runtime limits:
# max_iterations = 64
# max_tool_calls = 128
sessions_dir = "sessions"
log_file = "logs/combined.log"

[permissions]
mode = "default" # safe/default/auto/yolo; legacy solo remains accepted when reading
# auto = same Ask set as default, but a sticky reviewer expert answers approvals

# Optional reviewer model route used by permission mode "auto"
# [agents.reviewer]
# provider = "openai"
# model = "gpt-5.5"

# Optional local execution policy. Reviewed read tools may declare parallel support;
# all other tools stay exclusive unless their handler explicitly opts in.
[tools.parallelism]
# "fs__read" = "parallel"
# "web__fetch" = "exclusive"

[providers.openai]
api_key = "YOUR_API_KEY"
base_url = "https://api.openai.com/v1"
protocol = "responses" # responses/completions
default_model = "gpt-5.5"

[providers.openai.models."gpt-5.5"]
display_name = "GPT-5.5"
# context_window = 400000
# effective_input_limit_tokens = 256000 # optional provider/model route input budget
supports_tools = true
parallel_tool_calls = false # allow one model response to request multiple tools
supports_reasoning = true
reasoning_effort = "medium" # model default
# Optional: restrict selectable levels and TUI cycle order.
# Supported values: none, minimal, low, medium, high, xhigh, max
reasoning_efforts = ["none", "low", "medium", "high", "max"]
reasoning_summary = "auto"
text_verbosity = "medium"

Provider API keys and base URLs can also come from environment variables named from the provider, for example OPENAI_API_KEY / OPENAI_BASE_URL; for a provider named compat, use COMPAT_API_KEY / COMPAT_BASE_URL.

Relative sessions_dir and log_file paths are resolved relative to the config file directory.

Optional Langfuse/OpenTelemetry tracing is off by default. Enable it with LETCODE_LANGFUSE_ENABLED=true, and set LANGFUSE_PUBLIC_KEY, LANGFUSE_SECRET_KEY, and optional LANGFUSE_HOST (or the same variables in a local .env). Missing credentials leave tracing disabled without stopping the agent.

Sessions

Session transcripts are stored as append-only JSONL under sessions_dir and can be restored later. In the TUI, use /tree to browse history, /undo / /redo to move between completed user turns, and /help for all local commands. The line-based CLI supports /tree as a read-only listing; /undo and /redo require the TUI.

Changelog

See CHANGELOG.md for release notes.

License

This project is dual-licensed under the MIT License OR the Apache License 2.0. You may choose either license when using, modifying, or redistributing this project.

• MIT License: see LICENSE-MIT
• Apache License 2.0: see LICENSE-APACHE