CLAUDE.md

nano-code — CLAUDE.md

Minimal Rust coding agent. Single file: src/main.rs. ~185 LOC.

Architecture

• No async — uses reqwest::blocking. Keeps the code linear and readable.
• No abstraction layers — one struct (Msg), five functions (load_env, shell, read_file, write_file, dispatch, call_api, main).
• OpenAI-compatible API — works with OpenRouter, or any /chat/completions endpoint.
• Three tools — shell, read_file, write_file.
• Executor-mode system prompt — forces the model to act (write files, run commands) rather than describe.

Key design decisions

• Msg uses Option<Value> fields with skip_serializing_if so tool/assistant messages serialize correctly without extra variants.
• Tool results are pushed as individual role: "tool" messages with tool_call_id matching the request.
• .env is parsed manually (no dotenv crate) — just split_once('=').
• Full conversation history is sent every request — no summarization, no truncation.
• system field sent on every API call alongside messages.

Files

src/main.rs     # entire implementation
Cargo.toml      # 3 dependencies
.env            # runtime config (not committed)
.env.example    # template

Environment variables

• OPENROUTER_API_KEY — required
• INFERENCE_BASE_URL — API base (default: https://openrouter.ai/api/v1)
• MODEL_NAME — model string (default: anthropic/claude-sonnet-4-6)

Build & run

cargo build --release
./target/release/nano-code

Extending

To add a tool: add an entry to the tools array in call_api(), add a match arm in dispatch(). That's it.