harnessd

A long-lived coding-agent daemon. It owns the model loop, sessions, tools, and managed processes, and speaks JSON-RPC 2.0 over a WebSocket so any number of thin clients (CLI, TUI, web, or another agent over MCP) can attach to the same session at once.

npx harnessd
# [harnessd] listening on ws://127.0.0.1:7777 (pid 12345)

What it does

• Sessions — each is a conversation thread with its own history, working directory, tool policy, memory scratchpad, and context budget. Persisted as JSONL under ~/.harness/sessions, so a restart loses nothing.
• Tools — read, write, edit, bash (foreground or background), memory, and task (spawn a sub-agent, optionally on a different model).
• Processes — background commands are supervised, logged to disk, and survive daemon restarts; killable as a group.
• MCP — connects to MCP servers listed in ~/.config/mcp/mcp.json (stdio and streamable HTTP) and offers their tools to the model. Deferred loading plus a tool-search tool keeps large tool catalogs out of the context window.
• Context — cross-turn tool history, lossless tool-result eviction, and summarizing compaction, in that order.

Configuration

Credentials: ANTHROPIC_API_KEY, or HARNESS_PROVIDER=vertex with HARNESS_VERTEX_PROJECT and Google application default credentials.

| Env | Default | Meaning | | --- | --- | --- | | HARNESS_PORT / HARNESS_HOST | 7777 / 127.0.0.1 | Listen address | | HARNESS_PROVIDER | anthropic | anthropic or vertex | | HARNESS_MODEL | claude-sonnet-4-6 | Default model (per-session override) | | HARNESS_EFFORT | high | Reasoning effort; clamped per model | | HARNESS_THINKING | adaptive | hidden keeps thinking, drops the transcript; off disables | | HARNESS_CACHE | on | Prompt-cache breakpoints | | HARNESS_MCP_CONFIG | ~/.config/mcp/mcp.json | MCP config file(s), colon-separated; off disables | | HARNESS_TOOL_SEARCH | on | Defer MCP tool schemas behind a search tool | | HARNESS_SESSIONS_DIR | ~/.harness/sessions | Session logs | | HARNESS_WEB_DIR | unset | Also serve a web UI from this directory |

The daemon binds loopback and has no authentication of its own; put a tunnel with real auth in front if you want it reachable.

Status

Early. The protocol is stable enough to build on but not frozen.

Requires Node >= 22.