lightcraft/docs/mcp.md

LightCraft MCP server

lightcraft-cli mcp exposes LightCraft to AI agents through the Model Context Protocol: newline-delimited JSON-RPC 2.0 over stdio (protocol revision 2025-06-18; 2025-03-26 and 2024-11-05 clients are accepted). The server lives in crates/mcp (lightcraft-mcp, layer L5) and is hand-written: no async runtime, no C dependencies.

It runs in one of two modes:

Mode	Command	What it drives
Headless (default)	lightcraft-cli mcp [--demo] [FILES/FOLDERS…]	An in-process engine Session. Develop, render and export without a window.
Connect	lightcraft-cli mcp --connect [127.0.0.1:7980]	A running desktop app started with lightcraft --control 7980, through its loopback JSON-lines control channel (control-protocol.md). Adds the UI tools (screenshot, clicks, keys, pointer gestures).

Options: --library DIR opens (or creates) a persistent LightCraft library — the same crash-safe format the desktop app uses (~/Pictures/LightCraft Library by default there) — so ratings, edits and albums survive between sessions (with --demo, a new library is seeded with the demo photos); --demo starts the headless session with the procedurally generated demo library; --compact lists only the helper tools (see below). In connect mode the server starts even when the app is not running yet and connects on the first call (and reconnects if the app restarts).

Logs go to stderr; stdout carries only protocol messages.

Wiring it into a client

Build once: cargo build --release -p lightcraft-cli (binary: target/release/lightcraft-cli).

Claude Code

# headless, with a folder of photos imported at start
claude mcp add lightcraft -- /path/to/lightcraft/target/release/lightcraft-cli mcp ~/Pictures/shoot

# or: drive the running desktop app (start it with `lightcraft --control 7980`)
claude mcp add lightcraft-app -- /path/to/lightcraft/target/release/lightcraft-cli mcp --connect 127.0.0.1:7980

Or check a project-scoped .mcp.json into your repo:

{
  "mcpServers": {
    "lightcraft": {
      "command": "/path/to/lightcraft/target/release/lightcraft-cli",
      "args": ["mcp", "--connect", "127.0.0.1:7980"]
    }
  }
}

Other clients (Claude Desktop, Cursor, …)

Every stdio MCP client takes the same shape: a command plus args. For example claude_desktop_config.json:

{
  "mcpServers": {
    "lightcraft": {
      "command": "/path/to/lightcraft/target/release/lightcraft-cli",
      "args": ["mcp", "--demo"]
    }
  }
}

During development you can also point the client at cargo run --release -p lightcraft-cli -- mcp (with "cwd" set to the repository), at the cost of a slower start.

Tools

Helpers

Tool	Does
list_commands {filter?}	Every command: id, label, menu, shortcut, parameter doc, enabled now
run_command {command, params?}	Run any command by id
import {paths, album?}	Import files/folders (folders are scanned recursively); the first new photo becomes active
query_photos {filter?, sort?, offset?, limit?}	Photos in the current view (or matching a catalog Filter)
select_photos {ids, active?, mode?}	Set the selection / active photo
list_controls {section?}	Every develop slider: id (light.exposure…), range, default, current value
get_develop {id?}	Full develop-settings JSON
set_develop {id?, values?, settings?, label?}	values: {controlId: number}; settings: partial develop JSON deep-merged. Undoable
apply_preset {preset, amount?, ids?}	Apply a preset (ids from cmd_presets_list)
crop {id?, rect?, angle?, reset?}	Normalized crop rect [x0,y0,x1,y1] and straighten angle
render_photo {id?, size?, format?, path?}	Render with current settings → image content (PNG, or JPEG with format: "jpeg"), long edge size (default 1024)
export {path, id?, longEdge?, quality?}	Full-quality render to .png / .jpg / .tif / .webp (headless; the desktop app writes PNG)

Tools taking id make that photo active first; without it they act on the active photo.

Connect mode only

Tool	Does
screenshot {maxSize?, format?, path?}	The app window as an image, after pending renders finish
inspect_ui	View, panel, window/image rects, selection, status
set_ui {state}	Merge UI state, e.g. {"view": "detail"}
list_widgets {filter?} / click {widget | x,y, count?}	Widgets by automation id; real egui clicks
press_key {key, cmd?, shift?, alt?} / type_text {text}	Keyboard input (shortcuts)
pointer_gesture {events}	Gestures in normalized image coordinates (brush strokes, gradients, crop handles)

In headless mode these return a tool error explaining how to start the app.

One tool per command

Everything is a command in LightCraft, so tools/list also contains one tool per entry of the command registry (engine commands, plus the app's UI commands such as view.detail when connected): the id with . replaced by _ and a cmd_ prefix — photo.rate → cmd_photo_rate, develop.set → cmd_develop_set, edit.undo → cmd_edit_undo. Arguments are the command's JSON params (documented in each tool's description and by list_commands). Pass --compact to leave these out (about 90 headless / 140 connected) when a client struggles with many tools; run_command still reaches every command.

Resources

resources/list / resources/read serve JSON snapshots:

URI	Content
lightcraft://library	Source, filter, sort, selection, undo/redo labels (library.state)
lightcraft://photos	Photos in the current view (catalog.query)
lightcraft://photo/active	Everything about the active photo (photo.inspect)
lightcraft://develop/active	The active photo's develop settings (develop.get)
lightcraft://controls	Every develop control with its current value (develop.controls)

Example session

→ {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"me","version":"1"}}}
← {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-06-18","capabilities":{"tools":{…},"resources":{…}},"serverInfo":{"name":"lightcraft",…},"instructions":"…"}}
→ {"jsonrpc":"2.0","method":"notifications/initialized"}
→ {"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"set_develop","arguments":{"values":{"light.exposure":0.7}}}}
← {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"{…}"}],"isError":false,"structuredContent":{"ok":true,"controls":[{"id":"light.exposure","value":0.7}]}}}
→ {"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"render_photo","arguments":{"size":768}}}
← {"jsonrpc":"2.0","id":3,"result":{"content":[{"type":"image","data":"iVBORw0…","mimeType":"image/png"},{"type":"text","text":"{\"id\":21,\"width\":512,\"height\":768}"}],"isError":false}}

Errors from commands (unknown control, nothing selected, app not reachable) come back as tool results with isError: true so the model can read and correct them; malformed JSON-RPC gets the standard error codes (-32700 parse, -32600 invalid request, -32601 method not found, -32602 invalid params, -32002 resource not found).

Other CLI subcommands

lightcraft-cli render in.dng -o out.jpg --set light.exposure=0.5 --set light.contrast=20 --size 2048
lightcraft-cli render in.jpg -o out.png --settings look.json --preset <presetId>
lightcraft-cli commands [--json]   # the command registry
lightcraft-cli controls [--json]   # develop control ids and ranges

Tests

• crates/mcp/tests/e2e.rs — M0.9 acceptance: over the stdio framing, set exposure and render; checks the decoded PNG gets brighter/darker. Runs headless and through the TCP transport (Remote) against a stand-in control server; also import → render → JPEG export of a real file.
• apps/lightcraft-cli/tests/cli.rs — spawns lightcraft-cli mcp with real pipes; render; commands.