craft-anything/docs/extending.md

Extending Craft Anything

Every part people tend to change sits behind a seam: a trait or a table with built-in entries and one place to add yours. Follow AGENTS.md (never crash, layers, external programs through craft_anything_platform::process) and give each addition tests.

You want to add	Where	How
A tool	crates/engine/src/tools/<area>.rs	A ToolSpec (id per tool-ids.md, title, summary, beginner text, toolset, honest effects, a schemars params type, run), and one v.extend(<area>::specs()) line in tools/mod.rs. It's then a CLI command, an MCP tool and callable by the agent and the app. It runs only in Deep mode until it's listed in crates/engine/src/clean_room.rs; list it there only if it touches no target or watches from outside, as a visitor or user would.
A tool from a higher layer	that crate, e.g. crates/ai/src/rebuild/tools.rs	Static ToolSpecs passed to Engine::shared(session, extra); reach the engine through ctx.session.ext.get::<EngineRef>(). Never block a tool on work that calls back into the engine: start it and return.
A kind of thing to make	crates/ai/src/rebuild/kinds/	Implement TargetKind (detection score, default stack, brief sections, checks, refine message, the app's words) and KindRegistry::register it.
A way to judge a build	crates/ai/src/rebuild/kinds/	Implement Check (run(&mut CheckCx) -> CheckResult; applies to skip it in some runs, or wrap it in DeepOnly); return it from your kind's checks. Decide on passed/similarity, never on text.
A stack	crates/ai/src/rebuild/stacks.toml	One [[stack]] entry: name, the builder's steps, the phrases that ask for it, its engine, the package.json dependencies that show a build uses it. A component or styling library is a [[library]] entry (name, phrases, the stacks it works with, steps).
A coding agent	crates/ai/src/cli_agents/adapters.rs	Implement AgentAdapter (args, required flags, stream parsing, resume, sign-in check) and register it in AgentAdapters. Agents it doesn't know run with the plain adapter. An agent that speaks ACP needs only a row in cli_agents::acp::ACP_AGENTS (its id and the arguments that start ACP mode).
An asset provider	crates/engines/src/assets/	Implement AssetProvider (key_env names its environment variable); add its models and prices to assets.toml. Its key is its own: assets_key_set with provider, kept as assets:<id>.
An external engine	crates/engines/src/table.rs	One ExternalEngine entry (exes, known paths, what it's for, install pins with SHA-256 checked against the publisher), plus a tool_paths! id in settings.rs if people can point at their own install.
A front end for builds	anywhere above L5	Hold a rebuild::RunSet (it queues past its limit) and poll it; test it with RunSet::with_starter(StandIns::starter()), whose runs report only what the test sends.
A persisted file	its owner module	A version field and a migration step; see file-formats.md.

Plugins: external MCP servers

Any MCP server, local (stdio) or remote (Streamable HTTP), can add tools without changing code. Add it in Settings → Plugins, with plugins_add (craft-anything-cli plugins-add --id notes --command notes-mcp --args '["--stdio"]'), or list it in config.toml. A plugin added through the tool or the app always asks before each call; plugins_remove takes one away.

[[plugins.mcp]]
id = "notes"            # its toolset id; its tools become notes_<tool>
name = "Notes"          # shown to people (default: the id)
command = "notes-mcp"
args = ["--stdio"]
consent = true          # ask before each call (the default): it runs someone else's code
# enabled = false       # keep it configured but off
# env = { NOTES_DIR = "C:/notes" }

[[plugins.mcp]]
id = "docs"             # a remote server: url instead of command
url = "https://mcp.example.com/mcp"   # https, or http:// to this computer
headers = { Authorization = "Bearer …" }

Then craft-anything-cli plugins-refresh --allow-run starts it once and caches its tool list; from the next start its tools are offered everywhere, in a toolset of their own (MCP agents enable it with enable_toolset). plugins-list shows what's configured and cached.

• A local server starts on its first call and stays up for the session; one that exits is started again. A remote one is reached on its first call; its session id is sent back with each request.
• Effects come from the server's tool annotations (readOnlyHint, destructiveHint, openWorldHint), plus launching a process (local) or the network (remote). With consent = true every call asks first.
• env and headers values are never shown by settings_show or plugins_list.
• Another transport implements engines::mcp_client::Transport and is passed to McpClient::with_transport.
• Results are recorded as evidence from an external service (engine: plugin:<id>).
• Tool ids are <plugin id>_<tool name>, lower-cased with _ for anything else; a clash is skipped and logged.

Plugins: WASM components

A build with --features wasm-plugins (it compiles wasmtime, so it's off by default) also loads WASM components. A component exports craft-anything:plugin/tools from crates/engines/wit/plugin.wit: describe returns its tools (name, title, description, input schema) as JSON, and call(name, input) answers {"ok": …} or {"error": "…"}. List it in config.toml:

[[plugins.wasm]]
id = "wordtools"        # its toolset id; its tools become wordtools_<tool>
path = "C:/plugins/wordtools.wasm"

• It may also (or instead) export craft-anything:plugin/checks: ways of judging a build, run after the built-in checks of each round for the kinds of project they name. A check gets the build as JSON (every file's path and size, plus the text of the extensions it asks for) and answers passed, an optional 0-100 similarity and a summary; a failed one is shown to the builder to fix. Asset providers can't be plugins: they need the network.
• It may import nothing: no files, network, environment or clock, so its tools run without asking.
• Each call runs in a fresh instance with a work limit (fuel) and 256 MiB of memory; one that runs past them is stopped with an error.
• A plugin that can't be loaded, or a tool whose id is invalid or taken, is skipped and logged.
• Complete plugins in Rust, with build steps: a tool, examples/wasm-plugin-wordcount, and a check, examples/wasm-plugin-readme-check.

Plugin API versions

The plugin API is package craft-anything:plugin@<version> in plugin.wit, versioned with semver; a plugin exports the interfaces at the version it was built against (craft-anything:plugin/tools@0.1.0).

• Compatible means the same major version, and while it's 0.x the same minor too (0.1.0 and 0.1.7 work together; 0.2.0 doesn't). Craft Anything loads a compatible plugin whatever its patch version, and refuses an incompatible one by name ("exports … for API 0.2.0; this Craft Anything speaks 0.1.0") instead of silently skipping it.
• A patch version only adds optional JSON fields: in what a plugin returns (Craft Anything ignores fields it doesn't know) and in what it's given (plugins must ignore unknown fields too).
• A minor version (while 0.x) or a major one is anything else: a new or changed function, a field that becomes required, a changed meaning. The host's API_VERSION (engines::wasm_plugin) and the WIT package move together; a test checks they agree.

Code that defines tools at run time itself (a WASM host, a proxy for another protocol) uses registry::RuntimeTool with Engine::shared_with: owned names, a JSON schema, and a function that serves its tools, telling them apart by ctx.tool.