Mops CLI

Opinionated guide for Motoko projects. Covers project config, dependency management, type-checking, building, and linting.

Key Principles

1. No dfx — always pin moc in [toolchain]. Use the newest moc version.
2. No mo:base — it is deprecated. Always use mo:core (import Array "mo:core/Array").
3. All config in mops.toml — canisters, moc flags, toolchain versions, build settings.
4. Canister-centric workflow — define all canisters in [canisters]; never pass file paths to mops check. Exception: library packages (no [canisters]) use file paths directly: mops check src/**/*.mo.

Project Setup

Minimal mops.toml

[toolchain]
moc = "1.7.0"
lintoko = "0.10.0"

[dependencies]
core = "2.5.0"

[moc]
args = ["--default-persistent-actors", "-W=M0223,M0236,M0237"]

[canisters.backend]
main = "src/backend/main.mo"

[canisters.backend.migrations]
chain = "src/backend/migrations"
check-limit = 10   # optional — speeds up `mops check` when the chain gets long

[canisters.backend.check-stable]
path = ".old/src/backend/dist/backend.most"

[build]
outputDir = "src/backend/dist"
args = ["--release"]

check-stable verifies stable variable compatibility against a .most file from the deployed version. For a new project with no prior deployment, create a trivial .most file representing an empty actor:

// Version: 1.0.0
actor {
  
};

Optional canister fields: candid (path to .did for compatibility checking), initArg (Candid-encoded init args).

Warning Flags

-W=M0223,M0236,M0237 — redundant type instantiation (M0223), suggest contextual dot notation (M0236), suggest redundant explicit arguments (M0237). These are allowed (disabled) by default; -W= enables them as warnings.

Moc Args Layering

Flags are applied in this order (later overrides earlier):

1. [moc].args — global, all commands (check, build, test, etc.)
2. [build].args — build only (e.g. --release)
3. [canisters.<name>.migrations] — auto-injected --enhanced-migration (managed by mops)
4. [canisters.<name>].args — per-canister
5. CLI -- <flags> — one-off overrides

Core Commands

mops install

mops install

Run after cloning or after manual mops.toml edits. Updates mops.lock. In CI, uses --lock check by default (fails if lockfile is stale).

mops add <package>

mops add core             # latest version
mops add core@2.5.0       # specific version
mops add --dev test       # dev dependency

Updates mops.toml and mops.lock.

mops check

Primary correctness command — runs moc check, then check-stable (if configured), then lint (if lintoko is in toolchain).

mops check                # all canisters
mops check backend        # single canister
mops check --fix          # autofix + check + stable + lint
mops check --verbose      # show moc invocations
mops check -- -Werror     # treat warnings as errors

Always use canister names, not file paths. Per-canister args from mops.toml are applied automatically.

--fix applies machine-applicable fixes from both moc and lintoko in one pass.

mops build

mops build                # all canisters
mops build backend        # single canister
mops build --verbose      # show compiler commands
mops build -- --ai-errors # pass extra moc flags

Produces .wasm, .did, and .most files in [build].outputDir (default .mops/.build).

mops toolchain

mops toolchain use moc 1.7.0         # pin specific version
mops toolchain use moc latest        # pin latest version (non-interactive)
mops toolchain use lintoko 0.10.0    # pin specific version
mops toolchain update moc            # update to latest (requires existing [toolchain] entry)
mops toolchain update                # update all tools to latest
mops toolchain bin moc               # print path to binary

Agent note: toolchain use <tool> without a version opens an interactive picker — do not use in scripts or agents. Always pass a version or latest. toolchain update only works when the tool already has a [toolchain] entry.

Enhanced migrations

When [canisters.<name>.migrations] is configured, mops check, mops build, and mops check-stable automatically inject --enhanced-migration. Do not add --enhanced-migration to [canisters.<name>].args — mops will error.

Create migration files directly in the chain directory.

check-limit (optional) caps how many recent chain files mops check and mops lint consider — useful when the chain grows long and re-checking every old migration slows feedback down. mops build is unaffected by check-limit. When the limit kicks in, mops stages the included files into .migrations-<canister>/ next to the chain directory (auto-.gitignored). moc diagnostics may then print paths there — the real file lives in the chain directory with the same name.

mops remove <package>

mops remove base

Dependency Management

mops outdated             # list outdated dependencies (caret-bound)
mops update               # update all within caret bound (no major-version crossing)
mops update core          # update specific package within caret bound
mops update --major       # allow updates that cross major versions
mops sync                 # add missing / remove unused packages

Other Commands

mops test

Tests live in test/*.test.mo:

mops test                         # run all tests
mops test my-test                 # filter by name
mops test --mode wasi             # use wasmtime (for to_candid/from_candid)
mops test --reporter verbose      # show Debug.print output
mops test --watch                 # re-run on file changes

mops lint

Runs lintoko (also runs automatically as part of mops check when lintoko is in toolchain):

mops lint                 # lint all .mo files
mops lint --fix           # autofix lint issues
mops lint <name>          # filter to .mo files matching <name>

When [canisters.<name>.migrations].check-limit is set, mops lint skips the trimmed chain migrations to match what moc sees during mops check. To lint a trimmed migration on demand, pass an explicit filter (e.g. mops lint OldMigrationName).

mops format

mops format               # format all .mo files
mops format --check       # check formatting without modifying

Common Patterns

Warning suppression for a canister

Use per-canister args (not global) for suppressions:

[canisters.backend]
main = "src/backend/main.mo"
args = ["-A=M0198"]

New project

mops init -y
mops toolchain use moc latest        # pin latest moc (non-interactive)
mops toolchain use lintoko latest    # pin latest lintoko
mops add core

Then configure [moc].args, [canisters], and [build] in mops.toml.

To update tools later: mops toolchain update moc or mops toolchain update (all tools).