mops check

Check Motoko canisters or files for syntax errors and type issues

mops check [args...]

Runs the Motoko compiler in check-only mode (moc --check). All package sources from the project are automatically included.

Arguments can be canister names (as defined in [canisters]) or file paths. When no arguments are given, checks all canisters defined in the [canisters] section of mops.toml.

When checking canisters, per-canister [canisters.<name>].args from mops.toml are applied alongside global [moc].args.

Exits with a non-zero code if any file has errors, making it suitable for CI pipelines. Warnings do not cause a failure by default.

Examples

Check all canisters defined in mops.toml

mops check

Check a specific canister

mops check backend

Check a single file

mops check src/main.mo

Check multiple files

mops check src/main.mo src/types.mo

Check with verbose output (shows the compiler command being run)

mops check --verbose

Treat warnings as errors

mops check -- -Werror

Arguments

[args...]

Canister names or file paths to check.

• Canister names — resolved from [canisters.<name>] in mops.toml. Per-canister args are applied.
• File paths — .mo files to check directly. Only global [moc].args and CLI -- flags are applied.
• No arguments — checks all canisters defined in mops.toml.

You cannot mix canister names and file paths in the same invocation.

Options

--fix

Automatically apply fixes for supported diagnostics, including transitively imported files. Fixed files and the applied fix codes are printed to the console.

mops check --fix

After applying fixes, --fix re-checks all files and runs stable compatibility checks (if configured). If type-checking fails after fixing, stable checks are skipped.

--verbose

Print the full moc invocation before running it.

Passing flags to the Motoko compiler

Any arguments after -- are forwarded directly to moc. For example, to treat all warnings as errors:

mops check -- -Werror

tip

Global moc flags can be configured in mops.toml under [moc].args so they don't need to be passed on every invocation. See mops.toml reference.

Stable compatibility checking

When a canister has a [canisters.<name>.check-stable] section in mops.toml, mops check automatically runs a stable compatibility check after type-checking. This compares the deployed version against the current canister entrypoint to catch breaking changes to stable variables before deployment.

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

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

If the file at path doesn't exist, the check fails with an error. To silently skip the stable check when the file is missing (useful for initial deployments where no previous version exists), set skipIfMissing = true:

[canisters.backend.check-stable]
path = ".old/src/main.most"
skipIfMissing = true

For more details, see mops check-stable.

Enhanced migration support

When a canister has a [canisters.<name>.migrations] section in mops.toml, mops check automatically injects the --enhanced-migration flag. The frozen migration chain and any pending next migration are assembled into a temporary directory and passed to moc.

If a stable compatibility check fails and [migrations] is configured, a hint is shown suggesting to create a new migration with mops migrate new <Name>.

See mops migrate for the full migration workflow.

Lint integration

After type-checking succeeds, mops check automatically runs mops lint when lintoko is pinned in [toolchain].

This means mops check is the single command for all correctness checks — type errors and lint violations are both caught in one pass.

mops check --fix

--fix is forwarded to both the Motoko compiler and lintoko, so both type-level and lint fixes are applied in a single invocation.

note

When file paths are passed explicitly (e.g. mops check src/Main.mo), linting is scoped to those files. When checking canisters (by name or with no arguments), linting covers all .mo files in the project.

info

mops check only type-checks files — it does not produce any compiled output. To compile canisters, use mops build.