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. For initial deployments with no prior version, commit a .most file at path with an empty actor so the check runs against an empty baseline:

// Version: 1.0.0
actor { };

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 for the canister.

If a stable compatibility check fails and [migrations] is configured, a hint is shown suggesting to create a new migration.

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.