Exit Codes

tyc follows the conventional “0 = success, non-zero = failure” pattern. Specific values:

Code	Meaning
0	Success.
1	Compile-time or run-time failure — at least one error-severity diagnostic, a `tyc fmt --check` that would reformat, an unknown `tyc explain` code, a missing file or required tool (e.g. `uv` not installed for `tyc add`), or a program that exited non-zero under `tyc run`.
2	Invocation error — invalid arguments or an unknown subcommand (reported by the argument parser).
130	Interrupted (`Ctrl-C` / `SIGINT`).

Per-command notes

Command	0 means	Non-zero means
`tyc check`	No errors.	1 if any error-severity diagnostic, the input is invalid, or there was nothing to check (no `.ty` files under the paths); 2 if the command line is malformed.
`tyc check --stubs`	No drift.	1 if any `tyc::stub_mismatch` at error severity (the `stub-check` default).
`tyc build`	Build succeeded.	1 if any error-severity diagnostic stops emission.
`tyc fmt`	No changes / changes written.	1 (with `--check`) if any file would change.
`tyc run`	Script exited 0.	Propagates the script’s exit code verbatim.
`tyc repl`	Clean exit.	130 on Ctrl-C. With stdin piped (a script fed in), 1 if any snippet failed to compile or raised.
`tyc debug`	Debugger exited 0.	Whatever the debugger exits with.
`tyc trace`	Traceback remapped.	1 if the file is unreadable / not a valid traceback.
`tyc profile`	Instrumented build written.	1 if the build or the instrumentation failed. (`tyc profile` does not run the program — you do, and the profile is written on its exit.)
`tyc migrate`	Wrote `.ty`.	1 if the input is missing or unreadable. `--check` is a preview to stdout and exits 0 whenever the migration itself succeeds.
`tyc ty`	`ty` returned 0.	Propagates `ty`’s exit code.
`tyc stubtest`	`mypy.stubtest` returned 0.	Propagates mypy’s exit code.
`tyc add` / `remove` / `sync`	Manifest updated and `uv sync` succeeded.	1 if uv missing; non-zero from uv on resolution failure.
`tyc lsp`	The client sent `shutdown` before `exit`.	1 when the client sends `exit` without `shutdown` (the LSP spec’s rule) or closes stdin without it.

Severity-to-exit-code mapping

[strictness] keys control whether a finding is error, warn, or off. Only error-severity findings change the exit code.

[strictness]
unused-import = "warn"        # surfaces but doesn't fail CI
exhaustive-match = "error"    # fails CI

To gate CI on warnings, raise the relevant key to "error" — there is no --quiet switch to filter output with.

Where next

tyc check — the CI command.
Reading Diagnostics — diagnostic severities and format.
[strictness] config — every severity knob.