Deno "fmt --check ... Not formatted" gate failing in CI

deno fmt --check reformats in memory and compares against what is committed. When any file differs, it lists the files and exits non-zero, failing the CI gate without changing anything.

What this error means

The step fails with "error: Found N not formatted files in M files" and lists each file whose formatting differs.

deno

from /home/runner/work/app/app/src/main.ts:

error: Found 1 not formatted file in 42 files

Common causes

Committed code was not formatted

The files were edited without running the formatter, so they differ from deno fmt output.

A fmt config or version difference

A different fmt option in deno.json, or a different Deno version locally, produces output the CI formatter disagrees with.

How to fix it

Format and commit

Run deno fmt to rewrite the files.
Commit the formatting changes.
Re-run deno fmt --check to confirm it passes.

Terminal

deno fmt
deno fmt --check

Align fmt config and Deno version

Keep fmt options in deno.json and pin the same Deno version in CI so local and CI formatting agree.

.github/workflows/ci.yml

- uses: denoland/setup-deno@v2
  with:
    deno-version: v2.x

How to prevent it

Run deno fmt before committing, or in a pre-commit hook.
Keep fmt options in deno.json under version control.
Pin the Deno version so formatter output matches CI.