GitHub Actions "No event triggers defined in 'on'" in CI
A workflow must list at least one event under on. When on is empty, null, or its events failed to parse, the validator reports no triggers are defined.
What this error means
The run fails with "Invalid workflow file" and "No event triggers defined in 'on'". The workflow never starts on push or pull_request.
Invalid workflow file: .github/workflows/ci.yml
(Line: 3, Col: 1): No event triggers defined in 'on'Diagnose it: print the context before you change anything
Most workflow-expression bugs are not syntax errors, they are an expression reading something that is empty. GitHub resolves a missing property to an empty string instead of failing the run, so a wrong reference looks like a logic bug rather than a mistake. Dump the contexts first and you will usually see the answer immediately.
- name: Dump contexts
run: |
echo '--- github ---' ; echo '${{ toJSON(github) }}'
echo '--- needs ---' ; echo '${{ toJSON(needs) }}'
echo '--- steps ---' ; echo '${{ toJSON(steps) }}'
echo '--- matrix ---' ; echo '${{ toJSON(matrix) }}'
echo '--- inputs ---' ; echo '${{ toJSON(inputs) }}'Check the context is allowed where you used it
Contexts are not available everywhere. The same expression can be valid in a step if and invalid in a job if, which is why an expression that works in one workflow fails when moved.
| Where you wrote it | Contexts available there |
|---|---|
run-name | github, inputs, vars |
concurrency | github, inputs, vars |
Top-level env | github, secrets, inputs, vars |
jobs.<id>.if | github, needs, vars, inputs |
jobs.<id>.steps.if | github, needs, strategy, matrix, job, runner, env, vars, steps, inputs |
jobs.<id>.outputs | Full access, including secrets |
Reusable workflow outputs | github, jobs, vars, inputs |
Common causes
The on key is empty or null
Writing on: with nothing after it leaves the trigger list empty.
on was quoted away by YAML
Unquoted on: can be read as the boolean true in some YAML loaders, so the events go missing. Quoting "on": avoids this.
How to fix it
List concrete events under on
- Add at least one event such as
pushorpull_request. - Quote the key as
"on":if your tooling coerces it to a boolean. - Re-run by pushing or opening a PR.
"on":
push:
branches: [main]
pull_request:Use the list form for simple triggers
For multiple events without filters, a flow sequence is concise and unambiguous.
on: [push, pull_request]Catch it before it reaches CI
Every failure in this cluster is statically detectable. actionlint parses workflow expressions, checks context availability against the same rules above, and validates needs references, so these bugs never need to cost you a run.
# one-off
docker run --rm -v "$(pwd):/repo" --workdir /repo rhysd/actionlint:latest -color
# as a job, before anything expensive runs
- uses: actions/checkout@v4
- run: |
bash <(curl -s https://raw.githubusercontent.com/rhysd/actionlint/main/scripts/download-actionlint.bash)
./actionlint -colorHow to prevent it
- Always define at least one event under
on. - Quote
"on"when YAML boolean coercion is a risk. - Validate the workflow before pushing.