What causes "TODO: '%s' not implemented" in nektos/act?

Check the function name for typos — e.g. `success()` not `succes()`.

TODO: '%s' not implemented

Error message

TODO: '%s' not implemented

What it means

Thrown when an expression calls a function that the interpreter's evaluateFuncCall switch does not recognize. Only a fixed set of GitHub Actions functions (contains, startsWith, endsWith, format, join, toJSON, fromJSON, hashFiles, always, success, failure, cancelled) plus context-dependent status functions are implemented; anything else hits the default branch with the TODO message.

Source

Thrown at pkg/exprparser/interpreter.go:667

		if impl.config.Context == "job" {
			return impl.jobSuccess()
		}
		if impl.config.Context == "step" {
			return impl.stepSuccess()
		}
		return nil, fmt.Errorf("Context '%s' must be one of 'job' or 'step'", impl.config.Context)
	case "failure":
		if impl.config.Context == "job" {
			return impl.jobFailure()
		}
		if impl.config.Context == "step" {
			return impl.stepFailure()
		}
		return nil, fmt.Errorf("Context '%s' must be one of 'job' or 'step'", impl.config.Context)
	case "cancelled":
		return impl.cancelled()
	default:
		return nil, fmt.Errorf("TODO: '%s' not implemented", funcCallNode.Callee)
	}
}

View on GitHub (pinned to 4f41128141)

Solutions

Check the function name for typos — e.g. `success()` not `succes()`.
Compare against the list of supported functions in pkg/exprparser/interpreter.go evaluateFuncCall; replace unsupported functions with supported equivalents.
Upgrade act to the latest release where the function may have been implemented.
Move the logic into a run step (bash) if the expression language cannot express it.

Example fix

# before
if: ${{ startsWiths(github.ref, 'refs/tags/') }}
# after
if: ${{ startsWith(github.ref, 'refs/tags/') }}

Defensive patterns

Strategy: validation

Validate before calling

supported := map[string]bool{"contains": true, "startswith": true, "endswith": true, "format": true, "join": true, "tojson": true, "fromjson": true, "hashfiles": true, "always": true, "success": true, "failure": true, "cancelled": true}
func calleeSupported(expr string) bool { /* extract identifiers followed by '(' and check the map */ return true }

Try / catch

if err != nil && strings.Contains(err.Error(), "not implemented") {
    // surface a clear message: function X is unsupported by this act version
}

Prevention

Cross-check every function call in if:/env:/with: against the supported list before committing workflows.
Upgrade act when adopting newly released GitHub expression functions.
Prefer moving complex logic into run steps rather than exotic expression functions.

When it happens

Trigger: Using a newer GitHub Actions function that act has not implemented yet (e.g. a recently added builtin), misspelling a function name like `${{ succes() }}`, or calling custom/namespaced functions that do not exist in GitHub Actions expressions at all.

Common situations: Workflows written against newer GitHub features run on an older act binary; typos in function names; users expecting bash or JavaScript function syntax inside `${{ }}`.

Related errors

AI-assisted analysis of nektos/act@4f41128141 (2026-08-15). Data as JSON: /api/errors/d0b14da00fc3bab5. Report an issue: GitHub.