YAML errors in GitHub Actions, Kubernetes or docker-compose files: 'mapping values are not allowed here', 'did not find expected key', 'found character that cannot start any token'. How do I find and fix them?

Cause: YAML is indentation-sensitive and has a few characters with special meaning. These parser errors almost always come from: inconsistent indentation (a key indented one space more or less than its siblings), tabs used for indentation (YAML forbids tabs for indentation, hence 'cannot start any token'), a colon followed by a space inside an unquoted value (e.g. run: echo key: value, which YAML reads as a nested mapping: 'mapping values are not allowed here'), list items ('- ') mixed with keys at the wrong level, or special characters at the start of a value (*, &, !, %, @, `, {, [) that need quotes. Fix: 1. Go to the line (and the line before it) the error names; parsers often report the spot where they noticed the problem, just after the real mistake. 2. Use spaces only, consistently (2 per level is common), and make sure sibling keys line up exactly. 3. Quote values that contain ': ' or start with special characters: run: "echo key: value", or use a block scalar for commands: run: | followed by indented lines. 4. For GitHub Actions, expressions like ${{ ... }} at the start of a value are usually fine unquoted, but values starting with * or containing ': ' need quotes; check the workflow syntax docs for each key's expected type (string vs list vs mapping). 5. Validate before pushing: yamllint file.yaml, python -c "import yaml,sys; yaml.safe_load(open(sys.argv[1]))" file.yaml, kubectl apply --dry-run=client -f file.yaml, or docker compose config. Editors with YAML language support show errors as you type. 6. Watch for values YAML converts unexpectedly (yes/no/on/off, version numbers like 3.10 becoming 3.1); quote them when they must stay strings. Verify: the linter or dry run passes, and the tool (Actions, kubectl, compose) loads the file without errors. Sources: https://yaml.org/spec/1.2.2/ https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax