GitHub Actions: Error: Unable to process file command 'output' successfully. Invalid format 'x'

Meaning

These files accept NAME=value for single lines and NAME<<DELIMITER … DELIMITER for multi-line values. A value that contains a newline (a file's content, JSON, a commit message) breaks the format. Use a heredoc delimiter, or encode the value.

Common causes

• Multi-line value written as a single name=value line
• Value containing a newline from $(cat file) or command output
• Missing = between name and value
• Delimiter text appearing inside the value
• Windows line endings in the written text

Detailed fix by platform

Code examples

- id: notes
  run: |
    EOF=$(openssl rand -hex 8)
    {
      echo "body<<$EOF"
      git log -5 --pretty=format:'%s'
      echo
      echo "$EOF"
    } >> "$GITHUB_OUTPUT"
- run: echo "${{ steps.notes.outputs.body }}"

How to diagnose

1. File — output, env or state?
2. Value — Contains newlines?
3. Syntax — name=value or heredoc?

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