GITHUB_OUTPUT syntax error multiline output breaks workflow bug

• A step fails with an invalid format error while setting an output that contains newlines. - A downstream step receives only the first line of a multiline output. - A workflow still uses the deprecated set-output command and now errors on current runners.

Find the step that writes the output. Look for a line that appends a multiline variable, command output, or file contents to GITHUB_OUTPUT with the plain name=value form. Expected: you have the exact run block that produces the invalid format error or the truncated value.

Rewrite it with a unique delimiter. Use the delimiter form below, with a delimiter string that cannot appear inside the value; a random suffix or a guid works well. Success check: the step finishes without the file command error.

Remove any old set-output lines. The deprecated set-output workflow command is disabled on current runners and fails the step; every output must go through the GITHUB_OUTPUT file instead. Search the workflow for set-output and convert each one to the file form. Expected: no set-output text remains in the workflow files.

Check the reading side. Later steps and jobs read the value from the step outputs by the exact output name declared in the step and the job outputs mapping. A spelling difference between the written name and the read name gives an empty value with no error. Success check: the names match character by character in the write, the step outputs, and the job outputs.

Test with a value that has trailing newlines and special lines. Lines inside the value that look like name=value pairs are safe inside the delimiter block, but a line equal to the delimiter ends the value early. Run the workflow once with realistic content, then print the received value length in the next step. Expected: the length matches the source value and the content arrives whole.

For outputs built in a script file, write the file from the script. Scripts in any language can append the same delimiter block to the path stored in the GITHUB_OUTPUT environment variable. Success check: the script based step passes the same workflow test as the shell based step.

Resolved from the public problem: https://vectle.com/posts/pst_s64uwKdh8uSpUgqdCICzOw

Problem query: GITHUB_OUTPUT syntax error multiline output breaks workflow bug

TL;DR: GITHUB_OUTPUT is parsed line by line, so a raw multiline value written with the simple name=value form ends at the first newline and the remaining lines are read as new, invalid entries. Write multiline outputs with the delimiter form: a first line with the output name, a unique delimiter, and the value, then the value, then the delimiter again on its own line. Later steps then receive the full value, newlines included, through the step outputs.