GitHub Actions "fromJSON ... Unable to parse" in CI

By Kaveh Alemi·Latchkey

fromJSON expects a strictly valid JSON string. An empty value, single quotes instead of double quotes, or a trailing comma makes it fail with a parse error during expression evaluation.

What this error means

A job fails while evaluating an expression with "Unable to parse" and a JSON snippet, commonly when a dynamic matrix is built from a step output that was empty or malformed.

Error: Unable to parse '[ {os: ubuntu-latest, } ]':
Unexpected character encountered while parsing value

Common causes

The JSON uses single quotes or unquoted keys

JSON requires double-quoted keys and string values; {os: x} or '[...]' is not valid JSON.

The source output was empty

A step that should emit JSON produced nothing, so fromJSON receives an empty string it cannot parse.

How to fix it

Emit strict JSON from the producing step

1. Generate the value with a tool that writes valid JSON (e.g. jq -c).
2. Write it to $GITHUB_OUTPUT as a single line.
3. Reference it with fromJSON(steps.x.outputs.y).

matrix=$(jq -cn '{os:["ubuntu-latest","windows-latest"]}')
echo "matrix=$matrix" >> "$GITHUB_OUTPUT"

Guard against an empty output

Default to a valid empty JSON value so fromJSON never sees an empty string.

echo "matrix=${matrix:-[]}" >> "$GITHUB_OUTPUT"

How to prevent it

• Produce JSON with jq -c, not by string concatenation.
• Default to [] or {} when the source can be empty.
• Echo the output and validate it before using fromJSON.

Frequently asked questions

Related guides

References

• GitHub Actions documentation
• Workflow syntax for GitHub Actions