Format JSON

A postToolUse hook that formats JSON files after successful agent edits. There is no skill to invoke. It runs in Copilot CLI and in VS Code sessions that use the Copilot harness; see where each type works.

Install

copilot plugin marketplace add OWNER/REPO
copilot plugin install format-json@MARKETPLACE

Replace OWNER/REPO with the GitHub repository that hosts this marketplace and MARKETPLACE with the name in its marketplace.json. See Install plugins for VS Code.

How it works

The hook reads toolName, toolArgs, toolResult.resultType, and cwd from the Copilot CLI hook event, described in Hooks. Only a success result can trigger formatting. It supports these tool inputs:

toolArgs may be an object or JSON-encoded string. apply_patch also accepts the raw patch string and the runtime's input or patch object forms. Paths resolve against the event's cwd, not the hook process directory.

For each affected .json file, the hook uses Node's JSON.parse and JSON.stringify to produce two-space indentation, or the --indent you configure, and a final newline. It leaves already formatted files untouched. Invalid JSON stays unchanged, with a diagnostic on stderr and in the assistant's tool context. The diagnostic names the file and the line and column of the error, never the file's content, because Node's own parser messages can quote the input. Other file errors are reported the same way, without stopping the remaining files. Changed paths are returned as additionalContext so the assistant knows to re-read the file.

No directory scans, shell commands, formatter downloads, or commit checks. No permission decisions or tool-result replacements. Unrelated tools and failed edits return {}.

Requirements and limits

• Node.js 22.12+; no plugin runtime dependencies.
• A host that runs Copilot CLI hooks: Copilot CLI, or a VS Code session that uses the Copilot harness. VS Code's Local harness sends a different event shape, so the hook reports a diagnostic there instead of formatting.
• Only regular JSON files below the session working directory are eligible. Symbolic links and paths outside that directory produce a diagnostic.
• JSONC, comments, and trailing commas are not supported. Node's JSON number precision rules apply, so do not use this example for precision-sensitive numeric data.
• Shell-based writes and unrecognized edit tools are not detected. The hook does not roll back edits, enforce policy, or replace CI.

Customize

To change the indentation, add --indent N (1 to 10) to the command in hooks.json:

"command": "node \"${PLUGIN_ROOT}/com.github.copilot/hooks/format.mjs\" --indent 4"

Then change the last grader argument in the eval from '2' to '4', bump the version, and run npm run verify. A test fails if the configured command and the eval disagree. The script rejects any other argument with a diagnostic instead of ignoring it.

To use Prettier, Ruff, or another formatter, replace this script with your team's formatter invocation and declare its prerequisites. Keep the affected-file selection and report any content changed after the tool ran. Update test/hook.test.mjs and the grader in evals/graders/format-json.mjs to match the new output.

Host checks

Before a release, try these by hand in each host you support. Automated tests and evals don't prove that a host loads or shows everything.

• Edit JSON with a single-file edit, a multi-file edit, and apply_patch. Confirm two-space formatting (or your --indent), a final newline, and a message that names each changed file.
• Confirm failed edits and unrelated files stay untouched, invalid JSON stays unchanged with a visible diagnostic, and already formatted JSON isn't rewritten.
• In VS Code, try it in a session that uses the Copilot harness.