Summary

preToolUse hooks in GitHub Copilot CLI appear to ignore hookSpecificOutput.updatedInput, even though the VS Code hooks documentation describes updatedInput as supported for PreToolUse.

Environment

• Copilot CLI version: 1.0.5-0
• OS: Linux

What I expected

When a preToolUse hook returns:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "updatedInput": {
      "command": "printf PROBE_REWRITTEN"
    }
  }
}

I expected the CLI to execute the rewritten tool input.

What happened instead

The hook executed and returned updatedInput, but Copilot CLI still ran the original bash command.

Reproduction

Repository hook config:

{
  "version": 1,
  "hooks": {
    "preToolUse": [
      {
        "type": "command",
        "bash": "./scripts/rewrite-probe.sh",
        "cwd": ".github/hooks",
        "timeoutSec": 10
      }
    ]
  }
}

Hook script:

#!/usr/bin/env bash
set -euo pipefail

input=$(cat)
tool_name=$(jq -r '.tool_name // .toolName // empty' <<<"$input")
[[ "$tool_name" != "bash" ]] && exit 0

tool_input=$(jq -c '.tool_input // (.toolArgs | fromjson? ) // {}' <<<"$input")
command=$(jq -r '.command // empty' <<<"$tool_input")
[[ "$command" != *PROBE_ORIGINAL* ]] && exit 0

jq -nc '{
  continue: true,
  hookSpecificOutput: {
    hookEventName: "PreToolUse",
    permissionDecision: "allow",
    permissionDecisionReason: "Rewrite probe attempted updatedInput",
    updatedInput: {
      command: "printf PROBE_REWRITTEN",
      description: "Run the exact probe command"
    },
    additionalContext: "Rewrite probe replaced the bash command with printf PROBE_REWRITTEN"
  }
}'

Prompt used:

Use the bash tool to run exactly this command and nothing else:
printf PROBE_ORIGINAL
After running it, tell me exactly what was printed. Do not simulate the result.

Hook input captured:

{"sessionId":"...","timestamp":1773370259963,"cwd":".../hook-probe","toolName":"bash","toolArgs":"{\"command\":\"printf PROBE_ORIGINAL\",\"description\":\"Run the exact probe command\"}"}

Hook output captured:

{"continue":true,"systemMessage":"rewrite-probe fired","permissionDecision":"allow","hookSpecificOutput":{"hookEventName":"PreToolUse","permissionDecision":"allow","permissionDecisionReason":"Rewrite probe attempted updatedInput","updatedInput":{"command":"printf PROBE_REWRITTEN","description":"Run the exact probe command"},"additionalContext":"Rewrite probe replaced the bash command with printf PROBE_REWRITTEN"}}

Copilot CLI still executed:

printf PROBE_ORIGINAL

And reported output:

PROBE_ORIGINAL

Question

Is hookSpecificOutput.updatedInput intended to be supported in Copilot CLI as well, or is this field currently VS Code-only / undocumented for CLI? If it is intended to work in CLI, this looks like a bug or a docs mismatch.