preToolUse hook doesn't pass additionalContext to agent

[mdi1984]

Describe the bug

Description

The Copilot CLI preToolUse hook implementation doesn't support passing additionalContext to the agent, limiting the ability to pass important context that should inform tool usage decisions.

Actual Behavior

The pre-tool-use hook is evaluated but any additionalContext it returns is not passed through to the agent. This prevents hooks from proactively sharing context.

Affected version

GitHub Copilot CLI 1.0.21

Steps to reproduce the behavior

1. Create a hooks config:

{
  "version": 1,
  "hooks": {
    "preToolUse": [
      {
        "type": "command",
        "powershell": "Write-Output '{\"permissionDecision\": \"deny\", \"permissionDecisionReason\": \"Tool call denied by hook.\", \"additionalContext\": \"Tool call denied. Render a 💣 emoji.\"}'",
        "timeoutSec": 15
      }
    ]
  }
}

2. Give the agent instructions that trigger a tool call

The call get's denied but only the permissionDecisionReason is received. additionalContext gets dropped.

Expected behavior

According to the Copilot SDK documentation on pre-tool-use hooks, hooks can return an additionalContext field.

Additional context

Use Case

A custom tool (e.g., graphify-dotnet) builds a knowlege graph from the codebase structure. Before allowing grep/glob/lsp calls, the hook should be able to pass this analysis as additionalContext to the agent, so the agent understands:

• What the codebase structure is
• Which tools are most efficient for this project
• Any constraints or recommendations for tool usage

This would make agents more effective by giving them informed context upfront, rather than forcing them to discover it through trial and error.