Plugin SessionStart hooks don't surface hookSpecificOutput.additionalContext to Claude

Description

When a SessionStart hook is defined in a plugin's hooks.json (referenced via plugin.json), the hook executes successfully but the hookSpecificOutput.additionalContext content is not passed to Claude. Claude only receives a generic success message like "SessionStart:Callback hook success: Success" instead of the actual context.

Expected behavior

The additionalContext value should be injected into Claude's context, as documented in the hooks reference.

Actual behavior

• Hook executes and returns valid JSON with hookSpecificOutput.additionalContext
• Claude only sees the success callback message, not the context content

Reproduction

1. Create a plugin with hooks.json containing a SessionStart hook:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh"
          }
        ]
      }
    ]
  }
}

1. Hook script outputs valid JSON:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "test context that should be visible to Claude"
  }
}

1. Start a new Claude Code session
2. Claude does not receive the "test context" - only sees the success callback

Workaround

Adding the same hook directly to ~/.claude/settings.json (user config) works correctly - the additionalContext is properly surfaced to Claude.

Environment

• macOS Darwin 24.5.0
• Claude Code CLI

Impact

Plugins cannot provide session startup context to Claude, limiting use cases like:

• Loading previous session state
• Injecting project-specific instructions at startup
• Providing retrieval practice prompts
• Any plugin that needs to influence Claude's initial behavior