UserPromptSubmit hook additionalContext not injected into model context in VSCode extension

Environment

• Claude Code VSCode Extension (tested on v2.1.109+)
• Windows 11, Python 3.x
• Hook registered in ~/.claude/settings.json under hooks.UserPromptSubmit

Summary

A UserPromptSubmit hook that outputs valid {"additionalContext": "..."} JSON to stdout executes successfully but the content never reaches the model context. The same hook mechanism works as expected in CLI.

Reproduction

1. Register a UserPromptSubmit hook in settings.json:

``json
"UserPromptSubmit": [{
"hooks": [{
"type": "command",
"command": "python path/to/hook.py",
"timeout": 10
}]
}]
``

1. The hook script reads stdin JSON, builds context, outputs:

``json
{"additionalContext": "Current time: 2026-04-16 16:28 ..."}
``

1. Open VSCode, start a new Claude Code conversation
2. Ask the model: "What startup context did you receive from hooks?"
3. Model reports: none received

Evidence that the hook DID execute

• Hook writes a state file on successful execution → file exists with correct session_id and timestamp
• Running the hook manually with echo '{"session_id":"test"}' | python hook.py produces valid JSON output
• A second hook (session_reset.py) registered on the same event also executes and writes its own state file

Evidence that additionalContext is NOT injected

• Model explicitly reports no startup context received
• No <user-prompt-submit-hook> tag visible in conversation
• No time/memory/backlog content visible anywhere in model context

What works

• Hook triggering: ✅
• Hook script execution: ✅
• Hook stdout output (valid JSON with additionalContext): ✅
• Stop hook blocking feedback (non-zero exit): ✅ (reaches model)
• CLAUDE.md loading: ✅
• MCP tool calls: ✅

What doesn't work

• UserPromptSubmit hook additionalContext → model context: ❌

Expected behavior

Per Claude Code hooks documentation, additionalContext returned from a hook's stdout should be appended to the model's context for that turn. This works in CLI but not in the VSCode extension.