UserPromptSubmit hook additionalContext not injected into model context in VSCode extension

[qinyan1603-source]

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:

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

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

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

3. Open VSCode, start a new Claude Code conversation
4. Ask the model: "What startup context did you receive from hooks?"
5. 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.

[0xbrainkid]

UserPromptSubmit hook additionalContext working in CLI but not in the VS Code extension is a platform-specific hook output handling gap — the VS Code extension is executing the hook (evidenced by the state file being written) but not consuming the additionalContext from stdout.

The pattern is the same as several other VS Code extension permission and hook issues (#48724, #49047): the extension executes the hook mechanism but does not fully implement the output handling pipeline. The CLI and extension share hook execution but the extension's handling of hook output is missing or broken.

Root cause narrowing: The evidence is precise — hook executes (state file written with correct session_id), but model receives no additional context. This means the failure is not in hook execution but in the step that takes the hook's stdout JSON and injects additionalContext into the model's context window before the request is sent.

Likely location of the bug: The VS Code extension's UserPromptSubmit handler processes the user's message before forwarding it. In CLI, this handler includes a step to read hook stdout and merge additionalContext into the prompt context. The VS Code extension handler likely skips or misroutes this step.

Workaround: As an interim measure, the hook can write its context to a file that CLAUDE.md or a startup hook reads into the persistent context. This is indirect but achieves the same injection. The hook writes to ~/.claude/hook-context.md, and CLAUDE.md includes @~/.claude/hook-context.md — the model reads the context at session start even if per-message injection is broken.

The fix is to verify that the VS Code extension's UserPromptSubmit message processing path includes the same hook output consumption step as the CLI path.

[christian-drescher]

Claude Code CLI v2.1.144 on Linux also has this problem, MacOS, too (see #52876 which was falsely closed as duplicate).

[github-actions]

Closing for now — inactive for too long. Please open a new issue if this is still relevant.