SessionStart hook systemMessage no longer rendered in terminal

[joshwilhelmi]

Description

The systemMessage field returned by a SessionStart hook is no longer displayed in the terminal. Previously it rendered as:

⎿  SessionStart:startup says:
     Gobby Session ID: #4796 (4129ef90-...)
     Agent: default
     ├─ Rules: 72
     └─ Skills: 23

Now the hook executes successfully (exit 0, valid JSON with systemMessage field populated), but nothing is displayed. The additionalContext in hookSpecificOutput IS injected into the model context correctly — only the visual rendering of systemMessage is missing.

Steps to Reproduce

1. Configure a SessionStart hook that returns JSON with a systemMessage field:

{
  "continue": true,
  "systemMessage": "Session started successfully",
  "decision": "approve",
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "some context"
  }
}

2. Start a new Claude Code session
3. Observe: no visual output from the hook, despite additionalContext being injected

Expected Behavior

systemMessage should be displayed in the terminal, as documented ("optional warning message shown to the user").

Environment

• Claude Code: 2.1.88
• OS: macOS (Darwin 25.3.0)
• Terminal: tmux

Related

• [BUG] SessionEnd hook system message does not appear in the terminal screen #9090 (SessionEnd hook systemMessage not displaying)
• [FEATURE] Display SessionStart hook systemMessage in VS Code extension #15344 (Display SessionStart hook systemMessage in VS Code extension)

[yurukusa]

This appears to be a rendering regression — the systemMessage field is still accepted in the hook response JSON, but the terminal display was dropped at some point.

What still works:

• hookSpecificOutput.additionalContext — injects into the model's context (confirmed working by OP)
• Hook execution and JSON parsing — the hook runs fine

Workaround for visual feedback:

If you need the session info visible to the user, write it to stderr instead:

#!/bin/bash
INPUT=$(cat)
# Visual output to stderr (shown in terminal)
echo "Session #4796 started | Rules: 72 | Skills: 23" >&2

# Context injection for the model
cat <<'EOF'
{
  "continue": true,
  "decision": "approve",
  "hookSpecificOutput": {
    "additionalContext": "Session #4796 | Agent: default | Rules: 72 | Skills: 23"
  }
}
EOF

Stderr output from hooks appears inline in the terminal. Combined with additionalContext, this gives you both visual feedback and model context injection.

[arsmap]

Confirmed on Windows 10 Pro, Claude Code v2.1.119.

Additional findings:

• Not only systemMessage JSON — even plain text stdout (echo 'test') from SessionStart hook is not displayed in TUI
• The hook itself runs correctly and output IS injected into Claude's context
• The visual notification area ("SessionStart:startup says: ...") appears to have been removed entirely from the new minimal startup UI

The new startup UI (v2.1.119) shows only: logo + version + model + path + empty prompt. The area where hook output was previously displayed seems to be gone.

[arsmap]

Additional finding: the minimal UI vs full box UI appears to be intentional based on usage frequency.

• Users with extensive recent activity → minimal startup UI (no hook message display area)
• Users with no/little recent activity → full box UI with 'Recent activity' panel (hook messages visible)

Tested on same account (arsmap@gmail.com), same OS (Windows), same version (v2.1.119):

• Machine with heavy usage history → minimal UI, hook systemMessage not shown
• Fresh machine with no activity → full box UI shown

So the root issue is: the minimal UI mode has no area to display hook systemMessage, while the full box UI presumably still shows it. Heavy users who would benefit most from hook notifications are exactly the ones who can't see them.

[arsmap]

Update (2026-04-27, CC v2.1.119):

UserPromptSubmit hook systemMessage output is now appearing in the terminal (shown as UserPromptSubmit says: ...). This started today without any version change, so it may be a server-side rendering update.

However, SessionStart hook systemMessage still does not appear in the terminal — the original bug remains for SessionStart hooks.

Summary of current behavior:

• ✅ UserPromptSubmit systemMessage → now visible in terminal
• ❌ SessionStart systemMessage → still not visible in terminal

[ryu111]

+1 confirmed reproducible on:

• Claude Code: 2.1.132
• OS: macOS Darwin 25.4.0
• Terminal: VS Code integrated terminal → tmux → claude CLI (native, not VS Code extension)

Reproduction matches reporter's exactly: hook emits valid {systemMessage, hookSpecificOutput: {hookEventName: "SessionStart", additionalContext}}, additionalContext correctly injected into model context (verified via claude -p --include-hook-events --output-format=stream-json showing full payload in hook_response.output), but systemMessage banner silently dropped from terminal.

We initially mis-diagnosed this as our own multi-module hook dispatcher emitting multiple JSON lines (CC parses only first line — yes, that IS a separate bug in older versions), then as wrong envelope shape (additionalContext must be nested under hookSpecificOutput). Both turned out to be valid issues we fixed, but the underlying terminal rendering of systemMessage for SessionStart remains broken regardless of payload shape.

Currently affected hooks in our setup: tasklist injection, handoff active-consume, vault session log — all rely on banner for user visibility on session start.

Confirmed not a regression from our hook code: stream-json shows full output field populated, outcome:"success", exit_code:0. The CC binary side simply does not render the systemMessage for SessionStart events.