Desktop app crashes when running shell command as first message in new session

[shuv1337]

Problem

When starting a new session in the desktop app and immediately running a shell command with ! (before sending any plain text message), the app crashes and requires a reload.

Note: The shell command does execute successfully - results are visible when reopening the session after the crash.

Root Cause Analysis

This is a race condition in the shell command submission flow:

1. Shell commands are fire-and-forget: In prompt-input.tsx, sdk.client.session.shell() is called without await and no optimistic message is added
2. Navigation triggers sync immediately: After session creation, navigate(existing.id) triggers a session sync via createEffect in session.tsx
3. Sync returns empty messages: If the sync runs before the server processes the shell command, it returns no messages
4. Part component crashes on undefined: SessionTurn renders <Part part={assistantParts()[0]} ... /> where assistantParts()[0] is undefined, causing props.part.type access to crash

Why regular messages don't crash

Regular prompts use sync.session.addOptimisticMessage() to add a local message immediately, preventing the empty state. Shell commands skip this step.

Acceptance Criteria

• Running !<command> as the first message in a new session should not crash the app
• Shell command results should display immediately after execution (or show a loading state)
• No regression in regular shell command functionality

Implementation Details

Affected Files

File	Lines	Issue
packages/desktop/src/components/prompt-input.tsx	781-788	Shell command sent without await, no optimistic message
packages/ui/src/components/session-turn.tsx	427-428	Accesses assistantParts()[0] without null check
packages/ui/src/components/message-part.tsx	313-314	Accesses props.part.type without guard

Suggested Fix Options

Option 1: Add null guard in Part component (quick fix)

// packages/ui/src/components/message-part.tsx:313
export function Part(props: MessagePartProps) {
  if (!props.part) return null  // Add guard
  const component = createMemo(() => PART_MAPPING[props.part.type])
  // ...
}

Option 2: Add conditional rendering in SessionTurn (defensive)

// packages/ui/src/components/session-turn.tsx:427
<Match when={isShellMode() && assistantParts()[0]}>
  <Part part={assistantParts()[0]} message={msg()} defaultOpen />
</Match>

Option 3: Add optimistic message for shell commands (comprehensive, better UX)

// packages/desktop/src/components/prompt-input.tsx:781
if (isShellMode) {
  const messageID = Identifier.ascending("message")
  sync.session.addOptimisticMessage({
    sessionID: existing.id,
    messageID,
    parts: [{ type: "shell", command: text, sessionID: existing.id, messageID }],
    agent,
    model,
  })
  sdk.client.session.shell({...})
  return
}

Recommended Approach

Combine Option 1 or 2 (prevents crash) with Option 3 (improves UX with immediate feedback).

Tasks

• Add null check guard in Part component or SessionTurn shell mode
• Add optimistic message for shell commands (similar to regular prompts)
• Test shell commands as first message in new session
• Verify no regression in existing shell command functionality