fix(ai): seed system+history via initialPrompts; let session manage state

[dobrinyonkov]

Problem

Two related bugs trigger:

Failed to execute 'promptStreaming' on 'LanguageModel': The 'system' role message must be the first message of a session.

1. After clicking Reset history, sending a new prompt errors out.
2. On a normal multi-turn chat, the 3rd or later prompt errors out.

Root cause

The Chrome Prompt API session retains conversation history internally across promptStreaming calls. The previous code sent a fresh [system, ...history, user] messages array on every call. From the session's perspective, the system message wasn't first on subsequent turns — so the API rejected it.

Reference: https://developer.chrome.com/docs/ai/prompt-api — system role goes through initialPrompts at session creation; later prompt()/promptStreaming() calls take only the new user input.

Fix

Use the API the way it's meant to be used:

• createSession accepts initialPrompts — seeded with system prompt + any prior user/assistant turns.
• promptStreaming sends only the new user message string; session keeps history internally.
• _loadHistory re-seeds the session after loading stored messages so the model has context on chat reopen.

Test

• Send several prompts in a row → no error, context preserved.
• Click Reset history, send a prompt → no error, fresh session.
• Reopen the panel after history was stored → model remembers prior turns.
• 383 unit tests pass.

[cla-assistant]

All committers have signed the CLA.

[plamenivanov91]

I've inspected and tested the code. It fixes the issue but the AI fond some issues.

The core approach (seeding history via initialPrompts and letting the session manage state) is sound. Two confirmed bugs need fixing before merge, plus a few lower-priority observations.

🚨 Mandatory

1. _loadHistory never clears this._messages before appending

AIChat.js ~L1068

_addMessage always pushes to this._messages, and _loadHistory doesn't reset the array first. onTabActivated calls _loadHistory on every tab switch, so switching to the AI tab twice doubles every message in the array. _initializeSession then replays the bloated array as initialPrompts, feeding the model a duplicated conversation.

// Before the _addMessage loop in _loadHistory, add:
this._messages = [];
2. No session re-seed when navigating to a URL with no stored history

AIChat.js ~L1072

The destroy + _initializeSession block is inside if (messages.length > 0). Navigating from App A (has history) to App B (no history) skips the block entirely — the session retains App A's system prompt with App A's framework version, theme, and libraries.

// Move the re-seed block outside the messages.length guard:
if (this._sessionManager.hasActiveSession()) {
this._sessionManager.destroy();
await this._initializeSession();
}
if (messages.length > 0) {
// render messages...
}
💡 Nice to Have
3. No cap on initialPrompts replay

AIChat.js ~L229

All messages are replayed unconditionally. On a long conversation, LanguageModel.create() could be called with an initialPrompts array that hits or exceeds the session's inputQuota, leaving zero tokens for new input. Consider capping replay to the most recent N turns or checking tokensSoFar before pushing.

4. handleCreateSession destroys the session before the new one is ready

main.js ~L311

promptAPISession is set to null before initPromptAPISession is called. If initPromptAPISession throws, promptAPISession stays null permanently — every subsequent prompt returns "No active session" until the user manually retries. Consider creating the new session first and only destroying the old one on success.

5. Empty assistant placeholder replayed in initialPrompts mid-stream

AIChat.js ~L322

An { role: 'assistant', content: '' } placeholder is pushed to this._messages before streaming completes. onTabActivated has no _isStreaming guard, so switching tabs mid-stream triggers _loadHistory → _initializeSession, which replays the empty assistant turn into initialPrompts. The Chrome Prompt API may reject or behave oddly with blank-content turns. A simple fix: skip entries with empty content in the initialPrompts filter.

6. Race condition in _loadHistory re-seed

AIChat.js ~L1086

Between destroy() and the await _initializeSession() resolving, hasActiveSession() is false. A concurrent message send hits the _handleSendMessage lazy-init path and fires a second _initializeSession() in parallel. Both race to the background, which destroys the first session to create the second. Low probability in practice but worth a mutex/guard flag.