Issue with regenerate() — transport drops trigger, server doesn't delete stale messages

[alexander-zuev]

Bug

regenerate() from useAgentChat fails with Anthropic models. The AI SDK correctly truncates messages client-side (removes last assistant message), but two issues in @cloudflare/ai-chat prevent it from working:

1. ws-chat-transport.ts drops the trigger field — AI SDK passes trigger: 'regenerate-message' to the transport, but the transport never includes it in the WebSocket body
2. persistMessages only UPSERTs, never DELETEs — when the client sends a truncated array, the old assistant message row stays in SQLite

Result: this.messages on the server still ends with the stale assistant message → Anthropic rejects with 400:

AI_APICallError: This model does not support assistant message prefill.
The conversation must end with a user message.

statusCode: 400
requestBodyValues.messages: [{ role: 'user', ... }, { role: 'assistant', ... }]
                                                      ^^^^^^^^^^^^^^^^^^^^^^^^
                                                      stale — should have been deleted

Steps to reproduce

Repro repo: https://github.com/alexander-zuev/ai-chat-regenerate-bug-repro

pnpm install
npx wrangler dev

1. Send a message, wait for assistant response
2. Click "Regenerate Last"
3. Fails with the error above

The repro includes a side-by-side comparison: stock SDK (broken) vs a patched persistMessages that deletes stale rows (works).

Root cause

Transport (ws-chat-transport.ts):

// trigger is on options but never included in the body
const bodyPayload = JSON.stringify({
  messages: options.messages,
  ...extraBody,  // trigger is NOT here
});

Server (persistMessages in index.ts):

// UPSERT only — old rows never deleted
this.sql`
  insert into cf_ai_chat_agent_messages (id, message)
  values (${safe.id}, ${json})
  on conflict(id) do update set message = excluded.message
`;
// Then reloads ALL rows from SQLite, including stale ones
const persisted = this._loadMessagesFromDb();
this.messages = autoTransformMessages(persisted);

Suggested fix

1. ws-chat-transport.ts — include trigger in the body:

const bodyPayload = JSON.stringify({
  messages: options.messages,
  trigger: options.trigger,
  ...extraBody,
});

2. Server handler — when trigger === 'regenerate-message', delete DB rows not in the incoming array:

const incomingIds = new Set(mergedMessages.map(m => m.id));
const allDbIds = this.sql`SELECT id FROM cf_ai_chat_agent_messages`.toArray();
for (const row of allDbIds) {
  if (!incomingIds.has(row.id)) {
    this.sql`DELETE FROM cf_ai_chat_agent_messages WHERE id = ${row.id}`;
  }
}

Versions

• @cloudflare/ai-chat: 0.1.5
• agents: 0.6.0
• ai: 6.0.104

[alexander-zuev]

Note: Fix mentioned is just a very rough patch to verify the root cause

Raised this with Claude, but verified in source it doesn't hallucinate -> seems like the regenerate trigger not sent from the client hook + server doesn't handle it

Before / After calling regenerate():

[threepointone]

Thanks for digging in here, will fix