fix: filter empty messages from conversation history to prevent validation errors

The AI SDK can produce assistant messages with empty parts (parts:[]) when
a stream is aborted, and providers reject assistant messages with empty text
content. This adds a validation utility that filters both cases before
sending messages to createAgentUIStreamResponse and when persisting them.

packages/browseros-agent/apps/server/src/agent/message-validation.ts

@@ -0,0 +1,46 @@
+/**
+ * @license
+ * Copyright 2025 BrowserOS
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { UIMessage } from 'ai'
+
+/**
+ * Checks whether a UIMessage has meaningful content that can be sent
+ * to the AI provider without causing validation errors.
+ *
+ * Two layers of validation can reject messages:
+ *
+ * 1. **AI SDK** (`validate-ui-messages.ts`):
+ *    - `parts` array must be `.nonempty()` — rejects `parts: []`
+ *
+ * 2. **Provider API** (e.g. Gemini `generateContent`, Anthropic, OpenAI):
+ *    - Assistant messages with only empty-string text are rejected
+ *      as semantically empty, even though the SDK schema allows it
+ *
+ * This function guards against both layers so callers can filter
+ * messages before passing them to `createAgentUIStreamResponse`.
+ */
+export function hasMessageContent(message: UIMessage): boolean {
+  if (message.parts.length === 0) return false
+
+  // A message that contains any non-text part (tool invocation, reasoning,
+  // file, step-start, etc.) is always considered valid — those part types
+  // carry meaning regardless of text content.
+  const hasNonTextPart = message.parts.some((p) => p.type !== 'text')
+  if (hasNonTextPart) return true
+
+  // All parts are text — at least one must have non-whitespace content.
+  return message.parts.some(
+    (p) => p.type === 'text' && p.text.trim().length > 0,
+  )
+}
+
+/**
+ * Filters a UIMessage array, removing messages that would fail
+ * SDK validation or provider-level content checks.
+ */
+export function filterValidMessages(messages: UIMessage[]): UIMessage[] {
+  return messages.filter(hasMessageContent)
+}

packages/browseros-agent/apps/server/src/api/services/chat-service.ts

@@ -8,6 +8,7 @@ import { mkdir, utimes } from 'node:fs/promises'
 import path from 'node:path'
 import { createAgentUIStreamResponse, type UIMessage } from 'ai'
 import { AiSdkAgent } from '../../agent/ai-sdk-agent'
+import { filterValidMessages } from '../../agent/message-validation'
 import { formatUserMessage } from '../../agent/format-message'
 import type { SessionStore } from '../../agent/session-store'
 import type { ResolvedAgentConfig } from '../../agent/types'
@@ -139,6 +140,7 @@ export class ChatService {
 
     if (isNewSession && request.previousConversation?.length) {
       for (const msg of request.previousConversation) {
+        if (!msg.content.trim()) continue
         session.agent.messages.push({
           id: crypto.randomUUID(),
           role: msg.role === 'assistant' ? 'assistant' : 'user',
@@ -168,10 +170,10 @@ export class ChatService {
 
     return createAgentUIStreamResponse({
       agent: session.agent.toolLoopAgent,
-      uiMessages: session.agent.messages,
+      uiMessages: filterValidMessages(session.agent.messages),
       abortSignal,
       onFinish: async ({ messages }: { messages: UIMessage[] }) => {
-        session.agent.messages = messages
+        session.agent.messages = filterValidMessages(messages)
         logger.info('Agent execution complete', {
           conversationId: request.conversationId,
           totalMessages: messages.length,