fix(ai): filter empty error assistant messages in transformMessages

When 429/500 errors occur during tool execution, empty assistant messages
with stopReason='error' get persisted. These break the tool_use -> tool_result
chain for Claude/Gemini APIs.

Added centralized filtering in transformMessages to skip assistant messages
with empty content and no tool calls. Provider-level filters remain for
defense-in-depth.

packages/ai/CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Fixed OpenAI-compatible provider feature detection to use `model.provider` in addition to URL, allowing custom base URLs (e.g., proxies) to work correctly with provider-specific settings ([#774](https://github.com/badlogic/pi-mono/issues/774))
 - Fixed Bedrock tool call IDs to use only alphanumeric characters, avoiding API errors from invalid characters ([#781](https://github.com/badlogic/pi-mono/pull/781) by [@pjtf93](https://github.com/pjtf93))
+- Fixed empty error assistant messages (from 429/500 errors) breaking the tool_use to tool_result chain by filtering them in `transformMessages`
 
 ## [0.47.0] - 2026-01-16
 

packages/ai/src/providers/transform-messages.ts

@@ -118,6 +118,14 @@ export function transformMessages<TApi extends Api>(messages: Message[], model:
 				existingToolResultIds = new Set();
 			}
 
+			// Skip empty assistant messages (no content and no tool calls)
+			// This handles error responses (e.g., 429/500) that produced no content
+			// All providers already filter these in convertMessages, but we do it here
+			// centrally to prevent issues with the tool_use -> tool_result chain
+			if (assistantMsg.content.length === 0 && toolCalls.length === 0) {
+				continue;
+			}
+
 			result.push(msg);
 		} else if (msg.role === "toolResult") {
 			existingToolResultIds.add(msg.toolCallId);