fix: merge consecutive tool results for Anthropic API compatibility

When the Anthropic API returns multiple parallel tool calls in a single
assistant message, all tool_result blocks must be sent together in the
next message. Previously, each tool result was sent as a separate message,
causing Anthropic's API to reject the request with:
"tool_use ids were found without tool_result blocks immediately after"

This change modifies convertFromMessages() to merge consecutive tool
messages into a single message containing all tool_result content blocks.

Fixes #375

apps/x/packages/core/src/agents/runtime.ts

@@ -354,7 +354,8 @@ export async function loadAgent(id: string): Promise<z.infer<typeof Agent>> {
 
 export function convertFromMessages(messages: z.infer<typeof Message>[]): ModelMessage[] {
     const result: ModelMessage[] = [];
-    for (const msg of messages) {
+    for (let i = 0; i < messages.length; i++) {
+        const msg = messages[i];
         const { providerOptions } = msg;
         switch (msg.role) {
             case "assistant":
@@ -401,11 +402,19 @@ export function convertFromMessages(messages: z.infer<typeof Message>[]): ModelM
                     providerOptions,
                 });
                 break;
-            case "tool":
-                result.push({
-                    role: "tool",
-                    content: [
-                        {
+            case "tool": {
+                // Collect all consecutive tool messages into a single message
+                // This is required by Anthropic's API which expects all tool_result blocks
+                // for parallel tool calls to be in a single message
+                const toolResults: Array<{
+                    type: "tool-result";
+                    toolCallId: string;
+                    toolName: string;
+                    output: { type: "text"; value: string };
+                }> = [];
+
+                // Add current tool message
+                toolResults.push({
                     type: "tool-result",
                     toolCallId: msg.toolCallId,
                     toolName: msg.toolName,
@@ -413,13 +422,32 @@ export function convertFromMessages(messages: z.infer<typeof Message>[]): ModelM
                         type: "text",
                         value: msg.content,
                     },
+                });
+
+                // Collect any consecutive tool messages
+                while (i + 1 < messages.length && messages[i + 1].role === "tool") {
+                    i++;
+                    const nextMsg = messages[i] as z.infer<typeof ToolMessage>;
+                    toolResults.push({
+                        type: "tool-result",
+                        toolCallId: nextMsg.toolCallId,
+                        toolName: nextMsg.toolName,
+                        output: {
+                            type: "text",
+                            value: nextMsg.content,
                         },
-                    ],
+                    });
+                }
+
+                result.push({
+                    role: "tool",
+                    content: toolResults,
                     providerOptions,
                 });
                 break;
             }
         }
+    }
     // doing this because: https://github.com/OpenRouterTeam/ai-sdk-provider/issues/262
     return JSON.parse(JSON.stringify(result));
 }