diff --git a/src/agents/ollama-stream.ts b/src/agents/ollama-stream.ts
index f332ad1fd83..ccec6ea7d13 100644
--- a/src/agents/ollama-stream.ts
+++ b/src/agents/ollama-stream.ts
@@ -254,9 +254,21 @@ function extractToolCalls(content: unknown): OllamaToolCall[] {
   const result: OllamaToolCall[] = [];
   for (const part of parts) {
     if (part.type === "toolCall") {
-      result.push({ function: { name: part.name, arguments: part.arguments } });
+      // Ensure arguments are always an object, not a string (OpenAI format passes JSON string)
+      const args = part.arguments;
+      let parsedArgs: Record<string, unknown>;
+      if (typeof args === "string") {
+        try {
+          parsedArgs = JSON.parse(args);
+        } catch {
+          parsedArgs = {};
+        }
+      } else {
+        parsedArgs = args ?? {};
+      }
+      result.push({ function: { name: part.name, arguments: parsedArgs } });
     } else if (part.type === "tool_use") {
-      result.push({ function: { name: part.name, arguments: part.input } });
+      result.push({ function: { name: part.name, arguments: part.input ?? {} } });
     }
   }
   return result;
@@ -351,11 +363,17 @@ export function buildAssistantMessage(
   const toolCalls = response.message.tool_calls;
   if (toolCalls && toolCalls.length > 0) {
     for (const tc of toolCalls) {
+      // Normalize arguments: ensure it's always an object, not a string
+      const rawArgs = tc.function.arguments;
+      const normalizedArgs =
+        typeof rawArgs === "string"
+          ? (JSON.parse(rawArgs) as Record<string, unknown>)
+          : (rawArgs ?? {});
       content.push({
         type: "toolCall",
         id: `ollama_call_${randomUUID()}`,
         name: tc.function.name,
-        arguments: tc.function.arguments,
+        arguments: normalizedArgs,
       });
     }
   }