diff --git a/packages/ai/src/providers/ollama.ts b/packages/ai/src/providers/ollama.ts
index fb9f72092..4234a62f4 100644
--- a/packages/ai/src/providers/ollama.ts
+++ b/packages/ai/src/providers/ollama.ts
@@ -199,7 +199,18 @@ function convertMessages(model: Model<"ollama-chat">, context: Context): OllamaM
 		});
 	}
 	messages.push(...context.messages);
-	return transformMessages(messages, model).map(convertMessage);
+	const isCloud = model.provider === "ollama-cloud";
+	return transformMessages(messages, model).map(msg => {
+		const converted = convertMessage(msg);
+		// Ollama cloud rejects requests when assistant history messages contain the `thinking`
+		// field — it's valid in model responses but not accepted as a history input. Strip it
+		// to prevent HTTP 400 errors. Local Ollama instances are unaffected.
+		if (isCloud && converted.role === "assistant" && converted.thinking) {
+			const { thinking: _t, ...rest } = converted;
+			return rest;
+		}
+		return converted;
+	});
 }
 
 function convertTools(tools: Tool[] | undefined): OllamaFunctionTool[] | undefined {