diff --git a/src/adapter/openai-to-cli.ts b/src/adapter/openai-to-cli.ts
index c8ecaa1..89a80da 100644
--- a/src/adapter/openai-to-cli.ts
+++ b/src/adapter/openai-to-cli.ts
@@ -46,6 +46,27 @@ export function extractModel(model: string): ClaudeModel {
   return "opus";
 }
 
+/**
+ * Extract text content from OpenAI message content.
+ *
+ * OpenAI API supports both string content and array-style content blocks
+ * (e.g. [{"type": "text", "text": "..."}]). When content is an array,
+ * naive string interpolation produces "[object Object]". This helper
+ * extracts the actual text from both formats.
+ */
+function extractContent(content: string | Array<{type: string; text?: string}>): string {
+  if (typeof content === "string") {
+    return content;
+  }
+  if (Array.isArray(content)) {
+    return content
+      .filter((block) => block.type === "text")
+      .map((block) => block.text ?? "")
+      .join("\n");
+  }
+  return String(content ?? "");
+}
+
 /**
  * Convert OpenAI messages array to a single prompt string for Claude CLI
  *
@@ -59,17 +80,17 @@ export function messagesToPrompt(messages: OpenAIChatRequest["messages"]): strin
     switch (msg.role) {
       case "system":
         // System messages become context instructions
-        parts.push(`<system>\n${msg.content}\n</system>\n`);
+        parts.push(`<system>\n${extractContent(msg.content)}\n</system>\n`);
         break;
 
       case "user":
         // User messages are the main prompt
-        parts.push(msg.content);
+        parts.push(extractContent(msg.content));
         break;
 
       case "assistant":
         // Previous assistant responses for context
-        parts.push(`<previous_response>\n${msg.content}\n</previous_response>\n`);
+        parts.push(`<previous_response>\n${extractContent(msg.content)}\n</previous_response>\n`);
         break;
     }
   }