diff --git a/open-sse/executors/chatgpt-session/messages.ts b/open-sse/executors/chatgpt-session/messages.ts
index efbe49fc2d..6a44648c7d 100644
--- a/open-sse/executors/chatgpt-session/messages.ts
+++ b/open-sse/executors/chatgpt-session/messages.ts
@@ -34,18 +34,27 @@ function textFromContent(content: unknown): string {
   if (!Array.isArray(content)) return "";
   const chunks: string[] = [];
   for (const part of content) {
-    if (!part || typeof part !== "object") continue;
-    const typed = part as Record<string, unknown>;
-    if (typed.type === "text" && typeof typed.text === "string") {
+    const typed =
+      part && typeof part === "object" && !Array.isArray(part)
+        ? (part as Record<string, unknown>)
+        : null;
+    if (typed && typed.type === "text" && typeof typed.text === "string") {
       chunks.push(typed.text);
       continue;
     }
-    if (typed.type === "image_url" || typed.type === "image") {
+    if (typed && (typed.type === "image_url" || typed.type === "image")) {
       throw new ChatGptSessionInputError(
         "vision_unsupported",
         "ChatGPT Session does not accept image input yet"
       );
     }
+    // Every other part — file, input_audio, a future part type, or anything malformed — would
+    // otherwise be dropped in silence, and the model would answer about content it never
+    // received. Reject loudly instead.
+    throw new ChatGptSessionInputError(
+      "unsupported_content_part",
+      "ChatGPT Session does not accept this message content part type"
+    );
   }
   return chunks.join("\n");
 }
diff --git a/tests/unit/chatgpt-session-messages.test.ts b/tests/unit/chatgpt-session-messages.test.ts
index be0e91de58..d6b6de9f22 100644
--- a/tests/unit/chatgpt-session-messages.test.ts
+++ b/tests/unit/chatgpt-session-messages.test.ts
@@ -116,3 +116,21 @@ test("never carries tools into the parsed context", () => {
   });
   assert.equal(parsed.context.tools, undefined);
 });
+
+test("rejects any content part that is neither text nor an image", () => {
+  for (const part of [
+    { type: "file", file: { file_id: "f-1" } },
+    { type: "input_audio", input_audio: { data: "AA", format: "wav" } },
+  ]) {
+    assert.throws(
+      () =>
+        buildParsedRequest({
+          route,
+          messages: [{ role: "user", content: [part] }],
+          stream: false,
+        }),
+      (error: unknown) =>
+        error instanceof ChatGptSessionInputError && error.code === "unsupported_content_part"
+    );
+  }
+});