diff --git a/src/app/api/agent/cards/[cardId]/comments/route.ts b/src/app/api/agent/cards/[cardId]/comments/route.ts
new file mode 100644
index 0000000..3b755b8
--- /dev/null
+++ b/src/app/api/agent/cards/[cardId]/comments/route.ts
@@ -0,0 +1,55 @@
+import { getAdminPb } from "@/lib/pb/admin";
+import { requireAgentToken } from "@/lib/agent/auth";
+import { getOwnerUser } from "@/lib/agent/owner";
+import { jsonError, jsonOk } from "@/lib/http/json";
+import type { CardRecord } from "@/lib/pb/types";
+
+/** Matches the comments collection's own `body` field limit. */
+const MAX_BODY_LENGTH = 20000;
+
+/**
+ * POST /api/agent/cards/:cardId/comments — post a drafted plan as a comment.
+ *
+ * The stored body is always prefixed before being written, so no caller can
+ * accidentally or deliberately post something indistinguishable from the
+ * owner's own words. This prefix is the one thing this whole feature exists
+ * to guarantee — do not make it conditional or configurable.
+ */
+export async function POST(
+  request: Request,
+  context: { params: Promise<{ cardId: string }> }
+): Promise<Response> {
+  const denied = requireAgentToken(request);
+  if (denied) return denied;
+
+  const { cardId } = await context.params;
+
+  let json: unknown;
+  try {
+    json = await request.json();
+  } catch {
+    return jsonError("Invalid request body", 400);
+  }
+
+  const body = (json as { body?: unknown } | null)?.body;
+  if (typeof body !== "string" || !body.trim()) {
+    return jsonError("A non-empty body is required", 400);
+  }
+
+  const pb = await getAdminPb();
+  const card = await pb.collection("cards").getOne<CardRecord>(cardId).catch(() => null);
+  if (!card || card.archived) return jsonError("Card not found", 404);
+
+  const owner = await getOwnerUser();
+
+  const prefix = "🤖 Drafted by your agent:\n\n";
+  const trimmed = body.trim().slice(0, MAX_BODY_LENGTH - prefix.length);
+
+  const comment = await pb.collection("comments").create({
+    card: cardId,
+    author: owner.id,
+    body: `${prefix}${trimmed}`,
+  });
+
+  return jsonOk({ id: comment.id }, 201);
+}