diff --git a/src/codex/prompt-layers.ts b/src/codex/prompt-layers.ts
index 1e00afaa89..57ec8bde76 100644
--- a/src/codex/prompt-layers.ts
+++ b/src/codex/prompt-layers.ts
@@ -1028,7 +1028,10 @@ export function writeBaseVariant(
     ? input.id
     : input.id ?? newBaseVariantId(existing);
   if (!BASE_VARIANT_ID.test(targetId)) return { ok: false, error: "unknown_layer", detail: targetId };
-  if (deleting && !existing.some(v => v.id === targetId)) {
+  // A caller-supplied id is an edit (or delete), never an alternate create path.
+  // Requiring it to exist keeps the generated-id path as the sole place where a
+  // new variant can enter, and therefore makes the cap impossible to bypass.
+  if (input.id !== null && !existing.some(v => v.id === targetId)) {
     return { ok: false, error: "unknown_layer", detail: targetId };
   }
   if (!deleting && input.id === null && existing.length >= MAX_BASE_VARIANTS) {
diff --git a/src/server/management/codex-prompt-routes.ts b/src/server/management/codex-prompt-routes.ts
index 82a4b42540..0280310b34 100644
--- a/src/server/management/codex-prompt-routes.ts
+++ b/src/server/management/codex-prompt-routes.ts
@@ -397,7 +397,11 @@ export async function handleCodexPromptRoutes(ctx: ManagementContext): Promise<R
     }
     if (typeof body.title !== "string") return fail(ctx, "invalid_body", 400, "title must be a string");
     if (typeof body.body !== "string") return fail(ctx, "invalid_body", 400, "body must be a string");
-    return settle(ctx, writeBaseVariant({ id, title: body.title, body: body.body }, revision, paths(ctx)));
+    const normalized = normalizeBody(body.body);
+    if (utf8Bytes(normalized) > MAX_BODY_BYTES) {
+      return fail(ctx, "body_too_large", 400, `base variant exceeds ${MAX_BODY_BYTES} bytes`);
+    }
+    return settle(ctx, writeBaseVariant({ id, title: body.title, body: normalized }, revision, paths(ctx)));
   }
 
   if (url.pathname === "/api/codex-prompt/adopt" && req.method === "POST") {
diff --git a/tests/codex-prompt-base-variants.test.ts b/tests/codex-prompt-base-variants.test.ts
index 4371675a70..653aad519e 100644
--- a/tests/codex-prompt-base-variants.test.ts
+++ b/tests/codex-prompt-base-variants.test.ts
@@ -177,6 +177,15 @@ describe("base variant selection", () => {
     expect(writeBaseVariant({ id, title: "edited", body: "b2" }, rev(paths), paths).ok).toBe(true);
   });
 
+  test("a caller-supplied id cannot create a variant", () => {
+    const paths = fixture("model = \"x\"\n");
+    expect(writeBaseVariant({ id: "aaaaaa", title: "Injected", body: "b" }, rev(paths), paths)).toMatchObject({
+      ok: false,
+      error: "unknown_layer",
+    });
+    expect(readBaseVariants(paths)).toEqual([]);
+  });
+
   test("a stale revision is refused", () => {
     const paths = fixture("model = \"x\"\n");
     const stale = rev(paths);
diff --git a/tests/codex-prompt-route.test.ts b/tests/codex-prompt-route.test.ts
index 31f0dfd421..52d9f7c8c8 100644
--- a/tests/codex-prompt-route.test.ts
+++ b/tests/codex-prompt-route.test.ts
@@ -637,6 +637,20 @@ describe("020 coverage completions", () => {
     expect(editDefault.status).toBe(400);
     expect(editDefault.body.code).toBe("unknown_layer");
 
+    // A string id is edit-only. It cannot bypass the creation cap by naming a
+    // syntactically valid file that the server did not generate.
+    const editMissing = await call("PUT", "/api/codex-prompt/base", fx, {
+      id: "aaaaaa", title: "Missing", body: "b", revision: rev0,
+    });
+    expect(editMissing.status).toBe(400);
+    expect(editMissing.body.code).toBe("unknown_layer");
+
+    const oversized = await call("PUT", "/api/codex-prompt/base", fx, {
+      id: null, title: "Too large", body: "x".repeat(64 * 1024 + 1), revision: rev0,
+    });
+    expect(oversized.status).toBe(400);
+    expect(oversized.body.code).toBe("body_too_large");
+
     // An unknown variant would leave the key naming a file Codex cannot read.
     const unknown = await call("PUT", "/api/codex-prompt/base/select", fx, {
       kind: "variant", id: "zzzzzz", revision: rev0,