diff --git a/apps/web/app/api/assets/[id]/url/route.ts b/apps/web/app/api/assets/[id]/url/route.ts
index d58384f..bb401b5 100644
--- a/apps/web/app/api/assets/[id]/url/route.ts
+++ b/apps/web/app/api/assets/[id]/url/route.ts
@@ -2,7 +2,7 @@ import { z } from "zod";
 
 import { getDb } from "@tline/db";
 import { presignGetObjectUrl } from "@tline/minio";
-import { pickVariantKey } from "./variant";
+import { pickLegacyKeyForRequest, pickVariantKey } from "./variant";
 
 export const runtime = "nodejs";
 
@@ -13,6 +13,7 @@ const paramsSchema = z.object({
 const legacyVariantSchema = z.enum(["original", "thumb_small", "thumb_med", "poster"]);
 const kindSchema = z.enum(["original", "thumb", "poster", "video_mp4"]);
 const sizeSchema = z.coerce.number().int().positive();
+const videoMp4DefaultSize = 720;
 const legacyVariantMap = {
   original: { kind: "original" as const },
   thumb_small: { kind: "thumb" as const, size: 256 },
@@ -53,14 +54,18 @@ export async function GET(
     }
     requestedKind = kindParsed.data;
     if (requestedKind !== "original") {
-      const sizeParsed = sizeSchema.safeParse(sizeParam);
-      if (!sizeParsed.success) {
-        return Response.json(
-          { error: "invalid_query", issues: sizeParsed.error.issues },
-          { status: 400 },
-        );
+      if (requestedKind === "video_mp4" && !sizeParam) {
+        requestedSize = videoMp4DefaultSize;
+      } else {
+        const sizeParsed = sizeSchema.safeParse(sizeParam);
+        if (!sizeParsed.success) {
+          return Response.json(
+            { error: "invalid_query", issues: sizeParsed.error.issues },
+            { status: 400 },
+          );
+        }
+        requestedSize = sizeParsed.data;
       }
-      requestedSize = sizeParsed.data;
     }
   } else if (legacyVariantParam) {
     const legacyParsed = legacyVariantSchema.safeParse(legacyVariantParam);
@@ -113,14 +118,17 @@ export async function GET(
     return Response.json({ error: "not_found" }, { status: 404 });
   }
 
-  const legacyKey =
-    legacyVariant === "thumb_small"
-      ? asset.thumb_small_key
-      : legacyVariant === "thumb_med"
-        ? asset.thumb_med_key
-        : legacyVariant === "poster"
-          ? asset.poster_key
-          : null;
+  const legacyKey = legacyVariant
+    ? pickLegacyKeyForRequest(
+        { asset },
+        { kind: requestedKind, size: requestedSize ?? 0 },
+      )
+    : requestedSize !== null
+      ? pickLegacyKeyForRequest(
+          { asset },
+          { kind: requestedKind, size: requestedSize },
+        )
+      : null;
 
   const key =
     requestedKind === "original"
diff --git a/apps/web/app/api/assets/[id]/url/variant.ts b/apps/web/app/api/assets/[id]/url/variant.ts
index 6e01082..645357e 100644
--- a/apps/web/app/api/assets/[id]/url/variant.ts
+++ b/apps/web/app/api/assets/[id]/url/variant.ts
@@ -7,3 +7,25 @@ export function pickVariantKey(
   );
   return v?.key ?? null;
 }
+
+export function pickLegacyKeyForRequest(
+  input: {
+    asset: {
+      thumb_small_key: string | null;
+      thumb_med_key: string | null;
+      poster_key: string | null;
+    };
+  },
+  req: { kind: string; size: number },
+) {
+  if (req.kind === "thumb" && req.size === 256) {
+    return input.asset.thumb_small_key ?? null;
+  }
+  if (req.kind === "thumb" && req.size === 768) {
+    return input.asset.thumb_med_key ?? null;
+  }
+  if (req.kind === "poster" && req.size === 256) {
+    return input.asset.poster_key ?? null;
+  }
+  return null;
+}
diff --git a/apps/web/src/__tests__/variant-url-404.test.ts b/apps/web/src/__tests__/variant-url-404.test.ts
index 369fa08..17b66d9 100644
--- a/apps/web/src/__tests__/variant-url-404.test.ts
+++ b/apps/web/src/__tests__/variant-url-404.test.ts
@@ -1,9 +1,33 @@
 import { test, expect } from "bun:test";
 
-test("/api/assets/:id/url returns 404 when requested variant missing", async () => {
+test("variant lookup returns null when no matching variant", async () => {
   const { pickVariantKey } = await import(
     "../../app/api/assets/[id]/url/variant",
   );
   const key = pickVariantKey({ variants: [] }, { kind: "thumb", size: 256 });
   expect(key).toBeNull();
 });
+
+test("legacy fallback maps kind+size to asset keys", async () => {
+  const { pickLegacyKeyForRequest } = await import(
+    "../../app/api/assets/[id]/url/variant",
+  );
+  const asset = {
+    thumb_small_key: "thumb-small",
+    thumb_med_key: "thumb-med",
+    poster_key: "poster",
+  };
+
+  expect(
+    pickLegacyKeyForRequest({ asset }, { kind: "thumb", size: 256 }),
+  ).toBe("thumb-small");
+  expect(
+    pickLegacyKeyForRequest({ asset }, { kind: "thumb", size: 768 }),
+  ).toBe("thumb-med");
+  expect(
+    pickLegacyKeyForRequest({ asset }, { kind: "poster", size: 256 }),
+  ).toBe("poster");
+  expect(
+    pickLegacyKeyForRequest({ asset }, { kind: "thumb", size: 1024 }),
+  ).toBeNull();
+});