diff --git a/apps/web/src/myra-deploy.test.ts b/apps/web/src/myra-deploy.test.ts
index 60e9f7096a..8a3aca6d4c 100644
--- a/apps/web/src/myra-deploy.test.ts
+++ b/apps/web/src/myra-deploy.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { buildMyraDeployInput, MyraDeployError } from "./myra-deploy";
+import { buildMyraDeployInput, MyraDeployError, packTarball } from "./myra-deploy";
 import { MYRA_SOURCE_CONFIG } from "./myra-source";
 
 describe("buildMyraDeployInput", () => {
@@ -44,3 +44,20 @@ describe("buildMyraDeployInput", () => {
     ).toThrow(MyraDeployError);
   });
 });
+
+describe("packTarball", () => {
+  test("writes USTAR headers whose checksum verifies and whose typeflag survives", async () => {
+    const tarball = await packTarball({ "package.json": "{}" });
+    const tar = new Uint8Array(
+      await new Response(
+        new Blob([tarball as Uint8Array<ArrayBuffer>]).stream().pipeThrough(new DecompressionStream("gzip")),
+      ).arrayBuffer(),
+    );
+    const header = tar.slice(0, 512);
+    let expected = 0;
+    for (let i = 0; i < 512; i++) expected += i >= 148 && i < 156 ? 0x20 : header[i]!;
+    const stored = parseInt(new TextDecoder().decode(header.slice(148, 154)), 8);
+    expect(stored).toBe(expected);
+    expect(String.fromCharCode(header[156]!)).toBe("0");
+  });
+});
diff --git a/apps/web/src/myra-deploy.ts b/apps/web/src/myra-deploy.ts
index 144bb2e9e3..8c11d31dbb 100644
--- a/apps/web/src/myra-deploy.ts
+++ b/apps/web/src/myra-deploy.ts
@@ -125,7 +125,9 @@ function tarEntry(path: string, content: Uint8Array): Uint8Array {
 
   let checksum = 0;
   for (const byte of header) checksum += byte;
-  writeField(148, padOctal(checksum, 8).slice(0, 7) + "\0 ");
+  // The 8-byte checksum field is 6 octal digits, NUL, space; anything
+  // longer spills into the typeflag byte and fails every reader's check.
+  writeField(148, checksum.toString(8).padStart(6, "0") + "\0 ");
 
   const paddedLength = Math.ceil(content.length / TAR_BLOCK_SIZE) * TAR_BLOCK_SIZE;
   const body = new Uint8Array(paddedLength);