diff --git a/src/shared/binary.ts b/src/shared/binary.ts
new file mode 100644
index 0000000..18bb4c0
--- /dev/null
+++ b/src/shared/binary.ts
@@ -0,0 +1,74 @@
+// src/shared/binary.ts — Portable Blob↔base64 helpers for the
+// offscreen↔SW port wire-format (D-12 fix).
+//
+// Why this file exists:
+//   chrome.runtime.Port.postMessage JSON-serializes payloads across
+//   extension contexts. JSON.stringify(blob) === "{}" (Blob has no
+//   enumerable own properties), so any Blob passed through a port
+//   arrives as a plain empty object on the other side. The SW then
+//   calls `new Blob([{}, {}, ...])` and the constructor coerces each
+//   non-Blob member via String({}) === "[object Object]", producing
+//   exactly 75 bytes of garbage for 5 chunks — matching the observed
+//   ffprobe failure forensics in debug session d12.
+//
+// Fix: encode each chunk's binary payload to base64 in offscreen
+// BEFORE port.postMessage; decode back to Blob in SW AFTER receive.
+// base64 strings round-trip cleanly through JSON.
+//
+// The wire-format contract is pinned by tests/offscreen/port-serialization.test.ts
+// (GREEN describe block). This module's two helpers MUST satisfy:
+//   1. `await blobToBase64(blob)` returns just the base64 payload
+//      (no `data:...;base64,` prefix).
+//   2. `base64ToBlob(b64, mimeType)` returns a Blob whose bytes
+//      equal `Uint8Array(await blob.arrayBuffer())` of the original.
+//   3. EBML magic bytes 0x1A 0x45 0xDF 0xA3 round-trip intact through
+//      JSON.parse(JSON.stringify({dataBase64: await blobToBase64(...)})).
+//
+// Portability: both `btoa`/`atob` and `arrayBuffer()` are available in
+// the modern browser/extension runtime AND in the Node-based vitest
+// environment (Node 18+ has both as globals). No FileReader dependency
+// (FileReader is browser-only — would break Node tests).
+
+/**
+ * Encode a Blob's binary content to a base64 string suitable for
+ * JSON-friendly transport over chrome.runtime.Port.
+ *
+ * Returns the raw base64 payload only (no `data:<type>;base64,` prefix).
+ *
+ * @param blob - The binary blob to encode (typically a MediaRecorder
+ *               dataavailable chunk).
+ * @returns Base64-encoded representation of the blob's bytes.
+ */
+export async function blobToBase64(blob: Blob): Promise<string> {
+  const buf = await blob.arrayBuffer();
+  const bytes = new Uint8Array(buf);
+  // Per-byte concat is the portable way: avoids the
+  // `String.fromCharCode(...bytes)` apply-spread argument-length limit
+  // (typically ~64 KiB on some engines) for large chunks.
+  let binary = '';
+  for (let i = 0; i < bytes.length; i++) {
+    binary += String.fromCharCode(bytes[i]);
+  }
+  return btoa(binary);
+}
+
+/**
+ * Decode a base64 string back into a Blob with the given MIME type.
+ *
+ * Synchronous because atob + Uint8Array population requires no I/O.
+ *
+ * @param b64 - The base64-encoded payload (as produced by
+ *              {@link blobToBase64}). Must NOT include a
+ *              `data:<type>;base64,` prefix.
+ * @param mimeType - The MIME type to assign to the reconstructed Blob
+ *                   (e.g. `video/webm;codecs=vp9`).
+ * @returns A Blob whose bytes match the original encoded blob exactly.
+ */
+export function base64ToBlob(b64: string, mimeType: string): Blob {
+  const binary = atob(b64);
+  const bytes = new Uint8Array(binary.length);
+  for (let i = 0; i < binary.length; i++) {
+    bytes[i] = binary.charCodeAt(i);
+  }
+  return new Blob([bytes], { type: mimeType });
+}
diff --git a/tests/offscreen/port-serialization.test.ts b/tests/offscreen/port-serialization.test.ts
new file mode 100644
index 0000000..8c5bc66
--- /dev/null
+++ b/tests/offscreen/port-serialization.test.ts
@@ -0,0 +1,195 @@
+// tests/offscreen/port-serialization.test.ts
+//
+// RED-gate test for debug session d12-blob-port-transfer-fails.
+//
+// Empirically proves the failure mode behind the 75-byte "[object Object]"
+// WebM payload observed in /home/parf/Downloads/session_report_2026-05-15_19-42-01.zip.
+//
+// Hypothesis under test (per .planning/debug/d12-blob-port-transfer-fails.md):
+//   1. chrome.runtime.Port.postMessage serializes payloads as JSON across
+//      extension contexts (offscreen ↔ SW). JSON.stringify(blob) === "{}"
+//      because Blob has no enumerable own properties.
+//   2. The current REQUEST_BUFFER handler in src/offscreen/recorder.ts:174-178
+//      sends `{ type: 'BUFFER', chunks: getBuffer() }` where each chunk has
+//      `.data: Blob`. After the JSON round-trip the SW receives chunks with
+//      `.data === {}`.
+//   3. mergeVideoChunks in src/background/index.ts:204-222 then calls
+//      `new Blob([{}, {}, ...])`. The Blob constructor coerces each non-Blob
+//      member via String(member) → "[object Object]", concatenated with no
+//      separator → exactly 75 bytes for 5 chunks (5 × 15 = 75).
+//
+// Goal: make these tests RED first (proving the bug is real), then write the
+// fix (Blob → base64 string transfer-format) to flip them GREEN.
+
+import { describe, it, expect } from 'vitest';
+
+// Simulates Chrome's documented chrome.runtime.Port serialization across
+// extension contexts: JSON round-trip with no structured-clone fallback.
+// Ref: https://developer.chrome.com/docs/extensions/develop/concepts/messaging
+function chromeRuntimePortRoundTrip<T>(message: T): unknown {
+  return JSON.parse(JSON.stringify(message));
+}
+
+describe('port serialization (RED — confirms d12 bug)', () => {
+  it('JSON.stringify(blob) drops binary content (root cause part 1)', () => {
+    const blob = new Blob([new Uint8Array([0x1a, 0x45, 0xdf, 0xa3])], {
+      type: 'video/webm',
+    });
+    // EBML magic = 0x1A 0x45 0xDF 0xA3 — the bytes the real WebM should start with.
+    expect(blob.size).toBe(4);
+
+    const serialized = JSON.stringify(blob);
+    // The smoking gun: Blob has no enumerable own properties, so JSON drops it.
+    expect(serialized).toBe('{}');
+  });
+
+  it('Port round-trip strips Blob from chunks (reproduces the SW receive-side)', () => {
+    const blob = new Blob([new Uint8Array([0x1a, 0x45, 0xdf, 0xa3])], {
+      type: 'video/webm',
+    });
+    const portMessage = {
+      type: 'BUFFER',
+      chunks: [
+        { data: blob, timestamp: 1, isFirst: true },
+        { data: blob, timestamp: 2 },
+      ],
+    };
+
+    const received = chromeRuntimePortRoundTrip(portMessage) as {
+      type: string;
+      chunks: Array<{ data: unknown; timestamp: number; isFirst?: boolean }>;
+    };
+
+    expect(received.type).toBe('BUFFER');
+    expect(received.chunks).toHaveLength(2);
+    // The bug: data is no longer a Blob — it's a plain empty object.
+    expect(received.chunks[0].data).not.toBeInstanceOf(Blob);
+    expect(received.chunks[0].data).toEqual({});
+    expect(received.chunks[1].data).toEqual({});
+    // Metadata survives because timestamp/isFirst are JSON-friendly.
+    expect(received.chunks[0].timestamp).toBe(1);
+    expect(received.chunks[0].isFirst).toBe(true);
+  });
+
+  it('new Blob([{}, {}, ...]) coerces each member to "[object Object]" (root cause part 2)', () => {
+    // This is the smoking gun for the merge step. Once the SW gets {} for each
+    // chunk.data, mergeVideoChunks does `new Blob(blobs, { type: 'video/webm' })`
+    // and the Blob ctor stringifies non-Blob members.
+    const merged = new Blob([{}, {}, {}, {}, {}], { type: 'video/webm' });
+    expect(merged.size).toBe(75); // 5 × 15 = 75 — matches the observed payload!
+    return merged.text().then((text) => {
+      expect(text).toBe('[object Object][object Object][object Object][object Object][object Object]');
+    });
+  });
+
+  it('end-to-end: simulate full failure path — Blob → port → merge → 75-byte garbage', async () => {
+    // Build a fake recorder buffer of 5 real WebM-ish blobs.
+    const realChunks = [0x1a, 0x45, 0xdf, 0xa3, 0xa3].map((firstByte, i) => ({
+      data: new Blob([new Uint8Array([firstByte, 0x00, 0x00, 0x00])], {
+        type: 'video/webm',
+      }),
+      timestamp: 1000 + i,
+      isFirst: i === 0,
+    }));
+
+    // Send through the simulated port.
+    const message = { type: 'BUFFER', chunks: realChunks };
+    const received = chromeRuntimePortRoundTrip(message) as {
+      chunks: Array<{ data: unknown; timestamp: number }>;
+    };
+
+    // SW-side: mergeVideoChunks does this exact operation.
+    const blobs = received.chunks
+      .sort((a, b) => a.timestamp - b.timestamp)
+      .map((c) => c.data as Blob);
+    const merged = new Blob(blobs, { type: 'video/webm' });
+
+    // The observed failure: 75 bytes of "[object Object]" repeated.
+    expect(merged.size).toBe(75);
+    const text = await merged.text();
+    expect(text).toBe(
+      '[object Object][object Object][object Object][object Object][object Object]'
+    );
+    // The original 5 × 4 = 20 bytes of WebM data are GONE.
+    expect(merged.size).not.toBe(20);
+  });
+});
+
+describe('port serialization (GREEN — pins the eventual fix contract)', () => {
+  // These tests are forward-pinning: they describe the wire-format the fix
+  // MUST implement. They will run today and PASS — they test pure helpers, not
+  // the (still-buggy) recorder.ts handler.
+  //
+  // The fix must:
+  //   1. Define a TransferredVideoChunk wire-format that is JSON-friendly:
+  //        { dataBase64: string; type: string; timestamp: number; isFirst?: boolean }
+  //   2. Convert Blob → base64 in offscreen before postMessage.
+  //   3. Convert base64 → Blob in SW after receive.
+  //
+  // Until the fix lands, these helpers don't exist in src/. These tests live
+  // here as a contract the fix's reviewer can check against.
+
+  async function blobToBase64(blob: Blob): Promise<string> {
+    const buf = await blob.arrayBuffer();
+    const bytes = new Uint8Array(buf);
+    // Node + browser both support Buffer / btoa. Use a portable conversion.
+    let binary = '';
+    for (let i = 0; i < bytes.length; i++) {
+      binary += String.fromCharCode(bytes[i]);
+    }
+    // btoa exists in vitest's node env via globalThis polyfill in modern Node.
+    return typeof btoa === 'function'
+      ? btoa(binary)
+      : Buffer.from(binary, 'binary').toString('base64');
+  }
+
+  function base64ToBlob(b64: string, type: string): Blob {
+    const binary =
+      typeof atob === 'function'
+        ? atob(b64)
+        : Buffer.from(b64, 'base64').toString('binary');
+    const bytes = new Uint8Array(binary.length);
+    for (let i = 0; i < binary.length; i++) {
+      bytes[i] = binary.charCodeAt(i);
+    }
+    return new Blob([bytes], { type });
+  }
+
+  it('base64 round-trip preserves binary content across JSON serialization', async () => {
+    const original = new Blob([new Uint8Array([0x1a, 0x45, 0xdf, 0xa3])], {
+      type: 'video/webm',
+    });
+    const b64 = await blobToBase64(original);
+    expect(b64).toBe('GkXfow=='); // base64 of EBML magic
+    // JSON round-trip preserves the string.
+    const wire = { dataBase64: b64, type: 'video/webm' };
+    const received = JSON.parse(JSON.stringify(wire)) as {
+      dataBase64: string;
+      type: string;
+    };
+    const restored = base64ToBlob(received.dataBase64, received.type);
+    expect(restored.size).toBe(4);
+    const text = await restored.text();
+    // EBML magic ≈ "Eߣ" — bytes preserved.
+    expect(new Uint8Array(await restored.arrayBuffer())).toEqual(
+      new Uint8Array([0x1a, 0x45, 0xdf, 0xa3])
+    );
+  });
+
+  it('merging base64-decoded chunks produces a real WebM-prefixed blob', async () => {
+    const original = new Blob([new Uint8Array([0x1a, 0x45, 0xdf, 0xa3])], {
+      type: 'video/webm',
+    });
+    const b64 = await blobToBase64(original);
+    const wireChunks = [
+      { dataBase64: b64, type: 'video/webm', timestamp: 1, isFirst: true },
+    ];
+    const received = JSON.parse(JSON.stringify({ chunks: wireChunks })) as {
+      chunks: Array<{ dataBase64: string; type: string; timestamp: number }>;
+    };
+    const blobs = received.chunks.map((c) => base64ToBlob(c.dataBase64, c.type));
+    const merged = new Blob(blobs, { type: 'video/webm' });
+    expect(merged.size).toBe(4);
+    expect(merged.size).not.toBe(75); // the bug is gone in the fixed form.
+  });
+});