diff --git a/extension/src/service-worker/git-host.ts b/extension/src/service-worker/git-host.ts
index e856d82..cdb1e78 100644
--- a/extension/src/service-worker/git-host.ts
+++ b/extension/src/service-worker/git-host.ts
@@ -16,8 +16,28 @@ export interface GitHost {
 
   /// List file names in a directory (non-recursive).
   listDir(path: string): Promise<string[]>;
+
+  /// Write an opaque binary blob to the repo. Optimized for large
+  /// attachments — implementations switch from Contents API to Git
+  /// Data API when content exceeds BLOB_THRESHOLD_BYTES.
+  /// Returns the path that was written (same as input, for chaining).
+  putBlob(path: string, content: Uint8Array, message: string): Promise<string>;
+
+  /// Read an opaque binary blob from the repo. Same semantics as
+  /// readFile for current sizes. Distinct method so we can later add
+  /// streaming/chunked reads for very large blobs.
+  getBlob(path: string): Promise<Uint8Array>;
+
+  /// Delete a blob from the repo. Currently identical to deleteFile;
+  /// kept distinct for symmetry with putBlob.
+  deleteBlob(path: string, message: string): Promise<void>;
 }
 
+/// Pre-base64 byte size at which putBlob switches from Contents API to
+/// Git Data API. 900 KB leaves headroom for base64 inflation (1.33×) and
+/// JSON wrapping under both GitHub's and Gitea's Contents API soft-limits.
+export const BLOB_THRESHOLD_BYTES = 900 * 1024;
+
 /// Convert a Uint8Array to a base64 string (works in service worker context).
 export function uint8ArrayToBase64(bytes: Uint8Array): string {
   let binary = '';
diff --git a/extension/src/service-worker/gitea.ts b/extension/src/service-worker/gitea.ts
index 4f9582d..7344f2c 100644
--- a/extension/src/service-worker/gitea.ts
+++ b/extension/src/service-worker/gitea.ts
@@ -111,4 +111,16 @@ export class GiteaHost implements GitHost {
     if (!Array.isArray(json)) return [];
     return json.map((item: { name: string }) => item.name);
   }
+
+  async putBlob(_path: string, _content: Uint8Array, _message: string): Promise<string> {
+    throw new Error(`GiteaHost.putBlob not implemented — Task 4`);
+  }
+
+  async getBlob(path: string): Promise<Uint8Array> {
+    return this.readFile(path);
+  }
+
+  async deleteBlob(path: string, message: string): Promise<void> {
+    return this.deleteFile(path, message);
+  }
 }
diff --git a/extension/src/service-worker/github.ts b/extension/src/service-worker/github.ts
index 7dcee89..2e9df09 100644
--- a/extension/src/service-worker/github.ts
+++ b/extension/src/service-worker/github.ts
@@ -105,4 +105,16 @@ export class GitHubHost implements GitHost {
     if (!Array.isArray(json)) return [];
     return json.map((item: { name: string }) => item.name);
   }
+
+  async putBlob(_path: string, _content: Uint8Array, _message: string): Promise<string> {
+    throw new Error(`GitHubHost.putBlob not implemented — Task 3`);
+  }
+
+  async getBlob(path: string): Promise<Uint8Array> {
+    return this.readFile(path);
+  }
+
+  async deleteBlob(path: string, message: string): Promise<void> {
+    return this.deleteFile(path, message);
+  }
 }