diff --git a/.env.example b/.env.example
index 46202b553..e7cb93e20 100644
--- a/.env.example
+++ b/.env.example
@@ -36,6 +36,15 @@ FEISHU_REDIRECT_URI=http://localhost:3000/auth/feishu/callback
 # S3_MAX_POOL_CONNECTIONS=50
 # S3_WRITE_WORKERS=32
 
+# Google Cloud Storage (S3-compatible API) — set these instead of MinIO values:
+# STORAGE_BACKEND=s3
+# S3_BUCKET=your-gcs-bucket-name
+# S3_REGION=auto
+# S3_ENDPOINT_URL=https://storage.googleapis.com
+# S3_ACCESS_KEY_ID=your-hmac-access-key
+# S3_SECRET_ACCESS_KEY=your-hmac-secret
+# S3_PREFIX=agents
+
 # Local MinIO settings used by docker-compose.multi-instance.yml.
 # Change the password before exposing MinIO outside local development.
 MINIO_ROOT_USER=clawith
diff --git a/backend/app/services/storage_runtime/s3.py b/backend/app/services/storage_runtime/s3.py
index 6b569447a..1b6b105d2 100644
--- a/backend/app/services/storage_runtime/s3.py
+++ b/backend/app/services/storage_runtime/s3.py
@@ -177,13 +177,16 @@ async def read_bytes(self, key: str) -> bytes:
         return await asyncio.to_thread(body.read)
 
     async def write_bytes(self, key: str, data: bytes, content_type: str | None = None) -> None:
+        # GCS S3-compatible API requires an explicit Content-Type; without it
+        # the V4 signature body-hash is calculated on an empty content-type,
+        # but GCS applies a different default — causing SignatureDoesNotMatch.
+        resolved_ct = content_type or "application/octet-stream"
         kwargs: dict[str, Any] = {
             "Bucket": self.bucket,
             "Key": self._object_key(key),
             "Body": data,
+            "ContentType": resolved_ct,
         }
-        if content_type:
-            kwargs["ContentType"] = content_type
         async with self._async_client() as client:
             await client.put_object(**kwargs)
 
@@ -302,8 +305,10 @@ async def presign_download_url(self, key: str, filename: str | None = None, inli
             parsed_url = urlparse(url)
             parsed_endpoint = urlparse(self.endpoint_url)
             if parsed_url.netloc == parsed_endpoint.netloc:
+                # MinIO-style endpoint: rewrite path with /minio prefix
                 new_path = "/minio" + parsed_url.path
                 url = urlunparse(("", "", new_path, parsed_url.params, parsed_url.query, parsed_url.fragment))
+            # GCS (storage.googleapis.com): presigned URLs are already correct, no rewrite needed
         return url
 
 
diff --git a/deploy/.env.example b/deploy/.env.example
index efd6ecb2e..20b1591a5 100644
--- a/deploy/.env.example
+++ b/deploy/.env.example
@@ -36,6 +36,15 @@ FEISHU_REDIRECT_URI=http://localhost:3000/auth/feishu/callback
 # S3_MAX_POOL_CONNECTIONS=50
 # S3_WRITE_WORKERS=32
 
+# Google Cloud Storage (S3-compatible API) — set these instead of MinIO values:
+# STORAGE_BACKEND=s3
+# S3_BUCKET=your-gcs-bucket-name
+# S3_REGION=auto
+# S3_ENDPOINT_URL=https://storage.googleapis.com
+# S3_ACCESS_KEY_ID=your-hmac-access-key
+# S3_SECRET_ACCESS_KEY=your-hmac-secret
+# S3_PREFIX=agents
+
 
 API_PORT=8000