diff --git a/api/apps/restful_apis/agent_api.py b/api/apps/restful_apis/agent_api.py
index aff36cc621..2338b86a64 100644
--- a/api/apps/restful_apis/agent_api.py
+++ b/api/apps/restful_apis/agent_api.py
@@ -2357,7 +2357,7 @@ async def download_attachment(tenant_id=None, attachment_id=None):
 
     Mirrors the authorization model of the preview endpoint: the user must belong
     to the tenant that owns the document's knowledge base. A denial returns the
-    same "Document not found!" response so the endpoint cannot be used to
+    same "Attachment not found!" response so the endpoint cannot be used to
     enumerate doc ids across tenants.
     """
     try:
@@ -2365,6 +2365,15 @@ async def download_attachment(tenant_id=None, attachment_id=None):
         # pass `attachment_id` instead of the route parameter name.
         ext = request.args.get("ext", "markdown")
         data = await thread_pool_exec(settings.STORAGE_IMPL.get, tenant_id, attachment_id)
+        if not data:
+            # Storage object missing or empty (orphaned DB metadata, tenant
+            # mismatch). Without this guard `make_response(None)` raises
+            # `TypeError: response value cannot be None` and the caller
+            # sees HTTP 500 — same bug class as #15365 on document
+            # preview. Return the same "Attachment not found!" shape used
+            # by the preview route's missing-record path so byte-streaming
+            # endpoints respond consistently on a not-found.
+            return get_data_error_result(message="Attachment not found!")
         response = await make_response(data)
         content_type = CONTENT_TYPE_MAP.get(ext, f"application/{ext}")
         apply_safe_file_response_headers(response, content_type, ext)
diff --git a/test/unit_test/api/apps/restful_apis/test_attachment_download_missing_blob.py b/test/unit_test/api/apps/restful_apis/test_attachment_download_missing_blob.py
new file mode 100644
index 0000000000..b581918642
--- /dev/null
+++ b/test/unit_test/api/apps/restful_apis/test_attachment_download_missing_blob.py
@@ -0,0 +1,209 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+"""Regression test for `/api/v1/agents/attachments/<attachment_id>/download` (#15502).
+
+Without the empty-blob guard, `make_response(None)` raises `TypeError`
+and the handler's blanket `except Exception` converts it to HTTP 500.
+Same bug class as #15365 on document preview. This test pins the
+missing-storage path to a structured 4xx response.
+"""
+
+import asyncio
+import importlib.util
+import sys
+from pathlib import Path
+from types import ModuleType, SimpleNamespace
+
+import pytest
+
+
+class _PassthroughManager:
+    def route(self, *_args, **_kwargs):
+        return lambda func: func
+
+
+class _LenientModule(ModuleType):
+    """A stub module that yields a harmless placeholder for any attribute that
+    wasn't explicitly provided. agent_api.py's top-level `from <mod> import a, b`
+    only needs every imported name to exist; symbols not on the
+    download_attachment path are never called, so a no-op placeholder is safe.
+    This keeps the test from rotting each time agent_api.py grows an import.
+    """
+
+    def __getattr__(self, _name):
+        return lambda *_a, **_k: None
+
+
+def _stub(monkeypatch, name, **attrs):
+    mod = _LenientModule(name)
+    for key, value in attrs.items():
+        setattr(mod, key, value)
+    monkeypatch.setitem(sys.modules, name, mod)
+    return mod
+
+
+def _load_agent_api(monkeypatch, *, storage_get):
+    """Load agent_api.py with the minimum stubs to exercise download_attachment."""
+
+    async def _make_response(payload):
+        if payload is None:
+            raise TypeError("response value cannot be None")
+        return SimpleNamespace(payload=payload, headers={})
+
+    _stub(
+        monkeypatch, "api.apps",
+        current_user=SimpleNamespace(id="tenant-1"),
+        login_required=lambda func: func,
+    )
+    _stub(monkeypatch, "api.apps.services.canvas_replica_service", CanvasReplicaService=SimpleNamespace())
+    _stub(monkeypatch, "api.db", CanvasCategory=SimpleNamespace())
+    _stub(monkeypatch, "api.db.db_models", Task=SimpleNamespace())
+    _stub(
+        monkeypatch, "api.db.services.api_service",
+        API4ConversationService=SimpleNamespace(
+            get_by_id=lambda _id: (False, None),
+            save=lambda **_k: True,
+            delete_by_id=lambda *_a, **_k: True,
+            query=lambda **_k: [],
+        ),
+    )
+    _stub(
+        monkeypatch, "api.db.services.canvas_service",
+        CanvasTemplateService=SimpleNamespace(),
+        UserCanvasService=SimpleNamespace(accessible=lambda *_a, **_k: True, query=lambda **_k: []),
+        completion=lambda *_a, **_k: None,
+        completion_openai=lambda *_a, **_k: None,
+    )
+    _stub(monkeypatch, "api.db.services.document_service", DocumentService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.file_service", FileService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.knowledgebase_service", KnowledgebaseService=SimpleNamespace())
+    _stub(monkeypatch, "api.db.services.pipeline_operation_log_service", PipelineOperationLogService=SimpleNamespace())
+    _stub(
+        monkeypatch, "api.db.services.task_service",
+        CANVAS_DEBUG_DOC_ID="", TaskService=SimpleNamespace(), queue_dataflow=lambda *_a, **_k: None,
+    )
+    _stub(
+        monkeypatch, "api.db.services.user_service",
+        TenantService=SimpleNamespace(), UserService=SimpleNamespace(get_by_id=lambda *_a, **_k: (False, None)),
+    )
+    _stub(monkeypatch, "api.db.services.user_canvas_version", UserCanvasVersionService=SimpleNamespace())
+
+    _stub(
+        monkeypatch, "api.utils.api_utils",
+        construct_json_result=lambda **kw: {"kind": "json", **kw},
+        get_data_error_result=lambda message="", code=0, data=False: {"kind": "data_error", "message": message},
+        get_error_data_result=lambda *_a, **_k: {"kind": "error"},
+        get_result=lambda *_a, **_k: {"kind": "result"},
+        get_json_result=lambda *_a, **_k: {"kind": "json_result"},
+        server_error_response=lambda e: {"kind": "server_error", "error": str(e)},
+        add_tenant_id_to_kwargs=lambda func: func,
+        get_request_json=lambda: {},
+        # Used as `@validate_request(...)` decorator factory at module level, so it
+        # must return an identity decorator (the lenient fallback would return None
+        # and `@None` raises TypeError during import).
+        validate_request=lambda *_a, **_k: (lambda func: func),
+    )
+    _stub(
+        monkeypatch, "common.settings",
+        retriever=SimpleNamespace(), kg_retriever=SimpleNamespace(),
+        # download_attachment reads settings.STORAGE_IMPL.get after
+        # `from common import settings` rebinds the module's `settings` name.
+        STORAGE_IMPL=SimpleNamespace(get=storage_get),
+    )
+    _stub(monkeypatch, "common.ssrf_guard", assert_host_is_safe=lambda *_a, **_k: None)
+    _stub(monkeypatch, "common.constants", RetCode=SimpleNamespace())
+    _stub(monkeypatch, "api.utils.pagination_utils", validate_rest_api_page_size=lambda *_a, **_k: None)
+    _stub(monkeypatch, "peewee")
+
+    async def _thread_pool_exec(fn, *args, **kwargs):
+        # download_attachment does `await thread_pool_exec(STORAGE_IMPL.get, ...)`;
+        # run the callable inline so the storage stub's return value flows through.
+        return fn(*args, **kwargs)
+
+    _stub(monkeypatch, "common.misc_utils", get_uuid=lambda: "uuid", thread_pool_exec=_thread_pool_exec)
+    _stub(
+        monkeypatch, "api.utils.web_utils",
+        CONTENT_TYPE_MAP={"markdown": "text/markdown"},
+        apply_safe_file_response_headers=lambda *_a, **_k: None,
+    )
+
+    # Lenient: agent_api.py imports Response, jsonify, request, make_response from
+    # quart; only request and make_response are on the download path, the rest
+    # resolve to placeholders so the import line can't break the test.
+    quart_stub = _LenientModule("quart")
+    quart_stub.request = SimpleNamespace(method="GET", args={"ext": "markdown"})
+    quart_stub.make_response = _make_response
+    monkeypatch.setitem(sys.modules, "quart", quart_stub)
+
+    # parents[5] = repo root from test/unit_test/api/apps/restful_apis/<file>
+    repo_root = Path(__file__).resolve().parents[5]
+    module_path = repo_root / "api" / "apps" / "restful_apis" / "agent_api.py"
+    spec = importlib.util.spec_from_file_location("test_agent_api_module", module_path)
+    module = importlib.util.module_from_spec(spec)
+    # `manager` must exist before exec so the @manager.route decorators run.
+    module.manager = _PassthroughManager()
+    monkeypatch.setitem(sys.modules, "test_agent_api_module", module)
+    spec.loader.exec_module(module)
+
+    # Pin every module global that download_attachment resolves at call time.
+    # The sys.modules stubs above only guarantee the import *succeeds*. In the
+    # full test environment `from common import settings` (and the api_utils /
+    # misc_utils imports) can bind the REAL modules instead of our stubs — e.g.
+    # if another test already imported common.settings, it is an attribute on the
+    # already-loaded `common` package and the sys.modules stub is bypassed — so
+    # settings.STORAGE_IMPL would be the uninitialised real None and the handler
+    # would 500 with "'NoneType' object has no attribute 'get'". Overriding the
+    # globals on the loaded module makes behaviour identical in both the
+    # bare-stub (local) and full-dependency (CI) environments.
+    module.settings = SimpleNamespace(STORAGE_IMPL=SimpleNamespace(get=storage_get))
+    module.request = SimpleNamespace(method="GET", args={"ext": "markdown"})
+    module.make_response = _make_response
+    module.thread_pool_exec = _thread_pool_exec
+    module.get_data_error_result = lambda message="", **_k: {"kind": "data_error", "message": message}
+    module.server_error_response = lambda e: {"kind": "server_error", "error": str(e)}
+    module.CONTENT_TYPE_MAP = {"markdown": "text/markdown"}
+    module.apply_safe_file_response_headers = lambda *_a, **_k: None
+    return module
+
+
+@pytest.mark.p1
+class TestAttachmentDownloadMissingBlob:
+    """Regression for #15502: missing-blob → structured 4xx, not HTTP 500."""
+
+    def test_empty_blob_returns_not_found(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """Storage returns None (orphaned metadata) → 'Document not found!' 4xx,
+        not a TypeError 500 from make_response(None)."""
+        module = _load_agent_api(monkeypatch, storage_get=lambda *_a, **_k: None)
+        result = asyncio.run(module.download_attachment(tenant_id="t1", attachment_id="orphan"))
+        # 'data_error' is the get_data_error_result shape from our stub above.
+        # If the empty-blob guard is missing, make_response(None) raises and
+        # the result instead has `kind == "server_error"`.
+        assert isinstance(result, dict) and result.get("kind") == "data_error", result
+        assert "not found" in result["message"].lower()
+
+    def test_empty_bytes_returns_not_found(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """Same path when storage returns b'' instead of None."""
+        module = _load_agent_api(monkeypatch, storage_get=lambda *_a, **_k: b"")
+        result = asyncio.run(module.download_attachment(tenant_id="t1", attachment_id="empty"))
+        assert isinstance(result, dict) and result.get("kind") == "data_error"
+        assert "not found" in result["message"].lower()
+
+    def test_nonempty_blob_proceeds(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """Happy path: real bytes flow through make_response untouched."""
+        module = _load_agent_api(monkeypatch, storage_get=lambda *_a, **_k: b"PDFDATA")
+        result = asyncio.run(module.download_attachment(tenant_id="t1", attachment_id="good"))
+        # SimpleNamespace from our _make_response stub
+        assert hasattr(result, "payload") and result.payload == b"PDFDATA"