diff --git a/backend/packages/harness/deerflow/mcp/tools.py b/backend/packages/harness/deerflow/mcp/tools.py
index 19f5ffcec5..b77ef85129 100644
--- a/backend/packages/harness/deerflow/mcp/tools.py
+++ b/backend/packages/harness/deerflow/mcp/tools.py
@@ -604,7 +604,17 @@ def _convert_call_tool_result(
                 if mime and mime.startswith("image/"):
                     lc_content.append(create_image_block(base64=res.blob, mime_type=mime))
                 else:
-                    lc_content.append(create_file_block(base64=res.blob, mime_type=mime))
+                    # ``BlobResourceContents.mimeType`` is optional in the MCP spec, but
+                    # langchain-core refuses a base64 block without one. Degrade to a
+                    # generic binary type instead of raising, so the rest of the result —
+                    # including the text blocks the server returned — survives. The
+                    # sibling ``ResourceLink`` branch already degrades the same way.
+                    lc_content.append(
+                        create_file_block(
+                            base64=res.blob,
+                            mime_type=mime or "application/octet-stream",
+                        )
+                    )
             else:
                 lc_content.append(create_text_block(text=str(res)))
         else:
diff --git a/backend/tests/test_mcp_blob_missing_mime_type.py b/backend/tests/test_mcp_blob_missing_mime_type.py
new file mode 100644
index 0000000000..b1c0aa5d88
--- /dev/null
+++ b/backend/tests/test_mcp_blob_missing_mime_type.py
@@ -0,0 +1,51 @@
+"""An MCP blob without a mimeType must degrade, not fail the whole tool call."""
+
+import base64
+
+from mcp.types import BlobResourceContents, CallToolResult, EmbeddedResource, TextContent
+
+from deerflow.mcp.tools import _convert_call_tool_result
+
+BLOB = base64.b64encode(b"binary payload").decode("ascii")
+
+
+def _result(mime_type: str | None, *, with_text: bool = True) -> CallToolResult:
+    content = [TextContent(type="text", text="here is your report")] if with_text else []
+    content.append(
+        EmbeddedResource(
+            type="resource",
+            resource=BlobResourceContents(uri="file:///tmp/report.bin", blob=BLOB, mimeType=mime_type),
+        )
+    )
+    return CallToolResult(content=content, isError=False)
+
+
+def test_blob_without_mime_type_degrades_to_octet_stream():
+    lc_content, _artifact = _convert_call_tool_result(_result(None))
+
+    assert [block["type"] for block in lc_content] == ["text", "file"]
+    assert lc_content[0]["text"] == "here is your report"
+    assert lc_content[1]["mime_type"] == "application/octet-stream"
+    assert lc_content[1]["base64"] == BLOB
+
+
+def test_text_blocks_survive_the_fallback():
+    lc_content, _artifact = _convert_call_tool_result(_result(None))
+
+    assert [block["type"] for block in lc_content] == ["text", "file"]
+    assert lc_content[0]["text"] == "here is your report"
+    assert lc_content[1]["mime_type"] == "application/octet-stream"
+
+
+def test_declared_non_image_mime_type_is_preserved():
+    lc_content, _artifact = _convert_call_tool_result(_result("application/pdf"))
+
+    assert lc_content[1]["type"] == "file"
+    assert lc_content[1]["mime_type"] == "application/pdf"
+
+
+def test_declared_image_mime_type_still_becomes_an_image_block():
+    lc_content, _artifact = _convert_call_tool_result(_result("image/png"))
+
+    assert [block["type"] for block in lc_content] == ["text", "image"]
+    assert lc_content[1]["mime_type"] == "image/png"