diff --git a/pyrefly/lib/error/error.rs b/pyrefly/lib/error/error.rs
index c6989c755b..63ed836883 100644
--- a/pyrefly/lib/error/error.rs
+++ b/pyrefly/lib/error/error.rs
@@ -14,6 +14,7 @@ use std::path::Path;
 use itertools::Itertools;
 use lsp_types::CodeDescription;
 use lsp_types::Diagnostic;
+use lsp_types::DiagnosticTag;
 use lsp_types::Url;
 use pyrefly_python::ignore::Tool;
 use pyrefly_python::module::Module;
@@ -199,7 +200,11 @@ impl Error {
             message: self.msg().to_owned(),
             code: Some(lsp_types::NumberOrString::String(code)),
             code_description,
-            tags: None,
+            tags: if self.error_kind() == ErrorKind::Deprecated {
+                Some(vec![DiagnosticTag::DEPRECATED])
+            } else {
+                None
+            },
             ..Default::default()
         }
     }
diff --git a/pyrefly/lib/test/lsp/lsp_interaction/diagnostic.rs b/pyrefly/lib/test/lsp/lsp_interaction/diagnostic.rs
index 3de6866812..1a646771a4 100644
--- a/pyrefly/lib/test/lsp/lsp_interaction/diagnostic.rs
+++ b/pyrefly/lib/test/lsp/lsp_interaction/diagnostic.rs
@@ -1672,3 +1672,54 @@ fn test_no_diagnostics_for_non_open_files_in_open_files_only_mode() {
         non_open_file.display()
     );
 }
+
+#[test]
+fn test_deprecated_diagnostic_tag() {
+    let test_files_root = get_test_files_root();
+    let mut interaction = LspInteraction::new();
+    interaction.set_root(test_files_root.path().to_path_buf());
+    interaction
+        .initialize(InitializeSettings {
+            configuration: Some(None),
+            ..Default::default()
+        })
+        .unwrap();
+
+    interaction.client.did_change_configuration();
+
+    interaction
+        .client
+        .expect_configuration_request(None)
+        .unwrap()
+        .send_configuration_response(json!([
+            {"pyrefly": {"displayTypeErrors": "force-on"}}
+        ]));
+
+    interaction.client.did_open("deprecated_function.py");
+
+    interaction
+        .client
+        .diagnostic("deprecated_function.py")
+        .expect_response(json!({
+            "items": [
+                {
+                    "code": "deprecated",
+                    "codeDescription": {
+                        "href": "https://pyrefly.org/en/docs/error-kinds/#deprecated"
+                    },
+                    "message": "`old_function` is deprecated\n  use new_function instead",
+                    "range": {
+                        "start": {"line": 10, "character": 0},
+                        "end": {"line": 10, "character": 12}
+                    },
+                    "severity": 2,
+                    "source": "Pyrefly",
+                    "tags": [2]
+                }
+            ],
+            "kind": "full"
+        }))
+        .unwrap();
+
+    interaction.shutdown().unwrap();
+}
diff --git a/pyrefly/lib/test/lsp/lsp_interaction/test_files/deprecated_function.py b/pyrefly/lib/test/lsp/lsp_interaction/test_files/deprecated_function.py
new file mode 100644
index 0000000000..717adc1f3a
--- /dev/null
+++ b/pyrefly/lib/test/lsp/lsp_interaction/test_files/deprecated_function.py
@@ -0,0 +1,11 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+from warnings import deprecated
+
+@deprecated("use new_function instead")
+def old_function() -> None: ...
+
+old_function()