diff --git a/src/specify_cli/events/__init__.py b/src/specify_cli/events/__init__.py
index 4cec7997f9..02f1e0ddcf 100644
--- a/src/specify_cli/events/__init__.py
+++ b/src/specify_cli/events/__init__.py
@@ -2552,7 +2552,12 @@ def _safe_write_json(dst: Path, data: dict) -> None:
     """Write *data* as JSON to *dst* after validating the destination (#12)."""
     _ensure_safe_destination(dst)
     dst.parent.mkdir(parents=True, exist_ok=True)
-    dst.write_text(json.dumps(data, indent=2) + "\n", encoding="utf-8")
+    # A lone surrogate (\ud800) can't be UTF-8 encoded; write it back as its JSON escape.
+    dst.write_text(
+        json.dumps(data, indent=2, ensure_ascii=False) + "\n",
+        encoding="utf-8",
+        errors="backslashreplace",
+    )
 
 
 def _ensure_safe_destination(dst: Path) -> None:
diff --git a/src/specify_cli/integrations/copilot/__init__.py b/src/specify_cli/integrations/copilot/__init__.py
index 1a8285d7c4..9adaf5cd9d 100644
--- a/src/specify_cli/integrations/copilot/__init__.py
+++ b/src/specify_cli/integrations/copilot/__init__.py
@@ -664,6 +664,9 @@ def _merge_vscode_settings(src: Path, dst: Path) -> None:
         if not changed:
             return
 
+        # A lone surrogate (\ud800) can't be UTF-8 encoded; write it back as its JSON escape.
         dst.write_text(
-            json.dumps(existing, indent=4) + "\n", encoding="utf-8"
+            json.dumps(existing, indent=4, ensure_ascii=False) + "\n",
+            encoding="utf-8",
+            errors="backslashreplace",
         )
diff --git a/tests/integrations/test_integration_copilot.py b/tests/integrations/test_integration_copilot.py
index 35d30faf0e..cb882a71a8 100644
--- a/tests/integrations/test_integration_copilot.py
+++ b/tests/integrations/test_integration_copilot.py
@@ -84,6 +84,37 @@ def test_setup_merges_existing_vscode_settings(self, tmp_path):
         assert settings not in created
         assert not any("settings.json" in k for k in m.files)
 
+    def test_setup_merge_keeps_non_ascii_vscode_settings_readable(self, tmp_path):
+        from specify_cli.integrations.copilot import CopilotIntegration
+        copilot = CopilotIntegration()
+        vscode_dir = tmp_path / ".vscode"
+        vscode_dir.mkdir(parents=True)
+        existing = {"cSpell.words": ["naïve", "Привіт", "日本語"]}
+        settings = vscode_dir / "settings.json"
+        settings.write_text(json.dumps(existing, indent=4, ensure_ascii=False), encoding="utf-8")
+        m = IntegrationManifest("copilot", tmp_path)
+
+        copilot.setup(tmp_path, m, parsed_options={"commands": True})
+
+        text = settings.read_text(encoding="utf-8")
+        assert '"naïve"' in text and '"Привіт"' in text and '"日本語"' in text
+        assert json.loads(text)["cSpell.words"] == existing["cSpell.words"]
+
+    def test_setup_merge_keeps_escaped_lone_surrogate_in_vscode_settings(self, tmp_path):
+        from specify_cli.integrations.copilot import CopilotIntegration
+        copilot = CopilotIntegration()
+        vscode_dir = tmp_path / ".vscode"
+        vscode_dir.mkdir(parents=True)
+        settings = vscode_dir / "settings.json"
+        settings.write_text('{"custom.setting": "\\ud800"}', encoding="utf-8")
+        m = IntegrationManifest("copilot", tmp_path)
+
+        copilot.setup(tmp_path, m, parsed_options={"commands": True})
+
+        data = json.loads(settings.read_text(encoding="utf-8"))
+        assert data["custom.setting"] == "\ud800"
+        assert "chat.promptFilesRecommendations" in data
+
     def test_setup_preserves_non_utf8_vscode_settings(self, tmp_path, caplog):
         from specify_cli.integrations.copilot import CopilotIntegration
         copilot = CopilotIntegration()
diff --git a/tests/specify_cli/events/test_events.py b/tests/specify_cli/events/test_events.py
index d1f15bec5f..cb10ad3855 100644
--- a/tests/specify_cli/events/test_events.py
+++ b/tests/specify_cli/events/test_events.py
@@ -463,6 +463,49 @@ def test_remove_preserves_user_hooks(self, tmp_path):
         assert len(data["hooks"]["PreToolUse"]) == 1
         assert data["hooks"]["PreToolUse"][0]["matcher"] == "Bash"
 
+    def test_merge_keeps_non_ascii_user_settings_readable(self, tmp_path):
+        integration = ClaudeIntegration()
+        manifest = MagicMock(spec=IntegrationManifest)
+        manifest.files = {}
+        manifest.record_file = MagicMock()
+        manifest.record_existing = MagicMock()
+
+        config_path = tmp_path / ".claude/settings.json"
+        config_path.parent.mkdir(parents=True, exist_ok=True)
+        config_path.write_text(
+            json.dumps({"env": {"GREETING": "Привіт — ¿qué tal? 日本語"}}, ensure_ascii=False),
+            encoding="utf-8",
+        )
+
+        events = {
+            "pre_tool_use": [{"command": "speckit.tdd.validate"}],
+        }
+        install_integration_events(integration, tmp_path, manifest, events)
+
+        text = config_path.read_text(encoding="utf-8")
+        assert "Привіт — ¿qué tal? 日本語" in text
+        assert json.loads(text)["env"]["GREETING"] == "Привіт — ¿qué tal? 日本語"
+
+    def test_merge_keeps_escaped_lone_surrogate_in_user_settings(self, tmp_path):
+        integration = ClaudeIntegration()
+        manifest = MagicMock(spec=IntegrationManifest)
+        manifest.files = {}
+        manifest.record_file = MagicMock()
+        manifest.record_existing = MagicMock()
+
+        config_path = tmp_path / ".claude/settings.json"
+        config_path.parent.mkdir(parents=True, exist_ok=True)
+        config_path.write_text('{"env": {"ODD": "\\ud800"}}', encoding="utf-8")
+
+        events = {
+            "pre_tool_use": [{"command": "speckit.tdd.validate"}],
+        }
+        install_integration_events(integration, tmp_path, manifest, events)
+
+        data = json.loads(config_path.read_text(encoding="utf-8"))
+        assert data["env"]["ODD"] == "\ud800"
+        assert "PreToolUse" in data["hooks"]
+
 
 # -- Copilot events JSON writing --------------------------------------------