diff --git a/handler.py b/handler.py
index e92e287..5b70463 100755
--- a/handler.py
+++ b/handler.py
@@ -158,6 +158,11 @@ def handle_muEd_command(event: JsonType, command: str) -> HandlerResponse:
         }
         return wrap_muEd_response(error, event, 400)
 
+    except commands.InvalidSubmissionError as e:
+        detail = str(e) if str(e) else repr(e)
+        error = {"title": "Unprocessable submission", "message": detail, "code": ErrorCode.VALIDATION_ERROR}
+        return wrap_muEd_response(error, event, 422)
+
     except EvaluationException as e:
         detail = str(e) if str(e) else repr(e)
         error = {"title": "Internal server error", "message": detail, "code": ErrorCode.INTERNAL_ERROR}
diff --git a/schemas/muEd/openapi-v0_1_0.yml b/schemas/muEd/openapi-v0_1_0.yml
index 9004264..421ab05 100644
--- a/schemas/muEd/openapi-v0_1_0.yml
+++ b/schemas/muEd/openapi-v0_1_0.yml
@@ -368,6 +368,8 @@ paths:
           $ref: '#/components/responses/403-Forbidden'
         '406':
           $ref: '#/components/responses/406-VersionNotSupported'
+        '422':
+          $ref: '#/components/responses/422-UnprocessableSubmission'
         '500':
           $ref: '#/components/responses/500-InternalError'
         '501':
@@ -1778,6 +1780,30 @@ components:
                   requestedVersion: '0.0'
                   supportedVersions:
                     - 0.1.0
+    422-UnprocessableSubmission:
+      description: The request is valid, but the evaluation function cannot process the submission content (e.g. an unparseable expression).
+      headers:
+        X-Request-Id:
+          description: Request id for tracing this request across services.
+          schema:
+            type: string
+        X-Api-Version:
+          description: The API version that was used to serve this response.
+          schema:
+            type: string
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            unprocessableSubmission:
+              summary: Example unprocessable submission error
+              value:
+                title: Unprocessable submission
+                message: 'Failed to parse SymPy expression: A/(w*'
+                code: VALIDATION_ERROR
+                trace: null
+                details: null
     500-InternalError:
       description: Internal server error.
       headers:
diff --git a/tests/mued_handling_test.py b/tests/mued_handling_test.py
index d077414..c12e731 100644
--- a/tests/mued_handling_test.py
+++ b/tests/mued_handling_test.py
@@ -436,6 +436,78 @@ def test_presubmission_disabled_runs_normal_evaluation(self):
         self.assertIn("awardedPoints", body[0])
         self.assertNotIn("preSubmissionFeedback", body[0])
 
+    def test_preview_value_error_returns_unprocessable(self):
+        def preview_function(response, params):
+            raise ValueError(f"Failed to parse SymPy expression: {response}")
+
+        commands.preview_function = preview_function
+        event = {
+            "path": "/evaluate",
+            "body": {
+                "submission": {"type": "MATH", "content": {"expression": "A/(w*"}},
+                "preSubmissionFeedback": {"enabled": True},
+            },
+        }
+
+        response = handler(event)
+
+        self.assertEqual(response["statusCode"], 422)
+        body = json.loads(response["body"])
+        self.assertEqual(body["code"], "VALIDATION_ERROR")
+        self.assertEqual(body["message"], "Failed to parse SymPy expression: A/(w*")
+
+    def test_preview_unexpected_error_returns_internal_error(self):
+        def preview_function(response, params):
+            raise RuntimeError("boom")
+
+        commands.preview_function = preview_function
+        event = {
+            "path": "/evaluate",
+            "body": {
+                "submission": {"type": "MATH", "content": {"expression": "x+1"}},
+                "preSubmissionFeedback": {"enabled": True},
+            },
+        }
+
+        response = handler(event)
+
+        self.assertEqual(response["statusCode"], 500)
+        body = json.loads(response["body"])
+        self.assertEqual(body["code"], "INTERNAL_ERROR")
+
+    def test_evaluation_value_error_returns_unprocessable(self):
+        def evaluation_function(response, answer, params):
+            raise ValueError(f"Failed to parse SymPy expression: {response}")
+
+        commands.evaluation_function = evaluation_function
+        event = {
+            "path": "/evaluate",
+            "body": {"submission": {"type": "MATH", "content": {"expression": "A/(w*"}}},
+        }
+
+        response = handler(event)
+
+        self.assertEqual(response["statusCode"], 422)
+        body = json.loads(response["body"])
+        self.assertEqual(body["code"], "VALIDATION_ERROR")
+        self.assertEqual(body["message"], "Failed to parse SymPy expression: A/(w*")
+
+    def test_evaluation_unexpected_error_returns_internal_error(self):
+        def evaluation_function(response, answer, params):
+            raise RuntimeError("boom")
+
+        commands.evaluation_function = evaluation_function
+        event = {
+            "path": "/evaluate",
+            "body": {"submission": {"type": "MATH", "content": {"expression": "x+1"}}},
+        }
+
+        response = handler(event)
+
+        self.assertEqual(response["statusCode"], 500)
+        body = json.loads(response["body"])
+        self.assertEqual(body["code"], "INTERNAL_ERROR")
+
 
 class TestMuEdPreviewExtraction(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tools/commands.py b/tools/commands.py
index 8d8b21b..61aabc5 100644
--- a/tools/commands.py
+++ b/tools/commands.py
@@ -42,6 +42,10 @@ def preview_function(response: Any, params: Any) -> Dict:
     )
 
 
+class InvalidSubmissionError(Exception):
+    """Raised when the evaluation function cannot process the submitted response."""
+
+
 class CaseWarning(TypedDict, total=False):
     """Dictionary for reporting issues when testing cases"""
 
@@ -184,7 +188,10 @@ def _extract_muEd_submission(body: JsonType):
 
 def _run_muEd_preview(body: JsonType) -> List[Dict]:
     response, params, _ = _extract_muEd_submission(body)
-    preview_result = preview_function(response, params)
+    try:
+        preview_result = preview_function(response, params)
+    except ValueError as e:
+        raise InvalidSubmissionError(str(e)) from e
     return [{"preSubmissionFeedback": preview_result.get("preview", {})}]
 
 
@@ -200,7 +207,10 @@ def _run_muEd_evaluation(body: JsonType) -> List[Dict]:
     else:
         answer = None
 
-    result = _run_evaluation(response, answer, params)
+    try:
+        result = _run_evaluation(response, answer, params)
+    except ValueError as e:
+        raise InvalidSubmissionError(str(e)) from e
 
     is_correct = result.get("is_correct") or 0
     feedback_text = result.get("feedback", "")