diff --git a/changelog.d/calculate-malformed-payload-400.fixed.md b/changelog.d/calculate-malformed-payload-400.fixed.md
new file mode 100644
index 000000000..1a63c6479
--- /dev/null
+++ b/changelog.d/calculate-malformed-payload-400.fixed.md
@@ -0,0 +1 @@
+Return 400 instead of 500 when a calculate payload fails situation parsing.
diff --git a/policyengine_api/endpoints/household.py b/policyengine_api/endpoints/household.py
index 24e14661d..314871a00 100644
--- a/policyengine_api/endpoints/household.py
+++ b/policyengine_api/endpoints/household.py
@@ -10,6 +10,7 @@
     format_unrecognized_inputs_message,
 )
 from policyengine_api.utils.payload_validators import validate_country
+from policyengine_core.errors import SituationParsingError
 
 
 def get_countries():
@@ -45,7 +46,11 @@ def add_yearly_variables(household, country_id, countries=None):
                         if variables[variable]["isInputVariable"]:
                             household[entity_plural][entity][
                                 variables[variable]["name"]
-                            ] = {household_year: variables[variable]["defaultValue"]}
+                            ] = {
+                                household_year: variables[variable][
+                                    "defaultValue"
+                                ]
+                            }
                         else:
                             household[entity_plural][entity][
                                 variables[variable]["name"]
@@ -97,7 +102,9 @@ def get_household_year(household):
 
 
 @validate_country
-def get_household_under_policy(country_id: str, household_id: str, policy_id: str):
+def get_household_under_policy(
+    country_id: str, household_id: str, policy_id: str
+):
     """Get a household's output data under a given policy.
 
     Args:
@@ -274,6 +281,19 @@ def get_calculate(country_id: str, add_missing: bool = False) -> dict:
 
     try:
         result = country.calculate(household_json, policy_json)
+    except SituationParsingError as e:
+        # Malformed household payloads (e.g. a dict where a number belongs)
+        # are client errors, not server errors — mostly bot traffic.
+        response_body = dict(
+            status="error",
+            message=f"Invalid household payload: {e}",
+            result=None,
+        )
+        return Response(
+            json.dumps(response_body),
+            status=400,
+            mimetype="application/json",
+        )
     except Exception as e:
         logging.exception(e)
         response_body = dict(
diff --git a/tests/unit/endpoints/test_calculate_error_statuses.py b/tests/unit/endpoints/test_calculate_error_statuses.py
new file mode 100644
index 000000000..d37d72868
--- /dev/null
+++ b/tests/unit/endpoints/test_calculate_error_statuses.py
@@ -0,0 +1,99 @@
+from flask import Flask
+import pytest
+from policyengine_core.errors import SituationParsingError
+
+from policyengine_api.endpoints import household as household_endpoint
+
+from .test_calculate_deprecated_inputs import DummyCountry
+
+HOUSEHOLD = {
+    "people": {
+        "you": {
+            "age": {"2026": 40},
+            "employment_income": {"2026": 30_000},
+        }
+    }
+}
+
+
+class ParsingErrorCountry(DummyCountry):
+    def calculate(self, household, policy):
+        raise SituationParsingError(
+            ["people", "you", "employment_income", "2026"],
+            "Can't deal with value: expected type number, received '{}'.",
+        )
+
+
+class CrashingCountry(DummyCountry):
+    def calculate(self, household, policy):
+        raise RuntimeError("engine exploded")
+
+
+def make_client(monkeypatch, country, add_missing=False):
+    monkeypatch.setattr(
+        household_endpoint,
+        "get_countries",
+        lambda: {"us": country},
+    )
+    app = Flask(__name__)
+    if add_missing:
+        monkeypatch.setattr(
+            household_endpoint,
+            "add_yearly_variables",
+            lambda household, country_id: household,
+        )
+
+        def handler(country_id):
+            return household_endpoint.get_calculate(
+                country_id, add_missing=True
+            )
+
+        app.add_url_rule(
+            "/<country_id>/calculate-full",
+            "calculate_full",
+            handler,
+            methods=["POST"],
+        )
+    else:
+        app.add_url_rule(
+            "/<country_id>/calculate",
+            "calculate",
+            household_endpoint.get_calculate,
+            methods=["POST"],
+        )
+    return app.test_client()
+
+
+def test__calculate__returns_400_on_situation_parsing_error(monkeypatch):
+    client = make_client(monkeypatch, ParsingErrorCountry())
+
+    response = client.post("/us/calculate", json={"household": HOUSEHOLD})
+
+    assert response.status_code == 400
+    payload = response.get_json()
+    assert payload["status"] == "error"
+    assert payload["result"] is None
+    assert payload["message"].startswith("Invalid household payload")
+
+
+def test__calculate_full__returns_400_on_situation_parsing_error(monkeypatch):
+    client = make_client(monkeypatch, ParsingErrorCountry(), add_missing=True)
+
+    response = client.post("/us/calculate-full", json={"household": HOUSEHOLD})
+
+    assert response.status_code == 400
+    payload = response.get_json()
+    assert payload["status"] == "error"
+    assert payload["result"] is None
+    assert payload["message"].startswith("Invalid household payload")
+
+
+def test__calculate__returns_500_on_unexpected_error(monkeypatch):
+    client = make_client(monkeypatch, CrashingCountry())
+
+    response = client.post("/us/calculate", json={"household": HOUSEHOLD})
+
+    assert response.status_code == 500
+    payload = response.get_json()
+    assert payload["status"] == "error"
+    assert "engine exploded" in payload["message"]