diff --git a/Makefile b/Makefile
index 02271608..2f84cb35 100644
--- a/Makefile
+++ b/Makefile
@@ -430,7 +430,7 @@ prune:
 # Clean Cache Files
 .PHONY: clean
 clean:
-	@find . | grep -E "(__pycache__|\.pyc|\.pyo|\.pytest_cache|\.ruff_cache|\.mypy_cache|./data/)" | xargs rm -rf
+	@find . | grep -E "(__pycache__|\.pyc|\.pyo|\.pytest_cache|\.ruff_cache|\.mypy_cache|data/|site/)" | xargs rm -rf
 	@cd panel && rm -rf node_modules/ && rm -rf .next/ && rm -rf logs/ && rm -rf coverage/
 	@cd ..
 
diff --git a/roboco/api/middleware.py b/roboco/api/middleware.py
index fbbcc499..a97cfc58 100644
--- a/roboco/api/middleware.py
+++ b/roboco/api/middleware.py
@@ -12,6 +12,7 @@ from typing import Any, cast
 import structlog
 from fastapi import FastAPI, HTTPException, Request, Response
 from fastapi import status as http_status
+from fastapi.encoders import jsonable_encoder
 from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 from starlette.middleware.base import BaseHTTPMiddleware
@@ -431,7 +432,15 @@ async def request_validation_handler(request: Request, exc: Exception) -> JSONRe
         body=body_for_log,
         errors=errors,
     )
-    content: dict[str, Any] = {"detail": errors, "body": body}
+    # jsonable_encoder is mandatory: Pydantic v2 stashes the raw exception
+    # object under error['ctx']['error'] for any validator that raises
+    # ValueError (e.g. blocker_type), and a raw Exception is not JSON
+    # serializable — without this json.dumps crashes the 422 render into a
+    # 500. FastAPI's own default validation handler encodes for the same reason.
+    content: dict[str, Any] = {
+        "detail": jsonable_encoder(errors),
+        "body": jsonable_encoder(body),
+    }
     remediate = _uuid_field_remediation(errors)
     if remediate is not None:
         content["remediate"] = remediate
diff --git a/tests/unit/api/test_middleware.py b/tests/unit/api/test_middleware.py
index a516d638..baab77b6 100644
--- a/tests/unit/api/test_middleware.py
+++ b/tests/unit/api/test_middleware.py
@@ -12,7 +12,7 @@ from uuid import UUID  # noqa: TC003
 
 from fastapi import FastAPI, HTTPException
 from fastapi.testclient import TestClient
-from pydantic import BaseModel
+from pydantic import BaseModel, field_validator
 from roboco.api.middleware import (
     _uuid_field_remediation,
     get_status_code,
@@ -278,6 +278,43 @@ def test_request_validation_handler_returns_422_with_details() -> None:
     assert "body" in body
 
 
+class _EnumFieldBody(BaseModel):
+    """Mirrors IAmBlockedRequest: a field_validator that raises ValueError.
+
+    Pydantic v2 stashes the raw ValueError object in error['ctx']['error'],
+    which is not JSON-serializable — the handler must encode it (jsonable_encoder)
+    or json.dumps crashes the 422 render into a 500."""
+
+    kind: str
+
+    @field_validator("kind")
+    @classmethod
+    def _one_of(cls, v: str) -> str:
+        if v not in {"a", "b"}:
+            raise ValueError(f"kind must be one of: a | b. Got {v!r}.")
+        return v
+
+
+def test_validator_valueerror_returns_422_not_500() -> None:
+    """A field_validator ValueError (raw exc in ctx) must render a clean 422,
+    not crash the handler into a 500. Reproduces the live i_am_blocked
+    blocker_type='task_complete' crash."""
+    app = FastAPI()
+    setup_middleware(app)
+
+    @app.post("/enum")
+    async def _e(_data: _EnumFieldBody) -> Any:
+        return {"ok": True}
+
+    client = TestClient(app, raise_server_exceptions=False)
+    response = client.post("/enum", json={"kind": "task_complete"})
+    assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
+    body = response.json()
+    assert "detail" in body
+    # The human-readable validator message must survive serialization.
+    assert "kind must be one of" in str(body["detail"])
+
+
 # ---------------------------------------------------------------------------
 # secret scrubbing in the 422 log line
 # ---------------------------------------------------------------------------