feat: rework Exception/ApiError (#107)

* feat: rework Exception/ApiError

* docs: update exceptions module

* fix: docstring

docs/module/exceptions.md

@@ -21,30 +21,37 @@ init_exceptions_handlers(app=app)
 This registers handlers for:
 
 - [`ApiException`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ApiException) — all custom exceptions below
+- `HTTPException` — Starlette/FastAPI HTTP errors
 - `RequestValidationError` — Pydantic request validation (422)
 - `ResponseValidationError` — Pydantic response validation (422)
 - `Exception` — unhandled errors (500)
 
+It also patches `app.openapi()` to replace the default Pydantic 422 schema with a structured example matching the `ErrorResponse` format.
+
 ## Built-in exceptions
 
 | Exception | Status | Default message |
 |-----------|--------|-----------------|
 | [`UnauthorizedError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.UnauthorizedError) | 401 | Unauthorized |
 | [`ForbiddenError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ForbiddenError) | 403 | Forbidden |
-| [`NotFoundError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NotFoundError) | 404 | Not found |
+| [`NotFoundError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NotFoundError) | 404 | Not Found |
 | [`ConflictError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ConflictError) | 409 | Conflict |
-| [`NoSearchableFieldsError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NoSearchableFieldsError) | 400 | No searchable fields |
+| [`NoSearchableFieldsError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NoSearchableFieldsError) | 400 | No Searchable Fields |
-| [`InvalidFacetFilterError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidFacetFilterError) | 400 | Invalid facet filter |
+| [`InvalidFacetFilterError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidFacetFilterError) | 400 | Invalid Facet Filter |
+| [`InvalidOrderFieldError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidOrderFieldError) | 422 | Invalid Order Field |
+
+### Per-instance overrides
+
+All built-in exceptions accept optional keyword arguments to customise the response for a specific raise site without changing the class defaults:
+
+| Argument | Effect |
+|----------|--------|
+| `detail` | Overrides both `str(exc)` (log output) and the `message` field in the response body |
+| `desc` | Overrides the `description` field |
+| `data` | Overrides the `data` field |
 
 ```python
-from fastapi_toolsets.exceptions import NotFoundError
+raise NotFoundError(detail="User 42 not found", desc="No user with that ID exists in the database.")
-
-@router.get("/users/{id}")
-async def get_user(id: int, session: AsyncSession = Depends(get_db)):
-    user = await UserCrud.first(session=session, filters=[User.id == id])
-    if not user:
-        raise NotFoundError
-    return user
 ```
 
 ## Custom exceptions
@@ -58,12 +65,51 @@ from fastapi_toolsets.schemas import ApiError
 class PaymentRequiredError(ApiException):
     api_error = ApiError(
         code=402,
-        msg="Payment required",
+        msg="Payment Required",
         desc="Your subscription has expired.",
-        err_code="PAYMENT_REQUIRED",
+        err_code="BILLING-402",
     )
 ```
 
+!!! warning
+    Subclasses that do not define `api_error` raise a `TypeError` at **class creation time**, not at raise time.
+
+### Custom `__init__`
+
+Override `__init__` to compute `detail`, `desc`, or `data` dynamically, then delegate to `super().__init__()`:
+
+```python
+class OrderValidationError(ApiException):
+    api_error = ApiError(
+        code=422,
+        msg="Order Validation Failed",
+        desc="One or more order fields are invalid.",
+        err_code="ORDER-422",
+    )
+
+    def __init__(self, *field_errors: str) -> None:
+        super().__init__(
+            f"{len(field_errors)} validation error(s)",
+            desc=", ".join(field_errors),
+            data={"errors": [{"message": e} for e in field_errors]},
+        )
+```
+
+### Intermediate base classes
+
+Use `abstract=True` when creating a shared base that is not meant to be raised directly:
+
+```python
+class BillingError(ApiException, abstract=True):
+    """Base for all billing-related errors."""
+
+class PaymentRequiredError(BillingError):
+    api_error = ApiError(code=402, msg="Payment Required", desc="...", err_code="BILLING-402")
+
+class SubscriptionExpiredError(BillingError):
+    api_error = ApiError(code=402, msg="Subscription Expired", desc="...", err_code="BILLING-402-EXP")
+```
+
 ## OpenAPI response documentation
 
 Use [`generate_error_responses`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.generate_error_responses) to add error schemas to your endpoint's OpenAPI spec:
@@ -78,8 +124,7 @@ from fastapi_toolsets.exceptions import generate_error_responses, NotFoundError,
 async def get_user(...): ...
 ```
 
-!!! info
+Multiple exceptions sharing the same HTTP status code are grouped under one entry, each appearing as a named example keyed by its `err_code`. This keeps the OpenAPI UI readable when several error variants map to the same status.
-    The pydantic validation error is automatically added by FastAPI.
 
 ---
 

src/fastapi_toolsets/exceptions/exceptions.py

@@ -6,32 +6,46 @@ from ..schemas import ApiError, ErrorResponse, ResponseStatus
 
 
 class ApiException(Exception):
-    """Base exception for API errors with structured response.
+    """Base exception for API errors with structured response."""
-
-    Subclass this to create custom API exceptions with consistent error format.
-    The exception handler will use api_error to generate the response.
-
-    Example:
-        ```python
-        class CustomError(ApiException):
-            api_error = ApiError(
-                code=400,
-                msg="Bad Request",
-                desc="The request was invalid.",
-                err_code="CUSTOM-400",
-            )
-        ```
-    """
 
     api_error: ClassVar[ApiError]
 
-    def __init__(self, detail: str | None = None):
+    def __init_subclass__(cls, abstract: bool = False, **kwargs: Any) -> None:
+        super().__init_subclass__(**kwargs)
+        if not abstract and not hasattr(cls, "api_error"):
+            raise TypeError(
+                f"{cls.__name__} must define an 'api_error' class attribute. "
+                "Pass abstract=True when creating intermediate base classes."
+            )
+
+    def __init__(
+        self,
+        detail: str | None = None,
+        *,
+        desc: str | None = None,
+        data: Any = None,
+    ) -> None:
         """Initialize the exception.
 
         Args:
-            detail: Optional override for the error message
+            detail: Optional human-readable message
+            desc: Optional per-instance override for the ``description`` field
+                in the HTTP response body.
+            data: Optional per-instance override for the ``data`` field in the
+                HTTP response body.
         """
-        super().__init__(detail or self.api_error.msg)
+        updates: dict[str, Any] = {}
+        if detail is not None:
+            updates["msg"] = detail
+        if desc is not None:
+            updates["desc"] = desc
+        if data is not None:
+            updates["data"] = data
+        if updates:
+            object.__setattr__(
+                self, "api_error", self.__class__.api_error.model_copy(update=updates)
+            )
+        super().__init__(self.api_error.msg)
 
 
 class UnauthorizedError(ApiException):
@@ -92,14 +106,15 @@ class NoSearchableFieldsError(ApiException):
         """Initialize the exception.
 
         Args:
-            model: The SQLAlchemy model class that has no searchable fields
+            model: The model class that has no searchable fields configured.
         """
         self.model = model
-        detail = (
+        super().__init__(
-            f"No searchable fields found for model '{model.__name__}'. "
+            desc=(
-            "Provide 'search_fields' parameter or set 'searchable_fields' on the CRUD class."
+                f"No searchable fields found for model '{model.__name__}'. "
+                "Provide 'search_fields' parameter or set 'searchable_fields' on the CRUD class."
+            )
         )
-        super().__init__(detail)
 
 
 class InvalidFacetFilterError(ApiException):
@@ -116,16 +131,17 @@ class InvalidFacetFilterError(ApiException):
         """Initialize the exception.
 
         Args:
-            key: The unknown filter key provided by the caller
+            key: The unknown filter key provided by the caller.
-            valid_keys: Set of valid keys derived from the declared facet_fields
+            valid_keys: Set of valid keys derived from the declared facet_fields.
         """
         self.key = key
         self.valid_keys = valid_keys
-        detail = (
+        super().__init__(
-            f"'{key}' is not a declared facet field. "
+            desc=(
-            f"Valid keys: {sorted(valid_keys) or 'none — set facet_fields on the CRUD class'}."
+                f"'{key}' is not a declared facet field. "
+                f"Valid keys: {sorted(valid_keys) or 'none — set facet_fields on the CRUD class'}."
+            )
         )
-        super().__init__(detail)
 
 
 class InvalidOrderFieldError(ApiException):
@@ -142,15 +158,14 @@ class InvalidOrderFieldError(ApiException):
         """Initialize the exception.
 
         Args:
-            field: The unknown order field provided by the caller
+            field: The unknown order field provided by the caller.
-            valid_fields: List of valid field names
+            valid_fields: List of valid field names.
         """
         self.field = field
         self.valid_fields = valid_fields
-        detail = (
+        super().__init__(
-            f"'{field}' is not an allowed order field. Valid fields: {valid_fields}."
+            desc=f"'{field}' is not an allowed order field. Valid fields: {valid_fields}."
         )
-        super().__init__(detail)
 
 
 def generate_error_responses(
@@ -158,44 +173,39 @@ def generate_error_responses(
 ) -> dict[int | str, dict[str, Any]]:
     """Generate OpenAPI response documentation for exceptions.
 
-    Use this to document possible error responses for an endpoint.
-
     Args:
-        *errors: Exception classes that inherit from ApiException
+        *errors: Exception classes that inherit from ApiException.
 
     Returns:
-        Dict suitable for FastAPI's responses parameter
+        Dict suitable for FastAPI's ``responses`` parameter.
-
-    Example:
-        ```python
-        from fastapi_toolsets.exceptions import generate_error_responses, UnauthorizedError, ForbiddenError
-
-        @app.get(
-            "/admin",
-            responses=generate_error_responses(UnauthorizedError, ForbiddenError)
-        )
-        async def admin_endpoint():
-            ...
-        ```
     """
     responses: dict[int | str, dict[str, Any]] = {}
 
     for error in errors:
         api_error = error.api_error
+        code = api_error.code
 
-        responses[api_error.code] = {
+        if code not in responses:
-            "model": ErrorResponse,
+            responses[code] = {
-            "description": api_error.msg,
+                "model": ErrorResponse,
-            "content": {
+                "description": api_error.msg,
-                "application/json": {
+                "content": {
-                    "example": {
+                    "application/json": {
-                        "data": api_error.data,
+                        "examples": {},
-                        "status": ResponseStatus.FAIL.value,
-                        "message": api_error.msg,
-                        "description": api_error.desc,
-                        "error_code": api_error.err_code,
                     }
-                }
+                },
+            }
+
+        responses[code]["content"]["application/json"]["examples"][
+            api_error.err_code
+        ] = {
+            "summary": api_error.msg,
+            "value": {
+                "data": api_error.data,
+                "status": ResponseStatus.FAIL.value,
+                "message": api_error.msg,
+                "description": api_error.desc,
+                "error_code": api_error.err_code,
             },
         }
 

src/fastapi_toolsets/exceptions/handler.py

@@ -1,10 +1,14 @@
 """Exception handlers for FastAPI applications."""
 
+from collections.abc import Callable
 from typing import Any
 
 from fastapi import FastAPI, Request, Response, status
-from fastapi.exceptions import RequestValidationError, ResponseValidationError
+from fastapi.exceptions import (
-from fastapi.openapi.utils import get_openapi
+    HTTPException,
+    RequestValidationError,
+    ResponseValidationError,
+)
 from fastapi.responses import JSONResponse
 
 from ..schemas import ErrorResponse, ResponseStatus
@@ -14,43 +18,20 @@ from .exceptions import ApiException
 def init_exceptions_handlers(app: FastAPI) -> FastAPI:
     """Register exception handlers and custom OpenAPI schema on a FastAPI app.
 
-    Installs handlers for :class:`ApiException`, validation errors, and
-    unhandled exceptions, and replaces the default 422 schema with a
-    consistent error format.
-
     Args:
-        app: FastAPI application instance
+        app: FastAPI application instance.
 
     Returns:
-        The same FastAPI instance (for chaining)
+        The same FastAPI instance (for chaining).
-
-    Example:
-        ```python
-        from fastapi import FastAPI
-        from fastapi_toolsets.exceptions import init_exceptions_handlers
-
-        app = FastAPI()
-        init_exceptions_handlers(app)
-        ```
     """
     _register_exception_handlers(app)
-    app.openapi = lambda: _custom_openapi(app)  # type: ignore[method-assign]
+    _original_openapi = app.openapi
+    app.openapi = lambda: _patched_openapi(app, _original_openapi)  # type: ignore[method-assign]
     return app
 
 
 def _register_exception_handlers(app: FastAPI) -> None:
-    """Register all exception handlers on a FastAPI application.
+    """Register all exception handlers on a FastAPI application."""
-
-    Args:
-        app: FastAPI application instance
-
-    Example:
-        from fastapi import FastAPI
-        from fastapi_toolsets.exceptions import init_exceptions_handlers
-
-        app = FastAPI()
-        init_exceptions_handlers(app)
-    """
 
     @app.exception_handler(ApiException)
     async def api_exception_handler(request: Request, exc: ApiException) -> Response:
@@ -62,12 +43,25 @@ def _register_exception_handlers(app: FastAPI) -> None:
             description=api_error.desc,
             error_code=api_error.err_code,
         )
-
         return JSONResponse(
             status_code=api_error.code,
             content=error_response.model_dump(),
         )
 
+    @app.exception_handler(HTTPException)
+    async def http_exception_handler(request: Request, exc: HTTPException) -> Response:
+        """Handle Starlette/FastAPI HTTPException with a consistent error format."""
+        detail = exc.detail if isinstance(exc.detail, str) else "HTTP Error"
+        error_response = ErrorResponse(
+            message=detail,
+            error_code=f"HTTP-{exc.status_code}",
+        )
+        return JSONResponse(
+            status_code=exc.status_code,
+            content=error_response.model_dump(),
+            headers=getattr(exc, "headers", None),
+        )
+
     @app.exception_handler(RequestValidationError)
     async def request_validation_handler(
         request: Request, exc: RequestValidationError
@@ -90,7 +84,6 @@ def _register_exception_handlers(app: FastAPI) -> None:
             description="An unexpected error occurred. Please try again later.",
             error_code="SERVER-500",
         )
-
         return JSONResponse(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
             content=error_response.model_dump(),
@@ -105,11 +98,10 @@ def _format_validation_error(
     formatted_errors = []
 
     for error in errors:
-        field_path = ".".join(
+        locs = error["loc"]
-            str(loc)
+        if locs and locs[0] in ("body", "query", "path", "header", "cookie"):
-            for loc in error["loc"]
+            locs = locs[1:]
-            if loc not in ("body", "query", "path", "header", "cookie")
+        field_path = ".".join(str(loc) for loc in locs)
-        )
         formatted_errors.append(
             {
                 "field": field_path or "root",
@@ -131,34 +123,22 @@ def _format_validation_error(
     )
 
 
-def _custom_openapi(app: FastAPI) -> dict[str, Any]:
+def _patched_openapi(
-    """Generate custom OpenAPI schema with standardized error format.
+    app: FastAPI, original_openapi: Callable[[], dict[str, Any]]
-
+) -> dict[str, Any]:
-    Replaces default 422 validation error responses with the custom format.
+    """Generate the OpenAPI schema and replace default 422 responses.
 
     Args:
-        app: FastAPI application instance
+        app: FastAPI application instance.
+        original_openapi: The previous ``app.openapi`` callable to delegate to.
 
     Returns:
-        OpenAPI schema dict
+        Patched OpenAPI schema dict.
-
-    Example:
-        from fastapi import FastAPI
-        from fastapi_toolsets.exceptions import init_exceptions_handlers
-
-        app = FastAPI()
-        init_exceptions_handlers(app)  # Automatically sets custom OpenAPI
     """
     if app.openapi_schema:
         return app.openapi_schema
 
-    openapi_schema = get_openapi(
+    openapi_schema = original_openapi()
-        title=app.title,
-        version=app.version,
-        openapi_version=app.openapi_version,
-        description=app.description,
-        routes=app.routes,
-    )
 
     for path_data in openapi_schema.get("paths", {}).values():
         for operation in path_data.values():
@@ -168,20 +148,25 @@ def _custom_openapi(app: FastAPI) -> dict[str, Any]:
                         "description": "Validation Error",
                         "content": {
                             "application/json": {
-                                "example": {
+                                "examples": {
-                                    "data": {
+                                    "VAL-422": {
-                                        "errors": [
+                                        "summary": "Validation Error",
-                                            {
+                                        "value": {
-                                                "field": "field_name",
+                                            "data": {
-                                                "message": "value is not valid",
+                                                "errors": [
-                                                "type": "value_error",
+                                                    {
-                                            }
+                                                        "field": "field_name",
-                                        ]
+                                                        "message": "value is not valid",
-                                    },
+                                                        "type": "value_error",
-                                    "status": ResponseStatus.FAIL.value,
+                                                    }
-                                    "message": "Validation Error",
+                                                ]
-                                    "description": "1 validation error(s) detected",
+                                            },
-                                    "error_code": "VAL-422",
+                                            "status": ResponseStatus.FAIL.value,
+                                            "message": "Validation Error",
+                                            "description": "1 validation error(s) detected",
+                                            "error_code": "VAL-422",
+                                        },
+                                    }
                                 }
                             }
                         },

tests/test_crud_search.py

@@ -410,7 +410,7 @@ class TestNoSearchableFieldsError:
         from fastapi_toolsets.exceptions import NoSearchableFieldsError
 
         error = NoSearchableFieldsError(User)
-        assert "User" in str(error)
+        assert "User" in error.api_error.desc
         assert error.model is User
 
     def test_error_raised_when_no_fields(self):
@@ -434,7 +434,7 @@ class TestNoSearchableFieldsError:
             build_search_filters(NoStringModel, "test")
 
         assert exc_info.value.model is NoStringModel
-        assert "NoStringModel" in str(exc_info.value)
+        assert "NoStringModel" in exc_info.value.api_error.desc
 
 
 class TestGetSearchableFields:

tests/test_exceptions.py

@@ -2,6 +2,7 @@
 
 import pytest
 from fastapi import FastAPI
+from fastapi.exceptions import HTTPException
 from fastapi.testclient import TestClient
 
 from fastapi_toolsets.exceptions import (
@@ -36,8 +37,8 @@ class TestApiException:
         assert error.api_error.msg == "I'm a teapot"
         assert str(error) == "I'm a teapot"
 
-    def test_custom_detail_message(self):
+    def test_detail_overrides_msg_and_str(self):
-        """Custom detail overrides default message."""
+        """detail sets both str(exc) and api_error.msg; class-level msg is unchanged."""
 
         class CustomError(ApiException):
             api_error = ApiError(
@@ -47,8 +48,172 @@ class TestApiException:
                 err_code="BAD-400",
             )
 
-        error = CustomError("Custom message")
+        error = CustomError("Widget not found")
-        assert str(error) == "Custom message"
+        assert str(error) == "Widget not found"
+        assert error.api_error.msg == "Widget not found"
+        assert CustomError.api_error.msg == "Bad Request"  # class unchanged
+
+    def test_desc_override(self):
+        """desc kwarg overrides api_error.desc on the instance only."""
+
+        class MyError(ApiException):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Default.", err_code="ERR-400"
+            )
+
+        err = MyError(desc="Custom desc.")
+        assert err.api_error.desc == "Custom desc."
+        assert MyError.api_error.desc == "Default."  # class unchanged
+
+    def test_data_override(self):
+        """data kwarg sets api_error.data on the instance only."""
+
+        class MyError(ApiException):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Default.", err_code="ERR-400"
+            )
+
+        err = MyError(data={"key": "value"})
+        assert err.api_error.data == {"key": "value"}
+        assert MyError.api_error.data is None  # class unchanged
+
+    def test_desc_and_data_override(self):
+        """detail, desc and data can all be overridden together."""
+
+        class MyError(ApiException):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Default.", err_code="ERR-400"
+            )
+
+        err = MyError("custom msg", desc="New desc.", data={"x": 1})
+        assert str(err) == "custom msg"
+        assert err.api_error.msg == "custom msg"  # detail also updates msg
+        assert err.api_error.desc == "New desc."
+        assert err.api_error.data == {"x": 1}
+        assert err.api_error.code == 400  # other fields unchanged
+
+    def test_class_api_error_not_mutated_after_instance_override(self):
+        """Raising with desc/data does not mutate the class-level api_error."""
+
+        class MyError(ApiException):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Default.", err_code="ERR-400"
+            )
+
+        MyError(desc="Changed", data={"x": 1})
+        assert MyError.api_error.desc == "Default."
+        assert MyError.api_error.data is None
+
+    def test_subclass_uses_super_with_desc_and_data(self):
+        """Subclasses can delegate detail/desc/data to super().__init__()."""
+
+        class BuildValidationError(ApiException):
+            api_error = ApiError(
+                code=422,
+                msg="Build Validation Error",
+                desc="The build configuration is invalid.",
+                err_code="BUILD-422",
+            )
+
+            def __init__(self, *errors: str) -> None:
+                super().__init__(
+                    f"{len(errors)} validation error(s)",
+                    desc=", ".join(errors),
+                    data={"errors": [{"message": e} for e in errors]},
+                )
+
+        err = BuildValidationError("Field A is required", "Field B is invalid")
+        assert str(err) == "2 validation error(s)"
+        assert err.api_error.msg == "2 validation error(s)"  # detail set msg
+        assert err.api_error.desc == "Field A is required, Field B is invalid"
+        assert err.api_error.data == {
+            "errors": [
+                {"message": "Field A is required"},
+                {"message": "Field B is invalid"},
+            ]
+        }
+        assert err.api_error.code == 422  # other fields unchanged
+
+    def test_detail_desc_data_in_http_response(self):
+        """detail/desc/data overrides all appear correctly in the FastAPI HTTP response."""
+
+        class DynamicError(ApiException):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Default.", err_code="ERR-400"
+            )
+
+            def __init__(self, message: str) -> None:
+                super().__init__(
+                    message,
+                    desc=f"Detail: {message}",
+                    data={"reason": message},
+                )
+
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/error")
+        async def raise_error():
+            raise DynamicError("something went wrong")
+
+        client = TestClient(app)
+        response = client.get("/error")
+
+        assert response.status_code == 400
+        body = response.json()
+        assert body["message"] == "something went wrong"
+        assert body["description"] == "Detail: something went wrong"
+        assert body["data"] == {"reason": "something went wrong"}
+
+
+class TestApiExceptionGuard:
+    """Tests for the __init_subclass__ api_error guard."""
+
+    def test_missing_api_error_raises_type_error(self):
+        """Defining a subclass without api_error raises TypeError at class creation time."""
+        with pytest.raises(
+            TypeError, match="must define an 'api_error' class attribute"
+        ):
+
+            class BrokenError(ApiException):
+                pass
+
+    def test_abstract_subclass_skips_guard(self):
+        """abstract=True allows intermediate base classes without api_error."""
+
+        class BaseGroupError(ApiException, abstract=True):
+            pass
+
+        # Concrete child must still define it
+        class ConcreteError(BaseGroupError):
+            api_error = ApiError(
+                code=400, msg="Error", desc="Desc.", err_code="ERR-400"
+            )
+
+        err = ConcreteError()
+        assert err.api_error.code == 400
+
+    def test_abstract_child_still_requires_api_error_on_concrete(self):
+        """Concrete subclass of an abstract class must define api_error."""
+
+        class Base(ApiException, abstract=True):
+            pass
+
+        with pytest.raises(
+            TypeError, match="must define an 'api_error' class attribute"
+        ):
+
+            class Concrete(Base):
+                pass
+
+    def test_inherited_api_error_satisfies_guard(self):
+        """Subclass that inherits api_error from a parent does not need its own."""
+
+        class ConcreteError(NotFoundError):
+            pass
+
+        err = ConcreteError()
+        assert err.api_error.code == 404
 
 
 class TestBuiltInExceptions:
@@ -90,7 +255,7 @@ class TestGenerateErrorResponses:
         assert responses[404]["description"] == "Not Found"
 
     def test_generates_multiple_responses(self):
-        """Generates responses for multiple exceptions."""
+        """Generates responses for multiple exceptions with distinct status codes."""
         responses = generate_error_responses(
             UnauthorizedError,
             ForbiddenError,
@@ -101,15 +266,24 @@ class TestGenerateErrorResponses:
         assert 403 in responses
         assert 404 in responses
 
-    def test_response_has_example(self):
+    def test_response_has_named_example(self):
-        """Generated response includes example."""
+        """Generated response uses named examples keyed by err_code."""
         responses = generate_error_responses(NotFoundError)
-        example = responses[404]["content"]["application/json"]["example"]
+        examples = responses[404]["content"]["application/json"]["examples"]
 
-        assert example["status"] == "FAIL"
+        assert "RES-404" in examples
-        assert example["error_code"] == "RES-404"
+        value = examples["RES-404"]["value"]
-        assert example["message"] == "Not Found"
+        assert value["status"] == "FAIL"
-        assert example["data"] is None
+        assert value["error_code"] == "RES-404"
+        assert value["message"] == "Not Found"
+        assert value["data"] is None
+
+    def test_response_example_has_summary(self):
+        """Each named example carries a summary equal to api_error.msg."""
+        responses = generate_error_responses(NotFoundError)
+        example = responses[404]["content"]["application/json"]["examples"]["RES-404"]
+
+        assert example["summary"] == "Not Found"
 
     def test_response_example_with_data(self):
         """Generated response includes data when set on ApiError."""
@@ -124,9 +298,49 @@ class TestGenerateErrorResponses:
             )
 
         responses = generate_error_responses(ErrorWithData)
-        example = responses[400]["content"]["application/json"]["example"]
+        value = responses[400]["content"]["application/json"]["examples"]["BAD-400"][
+            "value"
+        ]
 
-        assert example["data"] == {"details": "some context"}
+        assert value["data"] == {"details": "some context"}
+
+    def test_two_errors_same_code_both_present(self):
+        """Two exceptions with the same HTTP code produce two named examples."""
+
+        class BadRequestA(ApiException):
+            api_error = ApiError(
+                code=400, msg="Bad A", desc="Reason A.", err_code="ERR-A"
+            )
+
+        class BadRequestB(ApiException):
+            api_error = ApiError(
+                code=400, msg="Bad B", desc="Reason B.", err_code="ERR-B"
+            )
+
+        responses = generate_error_responses(BadRequestA, BadRequestB)
+
+        assert 400 in responses
+        examples = responses[400]["content"]["application/json"]["examples"]
+        assert "ERR-A" in examples
+        assert "ERR-B" in examples
+        assert examples["ERR-A"]["value"]["message"] == "Bad A"
+        assert examples["ERR-B"]["value"]["message"] == "Bad B"
+
+    def test_two_errors_same_code_single_top_level_entry(self):
+        """Two exceptions with the same HTTP code produce exactly one top-level entry."""
+
+        class BadRequestA(ApiException):
+            api_error = ApiError(
+                code=400, msg="Bad A", desc="Reason A.", err_code="ERR-A"
+            )
+
+        class BadRequestB(ApiException):
+            api_error = ApiError(
+                code=400, msg="Bad B", desc="Reason B.", err_code="ERR-B"
+            )
+
+        responses = generate_error_responses(BadRequestA, BadRequestB)
+        assert len([k for k in responses if k == 400]) == 1
 
 
 class TestInitExceptionsHandlers:
@@ -250,13 +464,68 @@ class TestInitExceptionsHandlers:
         assert data["status"] == "FAIL"
         assert data["error_code"] == "SERVER-500"
 
-    def test_custom_openapi_schema(self):
+    def test_handles_http_exception(self):
-        """Customizes OpenAPI schema for 422 responses."""
+        """Handles starlette HTTPException with consistent ErrorResponse envelope."""
         app = FastAPI()
         init_exceptions_handlers(app)
 
+        @app.get("/protected")
+        async def protected():
+            raise HTTPException(status_code=403, detail="Forbidden")
+
+        client = TestClient(app)
+        response = client.get("/protected")
+
+        assert response.status_code == 403
+        data = response.json()
+        assert data["status"] == "FAIL"
+        assert data["error_code"] == "HTTP-403"
+        assert data["message"] == "Forbidden"
+
+    def test_handles_http_exception_404_from_route(self):
+        """HTTPException(404) raised inside a route uses the consistent ErrorResponse envelope."""
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/items/{item_id}")
+        async def get_item(item_id: int):
+            raise HTTPException(status_code=404, detail="Item not found")
+
+        client = TestClient(app)
+        response = client.get("/items/99")
+
+        assert response.status_code == 404
+        data = response.json()
+        assert data["status"] == "FAIL"
+        assert data["error_code"] == "HTTP-404"
+        assert data["message"] == "Item not found"
+
+    def test_handles_http_exception_forwards_headers(self):
+        """HTTPException with WWW-Authenticate header forwards it in the response."""
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/secure")
+        async def secure():
+            raise HTTPException(
+                status_code=401,
+                detail="Not authenticated",
+                headers={"WWW-Authenticate": "Bearer"},
+            )
+
+        client = TestClient(app)
+        response = client.get("/secure")
+
+        assert response.status_code == 401
+        assert response.headers.get("www-authenticate") == "Bearer"
+
+    def test_custom_openapi_schema(self):
+        """Customises OpenAPI schema for 422 responses using named examples."""
         from pydantic import BaseModel
 
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
         class Item(BaseModel):
             name: str
 
@@ -269,8 +538,128 @@ class TestInitExceptionsHandlers:
         post_op = openapi["paths"]["/items"]["post"]
         assert "422" in post_op["responses"]
         resp_422 = post_op["responses"]["422"]
-        example = resp_422["content"]["application/json"]["example"]
+        examples = resp_422["content"]["application/json"]["examples"]
-        assert example["error_code"] == "VAL-422"
+        assert "VAL-422" in examples
+        assert examples["VAL-422"]["value"]["error_code"] == "VAL-422"
+
+    def test_custom_openapi_preserves_app_metadata(self):
+        """_patched_openapi preserves custom FastAPI app-level metadata."""
+        app = FastAPI(
+            title="My API",
+            version="2.0.0",
+            description="Custom description",
+        )
+        init_exceptions_handlers(app)
+
+        schema = app.openapi()
+        assert schema["info"]["title"] == "My API"
+        assert schema["info"]["version"] == "2.0.0"
+
+    def test_handles_response_validation_error(self):
+        """Handles ResponseValidationError with a structured 422 response."""
+        from pydantic import BaseModel
+
+        class CountResponse(BaseModel):
+            count: int
+
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/broken", response_model=CountResponse)
+        async def broken():
+            return {"count": "not-a-number"}  # triggers ResponseValidationError
+
+        client = TestClient(app, raise_server_exceptions=False)
+        response = client.get("/broken")
+
+        assert response.status_code == 422
+        data = response.json()
+        assert data["status"] == "FAIL"
+        assert data["error_code"] == "VAL-422"
+        assert "errors" in data["data"]
+
+    def test_handles_validation_error_with_non_standard_loc(self):
+        """Validation error with empty loc tuple maps the field to 'root'."""
+        from fastapi.exceptions import RequestValidationError
+
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/root-error")
+        async def root_error():
+            raise RequestValidationError(
+                [
+                    {
+                        "type": "custom",
+                        "loc": (),
+                        "msg": "root level error",
+                        "input": None,
+                        "url": "",
+                    }
+                ]
+            )
+
+        client = TestClient(app)
+        response = client.get("/root-error")
+
+        assert response.status_code == 422
+        data = response.json()
+        assert data["data"]["errors"][0]["field"] == "root"
+
+    def test_openapi_schema_cached_after_first_call(self):
+        """app.openapi() returns the cached schema on subsequent calls."""
+        from pydantic import BaseModel
+
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        class Item(BaseModel):
+            name: str
+
+        @app.post("/items")
+        async def create_item(item: Item):
+            return item
+
+        schema_first = app.openapi()
+        schema_second = app.openapi()
+        assert schema_first is schema_second
+
+    def test_openapi_skips_operations_without_422(self):
+        """_patched_openapi leaves operations that have no 422 response unchanged."""
+        app = FastAPI()
+        init_exceptions_handlers(app)
+
+        @app.get("/ping")
+        async def ping():
+            return {"ok": True}
+
+        schema = app.openapi()
+        get_op = schema["paths"]["/ping"]["get"]
+        assert "422" not in get_op["responses"]
+        assert "200" in get_op["responses"]
+
+    def test_openapi_skips_non_dict_path_item_values(self):
+        """_patched_openapi ignores non-dict values in path items (e.g. path-level parameters)."""
+        from fastapi_toolsets.exceptions.handler import _patched_openapi
+
+        app = FastAPI()
+
+        def fake_openapi() -> dict:
+            return {
+                "paths": {
+                    "/items": {
+                        "parameters": [
+                            {"name": "q", "in": "query"}
+                        ],  # list, not a dict
+                        "get": {"responses": {"200": {"description": "OK"}}},
+                    }
+                }
+            }
+
+        schema = _patched_openapi(app, fake_openapi)
+        # The list value was skipped without error; the GET operation is intact
+        assert schema["paths"]["/items"]["parameters"] == [{"name": "q", "in": "query"}]
+        assert "422" not in schema["paths"]["/items"]["get"]["responses"]
 
 
 class TestExceptionIntegration:
@@ -352,12 +741,12 @@ class TestInvalidOrderFieldError:
         assert error.field == "unknown"
         assert error.valid_fields == ["name", "created_at"]
 
-    def test_message_contains_field_and_valid_fields(self):
+    def test_description_contains_field_and_valid_fields(self):
-        """Exception message mentions the bad field and valid options."""
+        """api_error.desc mentions the bad field and valid options."""
         error = InvalidOrderFieldError("bad_field", ["name", "email"])
-        assert "bad_field" in str(error)
+        assert "bad_field" in error.api_error.desc
-        assert "name" in str(error)
+        assert "name" in error.api_error.desc
-        assert "email" in str(error)
+        assert "email" in error.api_error.desc
 
     def test_handled_as_422_by_exception_handler(self):
         """init_exceptions_handlers turns InvalidOrderFieldError into a 422 response."""