diff --git a/src/sentry/api/endpoints/organization_trace_item_attributes.py b/src/sentry/api/endpoints/organization_trace_item_attributes.py
index 08ce0cafb2129e..044c3198459ecb 100644
--- a/src/sentry/api/endpoints/organization_trace_item_attributes.py
+++ b/src/sentry/api/endpoints/organization_trace_item_attributes.py
@@ -47,6 +47,7 @@
 from sentry.apidocs.constants import RESPONSE_FORBIDDEN, RESPONSE_NOT_FOUND, RESPONSE_UNAUTHORIZED
 from sentry.apidocs.examples.trace_item_attribute_examples import TraceItemAttributeExamples
 from sentry.apidocs.parameters import CursorQueryParam, GlobalParams
+from sentry.apidocs.response_types import ValidationErrorResponse, as_validation_errors
 from sentry.apidocs.utils import inline_sentry_response_serializer
 from sentry.auth.staff import is_active_staff
 from sentry.auth.superuser import is_active_superuser
@@ -362,7 +363,9 @@ class OrganizationTraceItemAttributesEndpoint(OrganizationTraceItemAttributesEnd
         },
         examples=TraceItemAttributeExamples.LIST_TRACE_ITEM_ATTRIBUTES,
     )
-    def get(self, request: Request, organization: Organization) -> Response:
+    def get(
+        self, request: Request, organization: Organization
+    ) -> Response[list[TraceItemAttributeKey]] | Response[ValidationErrorResponse]:
         """
         List the attribute keys available on a given trace item dataset (spans, logs,
         trace metrics, etc.), with optional substring and structured filtering.
@@ -372,7 +375,7 @@ def get(self, request: Request, organization: Organization) -> Response:
 
         serializer = OrganizationTraceItemAttributesEndpointSerializer(data=request.GET)
         if not serializer.is_valid():
-            return Response(serializer.errors, status=400)
+            return Response(as_validation_errors(serializer), status=400)
 
         try:
             snuba_params = self.get_snuba_params(request, organization)
diff --git a/src/sentry/api/endpoints/project_filters.py b/src/sentry/api/endpoints/project_filters.py
index 4222aa0d1cd821..197613aaa4c361 100644
--- a/src/sentry/api/endpoints/project_filters.py
+++ b/src/sentry/api/endpoints/project_filters.py
@@ -42,12 +42,12 @@ class ProjectFiltersEndpoint(ProjectEndpoint):
         },
         examples=ProjectExamples.GET_PROJECT_FILTERS,
     )
-    def get(self, request: Request, project) -> Response:
+    def get(self, request: Request, project) -> Response[list[ProjectFilterResponse]]:
         """
         Retrieve a list of filters for a given project.
         `active` will be either a boolean or a list for the legacy browser filters.
         """
-        results = []
+        results: list[ProjectFilterResponse] = []
         for flt in inbound_filters.get_all_filter_specs():
             results.append(
                 {
diff --git a/src/sentry/apidocs/response_types.py b/src/sentry/apidocs/response_types.py
index 877d928e1eed83..48736e24f6f6d7 100644
--- a/src/sentry/apidocs/response_types.py
+++ b/src/sentry/apidocs/response_types.py
@@ -1,16 +1,18 @@
-"""Shared TypedDicts for endpoint Response annotations.
+"""Shared response shapes and helpers for endpoint Response annotations.
 
-This module holds TypedDict shapes that recur across multiple endpoints.
-It is *not* authoritative — endpoints whose error/response shapes don't
-match anything here are expected to declare local TypedDicts in their
-own files. The structural linter at
-`sentry.apidocs._check_response_annotation_matches_schema` is name-agnostic;
-it does not special-case any TypedDict name in this module.
+This module holds TypedDicts, type aliases, and small helpers that recur
+across multiple endpoints in the Response[T] typing rollout. It is *not*
+authoritative — endpoints whose error/response shapes don't match anything
+here are expected to declare local types in their own files. The structural
+linter at `sentry.apidocs._check_response_annotation_matches_schema` is
+name-agnostic; it does not special-case any name in this module.
 
 `DetailResponse` is included because DRF's exception handler renders every
 uncaught `APIException` subclass as `{"detail": "..."}` and a non-trivial
-number of endpoints return that shape inline. Other shapes can graduate
-into this module as they emerge from real usage.
+number of endpoints return that shape inline.
+
+`ValidationErrorResponse` + `as_validation_errors()` cover the parallel
+case for DRF `Response(serializer.errors, status=400)` paths.
 
 The module is named `response_types` rather than `types` to avoid shadowing
 Python's stdlib `types` module under subprocess tooling (e.g. some prek hooks).
@@ -18,7 +20,9 @@
 
 from __future__ import annotations
 
-from typing import TypedDict
+from typing import Any, TypeAlias, TypedDict
+
+from rest_framework import serializers
 
 
 class DetailResponse(TypedDict):
@@ -31,3 +35,41 @@ class DetailResponse(TypedDict):
     """
 
     detail: str
+
+
+ValidationErrorResponse: TypeAlias = dict[str, Any]
+"""DRF's validation-error body shape: `{field_name: <errors>, ...}`.
+
+DRF emits a few different value shapes here depending on the serializer:
+- Flat field errors: `{"field": ["error msg", ...]}`
+- Nested (e.g. `Serializer` with a nested `Serializer` field):
+  `{"field": {"nested_field": ["error msg", ...]}}`
+- Non-field errors (raised in `validate()`):
+  `{"non_field_errors": ["error msg", ...]}`
+
+The alias is intentionally `dict[str, Any]` — narrower types like
+`dict[str, list[str]]` collapse the nested-dict case and lose the error
+messages at runtime. The runtime value of `serializer.errors` is a
+`ReturnDict[Any, Any]` that mypy can't structurally match against any
+typed `Response[T]` union arm, so use this alias as the union arm:
+
+    def post(...) -> Response[FooResponse] | Response[ValidationErrorResponse]:
+
+and produce the body via `as_validation_errors(serializer)` below.
+"""
+
+
+def as_validation_errors(
+    serializer: serializers.Serializer[Any],
+) -> ValidationErrorResponse:
+    """Project a DRF `Serializer.errors` ReturnDict into a structurally typed
+    `dict[str, Any]` so a `Response[ValidationErrorResponse]` union arm is
+    satisfied without `cast()`. The DRF error structure (flat or nested) is
+    preserved verbatim — only the static type is narrowed.
+
+    Use immediately after `not serializer.is_valid()`:
+
+        if not serializer.is_valid():
+            return Response(as_validation_errors(serializer), status=400)
+    """
+    return dict(serializer.errors)
diff --git a/src/sentry/replays/endpoints/project_replay_jobs_delete.py b/src/sentry/replays/endpoints/project_replay_jobs_delete.py
index 32dd4ccbbf127d..1750ba27fe6a84 100644
--- a/src/sentry/replays/endpoints/project_replay_jobs_delete.py
+++ b/src/sentry/replays/endpoints/project_replay_jobs_delete.py
@@ -18,6 +18,7 @@
 from sentry.apidocs.constants import RESPONSE_BAD_REQUEST, RESPONSE_FORBIDDEN, RESPONSE_NOT_FOUND
 from sentry.apidocs.examples.replay_examples import ReplayExamples
 from sentry.apidocs.parameters import GlobalParams, ReplayParams
+from sentry.apidocs.response_types import ValidationErrorResponse, as_validation_errors
 from sentry.apidocs.utils import inline_sentry_response_serializer
 from sentry.replays.endpoints.project_replay_endpoint import ProjectReplayEndpoint
 from sentry.replays.models import ReplayDeletionJobModel
@@ -109,7 +110,7 @@ class ProjectReplayDeletionJobsIndexEndpoint(ProjectEndpoint):
         },
         examples=ReplayExamples.GET_REPLAY_DELETION_JOBS,
     )
-    def get(self, request: Request, project) -> Response:
+    def get(self, request: Request, project) -> Response[ReplayDeletionJobListResponse]:
         """
         Retrieve a collection of replay delete jobs.
         """
@@ -146,7 +147,9 @@ def get(self, request: Request, project) -> Response:
         },
         examples=ReplayExamples.CREATE_REPLAY_DELETION_JOB,
     )
-    def post(self, request: Request, project) -> Response:
+    def post(
+        self, request: Request, project
+    ) -> Response[ReplayDeletionJobDetailResponse] | Response[ValidationErrorResponse]:
         """
         Create a new replay deletion job.
         """
@@ -155,7 +158,7 @@ def post(self, request: Request, project) -> Response:
 
         serializer = ReplayDeletionJobCreateSerializer(data=request.data)
         if not serializer.is_valid():
-            return Response(serializer.errors, status=400)
+            return Response(as_validation_errors(serializer), status=400)
 
         data = serializer.validated_data["data"]
 
@@ -186,7 +189,7 @@ def post(self, request: Request, project) -> Response:
         )
 
         response_data = serialize(job, request.user, ReplayDeletionJobSerializer())
-        response = {"data": response_data}
+        response: ReplayDeletionJobDetailResponse = {"data": response_data}
 
         return Response(response, status=201)
 
@@ -215,7 +218,9 @@ class ProjectReplayDeletionJobDetailEndpoint(ProjectReplayEndpoint):
         },
         examples=ReplayExamples.GET_REPLAY_DELETION_JOB,
     )
-    def get(self, request: Request, project, job_id: int) -> Response:
+    def get(
+        self, request: Request, project, job_id: int
+    ) -> Response[ReplayDeletionJobDetailResponse]:
         """
         Fetch a replay delete job instance.
         """