diff --git a/src/sentry/api/base.py b/src/sentry/api/base.py
index 1de6858d72bd..9d04c18503c6 100644
--- a/src/sentry/api/base.py
+++ b/src/sentry/api/base.py
@@ -268,6 +268,9 @@ class Endpoint(APIView):
 
     owner: ApiOwner = ApiOwner.UNOWNED
     publish_status: dict[HTTP_METHOD_NAME, ApiPublishStatus] = {}
+    # Non-safe methods that a client may resend after a timeout or 5xx without
+    # changing the outcome. Published as `x-sentry-idempotent`; see apidocs/README.md.
+    idempotent_methods: frozenset[HTTP_METHOD_NAME] = frozenset()
     rate_limits: RateLimitConfig | Callable[..., RateLimitConfig] = DEFAULT_RATE_LIMIT_CONFIG
     enforce_rate_limit: bool = settings.SENTRY_RATELIMITER_ENABLED
     servers: list[dict[str, Any]] | None = None
diff --git a/src/sentry/api/endpoints/chunk.py b/src/sentry/api/endpoints/chunk.py
index acc08153a121..92281ab1df86 100644
--- a/src/sentry/api/endpoints/chunk.py
+++ b/src/sentry/api/endpoints/chunk.py
@@ -132,6 +132,8 @@ class ChunkUploadEndpoint(OrganizationEndpoint):
         "GET": ApiPublishStatus.PRIVATE,
         "POST": ApiPublishStatus.PRIVATE,
     }
+    # Blobs are unique per checksum, so resending a chunk leaves the same single blob.
+    idempotent_methods = frozenset({"POST"})
     owner = ApiOwner.OWNERS_INGEST
 
     authentication_classes = OrganizationEndpoint.authentication_classes + (
diff --git a/src/sentry/api/endpoints/debug_files.py b/src/sentry/api/endpoints/debug_files.py
index bf6ec565f6a3..aac427d32140 100644
--- a/src/sentry/api/endpoints/debug_files.py
+++ b/src/sentry/api/endpoints/debug_files.py
@@ -985,6 +985,9 @@ class DifAssembleEndpoint(ProjectEndpoint):
     publish_status = {
         "POST": ApiPublishStatus.PRIVATE,
     }
+    # Assemble state is cached per checksum, so a resend reports it instead of
+    # starting a second job.
+    idempotent_methods = frozenset({"POST"})
     permission_classes = (ProjectReleasePermission,)
 
     def post(self, request: Request, project: Project) -> Response:
diff --git a/src/sentry/api/endpoints/organization_artifactbundle_assemble.py b/src/sentry/api/endpoints/organization_artifactbundle_assemble.py
index bc1a5b42e55c..d67459f8d735 100644
--- a/src/sentry/api/endpoints/organization_artifactbundle_assemble.py
+++ b/src/sentry/api/endpoints/organization_artifactbundle_assemble.py
@@ -28,6 +28,9 @@ class OrganizationArtifactBundleAssembleEndpoint(OrganizationReleasesBaseEndpoin
     publish_status = {
         "POST": ApiPublishStatus.PRIVATE,
     }
+    # Assemble state is cached per checksum, so a resend reports it instead of
+    # starting a second job.
+    idempotent_methods = frozenset({"POST"})
 
     def post(self, request: Request, organization) -> Response:
         """
diff --git a/src/sentry/apidocs/README.md b/src/sentry/apidocs/README.md
index c0a295c8d279..36aff8341162 100644
--- a/src/sentry/apidocs/README.md
+++ b/src/sentry/apidocs/README.md
@@ -45,3 +45,24 @@ An organization's public locality URL remains runtime information supplied by
 bootstrap, configuration, organization listings (`links.regionUrl`), or optional
 discovery. This extension contains no deployment URLs, credentials, private cell
 addresses, cache policy, or automatic host selection.
+
+# Idempotent operations
+
+An operation carries `"x-sentry-idempotent": true` when its endpoint lists the
+method in `idempotent_methods`:
+
+```python
+class DifAssembleEndpoint(ProjectEndpoint):
+    publish_status = {"POST": ApiPublishStatus.PRIVATE}
+    idempotent_methods = frozenset({"POST"})
+```
+
+It means a client may resend the identical request after a timeout or a 5xx
+response without changing the outcome, so generated clients can retry it like a
+`GET`. Only list methods that are not already safe (RFC 9110 §9.2.1): the build
+fails for `GET`, `HEAD`, `OPTIONS`, `TRACE`, or a method the endpoint does not
+declare in `publish_status`. A missing marker means "not known to be idempotent",
+including `PUT` and `DELETE`, whose handlers do not all live up to RFC 9110 §9.2.2.
+
+Like routing, declarations are collected for every endpoint and reset on every
+build. The marker appears only on operations that are in the generated schema.
diff --git a/src/sentry/apidocs/hooks.py b/src/sentry/apidocs/hooks.py
index 6075bd3fa6b7..d413163ac294 100644
--- a/src/sentry/apidocs/hooks.py
+++ b/src/sentry/apidocs/hooks.py
@@ -144,6 +144,13 @@ class RoutingMetadata(TypedDict):
     organizationParameter: NotRequired[str]
 
 
+# (canonical path, lowercased method) pairs whose endpoint lists the method in
+# `idempotent_methods`. Collected for unpublished endpoints too, like silos.
+_IDEMPOTENT_OPERATIONS: set[tuple[str, str]] = set()
+
+# RFC 9110 9.2.1: these never change server state, so clients may always resend them.
+_SAFE_METHODS = frozenset({"GET", "HEAD", "OPTIONS", "TRACE"})
+
 # Include non-spectacular endpoints so existing APPEND_PATHS operations can use
 # their backend declaration without changing which methods are published.
 _OPERATION_SILOS: dict[tuple[str, str], DeclaredSilo] = {}
@@ -176,6 +183,7 @@ def custom_preprocessing_hook(endpoints: Any) -> Any:  # TODO: organize method,
     _ENDPOINT_SERVERS.clear()
     _EXPERIMENTAL_OPERATIONS.clear()
     _OPERATION_SILOS.clear()
+    _IDEMPOTENT_OPERATIONS.clear()
 
     filtered = []
     ownership_data: dict[ApiOwner, dict] = {}
@@ -208,13 +216,26 @@ def custom_preprocessing_hook(endpoints: Any) -> Any:  # TODO: organize method,
                 f"All methods must declare a publish_status. Please add a valid publish status for Endpoint {callback.view_class} {method} method.",
             )
 
+        idempotent_methods: frozenset[str] = getattr(
+            callback.view_class, "idempotent_methods", frozenset()
+        )
+        if not idempotent_methods <= callback.view_class.publish_status.keys():
+            raise SentryApiBuildError(
+                f"Endpoint {callback.view_class} declares idempotent_methods that it does not implement: {sorted(idempotent_methods - callback.view_class.publish_status.keys())}.",
+            )
+        if idempotent_methods & _SAFE_METHODS:
+            raise SentryApiBuildError(
+                f"Endpoint {callback.view_class} lists safe methods in idempotent_methods: {sorted(idempotent_methods & _SAFE_METHODS)}. Safe methods are always idempotent; only list non-safe methods such as POST.",
+            )
+
         if any(path.startswith(p) for p in EXCLUSION_PATH_PREFIXES):
             pass
 
         else:
-            _OPERATION_SILOS[(_canonical_issue_path(path), method.lower())] = _declared_silo(
-                callback.view_class
-            )
+            operation_key = (_canonical_issue_path(path), method.lower())
+            _OPERATION_SILOS[operation_key] = _declared_silo(callback.view_class)
+            if method in idempotent_methods:
+                _IDEMPOTENT_OPERATIONS.add(operation_key)
             # endpoints that are documented via tooling
             status = callback.view_class.publish_status.get(method)
             if status is not None and status.is_published:
@@ -299,10 +320,11 @@ def custom_postprocessing_hook(result: Any, generator: Any, **kwargs: Any) -> An
     # Stamp before alias canonicalization so metadata stays with its operation.
     for path, operations in result["paths"].items():
         for method, operation in operations.items():
-            routing: RoutingMetadata = {
-                "silo": _OPERATION_SILOS.get((_canonical_issue_path(path), method), "unknown")
-            }
+            operation_key = (_canonical_issue_path(path), method)
+            routing: RoutingMetadata = {"silo": _OPERATION_SILOS.get(operation_key, "unknown")}
             operation["x-sentry-routing"] = routing
+            if operation_key in _IDEMPOTENT_OPERATIONS:
+                operation["x-sentry-idempotent"] = True
 
     # Add servers override from endpoint class definitions
     for path, servers in _ENDPOINT_SERVERS.items():
diff --git a/tests/apidocs/test_hooks.py b/tests/apidocs/test_hooks.py
index 20a8e5bb1ccb..b9e5f2b1eb2e 100644
--- a/tests/apidocs/test_hooks.py
+++ b/tests/apidocs/test_hooks.py
@@ -13,6 +13,7 @@
 from sentry.apidocs.hooks import (
     _ENDPOINT_SERVERS,
     _EXPERIMENTAL_OPERATIONS,
+    _IDEMPOTENT_OPERATIONS,
     _OPERATION_SILOS,
     EXPERIMENTAL_NOTICE,
     CustomEndpointEnumerator,
@@ -169,6 +170,106 @@ def test_experimental_marker_stamped_on_operation(self) -> None:
         assert processed["paths"]["/api/0/public/"]["get"]["description"] == "A stable endpoint"
 
 
+@patch("sentry.apidocs.hooks.__write_ownership_data")
+class IdempotentOperationsTest(TestCase):
+    """Declared idempotent methods are collected for every endpoint and stamped as a marker."""
+
+    def setUp(self) -> None:
+        _IDEMPOTENT_OPERATIONS.clear()
+        _OPERATION_SILOS.clear()
+
+    def tearDown(self) -> None:
+        _ENDPOINT_SERVERS.clear()
+        _EXPERIMENTAL_OPERATIONS.clear()
+        _IDEMPOTENT_OPERATIONS.clear()
+        _OPERATION_SILOS.clear()
+
+    def _endpoints(
+        self, path: str, methods: dict[str, ApiPublishStatus], idempotent: frozenset[str]
+    ) -> list[tuple[Any, Any, str, Any]]:
+        view_class = type(
+            "FakeEndpoint",
+            (),
+            {
+                "owner": ApiOwner.CRONS,
+                "publish_status": methods,
+                "idempotent_methods": idempotent,
+                "servers": None,
+            },
+        )
+        callback = type("FakeCallback", (), {"view_class": view_class})
+        return [(path, path, method, callback) for method in methods]
+
+    def _operation(self, operation_id: str) -> dict[str, Any]:
+        return {
+            "tags": ["Events"],
+            "description": "An endpoint",
+            "operationId": operation_id,
+            "parameters": [],
+        }
+
+    def test_only_declared_methods_are_marked(self, _write_ownership: Any) -> None:
+        custom_preprocessing_hook(
+            self._endpoints(
+                "/api/0/assemble/",
+                {"POST": ApiPublishStatus.PUBLIC, "PUT": ApiPublishStatus.PUBLIC},
+                frozenset({"POST"}),
+            )
+        )
+        result = {
+            "components": {"schemas": {}},
+            "paths": {
+                "/api/0/assemble/": {
+                    "post": self._operation("assemble"),
+                    "put": self._operation("update-assembly"),
+                },
+            },
+        }
+
+        processed = custom_postprocessing_hook(result, None)
+
+        operations = processed["paths"]["/api/0/assemble/"]
+        assert operations["post"]["x-sentry-idempotent"] is True
+        assert "x-sentry-idempotent" not in operations["put"]
+
+    def test_unpublished_methods_are_collected(self, _write_ownership: Any) -> None:
+        filtered = custom_preprocessing_hook(
+            self._endpoints(
+                "/api/0/private-assemble/",
+                {"POST": ApiPublishStatus.PRIVATE},
+                frozenset({"POST"}),
+            )
+        )
+
+        assert filtered == []
+        assert _IDEMPOTENT_OPERATIONS == {("/api/0/private-assemble/", "post")}
+
+    def test_next_build_does_not_reuse_previous_markers(self, _write_ownership: Any) -> None:
+        _IDEMPOTENT_OPERATIONS.add(("/api/0/stale/", "post"))
+
+        custom_preprocessing_hook(
+            self._endpoints("/api/0/fresh/", {"POST": ApiPublishStatus.PUBLIC}, frozenset())
+        )
+
+        assert _IDEMPOTENT_OPERATIONS == set()
+
+    def test_undeclared_method_raises(self, _write_ownership: Any) -> None:
+        endpoints = self._endpoints(
+            "/api/0/assemble/", {"POST": ApiPublishStatus.PUBLIC}, frozenset({"PUT"})
+        )
+
+        with pytest.raises(SentryApiBuildError, match="does not implement"):
+            custom_preprocessing_hook(endpoints)
+
+    def test_safe_method_raises(self, _write_ownership: Any) -> None:
+        endpoints = self._endpoints(
+            "/api/0/assemble/", {"GET": ApiPublishStatus.PUBLIC}, frozenset({"GET"})
+        )
+
+        with pytest.raises(SentryApiBuildError, match="safe methods"):
+            custom_preprocessing_hook(endpoints)
+
+
 class SummaryUniquenessTest(TestCase):
     def _operation(self, summary: str) -> dict[str, Any]:
         return {