diff --git a/CHANGELOG.md b/CHANGELOG.md
index 366d5fb..ca053cd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,12 @@ public API, a minor version adds to it, and a patch version fixes it.
 - `feature_flags.targeting_context()`, `feature_flags.lint_rule()` and `feature_flags.test_rule()`:
   the facts a targeting rule can read, the check the API runs before accepting one, and a dry run
   of a rule against a customer or an instance.
+- `feature_flags.update(..., slug=...)` renames a flag.
+
+### Fixed
+
+- `feature_flags.update` sends the flag's slug. Without it, Kaiten blanked the slug and the flag
+  could no longer be reached.
 
 ## [1.0.0] - 2026-10-01
 
diff --git a/src/kaitencloud/_async/resources/feature_flags.py b/src/kaitencloud/_async/resources/feature_flags.py
index ffa5a32..08df060 100644
--- a/src/kaitencloud/_async/resources/feature_flags.py
+++ b/src/kaitencloud/_async/resources/feature_flags.py
@@ -103,11 +103,16 @@ async def update(
         description: str | None = None,
         metadata: Mapping[str, Any] | None = None,
         event_name: str = "",
+        slug: str | None = None,
     ) -> None:
         """Replace a feature flag's definition.
 
         A full replacement: the variants, rules and metadata sent become the flag's whole
         definition. See :meth:`create` for what each field means.
+
+        Args:
+            slug: A new slug, to rename the flag; applications evaluating it by its old key
+                stop finding it. Omitted, the flag keeps ``feature_flag_slug``.
         """
         body = _flag(
             name=name,
@@ -120,6 +125,9 @@ async def update(
             metadata=metadata,
             event_name=event_name,
         )
+        # The slug is always sent. Kaiten writes the body's slug as the flag's own, so a
+        # body without one would blank it rather than keep it.
+        body["slug"] = slug or feature_flag_slug
         await self._api.send_empty("PUT", api_path("feature-flags", feature_flag_slug), body=body)
 
     async def targeting_context(self) -> TargetingContext:
diff --git a/src/kaitencloud/_sync/resources/feature_flags.py b/src/kaitencloud/_sync/resources/feature_flags.py
index b65a54e..b463a94 100644
--- a/src/kaitencloud/_sync/resources/feature_flags.py
+++ b/src/kaitencloud/_sync/resources/feature_flags.py
@@ -105,11 +105,16 @@ def update(
         description: str | None = None,
         metadata: Mapping[str, Any] | None = None,
         event_name: str = "",
+        slug: str | None = None,
     ) -> None:
         """Replace a feature flag's definition.
 
         A full replacement: the variants, rules and metadata sent become the flag's whole
         definition. See :meth:`create` for what each field means.
+
+        Args:
+            slug: A new slug, to rename the flag; applications evaluating it by its old key
+                stop finding it. Omitted, the flag keeps ``feature_flag_slug``.
         """
         body = _flag(
             name=name,
@@ -122,6 +127,9 @@ def update(
             metadata=metadata,
             event_name=event_name,
         )
+        # The slug is always sent. Kaiten writes the body's slug as the flag's own, so a
+        # body without one would blank it rather than keep it.
+        body["slug"] = slug or feature_flag_slug
         self._api.send_empty("PUT", api_path("feature-flags", feature_flag_slug), body=body)
 
     def targeting_context(self) -> TargetingContext:
diff --git a/tests/test_contract.py b/tests/test_contract.py
index 44ca9b6..6331982 100644
--- a/tests/test_contract.py
+++ b/tests/test_contract.py
@@ -302,7 +302,7 @@ class Case:
             variants=VARIANTS,
             default_variant=targeting.fixed("off"),
         ),
-        forbidden=frozenset({"slug"}),
+        required=frozenset({"slug"}),
     ),
     Case("delete-feature-flag", lambda c: c.feature_flags.delete("new-checkout")),
     Case("get-openfeature-manifest", lambda c: c.feature_flags.manifest()),
diff --git a/tests/test_resources.py b/tests/test_resources.py
index 132b80d..6446389 100644
--- a/tests/test_resources.py
+++ b/tests/test_resources.py
@@ -182,9 +182,31 @@ def test_a_feature_flag_needs_only_the_essentials(make_client: Callable[..., Any
         "description": None,
         "metadata": {},
         "event_name": "",
+        "slug": "new-checkout",
     }
 
 
+def test_a_feature_flag_update_keeps_its_slug_unless_renamed(
+    make_client: Callable[..., Any],
+) -> None:
+    # Kaiten writes the body's slug as the flag's own: sending none blanked it.
+    client, recorder = make_client(respond(204))
+    essentials = {
+        "name": "New checkout",
+        "type": "boolean",
+        "variants": [{"name": "on", "value": True}],
+        "default_variant": "on",
+    }
+
+    client.feature_flags.update("new-checkout", **essentials)
+    client.feature_flags.update("new-checkout", **essentials, slug="checkout-v2")
+
+    first, second = recorder
+    assert body(first)["slug"] == "new-checkout"
+    assert body(second)["slug"] == "checkout-v2"
+    assert second.url.path.endswith("/feature-flags/new-checkout")
+
+
 def test_usage_defaults_to_append(make_client: Callable[..., Any]) -> None:
     client, recorder = make_client(respond(200, USAGE))
     client.instances.report_usage("acme-production", "seats", 3)