chore(cli): release 5.21.0

Author: github-actions[bot]

…eased/add-x-fern-streaming-resumable.yml ….21.0/add-x-fern-streaming-resumable.ymlpackages/cli/cli/changes/unreleased/add-x-fern-streaming-resumable.yml renamed to packages/cli/cli/changes/5.21.0/add-x-fern-streaming-resumable.yml packages/cli/cli/changes/unreleased/add-x-fern-streaming-resumable.yml renamed to packages/cli/cli/changes/5.21.0/add-x-fern-streaming-resumable.yml

@@ -1,4 +1,20 @@
 # yaml-language-server: $schema=../../../fern-versions-yml.schema.json
+- version: 5.21.0
+  changelogEntry:
+    - summary: |
+        Add a new `resumable` sub-property to the `x-fern-streaming` OpenAPI extension
+        (and a corresponding `resumable` field on Fern Definition `response-stream`
+        blocks). When set on an SSE endpoint, the IR carries `resumable: true` on
+        the SSE chunk so generators can emit a client-side reconnect loop using
+        standard SSE primitives (`Last-Event-ID`, `retry:`).
+
+        The flag is inheritable: setting `x-fern-streaming.resumable: true` at the
+        OpenAPI document level applies to every SSE endpoint unless an operation
+        overrides it (silent fallback). Defaults to `false`. The IR field is
+        optional, so generators that don't read it are unaffected.
+      type: feat
+  createdAt: "2026-05-08"
+  irVersion: 66
 - version: 5.20.1
   changelogEntry:
     - summary: |