diff --git a/packages/sdk/src/areas/plugins.ts b/packages/sdk/src/areas/plugins.ts
index 30137c133..5dd21645b 100644
--- a/packages/sdk/src/areas/plugins.ts
+++ b/packages/sdk/src/areas/plugins.ts
@@ -235,12 +235,20 @@ export interface PluginsArea {
   ): Promise<PluginUpdateSettingsResult>;
 }
 
-function pluginSourceSelection(args: PluginInstallArgs): PluginSourceSelection {
+/**
+ * Only a non-root selection goes on the wire. A root install is the server's
+ * default, and omitting the key keeps the request accepted by servers that
+ * predate the `selection` field (bb-app <= 0.37.x validate the install body
+ * strictly and answer 422 on unknown keys, see #1662).
+ */
+function pluginSourceSelection(
+  args: PluginInstallArgs,
+): PluginSourceSelection | undefined {
   if (args.subdirectory !== undefined) {
     return { kind: "subdirectory", path: args.subdirectory };
   }
   if (args.plugin !== undefined) return { kind: "entry", name: args.plugin };
-  return { kind: "root" };
+  return undefined;
 }
 
 function pluginPath(pluginId: string, suffix = ""): string {
@@ -424,10 +432,14 @@ export function createPluginsArea(args: CreateSdkAreaArgs): PluginsArea {
           "plugin install accepts subdirectory or plugin, not both",
         );
       }
-      const body = pluginInstallSourceRequestSchema.parse({
-        source: input.source,
-        selection: pluginSourceSelection(input),
-      });
+      const selection = pluginSourceSelection(input);
+      const body = {
+        source: z.string().min(1).parse(input.source),
+        ...(selection === undefined ? {} : { selection }),
+      };
+      // The contract still validates the shape; its root default is the
+      // server's to fill, so the parsed value is not what gets sent.
+      pluginInstallSourceRequestSchema.parse(body);
       const response = await requestParsed(
         "/api/v1/plugins/install",
         pluginInstallResponseSchema,
diff --git a/packages/sdk/test/sdk.test.ts b/packages/sdk/test/sdk.test.ts
index 4d27c39ff..ba3d651a2 100644
--- a/packages/sdk/test/sdk.test.ts
+++ b/packages/sdk/test/sdk.test.ts
@@ -1420,10 +1420,9 @@ describe("@bb/sdk", () => {
         url: "http://bb.test/api/v1/plugins",
       },
       {
-        bodyText: JSON.stringify({
-          source: "npm:@bb/notes@^1",
-          selection: { kind: "root" },
-        }),
+        // A root install sends no `selection`: the server fills that default,
+        // and pre-0.38.0 servers reject the key outright (#1662).
+        bodyText: JSON.stringify({ source: "npm:@bb/notes@^1" }),
         method: "POST",
         url: "http://bb.test/api/v1/plugins/install",
       },