Fix endpoint type inference for OpenAPI server variable templates (#3664)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ardatan <20847995+ardatan@users.noreply.github.com>

Author: Copilot

packages/fets/src/client/types.ts

@@ -473,6 +473,20 @@ export interface ClientOptions {
   globalParams?: ClientRequestParams;
 }
 
+type ServerVariableType<TVarName extends string, TVariables> =
+  TVariables extends Record<string, unknown>
+    ? TVarName extends keyof TVariables
+      ? TVariables[TVarName] extends { enum: readonly (infer TEnum extends string)[] }
+        ? TEnum
+        : string
+      : string
+    : string;
+
+type ResolveServerUrl<TUrl extends string, TVariables> =
+  TUrl extends `${infer Before}{${infer VarName}}${infer After}`
+    ? `${Before}${ServerVariableType<VarName, TVariables>}${ResolveServerUrl<After, TVariables>}`
+    : TUrl;
+
 export type ClientOptionsWithStrictEndpoint<TOAS extends OpenAPIDocument> = Omit<
   ClientOptions,
   'endpoint'
@@ -489,15 +503,15 @@ export type ClientOptionsWithStrictEndpoint<TOAS extends OpenAPIDocument> = Omit
         endpoint: TEndpoint;
       }
     : TOAS extends {
-          servers: { url: infer TEndpoint extends string }[];
+          servers: { url: infer TEndpoint extends string; variables?: infer TVariables }[];
         }
       ? {
           /**
            * The base URL of the API defined in the OAS document.
            *
            * @see https://swagger.io/docs/specification/api-host-and-base-path/
            */
-          endpoint: TEndpoint;
+          endpoint: ResolveServerUrl<TEndpoint, TVariables>;
         }
       : TOAS extends {
             host: infer THost extends string;

packages/fets/tests/client/fixtures/example-server-variables-oas.ts

@@ -0,0 +1,51 @@
+export default {
+  openapi: '3.0.0',
+  info: {
+    title: 'Server Variables Test API',
+    version: '1.0.0',
+  },
+  servers: [
+    {
+      url: 'https://{username}.server.com:{port}/{version}',
+      variables: {
+        username: {
+          default: 'demo',
+          description: 'This value is assigned by the service provider.',
+        },
+        port: {
+          enum: ['8443', '443'],
+          default: '8443',
+        },
+        version: {
+          default: 'v1',
+        },
+      },
+    },
+  ],
+  paths: {
+    '/users': {
+      get: {
+        operationId: 'getUsers',
+        responses: {
+          200: {
+            description: 'A list of users',
+            content: {
+              'application/json': {
+                schema: {
+                  type: 'array',
+                  items: {
+                    type: 'object',
+                    properties: {
+                      id: { type: 'string' },
+                      name: { type: 'string' },
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    },
+  },
+} as const;

packages/fets/tests/client/server-variables-test.ts

@@ -0,0 +1,33 @@
+import { createClient, type NormalizeOAS } from 'fets';
+import type { ClientOptionsWithStrictEndpoint } from '../../src/client/types';
+import serverVariablesOas from './fixtures/example-server-variables-oas';
+
+type NormOAS = NormalizeOAS<typeof serverVariablesOas>;
+
+// Valid endpoint matching the server variable template (port from enum, username/version are strings)
+const client = createClient<NormOAS>({
+  endpoint: 'https://me.server.com:443/v1',
+});
+
+// Port from enum: '8443' also valid
+const client2 = createClient<NormOAS>({
+  endpoint: 'https://demo.server.com:8443/v2',
+});
+
+void client;
+void client2;
+
+// Verify the endpoint type is properly inferred via ClientOptionsWithStrictEndpoint
+// Port '444' is not in the enum ['8443', '443'] - this should be a type error
+const _invalidOpts: ClientOptionsWithStrictEndpoint<NormOAS> = {
+  // @ts-expect-error - port '444' is not in the enum ['8443', '443']
+  endpoint: 'https://me.server.com:444/v1',
+};
+void _invalidOpts;
+
+const getUsersRes = await client['/users'].get();
+
+if (getUsersRes.ok) {
+  const users = await getUsersRes.json();
+  void users[0]?.id;
+}