diff --git a/packages/openapi-fetch/src/index.d.ts b/packages/openapi-fetch/src/index.d.ts
index ea042b8de..c0bc767ed 100644
--- a/packages/openapi-fetch/src/index.d.ts
+++ b/packages/openapi-fetch/src/index.d.ts
@@ -38,7 +38,19 @@ export type QuerySerializer<T> = (
 
 export type BodySerializer<T> = (body: OperationRequestBodyContent<T>) => any;
 
-export type ParseAs = "json" | "text" | "blob" | "arrayBuffer" | "stream";
+type BodyType<T = unknown> = {
+  json: T;
+  text: Awaited<ReturnType<Response["text"]>>;
+  blob: Awaited<ReturnType<Response["blob"]>>;
+  arrayBuffer: Awaited<ReturnType<Response["arrayBuffer"]>>;
+  stream: Response["body"];
+};
+export type ParseAs = keyof BodyType;
+export type ParseAsResponse<T, O extends FetchOptions> = O extends {
+  parseAs: ParseAs;
+}
+  ? BodyType<T>[O["parseAs"]]
+  : T;
 
 export interface DefaultParamsOption {
   params?: {
@@ -62,9 +74,20 @@ export type RequestBodyOption<T> = OperationRequestBodyContent<T> extends never
 
 export type FetchOptions<T> = RequestOptions<T> & Omit<RequestInit, "body">;
 
-export type FetchResponse<T> =
+/** This type helper makes the 2nd function param required if params/requestBody are required; otherwise, optional */
+export type MaybeOptionalInit<
+  P extends {},
+  M extends keyof P,
+> = HasRequiredKeys<FetchOptions<FilterKeys<P, M>>> extends never
+  ? [(FetchOptions<FilterKeys<P, M>> | undefined)?]
+  : [FetchOptions<FilterKeys<P, M>>];
+
+export type FetchResponse<T, O extends FetchOptions> =
   | {
-      data: FilterKeys<SuccessResponse<ResponseObjectMap<T>>, MediaType>;
+      data: ParseAsResponse<
+        FilterKeys<SuccessResponse<ResponseObjectMap<T>>, MediaType>,
+        O
+      >;
       error?: never;
       response: Response;
     }
@@ -86,157 +109,69 @@ export default function createClient<Paths extends {}>(
   clientOptions?: ClientOptions,
 ): {
   /** Call a GET endpoint */
-  GET<P extends PathsWithMethod<Paths, "get">>(
+  GET<
+    P extends PathsWithMethod<Paths, "get">,
+    I extends MaybeOptionalInit<Paths[P], "get">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "get">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "get">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "get">>]
-  ): Promise<
-    FetchResponse<
-      "get" extends infer T
-        ? T extends "get"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["get"], I[0]>>;
   /** Call a PUT endpoint */
-  PUT<P extends PathsWithMethod<Paths, "put">>(
+  PUT<
+    P extends PathsWithMethod<Paths, "put">,
+    I extends MaybeOptionalInit<Paths[P], "put">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "put">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "put">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "put">>]
-  ): Promise<
-    FetchResponse<
-      "put" extends infer T
-        ? T extends "put"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["put"], I[0]>>;
   /** Call a POST endpoint */
-  POST<P extends PathsWithMethod<Paths, "post">>(
+  POST<
+    P extends PathsWithMethod<Paths, "post">,
+    I extends MaybeOptionalInit<Paths[P], "post">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "post">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "post">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "post">>]
-  ): Promise<
-    FetchResponse<
-      "post" extends infer T
-        ? T extends "post"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["post"], I[0]>>;
   /** Call a DELETE endpoint */
-  DELETE<P extends PathsWithMethod<Paths, "delete">>(
+  DELETE<
+    P extends PathsWithMethod<Paths, "delete">,
+    I extends MaybeOptionalInit<Paths[P], "delete">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "delete">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "delete">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "delete">>]
-  ): Promise<
-    FetchResponse<
-      "delete" extends infer T
-        ? T extends "delete"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["delete"], I[0]>>;
   /** Call a OPTIONS endpoint */
-  OPTIONS<P extends PathsWithMethod<Paths, "options">>(
+  OPTIONS<
+    P extends PathsWithMethod<Paths, "options">,
+    I extends MaybeOptionalInit<Paths[P], "options">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "options">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "options">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "options">>]
-  ): Promise<
-    FetchResponse<
-      "options" extends infer T
-        ? T extends "options"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["options"], I[0]>>;
   /** Call a HEAD endpoint */
-  HEAD<P extends PathsWithMethod<Paths, "head">>(
+  HEAD<
+    P extends PathsWithMethod<Paths, "head">,
+    I extends MaybeOptionalInit<Paths[P], "head">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "head">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "head">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "head">>]
-  ): Promise<
-    FetchResponse<
-      "head" extends infer T
-        ? T extends "head"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["head"], I[0]>>;
   /** Call a PATCH endpoint */
-  PATCH<P extends PathsWithMethod<Paths, "patch">>(
+  PATCH<
+    P extends PathsWithMethod<Paths, "patch">,
+    I extends MaybeOptionalInit<Paths[P], "patch">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "patch">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "patch">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "patch">>]
-  ): Promise<
-    FetchResponse<
-      "patch" extends infer T
-        ? T extends "patch"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["patch"], I[0]>>;
   /** Call a TRACE endpoint */
-  TRACE<P extends PathsWithMethod<Paths, "trace">>(
+  TRACE<
+    P extends PathsWithMethod<Paths, "trace">,
+    I extends MaybeOptionalInit<Paths[P], "trace">,
+  >(
     url: P,
-    ...init: HasRequiredKeys<
-      FetchOptions<FilterKeys<Paths[P], "trace">>
-    > extends never
-      ? [(FetchOptions<FilterKeys<Paths[P], "trace">> | undefined)?]
-      : [FetchOptions<FilterKeys<Paths[P], "trace">>]
-  ): Promise<
-    FetchResponse<
-      "trace" extends infer T
-        ? T extends "trace"
-          ? T extends keyof Paths[P]
-            ? Paths[P][T]
-            : unknown
-          : never
-        : never
-    >
-  >;
+    ...init: I
+  ): Promise<FetchResponse<Paths[P]["trace"], I[0]>>;
 };
 
 /** Serialize query params to string */
diff --git a/packages/openapi-fetch/src/index.js b/packages/openapi-fetch/src/index.js
index 23e069fa5..acd39dcf4 100644
--- a/packages/openapi-fetch/src/index.js
+++ b/packages/openapi-fetch/src/index.js
@@ -21,8 +21,8 @@ export default function createClient(clientOptions) {
 
   /**
    * Per-request fetch (keeps settings created in createClient()
-   * @param {string} url
-   * @param {import('./index.js').FetchOptions} fetchOptions
+   * @param {T} url
+   * @param {import('./index.js').FetchOptions<T>} fetchOptions
    */
   async function coreFetch(url, fetchOptions) {
     const {
@@ -50,6 +50,7 @@ export default function createClient(clientOptions) {
     );
 
     // fetch!
+    /** @type {RequestInit} */
     const requestInit = {
       redirect: "follow",
       ...baseOptions,
diff --git a/packages/openapi-fetch/test/index.test.ts b/packages/openapi-fetch/test/index.test.ts
index 764a17286..6f1aa2833 100644
--- a/packages/openapi-fetch/test/index.test.ts
+++ b/packages/openapi-fetch/test/index.test.ts
@@ -569,32 +569,56 @@ describe("client", () => {
       it("text", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "text" });
-        expect(data).toBe("{}");
+        const { data, error } = (await client.GET("/anyMethod", {
+          parseAs: "text",
+        })) satisfies { data?: string };
+        if (error) {
+          throw new Error(`parseAs text: error`);
+        }
+        expect(data.toLowerCase()).toBe("{}");
       });
 
       it("arrayBuffer", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", {
+        const { data, error } = (await client.GET("/anyMethod", {
           parseAs: "arrayBuffer",
-        });
-        expect(data instanceof ArrayBuffer).toBe(true);
+        })) satisfies { data?: ArrayBuffer };
+        if (error) {
+          throw new Error(`parseAs arrayBuffer: error`);
+        }
+        expect(data.byteLength).toBe(2);
       });
 
       it("blob", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "blob" });
-        // eslint-disable-next-line @typescript-eslint/no-explicit-any
-        expect((data as any).constructor.name).toBe("Blob");
+        const { data, error } = (await client.GET("/anyMethod", {
+          parseAs: "blob",
+        })) satisfies { data?: Blob };
+        if (error) {
+          throw new Error(`parseAs blob: error`);
+        }
+
+        expect(data.constructor.name).toBe("Blob");
       });
 
       it("stream", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "stream" });
+        const { data } = (await client.GET("/anyMethod", {
+          parseAs: "stream",
+        })) satisfies { data?: ReadableStream<Uint8Array> | null };
+        if (!data) {
+          throw new Error(`parseAs stream: error`);
+        }
+
         expect(data instanceof Buffer).toBe(true);
+        if (!(data instanceof Buffer)) {
+          throw Error("Data should be an instance of Buffer in Node context");
+        }
+
+        expect(data.byteLength).toBe(2);
       });
     });
   });
diff --git a/packages/openapi-fetch/test/v7-beta.test.ts b/packages/openapi-fetch/test/v7-beta.test.ts
index 871bcd62c..542da7b86 100644
--- a/packages/openapi-fetch/test/v7-beta.test.ts
+++ b/packages/openapi-fetch/test/v7-beta.test.ts
@@ -578,23 +578,30 @@ describe("client", () => {
       it("text", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "text" });
+        const { data }: { data?: string } = await client.GET("/anyMethod", {
+          parseAs: "text",
+        });
         expect(data).toBe("{}");
       });
 
       it("arrayBuffer", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", {
-          parseAs: "arrayBuffer",
-        });
+        const { data }: { data?: ArrayBuffer } = await client.GET(
+          "/anyMethod",
+          {
+            parseAs: "arrayBuffer",
+          },
+        );
         expect(data instanceof ArrayBuffer).toBe(true);
       });
 
       it("blob", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "blob" });
+        const { data }: { data?: Blob } = await client.GET("/anyMethod", {
+          parseAs: "blob",
+        });
         // eslint-disable-next-line @typescript-eslint/no-explicit-any
         expect((data as any).constructor.name).toBe("Blob");
       });
@@ -602,7 +609,10 @@ describe("client", () => {
       it("stream", async () => {
         const client = createClient<paths>();
         mockFetchOnce({ status: 200, body: "{}" });
-        const { data } = await client.GET("/anyMethod", { parseAs: "stream" });
+        const { data }: { data?: ReadableStream | null } = await client.GET(
+          "/anyMethod",
+          { parseAs: "stream" },
+        );
         expect(data instanceof Buffer).toBe(true);
       });
     });
diff --git a/packages/openapi-typescript-helpers/index.d.ts b/packages/openapi-typescript-helpers/index.d.ts
index b7f851f11..d47bcb0d1 100644
--- a/packages/openapi-typescript-helpers/index.d.ts
+++ b/packages/openapi-typescript-helpers/index.d.ts
@@ -77,9 +77,7 @@ export type ErrorResponse<T> = FilterKeys<
 // Generic TS utils
 
 /** Find first match of multiple keys */
-export type FilterKeys<Obj, Matchers> = {
-  [K in keyof Obj]: K extends Matchers ? Obj[K] : never;
-}[keyof Obj];
+export type FilterKeys<Obj, Matchers> = Obj[keyof Obj & Matchers];
 /** Return any `[string]/[string]` media type (important because openapi-fetch allows any content response, not just JSON-like) */
 export type MediaType = `${string}/${string}`;
 /** Filter objects that have required keys */