diff --git a/examples/openapi-ts-angular-common/src/client/client/types.gen.ts b/examples/openapi-ts-angular-common/src/client/client/types.gen.ts
index 18bc780efb..76668ecc1d 100644
--- a/examples/openapi-ts-angular-common/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-angular-common/src/client/client/types.gen.ts
@@ -96,6 +96,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-angular/src/client/client/types.gen.ts b/examples/openapi-ts-angular/src/client/client/types.gen.ts
index 18bc780efb..76668ecc1d 100644
--- a/examples/openapi-ts-angular/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-angular/src/client/client/types.gen.ts
@@ -96,6 +96,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-fastify/src/client/client/types.gen.ts b/examples/openapi-ts-fastify/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-fastify/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-fastify/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-fetch/src/client/client/types.gen.ts b/examples/openapi-ts-fetch/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-fetch/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-fetch/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-ky/src/client/client/types.gen.ts b/examples/openapi-ts-ky/src/client/client/types.gen.ts
index 5b83a0ab00..4d66bba196 100644
--- a/examples/openapi-ts-ky/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-ky/src/client/client/types.gen.ts
@@ -121,6 +121,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-next/src/client/client/client.gen.ts b/examples/openapi-ts-next/src/client/client/client.gen.ts
index 45519b96b9..67f64c401e 100644
--- a/examples/openapi-ts-next/src/client/client/client.gen.ts
+++ b/examples/openapi-ts-next/src/client/client/client.gen.ts
@@ -67,7 +67,6 @@ export const createClient = (config: Config = {}): Client => {
 
   // @ts-expect-error
   const request: Client['request'] = async (options) => {
-    // @ts-expect-error
     const { opts, url } = await beforeRequest(options);
 
     for (const fn of interceptors.request.fns) {
@@ -207,14 +206,18 @@ export const createClient = (config: Config = {}): Client => {
       method,
       onRequest: async (url, init) => {
         let request = new Request(url, init);
+        // Create options for interceptor - convert headers to Headers instance
+        // to match ResolvedRequestOptions type
         const requestInit = {
           ...init,
+          headers: new Headers(init.headers),
           method: init.method as Config['method'],
           url,
         };
         for (const fn of interceptors.request.fns) {
           if (fn) {
-            await fn(requestInit);
+            // Cast to ResolvedRequestOptions: structurally matches with headers: Headers
+            await fn(requestInit as ResolvedRequestOptions);
             request = new Request(requestInit.url, requestInit);
           }
         }
diff --git a/examples/openapi-ts-next/src/client/client/types.gen.ts b/examples/openapi-ts-next/src/client/client/types.gen.ts
index 6dcf29dca3..5c510c6459 100644
--- a/examples/openapi-ts-next/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-next/src/client/client/types.gen.ts
@@ -71,6 +71,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-ofetch/src/client/client/types.gen.ts b/examples/openapi-ts-ofetch/src/client/client/types.gen.ts
index a193bd1d75..6020d0a9d1 100644
--- a/examples/openapi-ts-ofetch/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-ofetch/src/client/client/types.gen.ts
@@ -153,6 +153,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-openai/src/client/client/types.gen.ts b/examples/openapi-ts-openai/src/client/client/types.gen.ts
index cb6d0d54a0..44b69e041f 100644
--- a/examples/openapi-ts-openai/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-openai/src/client/client/types.gen.ts
@@ -92,6 +92,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-pinia-colada/src/client/client/types.gen.ts b/examples/openapi-ts-pinia-colada/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-pinia-colada/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-pinia-colada/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-tanstack-angular-query-experimental/src/client/client/types.gen.ts b/examples/openapi-ts-tanstack-angular-query-experimental/src/client/client/types.gen.ts
index 18bc780efb..76668ecc1d 100644
--- a/examples/openapi-ts-tanstack-angular-query-experimental/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-tanstack-angular-query-experimental/src/client/client/types.gen.ts
@@ -96,6 +96,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-tanstack-react-query/src/client/client/types.gen.ts b/examples/openapi-ts-tanstack-react-query/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-tanstack-react-query/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-tanstack-react-query/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-tanstack-svelte-query/src/client/client/types.gen.ts b/examples/openapi-ts-tanstack-svelte-query/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-tanstack-svelte-query/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-tanstack-svelte-query/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/examples/openapi-ts-tanstack-vue-query/src/client/client/types.gen.ts b/examples/openapi-ts-tanstack-vue-query/src/client/client/types.gen.ts
index 61a8dffa32..79041f6378 100644
--- a/examples/openapi-ts-tanstack-vue-query/src/client/client/types.gen.ts
+++ b/examples/openapi-ts-tanstack-vue-query/src/client/client/types.gen.ts
@@ -89,6 +89,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/packages/custom-client/src/client.ts b/packages/custom-client/src/client.ts
index d0f5b2e49c..4d83b118f5 100644
--- a/packages/custom-client/src/client.ts
+++ b/packages/custom-client/src/client.ts
@@ -1,4 +1,4 @@
-import type { Client, Config, RequestOptions } from './types';
+import type { Client, Config, ResolvedRequestOptions } from './types';
 import {
   buildUrl,
   createConfig,
@@ -24,7 +24,7 @@ export const createClient = (config: Config = {}): Client => {
     return getConfig();
   };
 
-  const interceptors = createInterceptors<Request, Response, unknown, RequestOptions>();
+  const interceptors = createInterceptors<Request, Response, unknown, ResolvedRequestOptions>();
 
   // @ts-expect-error
   const request: Client['request'] = async (options) => {
diff --git a/packages/custom-client/src/types.ts b/packages/custom-client/src/types.ts
index 8cb353eb81..cac8883e06 100644
--- a/packages/custom-client/src/types.ts
+++ b/packages/custom-client/src/types.ts
@@ -53,6 +53,18 @@ export interface RequestOptions<
   url: Url;
 }
 
+export interface ResolvedRequestOptions<
+  ThrowOnError extends boolean = boolean,
+  Url extends string = string,
+> extends RequestOptions<ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
+  serializedBody?: string;
+}
+
 export type RequestResult<
   TData = unknown,
   TError = unknown,
@@ -96,7 +108,7 @@ type BuildUrlFn = <
 ) => string;
 
 export type Client = CoreClient<RequestFn, Config, MethodFn, BuildUrlFn> & {
-  interceptors: Middleware<Request, Response, unknown, RequestOptions>;
+  interceptors: Middleware<Request, Response, unknown, ResolvedRequestOptions>;
 };
 
 /**
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-angular/bundle/types.ts b/packages/openapi-ts/src/plugins/@hey-api/client-angular/bundle/types.ts
index 54a9dfe628..e9905be39a 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-angular/bundle/types.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-angular/bundle/types.ts
@@ -97,6 +97,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-fetch/bundle/types.ts b/packages/openapi-ts/src/plugins/@hey-api/client-fetch/bundle/types.ts
index f2789cbc52..4fd803261c 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-fetch/bundle/types.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-fetch/bundle/types.ts
@@ -90,6 +90,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-ky/bundle/types.ts b/packages/openapi-ts/src/plugins/@hey-api/client-ky/bundle/types.ts
index 136269181f..f63d63bbe6 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-ky/bundle/types.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-ky/bundle/types.ts
@@ -122,6 +122,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/client.ts b/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/client.ts
index 4e76ab03de..117c20f04e 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/client.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/client.ts
@@ -65,7 +65,6 @@ export const createClient = (config: Config = {}): Client => {
 
   // @ts-expect-error
   const request: Client['request'] = async (options) => {
-    // @ts-expect-error
     const { opts, url } = await beforeRequest(options);
 
     for (const fn of interceptors.request.fns) {
@@ -205,14 +204,18 @@ export const createClient = (config: Config = {}): Client => {
       method,
       onRequest: async (url, init) => {
         let request = new Request(url, init);
+        // Create options for interceptor - convert headers to Headers instance
+        // to match ResolvedRequestOptions type
         const requestInit = {
           ...init,
+          headers: new Headers(init.headers),
           method: init.method as Config['method'],
           url,
         };
         for (const fn of interceptors.request.fns) {
           if (fn) {
-            await fn(requestInit);
+            // Cast to ResolvedRequestOptions: structurally matches with headers: Headers
+            await fn(requestInit as ResolvedRequestOptions);
             request = new Request(requestInit.url, requestInit);
           }
         }
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/types.ts b/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/types.ts
index 2a8de33eca..ee3856ff62 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/types.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-next/bundle/types.ts
@@ -72,6 +72,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }
 
diff --git a/packages/openapi-ts/src/plugins/@hey-api/client-ofetch/bundle/types.ts b/packages/openapi-ts/src/plugins/@hey-api/client-ofetch/bundle/types.ts
index 38af322af3..30beb390aa 100644
--- a/packages/openapi-ts/src/plugins/@hey-api/client-ofetch/bundle/types.ts
+++ b/packages/openapi-ts/src/plugins/@hey-api/client-ofetch/bundle/types.ts
@@ -154,6 +154,11 @@ export interface ResolvedRequestOptions<
   ThrowOnError extends boolean = boolean,
   Url extends string = string,
 > extends RequestOptions<unknown, TResponseStyle, ThrowOnError, Url> {
+  /**
+   * Headers object after merging config and request headers.
+   * Always a Headers instance in request interceptors.
+   */
+  headers: Headers;
   serializedBody?: string;
 }