diff --git a/packages/gatsby-plugin-fastify/src/gatsby/header-builder.ts b/packages/gatsby-plugin-fastify/src/gatsby/header-builder.ts
index 54489343c..b70c80d90 100644
--- a/packages/gatsby-plugin-fastify/src/gatsby/header-builder.ts
+++ b/packages/gatsby-plugin-fastify/src/gatsby/header-builder.ts
@@ -4,11 +4,12 @@ import typeOf from "just-typeof";
 
 import { SECURITY_HEADERS, CACHING_HEADERS, IMMUTABLE_CACHING_HEADER } from "../utils/constants";
 import type { PluginData } from "../utils/plugin-data";
-import type { GatsbyNodeServerConfig } from "../utils/config";
+import type { GatsbyFastifyPluginOptions } from "../utils/config";
+import { HeadersOption } from "../utils/headers";
 
-function deepMerge(...headers) {
+function deepMerge(...headers: HeadersOption[]) {
   // eslint-disable-next-line unicorn/no-array-reduce
-  return headers.reduce((accumulator, header: { [key: string]: object }) => {
+  return headers.reduce((accumulator, header) => {
     for (let [key, value] of Object.entries(header)) {
       // console.log(typeOf(value));
       //merge needs empty object to prevent overwriting of references use across multiple routes
@@ -23,8 +24,8 @@ function deepMerge(...headers) {
 
 // program methods
 const applySecurityHeaders =
-  ({ mergeSecurityHeaders }: GatsbyNodeServerConfig) =>
-  (headers) => {
+  ({ mergeSecurityHeaders }: GatsbyFastifyPluginOptions) =>
+  (headers: HeadersOption) => {
     if (!mergeSecurityHeaders) {
       return headers;
     }
@@ -33,8 +34,8 @@ const applySecurityHeaders =
   };
 
 const applyCachingHeaders =
-  (pluginData: PluginData, { mergeCacheHeaders }: GatsbyNodeServerConfig) =>
-  (headers) => {
+  (pluginData: PluginData, { mergeCacheHeaders }: GatsbyFastifyPluginOptions) =>
+  (headers: HeadersOption) => {
     if (!mergeCacheHeaders) {
       return headers;
     }
@@ -53,7 +54,7 @@ const applyCachingHeaders =
 
     const files = chunks.flatMap((chunk) => pluginData.manifest[chunk]);
 
-    const cachingHeaders = {};
+    const cachingHeaders: HeadersOption = {};
 
     for (const file of files) {
       if (typeof file === `string`) {
@@ -63,7 +64,10 @@ const applyCachingHeaders =
     return deepMerge(cachingHeaders, CACHING_HEADERS, headers);
   };
 
-export function buildHeadersProgram(pluginData, pluginOptions) {
+export function buildHeadersProgram(
+  pluginData: PluginData,
+  pluginOptions: GatsbyFastifyPluginOptions
+) {
   return compose(
     applySecurityHeaders(pluginOptions),
     applyCachingHeaders(pluginData, pluginOptions)
diff --git a/packages/gatsby-plugin-fastify/src/utils/config.ts b/packages/gatsby-plugin-fastify/src/utils/config.ts
index 741d1399c..383b24346 100644
--- a/packages/gatsby-plugin-fastify/src/utils/config.ts
+++ b/packages/gatsby-plugin-fastify/src/utils/config.ts
@@ -10,6 +10,7 @@ import type { FastifyServerOptions } from "fastify";
 import { PathConfig } from "../plugins/client-routes";
 import { CONFIG_FILE_NAME, CONFIG_FILE_PATH } from "./constants";
 import { buildPrefixer } from "./plugin-data";
+import { HeadersOption } from "./headers";
 
 let config: Partial<GfConfig> = {};
 
@@ -22,15 +23,12 @@ export interface GatsbyFastifyPluginOptions extends PluginOptions {
     imageCdn: boolean;
   };
   fastify: FastifyServerOptions;
+  headers: HeadersOption;
 }
 export interface GatsbyNodeServerConfig extends GatsbyFastifyPluginOptions {
   clientSideRoutes: NoUndefinedField<PathConfig>[];
   serverSideRoutes: ServerSideRoute[];
-  headers: {
-    [path: string]: {
-      [name: string]: string;
-    };
-  };
+  headers: HeadersOption;
   mergeCacheHeaders?: boolean;
   mergeSecurityHeaders?: boolean;
   redirects: IRedirect[];
diff --git a/packages/gatsby-plugin-fastify/src/utils/headers.ts b/packages/gatsby-plugin-fastify/src/utils/headers.ts
index df43ca0b4..f4471e4d3 100644
--- a/packages/gatsby-plugin-fastify/src/utils/headers.ts
+++ b/packages/gatsby-plugin-fastify/src/utils/headers.ts
@@ -12,6 +12,12 @@ function appendHeader({ name, value }: { name: string; value: string }, reply: F
   }
 }
 
+export type HeadersOption = {
+  [key: string]: {
+    [key: string]: string;
+  };
+};
+
 export type Modules =
   | "DSG"
   | "SSR"