diff --git a/src/client.ts b/src/client.ts
index 6ea54adc..fe0c9834 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -325,7 +325,14 @@ export function createClient(config: CreateClientConfig): Base44Client {
     ...userModules,
 
     /** See {@link Base44Client.fetchWithAuth}. */
-    fetchWithAuth: createFetchWithAuth(axiosClient),
+    fetchWithAuth: createFetchWithAuth({
+      axios: axiosClient,
+      serviceRoleAxios: serviceRoleAxiosClient,
+      appId: String(appId),
+      serverUrl,
+      functionsVersion,
+      platformHeaders: optionalHeaders,
+    }),
 
     /**
      * Sets a new authentication token for all subsequent requests.
diff --git a/src/client.types.ts b/src/client.types.ts
index 3d36018c..31fa838a 100644
--- a/src/client.types.ts
+++ b/src/client.types.ts
@@ -13,6 +13,7 @@ import type { AppLogsModule } from "./modules/app-logs.types.js";
 import type { AppModule } from "./modules/app.types.js";
 import type { AnalyticsModule } from "./modules/analytics.types.js";
 import type { ActorsModule } from "./modules/actors.types.js";
+import type { FetchWithAuthInit } from "./utils/fetch-with-auth.js";
 
 /**
  * Options for creating a Base44 client.
@@ -148,24 +149,29 @@ export interface Base44Client {
   cleanup: () => void;
 
   /**
-   * Calls one of your app's own server routes with the signed-in user's access token attached.
+   * Calls one of your app's own server routes with this client's credentials attached.
    *
-   * Base44 keeps the user's access token in the browser's local storage, so a plain `fetch()` to your app's server routes arrives without it and the route sees an anonymous caller. `fetchWithAuth()` is the same `fetch()` with the `Authorization: Bearer <token>` header added, which is what lets a server route act on behalf of the signed-in user.
+   * Base44 keeps a user's access token in the browser's local storage, and the platform puts its own headers on a server request — so a plain `fetch()` to your app's routes carries neither, and the route sees an anonymous caller with no way to build a client. `fetchWithAuth()` is the same `fetch()` with whatever this client holds added, which is what lets the route act on behalf of the caller.
    *
-   * Requests are restricted to your app's own origin so the token is never sent to a third party: pass a relative path beginning with a single `/`, such as `/api/orders`. An absolute URL, a protocol-relative `//host`, or anything else that a URL parser would read as another origin throws. To call a Base44 backend function, use {@linkcode FunctionsModule.fetch | functions.fetch()}; for another origin, use plain `fetch()`.
+   * What that means depends on where the client came from, because a client can only send what it has:
    *
-   * The path is passed to `fetch` unchanged, so this also works in server code, where the runtime's `fetch` decides what a relative path means — a server-side client from {@linkcode createClientFromRequest | createClientFromRequest()} carries the caller's own token. Note that only the `Authorization` header is added: a route that builds its own client from the incoming request also needs the platform's `Base44-App-Id` and `Base44-Api-Url`, which a request you construct yourself does not have.
+   * - In a browser, from {@linkcode createClient | createClient()}: the signed-in user's `Authorization: Bearer <token>`. When nobody is signed in the request goes without it, so routes open to anonymous callers keep working.
+   * - In one of your server routes, from {@linkcode createClientFromRequest | createClientFromRequest()}: everything that function reads back — the caller's token, `Base44-App-Id`, `Base44-Api-Url`, `Base44-Functions-Version`, the signed `Base44-State`, `X-Data-Env`, and the app's per-request service credential. The callee's own `createClientFromRequest()` then rebuilds the client you are holding, service role included.
    *
-   * When no user is signed in the request is sent without an `Authorization` header, so routes that allow anonymous access keep working.
+   * That second case is why route-to-route calls need this. A sub-request carries nothing from the request that triggered it — your framework builds it from your arguments alone — so a route reached by a plain `fetch()` sees no headers at all and its `createClientFromRequest()` throws on the missing `Base44-App-Id`.
+   *
+   * Requests are restricted to your app's own origin, which is what keeps these credentials inside your app: pass a relative path beginning with a single `/`, such as `/api/orders`. An absolute URL, a protocol-relative `//host`, or anything else a URL parser would read as another origin throws. To call a Base44 backend function, use {@linkcode FunctionsModule.fetch | functions.fetch()}; for another origin, use plain `fetch()`.
+   *
+   * Two routes that need the same logic should call a shared function rather than each other — cheaper than an HTTP round trip, and it needs no headers at all. Hop when the hop is the point: rendering a page server-side, or going through a route for its own caching and route rules.
    *
    * @param path - A relative path on your app's own origin, such as `/api/orders`.
-   * @param init - Optional [`RequestInit`](https://developer.mozilla.org/en-US/docs/Web/API/RequestInit) options such as `method`, `headers`, `body`, and `signal`. The auth header is added automatically; an `Authorization` header you set yourself is kept.
+   * @param init - Optional [`RequestInit`](https://developer.mozilla.org/en-US/docs/Web/API/RequestInit) options such as `method`, `headers`, `body`, and `signal`, plus `fetch`: the transport that resolves a root-relative path against your app's routes. It defaults to the global `fetch`, which does that in a browser but not on a server — in Nitro pass its own (`import { fetch } from "nitro"`), which dispatches in-process with no network hop. Any header you set yourself is kept, so you can deliberately hand the callee a different identity.
    * @returns Promise resolving to a native [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response).
    * @throws {Error} When `path` is not a relative path on your app's own origin.
    *
    * @example
    * ```typescript
-   * // Call your app's own server route as the signed-in user
+   * // Browser: call your app's own server route as the signed-in user
    * const response = await base44.fetchWithAuth('/api/orders');
    * const orders = await response.json();
    * ```
@@ -183,8 +189,20 @@ export interface Base44Client {
    *   throw new Error(`Request failed: ${response.status}`);
    * }
    * ```
+   *
+   * @example
+   * ```typescript
+   * // Server-side render: reach the app's own route as this request
+   * import { fetch } from 'nitro';
+   * import { createClientFromRequest } from '@base44/sdk';
+   *
+   * const base44 = createClientFromRequest(event.req);
+   * const response = await base44.fetchWithAuth('/api/items', { fetch });
+   * const items = await response.json();
+   * ```
    */
-  fetchWithAuth(path: string, init?: RequestInit): Promise<Response>;
+  fetchWithAuth(path: string, init?: FetchWithAuthInit): Promise<Response>;
+
 
   /**
    * Sets a new authentication token for all subsequent requests.
diff --git a/src/utils/fetch-with-auth.ts b/src/utils/fetch-with-auth.ts
index 73c44be9..9b954982 100644
--- a/src/utils/fetch-with-auth.ts
+++ b/src/utils/fetch-with-auth.ts
@@ -1,41 +1,96 @@
 import type { AxiosInstance } from "axios";
 
+/** Options for {@link Base44Client.fetchWithAuth}. */
+export interface FetchWithAuthInit extends RequestInit {
+  /**
+   * The `fetch` that resolves a root-relative path against your app's own
+   * routes. Defaults to the global `fetch`, which does that in a browser but
+   * not on a server, where a path with no origin has nothing to resolve
+   * against. In Nitro pass its own, which routes a leading-slash path
+   * in-process: `import { fetch } from "nitro"`.
+   */
+  fetch?: (input: string, init?: RequestInit) => Promise<Response>;
+}
+
 /**
- * Builds the client's `fetchWithAuth`: a `fetch` that attaches the signed-in
- * user's access token to a request for the app's own origin.
+ * Builds the client's `fetchWithAuth`: a `fetch` to the app's own origin that
+ * carries whatever credentials this client holds.
+ *
+ * That is the user's access token in a browser, and from
+ * `createClientFromRequest()` the full set that function reads back — so a
+ * route reached this way rebuilds the caller's client, service role included.
+ * The service credential is minted per request for the app as a whole, not for
+ * one route, so a handler in the same worker already runs with that authority;
+ * reaching it through a route hop is the privilege it would have had by
+ * importing a shared function. What must never happen is a credential leaving
+ * the app, and the relative-path rule, not a shorter header list, is what
+ * prevents that.
  *
- * @param axios - The user-scoped axios instance. Its `Authorization` default is
- * the live token: it follows `setToken()` and is deleted on `logout()`, so a
- * request never carries a token the user no longer has. In a server-side client
- * from `createClientFromRequest()` it holds the caller's own token.
+ * @param axios - The user-scoped instance. Its `Authorization` default is the
+ * live token: it follows `setToken()` and is deleted on `logout()`, so a
+ * request never carries a token the user no longer has. From
+ * `createClientFromRequest()` it holds the caller's own token.
+ * @param serviceRoleAxios - The service-role instance, holding the app's
+ * per-request credential as its own `Authorization` default. A browser client
+ * has none, so nothing is sent.
  * @internal
  */
-export function createFetchWithAuth(axios: AxiosInstance) {
-  const currentToken = (): string | null => {
-    const header = axios.defaults.headers.common["Authorization"];
-    if (typeof header !== "string" || !header.startsWith("Bearer ")) {
-      return null;
-    }
-    return header.slice("Bearer ".length) || null;
+export function createFetchWithAuth({
+  axios,
+  serviceRoleAxios,
+  appId,
+  serverUrl,
+  functionsVersion,
+  platformHeaders,
+}: {
+  axios: AxiosInstance;
+  serviceRoleAxios: AxiosInstance;
+  appId: string;
+  serverUrl: string;
+  functionsVersion?: string;
+  platformHeaders?: Record<string, string>;
+}) {
+  const inherited = new Headers(platformHeaders);
+
+  const bearer = (client: AxiosInstance): string | null => {
+    const header = client.defaults.headers.common["Authorization"];
+    return typeof header === "string" && header.startsWith("Bearer ")
+      ? header
+      : null;
   };
 
   return async function fetchWithAuth(
     path: string,
-    init: RequestInit = {}
+    init: FetchWithAuthInit = {}
   ): Promise<Response> {
     assertOwnOriginPath(path);
 
+    const { fetch: transport = fetch, ...requestInit } = init;
     const headers = new Headers(init.headers);
-    const token = currentToken();
 
-    if (token && !headers.has("Authorization")) {
-      headers.set("Authorization", `Bearer ${token}`);
-    }
+    // A caller-supplied value always wins, so a route can hand the callee a
+    // different identity on purpose (say, dropping Authorization to render a
+    // page as anonymous).
+    const inherit = (name: string, value: string | null | undefined) => {
+      if (value && !headers.has(name)) headers.set(name, value);
+    };
+
+    // Exactly what createClientFromRequest reads, so the callee can rebuild
+    // this client. Keep the two in step.
+    inherit("Authorization", bearer(axios));
+    inherit("Base44-Service-Authorization", bearer(serviceRoleAxios));
+    inherit("Base44-App-Id", appId);
+    inherit("Base44-Api-Url", serverUrl);
+    inherit("Base44-Functions-Version", functionsVersion);
+    inherit("Base44-State", inherited.get("Base44-State"));
+    inherit("X-Data-Env", inherited.get("X-Data-Env"));
 
-    // Passed through untouched: resolving it here would need a document, and a
-    // root-relative path is already what a runtime that dispatches in-process
-    // (Nitro's `fetch`) expects.
-    return fetch(path, { ...init, headers });
+    // The path is passed through untouched: resolving it here would need a
+    // document, and a root-relative path is already what a runtime that
+    // dispatches in-process (Nitro's `fetch`) expects. `host` is deliberately
+    // never sent — such a runtime synthesizes the sub-request's origin from it,
+    // so forwarding the inbound one would point the hop at another host.
+    return transport(path, { ...requestInit, headers });
   };
 }
 
@@ -59,7 +114,7 @@ function assertOwnOriginPath(path: string): void {
     asParsed.startsWith("/\\")
   ) {
     throw new Error(
-      `fetchWithAuth() only sends requests to your app's own origin, so the access token never reaches a third party. "${path}" is not a path on it — pass a relative path such as '/api/orders'. Use base44.functions.fetch() to call a Base44 backend function, or plain fetch() for another origin.`
+      `fetchWithAuth() only sends requests to your app's own origin, so your app's credentials never reach a third party. "${path}" is not a path on it — pass a relative path such as '/api/orders'. Use base44.functions.fetch() to call a Base44 backend function, or plain fetch() for another origin.`
     );
   }
 }
diff --git a/tests/unit/fetch-with-auth.test.ts b/tests/unit/fetch-with-auth.test.ts
index 7534b843..73167ff2 100644
--- a/tests/unit/fetch-with-auth.test.ts
+++ b/tests/unit/fetch-with-auth.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
-import { createClient } from "../../src/index.ts";
+import { createClient, createClientFromRequest } from "../../src/index.ts";
 
 const appId = "test-app-id";
 const origin = "https://my-app.base44.app";
@@ -191,3 +191,147 @@ describe("fetchWithAuth", () => {
     expect(fetchMock).not.toHaveBeenCalled();
   });
 });
+
+const apiUrl = "https://base44.app";
+
+/** The header set the platform puts on a fullstack worker request. */
+function inboundRequest(
+  overrides: Record<string, string | undefined> = {}
+): Request {
+  const headers: Record<string, string> = {
+    Authorization: "Bearer caller-user-token",
+    "Base44-Service-Authorization": "Bearer service-credential",
+    "Base44-App-Id": appId,
+    "Base44-Api-Url": apiUrl,
+    "Base44-Functions-Version": "draft",
+    "Base44-State": "signed-state-jwt",
+    "X-Data-Env": "dev",
+    host: "my-app.base44.app",
+    cookie: "session=irrelevant",
+  };
+  for (const [name, value] of Object.entries(overrides)) {
+    if (value === undefined) delete headers[name];
+    else headers[name] = value;
+  }
+  return new Request(`${origin}/page`, { headers });
+}
+
+describe("fetchWithAuth from a server route", () => {
+  test("sends every header createClientFromRequest reads, so the callee rebuilds the same client", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    const { url, headers } = lastCall();
+    expect(url).toBe("/api/items");
+    expect(headers.get("Authorization")).toBe("Bearer caller-user-token");
+    expect(headers.get("Base44-App-Id")).toBe(appId);
+    expect(headers.get("Base44-Api-Url")).toBe(apiUrl);
+    expect(headers.get("Base44-Functions-Version")).toBe("draft");
+    expect(headers.get("Base44-State")).toBe("signed-state-jwt");
+    expect(headers.get("X-Data-Env")).toBe("dev");
+  });
+
+  test("carries the service credential, so asServiceRole works in the callee", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    expect(lastCall().headers.get("Base44-Service-Authorization")).toBe(
+      "Bearer service-credential"
+    );
+  });
+
+  test("does not forward host, which would repoint the sub-request's origin", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    expect(lastCall().headers.has("host")).toBe(false);
+  });
+
+  test("forwards nothing from the inbound request beyond that set", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    expect(lastCall().headers.has("cookie")).toBe(false);
+  });
+
+  test("stays anonymous when the caller is", async () => {
+    const base44 = createClientFromRequest(
+      inboundRequest({ Authorization: undefined })
+    );
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    const { headers } = lastCall();
+    expect(headers.has("Authorization")).toBe(false);
+    expect(headers.get("Base44-Service-Authorization")).toBe(
+      "Bearer service-credential"
+    );
+  });
+
+  test("omits headers the inbound request did not carry", async () => {
+    const base44 = createClientFromRequest(
+      inboundRequest({
+        "Base44-State": undefined,
+        "X-Data-Env": undefined,
+        "Base44-Functions-Version": undefined,
+      })
+    );
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    const { headers } = lastCall();
+    expect(headers.has("Base44-State")).toBe(false);
+    expect(headers.has("X-Data-Env")).toBe(false);
+    expect(headers.has("Base44-Functions-Version")).toBe(false);
+  });
+
+  test("renders as anonymous when the caller drops Authorization on purpose", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", {
+      fetch: fetchMock,
+      headers: { Authorization: "" },
+    });
+
+    expect(lastCall().headers.get("Authorization")).toBe("");
+  });
+
+  test("uses the given transport and does not pass it on as request init", async () => {
+    vi.stubGlobal("fetch", vi.fn());
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await base44.fetchWithAuth("/api/items", { fetch: fetchMock });
+
+    expect(fetchMock).toHaveBeenCalledOnce();
+    expect(lastCall().init).not.toHaveProperty("fetch");
+  });
+
+  test("refuses to send the app's credentials to another origin", async () => {
+    const base44 = createClientFromRequest(inboundRequest());
+
+    await expect(
+      base44.fetchWithAuth("https://evil.example/steal", { fetch: fetchMock })
+    ).rejects.toThrow(/only sends requests to your app's own origin/);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});
+
+describe("fetchWithAuth in a browser", () => {
+  // The reason one method can serve both: a browser client is built without a
+  // serviceToken, so there is no service credential for it to send. This is
+  // what makes the wider header set safe to apply everywhere.
+  test("sends no service credential, having none", async () => {
+    stubBrowser();
+    const base44 = createTestClient("user-token");
+
+    await base44.fetchWithAuth("/api/orders");
+
+    const { headers } = lastCall();
+    expect(headers.get("Authorization")).toBe("Bearer user-token");
+    expect(headers.has("Base44-Service-Authorization")).toBe(false);
+  });
+});