diff --git a/.changeset/prepare-request-sync-send.md b/.changeset/prepare-request-sync-send.md
new file mode 100644
index 000000000..e55854ba6
--- /dev/null
+++ b/.changeset/prepare-request-sync-send.md
@@ -0,0 +1,5 @@
+---
+"@solidjs/web": patch
+---
+
+A synchronous `prepareRequest` hook no longer defers the server-function request by a microtask, so reads adopted during hydration stay inside the hydration trace instead of refetching from the browser (#3959). An async hook is still awaited.
diff --git a/packages/web/server-functions/src/client.ts b/packages/web/server-functions/src/client.ts
index dbfda6c8e..1c16257c9 100644
--- a/packages/web/server-functions/src/client.ts
+++ b/packages/web/server-functions/src/client.ts
@@ -225,6 +225,10 @@ export interface ServerFunctionsClientConfig {
    *   }
    * });
    * ```
+   *
+   * Return synchronously when you can: hydration adopts a server-rendered
+   * read only if its request is sent in the same tick, so an async hook
+   * makes every hydrated read fetch again from the browser.
    */
   prepareRequest?: PrepareRequestHook;
   /**
@@ -608,7 +612,9 @@ async function createRequest(base, id, options, meta) {
   // RequestInit (transport headers included), so session policy can adjust
   // anything the transport is about to send.
   if (config.prepareRequest) {
-    const prepared = await config.prepareRequest(init, { id, meta });
+    let prepared = config.prepareRequest(init, { id, meta });
+    // A sync hook must send in this tick: hydration's fetch mock is only in place synchronously.
+    if (prepared && typeof prepared.then === "function") prepared = await prepared;
     // The hook's return is validated, not adopted wholesale (#3174): the
     // natural mistake — returning a fresh `{ headers }` instead of
     // spreading — used to silently drop the argument payload, the abort
diff --git a/packages/web/test/hydration/prepare-request-hydration-3959.spec.tsx b/packages/web/test/hydration/prepare-request-hydration-3959.spec.tsx
new file mode 100644
index 000000000..680f634d5
--- /dev/null
+++ b/packages/web/test/hydration/prepare-request-hydration-3959.spec.tsx
@@ -0,0 +1,56 @@
+/**
+ * @vitest-environment jsdom
+ *
+ * #3959: a synchronous `prepareRequest` hook must send inside hydration's
+ * synchronous trace, where the mocked fetch absorbs the adopted read. Before
+ * the fix the hook was always awaited, so the read reached the network.
+ */
+import { afterEach, expect, test, vi } from "vitest";
+import { createMemo, createRoot, enableHydration, flush, sharedConfig } from "solid-js";
+import {
+  configureServerFunctionsClient,
+  createServerReference
+} from "../../server-functions/src/client.js";
+
+enableHydration();
+
+const { has, load, gather } = sharedConfig as any;
+
+afterEach(() => {
+  sharedConfig.hydrating = false;
+  Object.assign(sharedConfig, { has, load, gather });
+  configureServerFunctionsClient({ prepareRequest: null as any });
+  vi.unstubAllGlobals();
+});
+
+test.each([
+  ["no prepareRequest", undefined],
+  ["a synchronous prepareRequest", (init: RequestInit) => init]
+])("an adopted server-function read does not refetch with %s", async (_, hook) => {
+  const data: Record<string, unknown> = { t0: "server" };
+  sharedConfig.hydrating = true;
+  (sharedConfig as any).has = (id: string) => id in data;
+  (sharedConfig as any).load = (id: string) => data[id];
+  (sharedConfig as any).gather = () => {};
+  const urls: string[] = [];
+  vi.stubGlobal("fetch", async (input: RequestInfo | URL) => {
+    urls.push(String(input));
+    return new Response("null");
+  });
+  if (hook) configureServerFunctionsClient({ prepareRequest: hook });
+
+  const read = createServerReference("prepare-request-3959");
+  let value!: () => unknown;
+  createRoot(
+    () => {
+      value = createMemo(() => read());
+    },
+    { id: "t" }
+  );
+  flush();
+  sharedConfig.hydrating = false;
+  await new Promise(resolve => setTimeout(resolve, 20));
+
+  expect(value()).toBe("server");
+  expect(urls).toEqual([]);
+});
diff --git a/packages/web/test/server/server-functions-extensions.spec.tsx b/packages/web/test/server/server-functions-extensions.spec.tsx
index 2ee22e543..e2bc4528e 100644
--- a/packages/web/test/server/server-functions-extensions.spec.tsx
+++ b/packages/web/test/server/server-functions-extensions.spec.tsx
@@ -258,6 +258,39 @@ describe("server-function extension surface (built bundles)", () => {
     }
   });
 
+  it("sends in the calling tick when prepareRequest returns synchronously", async () => {
+    registerServerFunction("ext-fetch-8", async (n?: number) => n ?? 0);
+    const restore = connectTransport();
+    const send = globalThis.fetch;
+    let sends = 0;
+    // hydration swaps the global fetch only for one synchronous compute (#3959)
+    globalThis.fetch = ((address: any, init?: RequestInit) => {
+      sends++;
+      return send(address, init);
+    }) as typeof fetch;
+    configureServerFunctionsClient({ prepareRequest: init => init });
+    const calls: Promise<unknown>[] = [];
+    try {
+      const plain = createServerReference("ext-fetch-8")();
+      const withArg = createServerReference("ext-fetch-8")(5);
+      calls.push(plain, withArg);
+      expect(sends).toBe(2);
+      expect(await plain).toBe(0);
+      expect(await withArg).toBe(5);
+
+      configureServerFunctionsClient({ prepareRequest: async init => init });
+      const deferred = createServerReference("ext-fetch-8")(7);
+      calls.push(deferred);
+      expect(sends).toBe(2);
+      expect(await deferred).toBe(7);
+      expect(sends).toBe(3);
+    } finally {
+      await Promise.allSettled(calls);
+      configureServerFunctionsClient({ prepareRequest: null as any });
+      restore();
+    }
+  });
+
   it("sends a GET-declared read through the configured fetch too", async () => {
     serverGET(
       createServerSideReference(registerServerReference("ext-fetch-5", async (n: number) => n * 2))