diff --git a/packages/fresh/src/build_cache.ts b/packages/fresh/src/build_cache.ts
index a63cbc5510e..e99f0ebc628 100644
--- a/packages/fresh/src/build_cache.ts
+++ b/packages/fresh/src/build_cache.ts
@@ -35,6 +35,8 @@ export interface BuildCache<State = any> {
   root: string;
   islandRegistry: ServerIslandRegistry;
   clientEntry: string;
+  /** Pathname for the HMR-only chunk (development only). Undefined in production. */
+  hmrClientEntry?: string;
   features: {
     errorOverlay: boolean;
   };
diff --git a/packages/fresh/src/context.ts b/packages/fresh/src/context.ts
index b89e21a2c07..782beb2a9ef 100644
--- a/packages/fresh/src/context.ts
+++ b/packages/fresh/src/context.ts
@@ -357,25 +357,34 @@ export class Context<State> {
         }
         throw err;
       } finally {
-        // Add preload headers
+        // Add preload headers only when client JS is actually emitted.
         const basePath = this.config.basePath;
-        const runtimeUrl = state.buildCache.clientEntry.startsWith(".")
-          ? state.buildCache.clientEntry.slice(1)
-          : state.buildCache.clientEntry;
-        let link = `<${
-          encodeURI(`${basePath}${runtimeUrl}`)
-        }>; rel="modulepreload"; as="script"`;
-        state.islands.forEach((island) => {
-          const specifier = `${basePath}${
-            island.file.startsWith(".") ? island.file.slice(1) : island.file
-          }`;
-          link += `, <${
-            encodeURI(specifier)
-          }>; rel="modulepreload"; as="script"`;
-        });
-
-        if (link !== "") {
-          headers.append("Link", link);
+        const linkParts: string[] = [];
+
+        if (
+          state.needsClientRuntime ||
+          state.buildCache.hmrClientEntry !== undefined
+        ) {
+          const runtimeUrl = state.buildCache.clientEntry.startsWith(".")
+            ? state.buildCache.clientEntry.slice(1)
+            : state.buildCache.clientEntry;
+          linkParts.push(
+            `<${
+              encodeURI(`${basePath}${runtimeUrl}`)
+            }>; rel="modulepreload"; as="script"`,
+          );
+          state.islands.forEach((island) => {
+            const specifier = `${basePath}${
+              island.file.startsWith(".") ? island.file.slice(1) : island.file
+            }`;
+            linkParts.push(
+              `<${encodeURI(specifier)}>; rel="modulepreload"; as="script"`,
+            );
+          });
+        }
+
+        if (linkParts.length > 0) {
+          headers.append("Link", linkParts.join(", "));
         }
 
         renderNonce = state.nonce;
diff --git a/packages/fresh/src/dev/builder.ts b/packages/fresh/src/dev/builder.ts
index 723a2e9d538..69b3711583c 100644
--- a/packages/fresh/src/dev/builder.ts
+++ b/packages/fresh/src/dev/builder.ts
@@ -314,6 +314,13 @@ export class Builder<State = any> {
       "fresh-runtime": new URL(runtimePath, import.meta.url).href,
     };
 
+    if (dev) {
+      entryPoints["fresh-hmr"] = new URL(
+        "../runtime/client/dev_hmr.ts",
+        import.meta.url,
+      ).href;
+    }
+
     const namer = new UniqueNamer();
     for (const spec of this.#islandSpecifiers) {
       const specName = specToName(spec);
@@ -353,6 +360,13 @@ export class Builder<State = any> {
       buildCache.islandModNameToChunk.get(name)!.browser = pathname;
     }
 
+    if (dev) {
+      const hmrChunkName = output.entryToChunk.get("fresh-hmr");
+      if (hmrChunkName !== undefined) {
+        buildCache.hmrClientEntry = `${prefix}${hmrChunkName}`;
+      }
+    }
+
     for (let i = 0; i < output.files.length; i++) {
       const file = output.files[i];
       const pathname = `${prefix}${file.path}`;
diff --git a/packages/fresh/src/dev/dev_build_cache.ts b/packages/fresh/src/dev/dev_build_cache.ts
index 2414886274c..b34098b55fd 100644
--- a/packages/fresh/src/dev/dev_build_cache.ts
+++ b/packages/fresh/src/dev/dev_build_cache.ts
@@ -63,6 +63,7 @@ export class MemoryBuildCache<State> implements DevBuildCache<State> {
   root: string;
   islandRegistry: ServerIslandRegistry = new Map();
   clientEntry: string;
+  hmrClientEntry: string | undefined = undefined;
   features = { errorOverlay: false };
 
   constructor(
@@ -236,6 +237,7 @@ export class DiskBuildCache<State> implements DevBuildCache<State> {
   root: string;
   islandRegistry: ServerIslandRegistry = new Map();
   clientEntry: string = "";
+  hmrClientEntry: string | undefined = undefined;
   features = { errorOverlay: false };
 
   constructor(
diff --git a/packages/fresh/src/runtime/client/dev.ts b/packages/fresh/src/runtime/client/dev.ts
index 8f993518c59..f96f681c82d 100644
--- a/packages/fresh/src/runtime/client/dev.ts
+++ b/packages/fresh/src/runtime/client/dev.ts
@@ -1,136 +1,3 @@
 import "preact/debug";
 export * from "./mod.ts";
-import { IS_BROWSER } from "../shared.ts";
-
-let ws: WebSocket;
-let revision = 0;
-
-let reconnectTimer: number;
-const backoff = [
-  // Wait 100ms initially, because we could also be
-  // disconnected because of a form submit.
-  100,
-  150,
-  200,
-  250,
-  300,
-  350,
-  400,
-  450,
-  500,
-  500,
-  605,
-  750,
-  1000,
-  1250,
-  1500,
-  1750,
-  2000,
-];
-let backoffIdx = 0;
-function reconnect() {
-  if (ws.readyState !== ws.CLOSED) return;
-
-  reconnectTimer = setTimeout(() => {
-    if (backoffIdx === 0) {
-      // deno-lint-ignore no-console
-      console.log(
-        `%c Fresh %c Connection closed. Trying to reconnect...`,
-        "background-color: #86efac; color: black",
-        "color: inherit",
-      );
-    }
-    backoffIdx++;
-
-    try {
-      connect();
-      clearTimeout(reconnectTimer);
-    } catch (_err) {
-      reconnect();
-    }
-  }, backoff[Math.min(backoffIdx, backoff.length - 1)]);
-}
-
-function onOpenWs() {
-  backoffIdx = 0;
-}
-
-function onCloseWs() {
-  disconnect();
-  reconnect();
-}
-
-function connect() {
-  const url = new URL("/_frsh/alive", location.origin.replace("http", "ws"));
-  ws = new WebSocket(
-    url,
-  );
-
-  ws.addEventListener("open", onOpenWs);
-  ws.addEventListener("close", onCloseWs);
-  ws.addEventListener("message", handleMessage);
-  ws.addEventListener("error", handleError);
-}
-
-function disconnect() {
-  ws.removeEventListener("open", onOpenWs);
-  ws.removeEventListener("close", onCloseWs);
-  ws.removeEventListener("message", handleMessage);
-  ws.removeEventListener("error", handleError);
-  ws.close();
-}
-
-function handleMessage(e: MessageEvent) {
-  const data = JSON.parse(e.data);
-  switch (data.type) {
-    case "initial-state": {
-      if (revision === 0) {
-        // deno-lint-ignore no-console
-        console.log(
-          `%c Fresh %c Connected to development server.`,
-          "background-color: #86efac; color: black",
-          "color: inherit",
-        );
-      }
-
-      if (revision === 0) {
-        revision = data.revision;
-      } else if (revision < data.revision) {
-        disconnect();
-        // Needs reload
-        location.reload();
-      }
-    }
-  }
-}
-
-function handleError(e: Event) {
-  // TODO
-  // deno-lint-ignore no-explicit-any
-  if (e && (e as any).code === "ECONNREFUSED") {
-    setTimeout(connect, 1000);
-  }
-}
-
-if (IS_BROWSER) {
-  connect();
-
-  addEventListener("message", (ev) => {
-    if (ev.origin !== location.origin) return;
-    if (typeof ev.data !== "string" || ev.data !== "close-error-overlay") {
-      return;
-    }
-
-    document.querySelector("#fresh-error-overlay")?.remove();
-  });
-
-  // Disconnect when the tab becomes inactive and re-connect when it
-  // becomes active again
-  addEventListener("visibilitychange", () => {
-    if (document.hidden) {
-      disconnect();
-    } else {
-      connect();
-    }
-  });
-}
+export * from "./dev_hmr.ts";
diff --git a/packages/fresh/src/runtime/client/dev_hmr.ts b/packages/fresh/src/runtime/client/dev_hmr.ts
new file mode 100644
index 00000000000..b86c98e0358
--- /dev/null
+++ b/packages/fresh/src/runtime/client/dev_hmr.ts
@@ -0,0 +1,134 @@
+import { IS_BROWSER } from "../shared.ts";
+
+let ws: WebSocket;
+let revision = 0;
+
+let reconnectTimer: number;
+const backoff = [
+  // Wait 100ms initially, because we could also be
+  // disconnected because of a form submit.
+  100,
+  150,
+  200,
+  250,
+  300,
+  350,
+  400,
+  450,
+  500,
+  500,
+  605,
+  750,
+  1000,
+  1250,
+  1500,
+  1750,
+  2000,
+];
+let backoffIdx = 0;
+function reconnect() {
+  if (ws.readyState !== ws.CLOSED) return;
+
+  reconnectTimer = setTimeout(() => {
+    if (backoffIdx === 0) {
+      // deno-lint-ignore no-console
+      console.log(
+        `%c Fresh %c Connection closed. Trying to reconnect...`,
+        "background-color: #86efac; color: black",
+        "color: inherit",
+      );
+    }
+    backoffIdx++;
+
+    try {
+      connect();
+      clearTimeout(reconnectTimer);
+    } catch (_err) {
+      reconnect();
+    }
+  }, backoff[Math.min(backoffIdx, backoff.length - 1)]);
+}
+
+function onOpenWs() {
+  backoffIdx = 0;
+}
+
+function onCloseWs() {
+  disconnect();
+  reconnect();
+}
+
+function connect() {
+  const url = new URL("/_frsh/alive", location.origin.replace("http", "ws"));
+  ws = new WebSocket(
+    url,
+  );
+
+  ws.addEventListener("open", onOpenWs);
+  ws.addEventListener("close", onCloseWs);
+  ws.addEventListener("message", handleMessage);
+  ws.addEventListener("error", handleError);
+}
+
+function disconnect() {
+  ws.removeEventListener("open", onOpenWs);
+  ws.removeEventListener("close", onCloseWs);
+  ws.removeEventListener("message", handleMessage);
+  ws.removeEventListener("error", handleError);
+  ws.close();
+}
+
+function handleMessage(e: MessageEvent) {
+  const data = JSON.parse(e.data);
+  switch (data.type) {
+    case "initial-state": {
+      if (revision === 0) {
+        // deno-lint-ignore no-console
+        console.log(
+          `%c Fresh %c Connected to development server.`,
+          "background-color: #86efac; color: black",
+          "color: inherit",
+        );
+      }
+
+      if (revision === 0) {
+        revision = data.revision;
+      } else if (revision < data.revision) {
+        disconnect();
+        // Needs reload
+        location.reload();
+      }
+    }
+  }
+}
+
+function handleError(e: Event) {
+  // TODO
+  // deno-lint-ignore no-explicit-any
+  if (e && (e as any).code === "ECONNREFUSED") {
+    setTimeout(connect, 1000);
+  }
+}
+
+if (IS_BROWSER) {
+  connect();
+
+  addEventListener("message", (ev) => {
+    if (ev.origin !== location.origin) return;
+    if (typeof ev.data !== "string" || ev.data !== "close-error-overlay") {
+      return;
+    }
+
+    document.querySelector("#fresh-error-overlay")?.remove();
+  });
+
+  // Disconnect when the tab becomes inactive and re-connect when it
+  // becomes active again
+  addEventListener("visibilitychange", () => {
+    if (document.hidden) {
+      disconnect();
+    } else {
+      connect();
+    }
+  });
+}
diff --git a/packages/fresh/src/runtime/server/preact_hooks.ts b/packages/fresh/src/runtime/server/preact_hooks.ts
index d33d61fee1a..b7ad8f8e733 100644
--- a/packages/fresh/src/runtime/server/preact_hooks.ts
+++ b/packages/fresh/src/runtime/server/preact_hooks.ts
@@ -82,6 +82,26 @@ export class RenderState {
   renderedHtmlBody = false;
   renderedHtmlHead = false;
   hasRuntimeScript = false;
+  /** Set to true when any element in the tree renders f-client-nav="true". */
+  clientNavEnabled = false;
+
+  /**
+   * True when the page needs Fresh's client runtime (islands, client nav, or
+   * `<Partial>` regions on a full document). Partial subresponses omit boot;
+   * `encounteredPartials` must not force runtime for those requests.
+   */
+  get needsClientRuntime(): boolean {
+    if (this.islands.size > 0 || this.clientNavEnabled) {
+      return true;
+    }
+    if (
+      !this.ctx.url.searchParams.has(PARTIAL_SEARCH_PARAM) &&
+      this.encounteredPartials.size > 0
+    ) {
+      return true;
+    }
+    return false;
+  }
 
   constructor(
     // deno-lint-ignore no-explicit-any
@@ -419,6 +439,22 @@ options[OptionsType.DIFF] = (vnode) => {
           break;
       }
 
+      // Detect f-client-nav="true" on any element in the rendered tree.
+      // We check here in the diff hook (not the vnode hook) so we catch both
+      // VNodes created inside component functions during rendering AND those
+      // pre-created in route handlers before setRenderState was called.
+      //
+      // The === "true" check relies on the vnode hook having normalized boolean
+      // f-client-nav on string elements via String(...) (see OptionsType.VNODE).
+      // Preact invokes the vnode hook before diff for a given VNode, so e.g.
+      // <html f-client-nav> becomes the string "true" before we run here.
+      if (
+        CLIENT_NAV_ATTR in (vnode.props as Record<string, unknown>) &&
+        (vnode.props as Record<string, unknown>)[CLIENT_NAV_ATTR] === "true"
+      ) {
+        RENDER_STATE!.clientNavEnabled = true;
+      }
+
       if (
         vnode.key !== undefined &&
         (RENDER_STATE!.partialDepth > 0 || hasIslandOwner(RENDER_STATE!, vnode))
@@ -623,6 +659,9 @@ function FreshRuntimeScript() {
 
   const islandArr = Array.from(islands);
 
+  // Partial responses only embed __FRSH_STATE__ JSON for the swapped fragment.
+  // We do not gate on needsClientRuntime: the parent full-document response is
+  // responsible for loading the client boot when islands or client nav require it.
   if (ctx.url.searchParams.has(PARTIAL_SEARCH_PARAM)) {
     const islands = islandArr.map((island) => {
       return {
@@ -647,7 +686,12 @@ function FreshRuntimeScript() {
         },
       })
     );
-  } else {
+  } else if (
+    RENDER_STATE!.needsClientRuntime ||
+    buildCache.hmrClientEntry !== undefined
+  ) {
+    // Full-document boot: islands / partials / client nav, or Vite/HMR dev
+    // (client entry must load so e.g. CSS side-effect imports run).
     const islandImports = islandArr.map((island) => {
       const named = island.exportName === "default"
         ? island.name
@@ -689,6 +733,9 @@ function FreshRuntimeScript() {
       )
     );
   }
+
+  // Production static page: no client JS at all.
+  return buildCache.features.errorOverlay ? h(ShowErrorOverlay, null) : null;
 }
 
 export function ShowErrorOverlay() {
diff --git a/packages/fresh/tests/no_client_js_test.tsx b/packages/fresh/tests/no_client_js_test.tsx
new file mode 100644
index 00000000000..6ffab3cdc08
--- /dev/null
+++ b/packages/fresh/tests/no_client_js_test.tsx
@@ -0,0 +1,181 @@
+/**
+ * Tests for the "zero client JS by default" feature.
+ *
+ * Fresh should not ship any client JavaScript when a page renders no islands
+ * and has no element with `f-client-nav="true"`. The full Fresh client runtime
+ * (fresh-runtime.js + inline boot script) must only appear when actually needed.
+ */
+import { App } from "fresh";
+import { signal } from "@preact/signals";
+import { Counter } from "./fixtures_islands/Counter.tsx";
+import { expect } from "@std/expect";
+import { FakeServer } from "../src/test_utils.ts";
+import {
+  ALL_ISLAND_DIR,
+  assertNotSelector,
+  assertSelector,
+  buildProd,
+  Doc,
+  parseHtml,
+} from "./test_utils.tsx";
+
+// Build once so all island tests share the same compiled output.
+const allIslandCache = await buildProd({ islandDir: ALL_ISLAND_DIR });
+
+function islandApp(): App<unknown> {
+  const app = new App().get("/", (ctx) => {
+    const sig = signal(0);
+    return ctx.render(
+      <Doc>
+        <Counter count={sig} />
+      </Doc>,
+    );
+  });
+  allIslandCache(app);
+  return app;
+}
+
+// ---------------------------------------------------------------------------
+// Static pages (no islands, no f-client-nav)
+// ---------------------------------------------------------------------------
+
+Deno.test("no-client-js - static page emits no module boot script", async () => {
+  const app = new App().get("/", (ctx) =>
+    ctx.render(
+      <Doc>
+        <h1>Hello</h1>
+      </Doc>,
+    ));
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+  const doc = parseHtml(html);
+
+  // No inline module script should be present.
+  assertNotSelector(doc, 'script[type="module"]');
+  // Confirm the page itself rendered fine.
+  assertSelector(doc, "h1");
+  expect(doc.querySelector("h1")?.textContent).toEqual("Hello");
+});
+
+Deno.test("no-client-js - static page does not reference fresh-runtime.js", async () => {
+  const app = new App().get("/", (ctx) =>
+    ctx.render(
+      <Doc>
+        <p>Hi</p>
+      </Doc>,
+    ));
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+
+  expect(html).not.toContain("fresh-runtime");
+  expect(html).not.toContain("boot(");
+});
+
+Deno.test("no-client-js - static page has no Link modulepreload header", async () => {
+  const app = new App().get("/", (ctx) =>
+    ctx.render(
+      <Doc>
+        <p>Hi</p>
+      </Doc>,
+    ));
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+
+  expect(res.headers.get("Link")).toBeNull();
+});
+
+// ---------------------------------------------------------------------------
+// Pages with f-client-nav="true" must still load the client runtime.
+// f-client-nav is typically placed in an appWrapper or layout component,
+// so the VNode is created INSIDE a component that runs during renderToString
+// (and therefore sees an active RENDER_STATE).
+// ---------------------------------------------------------------------------
+
+Deno.test("no-client-js - f-client-nav='true' page includes boot script", async () => {
+  const app = new App()
+    .appWrapper(({ Component }) => (
+      <html f-client-nav>
+        <head>
+          <meta charset="utf-8" />
+        </head>
+        <body>
+          <Component />
+        </body>
+      </html>
+    ))
+    .get("/", (ctx) => ctx.render(<a href="/other">Link</a>));
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+  const doc = parseHtml(html);
+
+  // A module script importing boot must be present.
+  assertSelector(doc, 'script[type="module"]');
+  expect(html).toContain("boot(");
+});
+
+Deno.test("no-client-js - f-client-nav='false' page emits no boot script", async () => {
+  const app = new App()
+    .appWrapper(({ Component }) => (
+      <html f-client-nav={false}>
+        <head>
+          <meta charset="utf-8" />
+        </head>
+        <body>
+          <Component />
+        </body>
+      </html>
+    ))
+    .get("/", (ctx) => ctx.render(<p>Static</p>));
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+
+  expect(html).not.toContain("boot(");
+  expect(html).not.toContain("fresh-runtime");
+});
+
+// ---------------------------------------------------------------------------
+// Pages with islands must still load the client runtime
+// ---------------------------------------------------------------------------
+
+Deno.test("no-client-js - island page includes boot script", async () => {
+  const app = islandApp();
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+  const doc = parseHtml(html);
+
+  assertSelector(doc, 'script[type="module"]');
+  expect(html).toContain("boot(");
+});
+
+Deno.test("no-client-js - island page references fresh-runtime.js", async () => {
+  const app = islandApp();
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+  const html = await res.text();
+
+  expect(html).toContain("fresh-runtime");
+});
+
+Deno.test("no-client-js - island page has Link modulepreload header", async () => {
+  const app = islandApp();
+
+  const server = new FakeServer(app.handler());
+  const res = await server.get("/");
+
+  const link = res.headers.get("Link");
+  expect(link).not.toBeNull();
+  expect(link).toContain("modulepreload");
+  expect(link).toContain("fresh-runtime");
+});
diff --git a/packages/plugin-vite/src/plugins/deno.ts b/packages/plugin-vite/src/plugins/deno.ts
index 97f6ad7c5e0..1c5f9f6d34e 100644
--- a/packages/plugin-vite/src/plugins/deno.ts
+++ b/packages/plugin-vite/src/plugins/deno.ts
@@ -302,6 +302,7 @@ function isJsMediaType(media: MediaType): boolean {
     case MediaType.Unknown:
       return false;
   }
+  return false;
 }
 
 export type DenoSpecifier = string & { __deno: string };
diff --git a/packages/plugin-vite/tests/dev_server_test.ts b/packages/plugin-vite/tests/dev_server_test.ts
index 6f446ed310d..b271c9f326a 100644
--- a/packages/plugin-vite/tests/dev_server_test.ts
+++ b/packages/plugin-vite/tests/dev_server_test.ts
@@ -203,7 +203,10 @@ Deno.test({
           waitUntil: "networkidle2",
         });
 
-        await page.locator("style[data-vite-dev-id$='style.css']").wait();
+        // Vite 6: data-vite-dev-id; Vite 7+: vite-module-id on injected style.
+        await page.locator(
+          "style[data-vite-dev-id$='style.css'], style[vite-module-id]",
+        ).wait();
       });
     });
   },
@@ -221,7 +224,9 @@ Deno.test({
           waitUntil: "networkidle2",
         });
 
-        await page.locator("style[data-vite-dev-id$='style.css']").wait();
+        await page.locator(
+          "style[data-vite-dev-id$='style.css'], style[vite-module-id]",
+        ).wait();
       });
     });
   },