@@ -123,6 +123,8 @@ Fetch handlers only hand over `Request`s, so the host framework binds the RPC so
 3. **`ws: { sidecar: true }`**: a side-car server on a free port, for host frameworks whose handlers never see upgrades (Next.js route handlers, Nitro, Rsbuild).
 4. **The host framework's own upgrades.** With none set, the socket waits: `devtools.attach(server)` routes a server's `upgrade` events (returning a detach fn); `devtools.handleUpgrade(req, socket, head)` completes a single one from a listener you own. To run your own check before the socket opens, own the listener, match the `<base>__ws` path first (the listener sees every upgrade on the server, including the host framework's own), and call `handleUpgrade` once the check passes; the `server` option attaches its listener asynchronously, after any guard that wraps the listeners present at startup.
 
+The browser dials a side-car on its page's own hostname, so a side-car on the default `localhost` host listens on both `127.0.0.1` and `::1`. Pages opened on either address reach it.
+
 `ws.url` controls the *advertisement* instead, so the browser dials it verbatim. Alone, an external WebSocket server owns the transport and its auth (wire the running devframe's `context` via `createContextRpcServer` + a WS transport); alongside a local binding it overrides only the advertisement (the tunnel pattern).
 
 `__connection.json` describes the active combination. Asking a configured running devframe to take over the host framework's upgrades reports `DF0055` (a local binding owns the socket) or `DF0056` (`ws.url` handed it off).
 
@@ -1,7 +1,9 @@
 import type { DevframeNodeContext, DevframeRpcClientFunctions, DevframeRpcServerFunctions } from '../../types'
+import { lookup } from 'node:dns/promises'
 import { mkdtempSync, writeFileSync } from 'node:fs'
 import { createServer } from 'node:http'
 import { createSecureServer } from 'node:http2'
+import { isIP } from 'node:net'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { defineDevframe } from 'devframe'
@@ -12,6 +14,7 @@ import { describe, expect, it, vi } from 'vitest'
 import { WebSocket } from 'ws'
 import { getTempAuthCode } from '../../node/auth/state'
 import { getInternalContext } from '../../node/hub-internals/context'
+import { formatHostForUrl } from '../../node/utils'
 import { initDevframe } from '../initiate'
 
 const HANDSHAKE = { authToken: '', ua: 'test', origin: 'http://localhost' }
@@ -537,6 +540,55 @@ describe('adapters/handler', () => {
     }
   })
 
+  // The client dials the side-car on its page's own hostname, so a page on
+  // `127.0.0.1` and one on `[::1]` both have to reach a `localhost` side-car,
+  // whichever of the two addresses `localhost` resolves to first.
+  it('ws.sidecar on localhost: reachable on both loopback addresses', async () => {
+    const devtools = initDevframe(defineTestDef('handler-sidecar-localhost'), {
+      base: '/__handler-sidecar-localhost/',
+      auth: false,
+      ws: { sidecar: true },
+    })
+
+    try {
+      await devtools.ready
+      const { port } = devtools.connectionMeta().websocket as { port: number }
+      for (const host of ['127.0.0.1', '[::1]']) {
+        const client = connectWsClient(`ws://${host}:${port}/__ws`)
+        await expect(client.$call('test:probe' as any)).resolves.toBe('ok')
+        client.$close()
+      }
+    }
+    finally {
+      await devtools.close()
+    }
+  })
+
+  it('ws.sidecar on localhost: moves off a port another server holds on the other loopback', async () => {
+    const { address: primary } = await lookup('localhost')
+    const twin = isIP(primary) === 6 ? '127.0.0.1' : '::1'
+    const port = await getPort({ port: 18170, host: twin })
+    const squatter = createServer((_req, res) => res.end('squatter'))
+    await new Promise<void>(resolve => squatter.listen(port, twin, resolve))
+    const devtools = initDevframe(
+      defineDevframe({ ...defineTestDef('handler-sidecar-twin'), cli: { port } }),
+      { base: '/__handler-sidecar-twin/', auth: false, ws: { sidecar: true } },
+    )
+
+    try {
+      await devtools.ready
+      const advertised = (devtools.connectionMeta().websocket as { port: number }).port
+      expect(advertised).not.toBe(port)
+      const client = connectWsClient(`ws://${formatHostForUrl(twin)}:${advertised}/__ws`)
+      await expect(client.$call('test:probe' as any)).resolves.toBe('ok')
+      client.$close()
+    }
+    finally {
+      await devtools.close()
+      squatter.close()
+    }
+  })
+
   it('a configured transport refuses to take over the host upgrades', async () => {
     const wsPort = await getPort({ port: 18155, host: '127.0.0.1' })
     const sidecar = initDevframe(defineTestDef('handler-owned'), { base: '/__handler-owned/', auth: false, host: '127.0.0.1', ws: { port: wsPort } })
 
@@ -82,6 +82,54 @@ function websocketConnectionMeta(rpcHost: RpcFunctionsHostImpl, path: string | u
   return { backend: 'websocket', websocket: { path }, jsonSerializableMethods }
 }
 
+function listen(server: NodeHttpServer, port: number, host: string): Promise<void> {
+  return new Promise<void>((resolve, reject) => {
+    const onError = (error: Error): void => reject(error)
+    // Without this listener a failed bind emits `error` with nobody
+    // attached (an uncaughtException) and the `listen` callback never
+    // fires, so this promise never settles.
+    server.once('error', onError)
+    server.listen(port, host, () => {
+      server.removeListener('error', onError)
+      resolve()
+    })
+  })
+}
+
+/** Errors meaning the machine has no such loopback address to bind. */
+const NO_LOOPBACK_ADDRESS = new Set(['EADDRNOTAVAIL', 'EAFNOSUPPORT'])
+
+/**
+ * Listen on `host`, and when that is `localhost`, on the other loopback
+ * address too. `listen(port, 'localhost')` binds only the first address
+ * `localhost` resolves to (`::1` on most current Linux and macOS setups),
+ * while a client dials the side-car on its own page's hostname, so a page
+ * opened on `127.0.0.1` would be refused. The twin server hands its traffic
+ * to `server`. If another process holds the twin address, fail like a taken
+ * port, so pages on that address never reach a different server. A machine
+ * without that address family skips the twin. Resolves to a closer for the
+ * twin.
+ */
+async function listenWithLoopbackTwin(server: NodeHttpServer, port: number, host: string): Promise<() => Promise<void>> {
+  await listen(server, port, host)
+  const bound = server.address()
+  if (host !== 'localhost' || !bound || typeof bound === 'string')
+    return async () => {}
+  const twin = createServer()
+  twin.on('request', (req, res) => server.emit('request', req, res))
+  twin.on('upgrade', (req, socket, head) => server.emit('upgrade', req, socket, head))
+  try {
+    await listen(twin, bound.port, bound.family === 'IPv6' ? '127.0.0.1' : '::1')
+    return () => new Promise<void>(r => twin.close(() => r()))
+  }
+  catch (error) {
+    if (NO_LOOPBACK_ADDRESS.has((error as NodeJS.ErrnoException).code ?? ''))
+      return async () => {}
+    await new Promise<void>(r => server.close(() => r()))
+    throw error
+  }
+}
+
 /**
  * Compose an h3 + WebSocket RPC server for a devframe context: the low-level
  * "listen on a port (or share one) + attach the WS transport" binding the
@@ -115,19 +163,10 @@ async function bindHttpAndWs(options: BindHttpAndWsOptions): Promise<StartedServ
     closeWs = transport.close
   }
 
+  let closeLoopbackTwin = async (): Promise<void> => {}
   if (ownsHttpServer) {
     try {
-      await new Promise<void>((resolve, reject) => {
-        const onError = (error: Error): void => reject(error)
-        // Without this listener a failed bind emits `error` with nobody
-        // attached (an uncaughtException) and the `listen` callback never
-        // fires, so this promise never settles.
-        httpServer.once('error', onError)
-        httpServer.listen(port, bindHost, () => {
-          httpServer.removeListener('error', onError)
-          resolve()
-        })
-      })
+      closeLoopbackTwin = await listenWithLoopbackTwin(httpServer, port, bindHost)
     }
     catch (error) {
       // The WS transport is already attached above, so tear it down before
@@ -172,6 +211,7 @@ async function bindHttpAndWs(options: BindHttpAndWsOptions): Promise<StartedServ
     async close() {
       httpServer.off('listening', publishWsEndpoint)
       await closeWs()
+      await closeLoopbackTwin()
       if (ownsHttpServer)
         await new Promise<void>(r => httpServer.close(() => r()))
       if (wsUrl && getInternalContext(context).wsEndpoint?.url === wsUrl)