diff --git a/apps/stage-tamagotchi/src/main/services/airi/auth.ts b/apps/stage-tamagotchi/src/main/services/airi/auth.ts
index b789317bf..aed816277 100644
--- a/apps/stage-tamagotchi/src/main/services/airi/auth.ts
+++ b/apps/stage-tamagotchi/src/main/services/airi/auth.ts
@@ -105,7 +105,7 @@ export function createAuthService(params: {
       const state = generateState()
 
       // Start loopback server to receive the callback
-      const loopback = await startLoopbackServer()
+      const loopback = await startLoopbackServer(state)
       closeLoopback = loopback.close
 
       // Use the server-side relay as redirect_uri. The relay page serves HTML
@@ -134,13 +134,7 @@ export function createAuthService(params: {
 
       // Wait for the callback in the background
       loopback.result
-        .then(async ({ code, state: returnedState }) => {
-          if (returnedState !== state) {
-            log.warn('State mismatch — possible CSRF attack')
-            params.windowAuthManager.broadcastAuthError('State mismatch')
-            return
-          }
-
+        .then(async ({ code }) => {
           const tokens = await exchangeCode(code, codeVerifier, redirectUri)
           params.windowAuthManager.broadcastAuthCallback(tokens)
           log.log('OIDC token exchange successful')
diff --git a/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.test.ts b/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.test.ts
index 1cf0219ac..f992005e3 100644
--- a/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.test.ts
+++ b/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.test.ts
@@ -2,6 +2,10 @@ import { afterEach, describe, expect, it } from 'vitest'
 
 import { startLoopbackServer } from './index'
 
+/**
+ * @example
+ * const server = await startLoopbackServer('expected-state')
+ */
 describe('startLoopbackServer', () => {
   const servers: Array<Awaited<ReturnType<typeof startLoopbackServer>>> = []
 
@@ -12,13 +16,45 @@ describe('startLoopbackServer', () => {
     }
   })
 
-  it('returns code and state from callback', async () => {
-    const server = await startLoopbackServer()
+  /** @example A callback with the expected state resolves the authorization code. */
+  it('returns the code from a callback with the expected state', async () => {
+    const server = await startLoopbackServer('state-1')
     servers.push(server)
 
     const response = await fetch(`http://127.0.0.1:${server.port}/callback?code=ok&state=state-1`)
     expect(response.status).toBe(200)
 
-    await expect(server.result).resolves.toEqual({ code: 'ok', state: 'state-1' })
+    await expect(server.result).resolves.toEqual({ code: 'ok' })
+  })
+
+  /** @example A forged callback cannot consume the one-shot server before the valid callback. */
+  it('rejects a mismatched state without settling the login attempt', async () => {
+    const server = await startLoopbackServer('expected-state')
+    servers.push(server)
+
+    const forgedResponse = await fetch(`http://127.0.0.1:${server.port}/callback?code=forged&state=wrong-state`)
+    expect(forgedResponse.status).toBe(400)
+
+    const validResponse = await fetch(`http://127.0.0.1:${server.port}/callback?code=valid&state=expected-state`)
+    expect(validResponse.status).toBe(200)
+
+    await expect(server.result).resolves.toEqual({ code: 'valid' })
+  })
+
+  /** @example The web relay receives ordinary CORS without the obsolete PNA response header. */
+  it('keeps standard CORS for the relay without private-network access headers', async () => {
+    const server = await startLoopbackServer('state-1')
+    servers.push(server)
+
+    const response = await fetch(`http://127.0.0.1:${server.port}/callback?code=ok&state=state-1`, {
+      headers: {
+        Origin: 'https://accounts.airi.build',
+      },
+    })
+
+    expect(response.status).toBe(200)
+    expect(response.headers.get('Access-Control-Allow-Origin')).toBe('*')
+    expect(response.headers.get('Access-Control-Allow-Private-Network')).toBeNull()
+    await expect(server.result).resolves.toEqual({ code: 'ok' })
   })
 })
diff --git a/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.ts b/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.ts
index 738c0153f..d0076b49c 100644
--- a/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.ts
+++ b/apps/stage-tamagotchi/src/main/services/airi/http-server/http/auth/index.ts
@@ -2,9 +2,12 @@ import { eventHandler, getQuery, H3, handleCors } from 'h3'
 
 import { createH3Server } from '../../server'
 
+/**
+ * Validated authorization data returned by the temporary loopback server.
+ */
 export interface LoopbackCallbackResult {
+  /** Authorization code accepted only after the OIDC state matches. */
   code: string
-  state: string
 }
 
 /**
@@ -14,13 +17,14 @@ export interface LoopbackCallbackResult {
  * - Exchanging authorization code from system browser callback
  *
  * Expects:
+ * - `expectedState` is the high-entropy state generated for this login attempt
  * - Callback request on `GET /callback?code=...&state=...`
- * - One-shot lifecycle; first successful callback closes the server
+ * - One-shot lifecycle; the first callback with matching state closes the server
  *
  * Returns:
  * - Random bound port, callback result promise, and manual cancellation method
  */
-export async function startLoopbackServer(): Promise<{
+export async function startLoopbackServer(expectedState: string): Promise<{
   port: number
   result: Promise<LoopbackCallbackResult>
   close: () => void
@@ -47,6 +51,15 @@ export async function startLoopbackServer(): Promise<{
     },
   } as const
 
+  // NOTICE:
+  // Standard CORS lets configured web relay origins read successful handoff responses.
+  // A simple cross-origin GET is still sent regardless of CORS response headers, so OIDC state validation is the authorization boundary.
+  // Source/context: `https://developer.chrome.com/blog/local-network-access`.
+  // Removal condition: the relay moves to same-origin transport or top-level navigation only.
+
+  /**
+   * Settles the one-shot callback result and stops the loopback listener.
+   */
   const finish = (callback: () => void) => {
     if (settled) {
       return
@@ -77,6 +90,14 @@ export async function startLoopbackServer(): Promise<{
     }
 
     const query = getQuery(event)
+    const state = typeof query.state === 'string' ? query.state : ''
+    if (!state || state !== expectedState) {
+      return new Response('<html><body><h2>Invalid state</h2></body></html>', {
+        status: 400,
+        headers: { 'Content-Type': 'text/html; charset=utf-8' },
+      })
+    }
+
     const error = typeof query.error === 'string' ? query.error : undefined
     if (error) {
       const description = typeof query.error_description === 'string' && query.error_description.length > 0
@@ -92,9 +113,7 @@ export async function startLoopbackServer(): Promise<{
     }
 
     const code = typeof query.code === 'string' ? query.code : ''
-    const state = typeof query.state === 'string' ? query.state : ''
-
-    if (!code || !state) {
+    if (!code) {
       return new Response('<html><body><h2>Missing parameters</h2></body></html>', {
         status: 400,
         headers: { 'Content-Type': 'text/html; charset=utf-8' },
@@ -102,7 +121,7 @@ export async function startLoopbackServer(): Promise<{
     }
 
     finish(() => {
-      resolveResult({ code, state })
+      resolveResult({ code })
     })
 
     return new Response('<html><body><h2>Authentication successful!</h2><p>You can close this window and return to the app.</p></body></html>', {