diff --git a/src/main/services/mcpOAuthService.ts b/src/main/services/mcpOAuthService.ts
index e05bb3d..f3e9bb8 100644
--- a/src/main/services/mcpOAuthService.ts
+++ b/src/main/services/mcpOAuthService.ts
@@ -5,7 +5,7 @@ import { shell } from 'electron';
 
 import type { McpOauthStaticClientConfig } from '@shared/domain/mcpAuth';
 
-import { storeToken, type McpOAuthToken } from './mcpTokenStore';
+import { storeToken, buildWellKnownUrl, type McpOAuthToken } from './mcpTokenStore';
 
 /* ── Public API ──────────────────────────────────────────────── */
 
@@ -39,8 +39,7 @@ export async function requiresOAuth(serverUrl: string): Promise<boolean> {
       return false;
     }
 
-    const base = serverUrl.replace(/\/+$/, '');
-    const prmUrl = `${base}/.well-known/oauth-protected-resource`;
+    const prmUrl = buildWellKnownUrl(serverUrl, 'oauth-protected-resource');
     console.log(`[aryx oauth] Checking PRM at ${prmUrl}…`);
     const prmResponse = await fetch(prmUrl, {
       signal: AbortSignal.timeout(5_000),
@@ -128,8 +127,7 @@ interface AuthServerMetadata {
 }
 
 async function discoverAuthorizationServer(serverUrl: string): Promise<string> {
-  const base = serverUrl.replace(/\/+$/, '');
-  const prmUrl = `${base}/.well-known/oauth-protected-resource`;
+  const prmUrl = buildWellKnownUrl(serverUrl, 'oauth-protected-resource');
 
   const response = await fetch(prmUrl);
   if (!response.ok) {
@@ -146,8 +144,7 @@ async function discoverAuthorizationServer(serverUrl: string): Promise<string> {
 }
 
 async function fetchAuthServerMetadata(authServerUrl: string): Promise<AuthServerMetadata> {
-  const base = authServerUrl.replace(/\/+$/, '');
-  const metadataUrl = `${base}/.well-known/oauth-authorization-server`;
+  const metadataUrl = buildWellKnownUrl(authServerUrl, 'oauth-authorization-server');
 
   const response = await fetch(metadataUrl);
   if (!response.ok) {
diff --git a/src/main/services/mcpTokenStore.ts b/src/main/services/mcpTokenStore.ts
index e6f106b..6252df1 100644
--- a/src/main/services/mcpTokenStore.ts
+++ b/src/main/services/mcpTokenStore.ts
@@ -47,3 +47,16 @@ function normalizeUrl(url: string): string {
     return url.toLowerCase().replace(/\/+$/, '');
   }
 }
+
+/**
+ * Constructs a well-known URL per RFC 9728 Section 3.
+ * The `.well-known/{suffix}` segment is inserted between the origin and the path.
+ *
+ * Example: `buildWellKnownUrl('https://api.example.com/mcp/', 'oauth-protected-resource')`
+ *   → `https://api.example.com/.well-known/oauth-protected-resource/mcp/`
+ */
+export function buildWellKnownUrl(baseUrl: string, wellKnownSuffix: string): string {
+  const parsed = new URL(baseUrl);
+  const path = parsed.pathname === '/' ? '' : parsed.pathname;
+  return `${parsed.origin}/.well-known/${wellKnownSuffix}${path}`;
+}
diff --git a/tests/main/mcpOAuthDiscovery.test.ts b/tests/main/mcpOAuthDiscovery.test.ts
new file mode 100644
index 0000000..9b19e8c
--- /dev/null
+++ b/tests/main/mcpOAuthDiscovery.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, test } from 'bun:test';
+
+import { buildWellKnownUrl } from '@main/services/mcpTokenStore';
+
+describe('buildWellKnownUrl', () => {
+  test('inserts well-known segment after origin for URL with path', () => {
+    expect(buildWellKnownUrl('https://api.githubcopilot.com/mcp/', 'oauth-protected-resource'))
+      .toBe('https://api.githubcopilot.com/.well-known/oauth-protected-resource/mcp/');
+  });
+
+  test('inserts well-known segment for URL with multi-level path', () => {
+    expect(buildWellKnownUrl('https://example.com/v1/mcp/api', 'oauth-protected-resource'))
+      .toBe('https://example.com/.well-known/oauth-protected-resource/v1/mcp/api');
+  });
+
+  test('handles origin-only URL without path', () => {
+    expect(buildWellKnownUrl('https://auth.example.com', 'oauth-authorization-server'))
+      .toBe('https://auth.example.com/.well-known/oauth-authorization-server');
+  });
+
+  test('handles origin-only URL with trailing slash', () => {
+    expect(buildWellKnownUrl('https://auth.example.com/', 'oauth-authorization-server'))
+      .toBe('https://auth.example.com/.well-known/oauth-authorization-server');
+  });
+
+  test('handles URL with port', () => {
+    expect(buildWellKnownUrl('https://mcp.example.com:8443/v1/', 'oauth-protected-resource'))
+      .toBe('https://mcp.example.com:8443/.well-known/oauth-protected-resource/v1/');
+  });
+
+  test('preserves path without trailing slash', () => {
+    expect(buildWellKnownUrl('https://example.com/mcp', 'oauth-protected-resource'))
+      .toBe('https://example.com/.well-known/oauth-protected-resource/mcp');
+  });
+});