diff --git a/auth_server/oauth2_providers.yml b/auth_server/oauth2_providers.yml
index 0ff84e9..09cc020 100644
--- a/auth_server/oauth2_providers.yml
+++ b/auth_server/oauth2_providers.yml
@@ -96,6 +96,7 @@ session:
   secure: true  # Set to false for development
   httponly: true
   samesite: "lax"
+  domain: "${SESSION_COOKIE_DOMAIN}"  # Set to your domain (with leading dot) to share cookies across subdomains, e.g., ".example.com"
 
 # Registry integration settings
 registry:
diff --git a/auth_server/server.py b/auth_server/server.py
index a19fef0..94f6497 100644
--- a/auth_server/server.py
+++ b/auth_server/server.py
@@ -1794,14 +1794,46 @@ async def oauth2_callback(
         response = RedirectResponse(url=redirect_url, status_code=302)
         
         # Set registry-compatible session cookie
-        response.set_cookie(
-            key="mcp_gateway_session",  # Same as registry SESSION_COOKIE_NAME
-            value=registry_session,
-            max_age=OAUTH2_CONFIG.get("session", {}).get("max_age_seconds", 28800),
-            httponly=OAUTH2_CONFIG.get("session", {}).get("httponly", True),
-            samesite=OAUTH2_CONFIG.get("session", {}).get("samesite", "lax"),
-            secure=OAUTH2_CONFIG.get("session", {}).get("secure", False)
-        )
+        cookie_secure = OAUTH2_CONFIG.get("session", {}).get("secure", False)
+        cookie_samesite = OAUTH2_CONFIG.get("session", {}).get("samesite", "lax")
+        cookie_domain = OAUTH2_CONFIG.get("session", {}).get("domain", "")
+
+        # Auto-infer domain from request if not explicitly configured
+        if not cookie_domain or cookie_domain == "${SESSION_COOKIE_DOMAIN}":
+            # Get the host from headers (ALB sets x-forwarded-host or use Host header)
+            host = request.headers.get("x-forwarded-host") or request.headers.get("host", "")
+
+            # Extract base domain for cross-subdomain cookies
+            # e.g., "auth-server.subdomain.domain.tld" -> ".domain.tld"
+            if host and "." in host:
+                # Remove port if present
+                host = host.split(":")[0]
+
+                # Split by dots and take last 2 parts for base domain
+                parts = host.split(".")
+                if len(parts) >= 2:
+                    # Skip if it's localhost or an IP address
+                    if not host.startswith("localhost") and not host.replace(".", "").isdigit():
+                        cookie_domain = f".{'.'.join(parts[-2:])}"
+                        logger.info(f"Auto-inferred cookie domain from host '{host}': {cookie_domain}")
+
+        logger.info(f"Auth server setting session cookie: secure={cookie_secure}, samesite={cookie_samesite}, domain={cookie_domain or 'not set'}, request_scheme={request.url.scheme}")
+
+        cookie_params = {
+            "key": "mcp_gateway_session",  # Same as registry SESSION_COOKIE_NAME
+            "value": registry_session,
+            "max_age": OAUTH2_CONFIG.get("session", {}).get("max_age_seconds", 28800),
+            "httponly": OAUTH2_CONFIG.get("session", {}).get("httponly", True),
+            "samesite": cookie_samesite,
+            "secure": cookie_secure,
+            "path": "/"  # Ensure cookie is sent for all paths
+        }
+
+        # Only set domain if configured or inferred (for cross-subdomain cookies)
+        if cookie_domain:
+            cookie_params["domain"] = cookie_domain
+
+        response.set_cookie(**cookie_params)
         
         # Clear temporary OAuth2 session
         response.delete_cookie("oauth2_temp_session")