diff --git a/docs/nixos-modernization-options.md b/docs/nixos-modernization-options.md
index e64d1660..9f82d5c8 100644
--- a/docs/nixos-modernization-options.md
+++ b/docs/nixos-modernization-options.md
@@ -199,8 +199,7 @@ defaults become less transparent than a file you can read in the diff; your deli
 
 ## A7. attic self-hosted binary cache
 
-**Current:** substituters are the two Chinese mirrors plus numtide's cache; a cachix endpoint exists
-in the config but is unused.
+**Current:** substituters are the two Chinese mirrors plus numtide's cache.
 
 **Proposed:** `services.networking.atticd` on youko.
 
diff --git a/hosts/12kingdoms-youko/homelab-services/nix-cache-proxy.nix b/hosts/12kingdoms-youko/homelab-services/nix-cache-proxy.nix
new file mode 100644
index 00000000..8a4c46f7
--- /dev/null
+++ b/hosts/12kingdoms-youko/homelab-services/nix-cache-proxy.nix
@@ -0,0 +1,86 @@
+{ myvars, ... }:
+let
+  cacheDir = "/data/apps/nix-cache-proxy";
+  inherit (myvars.ports) youkoNixCacheProxy;
+
+  # Nix clients verify narinfo signatures and NAR hashes, so this only needs to
+  # be a transparent HTTP cache, not a Nix-aware store. Keep it on the btrfs
+  # data volume, never on the /data tmpfs.
+  cacheConfig = ''
+    proxy_ssl_server_name on;
+    proxy_ssl_verify on;
+    proxy_ssl_verify_depth 3;
+    proxy_ssl_trusted_certificate /etc/ssl/certs/ca-bundle.crt;
+
+    proxy_cache nix_cache;
+    proxy_cache_key "$scheme$request_method$host$request_uri";
+    proxy_cache_valid 200 206 30d;
+    proxy_cache_valid 404 1m;
+    proxy_cache_lock on;
+    proxy_cache_lock_timeout 60s;
+    proxy_cache_use_stale
+      error timeout updating
+      http_500 http_502 http_503 http_504;
+
+    add_header X-Cache-Status $upstream_cache_status always;
+  '';
+
+  cacheLocation = upstream: {
+    proxyPass = "https://${upstream}/";
+    extraConfig = ''
+      proxy_set_header Host ${upstream};
+      ${cacheConfig}
+    '';
+  };
+in
+{
+  services.nginx = {
+    enable = true;
+
+    appendHttpConfig = ''
+      proxy_cache_path ${cacheDir}
+        levels=1:2
+        keys_zone=nix_cache:256m
+        max_size=300g
+        inactive=30d
+        use_temp_path=off;
+    '';
+
+    virtualHosts."nix-cache-proxy" = {
+      # Caddy owns the public listener and terminates HTTPS; nginx is internal.
+      listen = [
+        {
+          addr = "127.0.0.1";
+          port = youkoNixCacheProxy;
+        }
+      ];
+
+      locations = {
+        "/cache.nixos.org/" = cacheLocation "cache.nixos.org";
+        "/cache.numtide.com/" = cacheLocation "cache.numtide.com";
+      };
+    };
+  };
+
+  # `proxy_cache_path` is created at runtime; nginx owns it.
+  systemd.tmpfiles.rules = [
+    "d ${cacheDir} 0755 nginx nginx - -"
+  ];
+  systemd.services.nginx = {
+    unitConfig.RequiresMountsFor = [ "/data/apps" ];
+    # The NixOS nginx unit runs with ProtectSystem=strict; the custom cache
+    # path must be explicitly writable.
+    serviceConfig.ReadWritePaths = [ cacheDir ];
+  };
+
+  # Public Let's Encrypt wildcard, already managed by the Caddy module. This
+  # avoids distributing the private homelab CA to clients that only consume the
+  # cache (including macOS).
+  services.caddy.virtualHosts."nix-cache.writefor.fun".extraConfig = ''
+    tls /var/lib/acme/writefor.fun/fullchain.pem /var/lib/acme/writefor.fun/key.pem {
+      protocols tls1.3 tls1.3
+      curves x25519 secp384r1 secp521r1
+    }
+    reverse_proxy http://127.0.0.1:${toString youkoNixCacheProxy}
+  '';
+}
diff --git a/hosts/idols-aquamarine/asr.nix b/hosts/idols-aquamarine/asr.nix
index 99d9e393..1eaeae9e 100644
--- a/hosts/idols-aquamarine/asr.nix
+++ b/hosts/idols-aquamarine/asr.nix
@@ -8,7 +8,8 @@
 }:
 let
   inherit (myvars.networking.hostsAddr.aquamarine) ipv4;
-  port = 18080;
+  inherit (myvars.ports) aquamarineAsr;
+  port = aquamarineAsr;
 
   # OpenVINO publishes static IRs for Whisper only, and the NPU compiler accepts
   # nothing less: any other ASR family keeps a dynamic time axis and aborts it
diff --git a/modules/base/nix.nix b/modules/base/nix.nix
index 6318b016..d93435de 100644
--- a/modules/base/nix.nix
+++ b/modules/base/nix.nix
@@ -28,7 +28,7 @@
     # The official https://cache.nixos.org is always trusted and is appended
     # automatically, so it does not need to be listed here.
     substituters = [
-      # cache mirror located in China
+      # Domestic mirrors are fast from the LAN; query them directly first.
       # status: https://mirrors.ustc.edu.cn/status/
       "https://mirrors.ustc.edu.cn/nix-channels/store"
       # status: https://mirror.sjtu.edu.cn/
@@ -37,11 +37,14 @@
       # "https://mirrors.sustech.edu.cn/nix-channels/store"
       "https://mirrors.tuna.tsinghua.edu.cn/nix-channels/store"
 
+      # Pull-through cache for upstreams that are slow from the LAN. Nix
+      # appends the official cache.nixos.org last, so list its proxy here.
+      "https://nix-cache.writefor.fun/cache.nixos.org"
       # numtide's cache (niks3), which serves the prebuilt packages of the
       # numtide/llm-agents.nix flake input used for the AI agent CLIs.
+      "https://nix-cache.writefor.fun/cache.numtide.com"
+      # Direct fallback if the LAN proxy is unavailable.
       "https://cache.numtide.com"
-      # my own cache server, currently not used.
-      # "https://ryan4yin.cachix.org"
     ];
 
     # WARNING: every key here trusts *everything* that key signs. Add one only
@@ -49,7 +52,6 @@
     # a third party the ability to ship you arbitrary store paths.
     trusted-public-keys = [
       "niks3.numtide.com-1:DTx8wZduET09hRmMtKdQDxNNthLQETkc/yaX7M4qK0g="
-      "ryan4yin.cachix.org-1:Gbk27ZU5AYpGS9i3ssoLlwdvMIh0NxG0w8it/cv9kbU="
     ];
     builders-use-substitutes = true;
   };
diff --git a/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expected.nix b/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expected.nix
new file mode 100644
index 00000000..c1b3682c
--- /dev/null
+++ b/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expected.nix
@@ -0,0 +1,11 @@
+{
+  nginxEnabled = true;
+  caddyVhostExists = true;
+  ustcRemainsFirst = true;
+  nixosProxyConfigured = true;
+  numtideProxyConfigured = true;
+  numtideDirectFallback = true;
+  cacheLivesOnDataVolume = true;
+  nixosUpstream = "https://cache.nixos.org/";
+  numtideUpstream = "https://cache.numtide.com/";
+}
diff --git a/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expr.nix b/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expr.nix
new file mode 100644
index 00000000..9a8c16b7
--- /dev/null
+++ b/outputs/x86_64-linux/tests/youko-nix-cache-proxy/expr.nix
@@ -0,0 +1,21 @@
+{
+  lib,
+  outputs,
+  ...
+}:
+let
+  cfg = outputs.nixosConfigurations.youko.config;
+  nginx = cfg.services.nginx;
+  substituters = cfg.nix.settings.substituters;
+in
+{
+  nginxEnabled = nginx.enable;
+  caddyVhostExists = cfg.services.caddy.virtualHosts ? "nix-cache.writefor.fun";
+  ustcRemainsFirst = builtins.head substituters == "https://mirrors.ustc.edu.cn/nix-channels/store";
+  nixosProxyConfigured = builtins.elem "https://nix-cache.writefor.fun/cache.nixos.org" substituters;
+  numtideProxyConfigured = builtins.elem "https://nix-cache.writefor.fun/cache.numtide.com" substituters;
+  numtideDirectFallback = builtins.elem "https://cache.numtide.com" substituters;
+  cacheLivesOnDataVolume = lib.hasInfix "/data/apps/nix-cache-proxy" nginx.appendHttpConfig;
+  nixosUpstream = nginx.virtualHosts."nix-cache-proxy".locations."/cache.nixos.org/".proxyPass;
+  numtideUpstream = nginx.virtualHosts."nix-cache-proxy".locations."/cache.numtide.com/".proxyPass;
+}
diff --git a/vars/README.md b/vars/README.md
index 157b9c3f..bb28e9b7 100644
--- a/vars/README.md
+++ b/vars/README.md
@@ -8,7 +8,8 @@ Common variables and configuration used across my NixOS and nix-darwin configura
 vars/
 ├── README.md
 ├── default.nix         # Main variables entry point
-└── networking.nix      # Network configuration and host definitions
+├── networking.nix      # Network configuration and host definitions
+└── ports.nix           # Loopback HTTP backend port registry
 ```
 
 ## Components
diff --git a/vars/default.nix b/vars/default.nix
index 491ea505..90f81d4b 100644
--- a/vars/default.nix
+++ b/vars/default.nix
@@ -4,6 +4,7 @@
   userfullname = "Ryan Yin";
   useremail = "xiaoyin_c@qq.com";
   networking = import ./networking.nix { inherit lib; };
+  ports = import ./ports.nix;
   # Generated using: mkpasswd -m yescrypt --rounds=11
   # Password: long, strong random string (full charset)
   # Rotation policy: changed annually
diff --git a/vars/ports.nix b/vars/ports.nix
new file mode 100644
index 00000000..8aebcc1e
--- /dev/null
+++ b/vars/ports.nix
@@ -0,0 +1,6 @@
+{
+  # Loopback-only HTTP backends behind Caddy. Allocate services sequentially
+  # from 18080; keep this list sorted by port.
+  aquamarineAsr = 18080;
+  youkoNixCacheProxy = 18081;
+}