diff --git a/tests/all-package-defaults.nix b/tests/all-package-defaults.nix
index 0cf76048..d8ec4099 100644
--- a/tests/all-package-defaults.nix
+++ b/tests/all-package-defaults.nix
@@ -13,6 +13,10 @@ let
   };
 
   disabledPackages = [
+    # 2026-09-20: lean4 install prefix build failure
+    # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+    "lean4"
+
     # 2026-08-16: broken dependency "sv-lang"
     "veridian"
 
diff --git a/tests/lsp-servers.nix b/tests/lsp-servers.nix
index a6aa69c9..5a1a9c8a 100644
--- a/tests/lsp-servers.nix
+++ b/tests/lsp-servers.nix
@@ -37,6 +37,10 @@ let
     let
 
       disabled = [
+        # 2026-09-20: lean4 install prefix build failure
+        # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+        "lean3ls"
+
         # 2026-08-16: broken dependency "sv-lang"
         "veridian"
 
diff --git a/tests/test-sources/modules/dependencies.nix b/tests/test-sources/modules/dependencies.nix
index 3c731c40..7d6293f6 100644
--- a/tests/test-sources/modules/dependencies.nix
+++ b/tests/test-sources/modules/dependencies.nix
@@ -3,6 +3,10 @@ let
   inherit (pkgs.stdenv) hostPlatform;
 
   disabledDeps = [
+    # 2026-09-20: lean4 install prefix build failure
+    # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+    "lean"
+
     # TODO: 2025-10-03
     # Transient dependency `vmr` has a build failure
     # https://github.com/NixOS/nixpkgs/issues/431811
diff --git a/tests/test-sources/plugins/by-name/lean/default.nix b/tests/test-sources/plugins/by-name/lean/default.nix
index 192beae2..41b081de 100644
--- a/tests/test-sources/plugins/by-name/lean/default.nix
+++ b/tests/test-sources/plugins/by-name/lean/default.nix
@@ -1,5 +1,9 @@
 {
   empty = {
+    # 2026-09-20: lean4 install prefix build failure
+    # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+    dependencies.lean.enable = false;
+
     # TODO 2025-10-01
     # Calls `require("lspconfig")` which is deprecated, producing a warning
     test.runNvim = false;
@@ -10,6 +14,10 @@
   # Enable the `leanls` LSP directly from `plugins.lsp`. This implies explicitly disabling the lsp
   # in the `lean` plugin configuration.
   lspDisabled = {
+    # 2026-09-20: lean4 install prefix build failure
+    # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+    dependencies.lean.enable = false;
+
     plugins = {
       lsp = {
         enable = true;
@@ -29,6 +37,10 @@
   };
 
   default = {
+    # 2026-09-20: lean4 install prefix build failure
+    # Re-enable when a future lockfile includes https://github.com/NixOS/nixpkgs/pull/563148.
+    dependencies.lean.enable = false;
+
     # TODO 2025-10-01
     # Calls `require("lspconfig")` which is deprecated, producing a warning
     test.runNvim = false;