diff --git a/.luacheckrc b/.luacheckrc
index c48021fb..21c181c3 100644
--- a/.luacheckrc
+++ b/.luacheckrc
@@ -12,6 +12,8 @@ globals = {
     fields = {
       "contents",
       "message",
+      "is_falsy",
+      "is_truthy",
     }
   },
 }
diff --git a/lua/lean/config.lua b/lua/lean/config.lua
index c48e9477..35d237ff 100644
--- a/lua/lean/config.lua
+++ b/lua/lean/config.lua
@@ -52,6 +52,8 @@ local DEFAULTS = {
   ft = {
     nomodifiable = {
       '.*/src/lean/.*', -- Lean core library
+      '.*/.elan/.*', -- elan toolchains
+      '.*/.lake/.*', -- project dependencies
     },
 
     ---Check whether a given path should be modifiable.
diff --git a/spec/fixtures.lua b/spec/fixtures.lua
index af23d16f..5532f7cf 100644
--- a/spec/fixtures.lua
+++ b/spec/fixtures.lua
@@ -16,9 +16,17 @@ local fixtures = {
     some_nonexisting_file = child 'DoesNotExist.lean',
     some_nested_existing_file = child 'Test/Squares.lean',
     some_nested_nonexisting_file = child 'Test/DoesNotExist.lean',
+
+    some_dependency_file = child '.lake/packages/importGraph/ImportGraph/Imports.lean',
   },
 }
 
+assert.is_truthy(vim.uv.fs_stat(fixtures.project.some_existing_file))
+assert.is_falsy(vim.uv.fs_stat(fixtures.project.some_nonexisting_file))
+assert.is_truthy(vim.uv.fs_stat(fixtures.project.some_nested_existing_file))
+assert.is_falsy(vim.uv.fs_stat(fixtures.project.some_nested_nonexisting_file))
+assert.is_truthy(vim.uv.fs_stat(fixtures.project.some_dependency_file))
+
 function fixtures.project:files()
   return vim.iter {
     ['existing'] = self.some_existing_file,
diff --git a/spec/ft_spec.lua b/spec/ft_spec.lua
index 764e0a34..dc157b56 100644
--- a/spec/ft_spec.lua
+++ b/spec/ft_spec.lua
@@ -35,8 +35,13 @@ describe('ft.detect', function()
     assert.is_falsy(vim.bo.modifiable)
   end)
 
+  it('marks dependency files nomodifable by default', function()
+    vim.cmd.edit { project.some_dependency_file, bang = true }
+    assert.is_falsy(vim.bo.modifiable)
+  end)
+
   it('does not mark other lean files nomodifiable', function()
-    vim.cmd('edit! ' .. project.some_existing_file)
+    vim.cmd.edit { project.some_existing_file, bang = true }
     assert.is_truthy(vim.bo.modifiable)
   end)
 end)