diff --git a/test/prism/ruby/parser_test.rb b/test/prism/ruby/parser_test.rb
index ef82ce8c6d..8e87e496ac 100644
--- a/test/prism/ruby/parser_test.rb
+++ b/test/prism/ruby/parser_test.rb
@@ -167,7 +167,7 @@ def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compa
         ignore_warnings { Prism::Translation::Parser33.new.tokenize(buffer) }
 
       if expected_ast == actual_ast
-        if !compare_asts
+        if !compare_asts && !Fixture.custom_base_path?
           puts "#{fixture.path} is now passing"
         end
 
@@ -178,7 +178,7 @@ def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compa
         rescue Test::Unit::AssertionFailedError
           raise if compare_tokens
         else
-          puts "#{fixture.path} is now passing" if !compare_tokens
+          puts "#{fixture.path} is now passing" if !compare_tokens && !Fixture.custom_base_path?
         end
 
         assert_equal_comments(expected_comments, actual_comments) if compare_comments
diff --git a/test/prism/snapshots_test.rb b/test/prism/snapshots_test.rb
index 0744eafad3..3fbda1f86e 100644
--- a/test/prism/snapshots_test.rb
+++ b/test/prism/snapshots_test.rb
@@ -2,6 +2,10 @@
 
 require_relative "test_helper"
 
+# We don't want to generate snapshots when running
+# against files outside of the project folder.
+return if Prism::TestCase::Fixture.custom_base_path?
+
 module Prism
   class SnapshotsTest < TestCase
     # When we pretty-print the trees to compare against the snapshots, we want
diff --git a/test/prism/test_helper.rb b/test/prism/test_helper.rb
index b848500283..c5afdc0ea5 100644
--- a/test/prism/test_helper.rb
+++ b/test/prism/test_helper.rb
@@ -38,7 +38,7 @@ class TestCase < ::Test::Unit::TestCase
     # are used to define test methods that assert against each fixture in some
     # way.
     class Fixture
-      BASE = File.join(__dir__, "fixtures")
+      BASE = ENV.fetch("FIXTURE_BASE", File.join(__dir__, "fixtures"))
 
       attr_reader :path
 
@@ -63,9 +63,14 @@ def test_name
       end
 
       def self.each(except: [], &block)
-        paths = Dir[ENV.fetch("FOCUS") { File.join("**", "*.txt") }, base: BASE] - except
+        glob_pattern = ENV.fetch("FOCUS") { custom_base_path? ? File.join("**", "*.rb") : File.join("**", "*.txt") }
+        paths = Dir[glob_pattern, base: BASE] - except
         paths.each { |path| yield Fixture.new(path) }
       end
+
+      def self.custom_base_path?
+        ENV.key?("FIXTURE_BASE")
+      end
     end
 
     # Yield each encoding that we want to test, along with a range of the