Insta: Snapshot Testing for Ruby @ Rocky Mountain Ruby 2026, Boulder, Colorado

Most Ruby tests rely on manually maintained assertions. What if your test suite could show you exactly what changed and let you approve those changes with a single command?

Inspired by Rust’s popular Insta library, this talk introduces snapshot testing for Ruby and demonstrates how snapshots, diffs, and review workflows can make testing complex output simpler, more expressive, and easier to maintain. Extracted from Herb's test suite.

1. Marco Roth Insta: Snapshot Testing for Ruby Sep 29, 2026 - Rocky Mountain Ruby, Boulder, Colorado
2. Extracted from the Herb Test Suite
4. assert_equal where the expected value manages itself.
5. Where you want to assert the whole thing, without having to write an assertion for every detail.
6. UserSerializer.render(user)
7. UserSerializer.render(user) '{"id":42,"name":"Ada","role":"a dmin","created_at":"2026-01-15T0 9:30:00Z"}'
8. require "test_helper" class UserSerializerTest < Minitest::Test def test_user_json expected = '{"id":42,"name":"Ada","role":"admin","created_at":" assert_equal UserSerializer.render(ADA), expected end def test_admin_json expected = '{"id":7,"name":"Grace","role":"owner","created_at": assert_equal UserSerializer.render(GRACE), expected end end
10. Now, the UserSerializer, changes, and we also want the avatar_url serialized
11. So you update your UserSerializer and run your tests
12. rake test
14. Now, we would have to go through all tests and add avatar_url, even though we can see that the actual value was right
15. Lets abort here and revert this avatar_url change for now
16. This is where Insta comes in
17. require "test_helper" class UserSerializerTest < Minitest::Test def test_user_json assert_snapshot UserSerializer.render(ADA) end def test_admin_json assert_snapshot UserSerializer.render(GRACE) end end
18. rake test
21. Whenever an assertion fails, it writes a new pending .snap.new file
22. insta review
24. Accepting will write the snapshot file to the test/snapshots/ folder
26. So lets add our new avatar_url into the serializer again.
27. rake test
29. Since the assertions failed it wrote some new pending .snap.new files
30. insta review
32. This will update the snapshot file accordingly
34. Inline Snapshots assert_inline_snapshot()
35. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end
36. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end insta review
37. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end
38. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse, "dlroW olleH") end end
39. Technically, you could alias assert_inline_snapshot to assert_equal to have this apply to all asserts in your codebase
40. ✓ Minitest + RSpec ✓ file snapshots (.snap + YAML frontmatter, source metadata) ✓ inline snapshots (Prism-powered file rewriting) ✓ interactive review CLI (review · accept · reject · pending · status · clean) ✓ difftastic structural diffs ✓ serializers: to_s · inspect · json · yaml ✓ redactions (jq-like selectors) ✓ INSTA_UPDATE modes + CI auto-detection ✓ per-snapshot metadata (input · description · info)
41. Where snapshots shine: •API responses (the full JSON) •rendered HTML views •ASTs •compiled output •CLI output •error messages
42. Seems to be the perfect test companion for the AI age
43. gem "insta"
45. Thank you for the insta gem name ! Renan Garcia
46. Feedback Welcome!
47. Happy Testing
48. Thank you "  @marcoroth 🌐 marcoroth.dev  @marcoroth_  @marcoroth.dev  @marcoroth@ruby.social  /in/marco-roth