From b2039ab896a01ebcc539704f69974f7bcdfb1226 Mon Sep 17 00:00:00 2001
From: Matt Pocock <mattpocockvoice@gmail.com>
Date: Wed, 4 Feb 2026 14:09:07 +0000
Subject: [PATCH] Added horizontal slices issue to TDD skill

---
 tdd/SKILL.md | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/tdd/SKILL.md b/tdd/SKILL.md
index 5280417..78f5077 100644
--- a/tdd/SKILL.md
+++ b/tdd/SKILL.md
@@ -15,6 +15,31 @@ description: Test-driven development with red-green-refactor loop. Use when user
 
 See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines.
 
+## Anti-Pattern: Horizontal Slices
+
+**DO NOT write all tests first, then all implementation.** This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."
+
+This produces **crap tests**:
+
+- Tests written in bulk test _imagined_ behavior, not _actual_ behavior
+- You end up testing the _shape_ of things (data structures, function signatures) rather than user-facing behavior
+- Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
+- You outrun your headlights, committing to test structure before understanding the implementation
+
+**Correct approach**: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.
+
+```
+WRONG (horizontal):
+  RED:   test1, test2, test3, test4, test5
+  GREEN: impl1, impl2, impl3, impl4, impl5
+
+RIGHT (vertical):
+  RED→GREEN: test1→impl1
+  RED→GREEN: test2→impl2
+  RED→GREEN: test3→impl3
+  ...
+```
+
 ## Workflow
 
 ### 1. Planning