diff --git a/.gitignore b/.gitignore
index d624eddd..96934e54 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ node_modules/
 .claude/.simplify-ignore-cache/
 .claude/sdd-cache/
 evals/results/
+__pycache__/
diff --git a/README.md b/README.md
index 38c4b1e3..3700fbaa 100644
--- a/README.md
+++ b/README.md
@@ -280,7 +280,7 @@ Quick-reference material that skills pull in when needed:
 | Reference | Covers |
 |-----------|--------|
 | [definition-of-done.md](references/definition-of-done.md) | Project-wide standing bar every change clears, contrasted with per-task acceptance criteria |
-| [testing-patterns.md](references/testing-patterns.md) | Test structure, naming, mocking, React/API/E2E examples, anti-patterns |
+| [testing-patterns.md](references/testing-patterns.md) | Test structure, naming, mocking, React/API/E2E examples, anti-patterns (JavaScript/TypeScript) |
 | [security-checklist.md](references/security-checklist.md) | Pre-commit checks, auth, input validation, headers, CORS, OWASP Top 10 |
 | [performance-checklist.md](references/performance-checklist.md) | Core Web Vitals targets, frontend/backend checklists, measurement commands |
 | [accessibility-checklist.md](references/accessibility-checklist.md) | Keyboard nav, screen readers, visual design, ARIA, testing tools |
diff --git a/evals/cases/test-driven-development.json b/evals/cases/test-driven-development.json
index 3ecb7903..7b2e7e6a 100644
--- a/evals/cases/test-driven-development.json
+++ b/evals/cases/test-driven-development.json
@@ -52,6 +52,21 @@
         "A regression test covers the reported three-at-3.335 currency case",
         "The full suite is run after the minimal fix"
       ]
+    },
+    {
+      "id": 3,
+      "prompt": "Add debit entries to the ledger, test-first. A debit subtracts its amount from the balance and must not drive the balance below zero.",
+      "expected_output": "A failing unittest written and shown failing first, a minimal implementation, and the full suite run with the repository's own Python test command",
+      "files": [
+        "test-driven-development-ecosystem"
+      ],
+      "expectations": [
+        "The repository's stack (Python, unittest) is identified before any test command is chosen",
+        "Tests are run with the repository's own command (python3 -m unittest), not npm test or another ecosystem's tool",
+        "A failing test is written and shown failing before the implementation",
+        "The below-zero rule gets its own test case",
+        "The full suite is run after the change"
+      ]
     }
   ]
 }
diff --git a/evals/fixtures/test-driven-development-ecosystem/README.md b/evals/fixtures/test-driven-development-ecosystem/README.md
new file mode 100644
index 00000000..ce71454c
--- /dev/null
+++ b/evals/fixtures/test-driven-development-ecosystem/README.md
@@ -0,0 +1,9 @@
+# Ledger
+
+Small Python ledger utilities. No dependencies beyond the standard library.
+
+Run the tests:
+
+```
+python3 -m unittest
+```
diff --git a/evals/fixtures/test-driven-development-ecosystem/ledger.py b/evals/fixtures/test-driven-development-ecosystem/ledger.py
new file mode 100644
index 00000000..0b8c09b0
--- /dev/null
+++ b/evals/fixtures/test-driven-development-ecosystem/ledger.py
@@ -0,0 +1,15 @@
+"""Simple in-memory ledger utilities."""
+
+
+def apply_entries(balance, entries):
+    """Apply entries to a starting balance and return the result.
+
+    Entries are (kind, amount) tuples. Only "credit" entries are
+    supported; anything else raises ValueError.
+    """
+    for kind, amount in entries:
+        if kind == "credit":
+            balance += amount
+        else:
+            raise ValueError(f"unsupported entry kind: {kind}")
+    return balance
diff --git a/evals/fixtures/test-driven-development-ecosystem/test_ledger.py b/evals/fixtures/test-driven-development-ecosystem/test_ledger.py
new file mode 100644
index 00000000..617b0a01
--- /dev/null
+++ b/evals/fixtures/test-driven-development-ecosystem/test_ledger.py
@@ -0,0 +1,19 @@
+import unittest
+
+from ledger import apply_entries
+
+
+class ApplyEntriesTest(unittest.TestCase):
+    def test_applies_a_single_credit(self):
+        self.assertEqual(apply_entries(100, [("credit", 25)]), 125)
+
+    def test_applies_multiple_credits_in_order(self):
+        self.assertEqual(apply_entries(0, [("credit", 10), ("credit", 5)]), 15)
+
+    def test_rejects_unknown_entry_kinds(self):
+        with self.assertRaises(ValueError):
+            apply_entries(0, [("transfer", 10)])
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/references/testing-patterns.md b/references/testing-patterns.md
index b97de3aa..bc96e271 100644
--- a/references/testing-patterns.md
+++ b/references/testing-patterns.md
@@ -1,6 +1,6 @@
-# Testing Patterns Reference
+# Testing Patterns Reference (JavaScript/TypeScript)
 
-Quick reference for common testing patterns across the stack. Use alongside the `test-driven-development` skill.
+Quick reference of JavaScript/TypeScript testing patterns — Jest, React Testing Library, Supertest, and Playwright — illustrating the universal principles from the `test-driven-development` skill. The principles (Arrange-Act-Assert, naming, mock discipline, anti-patterns) apply in any ecosystem; the syntax and tooling shown here are JS/TS-specific. In another stack, follow the same principles with the repository's own test framework and commands.
 
 ## Table of Contents
 
diff --git a/skills/test-driven-development/SKILL.md b/skills/test-driven-development/SKILL.md
index c96a67f4..a0c9944d 100644
--- a/skills/test-driven-development/SKILL.md
+++ b/skills/test-driven-development/SKILL.md
@@ -21,6 +21,20 @@ Write a failing test before writing the code that makes it pass. For bug fixes,
 
 **Related:** For browser-based changes, combine TDD with runtime verification using Chrome DevTools MCP — see the Browser Testing section below.
 
+## Discover the Stack First
+
+The TDD cycle is universal; the commands are not. Before writing the first test, discover how *this* repository tests, and use its commands for every RED, GREEN, and verification step:
+
+- **Language and build system** — `package.json`, `pom.xml`/`build.gradle`, `pyproject.toml`, `go.mod`, `Cargo.toml`, `Gemfile`, a `Makefile`
+- **Checked-in wrappers** — prefer `./gradlew`, `./mvnw`, `make test`, or a repo script over globally installed tools
+- **Test framework and configuration** — and how it runs a single focused test vs the full suite
+- **Existing conventions** — where tests live, how files are named, what patterns neighboring tests follow
+- **Documented commands** — README, CONTRIBUTING, and CI workflows show the commands that actually gate merges
+
+Run the repository's focused-test command during the loop and its full-suite command before completion. Never assume a default like `npm test` — a Gradle, Cargo, or pytest project has its own equivalent.
+
+The examples below use TypeScript for illustration; the workflow is identical in any language once you've discovered the project's own tooling.
+
 ## The TDD Cycle
 
 ```
@@ -344,7 +358,7 @@ This separation ensures the test is written without knowledge of the fix, making
 
 ## See Also
 
-For detailed testing patterns, examples, and anti-patterns across frameworks, see `references/testing-patterns.md`.
+For JavaScript/TypeScript testing patterns illustrating these principles — Jest, React Testing Library, Supertest, Playwright — see `references/testing-patterns.md`. The principles transfer to any ecosystem; the syntax and tools there are JS/TS-specific.
 
 ## Common Rationalizations
 
@@ -361,6 +375,7 @@ For detailed testing patterns, examples, and anti-patterns across frameworks, se
 ## Red Flags
 
 - Writing code without any corresponding tests
+- Reaching for a default test command (`npm test`) without checking what this repository actually uses
 - Tests that pass on the first run (they may not be testing what you think)
 - "All tests pass" but no tests were actually run
 - Bug fixes without reproduction tests
@@ -374,7 +389,7 @@ For detailed testing patterns, examples, and anti-patterns across frameworks, se
 After completing any implementation:
 
 - [ ] Every new behavior has a corresponding test
-- [ ] All tests pass: `npm test`
+- [ ] The full suite passes, run with the repository's own test command (`npm test`, `./gradlew test`, `pytest`, `go test ./...`, ...)
 - [ ] Bug fixes include a reproduction test that failed before the fix
 - [ ] Test names describe the behavior being verified
 - [ ] No tests were skipped or disabled