The original docs/testing.md mentioned finding existing tests as step 1
of the checklist but never explicitly said "if existing coverage
already addresses your case, extend it; don't duplicate." Adds a
prominent "First principle" section that names extend-vs-new as the
preferred outcome and lists three duplicated init_and_load blocks as
the most common form of test rot.
Adds an extra checklist item: verify your change makes an *existing*
test fail before it makes a new one pass — if you can break the code
without breaking a test, that coverage gap is the bug to fix first.
Strengthens the AGENTS.md callout so the principle ("always check what
already covers it") is in scope from the top of every session.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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