diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 8213bb8cd..80d11f3a5 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -218,3 +218,13 @@ When documenting UI elements, follow these guidelines:
 - **Conceptual docs** — architecture overviews, explanations, mental models
 - **Release notes** — changes per version, grouped by type
 - **READMEs** — project and repository overviews
+
+## Red-Green Test-Driven Development
+
+When making code changes in this repository, follow a red-green TDD workflow:
+
+1. **Red**: Write or update a test that captures the desired behavior or reproduces the bug. Run the test (locally or let CI run it) and confirm it **fails**.
+2. **Green**: Write the minimal code change needed to make the failing test pass. Re-run the test (locally or in CI) and confirm it now **passes**.
+3. **Refactor**: With the passing test as a safety net, clean up the implementation as needed, re-running tests to ensure they remain green.
+
+Do not write the fix before the failing test exists. This ensures every change is backed by a test that would have caught the issue, and that the fix is verifiably correct.
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