diff --git a/AGENTS.md b/AGENTS.md
index e3efb88..a912b4d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -198,6 +198,9 @@ For changes that touch the cross-language pipeline (Haskell exporter and Rust ru
 2. `make examples`: runs `export-fixtures` and then `cargo test -p plan-runner --test examples`, which walks every regenerated fixture and verifies it
    against its `expected_bindings` oracle.
 
+For changes that touch `tools/plan-viewer`, run `make viewer-test`.
+It checks the viewer's JavaScript engine against every fixture oracle under Node; the Rust crates remain the correctness oracle.
+
 For Markdown-only changes, run a manual read-through and check that headings follow the writing style.
 
 ## Change Design Checklist