diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..002223a
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,3 @@
+# Repository instructions
+
+Read [README.md](README.md) before making changes to this repository.
diff --git a/README.md b/README.md
index 3d2f5f3..6985338 100644
--- a/README.md
+++ b/README.md
@@ -356,3 +356,19 @@ npm run test:sim -- test/simulator-btc.test.ts
 
 For build, test, simulator, protobuf, and contribution workflow details, see
 [CONTRIBUTING.md](CONTRIBUTING.md).
+
+## Updating the Bitcoin transaction test vectors
+
+The vectors in [test/data/btc-transaction-test-vectors.json](test/data/btc-transaction-test-vectors.json)
+are generated in the BitBox02 firmware repository. Do not modify this JSON file directly.
+
+The source of truth is the Rust constructors in
+[`src/rust/bitbox-test-vectors/src/btc_transaction/cases/`](https://github.com/BitBoxSwiss/bitbox02-firmware/tree/master/src/rust/bitbox-test-vectors/src/btc_transaction/cases).
+Make changes there and regenerate the canonical JSON following the
+[test vector generation instructions](https://github.com/BitBoxSwiss/bitbox02-firmware/blob/master/src/rust/bitbox-test-vectors/README.md).
+Then copy the generated file byte-for-byte into this repository:
+
+```sh
+cp /path/to/bitbox02-firmware/src/rust/bitbox-test-vectors/testdata/btc-transaction-test-vectors.json \
+    test/data/btc-transaction-test-vectors.json
+```