diff --git a/test/at/tid_test.gleam b/test/at/tid_test.gleam
new file mode 100644
index 0000000..ca545fa
--- /dev/null
+++ b/test/at/tid_test.gleam
@@ -0,0 +1,37 @@
+// IMPORTS ---------------------------------------------------------------------
+
+import at/tid
+import gleam/list
+
+// FIXTURES --------------------------------------------------------------------
+
+const valid = [
+  "3jzfcijpj2z2a",
+  "7777777777777",
+  "3zzzzzzzzzzzz",
+]
+
+const invalid = [
+  "3jzfcijpj2z21",
+  "0000000000000",
+  "3jzfcijpj2z2aa",
+  "3jzfcijpj2z2",
+  "3jzf-cij-pj2z-2a",
+  "zzzzzzzzzzzzz",
+  "kjzfcijpj2z2a",
+]
+
+// INTEROP TESTS ---------------------------------------------------------------
+
+pub fn atproto_interop_valid_tid_test() {
+  use input <- list.each(valid)
+  let assert Ok(tid) = tid.from_string(input) as input
+
+  assert tid.to_string(tid) == input
+}
+
+pub fn atproto_interop_invalid_tid_test() {
+  use input <- list.each(invalid)
+
+  assert tid.from_string(input) == Error(Nil) as input
+}