diff --git a/internal/hierarchy/selector_keys_test.go b/internal/hierarchy/selector_keys_test.go
index c0908ef..24f6acd 100644
--- a/internal/hierarchy/selector_keys_test.go
+++ b/internal/hierarchy/selector_keys_test.go
@@ -8,26 +8,33 @@ import (
 	"testing"
 )
 
+// selectorKeysGolden is the cross-runtime contract for object selectors: the
+// keys both runtimes accept, and the diagnostic both raise for a key neither
+// can match.
+type selectorKeysGolden struct {
+	Keys              []string `json:"keys"`
+	UnknownKeyExample []string `json:"unknownKeyExample"`
+	UnknownKeyMessage string   `json:"unknownKeyMessage"`
+}
+
 // Both runtimes reject an object-selector key they do not know, so the two key
 // lists have to be one list. Were they to drift, a spec would be accepted by
 // the runtime that lists the key and fail the run on the one that does not, and
 // the difference would only show on the platform nobody ran first.
 // pkg/spec/test/selector-keys.test.ts asserts the SAME file from the web side.
 func TestSelectorKeysMatchTheCrossRuntimeList(t *testing.T) {
-	path, err := filepath.Abs("../../pkg/spec/test/fixtures/selector-keys.json")
-	if err != nil {
-		t.Fatal(err)
+	golden := loadSelectorKeysGolden(t)
+	if got := SelectorKeys(); !slices.Equal(got, golden.Keys) {
+		t.Errorf("native key list\n got=%v\nwant=%v", got, golden.Keys)
 	}
-	body, err := os.ReadFile(path)
-	if err != nil {
-		t.Fatalf("read selector keys: %v", err)
-	}
-	var want []string
-	if err := json.Unmarshal(body, &want); err != nil {
-		t.Fatalf("decode selector keys: %v", err)
-	}
-	if got := SelectorKeys(); !slices.Equal(got, want) {
-		t.Errorf("native key list\n got=%v\nwant=%v", got, want)
+}
+
+// An author who hits this on Android and again on web must read one sentence,
+// not two dialects of it.
+func TestUnknownSelectorKeyMessageMatchesTheCrossRuntimeText(t *testing.T) {
+	golden := loadSelectorKeysGolden(t)
+	if got := UnknownSelectorKeyMessage(golden.UnknownKeyExample); got != golden.UnknownKeyMessage {
+		t.Errorf("native message\n got=%q\nwant=%q", got, golden.UnknownKeyMessage)
 	}
 }
 
@@ -37,3 +44,20 @@ func TestSelectorKeysAreSorted(t *testing.T) {
 		t.Errorf("keys must stay sorted so the two lists compare readably: %v", keys)
 	}
 }
+
+func loadSelectorKeysGolden(t *testing.T) selectorKeysGolden {
+	t.Helper()
+	path, err := filepath.Abs("../../pkg/spec/test/fixtures/selector-keys.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	body, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read selector keys: %v", err)
+	}
+	var golden selectorKeysGolden
+	if err := json.Unmarshal(body, &golden); err != nil {
+		t.Fatalf("decode selector keys: %v", err)
+	}
+	return golden
+}
diff --git a/pkg/spec/src/web-runtime.ts b/pkg/spec/src/web-runtime.ts
index e088a58..f4c60f1 100644
--- a/pkg/spec/src/web-runtime.ts
+++ b/pkg/spec/src/web-runtime.ts
@@ -694,6 +694,7 @@ export const __testing__ = {
   selectorFromString,
   selectorFromObject,
   SELECTOR_KEYS,
+  unknownSelectorKeyMessage,
   xpathStringLiteral,
 };
 
diff --git a/pkg/spec/test/fixtures/selector-keys.json b/pkg/spec/test/fixtures/selector-keys.json
index 57886b1..87b1bf7 100644
--- a/pkg/spec/test/fixtures/selector-keys.json
+++ b/pkg/spec/test/fixtures/selector-keys.json
@@ -1,38 +1,44 @@
-[
-  "accessibilityIdentifier",
-  "accessibilityLabel",
-  "accessibilityText",
-  "aria-label",
-  "ariaLabel",
-  "bounds",
-  "checked",
-  "class",
-  "className",
-  "clickable",
-  "content-desc",
-  "contentDescription",
-  "data-testid",
-  "desc",
-  "descPrefix",
-  "editable",
-  "elementType",
-  "enabled",
-  "focused",
-  "hintText",
-  "id",
-  "idPrefix",
-  "identifier",
-  "label",
-  "package",
-  "placeholder",
-  "placeholderValue",
-  "resource-id",
-  "scrollable",
-  "selected",
-  "tag",
-  "testID",
-  "testTag",
-  "text",
-  "title",
-  "value"
-]
+{
+  "keys": [
+    "accessibilityIdentifier",
+    "accessibilityLabel",
+    "accessibilityText",
+    "aria-label",
+    "ariaLabel",
+    "bounds",
+    "checked",
+    "class",
+    "className",
+    "clickable",
+    "content-desc",
+    "contentDescription",
+    "data-testid",
+    "desc",
+    "descPrefix",
+    "editable",
+    "elementType",
+    "enabled",
+    "focused",
+    "hintText",
+    "id",
+    "idPrefix",
+    "identifier",
+    "label",
+    "package",
+    "placeholder",
+    "placeholderValue",
+    "resource-id",
+    "scrollable",
+    "selected",
+    "tag",
+    "testID",
+    "testTag",
+    "text",
+    "title",
+    "value"
+  ],
+  "unknownKeyExample": [
+    "descripton"
+  ],
+  "unknownKeyMessage": "selector key \"descripton\" cannot match: no element carries that attribute, and it is not one of the accepted keys: accessibilityIdentifier, accessibilityLabel, accessibilityText, aria-label, ariaLabel, bounds, checked, class, className, clickable, content-desc, contentDescription, data-testid, desc, descPrefix, editable, elementType, enabled, focused, hintText, id, idPrefix, identifier, label, package, placeholder, placeholderValue, resource-id, scrollable, selected, tag, testID, testTag, text, title, value"
+}
diff --git a/pkg/spec/test/selector-keys.test.ts b/pkg/spec/test/selector-keys.test.ts
index 715292e..7f39aae 100644
--- a/pkg/spec/test/selector-keys.test.ts
+++ b/pkg/spec/test/selector-keys.test.ts
@@ -5,10 +5,14 @@ import { test } from "node:test";
 
 import { __testing__ } from "../src/web-runtime.ts";
 
-const { SELECTOR_KEYS } = __testing__;
+const { SELECTOR_KEYS, unknownSelectorKeyMessage } = __testing__;
 
 const goldenPath = fileURLToPath(new URL("./fixtures/selector-keys.json", import.meta.url));
-const golden: string[] = JSON.parse(readFileSync(goldenPath, "utf8"));
+const golden: {
+  keys: string[];
+  unknownKeyExample: string[];
+  unknownKeyMessage: string;
+} = JSON.parse(readFileSync(goldenPath, "utf8"));
 
 // Both runtimes reject an object-selector key they do not know, so the two key
 // lists have to be one list. Were they to drift, a spec would be accepted by
@@ -17,5 +21,11 @@ const golden: string[] = JSON.parse(readFileSync(goldenPath, "utf8"));
 // internal/hierarchy/selector_keys_test.go asserts the SAME file from the
 // native side.
 test("the web key list is the cross-runtime list", () => {
-  assert.deepEqual([...SELECTOR_KEYS], golden);
+  assert.deepEqual([...SELECTOR_KEYS], golden.keys);
+});
+
+// An author who hits this on Android and again on web must read one sentence,
+// not two dialects of it.
+test("the web diagnostic is the cross-runtime text", () => {
+  assert.equal(unknownSelectorKeyMessage(golden.unknownKeyExample), golden.unknownKeyMessage);
 });