diff --git a/src/tests/v2/v2.test.ts b/src/tests/v2/v2.test.ts
index 14c9446b..398aae4f 100644
--- a/src/tests/v2/v2.test.ts
+++ b/src/tests/v2/v2.test.ts
@@ -2433,7 +2433,7 @@ describe("ACP v2 core", () => {
     expect(client.sessionUpdates.at(-1)).toMatchObject({
       state: "idle",
       stopReason: "error",
-      error: { code: -32000 },
+      error: { code: -32000, message: "Authentication required" },
     });
   });
 
@@ -2442,7 +2442,7 @@ describe("ACP v2 core", () => {
       "an authentication failure as authRequired",
       "authentication_failed",
       "Failed to authenticate. API Error: 401",
-      { code: -32000 },
+      { code: -32000, message: "Authentication required: Failed to authenticate. API Error: 401" },
     ],
     [
       "another failure with its error kind",
diff --git a/src/v2/outcome.ts b/src/v2/outcome.ts
index 26077e82..23cdb663 100644
--- a/src/v2/outcome.ts
+++ b/src/v2/outcome.ts
@@ -91,7 +91,11 @@ export function idleOutcome(
       providerFailureCategory(kind) === "auth_required");
   return failed(
     authFailure
-      ? RequestError.authRequired()
+      ? // Claude Code's `/login` instruction is for its terminal, so that text
+        // is left out. Any other says why the login failed, e.g. "Failed to
+        // authenticate: this login reached its time limit and cannot be
+        // renewed."
+        RequestError.authRequired(undefined, ended.loginRequired ? undefined : ended.error)
       : RequestError.internalError(kind ? { errorKind: kind } : undefined, ended.error),
   );
 }