diff --git a/CHANGELOG.md b/CHANGELOG.md
index 19c1bc1..354f218 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -62,6 +62,12 @@ Newest entries on top, within each tool.
 
 ## aikit
 
+### 1.15.5 — 2026-07-03
+- ⚠ **Fix: `aikit auth <agent>` now exits non-zero when the agent is not installed**
+  (POK-90). Previously it printed the "is not installed. Run 'aikit install …' first."
+  error but exited `0`, misleading scripts and CI. The message is unchanged; exit
+  code is now `1` via `AikitError`, consistent with unknown-agent validation.
+
 ### 1.15.4 — 2026-07-02
 - **Fix: five POK-65 agents falsely showed "needs auth" when authenticated** (POK-78).
   `auggie`, `devin`, `qodo`, `openinterpreter`, and `plandex` were registered without
diff --git a/aikit b/aikit
index 8ca4a2b..0c3dcaf 100755
--- a/aikit
+++ b/aikit
@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 """
-aikit — AI Coding Agent CLI Installer & Manager  v1.15.4
+aikit — AI Coding Agent CLI Installer & Manager  v1.15.5
 Install, update, authenticate, and manage 31 AI coding agent CLIs from one tool.
 
 Architecture:
@@ -52,7 +52,7 @@ from rich.progress import Progress, SpinnerColumn, TextColumn
 from rich.table import Table
 from rich.text import Text
 
-__version__ = "1.15.4"
+__version__ = "1.15.5"
 
 import scriptkit as sk
 
@@ -2879,8 +2879,9 @@ def _do_auth_impl(agent_key):
     icon = agent["icon"]
 
     if not detect_agent_bin(agent_key):
-        print_error(f"{icon} {agent['name']} is not installed. Run 'aikit install {agent_key}' first.")
-        return
+        raise AikitError(
+            f"{icon} {agent['name']} is not installed. Run 'aikit install {agent_key}' first."
+        )
 
     auth_type = agent.get("auth_type", "unknown")
     auth_note = agent.get("auth_note", "")
diff --git a/tests/test_tools_characterization.py b/tests/test_tools_characterization.py
index 6a486f7..f425415 100644
--- a/tests/test_tools_characterization.py
+++ b/tests/test_tools_characterization.py
@@ -1311,6 +1311,18 @@ def test_aikit_uninstall_prunes_config_entry(tool_loader, monkeypatch, tmp_path)
     assert "kilo" not in saved.get("agents", {})
 
 
+def test_aikit_auth_signals_failure_when_agent_not_installed(tool_loader, monkeypatch):
+    # POK-90: auth for a known-but-not-installed agent must raise AikitError so
+    # `aikit auth` exits non-zero (previously it printed an error and returned 0).
+    m = tool_loader("aikit")
+
+    monkeypatch.setattr(m, "validate_agent_keys", lambda keys: keys)
+    monkeypatch.setattr(m, "detect_agent_bin", lambda _key: False)
+
+    with pytest.raises(m.AikitError, match="not installed"):
+        m._do_auth_impl("aider")
+
+
 def test_aikit_uninstall_signals_failure_when_no_uninstall_command(tool_loader, monkeypatch):
     # POK-82: an installed agent with no automated uninstall command must make
     # `aikit uninstall` exit non-zero (previously it exited 0 and misled callers).