diff --git a/src/clayde/claude.py b/src/clayde/claude.py
index d5c93a7..2f0b07a 100644
--- a/src/clayde/claude.py
+++ b/src/clayde/claude.py
@@ -43,6 +43,14 @@
     "hit your limit",
 ]
 
+# Text patterns that indicate an authentication error in CLI error output.
+_AUTH_ERROR_PATTERNS = [
+    "not logged in",
+    "failed to authenticate",
+    "authentication_error",
+    "invalid authentication credentials",
+]
+
 
 @dataclasses.dataclass
 class InvocationResult:
@@ -446,6 +454,12 @@ def _is_limit_error(text: str) -> bool:
     return any(p in t for p in _LIMIT_PATTERNS)
 
 
+def _is_auth_error(text: str) -> bool:
+    """Return True if text contains an authentication error pattern."""
+    t = text.lower()
+    return any(p in t for p in _AUTH_ERROR_PATTERNS)
+
+
 def _make_cli_env() -> dict[str, str]:
     """Build an environment dict for CLI subprocess calls."""
     env = os.environ.copy()
@@ -613,6 +627,11 @@ def invoke(
                     exc = UsageLimitError("Claude CLI usage limit hit")
                     span.record_exception(exc)
                     raise exc
+                if _is_auth_error(error_text):
+                    log.error("Claude CLI authentication failed")
+                    exc = RuntimeError("Claude CLI authentication failed")
+                    span.record_exception(exc)
+                    raise exc
 
             if result.returncode != 0:
                 log.error("Claude CLI exited with code %d", result.returncode)
@@ -652,8 +671,8 @@ def is_available(self) -> bool:
                 if _is_limit_error(error_text):
                     span.set_attribute("claude.available", False)
                     return False
-                if is_error and "not logged in" in error_text.lower():
-                    log.warning("Claude CLI is not logged in — marking unavailable")
+                if is_error and _is_auth_error(error_text):
+                    log.warning("Claude CLI authentication failed — marking unavailable")
                     span.set_attribute("claude.available", False)
                     return False
                 span.set_attribute("claude.available", True)
diff --git a/tests/test_claude.py b/tests/test_claude.py
index 7c330dd..7d1066c 100644
--- a/tests/test_claude.py
+++ b/tests/test_claude.py
@@ -888,6 +888,42 @@ def test_stale_session_no_conv_path_no_retry(self, tmp_path):
         assert result.output == ""
         assert mock_run.call_count == 1
 
+    def test_auth_error_raises_runtime_error(self, tmp_path):
+        """is_error=True with auth failure raises RuntimeError instead of returning error text."""
+        (tmp_path / "CLAUDE.md").write_text("identity")
+        auth_error_text = (
+            'Failed to authenticate. API Error: 401 {"type":"error","error":{'
+            '"type":"authentication_error","message":"Invalid authentication credentials"}}'
+        )
+        mock_result = MagicMock()
+        mock_result.stdout = json.dumps({"is_error": True, "result": auth_error_text})
+        mock_result.stderr = ""
+        mock_result.returncode = 0
+        backend = CliBackend()
+
+        with patch("clayde.claude.APP_DIR", tmp_path), \
+             patch("clayde.claude.get_settings", return_value=_mock_settings(backend="cli")), \
+             patch("clayde.claude._resolve_cli_bin", return_value="/usr/bin/claude"), \
+             patch("clayde.claude.subprocess.run", return_value=mock_result):
+            with pytest.raises(RuntimeError, match="authentication failed"):
+                backend.invoke("prompt", "/repo")
+
+    def test_not_logged_in_raises_runtime_error(self, tmp_path):
+        """is_error=True with 'not logged in' text raises RuntimeError."""
+        (tmp_path / "CLAUDE.md").write_text("identity")
+        mock_result = MagicMock()
+        mock_result.stdout = json.dumps({"is_error": True, "result": "Not logged in · Please run /login"})
+        mock_result.stderr = ""
+        mock_result.returncode = 0
+        backend = CliBackend()
+
+        with patch("clayde.claude.APP_DIR", tmp_path), \
+             patch("clayde.claude.get_settings", return_value=_mock_settings(backend="cli")), \
+             patch("clayde.claude._resolve_cli_bin", return_value="/usr/bin/claude"), \
+             patch("clayde.claude.subprocess.run", return_value=mock_result):
+            with pytest.raises(RuntimeError, match="authentication failed"):
+                backend.invoke("prompt", "/repo")
+
 
 class TestCliBackendIsAvailable:
     def test_available_on_success(self):
@@ -926,6 +962,25 @@ def test_unavailable_on_not_logged_in(self):
              patch("clayde.claude.subprocess.run", return_value=mock_result):
             assert backend.is_available() is False
 
+    def test_unavailable_on_failed_to_authenticate(self):
+        mock_result = MagicMock()
+        mock_result.stdout = json.dumps({
+            "is_error": True,
+            "result": (
+                "Failed to authenticate. API Error: 401 "
+                '{"type":"error","error":{"type":"authentication_error",'
+                '"message":"Invalid authentication credentials"}}'
+            ),
+        })
+        mock_result.stderr = ""
+        mock_result.returncode = 0
+        backend = CliBackend()
+
+        with patch("clayde.claude.get_settings", return_value=_mock_settings(backend="cli")), \
+             patch("clayde.claude._resolve_cli_bin", return_value="/usr/bin/claude"), \
+             patch("clayde.claude.subprocess.run", return_value=mock_result):
+            assert backend.is_available() is False
+
     def test_available_on_exception(self):
         backend = CliBackend()