diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 58abc438d..cb9bb4775 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -889,6 +889,26 @@ async def connect(self) -> None:
             error = CLINotFoundError(f"Claude Code not found at: {self._cli_path}")
             self._exit_error = error
             raise error from e
+        except (KeyError, PermissionError, ValueError) as e:
+            # These three are how subprocess.Popen(user=...) fails when
+            # options.user is set: KeyError from getpwnam() for an unknown
+            # account, PermissionError when the process may not switch to it,
+            # and ValueError on platforms without setreuid() (e.g. Windows).
+            # Callers who mistake `user` for a session/user identifier hit the
+            # KeyError case, so explain what the option actually does.
+            if self._options.user is not None:
+                error = CLIConnectionError(
+                    f"Failed to start Claude Code as OS user"
+                    f" {self._options.user!r}: {e}. The `user` option runs the"
+                    " CLI subprocess as that operating-system account (it is"
+                    " not a session or user identifier). The account must"
+                    " exist and the current process needs permission to"
+                    " switch to it; the option is not supported on Windows."
+                )
+            else:
+                error = CLIConnectionError(f"Failed to start Claude Code: {e}")
+            self._exit_error = error
+            raise error from e
         except Exception as e:
             error = CLIConnectionError(f"Failed to start Claude Code: {e}")
             self._exit_error = error
diff --git a/src/claude_agent_sdk/types.py b/src/claude_agent_sdk/types.py
index 308b76cb7..a961ca4f5 100644
--- a/src/claude_agent_sdk/types.py
+++ b/src/claude_agent_sdk/types.py
@@ -2139,7 +2139,15 @@ class ClaudeAgentOptions:
     """
 
     user: str | None = None
-    """Optional user identifier associated with the session."""
+    """Run the Claude Code subprocess as this operating-system user.
+
+    The value is passed to ``subprocess.Popen(user=...)``, which resolves it
+    via ``getpwnam()`` and switches the child process to that account before
+    the CLI starts. It is **not** a session or analytics identifier.
+
+    Requires the account to exist and the current process to have permission
+    to switch users (typically root). Not supported on Windows.
+    """
 
     include_partial_messages: bool = False
     """Include partial/streaming message events in the output.
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 2a0ed35bf..0ea46223c 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -1731,6 +1731,87 @@ async def _test():
 
         anyio.run(_test)
 
+    def test_connect_with_unknown_os_user_explains_user_option(self):
+        """A getpwnam failure for options.user must name the option and
+        explain that it is an OS account, not a session/user identifier."""
+        from claude_agent_sdk._errors import CLIConnectionError
+
+        async def _test():
+            options = make_options(user="customer-42")
+
+            with patch(
+                "anyio.open_process", new_callable=AsyncMock
+            ) as mock_open_process:
+                # Mock version check process
+                mock_version_process = MagicMock()
+                mock_version_process.stdout = MagicMock()
+                mock_version_process.stdout.receive = AsyncMock(
+                    return_value=b"2.0.0 (Claude Code)"
+                )
+                mock_version_process.terminate = MagicMock()
+                mock_version_process.wait = AsyncMock()
+
+                # Version check succeeds; the main spawn fails the way
+                # subprocess.Popen(user=...) fails for an unknown account.
+                mock_open_process.side_effect = [
+                    mock_version_process,
+                    KeyError("getpwnam(): name not found: 'customer-42'"),
+                ]
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=options,
+                )
+
+                with pytest.raises(CLIConnectionError) as exc_info:
+                    await transport.connect()
+
+                message = str(exc_info.value)
+                assert "'customer-42'" in message
+                assert "operating-system account" in message
+                assert "not a session or user identifier" in message
+
+        anyio.run(_test)
+
+    def test_connect_spawn_failure_without_user_keeps_generic_error(self):
+        """The OS-user explanation must not leak into failures unrelated to
+        options.user."""
+        from claude_agent_sdk._errors import CLIConnectionError
+
+        async def _test():
+            options = make_options()  # user not set
+
+            with patch(
+                "anyio.open_process", new_callable=AsyncMock
+            ) as mock_open_process:
+                # Mock version check process
+                mock_version_process = MagicMock()
+                mock_version_process.stdout = MagicMock()
+                mock_version_process.stdout.receive = AsyncMock(
+                    return_value=b"2.0.0 (Claude Code)"
+                )
+                mock_version_process.terminate = MagicMock()
+                mock_version_process.wait = AsyncMock()
+
+                mock_open_process.side_effect = [
+                    mock_version_process,
+                    ValueError("embedded null byte"),
+                ]
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=options,
+                )
+
+                with pytest.raises(CLIConnectionError) as exc_info:
+                    await transport.connect()
+
+                message = str(exc_info.value)
+                assert "Failed to start Claude Code: embedded null byte" in message
+                assert "operating-system account" not in message
+
+        anyio.run(_test)
+
     def test_build_command_with_sandbox_only(self):
         """Test building CLI command with sandbox settings (no existing settings)."""
         import json