diff --git a/api/runtime_adapter.py b/api/runtime_adapter.py
index 28a63d80..85e4c4e4 100644
--- a/api/runtime_adapter.py
+++ b/api/runtime_adapter.py
@@ -1,11 +1,13 @@
 """RuntimeAdapter seam for WebUI-owned run execution.
 
-This is the #1925 Slice 2 seam only.  The default WebUI chat path remains the
+This is the #1925 RuntimeAdapter seam.  The default WebUI chat path remains the
 legacy direct route; enabling ``HERMES_WEBUI_RUNTIME_ADAPTER=legacy-journal``
 routes through this protocol-translator facade over the same legacy execution
-path plus the Slice 1 run journal.  This module intentionally does not own
-AIAgent instances, cancellation flags, approval callbacks, clarify callbacks, or
-new long-lived queues.
+path plus the Slice 1 run journal.  Slice 4 adds a default-off runner-local
+selection point for tests and future runner backends, but live chat routes still
+stay on the legacy path until a separate route-wiring slice is reviewed.  This
+module intentionally does not own AIAgent instances, cancellation flags,
+approval callbacks, clarify callbacks, or new long-lived queues.
 """
 from __future__ import annotations
 
@@ -17,7 +19,12 @@ from typing import Any, Callable, Iterable, Literal, Protocol
 _RUNTIME_ADAPTER_ENV = "HERMES_WEBUI_RUNTIME_ADAPTER"
 _RUNTIME_ADAPTER_DIRECT = "legacy-direct"
 _RUNTIME_ADAPTER_JOURNAL = "legacy-journal"
-_VALID_RUNTIME_ADAPTER_MODES = {_RUNTIME_ADAPTER_DIRECT, _RUNTIME_ADAPTER_JOURNAL}
+_RUNTIME_ADAPTER_RUNNER_LOCAL = "runner-local"
+_VALID_RUNTIME_ADAPTER_MODES = {
+    _RUNTIME_ADAPTER_DIRECT,
+    _RUNTIME_ADAPTER_JOURNAL,
+    _RUNTIME_ADAPTER_RUNNER_LOCAL,
+}
 
 
 @dataclass(frozen=True)
@@ -106,6 +113,36 @@ def runtime_adapter_enabled(environ: dict[str, str] | None = None) -> bool:
     return runtime_adapter_mode(environ) == _RUNTIME_ADAPTER_JOURNAL
 
 
+def runtime_adapter_runner_enabled(environ: dict[str, str] | None = None) -> bool:
+    return runtime_adapter_mode(environ) == _RUNTIME_ADAPTER_RUNNER_LOCAL
+
+
+def build_runtime_adapter(
+    *,
+    environ: dict[str, str] | None = None,
+    legacy_adapter_factory: Callable[[], RuntimeAdapter] | None = None,
+    runner_client_factory: Callable[[], Any] | None = None,
+) -> RuntimeAdapter | None:
+    """Build the configured RuntimeAdapter without changing route behavior.
+
+    ``None`` means the safe default ``legacy-direct`` path should keep using the
+    existing direct route.  ``legacy-journal`` is opt-in and delegates to the
+    supplied legacy factory.  ``runner-local`` is also opt-in and only constructs
+    a ``RunnerRuntimeAdapter`` around an injected client; this function does not
+    create process-global runner state or wire live chat to the runner backend.
+    """
+    mode = runtime_adapter_mode(environ)
+    if mode == _RUNTIME_ADAPTER_DIRECT:
+        return None
+    if mode == _RUNTIME_ADAPTER_JOURNAL:
+        if legacy_adapter_factory is None:
+            raise NotImplementedError("legacy-journal mode requires a legacy adapter factory")
+        return legacy_adapter_factory()
+    if runner_client_factory is None:
+        raise NotImplementedError("runner-local mode requires a runner client factory")
+    return RunnerRuntimeAdapter(client=runner_client_factory())
+
+
 def _cursor_to_after_seq(cursor: str | None) -> int | None:
     if cursor in (None, ""):
         return None
diff --git a/docs/rfcs/hermes-run-adapter-contract.md b/docs/rfcs/hermes-run-adapter-contract.md
index 1796ba7e..4c0092f6 100644
--- a/docs/rfcs/hermes-run-adapter-contract.md
+++ b/docs/rfcs/hermes-run-adapter-contract.md
@@ -780,6 +780,12 @@ client/backend and explicit route selection.
 
 #### Slice 4c: Feature-flagged runner backend and restart/reattach harness
 
+Status as of PR #TBD: implementation proposed. The code adds a default-off
+`runner-local` adapter selection point plus factory wiring for an injected runner
+client, while keeping live browser chat routes on the legacy backend. The
+restart/reattach harness remains synthetic/fake-runner based until a later slice
+introduces a supervised runner process.
+
 After the facade exists, the next narrow implementation slice should add a real
 runner-client/backend selection point and a synthetic restart/reattach harness,
 without routing normal browser chat to that backend yet.
diff --git a/tests/test_runtime_adapter_seam.py b/tests/test_runtime_adapter_seam.py
index 26bd36de..641b4f31 100644
--- a/tests/test_runtime_adapter_seam.py
+++ b/tests/test_runtime_adapter_seam.py
@@ -26,9 +26,65 @@ def test_runtime_adapter_interface_and_legacy_journal_methods_exist():
     assert runtime.runtime_adapter_enabled({}) is False
     assert runtime.runtime_adapter_mode({"HERMES_WEBUI_RUNTIME_ADAPTER": "legacy-journal"}) == "legacy-journal"
     assert runtime.runtime_adapter_enabled({"HERMES_WEBUI_RUNTIME_ADAPTER": "legacy-journal"}) is True
+    assert runtime.runtime_adapter_mode({"HERMES_WEBUI_RUNTIME_ADAPTER": "runner-local"}) == "runner-local"
+    assert runtime.runtime_adapter_runner_enabled({"HERMES_WEBUI_RUNTIME_ADAPTER": "runner-local"}) is True
     assert runtime.runtime_adapter_mode({"HERMES_WEBUI_RUNTIME_ADAPTER": "sidecar"}) == "legacy-direct"
 
 
+def test_runtime_adapter_factory_selects_only_explicit_default_off_modes():
+    runtime = importlib.import_module("api.runtime_adapter")
+    calls = []
+
+    class FakeRunnerClient:
+        pass
+
+    def legacy_factory():
+        calls.append("legacy")
+        return runtime.LegacyJournalRuntimeAdapter(start_run_delegate=lambda request: {"stream_id": "s"})
+
+    def runner_factory():
+        calls.append("runner")
+        return FakeRunnerClient()
+
+    assert runtime.build_runtime_adapter(environ={}) is None
+
+    legacy = runtime.build_runtime_adapter(
+        environ={"HERMES_WEBUI_RUNTIME_ADAPTER": "legacy-journal"},
+        legacy_adapter_factory=legacy_factory,
+        runner_client_factory=runner_factory,
+    )
+    assert isinstance(legacy, runtime.LegacyJournalRuntimeAdapter)
+
+    runner = runtime.build_runtime_adapter(
+        environ={"HERMES_WEBUI_RUNTIME_ADAPTER": "runner-local"},
+        legacy_adapter_factory=legacy_factory,
+        runner_client_factory=runner_factory,
+    )
+    assert isinstance(runner, runtime.RunnerRuntimeAdapter)
+    assert calls == ["legacy", "runner"]
+
+
+def test_runner_local_factory_requires_injected_client_and_does_not_fallback_to_legacy():
+    runtime = importlib.import_module("api.runtime_adapter")
+    calls = []
+
+    def legacy_factory():
+        calls.append("legacy")
+        return runtime.LegacyJournalRuntimeAdapter(start_run_delegate=lambda request: {"stream_id": "s"})
+
+    try:
+        runtime.build_runtime_adapter(
+            environ={"HERMES_WEBUI_RUNTIME_ADAPTER": "runner-local"},
+            legacy_adapter_factory=legacy_factory,
+        )
+    except NotImplementedError as exc:
+        assert "runner client factory" in str(exc)
+    else:
+        raise AssertionError("runner-local must require an injected runner client factory")
+
+    assert calls == []
+
+
 def test_legacy_journal_adapter_start_run_delegates_without_owning_runtime_state():
     runtime = importlib.import_module("api.runtime_adapter")
     calls = []