diff --git a/CHANGELOG.md b/CHANGELOG.md
index 93fb8cdd..c87c3056 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Keep the default-off `HERMES_WEBUI_RUNTIME_ADAPTER=legacy-journal` chat-start path response-compatible with the legacy-direct path by not adding adapter-internal `run_id`, `status`, or `active_controls` fields to `/api/chat/start` responses. Fixes #2435.
+
 ## [v0.51.82] — 2026-05-17 — Release BF (stage-375 — 2-PR batch — table renderer pipe protection + Catppuccin appearance skin)
 
 ### Added
diff --git a/api/routes.py b/api/routes.py
index 677a6677..779a0176 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -7800,9 +7800,6 @@ def _handle_chat_start(handler, body, diag=None):
             response = dict(result.payload)
             response.setdefault("stream_id", result.stream_id)
             response.setdefault("session_id", result.session_id)
-            response.setdefault("run_id", result.run_id)
-            response.setdefault("status", result.status)
-            response.setdefault("active_controls", result.active_controls)
         else:
             response = _start_chat_stream_for_session(
                 s,
diff --git a/tests/test_runtime_adapter_seam.py b/tests/test_runtime_adapter_seam.py
index 296abd64..c261d54d 100644
--- a/tests/test_runtime_adapter_seam.py
+++ b/tests/test_runtime_adapter_seam.py
@@ -119,3 +119,22 @@ def test_chat_start_route_selects_adapter_only_when_flag_enabled():
     assert "LegacyJournalRuntimeAdapter" in start_body
     assert "_start_chat_stream_for_session(" in start_body
     assert "HERMES_WEBUI_RUNTIME_ADAPTER" not in start_body, "route should use runtime_adapter_enabled(), not inline env checks"
+
+
+def test_chat_start_adapter_path_preserves_legacy_response_shape():
+    """The RuntimeAdapter seam must be invisible to /api/chat/start callers.
+
+    The adapter can use run_id/status/controls internally, but the flagged
+    route must not add fields that the legacy-direct response does not expose.
+    """
+    routes = importlib.import_module("api.routes")
+    src = (routes.Path(__file__).parent.parent / "api" / "routes.py").read_text(encoding="utf-8")
+    branch_start = src.index("if runtime_adapter_enabled():")
+    branch_end = src.index("else:", branch_start)
+    adapter_branch = src[branch_start:branch_end]
+
+    assert 'response.setdefault("stream_id", result.stream_id)' in adapter_branch
+    assert 'response.setdefault("session_id", result.session_id)' in adapter_branch
+    assert 'response.setdefault("run_id", result.run_id)' not in adapter_branch
+    assert 'response.setdefault("status", result.status)' not in adapter_branch
+    assert 'response.setdefault("active_controls", result.active_controls)' not in adapter_branch