fix(server): make mixed runtime-config update atomic (#607 follow-up)

coding-agent/src/coding_agent/server/http_server.py

@@ -2594,21 +2594,7 @@ async def update_runtime_config(
     if getattr(session, "turn_in_progress", False):
         raise HTTPException(status_code=409, detail="Turn already in progress")
 
-    # Apply thinking and approval directly on live mutable session/runtime state.
-    # No pipeline rebuild needed for these: provider reads thinking per-turn from
-    # ctx.config, and approval is evaluated by the existing live plugin.
     no_rebuild_update = not (provided & {"model", "provider", "base_url"})
-    if body.thinking is not None:
-        session.thinking_config["enabled"] = body.thinking.enabled
-        session.thinking_config["effort"] = body.thinking.effort
-    if body.approval is not None:
-        approval_policy = ApprovalPolicy(body.approval)
-        plugin = _live_approval_plugin(session)
-        if plugin is not None:
-            plugin.set_policy(approval_policy)
-        session.approval_policy = approval_policy
-        await session_manager._persist_session_async(session)
-
     try:
         if no_rebuild_update:
             updated_session = session
@@ -2634,6 +2620,20 @@ async def update_runtime_config(
             status_code=500, detail=_http_exception_detail(exc)
         ) from exc
 
+    # Apply thinking and approval after the fallible rebuild. For rebuilds this
+    # targets the freshly attached runtime; for approval/thinking-only updates it
+    # keeps the existing no-rebuild fast path.
+    if body.thinking is not None:
+        updated_session.thinking_config["enabled"] = body.thinking.enabled
+        updated_session.thinking_config["effort"] = body.thinking.effort
+    if body.approval is not None:
+        approval_policy = ApprovalPolicy(body.approval)
+        plugin = _live_approval_plugin(updated_session)
+        if plugin is not None:
+            plugin.set_policy(approval_policy)
+        updated_session.approval_policy = approval_policy
+        await session_manager._persist_session_async(updated_session)
+
     return RuntimeConfigUpdateResponse(
         session_id=session_id,
         provider_name=getattr(updated_session, "provider_name", None),

coding-agent/tests/ui/test_http_server.py

@@ -2444,6 +2444,64 @@ async def fail_replace_session_runtime_config(*args, **kwargs):
             Approve,
         )
 
+    async def test_mixed_runtime_replacement_failure_leaves_approval_and_thinking_unchanged(
+        self, client, monkeypatch
+    ):
+        response = await client.post("/sessions", json={"approval_policy": "auto"})
+        session_id = response.json()["session_id"]
+        await session_manager.ensure_session_runtime(session_id)
+        session = session_manager.get_session(session_id)
+        original_thinking_config = dict(session.thinking_config)
+        persisted_before = session_manager._store.load(session_id)
+        assert persisted_before is not None
+        assert persisted_before["approval_policy"] == "auto"
+
+        plugin = session.runtime_pipeline._registry.get("approval")
+        assert isinstance(plugin, ApprovalPlugin)
+        assert isinstance(
+            plugin.approve_tool_call(tool_name="bash_run", arguments={}),
+            AskUser,
+        )
+
+        replace_calls: list[dict[str, object]] = []
+
+        async def fail_replace_session_runtime_config(*args, **kwargs):
+            replace_calls.append(dict(kwargs))
+            raise RuntimeError("runtime replacement failed")
+
+        monkeypatch.setattr(
+            session_manager,
+            "replace_session_runtime_config",
+            fail_replace_session_runtime_config,
+        )
+
+        update = await client.post(
+            f"/sessions/{session_id}/runtime-config",
+            json={
+                "approval": "yolo",
+                "thinking": {"enabled": False, "effort": "high"},
+                "model": "replacement-model",
+            },
+        )
+
+        assert update.status_code == 500
+        assert replace_calls == [
+            {
+                "model_name": "replacement-model",
+                "provider_name": None,
+                "base_url": http_server.UNSET,
+            }
+        ]
+        assert session.approval_policy == ApprovalPolicy.AUTO
+        assert session.thinking_config == original_thinking_config
+        assert isinstance(
+            plugin.approve_tool_call(tool_name="bash_run", arguments={}),
+            AskUser,
+        )
+        persisted_after = session_manager._store.load(session_id)
+        assert persisted_after is not None
+        assert persisted_after["approval_policy"] == "auto"
+
     async def test_approval_null_is_rejected(self, client):
         response = await client.post("/sessions", json={})
         session_id = response.json()["session_id"]