feat(code): trace experimental mode in metadata (#4705)

Deep Agents Code traces now include `dcode_experimental: true` when
`DEEPAGENTS_CODE_EXPERIMENTAL` is enabled.

---

`DEEPAGENTS_CODE_EXPERIMENTAL` changes agent behavior, but traces
currently do not record whether that mode was active. This adds
`dcode_experimental: true` to trace-wide metadata whenever the
environment flag resolves to a truthy value. The key remains absent
otherwise, so default trace metadata is unchanged.

libs/code/deepagents_code/config.py

@@ -1676,6 +1676,9 @@ def build_stream_config(
     This describes the Deep Agents package installed alongside the TUI, which
     can differ from a remote graph's Deep Agents runtime version.
 
+    Also records `dcode_experimental=True` when `DEEPAGENTS_CODE_EXPERIMENTAL`
+    is enabled, so experimental runs are filterable in trace metadata.
+
     Args:
         thread_id: The app session thread identifier. Set both on
             `configurable.thread_id` and as the top-level `metadata.thread_id`
@@ -1699,7 +1702,7 @@ def build_stream_config(
         logger.warning("Could not determine working directory", exc_info=True)
         cwd = ""
 
-    from deepagents_code._env_vars import USER_ID
+    from deepagents_code._env_vars import EXPERIMENTAL, USER_ID
 
     metadata: dict[str, Any] = build_coding_agent_metadata(
         thread_id=thread_id,
@@ -1711,6 +1714,10 @@ def build_stream_config(
         user_id=os.environ.get(USER_ID) or None,
     )
 
+    # Mark experimental runs so they are filterable in trace metadata.
+    if is_env_truthy(EXPERIMENTAL):
+        metadata["dcode_experimental"] = True
+
     # Legacy / diagnostic keys preserved for backward-compatibility during the
     # coding-agent-v1 rollout (not part of the contract).
     metadata["lc_versions"] = {

libs/code/tests/unit_tests/test_transcript_virtualization.py

@@ -200,7 +200,7 @@ class TestScrollDrivenHydration:
             # Archive the newest rows below the window (the state after the user
             # has scrolled up and older history was mounted in their place).
             monkeypatch.setattr(app._message_store, "WINDOW_SIZE", 3)
-            monkeypatch.setattr(app._message_store, "HYDRATE_BUFFER", 2)
+            monkeypatch.setattr(app._message_store, "HYDRATE_BUFFER", 20)
             messages = app.query_one("#messages", Container)
             await app._prune_messages_below_window(messages)
             await pilot.pause()

libs/code/tests/unit_tests/tui/test_textual_adapter.py

@@ -1092,6 +1092,26 @@ class TestBuildStreamConfig:
             config = build_stream_config("t-nouid", assistant_id=None)
         assert "user_id" not in config["metadata"]
 
+    def test_experimental_included_when_enabled(self) -> None:
+        """Experimental runs should be identifiable in trace metadata."""
+        with patch.dict("os.environ", {"DEEPAGENTS_CODE_EXPERIMENTAL": "true"}):
+            config = build_stream_config("t-experimental", assistant_id=None)
+        assert config["metadata"]["dcode_experimental"] is True
+
+    def test_experimental_absent_when_disabled(self) -> None:
+        """Default runs should not be labeled experimental."""
+        with patch.dict("os.environ", {"DEEPAGENTS_CODE_EXPERIMENTAL": "false"}):
+            config = build_stream_config("t-stable", assistant_id=None)
+        assert "dcode_experimental" not in config["metadata"]
+
+    def test_experimental_absent_when_unset(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """The default path (env var unset) is not labeled experimental."""
+        monkeypatch.delenv("DEEPAGENTS_CODE_EXPERIMENTAL", raising=False)
+        config = build_stream_config("t-default", assistant_id=None)
+        assert "dcode_experimental" not in config["metadata"]
+
 
 class TestGetGitBranch:
     """Tests for `_get_git_branch` caching."""