feat(cli): show sandbox provider in header sub_title (#3295)

Closes #2304      

---
Auto-populate `sub_title` with the active sandbox provider name (e.g.
`"Sandbox: Daytona"`) so the optional header bar indicates which sandbox
is in use. Display names use proper casing via a lookup dict, with a
`.title()` fallback for unrecognised providers. An explicitly passed
`sub_title` takes precedence over the auto-detected value.

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/cli/deepagents_cli/app.py

@@ -888,6 +888,15 @@ _COMMAND_URLS: dict[str, str] = {
 }
 """Slash-command to URL mapping for commands that just open a browser."""
 
+_SANDBOX_DISPLAY_NAMES: dict[str, str] = {
+    "agentcore": "AgentCore",
+    "daytona": "Daytona",
+    "langsmith": "LangSmith",
+    "modal": "Modal",
+    "runloop": "Runloop",
+}
+"""Human-readable display names for sandbox providers."""
+
 
 _toast_internals_warned: list[bool] = [False]
 """Single-slot flag; once `_Toast._notification` is missing, log warning once.
@@ -1315,6 +1324,12 @@ class DeepAgentsApp(App):
         self._sandbox_type: str | None = raw if raw and raw != "none" else None
         """Normalized sandbox type (or `None`), attached to trace metadata."""
 
+        if sub_title is None and self._sandbox_type is not None:
+            display = _SANDBOX_DISPLAY_NAMES.get(
+                self._sandbox_type, self._sandbox_type.title()
+            )
+            self.sub_title = f"Sandbox: {display}"
+
         # Per-turn model overrides
         self._model_override: str | None = None
         """Per-turn model override set via `/model`; `None` uses session default."""

libs/cli/tests/unit_tests/test_app.py

@@ -7717,6 +7717,43 @@ class TestHeaderAndTitle:
             assert not app.query(Header)
 
 
+class TestSandboxSubTitle:
+    """sub_title reflects the active sandbox provider."""
+
+    async def test_sandbox_sets_sub_title(self) -> None:
+        """When a sandbox is active, sub_title shows the provider name."""
+        app = DeepAgentsApp(server_kwargs={"sandbox_type": "daytona"})
+        assert app.sub_title == "Sandbox: Daytona"
+
+    async def test_sandbox_sub_title_proper_casing(self) -> None:
+        """Provider display names use proper casing."""
+        app = DeepAgentsApp(server_kwargs={"sandbox_type": "langsmith"})
+        assert app.sub_title == "Sandbox: LangSmith"
+
+        app2 = DeepAgentsApp(server_kwargs={"sandbox_type": "agentcore"})
+        assert app2.sub_title == "Sandbox: AgentCore"
+
+    async def test_explicit_sub_title_overrides_sandbox(self) -> None:
+        """An explicitly passed sub_title is not overwritten by sandbox info."""
+        app = DeepAgentsApp(sub_title="custom", server_kwargs={"sandbox_type": "modal"})
+        assert app.sub_title == "custom"
+
+    async def test_no_sandbox_leaves_sub_title_default(self) -> None:
+        """Without a sandbox, sub_title remains at its Textual default."""
+        app = DeepAgentsApp()
+        assert app.sub_title == ""
+
+    async def test_sandbox_none_does_not_set_sub_title(self) -> None:
+        """The argparse default `'none'` is treated as no sandbox."""
+        app = DeepAgentsApp(server_kwargs={"sandbox_type": "none"})
+        assert app.sub_title == ""
+
+    async def test_unknown_provider_uses_title_case_fallback(self) -> None:
+        """An unrecognized provider falls back to `.title()` casing."""
+        app = DeepAgentsApp(server_kwargs={"sandbox_type": "kubernetes"})
+        assert app.sub_title == "Sandbox: Kubernetes"
+
+
 class TestHandleExternalSignal:
     """Verify routing of `kind=signal` external events."""