fix(quickjs): ensure top-level title on subagent response schemas (#4155)

## Problem

Agent-generated `response_schema` values passed through `task({
responseSchema })` in the QuickJS REPL bridge often omit a top-level
`title`. Structured-output backends that treat a JSON schema as a
function (e.g. the OpenAI function-calling path) require a top-level
`title` to use as the function name, otherwise they raise:

> To use a JSON schema as a function, it must have a top-level 'title'
key.

This surfaced as a `ValueError` from `js_eval` when a subagent was
dispatched with a schema like `{'type': 'object', 'properties': {'word':
{'type': 'string'}}, 'required': ['word']}`.

## Fix

In `langchain_quickjs/_subagent.py`, inject a default `title`
(`"subagent_response"`) into the response schema when one is missing or
blank, after validation and before the schema is wrapped in
`AutoStrategy`. This is the single point where every agent-generated
`response_schema` enters the pipeline.

- Schemas with an existing non-empty string `title` are returned
untouched.
- Blank / non-string titles (`""`, `"   "`, `0`, `None`) are replaced.
- The caller's dict is never mutated (a copy is returned).

## Tests

Added unit tests in `tests/unit_tests/test_repl_middleware.py` covering
default injection, preservation of existing titles, replacement of
blank/invalid titles, and non-mutation of the input.

Full suite: 75 passed, ruff clean.

libs/partners/quickjs/langchain_quickjs/_subagent.py

@@ -70,6 +70,7 @@ async def call_subagent_task_tool(
     parse_json_output = response_schema is not None
     if response_schema is not None:
         _validate_response_schema(response_schema)
+        response_schema = _ensure_schema_title(response_schema)
         runtime = _runtime_with_response_format(runtime, response_schema)
 
     runtime = _runtime_with_tool_call_id(
@@ -121,6 +122,23 @@ def _validate_response_schema(schema: dict[str, Any]) -> None:
     _check(schema, 0, [0])
 
 
+_DEFAULT_SCHEMA_TITLE = "subagent_response"
+
+
+def _ensure_schema_title(schema: dict[str, Any]) -> dict[str, Any]:
+    """Ensure the response schema carries a non-empty top-level ``title``.
+
+    Structured output backends that treat a JSON schema as a function (for
+    example, the OpenAI function-calling path) require a top-level ``title`` to
+    use as the function name. Agent-generated ``response_schema`` values often
+    omit it, so inject a default when it is missing or blank.
+    """
+    existing = schema.get("title")
+    if isinstance(existing, str) and existing.strip():
+        return schema
+    return {**schema, "title": _DEFAULT_SCHEMA_TITLE}
+
+
 def _runtime_with_response_format(
     runtime: Any,
     response_schema: dict[str, Any],

libs/partners/quickjs/tests/unit_tests/test_repl_middleware.py

@@ -28,7 +28,10 @@ from quickjs_rs import Runtime, ThreadWorker
 from langchain_quickjs import CodeInterpreterMiddleware
 from langchain_quickjs._format import format_outcome
 from langchain_quickjs._repl import _clear_exception_references, _Registry, _ThreadREPL
-from langchain_quickjs._subagent import _runtime_with_response_format
+from langchain_quickjs._subagent import (
+    _ensure_schema_title,
+    _runtime_with_response_format,
+)
 
 if TYPE_CHECKING:
     from langchain_core.callbacks import CallbackManagerForLLMRun
@@ -888,6 +891,44 @@ def test_runtime_with_response_format_uses_configurable() -> None:
     assert strategy.schema == schema
 
 
+def test_ensure_schema_title_injects_default_when_missing() -> None:
+    schema = {
+        "type": "object",
+        "properties": {"word": {"type": "string"}},
+        "required": ["word"],
+    }
+
+    updated = _ensure_schema_title(schema)
+
+    assert updated["title"] == "subagent_response"
+    # Original schema is not mutated.
+    assert "title" not in schema
+    # Other keys are preserved unchanged.
+    assert updated["properties"] == schema["properties"]
+    assert updated["required"] == schema["required"]
+
+
+def test_ensure_schema_title_preserves_existing_non_empty_title() -> None:
+    schema = {"title": "MyWord", "type": "object"}
+
+    updated = _ensure_schema_title(schema)
+
+    assert updated is schema
+    assert updated["title"] == "MyWord"
+
+
+@pytest.mark.parametrize(
+    "title",
+    ["", "   ", 0, None],
+)
+def test_ensure_schema_title_replaces_blank_or_invalid_title(title: Any) -> None:
+    schema = {"title": title, "type": "object"}
+
+    updated = _ensure_schema_title(schema)
+
+    assert updated["title"] == "subagent_response"
+
+
 class _StructuredSubagentModel(GenericFakeChatModel):
     """Fake subagent model that calls the currently bound response-format tool."""