Python: Add background agent support to harness agent (#6155)

* Add background agent support to harness agent

* Address PR comments

python/packages/core/agent_framework/_harness/_agent.py

@@ -14,12 +14,13 @@ import logging
 from collections.abc import Callable, Sequence
 from typing import TYPE_CHECKING, Any
 
-from .._agents import Agent
+from .._agents import Agent, SupportsAgentRun
 from .._clients import SupportsWebSearchTool
 from .._compaction import CompactionProvider, ContextWindowCompactionStrategy, ToolResultCompactionStrategy
 from .._feature_stage import ExperimentalFeature, experimental
 from .._sessions import ContextProvider, HistoryProvider, InMemoryHistoryProvider
 from .._skills import SkillsProvider
+from ._background_agents import BackgroundAgentsProvider
 from ._memory import MemoryContextProvider, MemoryStore
 from ._mode import AgentModeProvider
 from ._todo import TodoProvider
@@ -103,6 +104,8 @@ def _assemble_context_providers(
     memory_store: MemoryStore | None,
     skills_provider: SkillsProvider | None,
     skills_paths: Sequence[str] | None,
+    background_agents: Sequence[SupportsAgentRun] | None,
+    background_agents_instructions: str | None,
     extra_context_providers: Sequence[ContextProvider] | None,
 ) -> list[ContextProvider]:
     """Assemble the ordered list of context providers."""
@@ -130,6 +133,10 @@ def _assemble_context_providers(
     if skills_paths:
         providers.append(SkillsProvider.from_paths(*skills_paths))
 
+    # Background agents are opt-in: only added when agents are provided.
+    if background_agents:
+        providers.append(BackgroundAgentsProvider(background_agents, instructions=background_agents_instructions))
+
     # Append any user-supplied additional providers.
     if extra_context_providers:
         providers.extend(extra_context_providers)
@@ -165,6 +172,8 @@ def create_harness_agent(
     memory_store: MemoryStore | None = None,
     skills_provider: SkillsProvider | None = None,
     skills_paths: Sequence[str] | None = None,
+    background_agents: Sequence[SupportsAgentRun] | None = None,
+    background_agents_instructions: str | None = None,
     disable_web_search: bool = False,
     otel_provider_name: str | None = None,
     context_providers: Sequence[ContextProvider] | None = None,
@@ -182,6 +191,7 @@ def create_harness_agent(
     - **AgentModeProvider** — plan/execute mode tracking
     - **MemoryContextProvider** — file-based durable memory (when ``memory_store`` provided)
     - **SkillsProvider** — skill discovery and progressive loading
+    - **BackgroundAgentsProvider** — delegate work to background sub-agents
     - **OpenTelemetry** — observability via ``AgentTelemetryLayer``
 
     Each feature can be disabled or customized via keyword arguments.
@@ -253,6 +263,13 @@ def create_harness_agent(
         skills_paths: Paths for file-based skill discovery (looks for SKILL.md files).
             Can be combined with ``skills_provider``. When neither ``skills_provider``
             nor ``skills_paths`` is provided, no SkillsProvider is added.
+        background_agents: Collection of agents available for background task delegation.
+            When provided, a ``BackgroundAgentsProvider`` is automatically included,
+            enabling the agent to start, monitor, and retrieve results from background tasks.
+            Each agent must have a non-empty, unique name (case-insensitive).
+        background_agents_instructions: Optional instruction override for the
+            ``BackgroundAgentsProvider``. May include ``{background_agents}`` placeholder
+            which will be replaced with the agent listing.
         disable_web_search: When True, skip automatic web search tool inclusion.
             When False (default), the web search tool is automatically added if the
             client implements SupportsWebSearchTool. A warning is logged if the client
@@ -302,6 +319,8 @@ def create_harness_agent(
         memory_store=memory_store,
         skills_provider=skills_provider,
         skills_paths=skills_paths,
+        background_agents=background_agents,
+        background_agents_instructions=background_agents_instructions,
         extra_context_providers=context_providers,
     )
 

python/packages/core/tests/core/test_harness_agent.py

@@ -394,3 +394,94 @@ def test_create_harness_agent_logs_warning_when_no_web_search(caplog: pytest.Log
             max_output_tokens=16_384,
         )
     assert any("SupportsWebSearchTool" in msg for msg in caplog.messages)
+
+
+# --- Background Agents Tests ---
+
+
+class _FakeBackgroundAgent:
+    """Minimal agent stub satisfying SupportsAgentRun for background agents tests."""
+
+    def __init__(self, name: str, description: str | None = None):
+        self.id = f"agent-{name}"
+        self.name = name
+        self.description = description
+
+    def create_session(self, *, session_id: str | None = None) -> AgentSession:
+        return AgentSession(session_id=session_id)
+
+    def get_session(self, service_session_id: str, *, session_id: str | None = None) -> AgentSession:
+        return AgentSession(service_session_id=service_session_id, session_id=session_id)
+
+    async def run(self, messages: Any = None, *, stream: bool = False, session: Any = None, **kwargs: Any) -> Any:
+        from agent_framework import AgentResponse
+
+        return AgentResponse(messages=[], response_id="fake-bg-response")
+
+
+def test_create_harness_agent_no_background_agents_by_default() -> None:
+    """No BackgroundAgentsProvider should be included when background_agents is not provided."""
+    from agent_framework._harness._background_agents import BackgroundAgentsProvider
+
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        max_context_window_tokens=128_000,
+        max_output_tokens=16_384,
+        disable_web_search=True,
+    )
+    providers = agent.context_providers or []
+    assert not any(isinstance(p, BackgroundAgentsProvider) for p in providers)
+
+
+def test_create_harness_agent_adds_background_agents_provider() -> None:
+    """BackgroundAgentsProvider should be included when background_agents are provided."""
+    from agent_framework._harness._background_agents import BackgroundAgentsProvider
+
+    bg_agent = _FakeBackgroundAgent("WebSearcher", "Searches the web")
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        max_context_window_tokens=128_000,
+        max_output_tokens=16_384,
+        disable_web_search=True,
+        background_agents=[bg_agent],
+    )
+    providers = agent.context_providers or []
+    bg_providers = [p for p in providers if isinstance(p, BackgroundAgentsProvider)]
+    assert len(bg_providers) == 1
+
+
+def test_create_harness_agent_background_agents_custom_instructions() -> None:
+    """Custom instructions should be passed to BackgroundAgentsProvider."""
+    from agent_framework._harness._background_agents import BackgroundAgentsProvider
+
+    custom_instructions = "## Custom\n\nUse agents wisely.\n\n{background_agents}"
+    bg_agent = _FakeBackgroundAgent("Helper", "A helper agent")
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        max_context_window_tokens=128_000,
+        max_output_tokens=16_384,
+        disable_web_search=True,
+        background_agents=[bg_agent],
+        background_agents_instructions=custom_instructions,
+    )
+    providers = agent.context_providers or []
+    bg_providers = [p for p in providers if isinstance(p, BackgroundAgentsProvider)]
+    assert len(bg_providers) == 1
+    # Verify the custom instructions were used (placeholder replaced with agent list).
+    assert "Custom" in bg_providers[0]._instructions
+    assert "Helper" in bg_providers[0]._instructions
+
+
+def test_create_harness_agent_empty_background_agents_list() -> None:
+    """An empty background_agents list should NOT add a BackgroundAgentsProvider."""
+    from agent_framework._harness._background_agents import BackgroundAgentsProvider
+
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        max_context_window_tokens=128_000,
+        max_output_tokens=16_384,
+        disable_web_search=True,
+        background_agents=[],
+    )
+    providers = agent.context_providers or []
+    assert not any(isinstance(p, BackgroundAgentsProvider) for p in providers)