microsoft/semantic-kernel · error · RuntimeError

Runtime is already started

Error message

Runtime is already started

What it means

InProcessRuntime.start() sets _run_context. If _run_context is already set (runtime was started and not stopped), calling start() again raises RuntimeError. The runtime must be stopped before it can be restarted.

Source

Thrown at python/semantic_kernel/agents/runtime/in_process/in_process_runtime.py:655

                                    sender=sender,
                                    receiver=recipient,
                                    kind=MessageKind.RESPOND,
                                )
                            )
                            future.set_exception(MessageDroppedException())
                            return
                        message_envelope.message = temp_message
                task = asyncio.create_task(self._process_response(message_envelope))
                self._background_tasks.add(task)
                task.add_done_callback(self._background_tasks.discard)

        # Yield control to the message loop to allow other tasks to run
        await asyncio.sleep(0)

    def start(self) -> None:
        """Start the runtime message processing loop. This runs in a background task."""
        if self._run_context is not None:
            raise RuntimeError("Runtime is already started")
        self._run_context = RunContext(self)

    async def close(self) -> None:
        """Calls :meth:`stop` if applicable and the :meth:`Agent.close` method on all instantiated agents."""
        # stop the runtime if it hasn't been stopped yet
        if self._run_context is not None:
            await self.stop()
        # close all the agents that have been instantiated
        for agent_id in self._instantiated_agents:
            agent = await self._get_agent(agent_id)
            await agent.close()

    async def stop(self) -> None:
        """Immediately stop the runtime message processing loop.

        The currently processing message will be completed, but all others following it will be discarded.
        """
        if self._run_context is None:

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Call await runtime.stop() (or stop_when_idle) before calling start() again.
2. Use runtime.close() for full teardown, which internally calls stop().
3. Create a fresh InProcessRuntime instance for each independent run rather than reusing one.

Example fix

# before
runtime.start()
# ... use runtime ...
runtime.start()  # raises 'Runtime is already started'

# after
runtime.start()
# ... use runtime ...
await runtime.stop_when_idle()
runtime.start()  # ok

# Check runtime state before starting
if runtime._run_context is not None:
    raise RuntimeError('Runtime already started — stop it first')
runtime.start()

null

try:
    runtime.start()
except RuntimeError:
    await runtime.stop()
    runtime.start()

When it happens

Trigger: Calling runtime.start() twice without an intervening runtime.stop() (or stop_when_idle/stop_when). Common in test teardown/re-setup cycles or when reusing a runtime instance across runs.

Common situations: A test fixture starts the runtime in setUp but forgets to stop it in tearDown, then a second test calls start() on the same instance. Or application code that restarts the runtime on reconnect/retry without stopping first.

Related errors

• Runtime is not started
• Agent with type {type} already exists.
• Agent factory must take 0 or 2 arguments.
• Agent with name {agentId.Type} not found.
• Thread has been deleted; call `create()` to recreate it.

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/8fde69f52ca7734e. Report an issue: GitHub.