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Tool Use & Parallel Execution

Tools are how an agent acts on the world. CubePi turns each AgentTool into a JSON Schema for the model, validates arguments with Pydantic, runs the work, and feeds the result back as a ToolResultMessage. By default tools run in parallel when the model calls more than one in a single turn.

Anatomy of a tool​

from pydantic import BaseModel, Field
from cubepi import AgentTool, AgentToolResult, TextContent


class SearchParams(BaseModel):
    query: str = Field(..., description="The natural-language query")
    limit: int = Field(10, ge=1, le=100)


async def search(tool_call_id, params: SearchParams, *, signal=None, on_update=None):
    results = await my_search_backend(params.query, params.limit)
    return AgentToolResult(
        content=[TextContent(text="\n".join(results))],
        details={"raw_results": results},   # passes through to ToolResultMessage.details
    )


search_tool = AgentTool(
    name="search",
    description="Search the internal knowledge base.",
    parameters=SearchParams,
    execute=search,
)

The description is shown to the model verbatim — write it for the model, not for humans. Pydantic Field(description=...) propagates into the JSON Schema and helps the model understand each parameter.

Parallel by default​

When the model emits multiple tool calls in one assistant message, CubePi schedules them on asyncio.create_task() and gathers them. That's almost always what you want.

agent = Agent(
    provider=provider,
    model=model,
    tools=[search_tool, fetch_url_tool, summarise_tool],
)

The event stream emits tool_execution_start for all of them up front, interleaved tool_execution_update events as each tool reports progress, then tool_execution_end per tool in completion order.

Forcing sequential execution​

There are two ways to opt out of parallel mode:

  1. Per agent — Agent(tool_execution="sequential"). All tool batches run one-by-one in the order the model emitted them.

  2. Per tool — set execution_mode="sequential" on the AgentTool. If any tool in the current batch is sequential, the whole batch falls back to sequential.

    write_db_tool = AgentTool(
        name="write_db",
        description="Persist a record.",
        parameters=WriteDbParams,
        execute=write_db,
        execution_mode="sequential",   # opt out of parallelism for safety
    )

The built-in ask_user HITL tool (see HITL guide) sets execution_mode="sequential" — it pauses the agent for human input, so the tool batch runs one-by-one.

Use sequential mode when tools mutate shared state (a DB, a counter) and you want a deterministic order.

Streaming tool progress​

Long-running tools can stream partial updates that show up as tool_execution_update events on the agent stream:

async def slow_search(tool_call_id, params, *, signal=None, on_update=None):
    for i, page in enumerate(await fetch_pages(params.query)):
        if signal and signal.is_set():
            break
        if on_update:
            on_update({"progress": i, "total": len(pages), "url": page.url})
        await process(page)
    return AgentToolResult(content=[TextContent(text="done")])

partial_result in the event is whatever object you handed to on_update. Use a small dict; it doesn't end up in the model's context.

Cancelling in-flight tools​

The signal argument is the same asyncio.Event that agent.abort() sets. Check it in any loop:

async def long_running(tool_call_id, params, *, signal=None, on_update=None):
    for chunk in big_dataset:
        if signal and signal.is_set():
            return AgentToolResult(content=[TextContent(text="cancelled")])
        await process_chunk(chunk)

If your work is one big await, wrap it in asyncio.wait_for(..., timeout=â€Ļ) or use the abort callbacks of the underlying library.

Returning errors​

Two ways:

  1. Raise an exception. CubePi catches it, turns it into an AgentToolResult with is_error=True and the exception string as TextContent.

  2. Return is_error=True explicitly. Useful when you want a structured error body:

    return AgentToolResult(
        content=[TextContent(text="Rate limit exceeded; try again in 60s")],
        is_error=True,
    )

Either way, the model receives a tool result that's clearly marked as an error and usually adapts (retries with different args, asks the user, etc.).

Ending the turn from a tool: terminate​

A tool can declare "after this, stop looping — don't ask the model again." Set terminate=True:

async def submit_final_answer(tool_call_id, params, *, signal=None, on_update=None):
    save_answer(params.answer)
    return AgentToolResult(
        content=[TextContent(text="submitted")],
        terminate=True,
    )

CubePi only terminates if every tool result in the current batch is terminate=True. The agent loop emits turn_end, then agent_end, and exits.

Common pitfalls​

  • Forgetting the keyword-only args — execute(tool_call_id, params) will work in dev but crash when the framework passes signal=. Keep *, signal=None, on_update=None in the signature.
  • Heavy details payloads — details is preserved through the agent event but is not shown to the model. Don't pack massive blobs there unless you have a downstream consumer.
  • Pydantic strictness surprises — Field(..., min_length=1) lets the model see the constraint via JSON Schema; constraints help, but remember the model still sometimes sends bad JSON. CubePi turns the ValidationError into a tool error result; you don't need to wrap validation yourself.
  • tools=[] and the model still asks for a tool — Usually means the prompt mentions one. Either remove the suggestion from the system prompt or pass the tool.

See also​