Tool Executors

Tool executors control whether tools from a single assistant turn run concurrently or sequentially. Both SDKs default to concurrent execution.

Concurrent Executor

Concurrent execution runs all tool calls from a single turn in parallel. This is the default in both SDKs — you get it without any extra configuration.

Python

from strands import Agent
from strands.tools.executors import ConcurrentToolExecutor

agent = Agent(
    tool_executor=ConcurrentToolExecutor(),
    tools=[weather_tool, time_tool]
)
# or simply Agent(tools=[weather_tool, time_tool])

agent("What is the weather and time in New York?")

TypeScript

import { Agent } from '@strands-agents/sdk'

const agent = new Agent({
  tools: [weatherTool, timeTool],
  toolExecutor: 'concurrent',
})
// or simply: new Agent({ tools: [weatherTool, timeTool] })

await agent.invoke('What is the weather and time in New York?')

Assuming the model returns weather_tool and time_tool use requests, the concurrent executor runs both at the same time. End-to-end latency scales with the slowest tool rather than their sum.

Sequential Behavior

On certain prompts, the model may decide to return one tool use request at a time. Under these circumstances, the tools will execute sequentially. Concurrency is only achieved if the model returns multiple tool use requests in a single response. Certain models however offer additional abilities to coerce a desired behavior. For example, Anthropic exposes an explicit parallel tool use setting (docs).

Sequential Executor

Use sequential execution when tool order matters — for example, when a later tool depends on a side effect of an earlier one:

Python

from strands import Agent
from strands.tools.executors import SequentialToolExecutor

agent = Agent(
    tool_executor=SequentialToolExecutor(),
    tools=[screenshot_tool, email_tool]
)

agent("Please take a screenshot and then email the screenshot to my friend")

TypeScript

import { Agent } from '@strands-agents/sdk'

const agent = new Agent({
  tools: [screenshotTool, emailTool],
  toolExecutor: 'sequential',
})

await agent.invoke('Take a screenshot and email it to my friend')

Assuming the model returns screenshot_tool and email_tool use requests, the sequential executor runs both in the order given.

Event Ordering

Both modes preserve per-tool event order. In concurrent mode, events from different tools may interleave across that per-tool sequence.

Python

Per-tool event order: BeforeToolCallEvent → ToolStreamEvent* → AfterToolCallEvent → ToolResultEvent.

TypeScript

Per-tool event order: BeforeToolCallEvent → ToolStreamUpdateEvent* → AfterToolCallEvent → ToolResultEvent.

Cancellation

Cancellation works identically in both modes. Call agent.cancel() to request cooperative cancellation. In TypeScript, this flips agent.cancelSignal.

Python

• Pre-launch cancel: set BeforeToolCallEvent.cancel_tool on a per-tool hook to produce an error result for that tool.
• Mid-flight cancel in sequential mode short-circuits not-yet-started tools. In concurrent mode, all tools have already launched, so each in-flight tool must cooperatively check for cancellation to stop early.

TypeScript

• Pre-launch cancel: set BeforeToolsEvent.cancel on the batch-level hook, or call agent.cancel() before tools start, to produce error results for every tool in the batch.
• Mid-flight cancel in sequential mode short-circuits not-yet-started tools. In concurrent mode, all tools have already launched, so each in-flight tool must cooperatively observe agent.cancelSignal to stop early.

Custom Executors

Custom tool executors are not currently supported but are planned for a future release. You can track progress on this feature at GitHub Issue #762.