Why does "Error executing tool" happen? Confusing

[ckgithub2019]

When using manager agent and hierarchical mode, the app often popped up:

Error executing tool. coworker mentioned not found, it must be one of the following options:

• manager
  ...
  Error executing tool. coworker mentioned not found, it must be one of the following options:
• manager

[theCyberTech]

How do you have your crew setup?

[KlausHeywinkel]

I implemented a manager example taken from your docs: https://docs.crewai.com/how-to/custom-manager-agent

It always leads to the error output:

Error executing tool. coworker mentioned not found, it must be one of the following options:

• researcher
• senior writer

This is the complete code:

# Warning control
import warnings
warnings.filterwarnings('ignore')

from crewai import Agent, Task, Crew, Process
import os


openai_api_key = "<hidden for you>"
os.environ["OPENAI_API_KEY"] = openai_api_key

import os
from crewai import Agent, Task, Crew, Process

researcher = Agent(
    llm="gpt-4o-mini",
    role="researcher",
    goal="Conduct thorough research and analysis on AI and AI agents",
    backstory="You're an expert researcher, specialized in technology, software engineering, AI, and startups. You work as a freelancer and are currently researching for a new client.",
    allow_delegation=False,
    verbose=True,
)

writer = Agent(
    llm="gpt-4o-mini",
    role="senior writer",
    goal="Create compelling content about AI and AI agents",
    backstory="You're a senior writer, specialized in technology, software engineering, AI, and startups. You work as a freelancer and are currently writing content for a new client.",
    allow_delegation=False,
    verbose=True,
)

task = Task(
    description="Generate a list of 2 interesting ideas for an article, then write one captivating paragraph for each idea that showcases the potential of a full article on this topic. Return the list of ideas with their paragraphs and your notes.",
    expected_output="5 bullet points, each with a paragraph and accompanying notes.",
    output_file="test.md",
)

manager = Agent(
    llm="gpt-4o-mini",
    role="Project Manager",
    goal="Efficiently manage the crew and ensure high-quality task completion",
    backstory="You're an experienced project manager, skilled in overseeing complex projects and guiding teams to success. Your role is to coordinate the efforts of the crew members, ensuring that each task is completed on time and to the highest standard.",
    allow_delegation=True,
    verbose=True,
)

crew = Crew(
    agents=[researcher, writer],
    tasks=[task],
    manager_agent=manager,
    process=Process.hierarchical,
)

result = crew.kickoff()