microsoft/semantic-kernel · error · AgentInitializationException

Plugin ' ' not found in kernel.

Error message

Plugin '{plugin_name}' not found in kernel.

What it means

Thrown by _validate_tools after splitting a tool id into plugin/function when kernel.plugins.get(plugin_name) returns nothing. The plugin namespace named in the tool id was never registered on the kernel, so the reference cannot resolve.

Solutions

1. Register the plugin under the exact name used in the tool id: `kernel.add_plugin(WebPlugin(), plugin_name='WebPlugin')`.
2. Correct the tool id prefix in the YAML to match the registered plugin name.
3. List `kernel.plugins.get_directory()` (or iterate kernel.plugins) to confirm available plugin names before loading.

Example fix

# before
kernel.add_plugin(WebPlugin(), plugin_name='Web')
# yaml: id: WebPlugin.search

# after
kernel.add_plugin(WebPlugin(), plugin_name='WebPlugin')
# yaml: id: WebPlugin.search

for t in spec.get('tools', []):
    if t.get('type') != 'function' or not t.get('id') or '.' not in t['id']:
        continue
    plugin_name = t['id'].split('.', 1)[0]
    assert kernel.plugins.get(plugin_name), f'Plugin {plugin_name!r} not registered on kernel'

def plugin_is_registered(tool: dict, kernel) -> bool:
    tid = tool.get('id', '')
    if '.' not in tid:
        return True
    return kernel.plugins.get(tid.split('.', 1)[0]) is not None

from semantic_kernel.exceptions.agent_exceptions import AgentInitializationException
try:
    agent = await AgentRegistry.create_from_yaml(yaml_str, kernel=kernel)
except AgentInitializationException as e:
    if 'not found in kernel' in str(e):
        name = str(e).split("'")[1]
        kernel.add_plugin(MyPlugin(), plugin_name=name)
        agent = await AgentRegistry.create_from_yaml(yaml_str, kernel=kernel)
    else:
        raise

When it happens

Trigger: Tool id `WebPlugin.search` but the kernel only has a plugin registered under a different name (e.g. 'Web') or no plugin registered at all.

Common situations: Plugin registered with a different plugin_name than the YAML expects; plugin registration code path skipped in the current config; typo in the plugin prefix.

Related errors

• Function ' ' not found in plugin ' '.
• Kernel instance is required for tool resolution.
• Missing or malformed 'vector_store_ids' in
• Tool id ' ' must be in format PluginName.FunctionName
• Tool spec must include a 'type' field.

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/fe9d5fabcc913e66. Report an issue: GitHub.

Appendix: source

Thrown at python/semantic_kernel/agents/agent.py:1063

        This validates the declared tools in the YAML spec, and only checks whether those references resolve
        properly in the current kernel.
        """
        if not kernel:
            raise AgentInitializationException("Kernel instance is required for tool resolution.")

        for tool in tools_list:
            tool_id = tool.get("id")
            if not tool_id or tool.get("type") != "function":
                continue

            if "." not in tool_id:
                raise AgentInitializationException(f"Tool id '{tool_id}' must be in format PluginName.FunctionName")

            plugin_name, function_name = tool_id.split(".", 1)

            plugin = kernel.plugins.get(plugin_name)
            if not plugin:
                raise AgentInitializationException(f"Plugin '{plugin_name}' not found in kernel.")

            if function_name not in plugin.functions:
                raise AgentInitializationException(f"Function '{function_name}' not found in plugin '{plugin_name}'.")


# endregion

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