Built-in Tools and Extensions

The Runtime currently registers nine built-in tools. File and command tools require a concrete workspace, while web tools remain available in directory-free global tasks.

Built-in tools

Tool	Purpose	Mutating	Requires workspace
`read_file`	Reads lines with optional `offset` and `limit`	no	yes
`write_file`	Creates or overwrites a complete file	yes	yes
`edit_file`	Replaces a unique `old_string` with `new_string`	yes	yes
`list_dir`	Lists a directory tree to a requested depth	no	yes
`glob`	Matches file paths and returns up to 200 recent entries	no	yes
`grep`	Searches file content with regex and optional Glob filtering	no	yes
`bash`	Runs a shell command in the workspace	yes	yes
`web_search`	Searches Google, Bing, or Baidu	no	no
`web_fetch`	Extracts text from an HTTP or HTTPS page	no	no

Descriptions are loaded from prompts/tool/<name>.txt, while arguments are validated through JSON Schema. Output budgets adapt to the model context window and remain capped by agent.maxToolOutputChars.

`edit_file` arguments

The current tool uses text matching rather than line-number patches:

{
  "path": "src/UserService.cs",
  "old_string": "public bool IsActive => false;",
  "new_string": "public bool IsActive => status.IsActive;",
  "replace_all": false
}

old_string must match and is required to be unique by default. Add context when it matches multiple places or set replace_all: true explicitly. A successful edit publishes a unified diff event for Desktop and CLI renderers.

Modes and scopes

Global tasks filter tools whose RequiresWorkspace value is true.
plan and readonly filter tools whose Mutating value is true.
edit and auto allow mutations, which can trigger build verification.
A workspace can disable tools by name through disabledTools.

Custom C# tools

Implement ITool and register the instance with IToolRegistry:

public sealed class ProjectSummaryTool : ITool
{
    public string Name => "project_summary";
    public string Description => "Summarize the current project";
    public JsonObject ParameterSchema => ToolSchema.Object(
        ("depth", ToolSchema.Integer("Maximum directory depth"), false));
    public bool Mutating => false;
    public bool RequiresWorkspace => true;
    public string StatusLabel => "Inspecting project";

    public Task<ToolResult> ExecuteAsync(
        JsonObject arguments,
        ToolContext context,
        CancellationToken ct)
    {
        var summary = $"Workspace: {context.Workspace.Root}";
        return Task.FromResult(ToolResult.Ok(summary));
    }
}

using var registration = toolRegistry.Register(new ProjectSummaryTool());

The returned IDisposable unregisters the tool. MCP reload uses the same mechanism to clean up stale registrations.

Built-in Tools and Extensions

Built-in tools

edit_file arguments

Modes and scopes

Custom C# tools

`edit_file` arguments