feat(mcp): add NewToolResultErrorf

[icholy]

Description

Add a NewToolResultErrof utility function

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly

Summary by CodeRabbit

• New Features
  • Added support for generating formatted error messages in tool results, improving error reporting clarity for users.

[coderabbitai]

Walkthrough

A new function, NewToolResultErrorf, was introduced in mcp/utils.go to generate a CallToolResult representing an error with a formatted message. This function formats error messages using a format string and arguments, and sets the IsError flag in the result.

Changes

File(s)	Change Summary
mcp/utils.go	Added NewToolResultErrorf function to create error results with formatted messages.

Possibly related PRs

• feat: add NewToolResultError #87: Adds a simple error message constructor for CallToolResult, which is extended by this PR with a formatted variant.
• introduce NewToolResultErrorWithErr and update docs #140: Introduces NewToolResultErrorFromErr for wrapping existing errors, related to error result construction in this PR.
• docs: use helper functions and return error as result #322: Utilizes error result constructors for consistent error handling, complementing the new formatted error result function added here.

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (1.64.8)

Error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2
Failed executing command with error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 4b71176 and 3541a25.

📒 Files selected for processing (1)

• mcp/utils.go (1 hunks)

🧰 Additional context used

🧬 Code Graph Analysis (1)

mcp/utils.go (2)

mcp/tools.go (1)

• CallToolResult (36-43)

mcp/types.go (3)

• Content (827-829)
• TextContent (833-838)
• TextContent (840-840)

🔇 Additional comments (1)

mcp/utils.go (1)

330-343: LGTM! Well-implemented utility function.

The NewToolResultErrorf function is correctly implemented and follows the established patterns in the codebase:

• Uses proper fmt.Sprintf formatting with variadic parameters
• Constructs TextContent and CallToolResult structures correctly
• Sets IsError: true as required for error results
• Documentation follows the existing style and conventions
• Function signature and naming are consistent with other NewToolResult* functions

This provides a valuable convenience function for creating formatted error messages in tool results.

✨ Finishing Touches

• 📝 Generate Docstrings

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