Fix: Use JSON serialization instead of Debug format in tool returns - #63
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…ol returns Fixes #62 Changes: - Modified utils.rs generate_error_handling() to use serde_json::to_string() instead of format!("{:?}") for tool return values - Now populates structured_content field per MCP 2025-06-18 specification - Graceful fallback to Debug format for types without Serialize trait - Added comprehensive test suite (8 tests) in json_serialization_test.rs Breaking Change: - Tool return types should implement Serialize trait for optimal JSON output - Types without Serialize will fallback to Debug format Before: "SearchResult { items: [...] }" After: {"items": [...]} Version bumped to 0.13.0 due to breaking change requirement.
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Fixes #62
Problem
The
#[mcp_tools]macro was using Rust's Debug trait ({:?}) instead of JSON serialization when formatting tool results, causing tool responses to contain Rust debug format instead of proper JSON.Before:
{ "content": [{ "type": "text", "text": "SearchResult { items: [SearchItem { id: 1, title: \"Bug\" }] }" }] }After:
{ "content": [{ "type": "text", "text": "{\"items\":[{\"id\":1,\"title\":\"Bug\"}]}" }], "structured_content": {"items": [{"id": 1, "title": "Bug"}]} }Changes
Core Fix (mcp-macros/src/utils.rs)
generate_error_handling()to useserde_json::to_string()instead offormat!("{:?}")structured_contentfield per MCP 2025-06-18 specificationSerializeTesting (mcp-macros/tests/json_serialization_test.rs)
Added comprehensive test suite with 8 tests covering:
structured_contentis populatedVersion & Documentation
Breaking Change
Tool return types should implement
Serializetrait for optimal JSON output:Types without
Serializewill gracefully fall back to Debug format.Test Results
Compliance