feat(protocol): allow additional fields in meta #293
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WalkthroughThe changes introduce a new exported Changes
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Actionable comments posted: 0
🧹 Nitpick comments (1)
mcp/types.go (1)
130-139: Consider defensive initialization in UnmarshalJSONThe implementation correctly extracts the progress token and stores remaining fields, but
AdditionalFieldsisn't initialized if it's nil.func (m *Meta) UnmarshalJSON(data []byte) error { raw := make(map[string]any) if err := json.Unmarshal(data, &raw); err != nil { return err } m.ProgressToken = raw["progressToken"] delete(raw, "progressToken") + if m.AdditionalFields == nil { + m.AdditionalFields = make(map[string]any) + } m.AdditionalFields = raw return nil }
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📒 Files selected for processing (3)
mcp/tools.go(1 hunks)mcp/types.go(2 hunks)mcp/types_test.go(1 hunks)
🧰 Additional context used
🧬 Code Graph Analysis (2)
mcp/tools.go (1)
mcp/types.go (1)
Meta(106-118)
mcp/types_test.go (1)
mcp/types.go (2)
Meta(106-118)ProgressToken(99-99)
🔇 Additional comments (5)
mcp/types.go (3)
104-118: Well-designed Meta struct implementationThe implementation of the
Metastruct is clean and well-documented. It supports both the standardizedProgressTokenfield and additional arbitrary fields, enabling extensibility while maintaining backward compatibility.
120-128: Effective JSON marshalling implementationThe marshalling logic properly handles both the protocol-defined
progressTokenfield and any additional fields. Using themaps.Copyfunction is efficient for merging the maps.
144-144: Good update to Request.Params.Meta typeChanging from an anonymous struct to the new
Metatype maintains API compatibility while enabling additional field support.mcp/tools.go (1)
49-49: Consistent Meta type usage across packageThe
CallToolRequest.Params.Metafield is properly updated to use the newMetatype, ensuring consistent handling of metadata across the package.mcp/types_test.go (1)
11-70: Comprehensive test cases for Meta marshallingThe test cases thoroughly cover various scenarios including empty structs, string tokens, and additional fields. The tests properly validate both marshalling and unmarshalling behaviors, including handling of numeric types in JSON.
Description
Replaces anonymous struct type with real type for
Metaon request params with custom JSON marshalling logic to allow fields in addition to progress token to be marshalled.Technically, it seems all types in a JSON-RPC protocol like MCP should be open to other fields and MCP's python/JS SDKs do allow this in most types. In practice,
_metais being used in several instrumentation frameworks for propagating trace contextmodelcontextprotocol/modelcontextprotocol#246
Defining a type with custom marshalling allows doing similar with mcp-go. As the structure of the existing anonymous struct is preserved, I believe this is a backwards compatible change.
Type of Change
Checklist
MCP Spec Compliance
Actually I couldn't find text in the specification itself but convention in the SDKs along with guidance in the attached issue. I think it is in principal spec compliance.
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/types.py#L25
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/types.py#L51
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/types.ts#L30 (
.passthrough())Additional Information
Summary by CodeRabbit
New Features
Bug Fixes
Tests