feat: add NewToolResultJSON #550
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WalkthroughAdds a new generic helper NewToolResultJSON[T any](data T) in package mcp that marshals the provided data to JSON, returns a CallToolResult with TextContent containing the JSON string and StructuredContent set to the original data, and returns an error on marshal failure. Docs updated to use the new return form. Changes
Estimated code review effort🎯 3 (Moderate) | ⏱️ ~15 minutes Possibly related PRs
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Actionable comments posted: 1
🧹 Nitpick comments (1)
mcp/utils.go (1)
256-258: Tighten doc comment phrasingGrammar and clarity nit: “a JSON content” is slightly awkward; also explicitly calling out “JSON-encoded text content” improves precision.
-// NewToolResultJSON creates a new CallToolResult with a JSON content. -// This function panics if the JSON marshalling fails. +// NewToolResultJSON creates a CallToolResult with JSON-encoded text content, +// and sets structuredContent to the original value.
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📒 Files selected for processing (1)
mcp/utils.go(1 hunks)
🧰 Additional context used
🧠 Learnings (4)
📓 Common learnings
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:107-137
Timestamp: 2025-03-04T06:59:43.882Z
Learning: Tool responses from the MCP server shouldn't contain RawInputSchema, which is why the UnmarshalJSON method for the Tool struct is implemented to handle only the structured InputSchema format.
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:0-0
Timestamp: 2025-03-04T07:00:57.111Z
Learning: The Tool struct in mark3labs/mcp-go handles both InputSchema and RawInputSchema formats. When unmarshaling JSON, it first tries to parse into a structured ToolInputSchema format, and if that fails, it falls back to using the raw schema format, providing symmetry with the MarshalJSON method.
📚 Learning: 2025-03-04T06:59:43.882Z
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:107-137
Timestamp: 2025-03-04T06:59:43.882Z
Learning: Tool responses from the MCP server shouldn't contain RawInputSchema, which is why the UnmarshalJSON method for the Tool struct is implemented to handle only the structured InputSchema format.
Applied to files:
mcp/utils.go
📚 Learning: 2025-03-04T07:00:57.111Z
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:0-0
Timestamp: 2025-03-04T07:00:57.111Z
Learning: The Tool struct in mark3labs/mcp-go handles both InputSchema and RawInputSchema formats. When unmarshaling JSON, it first tries to parse into a structured ToolInputSchema format, and if that fails, it falls back to using the raw schema format, providing symmetry with the MarshalJSON method.
Applied to files:
mcp/utils.go
📚 Learning: 2025-03-04T07:00:57.111Z
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:0-0
Timestamp: 2025-03-04T07:00:57.111Z
Learning: The Tool struct in the mark3labs/mcp-go project should handle both InputSchema and RawInputSchema consistently between MarshalJSON and UnmarshalJSON methods, even though the tools response from MCP server typically doesn't contain rawInputSchema.
Applied to files:
mcp/utils.go
🧬 Code Graph Analysis (1)
mcp/utils.go (3)
mcp/tools.go (1)
CallToolResult(40-51)mcp/types.go (3)
Content(885-887)TextContent(891-898)TextContent(900-900)mcp/consts.go (1)
ContentTypeText(4-4)
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Actionable comments posted: 0
🧹 Nitpick comments (3)
www/docs/pages/servers/advanced.mdx (2)
133-133: Return updated to use NewToolResultJSON — correct and idiomaticThis matches the new signature that returns (*mcp.CallToolResult, error). Returning the helper directly (without appending ", nil") is the right approach.
For consistency across this page, consider updating earlier examples that hand-roll JSON:
- Lines 77–83 (in handleCalculateTyped)
- Lines 288–294 (in the session-aware tool)
to use mcp.NewToolResultJSON(...) as well. It reduces boilerplate and ensures consistent behavior on JSON marshal errors.
133-133: Optional: add a brief note about error propagationSince NewToolResultJSON can fail on marshal, this example now propagates that error up (good). If you want to show explicit error handling in docs, you could expand this example to demonstrate how to wrap or transform the error in a domain-specific way. Optional.
www/docs/pages/transports/http.mdx (1)
497-497: Return updated to use NewToolResultJSON — correctThis aligns with the helper’s new two-value return and simplifies the example.
Elsewhere in this file, responses are built with fmt.Sprintf JSON strings (e.g., lines 109–111, 140–142, 188–190). Consider switching those to NewToolResultJSON with typed structs or maps to avoid brittle string construction and ensure valid JSON.
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Review profile: CHILL
Plan: Pro
💡 Knowledge Base configuration:
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📒 Files selected for processing (3)
mcp/utils.go(1 hunks)www/docs/pages/servers/advanced.mdx(2 hunks)www/docs/pages/transports/http.mdx(2 hunks)
🚧 Files skipped from review as they are similar to previous changes (1)
- mcp/utils.go
🧰 Additional context used
🧠 Learnings (2)
📚 Learning: 2025-06-23T11:10:42.948Z
Learnt from: floatingIce91
PR: mark3labs/mcp-go#401
File: server/server.go:1082-1092
Timestamp: 2025-06-23T11:10:42.948Z
Learning: In Go MCP server, ServerTool.Tool field is only used for tool listing and indexing, not for tool execution or middleware. During handleToolCall, only the Handler field is used, so dynamic tools don't need the Tool field populated.
Applied to files:
www/docs/pages/servers/advanced.mdx
📚 Learning: 2025-03-04T06:59:43.882Z
Learnt from: xinwo
PR: mark3labs/mcp-go#35
File: mcp/tools.go:107-137
Timestamp: 2025-03-04T06:59:43.882Z
Learning: Tool responses from the MCP server shouldn't contain RawInputSchema, which is why the UnmarshalJSON method for the Tool struct is implemented to handle only the structured InputSchema format.
Applied to files:
www/docs/pages/servers/advanced.mdx
🔇 Additional comments (3)
www/docs/pages/servers/advanced.mdx (2)
965-965: Docs formatting change is fineThe “Next Steps” bullet tweak is non-functional and looks good.
133-133: No lingering oldNewToolResultJSON(...), nilusages
A repo-wide search located only two call sites—both correctly using the new signature—and no instances of the old, nilpattern remain:
- www/docs/pages/transports/http.mdx:497
- www/docs/pages/servers/advanced.mdx:133
All clear.
www/docs/pages/transports/http.mdx (1)
696-696: Docs formatting change is fineThe “Next Steps” bullet formatting update is OK.
Description
This PR adds a new util
NewToolResultJSON. It is mentioned in https://mcp-go.dev/servers/advanced but the function doesn't really exist.Type of Change
Checklist
MCP Spec Compliance
Additional Information
Summary by CodeRabbit
New Features
Documentation