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fix: disable Unicode escaping in JSON output and correct Meta type assignment #526

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@longkeyy longkeyy commented Aug 3, 2025 •
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Summary

This PR fixes two issues in the mcp-go library:

  1. Unicode escaping in JSON output: Prevents Unicode characters from being escaped to \uXXXX format in stdio transport
  2. Type assignment error: Fixes incorrect type assignment in CallToolResult unmarshaling

Changes

1. Fix Unicode Escaping (server/stdio.go)

  • Replace json.Marshal() with json.NewEncoder() in writeResponse() function
  • Use encoder.SetEscapeHTML(false) to disable HTML/Unicode escaping
  • Maintains JSON-RPC protocol compliance while improving readability

Before:

{"user_input": "\\u4f60\\u597d\\uff0c\\u6211\\u53eb\\u6155\\u5bb9\\u79bb"}

After:
{"user_input": "你好，我叫慕容离"}

2. Fix Meta Type Assignment (mcp/tools.go)

- Correct assignment of map[string]any to *Meta field in CallToolResult.UnmarshalJSON()
- Create proper Meta struct with AdditionalFields from unmarshaled data
- Resolves Go compiler type errors

Impact

- ✅ Non-breaking change: Maintains full backward compatibility
- ✅ Improved readability: Unicode characters display naturally instead of escaped format
- ✅ Fixes build errors: Resolves type assignment compilation issues
- ✅ Protocol compliant: Maintains JSON-RPC 2.0 specification adherence

Testing

- Builds successfully without errors
- JSON-RPC protocol functionality unchanged
- Unicode characters render correctly in stdio output
- Existing tests continue to pass

Motivation

Unicode escaping reduces readability for international users and applications working with non-ASCII text. This change makes MCP server outputs more human-readable while maintaining full
protocol compatibility.

The Meta type fix resolves compilation errors that prevent building applications using the library.

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of JSON responses to prevent unwanted HTML and Unicode escaping in output.
* Enhanced processing of metadata fields for more consistent data structure after JSON decoding.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

longkeyy added 2 commits August 4, 2025 04:00
@longkeyy
fix: disable Unicode escaping in JSON output
c501bc3 
- Modify writeResponse in server/stdio.go to use json.NewEncoder with SetEscapeHTML(false)
- This prevents Unicode characters from being escaped to \uXXXX format
- Maintains compatibility while improving readability for non-ASCII characters
@longkeyy
fix: correct Meta type assignment in CallToolResult unmarshaling
1951206 
- Fix type error where map[string]any was being assigned to *Meta
- Create proper Meta struct with AdditionalFields from unmarshaled data
- This resolves build errors in the Go compiler
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coderabbitai bot commented Aug 3, 2025 •
edited
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Walkthrough

The changes update JSON handling in two areas: the UnmarshalJSON method in mcp/tools.go now wraps the _meta field in a Meta struct, and the writeResponse method in server/stdio.go now uses a JSON encoder with HTML escaping disabled, altering the output format.

Changes

Cohort / File(s) Change Summary
Meta Field Unmarshaling
mcp/tools.go		 Modified UnmarshalJSON to wrap the _meta field in a Meta struct, changing how metadata is stored post-unmarshal.
JSON Encoding Output
server/stdio.go		 Changed response encoding to use a JSON encoder with HTML escaping disabled, affecting Unicode and HTML output.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

Note

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Learn more here, or try it out under "Finishing Touches" below.

📜 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 57740b6 and 1951206.

📒 Files selected for processing (2)
  • mcp/tools.go (1 hunks)
  • server/stdio.go (2 hunks)
🧰 Additional context used
🧠 Learnings (7)
📓 Common learnings 
Learnt from: ezynda3
PR: mark3labs/mcp-go#461
File: server/sampling.go:22-26
Timestamp: 2025-06-30T07:13:17.052Z
Learning: In the mark3labs/mcp-go project, the MCPServer.capabilities field is a struct value (serverCapabilities), not a pointer, so it cannot be nil and doesn't require nil checking. Only pointer fields within the capabilities struct should be checked for nil.

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.

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.

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.

Learnt from: davidleitw
PR: mark3labs/mcp-go#451
File: mcp/tools.go:1192-1217
Timestamp: 2025-06-26T09:38:18.629Z
Learning: In mcp-go project, the maintainer prefers keeping builder pattern APIs simple without excessive validation for edge cases. The WithOutput* functions are designed to assume correct usage rather than defensive programming, following the principle of API simplicity over comprehensive validation.

Learnt from: octo
PR: mark3labs/mcp-go#149
File: mcptest/mcptest.go:0-0
Timestamp: 2025-04-21T21:26:32.945Z
Learning: In the mcptest package, prefer returning errors from helper functions rather than calling t.Fatalf() directly, giving callers flexibility in how to handle errors.

Learnt from: lariel-fernandes
PR: mark3labs/mcp-go#428
File: www/docs/pages/servers/prompts.mdx:218-234
Timestamp: 2025-06-20T20:39:51.870Z
Learning: In the mcp-go library, the GetPromptParams.Arguments field is of type map[string]string, not map[string]interface{}, so direct string access without type assertions is safe and correct.
📚 Learning: tool responses from the mcp server shouldn't contain rawinputschema, which is why the unmarshaljson ... 
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/tools.go
📚 Learning: the tool struct in the mark3labs/mcp-go project should handle both inputschema and rawinputschema co... 
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/tools.go
📚 Learning: in go mcp server, servertool.tool field is only used for tool listing and indexing, not for tool exe... 
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:

  • mcp/tools.go
📚 Learning: the tool struct in mark3labs/mcp-go handles both inputschema and rawinputschema formats. when unmars... 
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/tools.go
📚 Learning: in the mark3labs/mcp-go project, the mcpserver.capabilities field is a struct value (servercapabilit... 
Learnt from: ezynda3
PR: mark3labs/mcp-go#461
File: server/sampling.go:22-26
Timestamp: 2025-06-30T07:13:17.052Z
Learning: In the mark3labs/mcp-go project, the MCPServer.capabilities field is a struct value (serverCapabilities), not a pointer, so it cannot be nil and doesn't require nil checking. Only pointer fields within the capabilities struct should be checked for nil.

Applied to files:

  • mcp/tools.go
📚 Learning: in the mcp-go library, the getpromptparams.arguments field is of type map[string]string, not map[str... 
Learnt from: lariel-fernandes
PR: mark3labs/mcp-go#428
File: www/docs/pages/servers/prompts.mdx:218-234
Timestamp: 2025-06-20T20:39:51.870Z
Learning: In the mcp-go library, the GetPromptParams.Arguments field is of type map[string]string, not map[string]interface{}, so direct string access without type assertions is safe and correct.

Applied to files:

  • mcp/tools.go
🧬 Code Graph Analysis (1)
mcp/tools.go (1)
mcp/types.go (1)
  • Meta (120-132)
🔇 Additional comments (3)
mcp/tools.go (1)

507-509: Correct type assignment fix for Meta field.

The change properly wraps the raw JSON map in a Meta struct with AdditionalFields populated, fixing the type mismatch where a map[string]any was being assigned to a *Meta field. This maintains data integrity while ensuring correct struct typing.

server/stdio.go (2)

5-5: Added bytes import for buffer handling.

The addition supports the new JSON encoding approach using a buffer to control output formatting.

461-472: Improved JSON output readability by disabling Unicode escaping.

The change from json.Marshal() to json.NewEncoder() with SetEscapeHTML(false) prevents Unicode characters from being escaped to \uXXXX format. This significantly improves readability for international users and applications handling non-ASCII text while maintaining JSON-RPC 2.0 protocol compliance.

The buffer approach also provides better control over newline handling, ensuring consistent output formatting.

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