fix: marshal ToolInputSchema.Properties to {} when len=0

[cryo-zd]

Fix: #224
Related: #116
 This PR customize function MarshalJSON() for ToolInputSchema which ensures that we could marshal Properties to {} rather than null when a Tool's input argument is empty

The "omitempty" option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any array, slice, map, or string of length zero.
go package encoding/json.Marshal

How about deleting the omitempty field ?
  From #116, I think omitempty is necessary so that clients such as claude-desktop could work.

Summary by CodeRabbit

• Bug Fixes
  • Improved JSON serialization so that empty or nil fields in tool input schemas are now omitted, resulting in cleaner and more accurate JSON output.

[coderabbitai]

"""

Walkthrough

A custom MarshalJSON method was added to the ToolInputSchema struct in mcp/tools.go. This method ensures that during JSON serialization, the type field is always present, the properties field is included only if it is non-nil, and the required field is included only if it contains elements. This change modifies how empty or nil fields are handled during marshaling, specifically omitting them from the output JSON.

Changes

File(s)	Change Summary
mcp/tools.go	Added a custom MarshalJSON method to ToolInputSchema to control JSON serialization behavior.

Assessment against linked issues

Objective	Addressed	Explanation
Ensure ToolInputSchema omits properties field when nil and does not marshal it as null (#224)	✅

Possibly related PRs

• Tools can be built with predefined Input Schema #30: Adds MarshalJSON for Tool struct handling serialization of InputSchema and RawInputSchema, related to JSON marshaling in the same package.
• add UnmarshalJSON to Tool #35: Modifies Tool struct’s InputSchema JSON tag and adds UnmarshalJSON for Tool, complementing serialization and deserialization logic for tool-related types.
  """

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

mcp/tools.go (1)

116-131: Implementation meets the PR objective, but has a few minor issues.

The added MarshalJSON method properly ensures that Properties is serialized as {} rather than null when empty, which solves the stated issue in the PR objectives. The implementation appropriately includes custom logic for all fields, correctly maintaining the behavior of the omitempty tag for the Required field.

However, there are a couple of minor issues to address:

1. There's a typo in the comment on line 121: "euqals" should be "equals"
2. The indentation is inconsistent - lines 118-120 use spaces while the rest of the method uses tabs

// MarshalJSON implements the json.Marshaler interface for ToolInputSchema.
func (tis ToolInputSchema) MarshalJSON() ([]byte, error) {
-    m := make(map[string]interface{})
-    m["type"] = tis.Type
-
-	// Marshal Properties to '{}' rather than `nil` when its length euqals zero
+	m := make(map[string]interface{})
+	m["type"] = tis.Type
+
+	// Marshal Properties to '{}' rather than `nil` when its length equals zero
	if tis.Properties != nil {
		m["properties"] = tis.Properties
	}

	if len(tis.Required) > 0 {
		m["required"] = tis.Required
	}

	return json.Marshal(m)
}

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