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fix: ToolInputSchema inherits MarshalJSON from ToolArgumentsSchema #696

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fix: ToolInputSchema inherits MarshalJSON from ToolArgumentsSchema #696

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4 changes: 2 additions & 2 deletions mcp/tools.go
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Original file line number Diff line number Diff line change
Expand Up @@ -642,8 +642,8 @@ type ToolArgumentsSchema struct {
AdditionalProperties any `json:"additionalProperties,omitempty"`
}

type ToolInputSchema ToolArgumentsSchema // For retro-compatibility
type ToolOutputSchema ToolArgumentsSchema
type ToolInputSchema = ToolArgumentsSchema // For retro-compatibility (true alias, inherits methods)
type ToolOutputSchema = ToolArgumentsSchema
Comment on lines +645 to +646
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@coderabbitai coderabbitai bot Jan 17, 2026 •
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⚠️ Potential issue | 🟡 Minor

🧩 Analysis chain

🏁 Script executed:

# Check the current state of mcp/tools.go around lines 645-646
sed -n '640,650p' mcp/tools.go

Repository: mark3labs/mcp-go

Length of output: 583

🏁 Script executed:

# Search for context around ToolInputSchema and ToolOutputSchema
rg -B 3 -A 3 'type ToolInputSchema|type ToolOutputSchema' mcp/tools.go

Repository: mark3labs/mcp-go

Length of output: 424

🏁 Script executed:

# Check if there are GoDoc comments anywhere above these lines
sed -n '630,650p' mcp/tools.go

Repository: mark3labs/mcp-go

Length of output: 852

🏁 Script executed:

# Check how ToolInputSchema and ToolOutputSchema are used in the codebase
rg 'ToolInputSchema|ToolOutputSchema' mcp/tools.go -B 2 -A 2

Repository: mark3labs/mcp-go

Length of output: 1108

🏁 Script executed:

# Check if there are any other type alias examples in the codebase with GoDoc comments
rg 'type \w+ = \w+' --type go -B 2 -A 0

Repository: mark3labs/mcp-go

Length of output: 1111

Add GoDoc comments for the exported aliases.

ToolInputSchema and ToolOutputSchema are exported type aliases but lack GoDoc comments. Other type aliases in the codebase (e.g., OAuthConfig, Token in client/oauth.go) follow the pattern of having GoDoc comments starting with the identifier name. Align these aliases with the project's documentation standard.

✍️ Proposed fix 
+// ToolInputSchema is an alias of ToolArgumentsSchema for backward compatibility.
+// It inherits ToolArgumentsSchema's JSON marshaling behavior.
 type ToolInputSchema = ToolArgumentsSchema // For retro-compatibility (true alias, inherits methods)
+
+// ToolOutputSchema is an alias of ToolArgumentsSchema for backward compatibility.
+// It inherits ToolArgumentsSchema's JSON marshaling behavior.
 type ToolOutputSchema = ToolArgumentsSchema
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
type ToolInputSchema = ToolArgumentsSchema // For retro-compatibility (true alias, inherits methods)
type ToolOutputSchema = ToolArgumentsSchema
// ToolInputSchema is an alias of ToolArgumentsSchema for backward compatibility.
// It inherits ToolArgumentsSchema's JSON marshaling behavior.
type ToolInputSchema = ToolArgumentsSchema // For retro-compatibility (true alias, inherits methods)
// ToolOutputSchema is an alias of ToolArgumentsSchema for backward compatibility.
// It inherits ToolArgumentsSchema's JSON marshaling behavior.
type ToolOutputSchema = ToolArgumentsSchema
🤖 Prompt for AI Agents 
In `@mcp/tools.go` around lines 645 - 646, Add GoDoc comments for the exported
type aliases: prepend a comment starting with "ToolInputSchema" above the
declaration "type ToolInputSchema = ToolArgumentsSchema" and a comment starting
with "ToolOutputSchema" above "type ToolOutputSchema = ToolArgumentsSchema" that
briefly explains they are retro-compatible aliases of ToolArgumentsSchema and
inherit its methods; follow the project's style used for other aliases (e.g.,
OAuthConfig, Token) so the comments begin with the identifier name and
succinctly describe purpose/compatibility.

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@ezynda3 ezynda3 Jan 23, 2026

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@proyuen can you add these please?

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Seems like the humans are having a chat. I'll hop back into my burrow for now. If you need me again, just tag @coderabbitai in a new comment, and I'll come hopping out!

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@ezynda3 ezynda3 Feb 15, 2026

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@proyuen ping


// MarshalJSON implements the json.Marshaler interface for ToolInputSchema.
func (tis ToolArgumentsSchema) MarshalJSON() ([]byte, error) {
Expand Down
34 changes: 34 additions & 0 deletions mcp/tools_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1765,3 +1765,37 @@ func TestWithSchemaAdditionalProperties(t *testing.T) {
assert.NoError(t, err)
assert.Contains(t, string(data), `"additionalProperties":false`)
}

// TestToolInputSchema_MarshalJSON_EmptyProperties verifies that ToolInputSchema
// inherits the MarshalJSON method from ToolArgumentsSchema, ensuring that
// the 'properties' field is included in JSON output even when empty.
// This fixes issue #694.
func TestToolInputSchema_MarshalJSON_EmptyProperties(t *testing.T) {
schema := ToolInputSchema{
Type: "object",
Properties: map[string]any{}, // empty but not nil
}

b, err := json.Marshal(schema)
assert.NoError(t, err)

result := string(b)
// Ensure 'properties' field is present in JSON output
assert.Contains(t, result, `"properties"`, "Expected properties field in JSON output")
// Verify the full expected output
assert.Contains(t, result, `"properties":{}`, "Expected empty properties object in JSON output")
}

// TestToolOutputSchema_MarshalJSON_EmptyProperties verifies the same for ToolOutputSchema.
func TestToolOutputSchema_MarshalJSON_EmptyProperties(t *testing.T) {
schema := ToolOutputSchema{
Type: "object",
Properties: map[string]any{}, // empty but not nil
}

b, err := json.Marshal(schema)
assert.NoError(t, err)

result := string(b)
assert.Contains(t, result, `"properties":{}`, "Expected empty properties object in JSON output")
}