bug: not support responses api

[CCpro10]

when i request openai responses api using a tool which no Properties
i got this:

Responses API error: 400 {"error":{"message":"code: invalid_function_parameters; message: Invalid schema for function 'GetCurrentTime': In context=(), object schema missing properties.","type":"invalid_request_error","param":"tools[7].parameters","code":"-4003"}}

when properties is nil

// ToolArgumentsSchema represents a JSON Schema for tool arguments.
type ToolArgumentsSchema struct {
Defs map[string]any json:"$defs,omitempty"
Type string json:"type"
Properties map[string]any json:"properties,omitempty"
Required []string json:"required,omitempty"
}

the Properties json result str is empty

I wish is {} , not empty

[coderabbitai]

📝 CodeRabbit Plan Mode

Generate an implementation plan and prompts that you can use with your favorite coding agent.

✅ Successfully generated implementation plan.

💡 Use @coderabbitai <feedback> to replan

Examples

• Example 1
• Example 2

---

🔗 Similar Issues

Possible Duplicates

• When register a tool with no input argument (no need to pass any args) The properties field is missing #224

Related Issues

• bug: ToolInputSchema design flaw #355
• bug: MCP-Go Library Tool Schema Serialization Bug #671

🔗 Related PRs

#116 - fix(tools): add omitempty to properties [merged]
#225 - fix: marshal ToolInputSchema.Properties to {} when len=0 [merged]

👤 Suggested Assignees

• HChengH
• kaliwin
• Pilsertech
• jkoelker
• cryo-zd

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!

[coderabbitai]

Coding Plan

Approach

The plan fixes a JSON serialization issue where OpenAI's Responses API rejects tool schemas with omitted Properties fields. The solution modifies the ToolArgumentsSchema.MarshalJSON() method to always serialize Properties as an empty object {} instead of omitting it when nil, ensuring compatibility with strict schema validators.

Observations

This is an MCP (Model Context Protocol) implementation in Go with tools defined in mcp/tools.go. The ToolArgumentsSchema struct uses custom JSON marshaling methods to handle serialization. Tools are created using factory functions like NewTool() (which already initializes Properties correctly) and can also be created via direct struct literals. The codebase has comprehensive test coverage in mcp/tools_test.go.

Implementation Steps

Phase 1: Fix JSON Serialization for Properties Field

Modify the custom JSON marshaling logic to ensure Properties always appears in JSON output as an empty object, satisfying OpenAI's schema validation requirements.

Task 1: Update ToolArgumentsSchema.MarshalJSON Method

Modify the serialization logic to handle nil Properties

• Update mcp/tools.go in the MarshalJSON() method for ToolArgumentsSchema (around lines 649-672)
• Change the conditional Properties handling from if tis.Properties != nil to always include the properties field
• When Properties is nil, serialize it as an empty object {} instead of omitting it
• Ensure the serialization outputs: m["properties"] = tis.Properties when non-nil, and m["properties"] = map[string]any{} when nil
• Keep existing logic for other fields (Defs, Required, AdditionalProperties) unchanged

Task 2: Add Test Coverage for Nil Properties Serialization

Create test case validating OpenAI-compatible JSON output

• Add test in mcp/tools_test.go that explicitly creates a tool with nil Properties
• Verify the marshaled JSON contains "properties": {} (not omitted)
• Test should create ToolArgumentsSchema{Type: "object", Properties: nil} and assert JSON output includes the properties field
• Consider naming like TestToolArgumentsSchema_MarshalIncludesEmptyPropertiesWhenNil to clearly document the behavior

🤖 Prompt for AI agents

Modify the ToolArgumentsSchema.MarshalJSON() method to ensure the Properties
field always serializes as an empty object instead of being omitted when nil.

In mcp/tools.go:
- Locate the MarshalJSON() method for ToolArgumentsSchema
- Change the conditional Properties handling to always include the properties
field
- When Properties is nil, set it to an empty map: `m["properties"] =
map[string]any{}`
- When Properties is non-nil, use the existing value: `m["properties"] =
tis.Properties`
- Keep all other field serialization logic unchanged

In mcp/tools_test.go:
- Add a new test named
TestToolArgumentsSchema_MarshalIncludesEmptyPropertiesWhenNil
- Create a ToolArgumentsSchema with Type: "object" and Properties: nil
- Marshal to JSON and verify the output contains `"properties": {}`
- Assert the properties field is present and not omitted

Phase 2: Documentation and Validation

Ensure the behavior is documented and examples reflect best practices.

Task 1: Update Documentation Comments

Clarify Properties field serialization behavior

• Update the comment for ToolArgumentsSchema struct in mcp/tools.go to document that Properties always appears in JSON output (even when nil, it serializes as {})
• Add a note explaining this ensures compatibility with strict schema validators like OpenAI's Responses API

Task 2: Verify Existing Examples Follow Best Practices

Ensure example code uses proper initialization patterns

• Review examples in examples/roots_server/main.go and other example directories
• Confirm they initialize Properties as map[string]any{} when creating tools without parameters
• No code changes needed since exploration shows examples already follow this pattern (using empty maps, not nil)
• This task is for validation only to ensure consistency

🤖 Prompt for AI agents

Update documentation and verify examples to reflect the Properties serialization
behavior.

In mcp/tools.go:
- Update the ToolArgumentsSchema struct documentation comment
- Document that Properties always appears in JSON output, serializing as `{}`
when nil
- Add a note explaining this ensures compatibility with strict schema validators
like OpenAI's Responses API

Verification (no code changes):
- Review examples in examples/roots_server/main.go and other example directories
- Confirm they initialize Properties as `map[string]any{}` when creating tools
without parameters
- Ensure consistency with the new serialization behavior

---

🚀 Next Steps

🤖 All AI agent prompts combined

Task: 1

Modify the ToolArgumentsSchema.MarshalJSON() method to ensure the Properties
field always serializes as an empty object instead of being omitted when nil.

In mcp/tools.go:
- Locate the MarshalJSON() method for ToolArgumentsSchema
- Change the conditional Properties handling to always include the properties
field
- When Properties is nil, set it to an empty map: `m["properties"] =
map[string]any{}`
- When Properties is non-nil, use the existing value: `m["properties"] =
tis.Properties`
- Keep all other field serialization logic unchanged

In mcp/tools_test.go:
- Add a new test named
TestToolArgumentsSchema_MarshalIncludesEmptyPropertiesWhenNil
- Create a ToolArgumentsSchema with Type: "object" and Properties: nil
- Marshal to JSON and verify the output contains `"properties": {}`
- Assert the properties field is present and not omitted
===============================================================================

Task: 2

Update documentation and verify examples to reflect the Properties serialization
behavior.

In mcp/tools.go:
- Update the ToolArgumentsSchema struct documentation comment
- Document that Properties always appears in JSON output, serializing as `{}`
when nil
- Add a note explaining this ensures compatibility with strict schema validators
like OpenAI's Responses API

Verification (no code changes):
- Review examples in examples/roots_server/main.go and other example directories
- Confirm they initialize Properties as `map[string]any{}` when creating tools
without parameters
- Ensure consistency with the new serialization behavior

💡 Iterate on the plan with: @coderabbitai <feedback>

Example Feedback
- @coderabbitai You can skip phase 3. Add a simple unit test case for phase 2.
- @coderabbitai For assumption 1 go ahead with option 3 and replan.

---

💬 Have feedback or questions? Drop into our discord!