bug: GPT agent chats reject zero-argument MCP tools with properties: null #27252

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• I have searched the existing issues.

Current Behavior

Coder Agent chats using an OpenAI/GPT-backed model fail before tool execution when an MCP server exposes a zero-argument tool.

The tool used to reproduce this is Tidewave's get_ecto_schemas. Its MCP schema is valid and includes an explicit empty object:

{
  "type": "object",
  "properties": {}
}

In a GPT-backed Coder chat, attempting to use that tool fails with HTTP 400. In a Sonnet-backed chat connected to the same workspace and MCP endpoint, the tool executes successfully and returns the expected schema list. MCP tools with non-empty parameter schemas also work on both providers.

This appears to be the object-valued counterpart of #23538: an empty or nil properties map reaches the OpenAI function schema as "properties": null rather than "properties": {}. The current MCP wrapper passes tool.InputSchema.Properties through as fantasy.ToolInfo.Parameters; that path likely needs the same defensive normalization previously added for required.

Relevant Log Output

OpenAI returned an unexpected error.
Invalid schema for function 'tidewave__get_ecto_schemas': None is not of type 'object'.
HTTP 400

Expected Behavior

Parameter-free MCP tools should be accepted by OpenAI-backed chats. Coder should preserve or normalize their function parameter schema to an explicit empty object, for example:

{
  "type": "object",
  "properties": {},
  "required": []
}

At the provider boundary, object schemas with missing, nil, or null properties should likewise be normalized to {}.

Steps to Reproduce

1. Configure a Streamable HTTP MCP server in Coder that exposes a zero-argument tool with inputSchema: {"type":"object","properties":{}}.
2. Create a Coder Agent chat using an OpenAI/GPT-backed model.
3. Ask the agent to invoke the zero-argument tool.
4. Observe the OpenAI HTTP 400 before the MCP tool executes.
5. Switch to a Sonnet-backed model and invoke the same tool; it succeeds.

Environment

• Host OS: self-hosted Linux deployment
• Coder server: v2.35.2+5c2838a
• Coder client: v2.33.1+3e34ba7
• MCP transport: Streamable HTTP
• Affected provider: OpenAI/GPT
• Unaffected provider: Anthropic/Sonnet
• Reproduces consistently: yes

Additional Context

• The MCP endpoint was independently inspected and returns properties: {}, not null.
• The zero-argument tool was independently invoked successfully with {}.
• No repository-side proxy or adapter transforms the schema before Coder discovers it.
• Similar compatibility failures have been reported in other MCP-to-OpenAI bridges: MCP tool with empty inputSchema causes provider error: 'object' is not of type 'object', 'boolean' promptfoo/promptfoo#5960 and OpenAI Agents SDK calls OpenAI API with invalid schema when MCP tool definitions have no properties field openai/openai-agents-python#449.
• Add CI testing for MCP server with AI models #19828 tracks broader MCP/model compatibility testing; a regression test specifically covering a zero-argument MCP tool against the OpenAI payload would catch this case.