[BUG] MCP tools with empty parameter schemas display without tool name/args header

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When an MCP tool has an empty parameter schema (no parameters defined), Claude Code displays only the tool output without showing the tool name or arguments header. This makes it unclear which tool was called and breaks the visual consistency of tool call displays.

Tools with at least one parameter (even optional) display correctly with the full header showing tool name and arguments, but tools with completely empty schemas are rendered as bare output.

Please see these screenshots with the buggy truncated output and the correct full output.

<img width="1743" height="560" alt="Image" src="https://github.com/user-attachments/assets/56138e44-06bb-400f-915a-679d77723cfb" />

<img width="791" height="482" alt="Image" src="https://github.com/user-attachments/assets/9f289035-16e0-41f7-a6cc-f2cfd096bdff" />

What Should Happen?

All MCP tool calls should display with a consistent header showing:

• Tool name (e.g., 🔧 sync_documents)
• Arguments (even if empty: {})
• Output

Steps to Reproduce

1. Create an MCP tool with an empty parameter schema:

#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
pub struct TestToolRequest {}

#[tool(name = "test_tool", description = "Test tool")]
async fn test_tool(&self, params: Parameters<TestToolRequest>) -> Result<String, String> {
Ok("test output".to_string())
}

1. Build and connect the MCP server to Claude Code
2. Call the tool: mcp__server__test_tool
3. Observe: Only "test output" is displayed, without tool name header
4. Add a parameter to the schema:

pub struct TestToolRequest {
pub optional_param: Option<String>,
}

1. Rebuild, reload, and call again
2. Observe: Now displays correctly with 🔧 test_tool header

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Claude Code Version

2.0.14

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Zed integrated terminal