ServerCapabilities drops unknown fields such as events #1447

Describe the bug

rmcp 3.9.0 silently drops unknown top-level fields when it deserializes ServerCapabilities. An MCP gateway that reads and forwards capabilities can therefore lose fields advertised by the upstream server. For example, ChatGPT looks for capabilities.events in initialize or server/discover responses to discover MCP Events. MCP Events documentation

To reproduce

Create a Cargo binary and add these dependencies to Cargo.toml:

[dependencies]
rmcp = { version = "=3.9.0", default-features = false, features = ["server"] }
serde_json = "1"

Put this in src/main.rs and run cargo run:

use rmcp::model::ServerCapabilities;
use serde_json::json;

fn main() {
    let input = json!({"tools": {}, "events": {}});
    let capabilities: ServerCapabilities = serde_json::from_value(input.clone()).unwrap();
    let output = serde_json::to_value(capabilities).unwrap();

    println!("input:  {input}");
    println!("output: {output}");
}

Expected behavior

Both tools and events should survive the JSON round trip. The MCP schema permits additional server capability fields. MCP ServerCapabilities schema

Logs

Actual output with the published rmcp 3.9.0 crate:

input:  {"events":{},"tools":{}}
output: {"tools":{}}

Additional context

We carry a local #[serde(flatten)] patch to preserve additional fields. An earlier Rust SDK PR proposed the same capability behavior, and the Python SDK has a parallel report. Rust SDK PR #1170 · Python SDK issue #3640

AI assistance: I used Codex to search for prior reports, inspect the SDK source, write and run the minimal reproduction, and draft this issue.