Prism MCP Rust SDK

prism-mcp-rs is an async Rust SDK for building Model Context Protocol (MCP) clients and servers. Version 3 implements the stateless MCP 2026-07-28 lifecycle natively while retaining production-grade MCP 2025-11-25 interoperability.

Protocol compatibility

ProtocolMode::Auto is the default. Clients try 2026 discovery first and downgrade only when the peer explicitly returns JSON-RPC Method not found; protocol, authentication, transport, and malformed-response failures never trigger a downgrade. Servers accept both revisions by default. Use ModernOnly or LegacyOnly to pin a deployment. See Protocol Versions for the behavior matrix and known limits.

Modern clients automatically complete bounded input_required flows through ClientRequestHandler. Servers can originate those flows with MultiRoundToolHandler, which receives opaque continuation state and request-scoped client capabilities. HTTP and STDIO implement opt-in subscriptions/listen; HTTP uses request-scoped SSE and STDIO uses cancellation to close the long-lived request.

The official io.modelcontextprotocol/tasks extension is opt-in. Servers register durable work with add_task_tool or add_task_tool_with_fallback, or use add_composed_task_tool to transition from MRTR input into durable execution. Clients enable the extension, then use automatic task completion or the typed get_task, update_task, and cancel_task APIs. Task handles are caller-bound, TTL-limited, cancellable, and support multi-round input.

Capability status

The default request policy is backward-compatible and permits requests. Production services should authenticate at the transport boundary and install a deny-by-default policy. Native plugins execute in process and are not a security boundary.

Install

[dependencies]
prism-mcp-rs = "3"
tokio = { version = "1", features = ["full"] }
async-trait = "0.1"
serde_json = "1"

Enable only what the application uses:

prism-mcp-rs = {
    version = "3",
    features = ["http", "tls", "auth", "otel"]
}

full enables every optional feature. The minimum supported Rust version is 1.85.

Minimal STDIO server

use prism_mcp_rs::prelude::*;
use std::collections::HashMap;

struct Echo;

#[async_trait]
impl ToolHandler for Echo {
    async fn call(&self, arguments: HashMap<String, Value>) -> McpResult<ToolResult> {
        let message = arguments
            .get("message")
            .and_then(Value::as_str)
            .unwrap_or_default();

        Ok(ToolResult {
            content: vec![ContentBlock::text(message)],
            is_error: Some(false),
            structured_content: None,
            meta: None,
        })
    }
}

#[tokio::main]
async fn main() -> McpResult<()> {
    let server = McpServer::create("echo-server", "1.0.0");
    server
        .add_tool(
            "echo",
            Some("Echo a message"),
            json!({
                "type": "object",
                "properties": { "message": { "type": "string" } },
                "required": ["message"]
            }),
            Echo,
        )
        .await?;

    server
        .run_with_transport(StdioServerTransport::new())
        .await
}

STDIO servers must reserve stdout for MCP frames; write diagnostics through tracing or stderr.

Production controls

The SDK provides shared request context, deny-by-default RBAC, per-principal/per-method token-bucket limiting, TLS 1.3 mTLS, W3C trace propagation with OTLP export, and conservative endpoint failover. These controls are opt-in because identity verification, certificates, quotas, and availability policy belong to the host application.

See Production Controls for integration examples and explicit trust boundaries.

Verification

cargo fmt --all -- --check
cargo clippy --all-features --all-targets -- -D warnings
cargo test --all-features
cargo test --doc --all-features
cargo bench --features bench,plugin,http --bench all_benchmarks

CI also runs the pinned upstream MCP conformance referee against both adapters. The maintained 2026 server-stateless scenario passes 30/30 checks; selected client metadata, tool, header, and JSON Schema scenarios pass 29/29 checks with no baselines.

Benchmark results depend on hardware, enabled features, handlers, payloads, and network conditions. The checked-in benchmark report is a development snapshot, not an SLA.

Documentation

• Documentation index
• Getting started
• AI tool integration
• Architecture
• Deployment
• Production controls
• Troubleshooting
• Examples
• API reference

Contributing and security

Read CONTRIBUTING.md before opening a change. Report vulnerabilities privately according to SECURITY.md; do not use a public issue for a suspected vulnerability.

License

MIT. See LICENSE.