Summary

This PR adds the SEP entry for SEP-2290: Content Negotiation Extension, an Extensions Track proposal that introduces transparent content negotiation for MCP.

Abstract

Not all MCP clients are the same, clients differ in what they need from the server and how they can consume it.

Clients also have different context window sizes or latency requirements, which may necessitate varying levels of detail or verbosity in responses. For example, a client with a small context window may prefer concise
summaries, while one with a larger window can handle more verbose explanations.

The content needed by a human consumer (e.g., an end-user interface) may prioritize readability and conciseness, while an agent (e.g., a downstream tool or another model) may require structured data, detailed reasoning traces, or specific formats.

Especially locally running agents have limited compute resources and thus prefer more compact responses that minimize unnecessary information. E.g., an agent running an mobile device (e.g. on Gemini Nano) may want to avoid large reasoning traces that consume bandwidth and processing power.

Real-World Use Cases

Some examples:

• Journey Service: Structured data for agents, readable narratives for humans
• Geospatial/Mapping Service: GeoJSON coordinates for agents, readable descriptions for humans
• Multi-Agent Orchestration: Different specialists have different capabilities

Key Design Principles

1. Session-scoped: Negotiation happens once at initialization, not per-request
2. Capability-driven: Feature tags mirror client's actual MCP capabilities
3. RFC 2295-inspired: Borrows transparent negotiation concepts from HTTP
4. Fully backward compatible: Zero breaking changes, optional feature
5. Secure: Features are for content shape only, never for auth

Impact on MCP ecosystem

This extension also helps to significantly reduce bandwidth and latency as well as significantly reduce compute requirements. All data not sent by server is bandwidth and compute saved on both sides. These also reduces electrical power consumption and thus have positive environmental impact.

Full Specification

The complete specification is maintained at:
https://github.com/schlpbch/ext-content-negotiation (v0.9.4)

Includes: feature tag registry, protocol examples, RFC 2295 mapping, security analysis, TypeScript reference implementation, and comparison with SEP-2053 Server Variants.

Reference Implementation

A FastMCP (Python) reference implementation is available at:
https://github.com/schlpbch/ext-content-negotiation/blob/master/FASTMCP_REFERENCE.md

Demonstrates ContentNegotiationMiddleware, a Features dataclass with typed convenience properties, and a get_features(ctx) helper for tool handlers — all built on FastMCP v3's middleware and session-state APIs.

Current Status

Draft specification complete with:

• Detailed specification (14 standard feature tags, predicate syntax)
• 3 initialize scenarios (agent, human, legacy)
• 8+ tool response examples
• RFC 2295 mapping table
• Security analysis (5 risks + mitigations)
• Full backward compatibility analysis
• Implementation guidelines: TypeScript / MCP SDK, Python / FastMCP, Java / Spring AI 2.0, Rust / rmcp, Go / go-sdk, C# / .NET, Kotlin / kotlin-sdk, Swift / swift-sdk

Checklist

• SEP file added at seps/2290-content-negotiation-extension.md
• Status: Draft
• Full specification linked
• Authors identified
• Reference implementation available (FastMCP/Python)