feat(sep): add SEP-3221 Secure Parameters Extension - #3221

This pull request introduces a new Extensions Track Specification Enhancement Proposal: Secure Parameters Extension (io.modelcontextprotocol/secure-params) under seps/3221-secure-parameters.md.

Motivation and Context

In centralized MCP servers with broad ambient credentials (such as fleet-wide database access), exposing infrastructure coordinates (project_id, instance, tenant_id) in inputSchema forces them into model prompts. An attacker using prompt injection can trick the LLM into redirecting tool calls to unauthorized instances (the "Confused Deputy" problem).

This SEP defines a standard protocol mechanism to isolate application/infrastructure parameters from model context by introducing:

1. tools/list: A sibling secureInputSchema field alongside standard inputSchema.
2. tools/call: A top-level secureArguments field for host-injected values, separate from model-generated arguments.
3. Stateless Capability Negotiation: Per-request _meta["io.modelcontextprotocol/clientCapabilities"].extensions (2026-07-28+) and server/discover.
4. Deterministic Precedence: Strict server-side discarding of colliding keys in arguments.
5. Standard Error Codes: Standard -32021 (MISSING_REQUIRED_CLIENT_CAPABILITY) and -32602 (INVALID_PARAMS).

How Has This Been Tested?

• Prototyped and validated in Google Cloud MCP Toolbox (mcp-toolbox Go server and mcp-toolbox-sdk-python).
• Traceability mapping created for automated testing in modelcontextprotocol/conformance (src/seps/sep-3221.yaml).
• Wire format tested against stateless 2026-07-28 JSON-RPC schemas.

Breaking Changes

None. This is an opt-in, non-breaking extension under the Extensions Track. Un-negotiated clients and servers continue operating under standard MCP semantics.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• Track: Extensions Track
• Status: Draft
• Sponsor: @kvg
• Co-Author: @duwenxin
• Interest Group Alignment: Security Interest Group
• Associated Conformance PR: feat(conformance): add SEP-3221 Secure Parameters traceability mapping conformance#452