feat: dynamic MCP proxy config with backend overrides (Host/Path/Auth)

[pbonuboinasnow]

Description

Adds dynamic MCP route and backend configuration without relying solely on the static filter-config.yaml, enabling flexible multi-tenant MCP deployments.

Use Case: Multi-Tenant MCP Deployments

This feature supports multi-tenant deployments with thousands of MCP routes, where each tenant or deployment has different MCP backend configurations and rules. Instead of managing static routes for every tenant:

• A trusted Envoy ext_proc filter (or ext_authz) dynamically injects route and backend configuration per request
• Backend configurations include host, path, and auth overrides for each tenant
• Headers are injected by the Envoy data plane, not external clients, ensuring secure multi-tenant isolation

Technical Details

1. Dynamic Route Config via Header

The MCP proxy checks for the x-ai-eg-mcp-dynamic-route-config header on every incoming request. If present, its value is interpreted as a base64-encoded JSON representation of an MCPRoute object, which takes precedence over the statically loaded route. Falls back to static config if the header is missing or malformed. This header is expected to be set by a trusted upstream filter (e.g. an ext_proc filter in the Envoy filter chain) before reaching the MCP proxy, not by external clients directly.

2. Backend Overrides (Host / Path / Auth)

Three new optional fields on MCPBackend:

• host — Sets :authority/Host header for Envoy DFP DNS + TLS
• backendPath — Rewrites URL path
• auth — Injects Authorization header

All are optional. Overrides are applied in all 3 backend request paths: invokeJSONRPCRequest, sendRequestPerBackend, and session.Close().

Files changed:

• internal/internalapi/internalapi.go — MCPDynamicRouteConfigHeader constant
• internal/filterapi/mcpconfig.go — Host, BackendPath, Auth fields on MCPBackend
• internal/mcpproxy/config.go — parseDynamicRouteConfig() and buildConfigRoute()
• internal/mcpproxy/mcpproxy.go — routeConfig(), applyBackendOverrides()
• internal/mcpproxy/handlers.go — replaced static lookups with routeConfig()
• internal/mcpproxy/session.go — applied overrides in sendRequestPerBackend and Close()
• Unit tests for applyBackendOverrides and dynamic route config parsing
• Add examples/dynamic-mcp-config/ with architecture documentation and configuration examples

Related Issues/PRs (if applicable)

Special notes for reviewers (if applicable)

This enables supporting 1000s of MCP routes in multi-tenant deployments where each tenant has dynamic configuration rules.

[pbonuboinasnow]

@mathetake @nacx what's your opinion on this? we need a way to support 1000s (yes) of MCP routes as we are a multi tenant deployment with dynamic rules.