Routing

mcpr parses every JSON-RPC 2.0 message and routes it based on the protocol, not the URL path.

How routing works

Each proxy instance fronts one upstream MCP app.

Request type	Behavior
JSON-RPC (tools/call, resources/read, etc.)	Classified, forwarded to the upstream MCP server, response is recorded
Everything else	Forwarded as-is — mcpr never blocks traffic

mcp = "http://localhost:9000"

To proxy multiple MCP servers, write one mcpr.toml per upstream and launch each with mcpr proxy run <path>.

MCP method support

mcpr classifies 25+ MCP methods for routing, observability, and CSP rewriting:

Category	Methods	Proxy Behavior
Lifecycle	initialize, notifications/initialized, ping	Extracts client info, tracks session state
Tools	tools/list, tools/call, notifications/tools/list_changed	CSP rewriting, per-tool metrics, schema capture
Resources	resources/list, resources/templates/list, resources/read, resources/subscribe, resources/unsubscribe	CSP rewriting, resource URI logging
Prompts	prompts/list, prompts/get	Prompt name extraction
Utility	logging/setLevel, completion/complete, notifications/cancelled, notifications/progress	Forwarded, classified for logging

Unknown methods are forwarded as-is — mcpr never blocks traffic. See Supported MCP Methods for the full reference.

Per-tool care

Each tool gets its own metrics automatically: call count, error rate, p95/max latency, bytes in/out. No configuration needed — mcpr extracts the tool name from every tools/call request.

Inspect locally with sqlite3 ~/.mcpr/store.db, or stream to mcpr Cloud for per-tool tables, slow-call detection, and latency percentiles.

CSP & widgets

MCP Apps (ChatGPT Apps, Claude connectors) render widgets in sandboxed iframes with strict CSP. ChatGPT reads openai/widgetCSP, Claude reads ui.csp, and each interprets domain lists differently. mcpr handles this automatically:

• Rewrites CSP domain arrays in tools/list, tools/call, resources/list, resources/templates/list, and resources/read responses
• Replaces localhost/upstream URLs with the proxy domain
• Emits the CSP shape each platform expects
• Deep-scans nested structures for CSP arrays

CSP has three independent directives — connectDomains (fetch / WebSocket), resourceDomains (scripts, styles, images), and frameDomains (iframes) — each with its own mode: extend (merge with upstream) or replace (use only declared domains). Widget entries layer glob-matched overrides on top of the global policy.

[csp.connectDomains]
domains = ["api.example.com"]
mode    = "extend"

[csp.resourceDomains]
domains = ["cdn.example.com"]
mode    = "extend"

[csp.frameDomains]
domains = []
mode    = "replace"

[[csp.widget]]
match              = "ui://widget/payment*"
connectDomains     = ["api.stripe.com"]
connectDomainsMode = "extend"

Schema capture

mcpr passively captures your server’s tools, resources, and prompts by intercepting discovery responses. No extra requests, no added latency. Captured schemas land in the local SQLite store and stream to mcpr Cloud, where added/modified/removed entries and unused tools surface in the Tools tab.

See Events for the full event and capture system.

What’s coming

• Multi-server routing — route to multiple upstream MCP backends
• Multiple proxies in one process — single binary serving several upstreams ([[proxy]] config array)