Standardize provenance for app-initiated server tool calls

[krubenok]

Problem / motivation

For a server tool whose _meta.ui.visibility includes both "model" and "app", the server can receive tools/call requests initiated by either the model/agent or an embedded MCP App. Today those requests are indistinguishable at the server boundary.

A standardized, per-invocation provenance signal would help with observability, UX analytics, debugging, rate limiting, and policy decisions. It should be treated as provenance asserted by the MCP client/host, not as an authentication or authorization primitive.

Current spec behavior

The stable MCP Apps specification allows a View to send the core tools/call request to its Host, which may proxy that request to the MCP server.

Existing mechanisms answer related but different questions:

• _meta.ui.visibility controls whether a tool may be called by the model and/or App; it does not identify who initiated a particular call to a dual-visible tool.
• MCP 2026-07-28 per-request io.modelcontextprotocol/clientInfo identifies the MCP client/Host; it does not distinguish whether that Host's model/agent or embedded App initiated the request.
• Apps ui/initialize appInfo identifies the View to the Host, but it is not propagated to the MCP server on each tools/call.
• A custom App-supplied _meta marker is non-interoperable, may be dropped by Hosts or gateways, can collide with other conventions, and should not be considered authoritative.

As a result, proxying preserves the call parameters but carries no normative per-call origin signal.

Concrete example

Both paths collapse to the same server request:

Agent -> Host -> Server
App   -> Host -> Server

{
  "method": "tools/call",
  "params": {
    "name": "refresh_dashboard",
    "arguments": { "accountId": "123" }
  }
}

The server cannot tell whether the Host emitted this request because the agent selected the tool or because a user clicked a control in the embedded App.

Proposal: host-attested metadata

Define an official Apps-owned request _meta entry, for example:

{
  "_meta": {
    "io.modelcontextprotocol/ui": {
      "initiator": "app"
    }
  }
}

Normative behavior could be:

• The Host MUST add or overwrite this value when proxying a View-originated tools/call.
• The Host MUST NOT set initiator: "app" for model/agent-originated calls.
• Downstream proxies and gateways SHOULD preserve the standardized entry.
• Absence means unknown, not implicitly "model".
• The View should not be responsible for self-declaring this marker.

The smallest interoperable shape may only need an App-origin boolean/enum. Optional appInfo and/or resourceUri could be considered separately if their interoperability value outweighs privacy and cardinality concerns.

Alternatives considered

• clientInfo / appInfo: clientInfo identifies the Host, while appInfo identifies the View only within the Apps initialization boundary. Neither currently expresses per-invocation origin to the server.
• Separate connections: A Host could use a distinct MCP connection for App traffic, but that complicates session/state sharing and still lacks a portable semantic signal across gateways.
• OpenTelemetry baggage: Useful inside a deployment, but it is not an interoperable MCP protocol contract and may not survive all transports or proxies.
• Custom vendor _meta: Feasible for closed ecosystems, but merely relocates the custom semantics. Hosts may drop it, namespaces may diverge, and App-authored values are not authoritative.

Security and trust considerations

“Host-attested” means asserted by the MCP client/Host; it is not cryptographically verified. A server must not use this field for authorization unless it separately trusts and authenticates the Host making the assertion.

Hosts should overwrite any spoofed App-provided value when proxying the request. Provenance should remain low-cardinality. If identity-bearing fields such as appInfo or resourceUri are included, the specification should give explicit privacy, disclosure, and logging guidance.

Open questions

• What namespace and shape best fit current Apps and core _meta conventions?
• Is an App-only boolean sufficient, or should the field be a broader enum such as app, model, user, host, or unknown?
• Should appInfo and/or resourceUri be included, optional, or intentionally excluded?
• What preservation requirements should apply through gateways and nested proxies?
• How should provenance relate to _meta.ui.visibility enforcement without conflating the two?
• Should the design align only with modern per-request metadata conventions, or also define legacy compatibility behavior?

The goal is to standardize the smallest interoperable shape; the options above are intended to invite maintainers to choose the least disruptive protocol contract.

Related specifications and issues

• Stable MCP Apps specification (2026-01-26)
• Current MCP request _meta documentation
• #492: Provide a consistent way for MCP Apps to know which tool invoked them — related opposite direction (Host/tool identity to App), not a duplicate