Map the Gateway Before Adding Credentials

A connector gateway accepts a caller's request, applies a connector policy, and asks an upstream system to do work. If those responsibilities blur, a successful HTTP response can hide the wrong caller, wrong destination, or leaked credential. Today you will trace the smallest complete path through the fictional Atlas Connector Lab and produce SCG-R01, a boundary map backed by one allowed call and one upstream denial.

The recurring lab has two upstream interfaces: a Calendar REST API and a companion Model Context Protocol (MCP) server. MCP is a protocol through which a client can discover and call tools. The gateway may translate either request shape, but the external resource still owns its data and final authorization decision.

An anonymous fixture call is useful because it exposes routing before credentials complicate the picture. The gateway control layer resolves connector calendar-lab; the runtime sends the request; the external resource returns the result. A known public operation returns 200, while a protected operation returns 401. The gateway records both without claiming that it caused or fixed the denial.

GET /connectors/calendar-lab/operations/public-status HTTP/1.1
X-Trace-Id: scg-d01-normal

{
  "connectorId": "calendar-lab",
  "operation": "public-status",
  "upstreamStatus": 200,
  "credentialSource": "none"
}

The same route with operation list-private-events reaches the same external resource and receives 401. That response is evidence that routing worked and authority is still absent, not evidence that the caller should receive a token.

An evidence plane turns each boundary decision into a redacted record. Its job is to preserve actor, resource, scope, precondition, expected result, observed result, environment, timestamp, and immutable run ID. It must not store authorization headers, cookies, request bodies, or future secret values.

{
  "runId": "scg-d01-denied",
  "actor": "learner-client",
  "resource": "calendar-lab:list-private-events",
  "precondition": "route resolved; no credential configured",
  "expected": "upstream_unauthorized",
  "observed": "upstream_unauthorized",
  "secretFields": "redacted"
}

For the positive control, call public-status again after the denial and require another 200. Recovery means correcting a mistyped connector ID and replaying only that disposable request. Cleanup deletes the two local trace receipts; it does not mutate the upstream fixture.

The worked example supplies every fact except one: which actor rejected the protected call? Choose client entry, connector control, connector runtime, or external resource, and cite the observed status boundary. The deterministic answer is external resource because the trace proves the runtime executed the request and recorded the upstream 401.

Treating any 401 as a gateway routing failure is the target misconception. Replay the public control through the same route: its 200 separates healthy routing from missing upstream authority. Decline questions that ask you to design authorization, choose scopes, or add a secret; those are multiple unknowns and belong to later days.

SCG-R01 contains the five stable actors, the request and execute edges, paired 200/401 evidence, and scoped cleanup. Day 02 consumes this map to place OpenAPI validation and connector-draft storage at the control boundary.