revisão e complementação da documentação do projeto

RI/docs/interfaces/mcp-gateway.en-us.md

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+# Interface: MCP Gateway
+
+The Gateway's MCP surface — what any integrating MCP Client calls. Implementation:
+[`src/emcp_bus/gateway/server.py`](../../src/emcp_bus/gateway/server.py),
+[`gateway/canonical_request.py`](../../src/emcp_bus/gateway/canonical_request.py),
+[`gateway/cache.py`](../../src/emcp_bus/gateway/cache.py). See [`AS-BUILT.md`](../AS-BUILT.md#pep--mcp-gateway-ep-05)
+for the component's role in the architecture.
+
+## Authentication
+
+OAuth2/OIDC bearer token in the `Authorization` header. Validated against the JWKS of the configured
+IdP (`OIDC_JWKS_URL`/`OIDC_ISSUER`/`OIDC_AUDIENCE`) — RS256 signature, `exp`, `iss`, `aud`. The
+trusted identity is the token's `azp` claim (or, in its absence, `client_id`) — never a
+`clientInfo` field or any part of the request body.
+
+## Two deployment modes
+
+| Mode | Condition | Behavior |
+|---|---|---|
+| With OIDC configured | `OIDC_JWKS_URL` + `OIDC_ISSUER` set | The `mcp` SDK's auth middleware rejects any connection without a valid bearer token before the `initialize` handshake. |
+| Without OIDC configured | Env vars absent (default) | No auth middleware installed; every handler runs with `get_access_token() is None` and denies by default. |
+
+Both modes are deny-by-default; neither is an "incomplete mode" waiting on the other — see
+`tests/e2e/test_governed_gateway.py`.
+
+## `tools/list`
+
+Without a valid token: returns `tools: []`. With a valid token: returns the intersection of the
+entitled backends' catalog (`CapabilityRouter.distinct_backends_for`) with the client's
+`MaximumEntitlement` (`EntitlementManager.resolve(...).allowed_tools`) — never any backend's full
+catalog. Each call re-evaluates entitlement from scratch; there is no server-side catalog cache
+between clients.
+
+**Cache hint (SEP-2549):** the response carries `ttl_ms`/`cache_scope`, always
+`cache_scope="private"` — never `"public"`, since the result depends on the caller's authorization
+(`gateway/cache.py:tools_list_cache_hint`). **Verified limitation**: on the classic session
+transport this RI uses everywhere, the SDK drops `ttl_ms` during serialization (the field is only
+delivered on the wire by the protocol's 2026-07-28 *stateless per-request* mode) — the client
+always sees the default (`ttl_ms=0`), which is the most conservative fallback possible, never an
+unsafe one. The actual security property (never leaking a catalog between clients) does not
+depend on this — it comes from the Gateway recomputing entitlement on every call.
+
+## `tools/call`
+
+Reauthorized completely independently of `tools/list` — the prior list is never consulted to
+decide whether a call is allowed. Flow:
+
+1. No valid token → deny (see error table below).
+2. Unknown client (`ClientRegistration` not found) → deny.
+3. PDP consulted (`PDPClient.evaluate`, see [`pdp-opa.md`](pdp-opa.md)) with `client_profile`,
+   `tool`, `arguments`.
+4. `DENY` → deny with the PDP's `reason_code`.
+5. `REQUIRE_APPROVAL` → consults `ApprovalService.check_and_consume`; without a granted approval,
+   creates an `ApprovalRequest` (see [`approval-workflow.md`](approval-workflow.md)) and denies.
+6. `ALLOW` → resolves the backend via `CapabilityRouter.route` (see
+   [`registry-capability-manifest.md`](registry-capability-manifest.md)) and executes the call
+   against the domain server, using the outbound credential (never the client's inbound token — see
+   [`downstream-identity.md`](downstream-identity.md)).
+
+### Denial format
+
+Every denial is a structured `CallToolResult` (`is_error: true`), never a protocol error:
+
+```json
+{"is_error": true, "content": [{"type": "text", "text": "Access denied: <REASON_CODE>"}]}
+```
+
+| `REASON_CODE` | When |
+|---|---|
+| `UNAUTHENTICATED` | No valid token. |
+| `UNKNOWN_CLIENT` | Valid token, but `client_id` without a `ClientRegistration`. |
+| `PDP_UNAVAILABLE` | OPA unreachable — fail-closed, never an implicit ALLOW. |
+| `NOT_IN_ENTITLEMENT` | The tool is not in `ClientProfile.allowed_tools` (PDP decision). |
+| `TRANSACTION_POLICY_DENIED` | Entitlement OK, but the arguments violate `transaction.rego` (limit, region, classification). |
+| `REQUIRE_APPROVAL:<reason>:<approval_id>` | Above the `approval_threshold` — use `approval_id` to track/grant out of band. |
+| `UNROUTABLE_CAPABILITY` | No `ACTIVE` manifest for the tool, or the backend is not resolvable. |
+| `UNSUPPORTED_BACKEND_TYPE:<type>` | The manifest points to a `backend.type` with no live dispatch in this RI (e.g. `rest`, see `AS-BUILT.md`). |
+
+## Header/body consistency (EP-05-T08)
+
+If the request carries the `Mcp-Method`/`Mcp-Name` headers (SEP-2243), they must agree with the
+`method`/resource name in the JSON-RPC body, or the request is rejected with HTTP 400
+(`{"error": "header_body_mismatch", "detail": "..."}`) before reaching authentication. Missing
+headers are not an error — it's a consistency check, not a mandate to adopt the SEP.
+
+## Correlation
+
+Every decision carries, and propagates as OTel span attributes and in every audit event:
+`decision_id`, `policy_decision_id`, `entitlement_version`, `policy_version`, `mcp_request_id`.
+See [`audit-events.md`](audit-events.md).