diff --git a/CHANGELOG.md b/CHANGELOG.md
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+++ b/CHANGELOG.md
@@ -143,6 +143,7 @@
 
 ### 📝 Maintenance
 
+- **docs (architecture):** add `docs/architecture/ROUTER_BACKENDS.md` — an ADR pinning down how the routing engines (`ts` native, `bifrost`, `cliproxy`, `9router`, VibeProxy-compatible) relate to each other along two orthogonal axes (lifecycle: in-process / supervised / external vs. relay selection backend), answering the architecture questions raised in [#5603](https://github.com/diegosouzapw/OmniRoute/issues/5603) (backend interface model, why CLIProxy spawns a process, feature-flag swapping, actionable route-contract errors). The typed router-backend registry the ADR describes lands separately via [#5868](https://github.com/diegosouzapw/OmniRoute/pull/5868). ([#5891](https://github.com/diegosouzapw/OmniRoute/pull/5891))
 - **tests (autoCombo):** stabilize the `getTaskFitnessWithSource identifies fitness_table as source for known models` unit test, which flaked whenever the models.dev capabilities DB was populated in CI: the fixture model `gpt-4o` is a real models.dev catalog id, so the fitness resolution chain returned `models_dev_tier` instead of the expected static `fitness_table` source. The fixture now uses `claude-sonnet` (a shortened alias absent from the models.dev catalog, matching the sibling resolution-chain test), which deterministically falls through to the static table — the exact `source` and score assertions are preserved (`0.95` = `FITNESS_TABLE.coding["claude-sonnet"]`). ([#5890](https://github.com/diegosouzapw/OmniRoute/pull/5890)) — thanks @KooshaPari
 - **oauth (dead-code removal):** delete the superseded legacy OAuth **service-class** hierarchy under `src/lib/oauth/services/`. The live OAuth flow runs through `src/lib/oauth/providers.ts` + `src/lib/oauth/providers/` (wired into the generic `oauth/[provider]/[action]` route); the old per-provider `class *Service extends OAuthService` implementations plus their barrel had **zero** production or test references. Removed `oauth.ts` (base class), `openai.ts`, `github.ts`, `claude.ts`, `codex.ts`, `antigravity.ts`, `qwen.ts`, `qoder.ts`, and the `index.ts` barrel (−1559 LOC). Kept the three still-live files that routes import **directly** by path: `kiro.ts` (Kiro import/exchange routes), `cursor.ts` (Cursor import route), and `codexImport.ts` (utility fns for the Codex bulk-import route). Proven safe by `typecheck:core` staying green (any live reference would fail the build) + a filesystem guard `tests/unit/oauth-legacy-services-removed.test.ts` pinning the removal against re-introduction. Salvage of the closed PR [#5039](https://github.com/diegosouzapw/OmniRoute/pull/5039). gaps v3.8.42 — T10 (5.7).
 
diff --git a/docs/architecture/ROUTER_BACKENDS.md b/docs/architecture/ROUTER_BACKENDS.md
new file mode 100644
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+# Router Backends & Embedded Services — architecture contract (ADR)
+
+> **Status:** Accepted · **Context:** [#5670](https://github.com/diegosouzapw/OmniRoute/issues/5670),
+> [#5603](https://github.com/diegosouzapw/OmniRoute/issues/5603) · **Contract:** `src/domain/routing/routerBackends.ts`
+
+This ADR pins down how `ts` (native), `bifrost`, `cliproxy`, `9router`, and
+VibeProxy-compatible engines relate to each other, so contributors stop
+conflating two things that are architecturally distinct. It documents the typed
+registry introduced by the router-backend-registry work as the single source of
+truth for that model.
+
+## The core distinction — two orthogonal axes
+
+An engine's role is described by **two independent axes**, encoded together in the
+registry's `RouterBackendDefinition`:
+
+1. **Lifecycle** (`RouterBackendLifecycle`) — _how the engine runs_:
+   - `in-process` — runs inside the OmniRoute Node process (the native TS pipeline).
+   - `supervised` — a local child process OmniRoute installs/starts/stops/health-checks
+     via `ServiceSupervisor`, then consumes as a provider connection.
+   - `external` — an HTTP endpoint OmniRoute dispatches to but does **not** manage
+     (configured by an env base URL).
+   - `disabled` — registered but not selectable.
+2. **Selection axis** (relay routing backend) — _whether the relay dispatches to it_:
+   `RelayRoutingBackend = "ts" | "bifrost" | "auto"` in
+   `src/app/api/v1/relay/chat/completions/routingBackend.ts`.
+
+The mistake to avoid: treating "embedded service" and "routing backend" as one
+list. They are not. A `supervised` engine (9router/cliproxy) is a **provider
+connection consumed by the native pipeline**, not an alternate relay dispatch
+backend. `bifrost` is the reverse — a relay dispatch backend that (historically)
+was `external`-only.
+
+## The registry — single source of truth
+
+`src/domain/routing/routerBackends.ts` declares every engine once, with its
+lifecycle, capabilities, service identity, default port, health config, and
+telemetry support. Consumers look engines up via `getRouterBackend(id)`,
+`listRouterBackends()`, and `listRouterBackendsByCapability(cap)` instead of
+special-casing each sidecar.
+
+| Backend     | Lifecycle    | Service (axis A) | Relay backend (axis B) | Health        | Default port |
+| ----------- | ------------ | ---------------- | ---------------------- | ------------- | ------------ |
+| `ts`        | `in-process` | —                | `ts` (native)          | —             | —            |
+| `bifrost`   | `external`¹  | —¹               | `bifrost` / `auto`     | `/health`     | —            |
+| `cliproxy`  | `supervised` | `cliproxy`       | — (provider)           | `/v1/models`  | 8317         |
+| `9router`   | `supervised` | `9router`        | — (provider)           | `/api/health` | 20130        |
+| `vibeproxy` | `external`   | —                | — (provider adapter)   | `/v1/models`  | —            |
+
+¹ Bifrost's promotion to a `supervised` embedded service (installable/startable
+from `/api/services/bifrost/`) is tracked in
+[#5817](https://github.com/diegosouzapw/OmniRoute/pull/5817); until it merges,
+Bifrost is `external`-only (reachable solely via `BIFROST_BASE_URL`).
+
+`capabilities` (`chat`, `responses`, `streaming`, `tools`, `vision`,
+`oauth-backed`, `dashboard-embed`, `model-sync`, `native-hot-path`) let callers
+filter by what an engine can actually do rather than hard-coding per-id branches.
+
+## Axis A — embedded services (supervised process side)
+
+- **Registry of supervised processes:** `src/lib/services/bootstrap.ts` `SERVICES[]`
+  (today: `9router`, `cliproxy`).
+- **Lifecycle owner:** `src/lib/services/ServiceSupervisor.ts` — `start()` spawns the
+  child, gates on `waitForHealthy()`, taps stdout/stderr into a ring buffer;
+  `stop()` SIGTERM→SIGKILL; all serialized under a lock.
+- **State union** (`src/lib/services/types.ts`):
+  `not_installed | stopped | starting | running | stopping | error`, plus an
+  orthogonal `HealthState = healthy | unhealthy | unknown`.
+- **Why a separate process (not an in-proc SDK)?** Process isolation is what makes
+  install/start/stop/health/logs independently controllable per sidecar and lets the
+  loopback spawn-guard apply. Modeling an in-proc adapter is future work — the
+  `native-hot-path` capability flag is where that would be expressed.
+
+### Lifecycle route contract (`/api/services/<tool>/…`)
+
+Status codes are **state/verb/path-specific by design** — this is the contract, not
+inconsistency:
+
+| Call                         | Condition                       | Status                               |
+| ---------------------------- | ------------------------------- | ------------------------------------ |
+| `POST .../start`             | service `not_installed`         | **409** (precondition)               |
+| `POST .../stop`              | already stopped                 | **200** (idempotent no-op)           |
+| `GET .../status`             | OK                              | **200** (`live ?? row ?? "unknown"`) |
+| `POST .../start`             | spawn failure                   | **503** (transient)                  |
+| `GET .../status`, `.../stop` | uncaught error                  | **500**                              |
+| `GET /api/services/<x>/logs` | unknown tool `<x>`              | **404** `Service '<x>' not found`    |
+| `GET .../status?reveal=key`  | missing `X-Reveal-Confirm: yes` | **403** (9router only)               |
+| **any** `/api/services/*`    | caller not loopback/private-LAN | **403 LOCAL_ONLY**                   |
+
+All error bodies are shaped by `createErrorResponse()` →
+`{ error: { message, type }, requestId }`, where `type` is derived from the status
+(`500→server_error`, `404→not_found`, `409→conflict`, else `invalid_request`) and is
+the machine-actionable discriminator. Messages are pre-sanitized
+(`sanitizeErrorMessage()`, Hard Rule #12).
+
+**The loopback guard** is the most common source of a `403`: `/api/services/` is in
+`LOCAL_ONLY_API_PREFIXES` (`src/server/authz/routeGuard.ts`) and
+`src/server/authz/policies/management.ts` rejects any non-loopback / non-private-LAN
+caller **before auth**, because these routes spawn child processes (Hard Rules 15
+and 17). Reaching them through a public tunnel is `403` by design.
+
+## Axis B — relay routing backend (dispatch side)
+
+Only the relay proxy path `/api/v1/relay/chat/completions` selects a dispatch
+backend; the main `/api/v1/chat/completions` surface never consults
+`routingBackend.ts`.
+
+- **Selection** (`resolveRelayRoutingBackend`): a single global env toggle —
+  `OMNIROUTE_RELAY_BACKEND` / `RELAY_ROUTING_BACKEND` ∈ {`ts`, `bifrost`, `auto`}.
+  If unset, `auto` when Bifrost is configured+enabled, else `ts`.
+- **Behavior:**
+  - `bifrost` (forced): Bifrost failure → hard `502`, no fallback.
+  - `auto`: try Bifrost, on failure/cooldown silently fall through to native.
+  - `ts` / post-fallback: the native `open-sse` translator/executor pipeline.
+- **Cooldown:** per-`baseUrl` failure cooldown in `bifrostCooldown.ts`.
+
+Selection is **all-or-nothing at the relay level today** — there is no per-provider
+or per-request engine swap on `release/v3.8.43`. The per-request gate is being added
+by the sidecar-manifest work
+([#5869](https://github.com/diegosouzapw/OmniRoute/pull/5869) manifest +
+[#5870](https://github.com/diegosouzapw/OmniRoute/pull/5870) `shouldTryBifrostForRequest`),
+which lets `auto` route only manifest-eligible providers through Bifrost.
+
+## Dashboard integration
+
+The services dashboard polls `GET /api/services/<tool>/status` every 5s via
+`src/app/(dashboard)/dashboard/providers/services/hooks/useServiceStatus.ts`,
+returning `{ tool, state, pid, port, health, installedVersion, latestVersion,
+updateAvailable, autoStart, … }`. There is no shared availability-context provider —
+each component calls the hook per tool. On `!res.ok` the hook currently surfaces a
+bare `HTTP <status>`; mapping the `error.type` field to a human explanation is a
+tracked UX improvement, not a contract change.
+
+## Consequences
+
+- New engines register once in `ROUTER_BACKENDS`; consumers gain them via capability
+  queries without new per-id branches.
+- "Is this a service or a routing backend?" is answered by the `lifecycle` field, not
+  by which list an id happens to appear in.
+- The Bifrost supervision (#5817) and native hot-path migration (#5670) build on this
+  shared contract instead of special-casing each sidecar.