feat(client): accept a cached era verdict on ConnectOptions.prior (#2511)

docs/advanced/gateway.md

@@ -7,7 +7,7 @@ A **gateway** — a proxy, a worker pool, any process that fronts one MCP server
 
 ## Connect with a prior discover result
 
-`connect()` takes an optional `prior`: a persisted `DiscoverResult` from an earlier probe. With it, `connect()` adopts the server's advertisement directly and sends nothing on the wire.
+`connect()` takes an optional `prior`: a cached era verdict (`PriorDiscovery`). Its modern arm, `{ kind: 'modern', discover }`, wraps a persisted `DiscoverResult` from an earlier probe — `connect()` adopts the server's advertisement directly and sends nothing on the wire.
 
 ```ts source="../../examples/guides/advanced/gateway.examples.ts#connect_prior"
 import { Client, StreamableHTTPClientTransport } from '@modelcontextprotocol/client';
@@ -21,12 +21,12 @@ const persisted = JSON.stringify(bootstrap.getDiscoverResult());
 
 // … then every other client connects with zero round trips.
 const worker = new Client({ name: 'worker', version: '1.0.0' });
-await worker.connect(new StreamableHTTPClientTransport(url), { prior: JSON.parse(persisted) });
+await worker.connect(new StreamableHTTPClientTransport(url), { prior: { kind: 'modern', discover: JSON.parse(persisted) } });
 ```
 
 `worker` is connected: `callTool` works immediately, and the server has not heard from it yet.
 
-`connect({ prior })` is 2026-07-28+ only — see [Protocol versions](../protocol-versions.md).
+The modern verdict is 2026-07-28+ only — see [Protocol versions](../protocol-versions.md). For a server known to be legacy, supply the negative verdict instead — see [Skip the probe for a known-legacy server](#skip-the-probe-for-a-known-legacy-server).
 
 ## Probe once at bootstrap
 
@@ -50,17 +50,18 @@ The recorded value is the server's whole advertisement — supported versions, c
 ```
 
 ::: tip
-An already-connected client can re-probe at any time: `await client.discover()` sends `server/discover` and updates `getDiscoverResult()`. A default-mode connect never probes, so its `getDiscoverResult()` is `undefined` — [Protocol versions](../protocol-versions.md#pin-an-era) lists the negotiation modes.
+A client on a modern-era connection can re-probe at any time: `await client.discover()` sends `server/discover` and updates `getDiscoverResult()`. On a legacy-era connection `discover()` throws (`server/discover` is not a 2025-era method) — to re-check a legacy verdict, reconnect without a `prior` under `mode: 'auto'`, as in [Caching discovery verdicts](#caching-discovery-verdicts). A default-mode connect never probes, so its `getDiscoverResult()` is `undefined` — [Protocol versions](../protocol-versions.md#pin-an-era) lists the negotiation modes.
 :::
 
 ## Persist the advertisement
 
-The value is plain JSON. Write the string to Redis, a config map, or a process-local cache; parse it back wherever a client needs it.
+The value is plain JSON. Write the string to Redis, a config map, or a process-local cache; parse it back and wrap it as the modern verdict wherever a client needs it.
 
 ```ts source="../../examples/guides/advanced/gateway.examples.ts#persist_advertisement"
-import type { DiscoverResult } from '@modelcontextprotocol/client';
+import type { DiscoverResult, PriorDiscovery } from '@modelcontextprotocol/client';
 
-const prior = JSON.parse(persisted) as DiscoverResult;
+const discover = JSON.parse(persisted) as DiscoverResult;
+const prior: PriorDiscovery = { kind: 'modern', discover };
 ```
 
 Nothing about `prior` is tied to the process that probed: any client that can reach the same URL can adopt it.
@@ -100,7 +101,7 @@ Never share a persisted `DiscoverResult` across principals — key the blob on t
 
 ## Open a listen stream when a worker needs notifications
 
-`connect({ prior })` never auto-opens a `subscriptions/listen` stream — prior-connected clients are request-only until you open one yourself.
+A modern-verdict `connect({ prior })` never auto-opens a `subscriptions/listen` stream — the client is request-only until you open one yourself. (A legacy-verdict connect is an ordinary 2025-era connection: unsolicited notifications, no `listen()`.)
 
 ```ts source="../../examples/guides/advanced/gateway.examples.ts#listen_worker"
 const subscription = await worker.listen({ toolsListChanged: true });
@@ -116,21 +117,21 @@ The server acknowledges the filter it agreed to honor:
 From here the stream behaves like any other subscription — [Subscriptions](../clients/subscriptions.md) covers the notification handlers and the close semantics.
 
 ::: info
-A `listChanged` option configured on a prior-connected client registers its handlers but stays silent: no stream opens until you call `listen()`.
+A `listChanged` option configured on a modern-verdict client registers its handlers but stays silent: no stream opens until you call `listen()`.
 :::
 
 ## Handle a stale or incompatible advertisement
 
-A `prior` that shares no 2026-07-28+ revision with the client rejects with `SdkError(EraNegotiationFailed)` before anything reaches the transport.
+A modern verdict whose `discover` shares no 2026-07-28+ revision with the client rejects with `SdkError(EraNegotiationFailed)` before anything reaches the transport.
 
 ```ts source="../../examples/guides/advanced/gateway.examples.ts#prior_stale"
 import { SdkError, SdkErrorCode } from '@modelcontextprotocol/client';
 
-const stale: DiscoverResult = { ...prior, supportedVersions: ['2025-06-18'] };
+const stale: DiscoverResult = { ...discover, supportedVersions: ['2025-06-18'] };
 
 const late = new Client({ name: 'worker-d', version: '1.0.0' }, { versionNegotiation: { mode: 'auto' } });
 try {
-    await late.connect(new StreamableHTTPClientTransport(url), { prior: stale });
+    await late.connect(new StreamableHTTPClientTransport(url), { prior: { kind: 'modern', discover: stale } });
 } catch (error) {
     if (!(error instanceof SdkError) || error.code !== SdkErrorCode.EraNegotiationFailed) throw error;
     console.log(error.code);
@@ -150,11 +151,95 @@ re-probed: 2026-07-28
 
 Replace the persisted blob with the fresh `getDiscoverResult()` and the rest of the fleet recovers on its next read.
 
+## Skip the probe for a known-legacy server
+
+When out-of-band metadata already says the server is pre-2026 — a registry entry, an earlier connection's outcome — an `'auto'`-mode probe is a round trip that fails on every single connect. Supply the negative verdict instead: `PriorDiscovery`'s `{ kind: 'legacy' }` arm skips the probe and goes straight to the `initialize` handshake.
+
+```ts source="../../examples/guides/advanced/gateway.examples.ts#prior_legacy"
+const pinnedLegacy = new Client({ name: 'worker-e', version: '1.0.0' }, { versionNegotiation: { mode: 'auto' } });
+await pinnedLegacy.connect(new StreamableHTTPClientTransport(url), {
+    prior: { kind: 'legacy' }
+});
+console.log(pinnedLegacy.getProtocolEra());
+```
+
+```
+legacy
+```
+
+Freshness is your responsibility: the SDK adopts whatever verdict you hand it. A stale modern verdict fails loudly at the first request, but a stale legacy verdict succeeds silently forever — an upgraded server still answers `initialize`, so nothing ever corrects it. Date cached legacy verdicts in your own storage and stop supplying them past your policy horizon; with no `prior`, the configured mode decides again (under `mode: 'auto'` the connect re-probes, so the upgrade is discovered).
+
+## Caching discovery verdicts
+
+The pieces above compose into the full host-side loop: probe once, cache the verdict under your own timestamp, and gate every later connect on a freshness check you control.
+
+The first connect under `mode: 'auto'` pays one probe. Afterwards the outcome is readable on the client: `getDiscoverResult()` returns the `DiscoverResult` on a modern server, and on a connected client its absence means the era is legacy. Store that verdict together with when you stored it — the `Map` below keeps `storedAt` explicitly; in a real deployment the store does the dating for you (a Redis key TTL, where an expired read simply comes back empty, or a database row's `created_at` column).
+
+Before each connect, supply the cached verdict only while your own policy says it is fresh — a fixed horizon here, any predicate in practice. Supplying `undefined` under `mode: 'auto'` *is* the re-probe, and the fresh outcome re-populates the cache. The timestamp matters most for the legacy branch: a stale legacy verdict succeeds silently forever (an upgraded server still answers `initialize`), so only the timestamp ever retires it; a stale modern verdict fails loudly at the first request.
+
+```ts source="../../examples/guides/advanced/gateway.examples.ts#prior_cache"
+// Host-side verdict cache. A real deployment keeps this in Redis (a key TTL
+// does the dating: an expired read is no prior) or a database row with a
+// created_at column; the Map stores the timestamp explicitly.
+const verdicts = new Map<string, { verdict: PriorDiscovery; storedAt: number }>();
+const HORIZON_MS = 24 * 60 * 60 * 1000; // fixed-horizon freshness policy
+const fresh = (entry: { storedAt: number }): boolean => Date.now() - entry.storedAt < HORIZON_MS;
+
+// Count server/discover probes at the fetch layer (the wire trace).
+let probes = 0;
+const tracingFetch: typeof fetch = async (input, init) => {
+    if (typeof init?.body === 'string' && init.body.includes('"server/discover"')) probes++;
+    return fetch(input, init);
+};
+
+async function connectCached(key: string): Promise<Client> {
+    const entry = verdicts.get(key);
+    // An entry past the horizon is not supplied — under mode: 'auto' that IS
+    // the re-probe, and the fresh outcome below re-populates the cache.
+    const prior = entry && fresh(entry) ? entry.verdict : undefined;
+
+    const client = new Client({ name: 'cached-worker', version: '1.0.0' }, { versionNegotiation: { mode: 'auto' } });
+    await client.connect(new StreamableHTTPClientTransport(url, { fetch: tracingFetch }), { prior });
+
+    if (prior === undefined) {
+        // Fresh outcome: getDiscoverResult() is the DiscoverResult on a modern
+        // server; its absence on a connected client means the era is legacy.
+        const discover = client.getDiscoverResult();
+        // Date the entry with the host's own clock — only this timestamp ever
+        // retires a legacy verdict (a stale one succeeds silently forever).
+        verdicts.set(key, {
+            verdict: discover ? { kind: 'modern', discover } : { kind: 'legacy' },
+            storedAt: Date.now()
+        });
+    }
+    return client;
+}
+
+const first = await connectCached('gateway-target'); // no entry: probes
+console.log('probes after first connect:', probes);
+
+const second = await connectCached('gateway-target'); // fresh entry: verdict supplied, no probe
+console.log('probes after second connect:', probes);
+
+verdicts.get('gateway-target')!.storedAt -= HORIZON_MS + 1; // the horizon passes
+const third = await connectCached('gateway-target'); // stale: dropped, re-probed, re-cached
+console.log('probes after the horizon passes:', probes);
+```
+
+```
+probes after first connect: 1
+probes after second connect: 1
+probes after the horizon passes: 2
+```
+
+The wire trace shows the shape of the loop: one probe to fill the cache, none while the verdict is fresh, one more when the horizon forces rediscovery.
+
 ## Recap
 
-- `connect(transport, { prior })` adopts a persisted `DiscoverResult` with zero round trips.
+- `connect(transport, { prior: { kind: 'modern', discover } })` adopts a persisted `DiscoverResult` with zero round trips.
 - The advertisement comes from one `'auto'`-mode or pinned probe — or an explicit `client.discover()` — and `getDiscoverResult()` reads it back.
-- The value is plain JSON: stringify it into a shared cache, parse it in any process that fronts the same server.
+- The value is plain JSON: stringify it into a shared cache, parse it back and wrap it as the modern verdict in any process that fronts the same server.
 - Reuse a `DiscoverResult` only across clients that present the same authorization context.
-- Prior-connected clients are request-only; call `listen()` on the one that needs notifications.
+- Modern-verdict clients are request-only; call `listen()` on the one that needs notifications.
 - An incompatible `prior` rejects with `SdkError(EraNegotiationFailed)`; fall back to a fresh probe and re-persist.
+- A known-legacy server takes `prior: { kind: 'legacy' }` — no probe, straight to `initialize`. Stale legacy verdicts fail silently (an upgraded server still answers `initialize`), so date them in your own storage and stop supplying them past your policy horizon.