Modern subscriptions: say "the server closed the subscription without acknowledging it" instead of silently retrying eight times #2097

Split out of #2063, which resolved into a documentation/spec question everywhere except this one point.

The gap

A modern-era (2026-07-28) subscriptions/listen is a long-lived request. The first message on the stream must be notifications/subscriptions/acknowledged; the JSON-RPC result for the listen id is reserved as the graceful-closure marker (spec: Subscriptions → Graceful Closure).

A server that answers the listen request with a result immediately — never acknowledging — is therefore saying "acknowledged and closed in the same breath". In @modelcontextprotocol/client, Client._onresponse settles that subscription with cause graceful and rejects the pending acknowledgment promise, so the Inspector's re-listen rejects.

What the Inspector does with that today (core/mcp/inspectorClient.ts):

• refreshModernSubscription rejects → onModernReconnectFailed() → scheduleModernReconnect()
• up to MODERN_RECONNECT_MAX_ATTEMPTS = 8 consecutive failures, with capped exponential backoff
• then the stream badge lands on ended

From the user's side that is: the Subscriptions badge flickers reconnecting, eight subscriptions/listen requests with increasing SDK-assigned ids go out over roughly a minute, and it finally settles on ended — with no statement anywhere about why. That is exactly the report in #2063, where it read as the Inspector "accepting" an invalid response.

Expected

Say what happened. Something on the order of:

The server closed the subscription without acknowledging it. A subscriptions/listen request must first be answered with a notifications/subscriptions/acknowledged notification; a JSON-RPC result for the listen id means graceful closure.

Concretely:

1. Distinguish the case. A graceful settlement that arrives before any acknowledgment is not the same as a graceful shutdown of an established stream. Only the latter is the expected ended path.
2. Don't retry it. Eight re-lists against a server that will answer the same way each time is noise, not recovery — the condition is deterministic. This case should end the stream on the first occurrence.
3. Surface the reason in the UI. The Subscriptions section badge already carries a status; the never-acknowledged case needs an accompanying message rather than a bare ended.
4. Optionally note the missing resultType discriminator, which the reporter's payload also lacked and which the spec's example carries.

Notes

• The Inspector is not wrong to accept the message — it is a well-formed graceful-closure response. The defect is the silence, not the acceptance. Full analysis in Subscription protocol, documentation and inspector behavior questions #2063 (comment).
• test-servers/configs/subscriptions-modern-http.json is the natural place for a preset that reproduces this (answer subscriptions/listen with a bare result).