Ping on 2026-07-28 sessions sends a removed method and surfaces confusing errors - propose failing fast #1377

Follow-up to #1249 / #1174 / #1130, which closed the missing-_meta reports as by design because ping is removed from the 2026-07-28 protocol schema. This issue is about the client-side consequence of that ruling.

What did you do? Connected to a discovery-native 2026-07-28 server (e.g. https://mcp.svelte.dev/mcp) and called ClientSession.Ping.

What did you see? The client sends the removed ping method anyway and surfaces the server's rejection: MCP error -32020: Header mismatch: MCP-Protocol-Version does not match the request protocol version on transports that validate header/meta consistency, or MethodNotFound at dispatch — neither of which tells the caller that the method no longer exists in the negotiated protocol.

What did you expect to see? A clear local error before any round trip, pointing at a liveness check that still works on 2026-07-28 sessions.

SDK version: v1.8.0 (behavior identical on current main: Ping in mcp/client.go sends unconditionally).

Go version: go1.27.1.

Proposal: on sessions negotiated at 2026-07-28, Ping returns a descriptive local error without a round trip — mcp: ping is not supported on 2026-07-28 sessions (the method was removed); verify liveness with tools/list, resources/list, or prompts/list — instead of sending a method the spec removed. No working behavior is lost: every such ping fails today. If this counts as an exported-API behavior change under the proposal process, happy to treat this issue as the proposal seed.

I have a patch ready (guard + a test asserting no ping request is sent on a simulated 2026-07-28 session, plus a cmd_test readiness check that no longer relies on ping) and will send the PR once there's a maintainer read on the direction.