diff --git a/internal/mcp/transport.go b/internal/mcp/transport.go
index f8f92702..4266d28c 100644
--- a/internal/mcp/transport.go
+++ b/internal/mcp/transport.go
@@ -20,12 +20,34 @@ import (
 // the advertisement is the promise a client acts on, and nothing here has
 // been read against the modern revision, let alone tested.
 //
-// It is also not merely a documentation gap. pad_set_workspace pins a session
-// default workspace, and the stateless era has no sessions for that pin to
-// live in — so the modern era is not something pad happens not to advertise,
-// it is something pad is not known to be able to serve. Establishing what it
-// would take is a separate unit; until then the honest advertisement is the
-// era pad was built against and is tested against.
+// It is also not merely a documentation gap: something pad relies on is deleted
+// by that era. internal/server/middleware_mcp_session.go keys the
+// mcp-active-sessions gauge on the Mcp-Session-Id header, and the generate-only
+// session-id manager at this transport's call site exists SO THAT the header is
+// always minted and the gauge stays observable. SEP-2567 removes session IDs in
+// 2026-07-28 — a server serving that version never mints or echoes one — so in
+// that era nothing pad mints is available to key on. The tracker does fall back
+// to a client-supplied REQUEST header, so the honest claim is under-counting by
+// a margin nobody controls rather than a flat zero; either way it is missing
+// numbers rather than wrong ones, in the direction that reads as quiet. The
+// full statement is at that header's declaration. Whoever opens that era
+// re-keys the gauge first; the cost is recorded at the metric's definition.
+//
+// AN EARLIER VERSION OF THIS COMMENT GAVE A DIFFERENT AND FALSE REASON, and it
+// is worth the four lines because the false one is the plausible one. It said
+// pad_set_workspace pins a session default workspace that the stateless era has
+// nowhere to keep. That is true of the LOCAL stdio transport and false of this
+// one: cmd/pad builds the cloud dispatcher with a SHARED workspace state whose
+// ResolveDefault() returns "" by construction (BUG-1865, the cross-user
+// workspace bleed), so the pin is recorded and never consulted here, and
+// resolution is already per-request — the explicit workspace argument, else a
+// default derived from the caller's own OAuth identity and token allow-list.
+// This transport has therefore been stateless with respect to workspace
+// resolution since that bug was fixed, and the fix for a cross-user bug turns
+// out to be most of the work a stateless era would need.
+//
+// Until the gauge is re-keyed, the honest advertisement is the era pad was
+// built against and is tested against.
 //
 // DERIVED, NOT LISTED, and that is load-bearing. mcp.LegacyProtocolVersions()
 // is the SDK's own answer to "which revisions use the handshake", so a future
diff --git a/internal/server/middleware_mcp_session.go b/internal/server/middleware_mcp_session.go
index f2f52a68..a80d9337 100644
--- a/internal/server/middleware_mcp_session.go
+++ b/internal/server/middleware_mcp_session.go
@@ -269,6 +269,32 @@ func (s *Server) stopMCPSessionTracker() {
 // the audit row's status, so this matches the audit row's view.
 //
 // No-op when the tracker isn't wired (selfhost / tests).
+//
+// ACCEPTED COST, RECORDED HERE BECAUSE THIS IS WHERE THE KEY IS CHOSEN
+// (TASK-2977, ruled day 62). This gauge is keyed on a header the MCP protocol
+// DELETES: SEP-2567 removes session IDs in revision 2026-07-28, and a server
+// serving that revision never mints or echoes one. So in that era there is no
+// PROTOCOL-PROVIDED id for this to key on.
+//
+// Read trackMCPSession below before believing the stronger claim, which an
+// earlier draft of this comment made: the id falls back to the REQUEST header,
+// so a modern-era client that volunteers an Mcp-Session-Id anyway is still
+// tracked. The accurate statement is therefore not "that era is invisible" but
+// "the gauge stops depending on anything pad mints and starts depending on
+// whether clients keep sending a header the spec removed" — under-counting by a
+// margin nobody controls, in the direction that reads as quiet rather than as
+// breakage.
+//
+// It costs nothing today because pad's remote transport does not advertise that
+// revision: ServedProtocolVersions in internal/mcp restricts it to the
+// handshake era, and the gauge's observability is one of the two reasons stated
+// there. Named by path rather than as a symbol because it is not callable from
+// here — internal/mcp imports this package, not the other way round, which is
+// also why the transport is handed to the router as a plain http.Handler. The
+// obligation is on whoever opens that era — RE-KEY THIS GAUGE FIRST, on
+// something the modern era carries (the caller's identity plus a per-connection
+// value), because the era's arrival is exactly the moment a silently-flat gauge
+// would be read as "no MCP traffic" instead of "no measurement".
 const mcpSessionIDHeader = "Mcp-Session-Id"
 
 func (s *Server) trackMCPSession(reqHeader, respHeader func(string) string, method string, httpStatus int) {