fix(derive): take an MCP server's host from its walked parent chain

internal/derive/buckets.go

@@ -78,7 +78,7 @@ func PlaceProcesses(procs []model.Proc, sessions []model.Session, sigs *signatur
 		}
 		placement := ProcPlacement{PID: p.PID, Bucket: BucketOther}
 		placement.Memory, placement.Measure = ProcessMemory(p)
-		agent, likely, appBoundary := false, false, false
+		agent, likely, appBoundary, hostPending := false, false, false, false
 		var shared int32
 		bundle := ""
 		for cur := range processChain(byPID, p.PID) {
@@ -97,9 +97,13 @@ func PlaceProcesses(procs []model.Proc, sessions []model.Session, sigs *signatur
 			if !appBoundary {
 				// MCP evidence wins over the bundle: editors may run MCP servers
 				// with their own bundled runtime.
-				if mcp[cur.PID] && shared == 0 {
-					shared = cur.PPID
-					likely = true
+				if hostPending {
+					// The host is the next process the chain walked, so it is
+					// a sampled, valid parent and never a reused or dead PID.
+					shared, likely, hostPending = cur.PID, true, false
+				}
+				if mcp[cur.PID] && shared == 0 && !likely {
+					hostPending = true
 				} else if AppBundle(cur.Exe) != "" && !proven[cur.PID] {
 					appBoundary = true
 				}

internal/derive/buckets_test.go

@@ -67,6 +67,33 @@ func TestBucketsEditorExtensionHostMCP(t *testing.T) {
 		t.Fatal(got)
 	}
 }
+
+// An MCP server's shared host is the parent its chain walked: never a process
+// that reused the PID of its exited parent, and never a parent missing from
+// the sample. Without a host the server is not shared with an editor.
+func TestBucketsMCPHostIsAWalkedParent(t *testing.T) {
+	now := time.Now()
+	for _, tc := range []struct {
+		name   string
+		mutate func([]model.Proc) []model.Proc
+		host   int32
+		bucket Bucket
+	}{
+		{"live parent", func(ps []model.Proc) []model.Proc { return ps }, 1, BucketLikely},
+		{"reused parent PID", func(ps []model.Proc) []model.Proc {
+			ps[0].StartTime, ps[1].StartTime = now, now.Add(-time.Minute)
+			return ps
+		}, 0, BucketOther},
+		{"parent not sampled", func(ps []model.Proc) []model.Proc { return ps[1:] }, 0, BucketOther},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			got := PlaceProcesses(tc.mutate(editorBucketFixture()), nil, nil)[2]
+			if got.Bucket != tc.bucket || got.SharedHost != tc.host {
+				t.Fatal(got)
+			}
+		})
+	}
+}
 func TestBucketsEditorWorkerIsOther(t *testing.T) {
 	ps := editorBucketFixture()
 	ps[1].Cmdline = "node /x/tsserver.js"