fix(run): the FinalGuard nudge round can never replace a non-blank answer

Live regression (mort run d5ec39f4, 2026-07-15): a research run's page-cache
file false-positived the guard, and the nudge round's meta closer ("my
answer above stands as-is") REPLACED the real answer — hosts deliver
Output text plus separately-drained attachments, so the user received the
closer and never saw the answer.

New merge policy: the nudge round exists to make tool calls (queue the
delivery); its final text is discarded and recorded as a final_nudge_result
audit event, so a round that declined to queue stays observable without
reaching the user. The only exception is a blank original stop turn, where
the nudge round's text is the only answer there is. Messages/Usage still
merge (audit, PostRun, checkpoint continuity).

Additive on top of 007b753 (mort main pins that commit as a pseudo-version;
this branch must never be rewritten).

Co-Authored-By: Claude Fable 5 <[email protected]>

run/final_guard_test.go

@@ -41,17 +41,21 @@ func (r *eventRecorder) LogEvent(eventType string, _ map[string]any) {
 
 // TestFinalGuardNudgeRunsOneExtraRound is the core announce-then-stop fix: the
 // model stops with "Done — sent!" (nothing delivered), the guard returns a
-// nudge, and the SAME conversation runs one more round — here a tool call then
-// a corrected final answer. The guard is consulted exactly once (the nudge
-// round's own stop is final), and the audit log records the nudge.
+// nudge, and the SAME conversation runs one more round — here a tool call
+// (queueing the delivery) then a closing text turn. The guard is consulted
+// exactly once (the nudge round's own stop is final), the audit log records
+// the nudge, and the ORIGINAL answer text survives: the nudge round's closer
+// is discarded (recorded as final_nudge_result), because hosts deliver Output
+// text + separately-drained attachments — replacing Output with the closer is
+// the run-d5ec39f4 lost-answer regression.
 func TestFinalGuardNudgeRunsOneExtraRound(t *testing.T) {
 	fp := fake.New("fake")
 	fp.Enqueue("m",
 		fake.Reply("Done — sent you the file!"),
 		// Nudge round: an (unregistered → tolerated error result) delivery call,
-		// then the corrected final answer.
+		// then a meta closer that must NOT become the run output.
 		fake.ReplyWith(llm.Response{ToolCalls: []llm.ToolCall{{ID: "c1", Name: "send_attachments", Arguments: []byte(`{}`)}}}),
-		fake.Reply("delivered for real"),
+		fake.Reply("queued the file — answer above stands"),
 	)
 	m, _ := fp.Model("m")
 	rec := &eventRecorder{}
@@ -72,8 +76,11 @@ func TestFinalGuardNudgeRunsOneExtraRound(t *testing.T) {
 	if res.Err != nil {
 		t.Fatalf("run error: %v", res.Err)
 	}
-	if res.Output != "delivered for real" {
-		t.Fatalf("output = %q, want the nudge round's answer", res.Output)
+	if res.Output != "Done — sent you the file!" {
+		t.Fatalf("output = %q, want the ORIGINAL answer preserved (nudge closer discarded)", res.Output)
+	}
+	if n := fp.CallCount("m"); n != 3 {
+		t.Fatalf("model called %d times, want 3 (the nudge round must still run)", n)
 	}
 	if guard.calls != 1 {
 		t.Fatalf("guard consulted %d times, want exactly 1 (no re-guard of the nudge round)", guard.calls)
@@ -84,17 +91,51 @@ func TestFinalGuardNudgeRunsOneExtraRound(t *testing.T) {
 	if guard.state.Output != "Done — sent you the file!" {
 		t.Errorf("guard state.Output = %q, want the pre-nudge final text", guard.state.Output)
 	}
-	var sawNudgeEvent bool
+	var sawNudge, sawResult bool
 	for _, ev := range rec.events {
-		if ev == "final_nudge" {
-			sawNudgeEvent = true
+		switch ev {
+		case "final_nudge":
+			sawNudge = true
+		case "final_nudge_result":
+			sawResult = true
 		}
 	}
-	if !sawNudgeEvent {
+	if !sawNudge {
 		t.Errorf("audit events %v missing final_nudge", rec.events)
 	}
-	if rec.stats.Output != "delivered for real" {
-		t.Errorf("audit close output = %q, want the nudged output", rec.stats.Output)
+	if !sawResult {
+		t.Errorf("audit events %v missing final_nudge_result (the discarded closer must stay observable)", rec.events)
+	}
+	if rec.stats.Output != "Done — sent you the file!" {
+		t.Errorf("audit close output = %q, want the original answer", rec.stats.Output)
+	}
+}
+
+// TestFinalGuardNudgeFillsEmptyOriginal: when the ORIGINAL stop turn was blank
+// (a degenerate run), the nudge round's text is the only answer there is — it
+// becomes the output instead of being discarded.
+func TestFinalGuardNudgeFillsEmptyOriginal(t *testing.T) {
+	fp := fake.New("fake")
+	fp.Enqueue("m",
+		fake.Reply("  \n"),
+		fake.Reply("actual answer from the nudge round"),
+	)
+	m, _ := fp.Model("m")
+	guard := &guardFunc{nudge: func(RunInfo, FinalState) string { return "deliver or correct" }}
+	ex := New(Config{
+		Registry: tool.NewRegistry(),
+		Models:   func(ctx context.Context, _ string) (context.Context, llm.Model, error) { return ctx, m, nil },
+		Ports:    Ports{FinalGuard: guard},
+	})
+
+	res := ex.Run(context.Background(),
+		RunnableAgent{ModelTier: "m"}, tool.Invocation{RunID: "r"}, "hi")
+
+	if res.Err != nil {
+		t.Fatalf("run error: %v", res.Err)
+	}
+	if res.Output != "actual answer from the nudge round" {
+		t.Fatalf("output = %q, want the nudge round's text to fill a blank original", res.Output)
 	}
 }