Let the agent stop a merge for review after substantive conflict fixes

internal/agents/mergein.go

@@ -21,7 +21,9 @@ const maxMergeTitle = 72
 func mergeInPrompt(target string) string {
 	return fmt.Sprintf("Merge %[1]s into this branch: run `git merge --no-ff --no-commit %[1]s`. "+
 		"Resolve any conflicts keeping both sides' intent; regenerate generated files instead of hand-merging them. "+
-		"Check that the build and tests pass. Leave the merge uncommitted: byre concludes it when the turn ends.", target)
+		"Check that the build and tests pass. Leave the merge uncommitted: byre concludes it when the turn ends. "+
+		"If resolving the conflicts took substantive changes rather than combining both sides as written, stop there: "+
+		"end your turn with a description of those changes, so the review of the turn's snapshot covers them.", target)
 }
 
 func (m *Manager) MergeIn(ctx context.Context, task api.Task, user api.User, target string) (api.AgentSession, error) {
@@ -100,7 +102,8 @@ func (m *Manager) startMerge(ctx context.Context, task api.Task, project api.Pro
 	}
 	m.mergeHolders[project.Id] = task.Id
 
-	_, err = m.sendMergeIn(ctx, task, user, mergeInPrompt(project.DefaultBranch))
+	prompt := mergeInPrompt(project.DefaultBranch) + fmt.Sprintf(" When this turn ends, byre squashes this branch onto %s and pushes it; if you stop, call stop_merge first so it doesn't.", project.DefaultBranch)
+	_, err = m.sendMergeIn(ctx, task, user, prompt)
 	if err != nil {
 		m.takeMerge(task.Id)
 		delete(m.mergeHolders, project.Id)
@@ -395,6 +398,51 @@ func (m *Manager) setMergeTitle(ctx context.Context, l *live, raw json.RawMessag
 	return fmt.Sprintf("Merge commit title set to %q.", title), nil
 }
 
+type stopMergeArgs struct {
+	Reason string `json:"reason"`
+}
+
+func (m *Manager) stopMergeTool(l *live) mcp.Tool {
+	return mcp.Tool{
+		Name: "stop_merge",
+		Description: "Stop the merge byre is to make when this turn ends, so the task stays active and its snapshot is reviewed before anything is pushed. " +
+			"Call it during a Merge turn when resolving conflicts took substantive changes, then end your turn describing them. The human merges again after the review.",
+		InputSchema: json.RawMessage(`{
+			"type": "object",
+			"properties": {
+				"reason": {"type": "string", "description": "The substantive changes the resolution made, in a sentence or two."}
+			},
+			"required": ["reason"]
+		}`),
+		Handler: func(ctx context.Context, raw json.RawMessage) (string, error) {
+			return m.stopMerge(ctx, l, raw)
+		},
+	}
+}
+
+func (m *Manager) stopMerge(ctx context.Context, l *live, raw json.RawMessage) (string, error) {
+	args := stopMergeArgs{}
+	err := json.Unmarshal(raw, &args)
+	if err != nil {
+		return "", fmt.Errorf("malformed stop_merge call: %w", err)
+	}
+	reason := strings.TrimSpace(args.Reason)
+	if reason == "" {
+		return "", fmt.Errorf("%w: reason must say what the resolution changed", ErrInvalid)
+	}
+
+	if m.takeMerge(l.taskID) == "" {
+		return "No merge is pending for this task; just end your turn.", nil
+	}
+	go m.releaseMerge(l.projectID, l.taskID)
+
+	err = m.emit(ctx, l, agentActor, "task.merge_stopped", map[string]any{"id": l.taskID, "reason": reason})
+	if err != nil {
+		return "", err
+	}
+	return "Merge stopped: byre won't squash or push when this turn ends. End your turn now with a description of the changes.", nil
+}
+
 func cleanMergeTitle(raw string) (string, error) {
 	title := strings.TrimSpace(raw)
 	if title == "" || strings.ContainsAny(title, "\r\n") {