Let the agent set the merge commit title, defaulting to the task title

internal/agents/mergein.go

@@ -2,17 +2,26 @@ package agents
 
 import (
 	"context"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
+	"strings"
+	"unicode/utf8"
 
 	"g.fc.run/misc/byre/internal/api"
 	"g.fc.run/misc/byre/internal/git"
+	"g.fc.run/misc/byre/internal/mcp"
+	"g.fc.run/misc/byre/internal/store"
 )
 
-func mergeInPrompt(target string) string {
+const maxMergeTitle = 72
+
+func mergeInPrompt(target, mergeTitle 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. "+
+		"Finally, if the merge commit title (currently \"%[2]s\") no longer describes all the work on this branch, set a better one with set_merge_title.", target, mergeTitle)
 }
 
 func (m *Manager) MergeIn(ctx context.Context, task api.Task, user api.User, target string) (api.AgentSession, error) {
@@ -21,13 +30,14 @@ func (m *Manager) MergeIn(ctx context.Context, task api.Task, user api.User, tar
 		return api.AgentSession{}, err
 	}
 
+	prompt := mergeInPrompt(target, task.MergeTitle)
 	for _, sess := range sessions {
 		if sess.State != api.AgentSessionStateCompleted {
-			return m.Send(ctx, sess.Id, "byre", mergeInPrompt(target), "")
+			return m.Send(ctx, sess.Id, "byre", prompt, "")
 		}
 	}
 
-	return m.create(ctx, task, user, mergeInPrompt(target), "", "byre", nil)
+	return m.create(ctx, task, user, prompt, "", "byre", nil)
 }
 
 func (m *Manager) Merge(ctx context.Context, task api.Task, project api.Project, user api.User) (api.AgentSession, error) {
@@ -85,7 +95,7 @@ func (m *Manager) squashAndClose(ctx context.Context, taskID, login string) erro
 		return err
 	}
 
-	commit, err := git.SquashMerge(project.RepoPath, task.WorktreePath, project.DefaultBranch, task.Title, []string{
+	commit, err := git.SquashMerge(project.RepoPath, task.WorktreePath, project.DefaultBranch, task.MergeTitle, []string{
 		"GIT_AUTHOR_NAME=" + user.GitName,
 		"GIT_AUTHOR_EMAIL=" + user.GitEmail,
 		"GIT_COMMITTER_NAME=" + user.GitName,
@@ -111,3 +121,51 @@ func (m *Manager) RemoveTask(ctx context.Context, task api.Task, project api.Pro
 	}
 	return git.RemoveWorktree(project.RepoPath, task.WorktreePath, task.Branch)
 }
+
+type setMergeTitleArgs struct {
+	Title string `json:"title"`
+}
+
+func (m *Manager) setMergeTitleTool(l *live) mcp.Tool {
+	return mcp.Tool{
+		Name: "set_merge_title",
+		Description: "Set the title of the single commit this task becomes when it is merged onto the default branch. " +
+			"It starts as the task title. Write one line in the imperative mood that describes all the work on the branch, " +
+			"for example \"Add retry with backoff to webhook delivery\". It replaces any earlier title.",
+		InputSchema: json.RawMessage(`{
+			"type": "object",
+			"properties": {
+				"title": {"type": "string", "description": "One line, at most 72 characters."}
+			},
+			"required": ["title"]
+		}`),
+		Handler: func(ctx context.Context, raw json.RawMessage) (string, error) {
+			return m.setMergeTitle(ctx, l, raw)
+		},
+	}
+}
+
+func (m *Manager) setMergeTitle(ctx context.Context, l *live, raw json.RawMessage) (string, error) {
+	args := setMergeTitleArgs{}
+	err := json.Unmarshal(raw, &args)
+	if err != nil {
+		return "", fmt.Errorf("malformed set_merge_title call: %w", err)
+	}
+
+	title := strings.TrimSpace(args.Title)
+	if title == "" || strings.ContainsAny(title, "\r\n") {
+		return "", errors.New("title must be one non-empty line")
+	}
+	if utf8.RuneCountInString(title) > maxMergeTitle {
+		return "", fmt.Errorf("title is %d characters; keep it to %d", utf8.RuneCountInString(title), maxMergeTitle)
+	}
+
+	err = m.store.SetMergeTitle(ctx, l.taskID, title, agentActor)
+	if errors.Is(err, store.ErrNotFound) {
+		return "", errors.New("this task is no longer active")
+	}
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("Merge commit title set to %q.", title), nil
+}