Support multi-todoset projects end-to-end

.surface

@@ -263,6 +263,7 @@ CMD basecamp todos sweep
 CMD basecamp todos trash
 CMD basecamp todos uncomplete
 CMD basecamp todosets
+CMD basecamp todosets list
 CMD basecamp todosets show
 CMD basecamp tools
 CMD basecamp tools create
@@ -5172,6 +5173,7 @@ FLAG basecamp todo --stats type=bool
 FLAG basecamp todo --styled type=bool
 FLAG basecamp todo --to type=string
 FLAG basecamp todo --todolist type=string
+FLAG basecamp todo --todoset type=string
 FLAG basecamp todo --verbose type=count
 FLAG basecamp todolistgroups --account type=string
 FLAG basecamp todolistgroups --agent type=bool
@@ -5534,6 +5536,7 @@ FLAG basecamp todos create --stats type=bool
 FLAG basecamp todos create --styled type=bool
 FLAG basecamp todos create --to type=string
 FLAG basecamp todos create --todolist type=string
+FLAG basecamp todos create --todoset type=string
 FLAG basecamp todos create --verbose type=count
 FLAG basecamp todos list --account type=string
 FLAG basecamp todos list --agent type=bool
@@ -5699,8 +5702,26 @@ FLAG basecamp todosets --quiet type=bool
 FLAG basecamp todosets --stats type=bool
 FLAG basecamp todosets --styled type=bool
 FLAG basecamp todosets --todolist type=string
-FLAG basecamp todosets --todoset type=string
 FLAG basecamp todosets --verbose type=count
+FLAG basecamp todosets list --account type=string
+FLAG basecamp todosets list --agent type=bool
+FLAG basecamp todosets list --cache-dir type=string
+FLAG basecamp todosets list --count type=bool
+FLAG basecamp todosets list --hints type=bool
+FLAG basecamp todosets list --ids-only type=bool
+FLAG basecamp todosets list --in type=string
+FLAG basecamp todosets list --json type=bool
+FLAG basecamp todosets list --markdown type=bool
+FLAG basecamp todosets list --md type=bool
+FLAG basecamp todosets list --no-hints type=bool
+FLAG basecamp todosets list --no-stats type=bool
+FLAG basecamp todosets list --profile type=string
+FLAG basecamp todosets list --project type=string
+FLAG basecamp todosets list --quiet type=bool
+FLAG basecamp todosets list --stats type=bool
+FLAG basecamp todosets list --styled type=bool
+FLAG basecamp todosets list --todolist type=string
+FLAG basecamp todosets list --verbose type=count
 FLAG basecamp todosets show --account type=string
 FLAG basecamp todosets show --agent type=bool
 FLAG basecamp todosets show --cache-dir type=string
@@ -6869,6 +6890,7 @@ SUB basecamp todos sweep
 SUB basecamp todos trash
 SUB basecamp todos uncomplete
 SUB basecamp todosets
+SUB basecamp todosets list
 SUB basecamp todosets show
 SUB basecamp tools
 SUB basecamp tools create

.surface-breaking

@@ -2,6 +2,7 @@ ARG basecamp assign 00 <todo_id>
 ARG basecamp unassign 00 <todo_id>
 FLAG basecamp checkin answer create --question type=string
 FLAG basecamp checkins answer create --question type=string
+FLAG basecamp todosets --todoset type=string
 FLAG basecamp webhook create --url type=string
 FLAG basecamp webhooks create --url type=string
 ARG basecamp upload doc create 00 <title>

e2e/todosets.bats

@@ -36,11 +36,11 @@ load test_helper
   assert_output_contains "--project requires a value"
 }
 
-@test "todosets --todoset without value shows error" {
+@test "todosets show --todoset without value shows error" {
   create_credentials
   create_global_config '{}'
 
-  run basecamp todosets --todoset
+  run basecamp todosets show --todoset
   assert_failure
   assert_output_contains "--todoset requires a value"
 }

internal/commands/commands.go

@@ -36,7 +36,7 @@ func CommandCategories() []CommandCategory {
 				{Name: "projects", Category: "core", Description: "Manage projects", Actions: []string{"list", "show", "create", "update", "delete"}},
 				{Name: "todos", Category: "core", Description: "Manage to-dos", Actions: []string{"list", "show", "create", "complete", "uncomplete", "position", "trash", "archive", "restore"}},
 				{Name: "todolists", Category: "core", Description: "Manage to-do lists", Actions: []string{"list", "show", "create", "update", "trash", "archive", "restore"}},
-				{Name: "todosets", Category: "core", Description: "View to-do set containers", Actions: []string{"show"}},
+				{Name: "todosets", Category: "core", Description: "Manage to-do set containers", Actions: []string{"list", "show"}},
 				{Name: "todolistgroups", Category: "core", Description: "Manage to-do list groups", Actions: []string{"list", "show", "create", "update", "position"}},
 				{Name: "messages", Category: "core", Description: "Manage messages", Actions: []string{"list", "show", "create", "update", "pin", "unpin", "trash", "archive", "restore"}},
 				{Name: "campfire", Category: "core", Description: "Chat in Campfire rooms", Actions: []string{"list", "messages", "post", "upload", "line", "delete"}},

internal/commands/helpers.go

@@ -237,7 +237,7 @@ func ensureTodoset(cmd *cobra.Command, app *appctx.App, projectID, explicitTodos
 // ensureTodolist resolves the todolist ID if not already configured.
 // This enables interactive prompts when --list flag and config are both missing.
 // The project must be resolved first (call ensureProject before this).
-func ensureTodolist(cmd *cobra.Command, app *appctx.App, projectID string) (string, error) {
+func ensureTodolist(cmd *cobra.Command, app *appctx.App, projectID, explicitTodosetID string) (string, error) {
 	// Check if todolist is already set via flag or config
 	if app.Flags.Todolist != "" {
 		return app.Flags.Todolist, nil
@@ -247,13 +247,107 @@ func ensureTodolist(cmd *cobra.Command, app *appctx.App, projectID string) (stri
 	}
 
 	// Try interactive resolution
-	resolved, err := app.Resolve().Todolist(cmd.Context(), projectID)
+	resolved, err := app.Resolve().Todolist(cmd.Context(), projectID, explicitTodosetID)
 	if err != nil {
 		return "", err
 	}
 	return resolved.Value, nil
 }
 
+// resolveTodolistInTodoset resolves a todolist name or ID, scoped to a specific todoset
+// when explicitTodosetID is non-empty. This ensures that --todoset actually constrains
+// which todolists are visible for name resolution.
+//
+// When explicitTodosetID is empty, falls back to the project-wide name resolver.
+func resolveTodolistInTodoset(cmd *cobra.Command, app *appctx.App, todolist, projectID, explicitTodosetID string) (string, error) {
+	// No todoset constraint — use the standard project-wide resolver
+	if explicitTodosetID == "" {
+		resolved, _, err := app.Names.ResolveTodolist(cmd.Context(), todolist, projectID)
+		return resolved, err
+	}
+
+	// Numeric ID passthrough — trust it regardless of todoset
+	if isNumeric(todolist) {
+		return todolist, nil
+	}
+
+	// Todoset-scoped name resolution: fetch todolists from only the specified
+	// todoset and resolve the name within that set.
+	todosetID, err := strconv.ParseInt(explicitTodosetID, 10, 64)
+	if err != nil {
+		return "", output.ErrUsage("Invalid todoset ID")
+	}
+
+	todolistsPath := fmt.Sprintf("/todosets/%d/todolists.json", todosetID)
+	pages, err := app.Account().GetAll(cmd.Context(), todolistsPath)
+	if err != nil {
+		return "", convertSDKError(err)
+	}
+
+	// Build a name-resolution list from the scoped todolists
+	type entry struct {
+		id   int64
+		name string
+	}
+	var entries []entry
+	for _, raw := range pages {
+		var tl struct {
+			ID   int64  `json:"id"`
+			Name string `json:"name"`
+		}
+		if err := json.Unmarshal(raw, &tl); err != nil {
+			return "", fmt.Errorf("failed to parse todolist from todoset %s: %w", explicitTodosetID, err)
+		}
+		entries = append(entries, entry{id: tl.ID, name: tl.Name})
+	}
+
+	// Exact match
+	inputLower := strings.ToLower(todolist)
+	for _, e := range entries {
+		if e.name == todolist {
+			return strconv.FormatInt(e.id, 10), nil
+		}
+	}
+	// Case-insensitive match
+	var caseMatches []entry
+	for _, e := range entries {
+		if strings.ToLower(e.name) == inputLower {
+			caseMatches = append(caseMatches, e)
+		}
+	}
+	if len(caseMatches) == 1 {
+		return strconv.FormatInt(caseMatches[0].id, 10), nil
+	}
+	if len(caseMatches) > 1 {
+		matchNames := make([]string, len(caseMatches))
+		for i, m := range caseMatches {
+			matchNames[i] = m.name
+		}
+		return "", output.ErrAmbiguous("todolist", matchNames)
+	}
+	// Partial match
+	var partials []entry
+	for _, e := range entries {
+		if strings.Contains(strings.ToLower(e.name), inputLower) {
+			partials = append(partials, e)
+		}
+	}
+	if len(partials) == 1 {
+		return strconv.FormatInt(partials[0].id, 10), nil
+	}
+	if len(partials) > 1 {
+		matchNames := make([]string, len(partials))
+		for i, m := range partials {
+			matchNames[i] = m.name
+		}
+		return "", output.ErrAmbiguous("todolist", matchNames)
+	}
+
+	return "", output.ErrNotFoundHint("Todolist", todolist,
+		fmt.Sprintf("Not found in todoset %s. Use 'basecamp todolists --in %s --todoset %s' to see available lists",
+			explicitTodosetID, projectID, explicitTodosetID))
+}
+
 // ensurePersonInProject resolves a person ID interactively from project members.
 // This is useful when you want to limit the selection to people who have
 // access to a specific project.

internal/commands/todos.go

@@ -71,6 +71,7 @@ func NewReopenCmd() *cobra.Command {
 func NewTodoCmd() *cobra.Command {
 	var project string
 	var todolist string
+	var todoset string
 	var assignee string
 	var due string
 	var description string
@@ -127,9 +128,9 @@ func NewTodoCmd() *cobra.Command {
 			if todolist == "" {
 				todolist = app.Config.TodolistID
 			}
-			// If still no todolist, try interactive selection
+			// If still no todolist, try interactive selection (todoset-scoped)
 			if todolist == "" {
-				selectedTodolist, err := ensureTodolist(cmd, app, project)
+				selectedTodolist, err := ensureTodolist(cmd, app, project, todoset)
 				if err != nil {
 					return err
 				}
@@ -140,8 +141,8 @@ func NewTodoCmd() *cobra.Command {
 				return output.ErrUsage("--list is required (no default todolist found)")
 			}
 
-			// Resolve todolist name to ID
-			resolvedTodolist, _, err := app.Names.ResolveTodolist(cmd.Context(), todolist, project)
+			// Resolve todolist name to ID, scoped to --todoset when provided
+			resolvedTodolist, err := resolveTodolistInTodoset(cmd, app, todolist, project, todoset)
 			if err != nil {
 				return err
 			}
@@ -228,6 +229,7 @@ func NewTodoCmd() *cobra.Command {
 	cmd.Flags().StringVarP(&project, "project", "p", "", "Project ID or name")
 	cmd.Flags().StringVar(&project, "in", "", "Project ID (alias for --project)")
 	cmd.Flags().StringVarP(&todolist, "list", "l", "", "Todolist ID")
+	cmd.Flags().StringVarP(&todoset, "todoset", "t", "", "Todoset ID (for projects with multiple todosets)")
 	cmd.Flags().StringVar(&assignee, "assignee", "", "Assignee ID or name")
 	cmd.Flags().StringVar(&assignee, "to", "", "Assignee (alias for --assignee)")
 	cmd.Flags().StringVarP(&due, "due", "d", "", "Due date")
@@ -605,6 +607,7 @@ You can pass either a todo ID or a Basecamp URL:
 func newTodosCreateCmd() *cobra.Command {
 	var project string
 	var todolist string
+	var todoset string
 	var assignee string
 	var due string
 	var description string
@@ -661,9 +664,9 @@ func newTodosCreateCmd() *cobra.Command {
 			if todolist == "" {
 				todolist = app.Config.TodolistID
 			}
-			// If still no todolist, try interactive selection
+			// If still no todolist, try interactive selection (todoset-scoped)
 			if todolist == "" {
-				selectedTodolist, err := ensureTodolist(cmd, app, project)
+				selectedTodolist, err := ensureTodolist(cmd, app, project, todoset)
 				if err != nil {
 					return err
 				}
@@ -674,8 +677,8 @@ func newTodosCreateCmd() *cobra.Command {
 				return output.ErrUsage("--list is required (no default todolist found)")
 			}
 
-			// Resolve todolist name to ID
-			resolvedTodolist, _, err := app.Names.ResolveTodolist(cmd.Context(), todolist, project)
+			// Resolve todolist name to ID, scoped to --todoset when provided
+			resolvedTodolist, err := resolveTodolistInTodoset(cmd, app, todolist, project, todoset)
 			if err != nil {
 				return err
 			}
@@ -762,6 +765,7 @@ func newTodosCreateCmd() *cobra.Command {
 	cmd.Flags().StringVarP(&project, "project", "p", "", "Project ID or name")
 	cmd.Flags().StringVar(&project, "in", "", "Project ID (alias for --project)")
 	cmd.Flags().StringVarP(&todolist, "list", "l", "", "Todolist ID")
+	cmd.Flags().StringVarP(&todoset, "todoset", "t", "", "Todoset ID (for projects with multiple todosets)")
 	cmd.Flags().StringVar(&assignee, "assignee", "", "Assignee ID")
 	cmd.Flags().StringVar(&assignee, "to", "", "Assignee ID (alias for --assignee)")
 	cmd.Flags().StringVarP(&due, "due", "d", "", "Due date (YYYY-MM-DD)")