feat(worker): let Claude set the cheapest model per generated task via MCP

AddTask, planning CreateChildTask, and SuggestImprovement now accept an
optional alias-validated model (haiku/sonnet/opus; blank = inherit) so the
model is chosen at creation time instead of a follow-up set_task_config call.
The planning, system, and improvement prompts instruct Claude to pick the
cheapest capable model (haiku < sonnet < opus).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Models/ModelRegistry.cs

@@ -4,9 +4,26 @@ public static class ModelRegistry
 {
     public static readonly IReadOnlyList<string> Aliases = new[] { "sonnet", "opus", "haiku" };
 
+    /// <summary>Model aliases ordered cheapest → most capable. Single source for prompt cost guidance.</summary>
+    public static readonly IReadOnlyList<string> ByCostAscending = new[] { "haiku", "sonnet", "opus" };
+
     public const string DefaultAlias = "sonnet";
     public const string PlanningAlias = "opus";
 
     public const string ListDefaultSentinel = "(default)";
     public const string TaskInheritSentinel = "(inherit)";
+
+    /// <summary>
+    /// Validate a model alias from external input. Null/blank → null (inherit).
+    /// Returns the canonical lowercase alias; throws on an unknown value.
+    /// </summary>
+    public static string? NormalizeAlias(string? model)
+    {
+        var m = model?.Trim();
+        if (string.IsNullOrEmpty(m)) return null;
+        foreach (var alias in Aliases)
+            if (string.Equals(alias, m, StringComparison.OrdinalIgnoreCase))
+                return alias;
+        throw new ArgumentException($"Unknown model '{model}'. Allowed: {string.Join(", ", Aliases)}.");
+    }
 }

src/ClaudeDo.Data/PromptFiles.cs

@@ -82,7 +82,10 @@ public static class PromptFiles
         ## Out-of-scope improvements
         If you notice worthwhile work that is genuinely outside this task's scope
         (a refactor, a follow-up, tech debt), do NOT do it here. File it with
-        SuggestImprovement(title, description) and stay focused on the task at hand.
+        SuggestImprovement(title, description, model) and stay focused on the task at hand.
+        Set `model` to the cheapest model that can do the follow-up well — 'haiku' for
+        trivial/mechanical work, 'sonnet' for normal coding, 'opus' only for genuinely
+        complex work (cheapest to most capable: haiku < sonnet < opus).
 
         ## Working in the repo
         - Read a file before editing it. Match the conventions already in this codebase —
@@ -122,8 +125,8 @@ public static class PromptFiles
         # Out-of-scope follow-up
 
         You are an improvement follow-up that another task filed via SuggestImprovement.
-        It was deliberately scoped narrow. Do EXACTLY what this task's title and
-        description ask — nothing more.
+        It was deliberately scoped narrow, and is intentionally a small, cheap unit of
+        work. Do EXACTLY what this task's title and description ask — nothing more.
 
         - Make the smallest change that satisfies the task. No opportunistic refactors,
           renames, reformatting, or "while I'm here" cleanup beyond what is asked.
@@ -150,6 +153,14 @@ public static class PromptFiles
         Once the design is approved, create the child tasks with CreateChildTask, then
         call Finalize. Keep each subtask concrete and self-contained with a clear
         done-state, ordered so dependencies come first.
+
+        For each subtask, pass CreateChildTask's `model` argument set to the CHEAPEST
+        model that can do that subtask well. Models, cheapest to most capable:
+        haiku < sonnet < opus.
+        - haiku — trivial/mechanical work: doc tweaks, simple renames, small localized edits.
+        - sonnet — normal coding work; the sensible default when unsure.
+        - opus — only for genuinely complex, cross-cutting, or hard-to-debug work.
+        Do not default everything to opus — most subtasks are haiku or sonnet.
         """;
 
     private const string PlanningInitialDefault = """

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -197,6 +197,7 @@ public sealed class TaskRepository
         string? description,
         string? commitType,
         string? createdBy = null,
+        string? model = null,
         CancellationToken ct = default)
     {
         // AsNoTracking: SetPlanningStartedAsync mutates via ExecuteUpdate which
@@ -223,6 +224,7 @@ public sealed class TaskRepository
             ParentTaskId = parentId,
             SortOrder = (maxSort ?? -1) + 1,
             CreatedBy = createdBy,
+            Model = ModelRegistry.NormalizeAlias(model),
         };
         _context.Tasks.Add(child);
         await _context.SaveChangesAsync(ct);

src/ClaudeDo.Worker/CLAUDE.md

@@ -166,7 +166,7 @@ Loaded from `~/.todo-app/worker.config.json`:
 - `signalr_port` (default 47821)
 - `claude_bin` (path to claude CLI)
 
-Per-list config (`list_config` in DB) provides defaults for `model`, `system_prompt`, `agent_path`; tasks can override each individually.
+Per-list config (`list_config` in DB) provides defaults for `model`, `system_prompt`, `agent_path`; tasks can override each individually. Task-generating MCP tools (`AddTask`, planning `CreateChildTask`, `SuggestImprovement`) accept an optional `model` (alias-validated via `ModelRegistry.NormalizeAlias` — `haiku`/`sonnet`/`opus`, blank = inherit) so Claude assigns the cheapest capable model at creation time; the planning/system/improvement prompts instruct it to do so (`ModelRegistry.ByCostAscending` = the cost order).
 
 ## Notes
 

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -142,13 +142,18 @@ public sealed class ExternalMcpService
         return ToDto(task);
     }
 
-    [McpServerTool, Description("Create a new task in the given list. Set queueImmediately=true to enqueue it for agent execution.")]
+    [McpServerTool, Description(
+        "Create a new task in the given list. Set queueImmediately=true to enqueue it for agent execution. " +
+        "Set model to the cheapest model that can do the task well — 'haiku' for trivial/mechanical work, " +
+        "'sonnet' for normal coding (the default), 'opus' only for complex or cross-cutting work. " +
+        "Leave model null to inherit the list/global default.")]
     public async Task<TaskDto> AddTask(
         string listId,
         string title,
         string? description = null,
         string? createdBy = null,
         bool queueImmediately = false,
+        string? model = null,
         CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(listId))
@@ -169,6 +174,7 @@ public sealed class ExternalMcpService
             CreatedAt = DateTime.UtcNow,
             CommitType = list.DefaultCommitType,
             CreatedBy = createdBy.NullIfBlank() ?? "mcp",
+            Model = ModelRegistry.NormalizeAlias(model),
         };
         await _tasks.AddAsync(entity, cancellationToken);
 

src/ClaudeDo.Worker/Planning/PlanningMcpService.cs

@@ -37,15 +37,20 @@ public sealed class PlanningMcpService
     private Task BroadcastTaskUpdatedAsync(string taskId, CancellationToken ct)
         => _broadcaster.TaskUpdated(taskId);
 
-    [McpServerTool, Description("Create a new draft child task under the current planning session's parent task.")]
+    [McpServerTool, Description(
+        "Create a new draft child task under the current planning session's parent task. " +
+        "Set model to the cheapest model that can do this subtask well — 'haiku' for trivial/mechanical " +
+        "work, 'sonnet' for normal coding (the default), 'opus' only for complex or cross-cutting work. " +
+        "Leave model null to inherit the list/global default.")]
     public async Task<CreatedChildDto> CreateChildTask(
         string title,
         string? description,
         string? commitType,
+        string? model,
         CancellationToken cancellationToken)
     {
         var ctx = _contextAccessor.Current;
-        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, cancellationToken);
+        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, model: model, ct: cancellationToken);
         await BroadcastTaskUpdatedAsync(child.Id, cancellationToken);
         await BroadcastTaskUpdatedAsync(ctx.ParentTaskId, cancellationToken);
         return new CreatedChildDto(child.Id, child.Status.ToString());

src/ClaudeDo.Worker/Runner/TaskRunMcpService.cs

@@ -25,10 +25,13 @@ public sealed class TaskRunMcpService
         "File an out-of-scope improvement as a child task of the current task. The child runs " +
         "automatically after this task finishes and is surfaced for review alongside it. Use ONLY " +
         "for work that is genuinely outside this task's scope (a refactor, follow-up, or tech debt) " +
-        "— never for work that belongs to the current task.")]
+        "— never for work that belongs to the current task. Set model to the cheapest model that can " +
+        "do the follow-up well — 'haiku' for trivial/mechanical work, 'sonnet' for normal coding, " +
+        "'opus' only for complex work. Leave model null to inherit the list/global default.")]
     public async Task<SuggestedImprovementDto> SuggestImprovement(
         string title,
         string description,
+        string? model,
         CancellationToken cancellationToken)
     {
         var callerId = _ctx.Current.CallerTaskId;
@@ -39,7 +42,7 @@ public sealed class TaskRunMcpService
                 "A child task cannot suggest further improvements (improvements are one layer deep).");
 
         var child = await _tasks.CreateChildAsync(
-            callerId, title, description, commitType: null, createdBy: callerId, cancellationToken);
+            callerId, title, description, commitType: null, createdBy: callerId, model: model, ct: cancellationToken);
         await _broadcaster.TaskUpdated(child.Id);
         await _broadcaster.TaskUpdated(callerId);
         return new SuggestedImprovementDto(child.Id);