feat(parallel): worker model override for parallel milestone workers

Add parallel.worker_model preference so coordinators can assign a
cheaper model to parallel workers (e.g. Haiku for execution) instead
of inheriting the coordinator's model. The override is applied via
GSD_WORKER_MODEL env var during worker bootstrap.

- Add worker_model to ParallelConfig type and validation
- Inject GSD_WORKER_MODEL env in spawnWorker when configured
- Apply override in bootstrapAutoSession for parallel workers
- Document in preferences-reference.md

src/resources/extensions/gsd/auto-start.ts

@@ -610,6 +610,25 @@ export async function bootstrapAutoSession(
       };
     }
 
+    // Apply worker model override from parallel orchestrator (#worker-model).
+    // GSD_WORKER_MODEL is injected by the coordinator when parallel.worker_model
+    // is configured, so parallel milestone workers use a cheaper model than the
+    // coordinator session (e.g. Haiku for execution, Sonnet for planning).
+    const workerModelOverride = process.env.GSD_WORKER_MODEL;
+    if (workerModelOverride && process.env.GSD_PARALLEL_WORKER === "1") {
+      const availableModels = ctx.modelRegistry.getAvailable();
+      const { resolveModelId } = await import("./auto-model-selection.js");
+      const overrideModel = resolveModelId(workerModelOverride, availableModels, ctx.model?.provider);
+      if (overrideModel) {
+        const ok = await pi.setModel(overrideModel, { persist: false });
+        if (ok) {
+          // Update start model so all subsequent units use this as the baseline
+          s.autoModeStartModel = { provider: overrideModel.provider, id: overrideModel.id };
+          ctx.ui.notify(`Worker model override: ${overrideModel.provider}/${overrideModel.id}`, "info");
+        }
+      }
+    }
+
     // Snapshot installed skills
     if (resolveSkillDiscoveryMode() !== "off") {
       snapshotSkills();

src/resources/extensions/gsd/docs/preferences-reference.md

@@ -204,6 +204,7 @@ Setting `prefer_skills: []` does **not** disable skill discovery — it just mea
   - `budget_ceiling`: number — optional per-parallel-run budget ceiling.
   - `merge_strategy`: `"per-slice"` or `"per-milestone"` — when to merge worktree results back. Default: `"per-milestone"`.
   - `auto_merge`: `"auto"`, `"confirm"`, or `"manual"` — merge behavior after completion. `"auto"` merges immediately; `"confirm"` asks first; `"manual"` leaves branches for you. Default: `"confirm"`.
+  - `worker_model`: string — optional model override for parallel milestone workers. When set, workers use this model (e.g. `"claude-haiku-4-5"`) instead of inheriting the coordinator's model. Useful for cost savings on execution-heavy milestones.
 
 - `verification_commands`: string[] — shell commands to run as verification after task execution (e.g., `["npm test", "npm run lint"]`). Commands run in order; if any fails, the task is marked as needing fixes.
 

src/resources/extensions/gsd/parallel-orchestrator.ts

@@ -548,19 +548,27 @@ export function spawnWorker(
 
   let child: ChildProcess;
   try {
+    const workerEnv: Record<string, string | undefined> = {
+      ...process.env,
+      GSD_MILESTONE_LOCK: milestoneId,
+      // Pass the real project root so workers don't need to re-derive it.
+      // Without this, process.cwd() resolves symlinks and the worktree
+      // path heuristic can match the user-level ~/.gsd instead of the
+      // project .gsd, causing writes to ~ and corrupting user config.
+      GSD_PROJECT_ROOT: basePath,
+      // Prevent workers from spawning their own parallel sessions
+      GSD_PARALLEL_WORKER: "1",
+    };
+
+    // Apply worker model override if configured, so workers use a cheaper
+    // model (e.g. Haiku) rather than inheriting the coordinator's model.
+    if (state.config.worker_model) {
+      workerEnv.GSD_WORKER_MODEL = state.config.worker_model;
+    }
+
     child = spawn(process.execPath, [binPath, "headless", "--json", "auto"], {
       cwd: worker.worktreePath,
-      env: {
-        ...process.env,
-        GSD_MILESTONE_LOCK: milestoneId,
-        // Pass the real project root so workers don't need to re-derive it.
-        // Without this, process.cwd() resolves symlinks and the worktree
-        // path heuristic can match the user-level ~/.gsd instead of the
-        // project .gsd, causing writes to ~ and corrupting user config.
-        GSD_PROJECT_ROOT: basePath,
-        // Prevent workers from spawning their own parallel sessions
-        GSD_PARALLEL_WORKER: "1",
-      },
+      env: workerEnv,
       stdio: ["ignore", "pipe", "pipe"],
       detached: false,
     });

src/resources/extensions/gsd/preferences-validation.ts

@@ -492,6 +492,14 @@ export function validatePreferences(preferences: GSDPreferences): {
       }
     }
 
+    if (p.worker_model !== undefined) {
+      if (typeof p.worker_model === "string" && p.worker_model.length > 0) {
+        parallel.worker_model = p.worker_model;
+      } else {
+        errors.push("parallel.worker_model must be a non-empty string");
+      }
+    }
+
     if (Object.keys(parallel).length > 0) {
       validated.parallel = parallel as unknown as import("./types.js").ParallelConfig;
     }

src/resources/extensions/gsd/preferences.ts

@@ -537,5 +537,6 @@ export function resolveParallelConfig(prefs: GSDPreferences | undefined): import
     budget_ceiling: prefs?.parallel?.budget_ceiling,
     merge_strategy: prefs?.parallel?.merge_strategy ?? "per-milestone",
     auto_merge: prefs?.parallel?.auto_merge ?? "confirm",
+    worker_model: prefs?.parallel?.worker_model,
   };
 }

src/resources/extensions/gsd/types.ts

@@ -450,6 +450,8 @@ export interface ParallelConfig {
   budget_ceiling?: number;
   merge_strategy: MergeStrategy;
   auto_merge: AutoMergeMode;
+  /** Optional model override for parallel milestone workers (e.g. "claude-haiku-4-5"). */
+  worker_model?: string;
 }
 
 // ─── Reactive Task Execution Types ───────────────────────────────────────