Fix model fallback across main/background/sync agents

src/tools/delegate-task/background-task.ts

@@ -1,5 +1,6 @@
 import type { DelegateTaskArgs, ToolContextWithMetadata } from "./types"
 import type { ExecutorContext, ParentContext } from "./executor-types"
+import type { FallbackEntry } from "../../shared/model-requirements"
 import { getTimingConfig } from "./timing"
 import { storeToolMetadata } from "../../features/tool-metadata-store"
 import { formatDetailedError } from "./error-formatting"
@@ -12,7 +13,8 @@ export async function executeBackgroundTask(
   parentContext: ParentContext,
   agentToUse: string,
   categoryModel: { providerID: string; modelID: string; variant?: string } | undefined,
-  systemContent: string | undefined
+  systemContent: string | undefined,
+  fallbackChain?: FallbackEntry[],
 ): Promise<string> {
   const { manager } = executorCtx
 
@@ -27,6 +29,7 @@ export async function executeBackgroundTask(
       parentAgent: parentContext.agent,
       parentTools: getSessionTools(parentContext.sessionID),
       model: categoryModel,
+      fallbackChain,
       skills: args.load_skills.length > 0 ? args.load_skills : undefined,
       skillContent: systemContent,
       category: args.category,

src/tools/delegate-task/category-resolver.ts

@@ -1,6 +1,7 @@
 import type { ModelFallbackInfo } from "../../features/task-toast-manager/types"
 import type { DelegateTaskArgs } from "./types"
 import type { ExecutorContext } from "./executor-types"
+import type { FallbackEntry } from "../../shared/model-requirements"
 import { mergeCategories } from "../../shared/merge-categories"
 import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { resolveCategoryConfig } from "./categories"
@@ -16,6 +17,7 @@ export interface CategoryResolutionResult {
   modelInfo: ModelFallbackInfo | undefined
   actualModel: string | undefined
   isUnstableAgent: boolean
+  fallbackChain?: FallbackEntry[]  // For runtime retry on model errors
   error?: string
 }
 
@@ -177,5 +179,6 @@ Available categories: ${categoryNames.join(", ")}`,
     modelInfo,
     actualModel,
     isUnstableAgent,
+    fallbackChain: requirement?.fallbackChain,
   }
 }

src/tools/delegate-task/subagent-resolver.ts

@@ -8,6 +8,7 @@ import { getAgentDisplayName, getAgentConfigKey } from "../../shared/agent-displ
 import { normalizeSDKResponse } from "../../shared"
 import { log } from "../../shared/logger"
 import { getAvailableModelsForDelegateTask } from "./available-models"
+import type { FallbackEntry } from "../../shared/model-requirements"
 import { resolveModelForDelegateTask } from "./model-selection"
 
 export async function resolveSubagentExecution(
@@ -15,7 +16,7 @@ export async function resolveSubagentExecution(
   executorCtx: ExecutorContext,
   parentAgent: string | undefined,
   categoryExamples: string
-): Promise<{ agentToUse: string; categoryModel: { providerID: string; modelID: string; variant?: string } | undefined; error?: string }> {
+): Promise<{ agentToUse: string; categoryModel: { providerID: string; modelID: string; variant?: string } | undefined; fallbackChain?: FallbackEntry[]; error?: string }> {
   const { client, agentOverrides } = executorCtx
 
   if (!args.subagent_type?.trim()) {
@@ -46,6 +47,7 @@ Create the work plan directly - that's your job as the planning agent.`,
 
   let agentToUse = agentName
   let categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
+  let fallbackChain: FallbackEntry[] | undefined = undefined
 
   try {
     const agentsResult = await client.app.agents()
@@ -92,6 +94,7 @@ Create the work plan directly - that's your job as the planning agent.`,
     const agentOverride = agentOverrides?.[agentConfigKey as keyof typeof agentOverrides]
       ?? (agentOverrides ? Object.entries(agentOverrides).find(([key]) => key.toLowerCase() === agentConfigKey)?.[1] : undefined)
     const agentRequirement = AGENT_MODEL_REQUIREMENTS[agentConfigKey]
+    fallbackChain = agentRequirement?.fallbackChain
 
     if (agentOverride?.model || agentRequirement || matchedAgent.model) {
       const availableModels = await getAvailableModelsForDelegateTask(client)
@@ -135,5 +138,5 @@ Create the work plan directly - that's your job as the planning agent.`,
     }
   }
 
-  return { agentToUse, categoryModel }
+  return { agentToUse, categoryModel, fallbackChain }
 }

src/tools/delegate-task/sync-task.ts

@@ -3,7 +3,7 @@ import type { DelegateTaskArgs, ToolContextWithMetadata } from "./types"
 import type { ExecutorContext, ParentContext } from "./executor-types"
 import { getTaskToastManager } from "../../features/task-toast-manager"
 import { storeToolMetadata } from "../../features/tool-metadata-store"
-import { subagentSessions } from "../../features/claude-code-session-state"
+import { subagentSessions, syncSubagentSessions, setSessionAgent } from "../../features/claude-code-session-state"
 import { log } from "../../shared/logger"
 import { formatDuration } from "./time-formatter"
 import { formatDetailedError } from "./error-formatting"
@@ -40,6 +40,8 @@ export async function executeSyncTask(
     const sessionID = createSessionResult.sessionID
     syncSessionID = sessionID
     subagentSessions.add(sessionID)
+    syncSubagentSessions.add(sessionID)
+    setSessionAgent(sessionID, agentToUse)
 
     if (onSyncSessionCreated) {
       log("[task] Invoking onSyncSessionCreated callback", { sessionID, parentID: parentContext.sessionID })
@@ -59,6 +61,7 @@ export async function executeSyncTask(
     if (toastManager) {
       toastManager.addTask({
         id: taskId,
+        sessionID,
         description: args.description,
         agent: agentToUse,
         isBackground: false,
@@ -145,6 +148,7 @@ session_id: ${sessionID}
   } finally {
     if (syncSessionID) {
       subagentSessions.delete(syncSessionID)
+      syncSubagentSessions.delete(syncSessionID)
     }
   }
 }

src/tools/delegate-task/tools.ts

@@ -164,6 +164,7 @@ Prompts MUST be in English.`
       let modelInfo: import("../../features/task-toast-manager/types").ModelFallbackInfo | undefined
       let actualModel: string | undefined
       let isUnstableAgent = false
+      let fallbackChain: import("../../shared/model-requirements").FallbackEntry[] | undefined
 
       if (args.category) {
         const resolution = await resolveCategoryExecution(args, options, inheritedModel, systemDefaultModel)
@@ -176,6 +177,7 @@ Prompts MUST be in English.`
         modelInfo = resolution.modelInfo
         actualModel = resolution.actualModel
         isUnstableAgent = resolution.isUnstableAgent
+        fallbackChain = resolution.fallbackChain
 
         const isRunInBackgroundExplicitlyFalse = args.run_in_background === false || args.run_in_background === "false" as unknown as boolean
 
@@ -206,6 +208,7 @@ Prompts MUST be in English.`
         }
         agentToUse = resolution.agentToUse
         categoryModel = resolution.categoryModel
+        fallbackChain = resolution.fallbackChain
       }
 
       const systemContent = buildSystemContent({
@@ -217,7 +220,7 @@ Prompts MUST be in English.`
       })
 
       if (runInBackground) {
-        return executeBackgroundTask(args, ctx, options, parentContext, agentToUse, categoryModel, systemContent)
+        return executeBackgroundTask(args, ctx, options, parentContext, agentToUse, categoryModel, systemContent, fallbackChain)
       }
 
       return executeSyncTask(args, ctx, options, parentContext, agentToUse, categoryModel, systemContent, modelInfo)