diff --git a/src/tool.ts b/src/tool.ts
index dd68f74..cf7dbce 100644
--- a/src/tool.ts
+++ b/src/tool.ts
@@ -54,11 +54,18 @@ function makeErrorResult(text: string, mode: "single" | "parallel" | "chain") {
   };
 }
 
+import type { ModelSelection } from "./models.ts";
+
 export function createSubagentTool(deps: {
   listAvailableModelReferences?: typeof listAvailableModelReferences;
   normalizeAvailableModelReference?: typeof normalizeAvailableModelReference;
   parameters?: typeof SubagentParamsSchema;
-  resolveChildModel?: typeof resolveChildModel;
+  // Compatibility: accept injected resolveChildModel functions with either the
+  // new API ({ callModel?, presetModel? }) or the older test/hooks API
+  // ({ taskModel?, topLevelModel? }). We adapt at callsite below.
+  resolveChildModel?:
+    | typeof resolveChildModel
+    | ((input: { taskModel?: string; topLevelModel?: string }) => ModelSelection);
   runSingleTask?: (input: {
     cwd: string;
     meta: Record<string, unknown>;
@@ -129,6 +136,23 @@ export function createSubagentTool(deps: {
         step.model = normalizedStepModel;
       }
 
+      const callResolveChildModel = (input: {
+        callModel?: string;
+        presetModel?: string;
+        taskModel?: string;
+        topLevelModel?: string;
+      }) => {
+        // If an injected resolveChildModel exists, call it with the older-shape
+        // keys (taskModel/topLevelModel) for compatibility. Otherwise, use the
+        // internal resolveChildModel which expects { callModel, presetModel }.
+        if (deps.resolveChildModel) {
+          const injected = deps.resolveChildModel as unknown as (arg: { taskModel?: string; topLevelModel?: string }) => unknown;
+          return injected({ taskModel: input.callModel ?? input.taskModel, topLevelModel: input.presetModel ?? input.topLevelModel }) as ModelSelection;
+        }
+
+        return resolveChildModel({ callModel: input.callModel ?? input.taskModel, presetModel: input.presetModel ?? input.topLevelModel });
+      };
+
       const runTask = async (input: {
         task: string;
         cwd?: string;
@@ -137,9 +161,7 @@ export function createSubagentTool(deps: {
         step?: number;
         mode: "single" | "parallel" | "chain";
       }) => {
-        const model = (deps.resolveChildModel ?? resolveChildModel)({
-          // compatibility: newer resolveChildModel expects { callModel, presetModel }
-          // older hooks/tests used { taskModel, topLevelModel }
+        const model = callResolveChildModel({
           callModel: input.taskModel,
           presetModel: params.model,
           taskModel: input.taskModel,