diff --git a/js/ai/src/generate.ts b/js/ai/src/generate.ts
index b7b6289d4..f656ea222 100755
--- a/js/ai/src/generate.ts
+++ b/js/ai/src/generate.ts
@@ -37,7 +37,6 @@ import {
   ModelMiddleware,
   ModelReference,
   Part,
-  ToolDefinition,
 } from './model.js';
 import { ExecutablePrompt } from './prompt.js';
 import { resolveTools, ToolArgument, toToolDefinition } from './tool.js';
@@ -135,20 +134,25 @@ async function resolveModel(
   options: GenerateOptions
 ): Promise<ResolvedModel> {
   let model = options.model;
+  let out: ResolvedModel;
+  let modelId: string;
+
   if (!model) {
-    throw new Error('Model is required.');
+    throw new GenkitError({
+      status: 'INVALID_ARGUMENT',
+      message: 'Must supply a `model` to `generate()` calls.',
+    });
   }
   if (typeof model === 'string') {
-    return {
-      modelAction: (await registry.lookupAction(
-        `/model/${model}`
-      )) as ModelAction,
-    };
+    modelId = model;
+    out = { modelAction: await registry.lookupAction(`/model/${model}`) };
   } else if (model.hasOwnProperty('__action')) {
-    return { modelAction: model as ModelAction };
+    modelId = (model as ModelAction).__action.name;
+    out = { modelAction: model as ModelAction };
   } else {
     const ref = model as ModelReference<any>;
-    return {
+    modelId = ref.name;
+    out = {
       modelAction: (await registry.lookupAction(
         `/model/${ref.name}`
       )) as ModelAction,
@@ -158,6 +162,15 @@ async function resolveModel(
       version: ref.version,
     };
   }
+
+  if (!out.modelAction) {
+    throw new GenkitError({
+      status: 'NOT_FOUND',
+      message: `Model ${modelId} not found`,
+    });
+  }
+
+  return out;
 }
 
 export class GenerationResponseError extends GenkitError {
@@ -180,6 +193,59 @@ export class GenerationResponseError extends GenkitError {
   }
 }
 
+async function toolsToActionRefs(
+  registry: Registry,
+  toolOpt?: ToolArgument[]
+): Promise<string[] | undefined> {
+  if (!toolOpt) return;
+
+  let tools: string[] = [];
+
+  for (const t of toolOpt) {
+    if (typeof t === 'string') {
+      tools.push(await resolveFullToolName(registry, t));
+    } else if ((t as Action).__action) {
+      tools.push(
+        `/${(t as Action).__action.metadata?.type}/${(t as Action).__action.name}`
+      );
+    } else if (typeof (t as ExecutablePrompt).asTool === 'function') {
+      const promptToolAction = (t as ExecutablePrompt).asTool();
+      tools.push(`/prompt/${promptToolAction.__action.name}`);
+    } else if (t.name) {
+      tools.push(await resolveFullToolName(registry, t.name));
+    } else {
+      throw new Error(`Unable to determine type of tool: ${JSON.stringify(t)}`);
+    }
+  }
+  return tools;
+}
+
+function messagesFromOptions(options: GenerateOptions): MessageData[] {
+  const messages: MessageData[] = [];
+  if (options.system) {
+    messages.push({
+      role: 'system',
+      content: Message.parseContent(options.system),
+    });
+  }
+  if (options.messages) {
+    messages.push(...options.messages);
+  }
+  if (options.prompt) {
+    messages.push({
+      role: 'user',
+      content: Message.parseContent(options.prompt),
+    });
+  }
+  if (messages.length === 0) {
+    throw new GenkitError({
+      status: 'INVALID_ARGUMENT',
+      message: 'at least one message is required in generate request',
+    });
+  }
+  return messages;
+}
+
 /** A GenerationBlockedError is thrown when a generation is blocked. */
 export class GenerationBlockedError extends GenerationResponseError {}
 
@@ -206,66 +272,13 @@ export async function generate<
   const resolvedOptions: GenerateOptions<O, CustomOptions> =
     await Promise.resolve(options);
   const resolvedModel = await resolveModel(registry, resolvedOptions);
-  const model = resolvedModel.modelAction;
-  if (!model) {
-    let modelId: string;
-    if (typeof resolvedOptions.model === 'string') {
-      modelId = resolvedOptions.model;
-    } else if ((resolvedOptions.model as ModelAction)?.__action?.name) {
-      modelId = (resolvedOptions.model as ModelAction).__action.name;
-    } else {
-      modelId = (resolvedOptions.model as ModelReference<any>).name;
-    }
-    throw new Error(`Model ${modelId} not found`);
-  }
-
-  // convert tools to action refs (strings).
-  let tools: (string | ToolDefinition)[] | undefined;
-  if (resolvedOptions.tools) {
-    tools = [];
-    for (const t of resolvedOptions.tools) {
-      if (typeof t === 'string') {
-        tools.push(await resolveFullToolName(registry, t));
-      } else if ((t as Action).__action) {
-        tools.push(
-          `/${(t as Action).__action.metadata?.type}/${(t as Action).__action.name}`
-        );
-      } else if (typeof (t as ExecutablePrompt).asTool === 'function') {
-        const promptToolAction = (t as ExecutablePrompt).asTool();
-        tools.push(`/prompt/${promptToolAction.__action.name}`);
-      } else if (t.name) {
-        tools.push(await resolveFullToolName(registry, t.name));
-      } else {
-        throw new Error(
-          `Unable to determine type of of tool: ${JSON.stringify(t)}`
-        );
-      }
-    }
-  }
 
-  const messages: MessageData[] = [];
-  if (resolvedOptions.system) {
-    messages.push({
-      role: 'system',
-      content: Message.parseContent(resolvedOptions.system),
-    });
-  }
-  if (resolvedOptions.messages) {
-    messages.push(...resolvedOptions.messages);
-  }
-  if (resolvedOptions.prompt) {
-    messages.push({
-      role: 'user',
-      content: Message.parseContent(resolvedOptions.prompt),
-    });
-  }
+  const tools = await toolsToActionRefs(registry, resolvedOptions.tools);
 
-  if (messages.length === 0) {
-    throw new Error('at least one message is required in generate request');
-  }
+  const messages: MessageData[] = messagesFromOptions(resolvedOptions);
 
   const params: z.infer<typeof GenerateUtilParamSchema> = {
-    model: model.__action.name,
+    model: resolvedModel.modelAction.__action.name,
     docs: resolvedOptions.docs,
     messages,
     tools,