Move note about which tools you have access to into instructions prompt

Author: jahooma

packages/agent-runtime/src/__tests__/prompt-caching-subagents.test.ts

@@ -472,16 +472,16 @@ describe('Prompt Caching for Subagents with inheritParentSystemPrompt', () => {
       parentSystemPrompt,
     )
 
-    // Verify there's a message about subagent tools
-    const subagentToolsMessage = childMessages.find(
+    // Verify there's an instructions prompt message that includes subagent tools info
+    const instructionsMessage = childMessages.find(
       (msg) =>
         msg.role === 'user' &&
         msg.content[0].type === 'text' &&
         msg.content[0].text.includes('subagent') &&
         msg.content[0].text.includes('read_files') &&
         msg.content[0].text.includes('code_search'),
     )
-    expect(subagentToolsMessage).toBeTruthy()
+    expect(instructionsMessage).toBeTruthy()
   })
 
   it('should support both inheritParentSystemPrompt and includeMessageHistory together', async () => {

packages/agent-runtime/src/run-agent-step.ts

@@ -594,12 +594,17 @@ export async function loopAgentSteps(
   agentState.runId = runId
 
   let cachedAdditionalToolDefinitions: CustomToolDefinitions | undefined
+  // Use parent's tools for prompt caching when inheritParentSystemPrompt is true
+  const useParentTools =
+    agentTemplate.inheritParentSystemPrompt && parentTools !== undefined
+
   // Initialize message history with user prompt and instructions on first iteration
   const instructionsPrompt = await getAgentPrompt({
     ...params,
     agentTemplate,
     promptType: { type: 'instructionsPrompt' },
     agentTemplates: localAgentTemplates,
+    useParentTools,
     additionalToolDefinitions: async () => {
       if (!cachedAdditionalToolDefinitions) {
         cachedAdditionalToolDefinitions = await additionalToolDefinitions({
@@ -634,10 +639,6 @@ export async function loopAgentSteps(
           },
         })) ?? ''
 
-  // Use parent's tools for prompt caching when inheritParentSystemPrompt is true
-  const useParentTools =
-    agentTemplate.inheritParentSystemPrompt && parentTools !== undefined
-
   const tools = useParentTools
     ? parentTools
     : await getToolSet({
@@ -653,26 +654,13 @@ export async function loopAgentSteps(
         },
       })
 
-  // Build a message explaining the subagent's tool access when using parent's tools
-  const subagentToolsMessage = useParentTools
-    ? `You are a subagent that only has access to the following tools: ${agentTemplate.toolNames.join(', ')}. Do not attempt to use any other tools.`
-    : undefined
-
   const hasUserMessage = Boolean(
     prompt || (spawnParams && Object.keys(spawnParams).length > 0),
   )
 
   const initialMessages = buildArray<Message>(
     ...agentState.messageHistory,
 
-    // Add subagent tools message before user prompt when using parent's tools for caching
-    subagentToolsMessage &&
-      userMessage({
-        content: withSystemTags(subagentToolsMessage),
-        tags: ['SUBAGENT_TOOLS'],
-        keepDuringTruncation: true,
-      }),
-
     hasUserMessage && [
       {
         // Actual user message!

packages/agent-runtime/src/templates/strings.ts

@@ -158,6 +158,7 @@ export async function getAgentPrompt<T extends StringField>(
     agentTemplates: Record<string, AgentTemplate>
     additionalToolDefinitions: () => Promise<CustomToolDefinitions>
     logger: Logger
+    useParentTools?: boolean
   } & ParamsExcluding<
     typeof formatPrompt,
     'prompt' | 'tools' | 'spawnableAgents'
@@ -173,6 +174,7 @@ export async function getAgentPrompt<T extends StringField>(
     agentState,
     agentTemplates,
     additionalToolDefinitions,
+    useParentTools,
   } = params
 
   let promptValue = agentTemplate[promptType.type]
@@ -198,6 +200,12 @@ export async function getAgentPrompt<T extends StringField>(
 
   // Add tool instructions, spawnable agents, and output schema prompts to instructionsPrompt
   if (promptType.type === 'instructionsPrompt' && agentState.agentType) {
+    // Add subagent tools message when using parent's tools for prompt caching
+    if (useParentTools) {
+      addendum +=
+        `\n\nYou are a subagent that only has access to the following tools: ${agentTemplate.toolNames.join(', ')}. Do not attempt to use any other tools.`
+    }
+
     addendum +=
       '\n\n' +
       (await buildSpawnableAgentsDescription({