In messageHistory, add a message to introduce a spawned agent

Author: jahooma

.agents/context-pruner.ts

@@ -172,6 +172,12 @@ const definition: AgentDefinition = {
     if (lastInstructionsPromptIndex !== -1) {
       currentMessages.splice(lastInstructionsPromptIndex, 1)
     }
+    const lastSubagentSpawnIndex = currentMessages.findLastIndex((message) =>
+      message.tags?.includes('SUBAGENT_SPAWN'),
+    )
+    if (lastSubagentSpawnIndex !== -1) {
+      currentMessages.splice(lastSubagentSpawnIndex, 1)
+    }
 
     // Initial check - if already under limit, return
     const initialTokens = countMessagesTokens(currentMessages)

packages/agent-runtime/src/tools/handlers/tool/spawn-agent-utils.ts

@@ -4,7 +4,7 @@ import { generateCompactId } from '@codebuff/common/util/string'
 
 import { loopAgentSteps } from '../../../run-agent-step'
 import { getAgentTemplate } from '../../../templates/agent-registry'
-import { filterUnfinishedToolCalls } from '../../../util/messages'
+import { filterUnfinishedToolCalls, withSystemTags } from '../../../util/messages'
 
 import type { AgentTemplate } from '@codebuff/common/types/agent-template'
 import type { Logger } from '@codebuff/common/types/contracts/logger'
@@ -19,6 +19,7 @@ import type {
   AgentTemplateType,
   Subgoal,
 } from '@codebuff/common/types/session-state'
+import { Message } from '@codebuff/common/types/messages/codebuff-message'
 
 /**
  * Checks if a parent agent is allowed to spawn a child agent
@@ -166,9 +167,21 @@ export function createAgentState(
   // When including message history, filter out any tool calls that don't have
   // corresponding tool responses. This prevents the spawned agent from seeing
   // unfinished tool calls which throw errors in the Anthropic API.
-  const messageHistory = agentTemplate.includeMessageHistory
-    ? filterUnfinishedToolCalls(parentAgentState.messageHistory)
-    : []
+  let messageHistory: Message[] = []
+
+  if (agentTemplate.includeMessageHistory) {
+    messageHistory = filterUnfinishedToolCalls(parentAgentState.messageHistory)
+    messageHistory.push({
+      role: 'user',
+      content: [
+        {
+          type: 'text',
+          text: withSystemTags(`Subagent ${agentType} has been spawned.`),
+        },
+      ],
+      tags: ['SUBAGENT_SPAWN'],
+    })
+  }
 
   return {
     agentId,