diff --git a/packages/agentlayer-core/src/tools/subagent-fork.ts b/packages/agentlayer-core/src/tools/subagent-fork.ts
index 0c0f5b3..c56cb22 100644
--- a/packages/agentlayer-core/src/tools/subagent-fork.ts
+++ b/packages/agentlayer-core/src/tools/subagent-fork.ts
@@ -16,13 +16,24 @@ function invokesToolCall(message: ModelMessage, toolCallId: string): boolean {
 	)
 }
 
+function isInjectedSkillMessage(message: ModelMessage): boolean {
+	if (message.role !== 'user' || typeof message.content !== 'string') return false
+	const content = message.content.trim()
+	return /^<skill(?:\s|>)/.test(content) && content.endsWith('</skill>')
+}
+
+function isUserTurnBoundary(message: ModelMessage): boolean {
+	return message.role === 'user' && !isInjectedSkillMessage(message)
+}
+
 function removeInvokingTurn(messages: ReadonlyArray<ModelMessage>, toolCallId: string): ModelMessage[] {
 	const invokingAssistantIndex = messages.findIndex((message) => invokesToolCall(message, toolCallId))
 	if (invokingAssistantIndex < 0) return [...messages]
 
 	let triggeringUserIndex = -1
 	for (let index = invokingAssistantIndex - 1; index >= 0; index--) {
-		if (messages[index]?.role === 'user') {
+		const message = messages[index]
+		if (message && isUserTurnBoundary(message)) {
 			triggeringUserIndex = index
 			break
 		}
@@ -46,7 +57,8 @@ function isEligibleForkMessage(message: ModelMessage): boolean {
 function selectRecentTurns(messages: ModelMessage[], turns: number): ModelMessage[] {
 	let userTurns = 0
 	for (let index = messages.length - 1; index >= 0; index--) {
-		if (messages[index]?.role !== 'user') continue
+		const message = messages[index]
+		if (!message || !isUserTurnBoundary(message)) continue
 		userTurns += 1
 		if (userTurns === turns) return messages.slice(index)
 	}
diff --git a/packages/agentlayer-core/test/subagent-tool.test.ts b/packages/agentlayer-core/test/subagent-tool.test.ts
index 0627c13..105ab8a 100644
--- a/packages/agentlayer-core/test/subagent-tool.test.ts
+++ b/packages/agentlayer-core/test/subagent-tool.test.ts
@@ -823,6 +823,33 @@ describe('forking subagent tool', () => {
 		expect(projectForkMessages(messages, 'none', 'spawn-call')).toEqual([])
 	})
 
+	test('does not treat an injected skill message as the invoking user turn boundary', () => {
+		const messages: ModelMessage[] = [
+			{ role: 'user', content: 'old request' },
+			{ role: 'assistant', content: [{ type: 'text', text: 'old answer' }] },
+			{ role: 'user', content: 'actual triggering request' },
+			{
+				role: 'user',
+				content: '<skill name="investigate" baseDir="/skills/investigate">\nInvestigate carefully.\n</skill>',
+			},
+			{
+				role: 'assistant',
+				content: [
+					{ type: 'tool-call', toolCallId: 'spawn-call', toolName: 'subagent', input: { prompt: 'work' } },
+				],
+			},
+		]
+
+		expect(projectForkMessages(messages, 'all', 'spawn-call')).toEqual([
+			{ role: 'user', content: 'old request' },
+			{ role: 'assistant', content: [{ type: 'text', text: 'old answer' }] },
+		])
+		expect(projectForkMessages(messages, 1, 'spawn-call')).toEqual([
+			{ role: 'user', content: 'old request' },
+			{ role: 'assistant', content: [{ type: 'text', text: 'old answer' }] },
+		])
+	})
+
 	test('constructs isolated fork state without caller tool, pending, child, or context state', () => {
 		const caller: AgentState = {
 			messages: [{ role: 'user', content: { nested: ['original'] } } as any],