diff --git a/src/services/langfuse/__tests__/langfuse.test.ts b/src/services/langfuse/__tests__/langfuse.test.ts
index dab33fd60..6c5a87947 100644
--- a/src/services/langfuse/__tests__/langfuse.test.ts
+++ b/src/services/langfuse/__tests__/langfuse.test.ts
@@ -231,6 +231,7 @@ describe('Langfuse integration', () => {
 
     test('merges assistant tool calls from OpenAI-style array content', async () => {
       const { convertMessagesToLangfuse } = await import('../convert.js')
+      // Content part with embedded tool_calls is non-standard; cast for defensive test
       const result = convertMessagesToLangfuse([
         {
           role: 'assistant',
@@ -255,7 +256,7 @@ describe('Langfuse integration', () => {
             },
           ],
         },
-      ])
+      ] as any)
 
       expect(result).toEqual([
         {
diff --git a/src/services/langfuse/convert.ts b/src/services/langfuse/convert.ts
index ad324c2a0..411692bce 100644
--- a/src/services/langfuse/convert.ts
+++ b/src/services/langfuse/convert.ts
@@ -10,7 +10,8 @@
  *   - tool_result blocks → separate { role: 'tool' } messages
  */
 
-import type { AssistantMessage } from 'src/types/message.js'
+import type { AssistantMessage, UserMessage } from 'src/types/message.js'
+import type { ChatCompletionMessageParam } from 'openai/resources/chat/completions/completions.mjs'
 
 type LangfuseContentPart =
   | { type: 'text'; text: string }
@@ -79,6 +80,12 @@ function mergeToolCalls(
   return [...merged.values()]
 }
 
+/** Union of all message formats accepted by Langfuse converters. */
+type LangfuseInputMessage =
+  | UserMessage
+  | AssistantMessage
+  | ChatCompletionMessageParam
+
 /** Normalize a content block into a LangfuseContentPart (non-tool_use, non-tool_result) */
 function toContentPart(block: Record<string, unknown>): LangfuseContentPart | null {
   const type = block.type as string | undefined
@@ -178,7 +185,7 @@ function toRoleFromWrappedMessage(msg: Record<string, unknown>): 'user' | 'assis
 
 /** Convert internal or OpenAI-style messages → Langfuse input format */
 export function convertMessagesToLangfuse(
-  messages: readonly unknown[],
+  messages: readonly LangfuseInputMessage[],
   systemPrompt?: readonly string[],
 ): LangfuseChatMessage[] {
   const result: LangfuseChatMessage[] = []