diff --git a/packages/llm/src/protocols/anthropic-messages.ts b/packages/llm/src/protocols/anthropic-messages.ts
index 060e2e091a..234ccd5baf 100644
--- a/packages/llm/src/protocols/anthropic-messages.ts
+++ b/packages/llm/src/protocols/anthropic-messages.ts
@@ -335,7 +335,9 @@ const lowerToolResultContent = Effect.fn("AnthropicMessages.lowerToolResultConte
   // Text / json / error results stay as a string for backward compatibility
   // with existing cassettes and provider expectations.
   if (part.result.type !== "content") return ProviderShared.toolResultText(part)
-  return yield* Effect.forEach(part.result.value, lowerToolResultContentItem)
+  // Preserve the narrowed array element type when compiled through a consumer package.
+  const content: ReadonlyArray<ToolResultContentPart> = part.result.value
+  return yield* Effect.forEach(content, lowerToolResultContentItem)
 })
 
 const lowerMessages = Effect.fn("AnthropicMessages.lowerMessages")(function* (