@langchain/openai: Chat Completions `file` content blocks are silently dropped when using the Responses API

Finding: In an external report, @langchain/openai 1.6.2 (with @langchain/core 1.2.14; also seen on main at 62d6d6ba0) silently drops Chat Completions-style user content blocks of the form {type:"file", file:{filename, file_data}} (or file_id) when ChatOpenAI uses the Responses API (useResponsesApi: true, or a model routed there automatically). Source: https://github.com/langchain-ai/langchainjs/issues/11838 Reported symptom: The reporter captured the outgoing request body through a custom fetch, so no API key or live call was needed. The Responses input contained only an input_text part, and the file part was missing. No error was thrown, so the model answers as if no file was attached. The reporter expected an input_file part carrying filename and file_data (or file_id). The reporter also says the same block works on the Chat Completions API. This is the reporter's statement and is not independently checked. Reporter's suspected cause (hypothesis, not maintainer-confirmed): in convertMessagesToResponsesInput (libs/providers/langchain-openai/src/converters/responses.ts), the Chat Completions image_url shape is converted to input_image, but there is no equivalent for the file shape. The block is not a LangChain standard data block because it has no source_type, so isDataContentBlock skips it and it reaches the final return [], where a code comment suggests warning about dropped blocks later. Status: At the time of the supplied data the issue was open. There was no maintainer response, only a non-maintainer comment on 2026-10-04 asking to be assigned. The reporter said they have a fix and tests ready, but no PR, merged commit or release was supplied. Do not assume a fix has shipped. Practical diagnosis (source-backed, not verified by KFA): if a Responses-routed ChatOpenAI call ignores an attached file, capture the request body as the reporter did and check whether the input content has an input_file part. If it is absent and the block uses the Chat Completions file shape, this issue may apply. The reporter distinguishes it from #11065 (standard URL file blocks), #11248 (standard image blocks) and #11460 (audio blocks). Limitations: This is an external report. KFA did not reproduce it. The reproduction used a mocked fetch. The comments coverage is partial. No fix or release evidence is available.