ChatOpenAI drops streamed tool calls when the server sends `role: ""`

[Sparky579]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangChain.js documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain.js rather than my code.

Example Code

import { ChatOpenAI } from "@langchain/openai";

const chunk = (delta, finish = null) =>
  `data: ${JSON.stringify({
    id: "c",
    object: "chat.completion.chunk",
    created: 1,
    model: "m",
    choices: [{ index: 0, delta, finish_reason: finish }],
  })}\n\n`;

// A server that sends role: "" instead of "assistant".
const sse =
  chunk({
    role: "",
    tool_calls: [
      {
        index: 0,
        id: "call_1",
        type: "function",
        function: { name: "get_weather", arguments: '{"city":"BJ"}' },
      },
    ],
  }) +
  chunk({}, "tool_calls") +
  "data: [DONE]\n\n";

const model = new ChatOpenAI({
  model: "m",
  apiKey: "x",
  configuration: {
    baseURL: "https://example.invalid/v1",
    fetch: async () =>
      new Response(sse, {
        status: 200,
        headers: { "content-type": "text/event-stream" },
      }),
  },
});

for await (const c of await model.stream("hi")) {
  console.log(c.tool_call_chunks);
}

Prints undefined for every chunk. Changing role: "" to role: "assistant" in the same script prints the expected tool_call_chunk.

Error Message and Stack Trace (if applicable)

No error is raised. The tool call is dropped silently.

Description

ChatOpenAI loses streamed tool calls when a server sends role: "" instead of "assistant". convertCompletionsDeltaToBaseMessageChunk resolves the role with delta.role ?? defaultRole, and ?? only falls back on null/undefined, so an empty string reaches the final branch and the delta is converted to a ChatMessageChunk rather than an AIMessageChunk. Any tool_calls on that delta are discarded and tool_call_chunks is undefined. Plain text still streams normally, which makes this easy to miss: the response looks healthy and only tool calls disappear.

Empty delta.role is something OpenAI-compatible servers do in practice — anomalyco/opencode#28427 reports the same responses from the consumer side. #11157 fixes the closely related case where role is omitted entirely, but keeps ??, so with that PR applied the empty-string variant still fails. Coalescing with || instead covers both, and explicit "tool" / "system" / "user" roles keep working since those are truthy.

System Info

@langchain/openai 1.5.8
@langchain/core 1.2.9
node v20.19.5
linux x64