Handling leading whitespace in SSE data for Stream Generate Content

[hegdeashwin]

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Environment details

• Programming language: JavaScript / TypeScript / Node.js
• OS: MacOS
• Language runtime version:
• Package version: ^1.19.0

Steps to reproduce

Note: I see possibility to occur in enterprise in case we have some custom handling of SSE event and data in between GCP and our service like gateway.

1. If the stream response includes leading whitespace — for example:

'\n\ndata: {"candidates": [{"content": {"role": "model","parts": [{"text": "Sheldon"}]}}],"usageMetadata": {"trafficType": "ON_DEMAND"},"modelVersion": "gemini-2.0-flash","createTime": "2025-09-14T13:18:29.993482Z","responseId": "pcDGaMrRPIKrnvgP-_WisAs"}\n\n'

— then the condition in this section of the js-genai client evaluates to null, which leads to the error: Incomplete JSON segment at the end.

The regex here:

js-genai/src/_api_client.ts

Line 28 in 3e850fd

const responseLineRE = /^data: (.*)(?:\n\n|\r\r|\r\n\r\n)/;

seems to be the problem.

Allowing leading whitespace like following is fixing the stream generate content issue:

const responseLineRE = /^\s*data: (.*)(?:\n\n|\r\r|\r\n\r\n)/;

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!

[shmishra99]

Hi @hegdeashwin,

Thanks for bringing this to our attention. To help us investigate the issue, could you please provide a code example that reproduces the "Incomplete JSON segment at the end" error?

[hegdeashwin]

@shmishra99 Thanks for responding back. You can refer to the sample in the js-genai test case by adding escape characters before data: like for example: (LF) \n\ndata: or (CR) \r\rdata: or (SSE Event Delimiter) \r\n\r\ndata: etc.

I’ve also raised a PR to address the issue: #967 - Regex + Test Case.

Let me know how can I help further