[Backend] errorHandler does not guard res.headersSent - throws after a partial/streamed response

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

error.middleware.ts:9-56 unconditionally calls res.status(...).json(...) in every branch with no if (res.headersSent) return next(err) check. If an error surfaces after headers were already written (e.g. the SSE subscribe path writes res.writeHead(200) then can throw), this throws a secondary error and can crash the request instead of delegating to Express's default finalizer.

Acceptance criteria

• Return next(err) immediately when res.headersSent is true
• Add a unit test passing a response with headersSent=true and asserting next(err) is called and res.status is not invoked

Files to touch

• backend/src/middleware/error.middleware.ts

Out of scope

• Reworking error response shapes

[charlesedeh021-cell]

@charlesedeh021-cell has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainers!

I’d love to help out with this issue. I’ve checked the current codebase/documentation and know exactly what needs to be updated to make this clearer. I will ensure everything aligns perfectly with your existing contribution guidelines.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @charlesedeh021-cell to this issue.

[drips-wave]

Congratulations, @charlesedeh021-cell! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @charlesedeh021-cell: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[grantfox-oss]

🦊 GrantFox — @charlesedeh021-cell has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #819)
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