GitHub Copilot streamed tool-call arguments are obfuscated, stalling agentic turns in Codex clients

[Simon-Opopeee]

Summary

GitHub Copilot's /responses wire is not protocol-compatible with Responses clients for the vscode-chat integration:

• Streamed response.function_call_arguments.delta events carry ciphertext delta plus an obfuscation field.
• Reasoning items ship as encrypted_content (GitHub's own scheme) with no plaintext summary; clients cannot decrypt them and stay stuck on "thinking".
• The response id and every item id are re-encrypted per event, so the terminal response.completed payload disagrees with the streamed events (call_id is the only stable field).

Impact

Any Responses-only Copilot model (gpt-5.6-luna, gpt-5.6-sol, gpt-5.6-terra, gpt-5.4, gpt-5.4-mini, gpt-5.5, gpt-5.3-codex — see #748) stalls agentic turns in Codex clients: reasoning never finishes and tool calls are unreadable. Plain text works.

Fix

Provider-scoped SSE rewrite for github-copilot (PR #990): drop ciphertext deltas and re-emit plaintext arguments before done, neutralize encrypted reasoning items with an empty plaintext summary, and pin response/item ids to first-seen values.

[github-actions]

_{Automated translation bookkeeping — detected language: unknown.}

[Simon-Opopeee]

Fix implemented and opened: #990

[github-actions]

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