fix(anthropic-sse): finalize stream on mid-stream socket close (never-hang)

[jsboige]

Problem

Anthropic-compatible providers that speak native Anthropic SSE (Z.AI, MiniMax, Kimi) intermittently close the TCP socket mid-stream under load / burst limits. When that happens, reader.read() in the passthrough loop rejects, the rejection escapes to the outer catch (e) block, which does a bare controller.close() with no terminal message_stop event.

The Claude Code client then reports:

API Error: The socket connection was closed unexpectedly. For more information, pass verbose: true in the second argument to fetch()

…and freezes the agent turn — a hung agent, the worst proxy failure mode.

Root cause

In anthropic-sse.ts, the read loop's outer catch can only raw-close the controller. There was no path to emit the terminal message_stop the client requires to end a turn, because a finalization helper was never in scope.

Fix

Two changes, both additive:

1. finalizeWithError(errMsg, path) helper — declared inside the outer try, above the read loop. Emits a valid terminal Claude message so the client ends the turn cleanly:

   • no content reached the client yet → synthetic message_start + a content block carrying a short notice, so the closing events are well-formed
   • content already streaming → message_delta(end_turn) + message_stop (the client tolerates a missing content_block_stop on early termination far better than a missing message_stop, which hangs the turn)

2. Inner try/catch around the read loop — a mid-stream reader rejection is now caught where finalizeWithError is in scope and routed through it, instead of escaping to the bare outer close. The outer catch is preserved as a last-resort safety net.

Whatever streamed before the cutoff is preserved — no content loss.

Why this matters

A frozen agent is far worse than a clean error. With providers that drop connections under load (observed in production with Z.AI serving GLM models), this turns a transient network blip into a permanently stuck session that the user must manually reset. The fix degrades it to a clean end-of-turn.

Test

Regression test added to format-translation.test.ts:

never-hang: upstream socket close mid-stream still emits terminal message_stop

Replays an upstream body that emits message_start + a text delta, then errors (simulating socket close). Asserts:

• the partial content survives (not lost to the cutoff)
• a terminal message_stop is present
• a synthesized stop_reason: end_turn

Fails without the fix (no message_stop reaches the client), passes with it.

Scope

• 2 files, +148 lines, additions only — no behavior change on the happy path or the normal end-of-stream path
• Touched file: packages/cli/src/handlers/shared/stream-parsers/anthropic-sse.ts
• No dependencies on other PRs

🤖 Generated with Claude Code