fix(opencode): connection drops when background subagent dispatched

[zmlgit]

Problem

When cc-connect talks to official opencode serve and the LLM dispatches a background subagent (task(background=true)), the connection drops — the user sees no response to the background completion.

Symptom reported: "现在官方的cc-connect和官方的opencode配合使用时，一旦派发后台任务就断了".

Root cause

agent/opencode/session_http.go had two load-bearing failure modes:

1. Blocking POST /session/{id}/message

The POST held the connection open for the entire agent turn. When the LLM dispatched a background subagent, the parent turn ended immediately (the tool returns "Background task started" → parent loop goes idle), so the blocking POST returned. cc-connect then treated the turn as done — but the actual work (background completion, parent-session injection) was still pending.

2. Fatal GET /event SSE EOF

Any /event SSE stream termination (server restart, network blip, keepalive expiry) marked the session dead:

s.alive.Store(false)
s.emitHTTPEvent(core.Event{Type: core.EventError, Error: ...})

The engine received the error → tore down the session → cancelled s.ctx → killed the in-flight turn. No reconnect logic existed.

Fix — align with opencode's official streaming pattern

The official TUI client (packages/opencode/src/cli/cmd/run/stream.transport.ts) uses four rules. This PR adopts the two cc-connect was missing:

#	Official pattern	This PR
1	One long-lived SSE subscription for the whole session	readHTTPEventsLoop reconnects with exponential backoff (500ms → 30s cap) on stream EOF; only exits when session ctx is cancelled
2	Fire-and-forget prompt POSTs	sendHTTP switched from POST /message (blocking) to POST /prompt_async (204 No Content)
3	session.status: idle events as turn-completion signal	Already in place — unchanged
4	Track child sessions	Analyzed and intentionally omitted — opencode injects background completion into the parent sessionID, so the existing filter is correct (see TestOpencodeHTTPMode_StreamsBeforePromptReturnsAndKeepsBackgroundEvents). Child session events are intermediate noise the user doesn't need.

Server compatibility

Verified against opencode serve v1.18.14 (current release):

• POST /session/{id}/prompt_async ✓ exists, returns 204 fire-and-forget
• GET /event ✓ exists, real SSE global stream
• /api/session/{id}/event?after= ✗ dev-branch only, not on this version (so no cursor/replay; reconnect is blind but sufficient because the SSE stays open across background runs)

Verification

Unit tests

=== RUN   TestOpencodeHTTPMode_SSEReconnectsAfterEOF
2026/08/06 15:20:42 INFO opencode: /event stream ended, reconnecting backoff=500ms
--- PASS: TestOpencodeHTTPMode_SSEReconnectsAfterEOF (0.00s)

ok  github.com/chenhg5/cc-connect/agent/opencode  8.828s  # all 6 HTTP-mode tests pass, race-clean
ok  github.com/chenhg5/cc-connect/core            3.014s  # CUJ tests pass

Live end-to-end against opencode serve v1.18.14

TestLivePromptAsync:
  session id: ses_02a0a1c81ffe4wAkMHMC1f6ebc
  Send returned in 3.370805ms (prompt_async 204 ack)   # OLD: would block ~38s
  event: type="text" content="PONG"
  event: type="result" content=""
  PASS (37.78s — full turn duration, all via SSE)

TestLiveSSEReconnectAfterServerRestart:
  INFO opencode: /event stream ended, reconnecting backoff=500ms
  PASS: SSE reconnected after forced EOF (killCount=2)

Test changes

• NEW TestOpencodeHTTPMode_SSEReconnectsAfterEOF — regression for the central fix
• NEW TestLivePromptAsync, TestLiveSSEReconnectAfterServerRestart — env-gated live tests
• UPDATED TestOpencodeHTTPMode_StreamsBeforePromptReturnsAndKeepsBackgroundEvents — adapted for prompt_async (Send returns immediately; no more releasePrompt synchronization)
• RENAMED TestOpencodeHTTPMode_SendReturnsSSEErrorWhenMessageEndpointFails → ...PromptAsyncFails — endpoint follow rename

Commit-by-commit

1. 23036102 — fix opencode background streaming over HTTP (baseline)
2. 5145e134 — fix opencode HTTP agent error reporting (baseline)
3. 9787dab0 — fix opencode HTTP lint errors (baseline)
4. 7204f7d0 — surface opencode HTTP retry limit errors (baseline)
5. 5e14fe5e — reset agent session after idle timeout (baseline)
6. 3f081e26 — split HTTP client (60s API) from streamClient (no-timeout SSE)
7. 165743b9 — core fix: prompt_async + SSE auto-reconnect

Out of scope (YAGNI)

• dev-branch ?after=<seq> cursor replay — server v1.18.14 doesn't expose it
• Child session event forwarding — opencode injects background results into the parent session; existing filter is correct
• CLI subprocess mode — only HTTP/SSE mode touches this issue

Pre-merge checklist

• Build passes: go build ./...
• Tests pass: go test ./agent/opencode/ -race
• CUJ tests pass: go test ./core/ -run TestCUJ
• Bug fix has regression test: TestOpencodeHTTPMode_SSEReconnectsAfterEOF
• No new hardcoded platform/agent names in core
• No new user-facing strings
• No secrets in code