feat(server): SSE heartbeat on streaming path (#47) (#49)

* docs(spec): design for #47 SSE heartbeat on streaming path

Draft spec for an opt-in idle-watchdog SSE heartbeat covering both
pre-first-byte and mid-stream silent windows. Default disabled,
controlled by CLAUDE_HEARTBEAT_INTERVAL. Targets ~40 LOC.

Decisions captured: D1 whole-stream reset-on-byte; D2 SSE comment
frame; D3 default disabled; D4 relocate ensureHeaders() to post-spawn;
D5 X-Accel-Buffering: no on both SSE header sites; D6 single log line
per affected request.

Scope-locked: does not touch CLAUDE_TIMEOUT semantics, the separate
server.mjs:480-489 dangling-client bug, issues #41/#42, or the
non-streaming path.

Includes privacy preflight and cloud-testing plan per maintainer
feedback.

Refs: #47

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(plan): implementation plan for #47 SSE heartbeat

6 phases: pre-work (file 480-bug), implementation (5 tasks on
feat/47-sse-heartbeat), opus fresh-context review, cloud verification
on Mac rig, privacy preflight, PR+release.

Each implementation task carries concrete code, syntax check, and a
scoped commit message. LOC budget enforced in Task 1.6 gate (~45
server.mjs lines max). Reviewer checklist scopes scope-lock, ALIGNMENT,
privacy, and heartbeat-cannot-abort discipline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(server): add startHeartbeat helper + HEARTBEAT_INTERVAL env var

Per design doc (refs #47). Helper is a per-request idle watchdog that
emits `: keepalive\n\n` SSE comment frames; returns a {reset, stop} handle.
No wiring yet — helper is unused, safe to commit in isolation.

cli.js citation: N/A — SSE response shaping is an OCP-owned translation
layer, not a cli.js operation. See AGENTS.md and ALIGNMENT.md Rule 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* refactor(server): eagerly send SSE headers post-spawn (D4, refs #47)

Moves the ensureHeaders() call from "on first stdout byte" to
"immediately after successful spawn." This is a prerequisite for the
heartbeat covering the pre-first-byte silent window (the 'processing
large contexts' failure mode in #47).

Behavioral consequence: the narrow "spawn succeeded but subprocess died
before any byte" branch at server.mjs:610-611 becomes effectively dead
in the common case. The post-headers SSE-stop path (612-619) handles
it instead. The branch remains defensively for the client-closed-before-
ensureHeaders race.

Isolated commit per design doc §D4 so reviewer can focus on this one
behavior change.

cli.js citation: N/A — SSE header emission is OCP response-shaping.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(server): wire heartbeat into streaming path (refs #47)

- sendSSE() accepts optional hb handle and calls hb.reset() before write
- callClaudeStreaming starts heartbeat after ensureHeaders() and passes
  hb to the three streaming sendSSE call sites
- All three exit paths (proc close, proc error, res close) call
  hb.stop() to guarantee timer cleanup; no-op handle when disabled
  means zero runtime cost when CLAUDE_HEARTBEAT_INTERVAL=0

Heartbeat never aborts — only writes comment frames and re-arms. Aligns
with v3.3 timeout discipline (single CLAUDE_TIMEOUT, no secondary
client-killing timers).

cli.js citation: N/A — SSE response shaping is OCP translation layer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(server): add X-Accel-Buffering: no to SSE response headers (refs #47)

nginx (and many LBs / Cloudflare) default to proxy_buffering=on, which
would buffer heartbeat comment frames indefinitely and defeat the
feature silently. This header hints no-buffering; other stacks ignore
it. Applied at both SSE header sites (real streaming + cache-hit).

cli.js citation: N/A — response header shaping is OCP translation layer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore(release): v3.12.0 — streaming heartbeat (refs #47)

Bundles the release-kit companion files per Iron Rule 5.2 / 11 example:
version bump across package.json + ocp-plugin + openclaw.plugin.json,
CHANGELOG v3.12.0 section, README env var row + "Streaming heartbeat"
explainer.

Tag push to v3.12.0 triggers .github/workflows/release.yml to create
the GitHub Release automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(server): ensureHeaders returns true when already sent (refs #47)

Phase 3 smoke test revealed every content chunk was being dropped
after the D4 eager ensureHeaders() call: the stdout.on('data') guard
`if (!ensureHeaders()) return;` early-returned on every chunk because
ensureHeaders returned false for the already-sent case (conflated with
the dead-connection case).

Split the two conditions explicitly: return false only when res is
ended/destroyed; return true when headers are (already or now) sent.
This also fixes a latent multi-chunk bug on main that was masked
because claude CLI typically outputs in one stdout chunk.

Verified: node -c server.mjs; subsequent re-run of Phase 3 smoke test
now sees streaming content chunks + heartbeat frames.

cli.js citation: N/A — SSE response shaping is OCP translation layer.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,22 @@
 # Changelog
 
+## v3.12.0 (2026-04-25)
+
+### Features
+
+- **Streaming heartbeat** — opt-in SSE comment frame (`: keepalive\n\n`) emitted during silent windows on the streaming response. Controlled by `CLAUDE_HEARTBEAT_INTERVAL` env var (ms; `0` = disabled, default). Covers both pre-first-byte and mid-stream tool-use pauses. Addresses #47. See [design doc](docs/superpowers/specs/2026-04-25-47-sse-heartbeat-design.md).
+- **`X-Accel-Buffering: no`** response header added to SSE responses so heartbeats survive nginx/Cloudflare default buffering.
+
+### Behavior changes
+
+- SSE headers are now sent immediately after the claude CLI spawns successfully, not on first stdout byte. The rare "spawn succeeded but subprocess died before any byte" path now closes the SSE stream cleanly rather than returning a JSON error.
+
+### Config additions
+
+| Variable | Default | Description |
+|---|---|---|
+| `CLAUDE_HEARTBEAT_INTERVAL` | `0` (disabled) | Interval in ms for SSE keepalive comment frames on streaming path. Resets on every real frame. |
+
 ## v3.11.1 — 2026-04-21
 
 ### Fixes