feat(tasks): add tasks-agent-proxy-keep-stream-open flag

[charlesvien]

Problem

Task-run events stream to the agent-proxy as one long-lived NDJSON upload. In @posthog/code the sender was changed to optionally close that upload per drained batch (companion PR PostHog/code#3067) so a buffering ingress forwards each batch within a round-trip instead of stranding events in a long-lived upload. We want to roll that out gradually rather than flip it for every run, so it needs a server-evaluated feature flag.

Changes

Introduces the tasks-agent-proxy-keep-stream-open feature flag and threads its evaluated value to the sandbox agent-server, mirroring the existing tasks-cloud-runs-sandbox-event-ingest wiring:

• Add the flag constant and _is_agent_proxy_keep_stream_open_enabled(...), evaluated once in get_task_processing_context and captured on TaskProcessingContext.agent_proxy_keep_stream_open so it stays deterministic across activity retries.
• Pass it through _prepare_launch -> sandbox.start_agent_server -> build_agent_runtime_env_prefix, which sets POSTHOG_TASK_RUN_EVENT_INGEST_KEEP_STREAM_OPEN=true only when enabled (unset otherwise).
• The agent-server reads that env var and keeps the proxy ingest stream open when set (companion PR feat(agent): add keep-proxy-stream-open ingest flag code#3067).

Off by default: no env var is emitted unless the flag resolves true, so existing runs are unaffected.

How did you test this code?

Automated (ran locally, all green):

• pytest -k keep_stream_open across test_docker_sandbox.py, test_modal_sandbox.py, test_get_task_processing_context.py -> 9 passed.
• ruff check + ruff format --check on all changed files -> clean.
• Pre-commit hook ran ruff lint/format + ty type check -> clean.

I did not run the full tasks Temporal suite or exercise a live sandbox.

The docker/modal tests catch a regression where the flag stops reaching the sandbox launch command; the evaluator tests catch the flag failing open on a flag-service error or ignoring the run-state override.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

No user-facing docs; internal rollout flag.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Tool: Claude Code (Opus), directed by @charlesvien to put the @posthog/code per-batch proxy stream-close behavior behind a real PostHog feature flag, off by default.
• Approach: mirrored the established tasks-cloud-runs-sandbox-event-ingest mechanism (flag constant -> context evaluator -> TaskProcessingContext field -> _prepare_launch -> sandbox env prefix) so the flag evaluates server-side and reaches the sandbox as an env var, rather than inventing a new path.
• Polarity chosen deliberately: flag on = keep the proxy stream open (buffered); default off = today's per-batch close, so enabling is the opt-in.

[github-actions]

Purely additive feature-flag-gated change: new parameter defaults to False throughout, env var set via shlex.quote (no injection risk), fail-closed flag check, and well-tested with parametrized coverage. No behavioral change to existing code paths.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "wire tasks-agent-proxy-keep-stream-open ..." | Re-trigger Greptile}

New commits pushed (delta classified non_linear_history) — stamphog approval dismissed; re-review running automatically.

[github-actions]

Purely additive feature-flag-gated change: new parameter defaults to False throughout, env var set via shlex.quote (no injection risk), fail-closed flag check, and well-tested with parameterized coverage. No behavioral change to existing code paths. The greptile comments about parameterized tests were addressed and marked resolved.

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 1 · PR risk: 0/10

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-07-01 19:19 UTC	Run
prod-us	✅ Deployed	2026-07-01 19:35 UTC	Run
prod-eu	✅ Deployed	2026-07-01 19:37 UTC	Run