Best possible solution:

Land the heartbeat prompt split once maintainers approve the runtime fallback policy change, or split the fallback removal into a separate policy PR and land the narrower heartbeat fix first.

Do we have a high-confidence way to reproduce the issue?

Yes, by source inspection. Current main combines the default HEARTBEAT_OK heartbeat prompt with appended heartbeat_respond guidance, and Codex heartbeat turns force the structured tool.

Is this the best way to solve the issue?

Unclear as a product decision. The heartbeat prompt split is the narrow fix, but retiring the public fallback config/env surface is a broader compatibility change that should be explicitly accepted by maintainers.

What I checked:

• Current main mixes heartbeat contracts: The default heartbeat prompt still asks quiet turns to reply HEARTBEAT_OK, while the runner appends heartbeat_respond tool guidance before dispatching the heartbeat turn. (src/auto-reply/heartbeat.ts:14, dda2cf4e73b3)
• Codex heartbeat turns force the structured tool: The Codex app-server runtime enables and forces the heartbeat response tool when the trigger is heartbeat, making the mixed text/tool prompt observable for Codex heartbeat turns on current main. (extensions/codex/src/app-server/run-attempt.ts:1485, dda2cf4e73b3)
• PR head uses the tool-mode heartbeat prompt: At the PR head, resolveHeartbeatPromptForResponseTool creates a heartbeat_respond/notify=false prompt without the default HEARTBEAT_OK sentinel, and resolveHeartbeatRunPrompt selects it for message-tool/Codex heartbeat paths. (src/auto-reply/heartbeat.ts:17, ca20f880f6aa)
• Runtime fallback removal is an intentional branch behavior: The PR head changes missing explicit plugin harness selection to throw immediately, removes fallback from the runtime zod schema, and adds doctor migration logic to remove legacy agentRuntime.fallback. (src/agents/harness/selection.ts:114, ca20f880f6aa)
• Protected maintainer label and clean PR state: GitHub's pull request API reports the PR is open, non-draft, mergeable clean, and labeled maintainer; exact-head check runs for ca20f880f6aa413dff66c5e7b4f596b5a7cfb758 are completed successfully aside from expected skipped/neutral jobs. (ca20f880f6aa)

Likely related people:

• pashpashpash: Local blame and GitHub path history tie current heartbeat prompt/tool and explicit harness-failure behavior to recent merged commits by this handle, including structured heartbeat responses and explicit harness failure work. (role: introduced relevant heartbeat/runtime behavior and PR author with prior merged history; confidence: high; commits: 439d8edf68e2, 11804a484ded; files: src/infra/heartbeat-runner.ts, src/auto-reply/heartbeat.ts, src/agents/harness/selection.ts)
• steipete: GitHub path history shows recent commits by this handle in heartbeat runner scheduling/routing, agent runtime config migration, and Codex harness selection. (role: recent maintainer for heartbeat, runtime config, and Codex harness areas; confidence: medium; commits: 5b9be2cdb1c0, 47c0ce5f8531; files: src/infra/heartbeat-runner.ts, src/agents/harness/selection.ts, extensions/codex/harness.ts)
• vincentkoc: Recent runtime-selection history includes provider/runtime normalization work by this handle, and the PR timeline explicitly requested review from this handle. (role: adjacent runtime selection maintainer; confidence: medium; commits: aa27e27f3606; files: src/agents/harness/selection.ts)

Remaining risk / open question:

• The PR removes the public agentRuntime.fallback and OPENCLAW_AGENT_HARNESS_FALLBACK compatibility surface; that is intentional in the discussion but still needs maintainer approval because existing configs may rely on it.

Codex review notes: model gpt-5.5, reasoning high; reviewed against dda2cf4e73b3.