Summary

Since 2026.4.22 (warm stdio sessions for claude-cli, #69679), embedded agents using claude-cli provider intermittently fail with:

Embedded agent failed before reply: Claude CLI stdout buffer exceeded limit.

The error is non-recoverable for the current session (user has to /new) and surfaces to channel users as the generic ⚠️ Something went wrong while processing your request. Please try again, or use /new to start a fresh session.

Root cause: the live-session stdout reader has hardcoded character limits with no config knob.

Affected versions

• 2026.4.22, 2026.4.23, 2026.4.24 — confirmed by inspecting tarballs.
• 2026.4.21 and earlier — not affected (the live-session reader did not exist).

Repro

Any embedded claude-cli turn whose stdout produces a single chunk/line longer than 256 KB before a newline triggers the abort. In practice this fires when:

• A tool result returned to the CLI is large (web fetches, big file reads, screenshots-in-prompt, large sessions_history results).
• A long assistant message is streamed in a single chunk.

In my case it fired 3 times in 7 minutes (turns of 101s, 77s, 119s before abort), all with the same errorHash sha256:b2802736e694. Same hash repeated across days.

Evidence

From the gateway log:

2026-04-25T18:57:58 WARN model-fallback/decision  reason=format status=400
  errorPreview="Claude CLI stdout buffer exceeded limit."
  errorHash=sha256:b2802736e694
2026-04-25T18:57:58 ERROR Embedded agent failed before reply:
  Claude CLI stdout buffer exceeded limit.

Source (in 2026.4.24 dist):

// dist/execute.runtime-ix6AkkxA.js
const CLAUDE_LIVE_MAX_STDOUT_BUFFER_CHARS = 256 * 1024;   // 256 KB
const CLAUDE_LIVE_MAX_STDERR_CHARS         = 64  * 1024;
const CLAUDE_LIVE_MAX_TURN_RAW_CHARS       = 2   * 1024 * 1024;
const CLAUDE_LIVE_MAX_TURN_LINES           = 5000;

if (session.stdoutBuffer.length > CLAUDE_LIVE_MAX_STDOUT_BUFFER_CHARS) {
  closeLiveSession(session, "abort",
    createOutputLimitError(session, "Claude CLI stdout buffer exceeded limit."));
  return;
}

There is no path in openclaw.json to override these — they are module-level constants.

Why 256 KB is too low for the stdout buffer

The buffer holds bytes received between newlines. Claude CLI's JSONL stream emits one line per event, but tool_use/tool_result payloads (especially media, file contents, or large search results) can easily exceed 256 KB in a single line before the \n arrives. Hitting the limit aborts the entire turn even though the line is well-formed JSONL that would parse fine.

Suggested fix

1. Bump defaults — at minimum match CLAUDE_LIVE_MAX_TURN_RAW_CHARS (2 MB) for stdout, since a single JSONL line can legitimately approach turn-level size. Suggested:
   • STDOUT_BUFFER: 2 MB (or 4 MB)
   • STDERR: 256 KB
2. Make all four constants configurable via agents.cli.claudeLive.{stdoutBufferBytes,stderrBytes,turnRawBytes,turnLines} so operators can tune without forking dist.
3. Improve the channel-facing error: include the underlying reason (or a /diagnostics hint) instead of the generic "Something went wrong", so users don't have to dig through gateway logs.

Happy to send a PR if useful.

Workaround for users on 2026.4.22+

Downgrade to 2026.4.21 (no warm stdio sessions, no buffer) — confirmed clean. Trade-off: lose stdio session warm-up and other 4.22+ improvements.

Environment

• openclaw 2026.4.24 (ed28607)
• macOS 26.4.1 / arm64 / node v25.8.2
• claude-cli provider, claude-opus-4-7 (also reproduced on claude-sonnet-4-6)
• Telegram channel