Bug Report: TypeError: Cannot read properties of undefined (reading 'length') at prompt assembly stage

To file: https://github.com/openclaw/openclaw/issues/new

Description

When @martian-engineering/lossless-claw v0.9.2 is enabled, all model calls fail at the prompt assembly stage with TypeError: Cannot read properties of undefined (reading 'length'). The error occurs before any provider API call is made — it's a crash in the core prompt builder triggered by an undefined field returned from lossless-claw's context assembly.

The same error hash (sha256:e980ad1b24fa) hits all models in the fallback chain, confirming it's not provider-specific.

Environment

• OpenClaw version: 2026.4.27 (built from go-ooa, commit 4eca5026e977)
• Node.js: 22.22.0
• OS: Linux 5.15.0-171-generic (x64)
• Lossless-claw version: 0.9.2 (installed via openclaw plugins install @martian-engineering/lossless-claw)
• Lossless-claw enabled: yes
• Channels affected: Telegram, WhatsApp

Logs

[agent/embedded] embedded_run_failover_decision
  stage: "prompt"
  decision: "surface_error"
  providerRuntimeFailureKind: "unclassified"
  rawErrorPreview: "Cannot read properties of undefined (reading 'length')"
  rawErrorHash: "sha256:e980ad1b24fa"

[diagnostic] lane task error:
  lane=main
  durationMs=6177
  error="TypeError: Cannot read properties of undefined (reading 'length')"

Crash source: subsystem-Izr-DYZz.js (core prompt assembly subsystem, not lossless-claw code).

Pattern

Immediately before every crash, lossless-claw's [lcm] assemble-debug log completes successfully, then the core prompt builder crashes. This suggests lossless-claw's context engine is returning a message structure where a field expected to be an array is undefined.

Fallback chain (all fail identically)

Workaround

1. openclaw doctor --fix (clears corrupt session state)
2. Disable lossless-claw in config: plugins.entries.lossless-claw.enabled = false
3. openclaw gateway restart

Related

Possible re-emergence of GitHub #66591 (reported fixed but may have regressed in 2026.4.27).