openclaw - 💡(How to fix) Fix Plugin runtime: prependSystemContext via before_prompt_build causes occasional empty model outputs [1 comments, 2 participants]

Bug Report

Severity: High — causes silent session corruption

Summary: Using prependSystemContext via the before_prompt_build hook causes occasional empty model outputs (stop=stop, content=[]). The issue was reproduced twice in the same day after the hook was enabled.

Affected versions: Confirmed on current production build (Apr 2026)

Reproduction:

1. Create a plugin that uses the before_prompt_build hook to set prependSystemContext
2. Run any prompt (tested with both anthropic/claude-haiku-4-5 and anthropic/claude-sonnet-4-6)
3. Approximately 1 in 5–10 runs returns stop_reason=stop with empty content=[]

Key detail: This is NOT a payload size issue. Tested with a minimal 220-character injection and still reproduced. The issue is an architectural interaction between the plugin hook and the model runtime pipeline.

Evidence:

• Session f8ee6b28 at approximately 19:07 CDT and 18:05 CDT on 2026-04-23
• Both haiku-4-5 and sonnet-4-6 affected
• Manifest in gateway.err.log as: incomplete turn detected: ... stopReason=stop payloads=0

Impact: When empty outputs occur, OpenClaw writes them into the session transcript, seeding LCM with structurally malformed entries. Combined with delivery-mirror (see related bug), this creates a corruption chain that renders the session unrecoverable without /new.

Workaround: Disable the plugin entirely. No config-level mitigation exists.

Request: Fix the root interaction between prependSystemContext and the model runtime, or provide a safe pre-prompt injection path that doesn't trigger empty outputs.