openclaw - 💡(How to fix) Fix Empty-text ContentBlock persisted to transcript permanently bricks a session (Bedrock validation error: messages.N.content.0 is blank) [1 participants]

Summary

An empty-text user ContentBlock can be persisted to a session transcript, permanently poisoning the session. Every subsequent LLM call fails with a Bedrock/Anthropic validation error and the session becomes unusable until the transcript is manually edited.

Validation error: The text field in the ContentBlock object at messages.N.content.0 is blank. Add text to the text field, and try again.

Environment

• OpenClaw version: 2026.4.24
• Model: amazon-bedrock/global.anthropic.claude-opus-4-7 (Bedrock Converse streaming)
• Channel: Discord group session
• OS: Linux 6.17.0-1010-aws (arm64), Node v22.22.0

Reproduction

1. Start a Discord group session where the main agent spawns background subagent tasks (ACP — in our case a claude / Claude Code child via sessions_spawn({ runtime: "acp", thread: true })).
2. Let one subagent task complete (internal "subagent task completed" system event is injected as a user message → handled with NO_REPLY).
3. Something (see "Likely source" below) causes the runtime to write a user message to the transcript with content:

{
  "role": "user",
  "content": [{ "type": "text", "text": "" }]
}

1. From this point on, every new turn — including a simple @Leo hi — fails immediately at provider-invoke time because Bedrock rejects the blank text block.

Evidence (redacted transcript around the poisoning event)

Session transcript: ~/.openclaw/agents/main/sessions/<sessionId>.jsonl

From line 152 onwards, every turn errors out with the messages.N.content.0 is blank validation error. OpenClaw created several .bak-<pid>-<epoch> backups of the transcript but did not self-heal.

Likely source

Based on timestamps, the poisoned record appears immediately after a subagent completion delivery and just before a cascade of subagent-run failures (Permission prompt unavailable in non-interactive mode). Suspect the empty block came from one of:

• A subagent task-completion / agent-to-agent announce event whose result payload serialized to an empty string.
• A Discord inbound envelope whose body was empty (e.g. attachment-only or autoscrubbed content).
• An internal system-event injection with an empty text field.

In any case, the ingestion layer does not validate text-block content before writing to the transcript.

Impact

• Session hard-fails until manual transcript surgery. The only recovery we found was to delete the offending line from the .jsonl and restart the turn. Users cannot realistically self-recover.
• High blast radius. Any surface that spawns subagents / handles system events can trip this.
• Silent. No actionable error surfaced to the user — only the cryptic Bedrock validation string on the next reply attempt.

Suggested fix

1. Guard at ingestion. When receiving any message (user, tool-result, system-event injection), drop or coerce empty text ContentBlocks before persistence:
   • Drop blocks where block.type === "text" && (block.text ?? "").trim().length === 0.
   • If that would leave content: [], either drop the whole message or substitute a single-block placeholder like "[empty message]".
2. Guard at send-time (defense-in-depth). In the Bedrock / Anthropic adapter that builds the provider request, filter out empty text blocks as a last-mile safety net so a legacy poisoned transcript cannot brick a session.
3. Self-heal on retry. When the provider returns this specific validation error, consider scrubbing the offending block in-memory (and optionally emitting a warning) so the user isn't stuck.

Workaround until fix lands

# backup, remove the offending line (adjust N), verify, restore in place
cp ~/.openclaw/agents/<agent>/sessions/<id>.jsonl{,.bak}
awk 'NR!=N' ~/.openclaw/agents/<agent>/sessions/<id>.jsonl.bak > ~/.openclaw/agents/<agent>/sessions/<id>.jsonl

To find N:

jq -c 'select(.type=="message") | {idx: input_line_number, role: .message.role, t0: .message.content[0].type, text_len: ((.message.content[0].text // "")|length)}' \
  ~/.openclaw/agents/<agent>/sessions/<id>.jsonl | grep '"text_len":0'