Summary

Fix Bedrock messages.N is empty validation rejection by extending the assemble pass's empty-content filter to cover user and toolResult roles, not only assistant.

The pre-existing cleanedEntries filter at src/assembler.ts only guarded the assistant role. An empty content array produced upstream for a user or toolResult message during incremental compaction could survive the cleaned-tail pass and reach Bedrock Converse, which rejects it with the literal wording:

Validation error: The content field in the Message object at messages.0 is empty. Add a ContentBlock object to the content field and try again.

A direct probe against eu.anthropic.claude-haiku-4-5-20251001-v1:0 via ConverseCommand confirms this exact wording is reproducible only when content === [] (empty array); bare strings produce a different SerializationException error, so the production wording is specific to the empty-array shape.

The asymmetry between the assistant guard and the missing user/toolResult guard explains why the v0.9.3 prefill safety net (#572) and the v0.9.4 blank-text guard did not fully close the validation surface.

Production reproduction signal

Observed in a long-running OpenClaw session against eu.anthropic.claude-opus-4-6-v1 via bedrock-converse-stream. The failed turn was recorded as:

{
  "type": "message",
  "timestamp": "2026-05-01T07:50:52.195Z",
  "message": {
    "role": "assistant",
    "content": [{"type":"text","text":"[assistant turn failed before producing content]"}],
    "api": "bedrock-converse-stream",
    "provider": "amazon-bedrock",
    "model": "eu.anthropic.claude-opus-4-6-v1",
    "stopReason": "error",
    "errorMessage": "Validation error: The content field in the Message object at messages.0 is empty. Add a ContentBlock object to the content field and try again."
  }
}

Timeline (the LCM conversations row for this session at the time of failure):

The ≤1 s delta between summary commit and failure points to a transient mid-compaction state rather than a persisted shape — replaying the same conversation's stored lcm.db state today through the assemble pipeline produces zero empty-array messages, so the empty array is produced and consumed within the compaction window.

What changed

1. New helper isEmptyMessageContent(message) in src/assembler.ts (exported with @internal JSDoc, matching the existing toolCallBlockFromPart / toolResultBlockFromPart / blockFromPart / contentFromParts convention). It returns true when content is:

   • undefined or null
   • empty string "" or whitespace-only string
   • empty array [] for any role
   • thinking-only or blank-text-only content for the assistant role specifically (preserves the existing v0.9.3 / v0.9.4 guards)

2. Replace the cleanedEntries filter to use the new helper. The filter now drops empty content for any role.

3. Extended comment on the filter explains the new compaction-window failure mode and notes that dropping a toolResult is safe because sanitizeToolUseResultPairing runs immediately afterwards and re-pairs missing results with the existing synthetic [lossless-claw] missing tool result … placeholder.

4. Regression test test/assembler-empty-content.test.ts (28 cases):

   • universal empty handling across user/assistant/toolResult
   • non-empty preservation across the same three roles
   • assistant-only thinking-only / blank-text guards remain in place
   • the exact production failure shape (empty-array user and toolResult)

5. Changeset (patch bump per AGENTS.md guidance for fixes / compatibility work).

Risk and compatibility

• The helper preserves all pre-existing behaviour for the assistant role (thinking-only and blank-text-only guards still fire).
• New filtering is strictly additive — empty content arrays for user/toolResult were always rejected by the downstream provider; we now drop them earlier rather than producing an unrecoverable rejection.
• Downstream sanitizeToolUseResultPairing already handles missing tool results via synthetic placeholders, so a dropped empty toolResult is recoverable.
• No public API change.

Test plan

• npm test — 879 / 886 tests pass on Windows. The 7 failures are pre-existing Windows-specific path/HOME issues (backslashes vs forward slashes, HOME env handling) unrelated to this change. Will run green on CI's ubuntu-latest.
• npm test -- test/assembler-empty-content.test.ts test/assembler-blocks.test.ts test/transcript-repair.test.ts — 75 / 75 green.
• npm run build — bundle builds cleanly (688.4 KB).
• CI green (will run on push).

Related

• v0.9.3 hotfix [hotfix] v0.9.3 prefill safety net + Codex/Ollama silent redirects (A/8) #572 (prefill safety net + blank-text guard) — sibling fix, addresses the text field … is blank wording for assistant content
• The pre-existing isThinkingOnlyContent / isBlankContent helpers (lines 97–124) — the new isEmptyMessageContent helper builds on them and keeps their assistant-specific scope intact

Happy to iterate on naming, helper placement, or the changeset wording — and to extend the filter to additional empty shapes if maintainers know of other transient post-compaction states that produce them.

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