Bug Report

OpenClaw Version: 2026.3.2 (inferred from recent upgrade)
Session ID: 2d882ee3-8503-4685-8eff-1cffc1c5835c
Provider/Model: anthropic/claude-sonnet-4-6
Date: 2026-03-06 ~12:28 EST

Summary

A long-running EPIC-002 Hive build session entered a permanent, unrecoverable error loop after being aborted mid-generation. Every subsequent request to the session returned HTTP 400 from the Anthropic API with the same error about an orphaned tool_use ID. The session had to be force-reset (.reset file created at 2026-03-06T13:06:47Z). All other concurrent sessions were unaffected.

Error Message

400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.259: `tool_use` ids were found without `tool_result` blocks immediately after: exec1772800125283396. Each `tool_use` block must have a corresponding `tool_result` block in the next message."}}

Root Cause (Reconstructed from Session JSONL)

The session JSONL (2d882ee3-8503-4685-8eff-1cffc1c5835c.jsonl) contained 291 entries. Timeline of failure:

The abort at entry 268 resulted in an empty content array being persisted to the JSONL. When OpenClaw reconstructs the session context for the Anthropic API, the serialized history creates a position (messages.259 in API context, 0-indexed) where exec1772800125283396 appears as a tool_use block without an immediately following tool_result block.

Notably, entry 260 already contained text:[openclaw] missing tool result in session history, indicating OpenClaw's placeholder injection mechanism was active — but it did not catch this second abort, or the placeholder was applied to an earlier orphan while the 12:28:51 abort created a new one.

Impact

• Session rendered permanently unrecoverable (100% 400 error rate on every request)
• User unable to resume active work context; required manual force-reset
• All EPIC-002 in-progress session state lost
• Non-trivial to diagnose/recover without digging into raw JSONL

Reproduction Steps

1. Start a long-running session with heavy tool calls (exec/process in particular)
2. Abort/interrupt the session mid-stream (connection drop, TUI disconnect) while an exec toolCall is the last item in the assistant's streaming buffer
3. Session is persisted with an empty content array [] for the aborted assistant message
4. Send any new message — Anthropic returns 400 on all future requests to the session
5. Note: the [openclaw] missing tool result placeholder may already have fired for a prior orphan; this new abort creates a second orphan that is not repaired

Expected Behavior

When an abort produces an empty assistant content array ([]) after a tool_use entry:

• OpenClaw's context reconstruction should detect the preceding tool_use has no matching tool_result
• Inject a synthetic error result: {"type":"tool_result","tool_use_id":"...","content":"[session interrupted]","is_error":true}
• Session should remain fully operational after an abort, not brick

Suggested Fix

In the session context serializer (the code that reconstructs message history for the Anthropic API):

1. After reconstruction, walk the serialized message array
2. For every assistant message containing a tool_use block, verify the immediately following message contains a tool_result with the matching tool_use_id
3. If not, inject a synthetic error tool_result before the next user message
4. Ensure this validation runs on every API context reconstruction, not only on explicit missing tool result events
5. Consider also: do not persist empty-content assistant messages (content=[]) to JSONL on abort — or flag them so the reconstructor knows to handle them specially

Workaround

Force-reset the session via the TUI or by renaming/deleting the session JSONL. This loses all session context.

Environment

• OS: macOS Darwin 25.3.0 (arm64)
• Node: v25.6.1
• Provider: Anthropic (claude-sonnet-4-6)
• Session length at failure: 279 message entries, 268 before error loop
• Concurrent sessions: unaffected (confirms session-scoped, not global corruption)

Filed by Cypher (OpenClaw Brain) on behalf of Bryan (Principal Systems Architect)