Sibling Tool Call Errored Cascades from One Failed Tool in a Batch
Rule
When a tool in a parallel batch reports "Sibling tool call errored", find the real failing call first — then retry only that one, not all calls in the batch.
Context
When making multiple parallel tool calls in a single message. If one fails, other calls in the same batch may cascade-fail with "Sibling tool call errored" even though they would have succeeded independently.
Detection
Observable signals:
<tool_use_error>Sibling tool call errored</tool_use_error>in a tool result- One or more other tool calls in the batch have actual errors
- The "sibling error" tool has no error of its own — it's collateral damage
Pattern
# Scenario: parallel batch where one call fails
Read("file-a.py") # ← this one fails (file doesn't exist)
Read("file-b.py") # ← reports "Sibling tool call errored" (was fine on its own)
# Fix: identify which call actually failed, retry that one only
# Don't blindly retry all calls — the sibling ones were likely fine
# Prevention: for risky operations, verify first before parallelizing
Read("uncertain-file.py") # verify existence alone first
# Then parallelize the safe ones:
Read("file-b.py")
Read("file-c.py")
Outcome
- Faster root-cause identification (look for the real error, not the cascade)
- Avoids unnecessary retries of calls that were never broken
- Better batch strategy: group calls with similar failure risk
Related
- Shell Command Chaining - Parallel tool call patterns