Summary

The delegate_task tool was silently ignoring the per-task model override field, causing all child subagents to always use the parent's model regardless of per-task specification.

Root Cause

In _build_child_agent() calls within delegate_task(), the model parameter always resolved to creds["model"] (the global delegation model), ignoring any per-task model field. Other per-task fields like role and toolsets correctly used the t.get(...) or global_default pattern.

Fix

Apply the same fallback pattern used for role and toolsets:

# Before (broken):
model=creds["model"],

# After (fixed):
model=t.get("model") or creds["model"],

This allows each task dict in a batch delegation to optionally specify its own model while preserving the default behavior.

Verification

• python3 -m py_compile tools/delegate_tool.py passes
• 29 existing subagent/delegate tests pass (tests/agent/test_subagent_*.py, tests/cli/test_cli_interrupt_subagent.py)

Closes #17685