[BUG] Agent tool model parameter silently ignored in v2.1.69+ (regression from v2.1.68)

Description

The Agent tool's model parameter (e.g., "sonnet", "haiku") is silently ignored starting in v2.1.69. All subagents run on the parent session's model regardless of what is specified. The parameter still exists in the tool schema but has no effect.

Steps to reproduce

1. Start a Claude Code session on v2.1.69 or v2.1.70
2. Ask Claude to launch a subagent with "model": "sonnet"
3. Observe that the subagent runs on Opus (the parent model) instead of Sonnet

Expected behavior

The subagent should run on the specified model (Sonnet 4.6 in this case), as it did in v2.1.68.

Actual behavior

The model parameter is silently ignored. All subagents run on Opus 4.6 regardless of the specified model. No error or warning is produced.

Version comparison

• v2.1.68: "model": "sonnet" works correctly, subagent dispatches to Sonnet 4.6
• v2.1.69: Same parameter is silently ignored, subagent runs on Opus 4.6
• v2.1.70: Still broken

Environment

• Platform: Linux (WSL2)
• Plan: Claude Max

Related issues

• #18873 (model parameter returns 404 / blocks cost-optimized workflows)
• #16115 (subagents fail with 404 for model: haiku)
• #11682 (model parameter causes API 404)
• #9243 (model name resolution failure)

The model parameter appears to have been fixed at some point before v2.1.68, then re-broken in v2.1.69. This is likely a regression rather than an intentional removal, since the parameter was not removed from the tool schema and no changelog entry documents the change.