Subagent model resolution strips [1m] context window suffix

Summary

When spawning subagents (via the Agent tool or agent teams), the [1m] context window suffix is stripped during model alias resolution. Subagents always get the default ~200k context window, even when the parent session runs with 1M context.

Reproduction

1. Set "model": "opus[1m]" in ~/.claude/settings.json
2. Parent session correctly runs as claude-opus-4-6[1m] with 1M context
3. Spawn a subagent via the Agent tool (any type — custom agent, general-purpose, etc.)
4. The subagent's system prompt shows claude-opus-4-6 (no [1m]) and no context window size is mentioned

Evidence

The subprocess command that spawns the subagent passes:

--model claude-opus-4-6

instead of:

--model claude-opus-4-6[1m]

What we tested

| Configuration | Result |
|---|---|
| Parent opus[1m], no agent override | Subagent gets claude-opus-4-6 (no 1M) |
| Agent frontmatter model: opus[1m] | Subagent gets claude-opus-4-6 (no 1M) |
| Agent tool model: "opus" | Subagent gets claude-opus-4-6 (no 1M) |

All three approaches produce the same result — the [1m] suffix is dropped.

Expected behavior

The model alias resolution should preserve the [1m] suffix:

opus[1m] → claude-opus-4-6[1m]   (expected)
opus[1m] → claude-opus-4-6        (actual)

Subagents should be able to inherit or be explicitly configured to use the 1M context window, through any of:

• Inheriting from parent session's model setting
• model: opus[1m] in agent definition frontmatter
• model parameter on the Agent tool

Environment

• Claude Code v2.1.96
• macOS (Darwin 23.1.0)
• Model: opus[1m] (parent), claude-opus-4-6 (subagents)