opusplan mode: 100% traffic routed to Opus 4.6, Sonnet quota never consumed

Summary

When using opusplan mode in Claude Code, all requests (including tool calls and sub-agent spawning) are routed to Claude Opus 4.6. The Sonnet 4.6 quota remains completely unchanged regardless of workload, while only the total weekly usage increases.

Expected Behavior

Per the documentation, opusplan mode should:

• Use Opus 4.6 for planning/reasoning phases
• Switch to Sonnet 4.6 for execution, code generation, and tool calls

This hybrid routing is intended to provide cost efficiency while retaining Opus-quality planning.

Actual Behavior

• 100% of traffic billed to Opus 4.6
• Sonnet 4.6 quota: 16% — unchanged before and after a full multi-hour coding session
• Weekly total usage increases normally, but Sonnet allocation is never touched
• Tool calls, sub-agents (Task tool), and regular execution turns all appear to consume Opus quota

Environment

• Claude Code version: latest (as of 2026-02-21)
• Model mode: opusplan
• Plan: Claude Max

Related Issues

• #6108 was closed as "COMPLETED" but the fix only addressed system prompt display — the underlying model routing for tool use and sub-agents appears to remain unresolved
• #10993 (subagent model selection ambiguity — closed as "not planned")

Evidence

Sonnet quota was 16% before starting a heavy multi-file coding session using opusplan mode. After completing the session (multiple file edits, several Task/sub-agent invocations, git operations), Sonnet quota remained at exactly 16%. Only the weekly Opus usage counter increased, confirming all traffic went through Opus.

Impact

Users selecting opusplan mode for its stated cost-efficiency benefit receive no savings — they pay Opus pricing for all operations including simple tool calls that should be handled by Sonnet.