[BUG] cost_usage_total metric applies Opus pricing to Sonnet 4.6 model

Bug Description

The claude_code.cost.usage (Prometheus: claude_code_cost_usage_total) metric applies Opus 4.6 pricing ($5/$25 per MTok) to the Sonnet 4.6 model, instead of the correct Sonnet pricing ($3/$15 per MTok). This results in cost estimates ~1.67x higher than actual for Sonnet 4.6.

Version

• Claude Code: 2.1.51 (also confirmed on 2.1.49)
• Platform: Linux (WSL2)

Evidence

1. Mathematical proof from Prometheus data

Running claude-sonnet-4-6 and collecting Prometheus metrics:

claude_code_cost_usage_total{model="claude-sonnet-4-6"} 0.6554
claude_code_token_usage_total{model="claude-sonnet-4-6",type="input"} 15
claude_code_token_usage_total{model="claude-sonnet-4-6",type="output"} 802
claude_code_token_usage_total{model="claude-sonnet-4-6",type="cacheRead"} 176003
claude_code_token_usage_total{model="claude-sonnet-4-6",type="cacheCreation"} 87565

Computing with Opus pricing ($5/$25/$0.50/$6.25 per MTok):

15×5/1M + 802×25/1M + 176003×0.5/1M + 87565×6.25/1M = 0.6554 ← matches reported value

Computing with correct Sonnet pricing ($3/$15/$0.30/$3.75 per MTok):

15×3/1M + 802×15/1M + 176003×0.3/1M + 87565×3.75/1M = 0.3932 ← expected value

2. Root cause in binary

Extracted from the v2.1.51 binary using strings:

Pricing tables:

TEH={inputTokens:3,outputTokens:15,promptCacheWriteTokens:3.75,promptCacheReadTokens:0.3,...}  // Sonnet pricing
u2H={inputTokens:5,outputTokens:25,promptCacheWriteTokens:6.25,promptCacheReadTokens:0.5,...}  // Opus 4.5/4.6 pricing

Model-to-pricing mapping (xJ$):

xJ$={
  [VU(UNH.firstParty)]:TEH,  // claude-3-5-sonnet-20241022 → $3/$15 ✓
  [VU(GNH.firstParty)]:TEH,  // claude-3-7-sonnet-20250219 → $3/$15 ✓
  [VU(XIH.firstParty)]:TEH,  // claude-sonnet-4-20250514   → $3/$15 ✓
  [VU(VNH.firstParty)]:u2H,  // claude-sonnet-4-6          → $5/$25 ✗ (should be TEH)
  ...
}

Sonnet 4 and earlier are correctly mapped to TEH ($3/$15), but Sonnet 4.6 is incorrectly mapped to u2H ($5/$25, which is Opus 4.6 pricing).

Impact

• Prometheus cost_usage_total metric reports 1.67x the actual cost for Sonnet 4.6
• Any monitoring/budgeting tools relying on these metrics are affected

Expected Behavior

claude-sonnet-4-6 should be mapped to TEH (Sonnet pricing: $3/$15/$3.75/$0.30 per MTok), consistent with the official pricing page and consistent with how earlier Sonnet models are mapped.