refactor(pricing): derive fast-tier cache rates via buildRates helper

Address review feedback on the per-service-tier pricing:

- Extract `buildRates(input, output)` plus named CACHE_5M_WRITE_MULTIPLIER
  / CACHE_READ_MULTIPLIER constants so fast-tier cache rates are derived
  from Anthropic's documented multipliers instead of hand-computed numbers.
  This removes the inconsistency where Opus 4.6 fast-tier lacked the
  explanatory inline comments that 4.7/4.8 carried (presto suggestion #3).
- Document in `applyServiceTier` that no production caller passes
  `serviceTier` yet (Anthropic's service_tier is not captured on
  CliproxyRequestDetail), so fast-mode usage is currently billed at the
  standard rate until the usage pipeline records the tier.
- Add a combined date-suffix + fast-tier lookup test to prove the two
  resolution stages compose.

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src/web-server/model-pricing.ts

@@ -50,6 +50,26 @@ export interface PricingLookupOptions extends ModelsDevPricingLookupOptions {
   serviceTier?: string;
 }
 
+// Anthropic prompt-caching multipliers, expressed relative to the base input
+// rate (see https://platform.claude.com/docs/en/about-claude/pricing#prompt-caching).
+// Keeping them as named constants avoids drift when deriving per-tier cache rates.
+const CACHE_5M_WRITE_MULTIPLIER = 1.25;
+const CACHE_READ_MULTIPLIER = 0.1;
+
+/**
+ * Build a full rate set from input/output rates, deriving cache rates from the
+ * documented Anthropic multipliers so fast-tier entries stay in sync with the
+ * base-rate math instead of carrying hand-computed cache numbers.
+ */
+function buildRates(inputPerMillion: number, outputPerMillion: number): PricingRates {
+  return {
+    inputPerMillion,
+    outputPerMillion,
+    cacheCreationPerMillion: inputPerMillion * CACHE_5M_WRITE_MULTIPLIER,
+    cacheReadPerMillion: inputPerMillion * CACHE_READ_MULTIPLIER,
+  };
+}
+
 // ============================================================================
 // USER-EDITABLE PRICING TABLE
 // Update rates below (per million tokens in USD)
@@ -247,12 +267,7 @@ const PRICING_REGISTRY: Record<string, ModelPricing> = {
     cacheCreationPerMillion: 6.25,
     cacheReadPerMillion: 0.5,
     serviceTiers: {
-      fast: {
-        inputPerMillion: 30.0,
-        outputPerMillion: 150.0,
-        cacheCreationPerMillion: 37.5,
-        cacheReadPerMillion: 3.0,
-      },
+      fast: buildRates(30.0, 150.0),
     },
   },
   'claude-opus-4-6-thinking': {
@@ -268,12 +283,7 @@ const PRICING_REGISTRY: Record<string, ModelPricing> = {
     cacheCreationPerMillion: 6.25,
     cacheReadPerMillion: 0.5,
     serviceTiers: {
-      fast: {
-        inputPerMillion: 30.0,
-        outputPerMillion: 150.0,
-        cacheCreationPerMillion: 37.5, // 30 * 1.25 (5m cache write multiplier)
-        cacheReadPerMillion: 3.0, // 30 * 0.1 (cache read multiplier)
-      },
+      fast: buildRates(30.0, 150.0),
     },
   },
   // Claude 4.8 Opus ($5/$25) — fast mode ($10/$50, 2x premium per Anthropic docs)
@@ -283,12 +293,7 @@ const PRICING_REGISTRY: Record<string, ModelPricing> = {
     cacheCreationPerMillion: 6.25,
     cacheReadPerMillion: 0.5,
     serviceTiers: {
-      fast: {
-        inputPerMillion: 10.0,
-        outputPerMillion: 50.0,
-        cacheCreationPerMillion: 12.5, // 10 * 1.25 (5m cache write multiplier)
-        cacheReadPerMillion: 1.0, // 10 * 0.1 (cache read multiplier)
-      },
+      fast: buildRates(10.0, 50.0),
     },
   },
   'claude-opus-4-7-thinking': {
@@ -944,6 +949,13 @@ function hasProviderContext(model: string, options: PricingLookupOptions): boole
  * Apply per-service-tier rates if the resolved model declares them and the
  * caller requested a matching tier. Unknown tiers transparently fall through
  * to the base rates so existing callers stay unaffected.
+ *
+ * TODO(opus-fast-mode): No production caller currently passes `serviceTier`.
+ * Anthropic's `service_tier` is not yet captured on CliproxyRequestDetail, so
+ * usage transformers (cliproxy-usage-transformer.ts, data-aggregator.ts) bill
+ * fast-mode requests at the standard rate — i.e. fast Opus is under-reported by
+ * the tier premium ($10/$50 vs $5/$25). Wire `serviceTier` through once the
+ * usage pipeline records it. The schema below is ready for that integration.
  */
 function applyServiceTier(pricing: ModelPricing, tier: string | undefined): ModelPricing {
   if (!tier) return pricing;

tests/unit/model-pricing.test.ts

@@ -202,6 +202,16 @@ describe('model-pricing', () => {
       expect(opus47fast.outputPerMillion).toBe(150.0);
     });
 
+    it('should apply fast-tier pricing together with date-suffix stripping', () => {
+      // stripDateSuffix (base lookup) and applyServiceTier run independently;
+      // confirm they compose for a date-stamped id requesting the fast tier.
+      const opus48fastDated = getModelPricing('claude-opus-4-8-20260530', { serviceTier: 'fast' });
+      expect(opus48fastDated.inputPerMillion).toBe(10.0);
+      expect(opus48fastDated.outputPerMillion).toBe(50.0);
+      expect(opus48fastDated.cacheCreationPerMillion).toBe(12.5);
+      expect(opus48fastDated.cacheReadPerMillion).toBe(1.0);
+    });
+
     it('should fall back to standard rates when serviceTier is unknown', () => {
       // Unknown tier names must not throw; revert to base pricing transparently.
       const opus48 = getModelPricing('claude-opus-4-8', { serviceTier: 'enterprise-mythos' });