feat: add Claude Fable 5.1 as a benchmark pricing model

cmd/cc-session/benchmark.go

@@ -30,7 +30,7 @@ func runBenchmark(args []string, out io.Writer, errOut io.Writer, store parser.S
 	days := fs.Int("days", 30, "how far back to scan")
 	minKB := fs.Int("min-kb", 100, "minimum JSONL file size in KB")
 	maxN := fs.Int("n", 10, "max sessions to include")
-	model := fs.String("model", "opus", "model: opus, opus-4-6, opus-4-7, opus-4-8, or sonnet")
+	model := fs.String("model", "opus", "model: opus, opus-4-6, opus-4-7, opus-4-8, sonnet, or fable (fable-5-1)")
 	overhead := fs.Int("overhead", 0, "session overhead tokens (system+tools+CLAUDE.md); measure with a 1-turn session")
 	isNoAPI := fs.Bool("no-api", false, "skip API calls; estimate filtered-text tokens with chars/2 (offline fallback)")
 	if err := fs.Parse(reorderArgs(args)); err != nil {

docs/benchmark.md

@@ -36,7 +36,7 @@ All other parameters are derived automatically from your real session data.
 | `--days` | 30 | How far back to scan for sessions |
 | `--min-kb` | 100 | Minimum JSONL file size in KB |
 | `--n` | 10 | Max successful session results to report |
-| `--model` | opus | Pricing and token-counting model: `opus`, `opus-4-6`, `opus-4-7`, `opus-4-8`, or `sonnet` |
+| `--model` | opus | Pricing and token-counting model: `opus`, `opus-4-6`, `opus-4-7`, `opus-4-8`, `sonnet`, or `fable` (`fable-5-1`) |
 
 ### Example output
 
@@ -164,10 +164,15 @@ the historical one-shot `NewCtx × CacheWrite` behavior.
 counting API. The `--model` flag controls both pricing and the tokenizer used by
 the token counting API. `opus` is an alias for `opus-4-8`; explicit Opus versions
 map to `claude-opus-4-6`, `claude-opus-4-7`, or `claude-opus-4-8`; `sonnet` maps
-to `claude-sonnet-4-6`. Opus 4.6, 4.7, and 4.8 use the same Opus pricing rates.
+to `claude-sonnet-4-6`; `fable` and `fable-5-1` both map to `claude-fable-5-1`.
+Opus 4.6, 4.7, and 4.8 use the same Opus pricing rates.
 Fallback constants are used only for behavior that cannot be read directly from
 transcript usage, such as sparse tool I/O data.
 
+Claude Fable 5.1's cache read is 2.5% of base input, not the 10% every other
+model here uses, so its cost-savings numbers are not directly comparable to the
+Opus/Sonnet rows.
+
 ### Simplifications
 
 - **Uncached input (`input_tokens`) omitted**: with auto-caching the breakpoint sits on the last cacheable block, making the uncached tail near zero. This is an inference from the docs (not an explicit statement). Impact on results: < 2% based on sensitivity analysis.

internal/benchmark/model.go

@@ -12,11 +12,18 @@ type Pricing struct {
 var PricingOpus = Pricing{CachedRead: 0.50, CacheWrite: 6.25, BaseInput: 5.00}
 var PricingSonnet = Pricing{CachedRead: 0.30, CacheWrite: 3.75, BaseInput: 3.00}
 
+// PricingFable is Claude Fable 5.1's pricing. Unlike every other tier here, its
+// cache read is 2.5% of base input rather than the usual 10% (source: Anthropic
+// pricing docs, "a cache hit costs 2.5% of the standard input price"), so its
+// cost-savings numbers are not directly comparable to the Opus/Sonnet rows.
+var PricingFable = Pricing{CachedRead: 0.25, CacheWrite: 12.50, BaseInput: 10.00}
+
 const (
 	TokenCountModelOpus46 = "claude-opus-4-6"
 	TokenCountModelOpus47 = "claude-opus-4-7"
 	TokenCountModelOpus48 = "claude-opus-4-8"
 	TokenCountModelSonnet = "claude-sonnet-4-6"
+	TokenCountModelFable  = "claude-fable-5-1"
 )
 
 // ModelConfig bundles the pricing and token-counting model for a given model alias.
@@ -36,8 +43,10 @@ func ResolveModel(model string) (ModelConfig, error) {
 		return ModelConfig{Pricing: PricingOpus, TokenCountModel: TokenCountModelOpus47}, nil
 	case "opus-4-6":
 		return ModelConfig{Pricing: PricingOpus, TokenCountModel: TokenCountModelOpus46}, nil
+	case "fable", "fable-5-1":
+		return ModelConfig{Pricing: PricingFable, TokenCountModel: TokenCountModelFable}, nil
 	default:
-		return ModelConfig{}, fmt.Errorf("unknown model %q: must be opus, opus-4-6, opus-4-7, opus-4-8, or sonnet", model)
+		return ModelConfig{}, fmt.Errorf("unknown model %q: must be opus, opus-4-6, opus-4-7, opus-4-8, sonnet, fable, or fable-5-1", model)
 	}
 }
 

internal/benchmark/model_test.go

@@ -0,0 +1,93 @@
+package benchmark
+
+import (
+	"strings"
+	"testing"
+)
+
+func Test_ResolveModel_GivenKnownAlias_ThenReturnsExpectedConfig(t *testing.T) {
+	tests := []struct {
+		name           string
+		alias          string
+		wantPricing    Pricing
+		wantTokenCount string
+	}{
+		{
+			name:           "sonnet",
+			alias:          "sonnet",
+			wantPricing:    PricingSonnet,
+			wantTokenCount: TokenCountModelSonnet,
+		},
+		{
+			name:           "opus",
+			alias:          "opus",
+			wantPricing:    PricingOpus,
+			wantTokenCount: TokenCountModelOpus48,
+		},
+		{
+			name:           "opus-4-8",
+			alias:          "opus-4-8",
+			wantPricing:    PricingOpus,
+			wantTokenCount: TokenCountModelOpus48,
+		},
+		{
+			name:           "opus-4-7",
+			alias:          "opus-4-7",
+			wantPricing:    PricingOpus,
+			wantTokenCount: TokenCountModelOpus47,
+		},
+		{
+			name:           "opus-4-6",
+			alias:          "opus-4-6",
+			wantPricing:    PricingOpus,
+			wantTokenCount: TokenCountModelOpus46,
+		},
+		{
+			name:           "fable",
+			alias:          "fable",
+			wantPricing:    PricingFable,
+			wantTokenCount: TokenCountModelFable,
+		},
+		{
+			name:           "fable-5-1",
+			alias:          "fable-5-1",
+			wantPricing:    PricingFable,
+			wantTokenCount: TokenCountModelFable,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ResolveModel(tt.alias)
+			if err != nil {
+				t.Fatalf("ResolveModel(%q) returned error: %v", tt.alias, err)
+			}
+			if got.Pricing != tt.wantPricing {
+				t.Errorf("Pricing = %+v, want %+v", got.Pricing, tt.wantPricing)
+			}
+			if got.TokenCountModel != tt.wantTokenCount {
+				t.Errorf("TokenCountModel = %q, want %q", got.TokenCountModel, tt.wantTokenCount)
+			}
+		})
+	}
+}
+
+// Regression guard: PricingFable's cache read must stay at 2.5% of base input
+// ($0.25/MTok), not the 10% ratio every other tier in this file uses.
+func Test_PricingFable_ThenCacheReadIsTwoPointFivePercentOfBaseInput(t *testing.T) {
+	want := PricingFable.BaseInput * 0.025
+	if PricingFable.CachedRead != want {
+		t.Errorf("PricingFable.CachedRead = %v, want %v (2.5%% of BaseInput %v)",
+			PricingFable.CachedRead, want, PricingFable.BaseInput)
+	}
+}
+
+func Test_ResolveModel_GivenUnknownAlias_ThenErrorListsFableAliases(t *testing.T) {
+	_, err := ResolveModel("nonsense")
+	if err == nil {
+		t.Fatal("ResolveModel(\"nonsense\") returned nil error, want unknown model error")
+	}
+	if !strings.Contains(err.Error(), "fable") {
+		t.Errorf("error = %v, want it to list the fable aliases", err)
+	}
+}