feat(providers): add Claude Fable 5 support

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### ✨ New Features
+
+- **feat(providers):** add Claude Fable 5 (`claude-fable-5`) — wires the new flagship model across the full pipeline: `cc` and `kiro` provider registries (1M context, 128k output), pricing constants, model spec (adaptive thinking, vision, tool use), fast mode, 1M-context beta header, fallback chain (`claude-fable-5 → claude-opus-4-8 → claude-opus-4-7 → claude-sonnet-4-6`), and cost data.
+
 ---
 
 ## [3.8.20] — Unreleased

open-sse/config/providerRegistry.ts

@@ -660,6 +660,12 @@ const _REGISTRY_EAGER: Record<string, RegistryEntry> = {
       tokenUrl: "https://api.anthropic.com/v1/oauth/token",
     },
     models: [
+      {
+        id: "claude-fable-5",
+        name: "Claude Fable 5",
+        contextLength: 1000000,
+        maxOutputTokens: 128000,
+      },
       {
         id: "claude-opus-4-8",
         name: "Claude Opus 4.8",
@@ -1142,6 +1148,12 @@ const _REGISTRY_EAGER: Record<string, RegistryEntry> = {
     },
     models: [
       { id: "auto-kiro", name: "Auto (Kiro picks best model)" },
+      {
+        id: "claude-fable-5",
+        name: "Claude Fable 5",
+        contextLength: 1000000,
+        maxOutputTokens: 128000,
+      },
       {
         id: "claude-opus-4.8",
         name: "Claude Opus 4.8",

open-sse/services/claudeCodeCompatible.ts

@@ -52,7 +52,7 @@ const CLAUDE_CODE_COMPATIBLE_DEFAULT_SYSTEM_BLOCKS = [
     text: "You are a Claude agent, built on Anthropic's Claude Agent SDK.",
   },
 ];
-const CONTEXT_1M_SUPPORTED_MODELS = ["claude-opus-4-8", "claude-opus-4-7", "claude-opus-4-6"];
+const CONTEXT_1M_SUPPORTED_MODELS = ["claude-fable-5", "claude-opus-4-8", "claude-opus-4-7", "claude-opus-4-6"];
 export const CLAUDE_CODE_COMPATIBLE_STAINLESS_TIMEOUT_SECONDS = getStainlessTimeoutSeconds(
   process.env
 );

open-sse/services/modelFamilyFallback.ts

@@ -14,6 +14,7 @@
 import { getModelContextLimit } from "../../src/lib/modelCapabilities";
 import { parseModel } from "./model.ts";
 import { CONTEXT_OVERFLOW_REGEX } from "./errorClassifier.ts";
+import { getRegistryEntry } from "../config/providerRegistry.ts";
 
 // ── Model Family Definitions ─────────────────────────────────────────────────
 
@@ -72,6 +73,10 @@ const MODEL_FAMILIES: Record<string, string[]> = {
   "gemini-2.5-pro": ["gemini-2.5-pro-preview-06-05", "gemini-2.5-pro-exp-03-25"],
   "gemini-2.5-pro-preview-06-05": ["gemini-2.5-pro", "gemini-2.5-pro-exp-03-25"],
 
+  // Claude Mythos family (Fable 5) — flagship falls to the next-best Opus
+  // tiers before the cheaper Sonnet, matching the Opus family ordering.
+  "claude-fable-5": ["claude-opus-4-8", "claude-opus-4-7", "claude-sonnet-4-6"],
+
   // Claude Opus family
   "claude-opus-4-8": ["claude-opus-4-7", "claude-opus-4-6", "claude-sonnet-4-6"],
   "claude-opus-4-7": ["claude-opus-4-6", "claude-opus-4-5-20251101", "claude-sonnet-4-6"],
@@ -142,14 +147,28 @@ export function getNextFamilyFallback(
 ): string | null {
   const parsed = parseModel(currentModel);
   const bareModel = parsed.model || currentModel;
-  const prefix =
-    parsed.provider || parsed.providerAlias ? `${parsed.provider || parsed.providerAlias}/` : "";
+  const provider = parsed.provider || parsed.providerAlias || "";
+  const prefix = provider ? `${provider}/` : "";
 
-  const family = MODEL_FAMILIES[bareModel];
+  // Normalize dots to hyphens for the lookup so kiro/claude-opus-4.8 finds the right entry
+  const lookupKey = bareModel.replace(/\./g, "-");
+  const family = MODEL_FAMILIES[lookupKey];
   if (!family) return null;
 
+  // Resolve the provider's supported model IDs so we can match notation (dot vs hyphen)
+  const registryEntry = provider ? getRegistryEntry(provider) : null;
+  const supportedIds = registryEntry ? new Set(registryEntry.models.map((m) => m.id)) : null;
+
   for (const candidate of family) {
-    const fullCandidate = `${prefix}${candidate}`;
+    let resolvedCandidate = candidate;
+    if (supportedIds && !supportedIds.has(candidate)) {
+      // Try dot-notation variants: claude-opus-4-8 → claude-opus-4.8
+      const dotVariant = candidate.replace(/-(\d+)-(\d+)$/, "-$1.$2");
+      const dotVariant2 = candidate.replace(/-(\d+)-(\d+)-/, "-$1.$2-");
+      if (supportedIds.has(dotVariant)) resolvedCandidate = dotVariant;
+      else if (supportedIds.has(dotVariant2)) resolvedCandidate = dotVariant2;
+    }
+    const fullCandidate = `${prefix}${resolvedCandidate}`;
     if (!triedModels.has(fullCandidate)) {
       return fullCandidate;
     }

open-sse/services/providerCostData.ts

@@ -11,6 +11,7 @@ export interface ModelPricing {
 export const KNOWN_MODEL_PRICING: Record<string, ModelPricing> = {
   "gpt-4o": { inputCostPer1M: 2.5, outputCostPer1M: 10.0, isFree: false },
   "gpt-4o-mini": { inputCostPer1M: 0.15, outputCostPer1M: 0.6, isFree: false },
+  "claude-fable-5": { inputCostPer1M: 15.0, outputCostPer1M: 75.0, isFree: false },
   "claude-opus-4-8": { inputCostPer1M: 15.0, outputCostPer1M: 75.0, isFree: false },
   "claude-opus-4-7": { inputCostPer1M: 15.0, outputCostPer1M: 75.0, isFree: false },
   "claude-sonnet-4-6": { inputCostPer1M: 3.0, outputCostPer1M: 15.0, isFree: false },

src/lib/db/settings.ts

@@ -106,7 +106,7 @@ export async function getSettings() {
     codexServiceTier: { enabled: false },
     claudeFastMode: {
       enabled: false,
-      supportedModels: ["claude-opus-4-8", "claude-opus-4-7", "claude-opus-4-6"],
+      supportedModels: ["claude-fable-5", "claude-opus-4-8", "claude-opus-4-7", "claude-opus-4-6"],
     },
     codexSessionAffinityTtlMs: 0,
     responsesPreviousResponseIdMode: DEFAULT_RESPONSES_PREVIOUS_RESPONSE_ID_MODE,

src/lib/providers/claudeFastMode.ts

@@ -7,6 +7,7 @@ type JsonRecord = Record<string, unknown>;
  * only the latest Opus tiers can request the priority service path.
  */
 export const CLAUDE_FAST_MODE_DEFAULT_MODELS = [
+  "claude-fable-5",
   "claude-opus-4-8",
   "claude-opus-4-7",
   "claude-opus-4-6",

src/lib/providers/staticModels.ts

@@ -33,6 +33,7 @@ const STATIC_MODEL_PROVIDERS: Record<string, () => Array<{ id: string; name: str
   ],
   antigravity: () => ANTIGRAVITY_PUBLIC_MODELS.map((model) => ({ ...model })),
   claude: () => [
+    { id: "claude-fable-5", name: "Claude Fable 5" },
     { id: "claude-opus-4-8", name: "Claude Opus 4.8" },
     { id: "claude-opus-4-7", name: "Claude Opus 4.7" },
     { id: "claude-opus-4-6", name: "Claude Opus 4.6" },

src/shared/constants/modelSpecs.ts

@@ -212,6 +212,18 @@ export const MODEL_SPECS: Record<string, ModelSpec> = {
     aliases: BEDROCK_CLAUDE_ALIASES("claude-opus-4-7", "claude-opus-4.7"),
   },
 
+  // ── Claude Fable 5 ──────────────────────────────────────────────
+  "claude-fable-5": {
+    maxOutputTokens: 128000,
+    contextWindow: 1000000,
+    defaultThinkingBudget: 32000,
+    thinkingBudgetCap: 120000,
+    supportsThinking: true,
+    supportsTools: true,
+    supportsVision: true,
+    aliases: BEDROCK_CLAUDE_ALIASES("claude-fable-5"),
+  },
+
   // ── Claude Opus 4.8 ─────────────────────────────────────────────
   "claude-opus-4-8": {
     maxOutputTokens: 128000,

src/shared/constants/pricing.ts

@@ -19,6 +19,14 @@ const GPT_5_5_PRICING = {
   cache_creation: 5.0,
 };
 
+const CLAUDE_FABLE_5_PRICING = {
+  input: 15.0,
+  output: 75.0,
+  cached: 7.5,
+  reasoning: 112.5,
+  cache_creation: 15.0,
+};
+
 const CLAUDE_OPUS_4_PRICING = {
   input: 15.0,
   output: 75.0,
@@ -129,6 +137,13 @@ export const DEFAULT_PRICING = {
 
   // Claude Code (cc)
   cc: {
+    "claude-fable-5": {
+      input: 5.0,
+      output: 25.0,
+      cached: 2.5,
+      reasoning: 37.5,
+      cache_creation: 5.0,
+    },
     "claude-opus-4-8": {
       input: 5.0,
       output: 25.0,
@@ -717,6 +732,7 @@ export const DEFAULT_PRICING = {
     // Common model IDs (without dates) used across providers
     // Intentional duplicates of dot-notation variants (e.g. claude-opus-4.6)
     // to cover hyphen-notation IDs (claude-opus-4-6) used by some clients
+    "claude-fable-5": CLAUDE_FABLE_5_PRICING,
     "claude-opus-4.8": CLAUDE_OPUS_4_PRICING,
     "claude-opus-4-8": CLAUDE_OPUS_4_PRICING,
     "claude-opus-4-7": CLAUDE_OPUS_4_PRICING,
@@ -1282,6 +1298,13 @@ export const DEFAULT_PRICING = {
   },
 
   kiro: {
+    "claude-fable-5": {
+      input: 15.0,
+      output: 75.0,
+      cached: 7.5,
+      reasoning: 112.5,
+      cache_creation: 15.0,
+    },
     "claude-sonnet-4.5": {
       input: 3.0,
       output: 15.0,

tests/unit/catalog-updates-v3x.test.ts

@@ -47,6 +47,25 @@ test("NVIDIA catalog includes the verified 2026 additions and GPT OSS 20B alias
   });
 });
 
+test("Fable 5 catalog exposes claude-fable-5 in cc and kiro providers with matching pricing", () => {
+  const ccIds = new Set(getModelsByProviderId("cc").map((m) => m.id));
+  assert.ok(ccIds.has("claude-fable-5"), "cc must expose claude-fable-5");
+
+  const kiroModels = getModelsByProviderId("kiro");
+  const kiroIds = new Set(kiroModels.map((m) => m.id));
+  assert.ok(kiroIds.has("claude-fable-5"), "kiro must expose claude-fable-5");
+
+  const fable = kiroModels.find((m) => m.id === "claude-fable-5");
+  assert.equal(fable?.contextLength, 1000000);
+  assert.equal(fable?.maxOutputTokens, 128000);
+
+  const ccPricing = (DEFAULT_PRICING as Record<string, Record<string, unknown>>).cc;
+  assert.ok(ccPricing["claude-fable-5"], "cc pricing must include claude-fable-5");
+
+  const kiroPricing = (DEFAULT_PRICING as Record<string, Record<string, unknown>>).kiro;
+  assert.ok(kiroPricing["claude-fable-5"], "kiro pricing must include claude-fable-5");
+});
+
 test("Kiro catalog exposes Claude Opus 4.8 alongside 4.7 with matching pricing", () => {
   const models = getModelsByProviderId("kiro");
   const ids = new Set(models.map((model) => model.id));

tests/unit/model-family-fallback-notation.test.ts

@@ -0,0 +1,39 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// Covers getNextFamilyFallback's dot-vs-hyphen notation resolution (the hunk
+// added alongside Claude Fable 5 in #3524 that affects ALL families): the
+// lookup normalizes dots→hyphens, and each candidate is resolved to the
+// notation the provider's registry actually exposes (kiro uses dot notation
+// `claude-opus-4.8`, cc uses hyphen `claude-opus-4-8`).
+const { getNextFamilyFallback } = await import("../../open-sse/services/modelFamilyFallback.ts");
+
+test("Fable 5 falls back to the next-best Opus tier first (not Sonnet) — cc→claude", () => {
+  // `cc` is an alias parseModel normalizes to the `claude` provider.
+  const next = getNextFamilyFallback("cc/claude-fable-5", new Set(["cc/claude-fable-5"]));
+  assert.equal(next, "claude/claude-opus-4-8");
+});
+
+test("Fable 5 fallback resolves to kiro's dot-notation model id", () => {
+  // kiro registry exposes `claude-opus-4.8` (dot), not `claude-opus-4-8`.
+  const next = getNextFamilyFallback("kiro/claude-fable-5", new Set(["kiro/claude-fable-5"]));
+  assert.equal(next, "kiro/claude-opus-4.8");
+});
+
+test("dot-notation current model is normalized for the family lookup", () => {
+  // kiro/claude-opus-4.8 must find the claude-opus-4-8 family entry.
+  const next = getNextFamilyFallback("kiro/claude-opus-4.8", new Set(["kiro/claude-opus-4.8"]));
+  assert.equal(next, "kiro/claude-opus-4.7");
+});
+
+test("skips already-tried candidates and advances down the Fable chain", () => {
+  const next = getNextFamilyFallback(
+    "cc/claude-fable-5",
+    new Set(["cc/claude-fable-5", "claude/claude-opus-4-8"])
+  );
+  assert.equal(next, "claude/claude-opus-4-7");
+});
+
+test("returns null for an unknown family", () => {
+  assert.equal(getNextFamilyFallback("cc/not-a-real-model", new Set()), null);
+});