fix(claude): support Fable 5.1

- add claude-fable-5-1 to the Claude Code model catalog (1M context,
  permanent adaptive thinking)
- centralize the spoofed Claude Code version and update both request
  and billing identities to 2.1.257 (Fable 5.1 rejects < 2.1.251)
- send output_config.effort without the redundant thinking switch for
  permanently adaptive models
- add regression coverage for capabilities, headers, billing identity
  and adaptive-effort payload

# Conflicts:
#	open-sse/providers/registry/claude.js
#	open-sse/providers/shared.js
#	open-sse/utils/claudeCloaking.js
#	tests/__baseline__/providers-baseline.json

CHANGELOG.md

@@ -40,6 +40,9 @@
 - **i18n**: pt-BR expanded to 1132 terms
 
 ## Fixes
+- **Claude Code**: add Claude Fable 5.1 and advertise Claude Code 2.1.258 in
+  both the request header and billing identity; use its permanent adaptive-thinking
+  mode with `output_config.effort`
 - **Stream**: record usage when a client closes on the terminal event — the
   Responses API has no [DONE] sentinel, so codex closed the socket on
   `response.completed` and cancelled the reader before flush() ran its usage
@@ -738,4 +741,4 @@
 # v0.4.46 (2026-05-15)
 
 ## Breaking Changes
-- Tunnel public URL changed — old tunnel links no longer work, please reconnect to get the new URL
+- Tunnel public URL changed — old tunnel links no longer work, please reconnect to get the new URL

open-sse/providers/capabilities.js

@@ -83,7 +83,8 @@ export function capabilitiesFromServiceKind(kind) {
  * otherwise mis-match. Only declare deltas vs DEFAULT.
  */
 export const MODEL_CAPABILITIES = {
-  // Claude Opus 5, 4.6/4.7/4.8, and Kiro Sonnet 5 have 1M context + adaptive thinking (override generic claude pattern)
+  // Claude Fable 5.1, Opus 5, 4.6/4.7/4.8, and Kiro Sonnet 5 have 1M context + adaptive thinking (override generic claude pattern)
+  "claude-fable-5-1": { vision: true, reasoning: true, search: true, thinkingFormat: "claude-adaptive", thinkingCanDisable: false, contextWindow: 1000000, maxOutput: 128000 },
   "claude-opus-5":     { vision: true, reasoning: true, search: true, thinkingFormat: "claude-adaptive", contextWindow: 1000000, maxOutput: 128000 },
   "claude-opus-5-thinking": { vision: true, reasoning: true, search: true, thinkingFormat: "claude-adaptive", contextWindow: 1000000, maxOutput: 128000 },
   "claude-opus-5-agentic": { vision: true, reasoning: true, search: true, thinkingFormat: "claude-adaptive", contextWindow: 1000000, maxOutput: 128000 },

open-sse/providers/registry/claude.js

@@ -1,4 +1,4 @@
-import { CLAUDE_CLI_SPOOF_HEADERS } from "../shared.js";
+import { CLAUDE_CLI_VERSION } from "../shared.js";
 
 export default {
   id: "claude",
@@ -25,7 +25,7 @@ export default {
       "Anthropic-Version": "2023-06-01",
       "Anthropic-Beta": "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24,structured-outputs-2025-12-15,fast-mode-2026-02-01,redact-thinking-2026-02-12,token-efficient-tools-2026-03-28",
       "Anthropic-Dangerous-Direct-Browser-Access": "true",
-      "User-Agent": "claude-cli/2.1.258 (external, sdk-cli)",
+      "User-Agent": `claude-cli/${CLAUDE_CLI_VERSION} (external, sdk-cli)`,
       "X-App": "cli",
       "X-Stainless-Helper-Method": "stream",
       "X-Stainless-Retry-Count": "0",
@@ -58,6 +58,7 @@ export default {
   },
   models: [
     { id: "claude-opus-5", name: "Claude Opus 5" },
+    { id: "claude-fable-5-1", name: "Claude Fable 5.1" },
     { id: "claude-fable-5", name: "Claude Fable 5" },
     { id: "claude-sonnet-5", name: "Claude Sonnet 5" },
     { id: "claude-haiku-4-5-20251001", name: "Claude 4.5 Haiku" },

open-sse/providers/shared.js

@@ -22,6 +22,7 @@ export function mapStainlessArch() {
 
 // Anthropic API version (single source — reused across claude-format providers/executors)
 export const ANTHROPIC_API_VERSION = "2023-06-01";
+export const CLAUDE_CLI_VERSION = "2.1.258";
 
 // Shared Claude-compatible API headers (reused across claude-format providers)
 export const CLAUDE_API_HEADERS = {
@@ -34,7 +35,7 @@ export const CLAUDE_CLI_SPOOF_HEADERS = {
   "Anthropic-Version": ANTHROPIC_API_VERSION,
   "Anthropic-Beta": "claude-code-20250219,oauth-2025-04-20,interleaved-thinking-2025-05-14,context-management-2025-06-27,prompt-caching-scope-2026-01-05,advanced-tool-use-2025-11-20,effort-2025-11-24,structured-outputs-2025-12-15,fast-mode-2026-02-01,redact-thinking-2026-02-12,token-efficient-tools-2026-03-28",
   "Anthropic-Dangerous-Direct-Browser-Access": "true",
-  "User-Agent": "claude-cli/2.1.258 (external, sdk-cli)",
+  "User-Agent": `claude-cli/${CLAUDE_CLI_VERSION} (external, sdk-cli)`,
   "X-App": "cli",
   "X-Stainless-Helper-Method": "stream",
   "X-Stainless-Retry-Count": "0",

open-sse/translator/concerns/thinkingUnified.js

@@ -237,12 +237,10 @@ function applyFormat(fmt, body, cfg, caps, supportedLevels) {
     }
     case "claude-adaptive": {
       if (none && canDisable) { body.thinking = { type: "disabled" }; break; }
-      // output_config.effort alone does NOT turn thinking on: Anthropic requires
+      // Models that can disable thinking need the explicit adaptive switch.
-      // an explicit thinking:{type:"adaptive"} on Opus 4.6/4.7/4.8 and Sonnet 4.6
+      // Permanently adaptive models such as Fable 5.1 accept effort directly.
-      // ("thinking is off unless you explicitly set it"), and Anthropic-compatible
+      if (canDisable) body.thinking = { type: "adaptive" };
-      // shims (e.g. GitHub Copilot /v1/messages) default thinking off even for
+      else delete body.thinking;
-      // Sonnet 5. Send both fields — the documented adaptive-thinking shape.
-      body.thinking = { type: "adaptive" };
       const level = toLevel(eff);
       body.output_config = { effort: level === "xhigh" ? "high" : level };
       break;

open-sse/utils/claudeCloaking.js

@@ -1,16 +1,16 @@
 import { createHash, randomBytes, randomUUID } from "crypto";
 import { CLAUDE_TOOL_SUFFIX, CC_DEFAULT_TOOLS } from "../config/appConstants.js";
+import { CLAUDE_CLI_VERSION } from "../providers/shared.js";
 
-const CLAUDE_VERSION = "2.1.258";
 const CC_ENTRYPOINT = "sdk-cli";
 
-// Generate billing header matching real Claude Code 2.1.258+ format:
+// Generate the billing header expected from current Claude Code clients.
 // x-anthropic-billing-header: cc_version=<ver>.<build>; cc_entrypoint=sdk-cli; cch=<hash>;
 function generateBillingHeader(payload) {
   const content = JSON.stringify(payload);
   const cch = createHash("sha256").update(content).digest("hex").slice(0, 5);
   const buildHash = randomBytes(2).toString("hex").slice(0, 3);
-  return `x-anthropic-billing-header: cc_version=${CLAUDE_VERSION}.${buildHash}; cc_entrypoint=${CC_ENTRYPOINT}; cch=${cch};`;
+  return `x-anthropic-billing-header: cc_version=${CLAUDE_CLI_VERSION}.${buildHash}; cc_entrypoint=${CC_ENTRYPOINT}; cch=${cch};`;
 }
 
 // Derive a deterministic UUID-v4-shaped string from a seed (stable per account)
@@ -19,7 +19,7 @@ function deriveUuid(seed) {
   return `${h.slice(0, 8)}-${h.slice(8, 12)}-4${h.slice(13, 16)}-${((parseInt(h[16], 16) & 0x3) | 0x8).toString(16)}${h.slice(17, 20)}-${h.slice(20, 32)}`;
 }
 
-// Generate fake user ID in Claude Code 2.1.258+ JSON format:
+// Generate fake user ID in the current Claude Code JSON format:
 // {"device_id":"<64hex>","account_uuid":"<uuid>","session_id":"<uuid>"}
 // device_id/account_uuid derive from apiKey (stable per account), session_id per-conversation
 function generateFakeUserID(sessionId, apiKey) {

tests/__baseline__/providers-baseline.json

@@ -1009,4 +1009,4 @@
     },
     "format": "openai"
   }
-}
+}

tests/translator/thinking-unified.test.js

@@ -82,6 +82,11 @@ describe("applyThinking per provider format", () => {
     // Sonnet 5). Both fields together are the documented adaptive shape.
     expect(out.thinking).toEqual({ type: "adaptive" });
   });
+  it("Fable 5.1 → effort without a redundant thinking switch", () => {
+    const out = apply("claude", "claude-fable-5-1", { reasoning_effort: "high" }, "claude");
+    expect(out.output_config).toEqual({ effort: "high" });
+    expect(out.thinking).toBeUndefined();
+  });
   it("claude haiku → enabled+budget", () => {
     const out = apply("claude", "claude-haiku-4.5", { reasoning_effort: "high" }, "claude");
     expect(out.thinking).toEqual({ type: "enabled", budget_tokens: 24576 });

tests/unit/capabilities.test.js

@@ -32,6 +32,13 @@ describe("getCapabilitiesForModel", () => {
     }
   });
 
+  it("reports Claude Fable 5.1 as a permanent adaptive-thinking model", () => {
+    expect(getCapabilitiesForModel("claude", "claude-fable-5-1")).toMatchObject({
+      ...claudeSonnet5Expected,
+      thinkingCanDisable: false,
+    });
+  });
+
   it("reports Kiro Claude Opus 4.8 as a 1M context model", () => {
     expect(getCapabilitiesForModel("kiro", "claude-opus-4.8").contextWindow).toBe(1000000);
     expect(getCapabilitiesForModel("kiro", "anthropic/claude-opus-4.8").contextWindow).toBe(1000000);

tests/unit/claude-cloaking.test.js

@@ -7,9 +7,14 @@
  */
 
 import { describe, it, expect } from "vitest";
-import { cloakClaudeTools, decloakStreamChunk } from "../../open-sse/utils/claudeCloaking.js";
+import { applyCloaking, cloakClaudeTools, decloakStreamChunk } from "../../open-sse/utils/claudeCloaking.js";
 import { CLAUDE_TOOL_SUFFIX } from "../../open-sse/config/appConstants.js";
 
+it("advertises a Claude Code version accepted by Fable 5.1", () => {
+  const body = applyCloaking({ messages: [] }, "sk-ant-oat-test", "session-id");
+  expect(body.system[0].text).toMatch(/^x-anthropic-billing-header: cc_version=2.1.258\./);
+});
+
 describe("cloakClaudeTools", () => {
   const baseBody = {
     tools: [{ name: "todo_write", description: "write todos", input_schema: { type: "object", properties: {} } }],

tests/unit/claude-header-forwarding.test.js

@@ -29,6 +29,7 @@ describe("DefaultExecutor.buildHeaders() — claude provider", () => {
       headers["Anthropic-Version"] === "2023-06-01" ||
       headers["anthropic-version"] === "2023-06-01";
     expect(hasVersion).toBe(true);
+    expect(headers["User-Agent"]).toBe("claude-cli/2.1.258 (external, sdk-cli)");
   });
 
   it("includes heavy-agent beta flags for claude-opus-5", () => {