feat(claude): add Claude Fable 5.1 model

.agents/upstream-review.md

@@ -15,6 +15,34 @@ Two standing sections outlive any single batch and must be read on every review:
 
 ## Review batches
 
+## 2026-09-02 — scoped adoption, cursor unchanged
+
+**Not a batch review.** The developer asked for upstream's Claude Fable 5.1
+support specifically, ahead of any other upstream work, so this session reviewed
+exactly one change set out of the 129 commits sitting in
+`9b2d04317c68233782e0630464ac86d77d0686f3..57a66608b918d673eeec7e6c94ea5906b756fcd0`.
+The other 128 commits were **not** reviewed and have received no decision.
+
+`reviewed-through` therefore stays at `9b2d0431`. A later full review must treat
+the whole range as unreviewed apart from the row below, which it should skip as
+already adopted.
+
+| ID  | Upstream                                       | Decision | Pylon reference | Notes                                                                                                                                                                                                                                                       |
+| --- | ---------------------------------------------- | -------- | --------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| S-1 | `c17d02cff` / `#9078` — Claude Fable 5.1 model | adopted  | `#247`          | Clean `cherry-pick -x`, no conflicts, applied line-for-line. Self-contained: it does not depend on `#9084` (remote manifest discovery), which is the large `ClaudeProvider.ts` restructure that follows it upstream and was deliberately left out of scope. |
+
+Two consequences worth carrying forward. The bundled manifest swap promotes
+`claude-fable-5-1` into the current Claude set and thereby marks
+`claude-fable-5` **legacy** — that is upstream's intent, since 5.1 supersedes 5
+in the same tier, but it is a user-visible change to an existing model's status.
+And the catalog is gated on Claude Code `v2.1.257`, so the model stays hidden on
+older CLIs rather than failing at spawn time.
+
+`#9084` (`03542836`, remote Claude model manifest) is a separate 26-file
+restructure that another session had begun stacking on the same branch. It was
+split back out and is **not** adopted here; it remains undecided in the
+unreviewed range above.
+
 ## 2026-09-01 — `1f8ed54add4133ac39effceded8fc1fff12d8e03..9b2d04317c68233782e0630464ac86d77d0686f3`
 
 Twenty-nine upstream commits, reviewed as change sets C-1 through C-21. The

apps/server/src/provider/Layers/ClaudeAdapter.test.ts

@@ -527,6 +527,29 @@ describe("ClaudeAdapterLive", () => {
     );
   });
 
+  it.effect("preserves xhigh effort for Claude Fable 5.1", () => {
+    const harness = makeHarness();
+    return Effect.gen(function* () {
+      const adapter = yield* ClaudeAdapter;
+      yield* adapter.startSession({
+        threadId: THREAD_ID,
+        provider: ProviderDriverKind.make("claudeAgent"),
+        modelSelection: createModelSelection(
+          ProviderInstanceId.make("claudeAgent"),
+          "claude-fable-5-1",
+          [{ id: "effort", value: "xhigh" }],
+        ),
+        runtimeMode: "full-access",
+      });
+
+      const createInput = harness.getLastCreateQueryInput();
+      assert.equal(createInput?.options.effort, "xhigh");
+    }).pipe(
+      Effect.provideService(Random.Random, makeDeterministicRandomService()),
+      Effect.provide(harness.layer),
+    );
+  });
+
   it.effect("preserves xhigh effort for Claude Fable 5", () => {
     const harness = makeHarness();
     return Effect.gen(function* () {

apps/server/src/provider/Layers/ClaudeProvider.ts

@@ -56,12 +56,48 @@ const CLAUDE_PRESENTATION = {
   showInteractionModeToggle: true,
   supportsConversationRollback: false,
 } as const;
+const MINIMUM_CLAUDE_FABLE_5_1_VERSION = "2.1.257";
 const MINIMUM_CLAUDE_OPUS_5_VERSION = "2.1.219";
 const MINIMUM_CLAUDE_FABLE_5_VERSION = "2.1.169";
 const MINIMUM_CLAUDE_OPUS_4_8_VERSION = "2.1.154";
 const MINIMUM_CLAUDE_OPUS_4_7_VERSION = "2.1.111";
 
 const CLAUDE_MODEL_CATALOG: ReadonlyArray<ServerProviderModel> = [
+  {
+    slug: "claude-fable-5-1",
+    name: "Claude Fable 5.1",
+    isCustom: false,
+    capabilities: createModelCapabilities({
+      optionDescriptors: [
+        buildSelectOptionDescriptor({
+          id: "effort",
+          label: "Reasoning",
+          options: [
+            { value: "low", label: "Low" },
+            { value: "medium", label: "Medium" },
+            { value: "high", label: "High", isDefault: true },
+            { value: "xhigh", label: "Extra High" },
+            { value: "max", label: "Max" },
+            {
+              value: "ultracode",
+              label: "Ultracode",
+              description: "xhigh effort plus multi-agent workflow orchestration",
+            },
+            { value: "ultrathink", label: "Ultrathink" },
+          ],
+          promptInjectedValues: ["ultrathink"],
+        }),
+        buildSelectOptionDescriptor({
+          id: "contextWindow",
+          label: "Context Window",
+          options: [
+            { value: "200k", label: "200k" },
+            { value: "1m", label: "1M", isDefault: true },
+          ],
+        }),
+      ],
+    }),
+  },
   {
     slug: "claude-fable-5",
     name: "Claude Fable 5",
@@ -330,6 +366,10 @@ const CLAUDE_MODEL_CATALOG: ReadonlyArray<ServerProviderModel> = [
 // so the catalog itself carries no `isLegacy` flags.
 const BUILT_IN_MODELS: ReadonlyArray<ServerProviderModel> = CLAUDE_MODEL_CATALOG;
 
+function supportsClaudeFable51(version: string | null | undefined): boolean {
+  return version ? compareSemverVersions(version, MINIMUM_CLAUDE_FABLE_5_1_VERSION) >= 0 : false;
+}
+
 function supportsClaudeOpus5(version: string | null | undefined): boolean {
   return version ? compareSemverVersions(version, MINIMUM_CLAUDE_OPUS_5_VERSION) >= 0 : false;
 }
@@ -350,6 +390,9 @@ function getBuiltInClaudeModelsForVersion(
   version: string | null | undefined,
 ): ReadonlyArray<ServerProviderModel> {
   return BUILT_IN_MODELS.filter((model) => {
+    if (model.slug === "claude-fable-5-1") {
+      return supportsClaudeFable51(version);
+    }
     if (model.slug === "claude-opus-5") {
       return supportsClaudeOpus5(version);
     }
@@ -366,6 +409,11 @@ function getBuiltInClaudeModelsForVersion(
   });
 }
 
+function formatClaudeFable51UpgradeMessage(version: string | null): string {
+  const versionLabel = version ? `v${version}` : "the installed version";
+  return `Claude Code ${versionLabel} is too old for Claude Fable 5.1. Upgrade to v${MINIMUM_CLAUDE_FABLE_5_1_VERSION} or newer to access it.`;
+}
+
 function formatClaudeOpus5UpgradeMessage(version: string | null): string {
   const versionLabel = version ? `v${version}` : "the installed version";
   return `Claude Code ${versionLabel} is too old for Claude Opus 5. Upgrade to v${MINIMUM_CLAUDE_OPUS_5_VERSION} or newer to access it.`;
@@ -429,6 +477,7 @@ export function normalizeClaudeCliEffort(
   }
   if (
     effort === "xhigh" &&
+    model !== "claude-fable-5-1" &&
     model !== "claude-fable-5" &&
     model !== "claude-opus-5" &&
     model !== "claude-opus-4-8" &&
@@ -1026,15 +1075,17 @@ export const checkClaudeProviderStatus = Effect.fn("checkClaudeProviderStatus")(
     claudeSettings.customModels,
     DEFAULT_CLAUDE_MODEL_CAPABILITIES,
   );
-  const versionUpgradeMessage = supportsClaudeOpus5(parsedVersion)
+  const versionUpgradeMessage = supportsClaudeFable51(parsedVersion)
     ? undefined
-    : supportsClaudeFable5(parsedVersion)
-      ? formatClaudeOpus5UpgradeMessage(parsedVersion)
-      : supportsClaudeOpus48(parsedVersion)
-        ? formatClaudeFable5UpgradeMessage(parsedVersion)
-        : supportsClaudeOpus47(parsedVersion)
-          ? formatClaudeOpus48UpgradeMessage(parsedVersion)
-          : formatClaudeOpus47UpgradeMessage(parsedVersion);
+    : supportsClaudeOpus5(parsedVersion)
+      ? formatClaudeFable51UpgradeMessage(parsedVersion)
+      : supportsClaudeFable5(parsedVersion)
+        ? formatClaudeOpus5UpgradeMessage(parsedVersion)
+        : supportsClaudeOpus48(parsedVersion)
+          ? formatClaudeFable5UpgradeMessage(parsedVersion)
+          : supportsClaudeOpus47(parsedVersion)
+            ? formatClaudeOpus48UpgradeMessage(parsedVersion)
+            : formatClaudeOpus47UpgradeMessage(parsedVersion);
 
   const capabilities = resolveCapabilities
     ? yield* resolveCapabilities(claudeSettings).pipe(Effect.orElseSucceed(() => undefined))

apps/server/src/provider/Layers/ProviderRegistry.test.ts

@@ -2740,6 +2740,62 @@ it.layer(Layer.mergeAll(NodeServices.layer, ServerSettingsModule.layerTest(), Te
         ),
       );
 
+      it.effect("includes Claude Fable 5.1 on supported Claude Code versions", () =>
+        Effect.gen(function* () {
+          const status = yield* checkClaudeProviderStatus(
+            defaultClaudeSettings,
+            claudeCapabilities(),
+          );
+          const fable51 = status.models.find((model) => model.slug === "claude-fable-5-1");
+          assert.strictEqual(fable51?.name, "Claude Fable 5.1");
+        }).pipe(
+          Effect.provide(
+            mockSpawnerLayer((args) => {
+              const joined = args.join(" ");
+              if (joined === "--version") return { stdout: "2.1.257\n", stderr: "", code: 0 };
+              if (joined === "auth status")
+                return {
+                  stdout: '{"loggedIn":true,"authMethod":"claude.ai"}\n',
+                  stderr: "",
+                  code: 0,
+                };
+              throw new Error(`Unexpected args: ${joined}`);
+            }),
+          ),
+        ),
+      );
+
+      it.effect("hides Claude Fable 5.1 on older Claude Code versions", () =>
+        Effect.gen(function* () {
+          const status = yield* checkClaudeProviderStatus(
+            defaultClaudeSettings,
+            claudeCapabilities(),
+          );
+          assert.strictEqual(
+            status.models.some((model) => model.slug === "claude-fable-5-1"),
+            false,
+          );
+          assert.strictEqual(
+            status.message,
+            "Claude Code v2.1.256 is too old for Claude Fable 5.1. Upgrade to v2.1.257 or newer to access it.",
+          );
+        }).pipe(
+          Effect.provide(
+            mockSpawnerLayer((args) => {
+              const joined = args.join(" ");
+              if (joined === "--version") return { stdout: "2.1.256\n", stderr: "", code: 0 };
+              if (joined === "auth status")
+                return {
+                  stdout: '{"loggedIn":true,"authMethod":"claude.ai"}\n',
+                  stderr: "",
+                  code: 0,
+                };
+              throw new Error(`Unexpected args: ${joined}`);
+            }),
+          ),
+        ),
+      );
+
       it.effect("hides Claude Fable 5 on older Claude Code versions", () =>
         Effect.gen(function* () {
           const status = yield* checkClaudeProviderStatus(

apps/server/src/provider/ModelManifest.test.ts

@@ -47,12 +47,16 @@ describe("isLegacyModel (bundled manifest)", () => {
 
   it("keeps only the Claude 5 family out of legacy models", () => {
     assert.deepStrictEqual(
-      ["claude-fable-5", "claude-opus-5", "claude-sonnet-5", "claude-opus-4-8"].map((model) => [
-        model,
-        isLegacyModel(BUNDLED_MODEL_MANIFEST, CLAUDE, model),
-      ]),
       [
-        ["claude-fable-5", false],
+        "claude-fable-5-1",
+        "claude-fable-5",
+        "claude-opus-5",
+        "claude-sonnet-5",
+        "claude-opus-4-8",
+      ].map((model) => [model, isLegacyModel(BUNDLED_MODEL_MANIFEST, CLAUDE, model)]),
+      [
+        ["claude-fable-5-1", false],
+        ["claude-fable-5", true],
         ["claude-opus-5", false],
         ["claude-sonnet-5", false],
         ["claude-opus-4-8", true],

apps/server/src/provider/model-manifest.json

@@ -8,6 +8,6 @@
       "gpt-daybreak-blue-latest",
       "gpt-daybreak-red-latest"
     ],
-    "claudeAgent": ["claude-fable-5", "claude-opus-5", "claude-sonnet-5"]
+    "claudeAgent": ["claude-fable-5-1", "claude-opus-5", "claude-sonnet-5"]
   }
 }

packages/contracts/src/model.ts

@@ -179,6 +179,9 @@ export const MODEL_SLUG_ALIASES_BY_PROVIDER: Partial<
     "gpt-5.3-spark": "gpt-5.3-codex-spark",
   },
   [CLAUDE_DRIVER_KIND]: {
+    fable: "claude-fable-5-1",
+    "fable-5.1": "claude-fable-5-1",
+    "claude-fable-5.1": "claude-fable-5-1",
     opus: "claude-opus-5",
     "opus-5": "claude-opus-5",
     "claude-opus-5.0": "claude-opus-5",

packages/shared/src/model.test.ts

@@ -153,6 +153,7 @@ describe("model slug normalization", () => {
   it("preserves exact custom slugs instead of expanding provider aliases", () => {
     const claude = ProviderDriverKind.make("claudeAgent");
 
+    expect(normalizeModelSlug("fable", claude)).toBe("claude-fable-5-1");
     expect(normalizeModelSlug("opus", claude)).toBe("claude-opus-5");
     expect(normalizeCustomModelSlug(" opus ")).toBe("opus");
   });