fix: list a resumed Fable 5.1 session's model as fable (#1548)

## Problem The Claude Code CLI gives Fable 5.1 a different option value depending on how the session starts: - **New session:** listed as `fable` (`resolvedModel: "claude-fable-5-1"`). - **Resumed session whose transcript last ran Fable 5.1:** the same row is listed as `claude-fable-5-1`, and there is no `fable` row. Reproduced with CLI 2.1.310. Other families don't do this: a session resumed on `claude-opus-5-5` still lists `opus`. Clients store the option value as the session's choice. A choice saved in a new session (`fable`) is not among the options of a resumed one, and the reverse. A client that checks its saved value against the options then refuses to apply it, and the model choice keeps flipping between the two ids. ## Fix `aliasResumedModelRow` in `session-model.ts` gives the resumed model's row its family alias back. It changes a row only when: - the row's value is the model read from the resumed transcript, equals its own `resolvedModel` and has no context hint, - no row already has the alias, and - no newer version of the family is listed, since the alias runs the family's newest model. `createSession` applies it before the allowlist and model selection, so new, resumed and loaded sessions over ACP v1 and v2 all get the same list. This is a workaround for the CLI listing the row under a different value. The CLI also lists rows this way when `ANTHROPIC_MODEL` or `settings.model` pins `claude-fable-5-1`. Those rows keep the pinned id on purpose: relabelling them would turn the user's pinned version into the alias, which moves to a newer Fable when one ships. ## Tests - `acp-agent-settings.test.ts`: a session resumed on `claude-fable-5-1` offers `fable` as the current value and option, with no `setModel` call. Fails without the fix. - `model-resolution.test.ts`: unit cases for the relabel and for each case that keeps the list as it is. - Checked end to end with the real CLI: a Fable turn, then `session/load`, reports `fable`.
2026-11-09 19:02:05 +00:00 · 2026-11-09 13:47:11 +01:00
parent ef65ab31b4
commit 4cceb06b31
4 changed files with 188 additions and 18 deletions
--- a/src/acp-agent.ts
+++ b/src/acp-agent.ts
@@ -362,6 +362,7 @@ import {
  SessionModeManager,
 } from "./session-mode.js";
 import {
+  aliasResumedModelRow,
  applyAvailableModelsAllowlist,
  buildModelConfigOption,
  getAvailableModels,
@@ -12519,14 +12520,21 @@ export class ClaudeAcpAgent {
      // configured. Managed `deniedModels` drops entries the CLI would refuse.
      const settingsAvailableModels = settingsManager.getSettings().availableModels;
      const settingsModelOverrides = settingsManager.getSettings().modelOverrides;
+      const resumedModel = await resumedModelHint;
+      // A resumed Fable 5.1 session lists Fable 5.1 as `fable`, as a new
+      // session does (see `aliasResumedModelRow`).
+      const sdkModels = aliasResumedModelRow(
+        initializationResult.models,
+        creationOpts.resume !== undefined ? resumedModel : undefined,
+      );
      const settingsAllowedModels = Array.isArray(settingsAvailableModels)
        ? applyAvailableModelsAllowlist(
-            initializationResult.models,
+            sdkModels,
            settingsAvailableModels,
            settingsModelOverrides,
            settingsManager.getManagedDeniedModels(),
          )
-        : initializationResult.models;
+        : sdkModels;
      // Then keep only the models the `providers/set` gateway listed.
      const allowedModels = resolvedProvider?.listedModels
        ? narrowToGatewayModels(
@@ -12544,7 +12552,7 @@ export class ClaudeAcpAgent {
        this.logger,
        creationOpts.resume !== undefined,
        sessionId,
-        await resumedModelHint,
+        resumedModel,
        resolvedProvider?.listedModels !== undefined,
      );
      timing.phase("models");
@@ -12560,7 +12568,7 @@ export class ClaudeAcpAgent {
      const allowlistedModelInfo = allowedModels.find((m) => m.value === models.currentModelId);
      const fallbackModelInfo = allowlistedModelInfo
        ? undefined
-        : (resolveModelPreference(initializationResult.models, models.currentModelId) ?? undefined);
+        : (resolveModelPreference(sdkModels, models.currentModelId) ?? undefined);
      const currentModelInfo = allowlistedModelInfo ?? fallbackModelInfo;
      // Register the fallback-resolved capabilities under the verbatim live id
      // so every modelInfos consumer (buildConfigOptions' effort lookup, later
--- a/src/session-model.ts
+++ b/src/session-model.ts
@@ -281,6 +281,66 @@ function resolveSettingsModel(

 const MODEL_FAMILY_ALIASES = new Set(["opus", "sonnet", "haiku", "fable"]);

+/** The family and version of a `claude-<family>-<major>[-<minor>]` id:
+ *  `claude-fable-5-1` is fable 5.1, `claude-haiku-4-5-20251001` haiku 4.5. */
+function familyVersionOf(id: string): { family: string; major: number; minor: number } | null {
+  const match = id.toLowerCase().match(/^claude-([a-z]+)-(\d+)(?:-(\d{1,2}))?(?![\d.])/);
+  if (!match) return null;
+  return { family: match[1], major: Number(match[2]), minor: Number(match[3] ?? 0) };
+}
+
+/**
+ * Give back the family alias value to the row the CLI lists under a resumed
+ * session's model id.
+ *
+ * The CLI lists Fable 5.1 as `fable`, but when it resumes a session that ran
+ * Fable 5.1 it lists the same row as `claude-fable-5-1` and has no `fable`
+ * row. Clients store the option value as the session's choice, so a value
+ * from a new session would not be offered by a resumed one, and the reverse.
+ *
+ * A row gets its family's alias as its value when its `value` is
+ * `resumedModel`, equals its own `resolvedModel` and has no context hint, no
+ * row has that alias, and no row of the family has a newer version (an alias
+ * runs its family's newest model). Every other row is returned as it is.
+ *
+ * The CLI lists a row the same way when `ANTHROPIC_MODEL` or `settings.model`
+ * pins `claude-fable-5-1`. Those rows keep the pinned id, so a client's choice
+ * stays on that version after a newer Fable becomes `fable`.
+ */
+export function aliasResumedModelRow(
+  models: ModelInfo[],
+  resumedModel: string | undefined,
+): ModelInfo[] {
+  if (resumedModel === undefined) return models;
+  const resumed = canonicalizeModelId(resumedModel);
+  const values = new Set(models.map((model) => model.value));
+  const versions = models.flatMap((model) => {
+    const version = familyVersionOf(model.resolvedModel ?? model.value);
+    return version ? [version] : [];
+  });
+  return models.map((model) => {
+    const version = familyVersionOf(model.value);
+    if (
+      version === null ||
+      !MODEL_FAMILY_ALIASES.has(version.family) ||
+      values.has(version.family) ||
+      model.resolvedModel !== model.value ||
+      contextHintOf(model.value) !== null ||
+      canonicalizeModelId(model.value) !== resumed ||
+      versions.some(
+        (other) =>
+          other.family === version.family &&
+          (other.major > version.major ||
+            (other.major === version.major && other.minor > version.minor)),
+      )
+    ) {
+      return model;
+    }
+    values.add(version.family);
+    return { ...model, value: version.family };
+  });
+}
+
 /** A model id without the spellings that name the same model: provider
 *  prefixes, dates, version suffixes, `:variant` suffixes, dotted versions,
 *  `-fast` and context hints. `deniedModels` and gateway listings both match on
--- a/src/tests/acp-agent-settings.test.ts
+++ b/src/tests/acp-agent-settings.test.ts
@@ -30,22 +30,22 @@ describe("ClaudeAcpAgent settings", () => {
    } as unknown as AcpClient;
  }

-  function mockQuery() {
+  function mockQuery(
+    models: unknown[] = [
+      {
+        value: "claude-sonnet-4-6",
+        displayName: "Claude Sonnet 4.5",
+        description: "Default",
+        supportsAutoMode: true,
+      },
+    ],
+  ) {
    let capturedOptions: any;
    const setModelSpy = vi.fn();
    querySpy.mockImplementation(({ options }: any) => {
      capturedOptions = options;
      return makeMockQuery({
-        initializationResult: async () => ({
-          models: [
-            {
-              value: "claude-sonnet-4-6",
-              displayName: "Claude Sonnet 4.5",
-              description: "Default",
-              supportsAutoMode: true,
-            },
-          ],
-        }),
+        initializationResult: async () => ({ models }),
        setModel: setModelSpy,
      }) as any;
    });
@@ -432,7 +432,7 @@ describe("ClaudeAcpAgent settings", () => {
  describe("on resume", () => {
    const sessionId = "resumed-mode-session";

-    async function setup(records: unknown[]) {
+    async function setup(records: unknown[], models?: unknown[]) {
      await fs.promises.writeFile(
        path.join(tempDir, "settings.json"),
        JSON.stringify({ permissions: { defaultMode: "plan" } }),
@@ -446,11 +446,11 @@ describe("ClaudeAcpAgent settings", () => {
        records.map((record) => JSON.stringify(record)).join("\n") + "\n",
      );

-      const { getCapturedOptions } = mockQuery();
+      const { getCapturedOptions, setModelSpy } = mockQuery(models);
      const { ClaudeAcpAgent } = await import("../acp-agent.js");
      const agent: ClaudeAcpAgentType = new ClaudeAcpAgent(createMockClient());
      (agent as any).logger = { log: () => {}, error: () => {} };
-      return { agent, projectDir, getCapturedOptions };
+      return { agent, projectDir, getCapturedOptions, setModelSpy };
    }

    async function resume(records: unknown[], meta: object = {}) {
@@ -479,6 +479,26 @@ describe("ClaudeAcpAgent settings", () => {
      previousUuid = uuid;
      return record;
    };
+    const assistant = (model: string) => {
+      const uuid = crypto.randomUUID();
+      const record = {
+        type: "assistant",
+        uuid,
+        parentUuid: previousUuid,
+        sessionId,
+        timestamp: new Date().toISOString(),
+        isSidechain: false,
+        message: {
+          id: `msg_${uuid}`,
+          type: "message",
+          role: "assistant",
+          model,
+          content: [{ type: "text", text: "ok" }],
+        },
+      };
+      previousUuid = uuid;
+      return record;
+    };

    it("continues in the last permission mode of the transcript", async () => {
      const { permissionMode, response } = await resume([user("plan"), user("acceptEdits")]);
@@ -513,6 +533,41 @@ describe("ClaudeAcpAgent settings", () => {
      expect(response.modes.currentModeId).toBe("plan");
    });

+    it("offers Fable 5.1 as `fable` when the CLI resumed on it", async () => {
+      // The CLI names the Fable 5.1 row after its startup model, which on
+      // resume is the transcript's model; a fresh session lists it as `fable`.
+      const fable = (value: string, resolvedModel: string, displayName: string) => ({
+        value,
+        resolvedModel,
+        displayName,
+        description: "",
+      });
+      const { agent, projectDir, setModelSpy } = await setup(
+        [user(), assistant("claude-fable-5-1")],
+        [
+          fable("default", "claude-opus-5-5", "Default (recommended)"),
+          fable("claude-fable-5-1", "claude-fable-5-1", "Fable 5.1"),
+          fable("claude-fable-5", "claude-fable-5", "Fable 5"),
+        ],
+      );
+
+      const response = await (agent as any).createSession(
+        { cwd: projectDir, mcpServers: [], _meta: { disableBuiltInTools: true } },
+        { resume: sessionId },
+      );
+
+      const model = response.configOptions.find((option: any) => option.id === "model");
+      expect(model).toMatchObject({
+        currentValue: "fable",
+        options: [
+          { value: "default" },
+          { value: "fable", name: "Fable 5.1" },
+          { value: "claude-fable-5", name: "Fable 5" },
+        ],
+      });
+      expect(setModelSpy).not.toHaveBeenCalled();
+    });
+
    it("continues in the last permission mode on session/load", async () => {
      const { agent, projectDir, getCapturedOptions } = await setup([
        user("plan"),
--- a/src/tests/model-resolution.test.ts
+++ b/src/tests/model-resolution.test.ts
@@ -4,6 +4,7 @@ import { query } from "@anthropic-ai/claude-agent-sdk";
 import type { ModelInfo } from "@anthropic-ai/claude-agent-sdk";
 import type { SessionConfigOption } from "@agentclientprotocol/sdk/unstable";
 import {
+  aliasResumedModelRow,
  resolveModelPreference,
  applyAvailableModelsAllowlist,
  isDeniedModel,
@@ -196,6 +197,52 @@ describe("matchResumedModel", () => {
  });
 });

+describe("aliasResumedModelRow", () => {
+  const row = (value: string, resolvedModel = value): ModelInfo => ({
+    value,
+    resolvedModel,
+    displayName: value,
+    description: "",
+  });
+  // What the CLI lists after resuming a session on `claude-fable-5-1`.
+  const STARTED_ON_FABLE = [
+    row("default", "claude-opus-5-5"),
+    row("claude-fable-5-1"),
+    row("claude-fable-5"),
+  ];
+
+  it("lists the resumed model's row under its family alias", () => {
+    expect(aliasResumedModelRow(STARTED_ON_FABLE, "claude-fable-5-1").map((m) => m.value)).toEqual([
+      "default",
+      "fable",
+      "claude-fable-5",
+    ]);
+  });
+
+  it.each([
+    [
+      "the alias already has a row",
+      [row("fable", "claude-fable-5-1"), row("claude-fable-5")],
+      "claude-fable-5",
+    ],
+    ["the row is not the resumed model", STARTED_ON_FABLE, "claude-opus-5-5"],
+    ["the session is not resumed", STARTED_ON_FABLE, undefined],
+    [
+      "the family has a newer row",
+      [row("claude-fable-5-1"), row("claude-fable-5")],
+      "claude-fable-5",
+    ],
+    [
+      "the row carries a context hint",
+      [row("claude-fable-5-1[1m]", "claude-fable-5-1")],
+      "claude-fable-5-1[1m]",
+    ],
+    ["the row is not a family model", [row("claude-gpt-9")], "claude-gpt-9"],
+  ])("keeps every value when %s", (_case, models, resumedModel) => {
+    expect(aliasResumedModelRow(models, resumedModel)).toEqual(models);
+  });
+});
+
 describe("applyAvailableModelsAllowlist - resolvedModel matching", () => {
  it("resolves an allowlist entry pinned to a canonical resolvedModel id", () => {
    const result = applyAvailableModelsAllowlist(LIVE_SHAPED_MODELS, ["claude-sonnet-5"]);