fix: explain why Claude Code turns Fast mode off (#1602)

When the user turns Fast mode on, Claude Code can report
`fast_mode_state: "off"` on the next turn's `system`/init. The adapter
mirrored that into the option but explained it only for reasons listed
in `FAST_MODE_UNAVAILABLE_EXPLANATIONS`. `preference`, `unknown` and any
newer code were dropped, so the toggle snapped off with no reason. This
happens on every turn behind an API gateway that authenticates with
`ANTHROPIC_AUTH_TOKEN` only: Claude Code treats Fast mode as disabled by
the organization (`preference`) without asking the server.

Changes:
- `preference` ("not enabled for your organization or API gateway") and
`unknown` ("currently unavailable") are explained. A reason code the SDK
adds later gets a generic `not available (<code>)`.
- Only `sdk_opt_in_required` (every SDK session starts there; the toggle
is the opt-in) and `pending` (eligibility still resolving) stay
unexplained.
- The notice rule is unchanged: Fast mode was on, then off with an
explained reason. Off without a reason stays quiet, because a `result`
can carry the state from the start of the turn.
- One shared function in `session-fast-mode.ts` serves v1 and v2. In
`src/v1/acp-agent.ts` only comments change.

Limitation: the adapter still offers the toggle up front. Until a
session opts in, Claude Code reports `sdk_opt_in_required` and hides the
account or organization blocker, so the adapter can't tell in advance
that Fast mode will be refused.

Tests: unit tests for each reason, plus v1 and v2 flow tests (set `fast`
on, prompt, init reports off/`preference`). The flow tests fail without
the fix.

src/session-fast-mode.ts

@@ -28,16 +28,9 @@ export function fastModeStateEnabled(state: FastModeState): boolean {
   return state !== "off";
 }
 
-/** User-facing explanations for the SDK's `fast_mode_disabled_reason` values
- *  that a user can act on (or at least wants to know about). Deliberately
- *  partial — the omitted reasons are not worth surfacing:
- *   - `sdk_opt_in_required`: every SDK session starts here (the toggle IS the
- *     opt-in), so it describes the default, not a problem.
- *   - `preference`: the user turned Fast mode off themselves.
- *   - `pending`: eligibility is still resolving; the next report supersedes it.
- *   - `unknown`: nothing meaningful to say.
- *  Unknown future reasons fall through the same way (open set — the SDK's docs
- *  say to ignore values you don't handle). */
+/** User-facing explanations for the SDK's `fast_mode_disabled_reason` values.
+ *  Every reason is explained except the two in {@link FAST_MODE_UNEXPLAINED_REASONS};
+ *  a code missing here gets the generic text from {@link explainFastModeDisabledReason}. */
 const FAST_MODE_UNAVAILABLE_EXPLANATIONS: Partial<Record<FastModeDisabledReason, string>> = {
   free: "not available on the free plan",
   extra_usage_disabled: "requires extra usage to be enabled for this account",
@@ -45,17 +38,38 @@ const FAST_MODE_UNAVAILABLE_EXPLANATIONS: Partial<Record<FastModeDisabledReason,
   not_first_party: "not available on this API provider",
   disabled_by_env: "disabled by environment configuration",
   network_error: "eligibility could not be verified (network error)",
+  preference: "not enabled for your organization or API gateway",
+  unknown: "currently unavailable",
 };
 
+/** Reasons that are never explained:
+ *   - `sdk_opt_in_required`: every SDK session starts here (the toggle IS the
+ *     opt-in), so it describes the default, not a problem.
+ *   - `pending`: eligibility is still resolving; the next report supersedes it. */
+const FAST_MODE_UNEXPLAINED_REASONS: ReadonlySet<FastModeDisabledReason> = new Set([
+  "sdk_opt_in_required",
+  "pending",
+]);
+
+/** The user-facing explanation of a `fast_mode_disabled_reason`: `undefined`
+ *  for no reason, `sdk_opt_in_required` or `pending`; the table text when there
+ *  is one; else a generic text naming the code (a reason a newer SDK added). */
+export function explainFastModeDisabledReason(
+  reason: FastModeDisabledReason | undefined,
+): string | undefined {
+  if (reason === undefined || FAST_MODE_UNEXPLAINED_REASONS.has(reason)) return undefined;
+  return FAST_MODE_UNAVAILABLE_EXPLANATIONS[reason] ?? `not available (${reason})`;
+}
+
 /** Normalize an SDK-reported `fast_mode_disabled_reason` to the one we retain:
- *  a reason we have an explanation for, else `undefined`. Keeping only
- *  explainable reasons means state comparisons (see `fastModeStateChange`) track
- *  exactly what the user can see, so routine reports like
- *  `sdk_opt_in_required` never churn the config option. */
+ *  a reason {@link explainFastModeDisabledReason} explains (including codes a
+ *  newer SDK adds), else `undefined`. State comparisons (see
+ *  `fastModeStateChange`) then track exactly what the user can see, so
+ *  `sdk_opt_in_required` and `pending` never churn the config option. */
 export function normalizeFastModeDisabledReason(
   reason: FastModeDisabledReason | undefined,
 ): FastModeDisabledReason | undefined {
-  return reason && FAST_MODE_UNAVAILABLE_EXPLANATIONS[reason] ? reason : undefined;
+  return explainFastModeDisabledReason(reason) !== undefined ? reason : undefined;
 }
 
 /** Build the Fast mode config option. When the Client supports boolean config
@@ -73,9 +87,7 @@ export function createFastModeConfigOption(
   useBooleanOption: boolean,
   disabledReason?: FastModeDisabledReason,
 ): SessionConfigOption {
-  const explanation = enabled
-    ? undefined
-    : disabledReason && FAST_MODE_UNAVAILABLE_EXPLANATIONS[disabledReason];
+  const explanation = enabled ? undefined : explainFastModeDisabledReason(disabledReason);
   const base = {
     id: FAST_MODE_CONFIG_ID,
     name: "Fast mode",
@@ -141,15 +153,17 @@ export type FastModeOptionState = {
  *  off), or the same state with the same explainable reason.
  *
  *  `reason` is the SDK's `fast_mode_disabled_reason`, reported alongside the
- *  state. Only explainable reasons are retained (see
- *  {@link normalizeFastModeDisabledReason}), so a routine
- *  `sdk_opt_in_required` report on every turn's result can't churn the
- *  option, while a real blocker updates the description even when the
+ *  state. Every reason except `sdk_opt_in_required` and `pending` is retained
+ *  (see {@link normalizeFastModeDisabledReason}; unknown future codes get the
+ *  generic text), so `sdk_opt_in_required` on every turn's result can't churn
+ *  the option, while a real blocker updates the description even when the
  *  toggle's own value is unchanged.
  *
  *  `notice` is set when the user had Fast mode on and the SDK turned it off
- *  for an explainable reason: the description carries the same explanation,
- *  but a toggle silently snapping back is worth saying out loud once. */
+ *  with a retained reason: the description carries the same explanation, but a
+ *  toggle silently snapping back is worth saying out loud once. Off with no
+ *  reason, `sdk_opt_in_required` or `pending` stays quiet: a `result` can carry
+ *  the turn-start snapshot, so a notice there would falsely say "turned off". */
 export function fastModeStateChange(
   current: { enabled: boolean; disabledReason?: FastModeDisabledReason },
   state: FastModeState | undefined,
@@ -163,8 +177,7 @@ export function fastModeStateChange(
   // so it can't decorate the option the next time fast mode goes off.
   const disabledReason = enabled ? undefined : normalizeFastModeDisabledReason(reason);
   if (enabled === current.enabled && disabledReason === current.disabledReason) return undefined;
-  const explanation =
-    disabledReason !== undefined ? FAST_MODE_UNAVAILABLE_EXPLANATIONS[disabledReason] : undefined;
+  const explanation = explainFastModeDisabledReason(disabledReason);
   if (!current.enabled || enabled || explanation === undefined) {
     return { enabled, disabledReason };
   }

src/tests/v1/acp-agent.test.ts

@@ -28,11 +28,8 @@ import { nodeToWebWritable, nodeToWebReadable } from "../../utils.js";
 import { createPostToolUseHook, markdownEscape, toDisplayPath } from "../../tools.js";
 import { toolUpdateFromDiffToolResponse } from "../../diff.js";
 import { ToolCallFieldTracker } from "../../tool-calls/field-tracker.js";
-import {
-  ClaudeAcpAgent,
-  computeSessionFingerprint,
-  runPromptWithCancellation,
-} from "../../v1/acp-agent.js";
+import { ClaudeAcpAgent, runPromptWithCancellation } from "../../v1/acp-agent.js";
+import { computeSessionFingerprint } from "../../v1/session-fingerprint.js";
 import type { SteerRequest } from "../../v1/steering.js";
 import { buildConfigOptions, discoverCustomAgents } from "../../session-config-options.js";
 import { createFastModeConfigOption } from "../../session-fast-mode.js";

src/tests/v1/fast-mode-config.test.ts

@@ -16,6 +16,14 @@ import {
   FAST_MODE_OFF,
 } from "../../session-fast-mode.js";
 import type { AcpClient } from "../../acp-client.js";
+import { Pushable } from "../../utils.js";
+import { initializeClient } from "../helpers.js";
+import {
+  mockSessionState,
+  successfulResultMessage,
+  userEcho,
+  wrapQuery,
+} from "./session-doubles.js";
 
 const MODES = {
   currentModeId: "default",
@@ -33,6 +41,8 @@ const MODEL_INFOS: ModelInfo[] = [
   { value: "claude-opus-4-8", displayName: "Claude Opus", description: "Most capable" },
 ];
 
+const FUTURE_REASON = "some_future_reason" as FastModeDisabledReason;
+
 describe("createFastModeConfigOption", () => {
   it("produces a native boolean toggle when the client opted in", () => {
     expect(createFastModeConfigOption(true, true)).toEqual({
@@ -45,20 +55,40 @@ describe("createFastModeConfigOption", () => {
     });
   });
 
-  it("explains a blocking disabled reason while the toggle reads off", () => {
-    const option = createFastModeConfigOption(false, true, "free");
-    expect(option.description).toBe(
-      "Faster responses on supported models — not available on the free plan",
-    );
-  });
+  /**
+   * Scenario:
+   * 1. Build the option with the toggle off and a disabled reason.
+   * Expected:
+   * - The description is the base text plus the reason's explanation.
+   * - A code the table lacks gets the generic text naming the code.
+   */
+  it.each([
+    ["free", "not available on the free plan"],
+    ["preference", "not enabled for your organization or API gateway"],
+    ["unknown", "currently unavailable"],
+    [FUTURE_REASON, "not available (some_future_reason)"],
+  ] as [FastModeDisabledReason, string][])(
+    "explains the disabled reason %s while the toggle reads off",
+    (reason, explanation) => {
+      const option = createFastModeConfigOption(false, true, reason);
+      expect(option.description).toBe(`Faster responses on supported models — ${explanation}`);
+    },
+  );
 
-  it("ignores routine and on-state reasons", () => {
+  /**
+   * Scenario:
+   * 1. Build the option, toggle off, with `sdk_opt_in_required` or `pending`.
+   * 2. Build the option, toggle on, with `free`.
+   * Expected:
+   * - The description stays the base text in all three cases.
+   */
+  it("ignores unexplained and on-state reasons", () => {
     // Every SDK session starts at `sdk_opt_in_required` — the toggle IS the
     // opt-in, so it must not read as a blocker.
     expect(createFastModeConfigOption(false, true, "sdk_opt_in_required").description).toBe(
       "Faster responses on supported models",
     );
-    expect(createFastModeConfigOption(false, true, "preference").description).toBe(
+    expect(createFastModeConfigOption(false, true, "pending").description).toBe(
       "Faster responses on supported models",
     );
     // A reason riding an `on` state isn't blocking anything right now.
@@ -133,25 +163,38 @@ describe("fastModeStateEnabled", () => {
 });
 
 describe("normalizeFastModeDisabledReason", () => {
-  it("retains only reasons we have an explanation for", () => {
-    expect(normalizeFastModeDisabledReason("free")).toBe("free");
-    expect(normalizeFastModeDisabledReason("extra_usage_disabled")).toBe("extra_usage_disabled");
-    expect(normalizeFastModeDisabledReason("model_not_allowed")).toBe("model_not_allowed");
-    expect(normalizeFastModeDisabledReason("not_first_party")).toBe("not_first_party");
-    expect(normalizeFastModeDisabledReason("disabled_by_env")).toBe("disabled_by_env");
-    expect(normalizeFastModeDisabledReason("network_error")).toBe("network_error");
+  /**
+   * Scenario:
+   * 1. Normalize each explained reason, including `preference`, `unknown` and a future code.
+   * Expected:
+   * - Each reason is returned unchanged.
+   */
+  it.each([
+    "free",
+    "extra_usage_disabled",
+    "model_not_allowed",
+    "not_first_party",
+    "disabled_by_env",
+    "network_error",
+    "preference",
+    "unknown",
+    FUTURE_REASON,
+  ] as FastModeDisabledReason[])("retains the explained reason %s", (reason) => {
+    expect(normalizeFastModeDisabledReason(reason)).toBe(reason);
   });
 
-  it("drops routine states and unknown values", () => {
-    expect(normalizeFastModeDisabledReason("sdk_opt_in_required")).toBeUndefined();
-    expect(normalizeFastModeDisabledReason("preference")).toBeUndefined();
-    expect(normalizeFastModeDisabledReason("pending")).toBeUndefined();
-    expect(normalizeFastModeDisabledReason("unknown")).toBeUndefined();
-    expect(normalizeFastModeDisabledReason(undefined)).toBeUndefined();
-    expect(
-      normalizeFastModeDisabledReason("some_future_reason" as FastModeDisabledReason),
-    ).toBeUndefined();
-  });
+  /**
+   * Scenario:
+   * 1. Normalize `sdk_opt_in_required`, `pending` and `undefined`.
+   * Expected:
+   * - Each normalizes to `undefined`.
+   */
+  it.each(["sdk_opt_in_required", "pending", undefined] as (FastModeDisabledReason | undefined)[])(
+    "drops the unexplained reason %s",
+    (reason) => {
+      expect(normalizeFastModeDisabledReason(reason)).toBeUndefined();
+    },
+  );
 });
 
 describe("buildConfigOptions Fast mode", () => {
@@ -403,29 +446,61 @@ describe("syncFastModeState (SDK-driven state changes)", () => {
     expect(sessionUpdates).toHaveLength(0);
   });
 
-  it("explains a blocking reason once when the SDK refuses a toggle the user turned on", async () => {
+  /**
+   * Scenario:
+   * 1. The toggle is on and the SDK reports `off` with a reason.
+   * 2. The SDK reports the same `off` and reason again.
+   * Expected:
+   * - The toggle is cleared and the reason retained.
+   * - One transcript line carries the explanation, then one `config_option_update`.
+   * - The repeat report sends nothing.
+   */
+  it.each([
+    ["extra_usage_disabled", "requires extra usage to be enabled for this account"],
+    ["preference", "not enabled for your organization or API gateway"],
+    ["unknown", "currently unavailable"],
+    [FUTURE_REASON, "not available (some_future_reason)"],
+  ] as [FastModeDisabledReason, string][])(
+    "explains the blocking reason %s once when the SDK refuses a toggle the user turned on",
+    async (reason, explanation) => {
+      const { sync, session, sessionUpdates } = setup({ fastModeEnabled: true, withOption: true });
+
+      await sync(SESSION_ID, session, "off", reason);
+
+      expect(session.fastModeEnabled).toBe(false);
+      expect(session.fastModeDisabledReason).toBe(reason);
+      expect(sessionUpdates).toHaveLength(2);
+      expect(sessionUpdates[0].update).toEqual({
+        sessionUpdate: "agent_message_chunk",
+        content: {
+          type: "text",
+          text: `\n\n> **Fast mode turned off:** ${explanation}.\n\n`,
+        },
+      });
+      expect(sessionUpdates[1].update).toMatchObject({ sessionUpdate: "config_option_update" });
+      expect(session.configOptions).toContainEqual(createFastModeConfigOption(false, true, reason));
+
+      // The same report again changes nothing the user can see: no repeat notice.
+      await sync(SESSION_ID, session, "off", reason);
+      expect(sessionUpdates).toHaveLength(2);
+    },
+  );
+
+  /**
+   * Scenario:
+   * 1. The toggle is on and the SDK reports `off` with `sdk_opt_in_required`.
+   * Expected:
+   * - The toggle is cleared and no reason is retained.
+   * - Only a `config_option_update` is sent: no notice or transcript line.
+   */
+  it("clears the toggle quietly when the SDK reports off with sdk_opt_in_required", async () => {
     const { sync, session, sessionUpdates } = setup({ fastModeEnabled: true, withOption: true });
 
-    await sync(SESSION_ID, session, "off", "extra_usage_disabled");
+    await sync(SESSION_ID, session, "off", "sdk_opt_in_required");
 
     expect(session.fastModeEnabled).toBe(false);
-    expect(session.fastModeDisabledReason).toBe("extra_usage_disabled");
-    expect(sessionUpdates).toHaveLength(2);
-    expect(sessionUpdates[0].update).toEqual({
-      sessionUpdate: "agent_message_chunk",
-      content: {
-        type: "text",
-        text: "\n\n> **Fast mode turned off:** requires extra usage to be enabled for this account.\n\n",
-      },
-    });
-    expect(sessionUpdates[1].update).toMatchObject({ sessionUpdate: "config_option_update" });
-    expect(session.configOptions).toContainEqual(
-      createFastModeConfigOption(false, true, "extra_usage_disabled"),
-    );
-
-    // The same report again changes nothing the user can see: no repeat notice.
-    await sync(SESSION_ID, session, "off", "extra_usage_disabled");
-    expect(sessionUpdates).toHaveLength(2);
+    expect(session.fastModeDisabledReason).toBeUndefined();
+    expect(sessionUpdates.map((n) => n.update.sessionUpdate)).toEqual(["config_option_update"]);
   });
 
   it("explains the flip as a live notice to a client on the notice contract", async () => {
@@ -493,3 +568,89 @@ describe("syncFastModeState (SDK-driven state changes)", () => {
     expect(disabledCase.sessionUpdates).toHaveLength(0);
   });
 });
+
+describe("Fast mode that Claude Code refuses during a turn", () => {
+  const SESSION_ID = "test-session";
+
+  /** A session whose Claude Code answers the first prompt with a `system`/init
+   *  that reports Fast mode off with `reason`, as it does on every turn. */
+  async function setupTurn(reason: FastModeDisabledReason) {
+    const updates: SessionNotification[] = [];
+    const client = {
+      sessionUpdate: async (n: SessionNotification) => {
+        updates.push(n);
+      },
+    } as unknown as AcpClient;
+    const agent = new ClaudeAcpAgent(client, { log: () => {}, error: () => {} });
+    await initializeClient(agent, { session: { configOptions: { boolean: {} } } });
+
+    const input = new Pushable<any>();
+    async function* messages() {
+      const { value: prompt } = await input[Symbol.asyncIterator]().next();
+      yield userEcho(prompt);
+      yield {
+        type: "system",
+        subtype: "init",
+        fast_mode_state: "off",
+        fast_mode_disabled_reason: reason,
+        session_id: SESSION_ID,
+      };
+      yield successfulResultMessage();
+      yield { type: "system", subtype: "session_state_changed", state: "idle" };
+    }
+    const query = Object.assign(wrapQuery(messages()), { applyFlagSettings: async () => {} });
+    agent.sessions[SESSION_ID] = mockSessionState({
+      query,
+      input,
+      fastModeEnabled: false,
+      configOptions: [createFastModeConfigOption(false, true)],
+    });
+    return { agent, updates };
+  }
+
+  /**
+   * Scenario:
+   * 1. The client, without the notices capability, turns Fast mode on.
+   * 2. It sends a prompt, and Claude Code's init reports Fast mode off, reason `preference`.
+   * Expected:
+   * - The transcript gets one line that explains the organization or gateway has not enabled it.
+   * - Then one `config_option_update` where `fast` reads off, with the explanation in its description.
+   */
+  it("explains a Fast mode that the organization or gateway has not enabled", async () => {
+    const { agent, updates } = await setupTurn("preference");
+
+    await agent.setSessionConfigOption({
+      sessionId: SESSION_ID,
+      configId: "fast",
+      type: "boolean",
+      value: true,
+    });
+    updates.length = 0;
+    await agent.prompt({ sessionId: SESSION_ID, prompt: [{ type: "text", text: "hi" }] });
+
+    const explained = updates
+      .map((n) => n.update)
+      .filter((u) => u.sessionUpdate !== "usage_update");
+    expect(explained).toEqual([
+      {
+        sessionUpdate: "agent_message_chunk",
+        content: {
+          type: "text",
+          text: "\n\n> **Fast mode turned off:** not enabled for your organization or API gateway.\n\n",
+        },
+      },
+      {
+        sessionUpdate: "config_option_update",
+        configOptions: [
+          expect.objectContaining({
+            id: "fast",
+            currentValue: false,
+            description: expect.stringMatching(
+              / — not enabled for your organization or API gateway$/,
+            ),
+          }),
+        ],
+      },
+    ]);
+  });
+});

src/tests/v2/acp-v2.test.ts

@@ -1445,6 +1445,51 @@ describe("ACP v2 sessions", () => {
       });
     });
 
+    /**
+     * Scenario:
+     * 1. The client turns Fast mode on and sends a prompt.
+     * 2. Claude Code's `system`/init of that turn reports Fast mode off, reason `preference`.
+     * Expected:
+     * - One warning notice explains that the organization or gateway has not enabled it.
+     * - The `fast` option reads off, and its description ends with the explanation.
+     */
+    it("explains a Fast mode that the organization or gateway has not enabled", async () => {
+      scriptTurns(async function* (options) {
+        yield fastModeInit(options, "off", "preference");
+        yield assistantText(options, "Done.");
+        yield result(options);
+        yield idleState(options);
+      });
+      const client = v2Client();
+      await client.app.connectWith(connectRouter(), async (agent) => {
+        await initializeV2(agent);
+        const { sessionId } = await agent.request(v2.methods.agent.session.new, { cwd });
+        await agent.request(v2.methods.agent.session.setConfigOption, {
+          sessionId,
+          configId: "fast",
+          type: "boolean",
+          value: true,
+        });
+        await agent.request(v2.methods.agent.session.prompt, { sessionId, prompt: text("hi") });
+        await vi.waitFor(() => expect(turnTrace(client.sessionUpdates)).toContain("idle end_turn"));
+        await client.authUpdate(1);
+      });
+
+      expect(client.updates("notice")).toEqual([
+        {
+          sessionUpdate: "notice",
+          severity: "warning",
+          title: "Fast mode turned off",
+          description: "Not enabled for your organization or API gateway.",
+        },
+      ]);
+      expect(fastOption(client.updates("config_option_update").at(-1))).toMatchObject({
+        currentValue: false,
+        description:
+          "Faster responses on supported models — not enabled for your organization or API gateway",
+      });
+    });
+
     it("reports a refusal fallback with a notice and follows the model it swapped to", async () => {
       sdk.initializationResult = async () => ({ models: [OPUS, HAIKU] });
       scriptTurns(async function* (options) {
@@ -1691,6 +1736,23 @@ function stream(options: Options, event: Record<string, unknown>) {
   };
 }
 
+/** Claude Code's per-turn `system`/init, reporting the Fast mode state. */
+function fastModeInit(options: Options, state: string, reason: string) {
+  return {
+    type: "system",
+    subtype: "init",
+    fast_mode_state: state,
+    fast_mode_disabled_reason: reason,
+    session_id: options.sessionId,
+    uuid: randomUUID(),
+  };
+}
+
+/** The `fast` option of a `config_option_update`. */
+function fastOption(update: { configOptions: { configId: string }[] } | undefined) {
+  return update?.configOptions.find((option) => option.configId === "fast");
+}
+
 function idleState(options: Options) {
   return {
     type: "system",

src/tests/v2/v2.test.ts

@@ -2433,7 +2433,7 @@ describe("ACP v2 core", () => {
     expect(client.sessionUpdates.at(-1)).toMatchObject({
       state: "idle",
       stopReason: "error",
-      error: { code: -32000 },
+      error: { code: -32000, message: "Authentication required" },
     });
   });
 
@@ -2442,7 +2442,7 @@ describe("ACP v2 core", () => {
       "an authentication failure as authRequired",
       "authentication_failed",
       "Failed to authenticate. API Error: 401",
-      { code: -32000 },
+      { code: -32000, message: "Authentication required: Failed to authenticate. API Error: 401" },
     ],
     [
       "another failure with its error kind",
@@ -2505,6 +2505,53 @@ describe("ACP v2 core", () => {
     },
   );
 
+  it("reports a failed API request that the turn goes on past where it failed", async () => {
+    const text = "API Error: Connection lost mid-response. The response above may be incomplete.";
+    const client = v2Client();
+    await client.connect(async (agent) => {
+      const { claude } = await openSession(agent);
+      claude.send(cycleStart());
+      // Claude Code keeps the tool call of the response that broke off, and runs it.
+      claude.send(
+        assistant("msg_partial", [
+          { type: "tool_use", id: "toolu_kept", name: "Bash", input: { command: "ls" } },
+        ]),
+      );
+      claude.send({
+        ...assistant("msg_api_error", [{ type: "text", text }]),
+        message: {
+          id: "msg_api_error",
+          type: "message",
+          role: "assistant",
+          model: "<synthetic>",
+          content: [{ type: "text", text }],
+          usage: {},
+        },
+        error: "server_error",
+      });
+      claude.send({
+        type: "user",
+        message: {
+          role: "user",
+          content: [{ type: "tool_result", tool_use_id: "toolu_kept", content: "src" }],
+        },
+        parent_tool_use_id: null,
+        uuid: randomUUID(),
+      });
+      claude.send(assistant("msg_after", [{ type: "text", text: "Done." }]));
+      claude.send(result({ result: "Done." }));
+      await vi.waitFor(() => expect(client.states()).toEqual(["running", "idle"]));
+    });
+
+    const notice = { sessionUpdate: "notice", severity: "error", title: text };
+    const shown = client.sessionUpdates.filter(
+      (update) =>
+        update.sessionUpdate === "notice" || update.sessionUpdate === "agent_message_chunk",
+    );
+    expect(shown).toMatchObject([notice, { sessionUpdate: "agent_message_chunk" }]);
+    expect(shown[0]).toEqual(notice);
+  });
+
   it.each([
     [
       "an error at the turn limit",

src/v1/acp-agent.ts

@@ -435,6 +435,7 @@ import {
   stripLocalCommandMetadata,
 } from "../message-filters.js";
 import type { NewSessionMeta, SDKMessageFilter } from "../new-session-meta.js";
+import { computeSessionFingerprint } from "./session-fingerprint.js";
 import { promptToClaude, SUPPORTED_PROMPT_CONTENT_TYPES } from "../prompt-to-claude.js";
 import { AUTONOMOUS_RESULT_ORIGINS, isAbortedResult } from "../result-end.js";
 import {
@@ -1065,12 +1066,13 @@ export type Session = {
    *  independent of user ownership: opted-in clients also apply automatic
    *  recommendations and settings-derived values. */
   appliedEffortLevel?: string;
-  /** Why the SDK currently can't serve Fast mode, when the reason is one worth
-   *  telling the user about (see {@link FAST_MODE_UNAVAILABLE_EXPLANATIONS} —
-   *  routine states like the SDK's own opt-in requirement normalize to
-   *  `undefined`). Refreshed from every `fast_mode_disabled_reason` the SDK
-   *  reports on `system`/init and user-turn `result`s; surfaced in the Fast mode
-   *  option's description so a toggle that snaps back off explains itself. */
+  /** Why the SDK currently can't serve Fast mode: the last
+   *  `fast_mode_disabled_reason` that `explainFastModeDisabledReason` explains.
+   *  {@link normalizeFastModeDisabledReason} drops the two it doesn't,
+   *  `sdk_opt_in_required` and `pending`, to `undefined`. Refreshed from every
+   *  `fast_mode_disabled_reason` the SDK reports on `system`/init and user-turn
+   *  `result`s; surfaced in the Fast mode option's description so a toggle that
+   *  snaps back off explains itself. */
   fastModeDisabledReason?: FastModeDisabledReason;
   abortController: AbortController;
   /** Signal the consumer races `query.next()` against. Aborted by cancel()
@@ -1462,149 +1464,6 @@ function disarmForceCancel(session: Session): void {
   }
 }
 
-/** Normalize skills without changing their SDK semantics. Array order and
- *  duplicates do not affect the selected skill set, so neither should rebuild
- *  the underlying Query process. */
-function normalizeSkills(skills: Options["skills"]): Options["skills"] {
-  return Array.isArray(skills) ? [...new Set(skills)].sort() : skills;
-}
-
-/** Whether a changed `_meta.claudeCode.options` key on session/load or
- *  session/resume must rebuild the Query process. Almost every option is read
- *  only when the process starts, so the default is `true`. Every key of the
- *  SDK's `Options` must be classified: an SDK bump that adds or removes an
- *  option fails to compile until it is decided here. */
-const OPTION_REBUILDS_SESSION = {
-  additionalDirectories: true,
-  agentProgressSummaries: true,
-  agents: true,
-  allowDangerouslySkipPermissions: true,
-  allowedTools: true,
-  betas: true,
-  debug: true,
-  debugFile: true,
-  disallowedTools: true,
-  effort: true,
-  enableFileCheckpointing: true,
-  env: true,
-  executableArgs: true,
-  extraArgs: true,
-  fallbackModel: true,
-  forwardSubagentText: true,
-  includeHookEvents: true,
-  managedSettings: true,
-  maxBudgetUsd: true,
-  maxThinkingTokens: true,
-  maxTurns: true,
-  mcpServers: true,
-  model: true,
-  outputFormat: true,
-  pathToClaudeCodeExecutable: true,
-  permissionPrompts: true,
-  permissionPromptToolName: true,
-  persistSession: true,
-  perTaskStopAffordance: true,
-  planModeInstructions: true,
-  pluginDelivery: true,
-  plugins: true,
-  projectConfigRoot: true,
-  promptSuggestions: true,
-  sandbox: true,
-  sessionStoreFlush: true,
-  settings: true,
-  settingSources: true,
-  skills: true,
-  strictMcpConfig: true,
-  supportedDialogKinds: true,
-  systemPrompt: true,
-  taskBudget: true,
-  thinking: true,
-  toolAliases: true,
-  toolConfig: true,
-  tools: true,
-  verbatimPrompts: true,
-  // Managed or ignored by the adapter.
-  agent: false,
-  cwd: false,
-  executable: false,
-  includePartialMessages: false,
-  permissionMode: false,
-  // Per-call controls for attaching to the stored conversation.
-  continue: false,
-  forkSession: false,
-  resume: false,
-  resumeDropsTurn: false,
-  resumeSessionAt: false,
-  sessionId: false,
-  // Callbacks and live objects: they cannot cross JSON-RPC and have no stable serialized form.
-  abortController: false,
-  canUseTool: false,
-  hooks: false,
-  onElicitation: false,
-  onUserDialog: false,
-  sessionStore: false,
-  spawnClaudeCodeProcess: false,
-  stderr: false,
-  // No effect on an already-created session.
-  loadTimeoutMs: false,
-  title: false,
-} as const satisfies Record<keyof Options, boolean>;
-
-/** A JSON value with object keys sorted, so key order never changes a
- *  fingerprint. Functions and class instances (hook callbacks, abort
- *  controllers, in-process MCP servers) cannot cross JSON-RPC and have no
- *  stable serialized form, so they are left out. */
-function canonicalJson(value: unknown): unknown {
-  if (typeof value === "function") return undefined;
-  if (value === null || typeof value !== "object") return value;
-  if (Array.isArray(value)) return value.map(canonicalJson);
-  const proto = Object.getPrototypeOf(value);
-  if (proto !== Object.prototype && proto !== null) return undefined;
-  return Object.fromEntries(
-    Object.keys(value)
-      .sort()
-      .map((key) => [key, canonicalJson((value as Record<string, unknown>)[key])])
-      .filter(([, entry]) => entry !== undefined),
-  );
-}
-
-/** Compute a stable fingerprint of the session-defining params so we can
- *  detect when a loadSession/resumeSession call requires tearing down and
- *  recreating the underlying Query process. Every option classified in
- *  {@link OPTION_REBUILDS_SESSION} is covered, so a warm resume never silently
- *  keeps stale values. MCP servers are sorted by name, and skills are
- *  normalized as a set, so ordering differences don't trigger unnecessary
- *  recreations. */
-export function computeSessionFingerprint(params: {
-  cwd: string;
-  mcpServers?: NewSessionRequest["mcpServers"];
-  additionalDirectories?: NewSessionRequest["additionalDirectories"];
-  systemPrompt?: NewSessionRequest["systemPrompt"];
-  _meta?: NewSessionRequest["_meta"];
-}): string {
-  const servers = [...(params.mcpServers ?? [])].sort((a, b) => a.name.localeCompare(b.name));
-  const meta = params._meta as (NewSessionMeta & Record<string, unknown>) | undefined;
-  const options: Record<string, unknown> = Object.fromEntries(
-    Object.entries(meta?.claudeCode?.options ?? {}).filter(
-      ([key]) => (OPTION_REBUILDS_SESSION as Record<string, boolean>)[key] === true,
-    ),
-  );
-  options.skills = normalizeSkills(options.skills as Options["skills"]);
-  return JSON.stringify(
-    canonicalJson({
-      cwd: params.cwd,
-      mcpServers: servers,
-      additionalDirectories: params.additionalDirectories,
-      additionalRoots: meta?.additionalRoots,
-      systemPrompt: meta?.systemPrompt,
-      clientSystemPrompt: params.systemPrompt ?? undefined,
-      disableBuiltInTools: meta?.disableBuiltInTools,
-      emitRawSDKMessages: meta?.claudeCode?.emitRawSDKMessages,
-      options,
-    }),
-  );
-}
-
 /** The JSON-RPC code ACP assigns to `authRequired`. Read from the SDK so it
  *  cannot drift from the errors this agent throws. */
 const AUTH_REQUIRED_CODE = RequestError.authRequired().code;
@@ -10452,7 +10311,8 @@ export class ClaudeAcpAgent {
       // `fast_mode_disabled_reason` reflects the post-switch model since SDK
       // 0.3.219 (the initialize response used to answer from the spawn-time
       // model). A fresh SDK session reports `sdk_opt_in_required` — the toggle IS
-      // the opt-in — which normalizes away, so only real blockers are retained.
+      // the opt-in — which normalizes to `undefined`, as does `pending`; every
+      // other reason is retained.
       const fastModeDisabledReason = fastModeEnabled
         ? undefined
         : normalizeFastModeDisabledReason(initializationResult.fast_mode_disabled_reason);

src/v1/session-fingerprint.ts

@@ -0,0 +1,147 @@
+import type { Options } from "@anthropic-ai/claude-agent-sdk";
+import type { NewSessionRequest } from "@agentclientprotocol/sdk/unstable";
+
+import type { NewSessionMeta } from "../new-session-meta.js";
+
+/** Normalize skills without changing their SDK semantics. Array order and
+ *  duplicates do not affect the selected skill set, so neither should rebuild
+ *  the underlying Query process. */
+function normalizeSkills(skills: Options["skills"]): Options["skills"] {
+  return Array.isArray(skills) ? [...new Set(skills)].sort() : skills;
+}
+
+/** Whether a changed `_meta.claudeCode.options` key on session/load or
+ *  session/resume must rebuild the Query process. Almost every option is read
+ *  only when the process starts, so the default is `true`. Every key of the
+ *  SDK's `Options` must be classified: an SDK bump that adds or removes an
+ *  option fails to compile until it is decided here. */
+const OPTION_REBUILDS_SESSION = {
+  additionalDirectories: true,
+  agentProgressSummaries: true,
+  agents: true,
+  allowDangerouslySkipPermissions: true,
+  allowedTools: true,
+  betas: true,
+  debug: true,
+  debugFile: true,
+  disallowedTools: true,
+  effort: true,
+  enableFileCheckpointing: true,
+  env: true,
+  executableArgs: true,
+  extraArgs: true,
+  fallbackModel: true,
+  forwardSubagentText: true,
+  includeHookEvents: true,
+  managedSettings: true,
+  maxBudgetUsd: true,
+  maxThinkingTokens: true,
+  maxTurns: true,
+  mcpServers: true,
+  model: true,
+  outputFormat: true,
+  pathToClaudeCodeExecutable: true,
+  permissionPrompts: true,
+  permissionPromptToolName: true,
+  persistSession: true,
+  perTaskStopAffordance: true,
+  planModeInstructions: true,
+  pluginDelivery: true,
+  plugins: true,
+  projectConfigRoot: true,
+  promptSuggestions: true,
+  sandbox: true,
+  sessionStoreFlush: true,
+  settings: true,
+  settingSources: true,
+  skills: true,
+  strictMcpConfig: true,
+  supportedDialogKinds: true,
+  systemPrompt: true,
+  taskBudget: true,
+  thinking: true,
+  toolAliases: true,
+  toolConfig: true,
+  tools: true,
+  verbatimPrompts: true,
+  // Managed or ignored by the adapter.
+  agent: false,
+  cwd: false,
+  executable: false,
+  includePartialMessages: false,
+  permissionMode: false,
+  // Per-call controls for attaching to the stored conversation.
+  continue: false,
+  forkSession: false,
+  resume: false,
+  resumeDropsTurn: false,
+  resumeSessionAt: false,
+  sessionId: false,
+  // Callbacks and live objects: they cannot cross JSON-RPC and have no stable serialized form.
+  abortController: false,
+  canUseTool: false,
+  hooks: false,
+  onElicitation: false,
+  onUserDialog: false,
+  sessionStore: false,
+  spawnClaudeCodeProcess: false,
+  stderr: false,
+  // No effect on an already-created session.
+  loadTimeoutMs: false,
+  title: false,
+} as const satisfies Record<keyof Options, boolean>;
+
+/** A JSON value with object keys sorted, so key order never changes a
+ *  fingerprint. Functions and class instances (hook callbacks, abort
+ *  controllers, in-process MCP servers) cannot cross JSON-RPC and have no
+ *  stable serialized form, so they are left out. */
+function canonicalJson(value: unknown): unknown {
+  if (typeof value === "function") return undefined;
+  if (value === null || typeof value !== "object") return value;
+  if (Array.isArray(value)) return value.map(canonicalJson);
+  const proto = Object.getPrototypeOf(value);
+  if (proto !== Object.prototype && proto !== null) return undefined;
+  return Object.fromEntries(
+    Object.keys(value)
+      .sort()
+      .map((key) => [key, canonicalJson((value as Record<string, unknown>)[key])])
+      .filter(([, entry]) => entry !== undefined),
+  );
+}
+
+/** Compute a stable fingerprint of the session-defining params so we can
+ *  detect when a loadSession/resumeSession call requires tearing down and
+ *  recreating the underlying Query process. Every option classified in
+ *  {@link OPTION_REBUILDS_SESSION} is covered, so a warm resume never silently
+ *  keeps stale values. MCP servers are sorted by name, and skills are
+ *  normalized as a set, so ordering differences don't trigger unnecessary
+ *  recreations. */
+export function computeSessionFingerprint(params: {
+  cwd: string;
+  mcpServers?: NewSessionRequest["mcpServers"];
+  additionalDirectories?: NewSessionRequest["additionalDirectories"];
+  systemPrompt?: NewSessionRequest["systemPrompt"];
+  _meta?: NewSessionRequest["_meta"];
+}): string {
+  const servers = [...(params.mcpServers ?? [])].sort((a, b) => a.name.localeCompare(b.name));
+  const meta = params._meta as (NewSessionMeta & Record<string, unknown>) | undefined;
+  const options: Record<string, unknown> = Object.fromEntries(
+    Object.entries(meta?.claudeCode?.options ?? {}).filter(
+      ([key]) => (OPTION_REBUILDS_SESSION as Record<string, boolean>)[key] === true,
+    ),
+  );
+  options.skills = normalizeSkills(options.skills as Options["skills"]);
+  return JSON.stringify(
+    canonicalJson({
+      cwd: params.cwd,
+      mcpServers: servers,
+      additionalDirectories: params.additionalDirectories,
+      additionalRoots: meta?.additionalRoots,
+      systemPrompt: meta?.systemPrompt,
+      clientSystemPrompt: params.systemPrompt ?? undefined,
+      disableBuiltInTools: meta?.disableBuiltInTools,
+      emitRawSDKMessages: meta?.claudeCode?.emitRawSDKMessages,
+      options,
+    }),
+  );
+}

src/v2/outcome.ts

@@ -91,7 +91,11 @@ export function idleOutcome(
       providerFailureCategory(kind) === "auth_required");
   return failed(
     authFailure
-      ? RequestError.authRequired()
+      ? // Claude Code's `/login` instruction is for its terminal, so that text
+        // is left out. Any other says why the login failed, e.g. "Failed to
+        // authenticate: this login reached its time limit and cannot be
+        // renewed."
+        RequestError.authRequired(undefined, ended.loginRequired ? undefined : ended.error)
       : RequestError.internalError(kind ? { errorKind: kind } : undefined, ended.error),
   );
 }
@@ -132,8 +136,9 @@ export class TurnReport {
    * - Claude Code's sign-in message. Its `/login` instruction is for Claude
    *   Code's terminal, and the turn's idle fails with `authRequired` instead.
    * - The message that Claude Code wrote for a failed API request (`error` on
-   *   a `<synthetic>` message). The turn's idle carries it when the turn
-   *   fails, and a notice otherwise (see {@link takeApiError}).
+   *   a `<synthetic>` message). A notice carries it where the turn goes on
+   *   past it. At the turn's end, the idle carries it when the turn fails,
+   *   and a notice otherwise (see {@link takeApiError}).
    */
   assistant(message: SDKAssistantMessage): boolean {
     if (message.error) this.errorKind = message.error;

src/v2/session.ts

@@ -903,6 +903,7 @@ export class V2Session implements RequestTarget {
       this.tasks.toolResults(this.output, message);
       return;
     }
+    this.wentOnPastApiError();
     // The rewrite applies to the updates of this message only.
     this.output.rewrite = this.clearContext.toolResults(message);
     try {
@@ -935,6 +936,7 @@ export class V2Session implements RequestTarget {
         if (this.stopped.drops(message)) return;
         if (message.parent_tool_use_id === null) {
           this.outputStarts();
+          this.wentOnPastApiError();
           this.reports.streamEvent(message);
         }
         this.output.streamEvent(message);
@@ -947,6 +949,7 @@ export class V2Session implements RequestTarget {
           // request are not shown as Claude's text (see `TurnReport`).
           if (this.turnReport.assistant(message)) return;
           this.outputStarts();
+          this.wentOnPastApiError();
           this.reports.assistant(message);
         }
         this.output.assistant(message);
@@ -1208,10 +1211,21 @@ export class V2Session implements RequestTarget {
   }
 
   /**
-   * Sends the text of a failed API request that Claude Code wrote into the
-   * conversation (see `TurnReport.assistant`) as a notice. A turn whose idle
-   * fails sends none: its error carries the failure. `end` is the turn's
-   * result, if it has one.
+   * The turn went on past a failed API request: Claude Code kept the tool
+   * calls of a response that broke off ("Connection lost mid-response. The
+   * response above may be incomplete.") and ran them. Its notice goes out
+   * here, after the output it is about, and not at the turn's end.
+   */
+  private wentOnPastApiError(): void {
+    const text = this.turnReport.takeApiError();
+    if (text) this.reports.apiError(text);
+  }
+
+  /**
+   * At the turn's end, sends the text of a failed API request that Claude
+   * Code wrote into the conversation (see `TurnReport.assistant`) as a
+   * notice. A turn whose idle fails sends none: its error carries the
+   * failure. `end` is the turn's result, if it has one.
    */
   private reportApiError(end: TurnEnd | undefined): void {
     const text = this.turnReport.takeApiError();