diff --git a/packages/agent/src/adapters/claude/claude-agent.permission-mode.test.ts b/packages/agent/src/adapters/claude/claude-agent.permission-mode.test.ts
new file mode 100644
index 0000000000..adfc6415f9
--- /dev/null
+++ b/packages/agent/src/adapters/claude/claude-agent.permission-mode.test.ts
@@ -0,0 +1,151 @@
+import type { AgentSideConnection } from "@agentclientprotocol/sdk";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  CODE_EXECUTION_MODES,
+  type CodeExecutionMode,
+} from "../../execution-mode";
+import { createMockQuery, type MockQuery } from "../../test/mocks/claude-sdk";
+import { Pushable } from "../../utils/streams";
+import { toSdkPermissionMode } from "./tools";
+
+vi.mock("@anthropic-ai/claude-agent-sdk", () => ({
+  query: vi.fn(),
+}));
+
+vi.mock("./mcp/tool-metadata", () => ({
+  fetchMcpToolMetadata: vi.fn().mockResolvedValue(undefined),
+  getConnectedMcpServerNames: vi.fn().mockReturnValue([]),
+  setMcpToolApprovalStates: vi.fn(),
+  isMcpToolReadOnly: vi.fn().mockReturnValue(false),
+  getMcpToolMetadata: vi.fn().mockReturnValue(undefined),
+  getMcpToolApprovalState: vi.fn().mockReturnValue(undefined),
+}));
+
+const { ClaudeAcpAgent } = await import("./claude-agent");
+type Agent = InstanceType<typeof ClaudeAcpAgent>;
+
+interface ClientMocks {
+  sessionUpdate: ReturnType<typeof vi.fn>;
+  extNotification: ReturnType<typeof vi.fn>;
+}
+
+function makeAgent(): { agent: Agent; client: ClientMocks } {
+  const client: ClientMocks = {
+    sessionUpdate: vi.fn().mockResolvedValue(undefined),
+    extNotification: vi.fn().mockResolvedValue(undefined),
+  };
+  const agent = new ClaudeAcpAgent(client as unknown as AgentSideConnection);
+  return { agent, client };
+}
+
+function installFakeSession(
+  agent: Agent,
+  sessionId: string,
+  permissionMode: CodeExecutionMode = "default",
+): MockQuery {
+  const query = createMockQuery();
+  const input = new Pushable();
+  const abortController = new AbortController();
+
+  const session = {
+    query,
+    queryOptions: { sessionId, cwd: "/tmp/repo", abortController },
+    buildInProcessMcpServers: () => ({}),
+    localToolsServerNames: [] as string[],
+    input,
+    cancelled: false,
+    interruptReason: undefined,
+    settingsManager: { dispose: vi.fn(), getRepoRoot: () => "/tmp/repo" },
+    permissionMode,
+    abortController,
+    accumulatedUsage: {
+      inputTokens: 0,
+      outputTokens: 0,
+      cachedReadTokens: 0,
+      cachedWriteTokens: 0,
+    },
+    sessionResources: new Set(),
+    configOptions: [],
+    turnQueue: [],
+    activeTurn: null,
+    pendingOrphanResults: 0,
+    queryGeneration: 0,
+    cwd: "/tmp/repo",
+    notificationHistory: [] as unknown[],
+    taskRunId: "run-1",
+    lastContextWindowSize: 200_000,
+    modelId: "claude-sonnet-4-6",
+    knownSlashCommands: undefined,
+  };
+
+  (agent as unknown as { session: typeof session }).session = session;
+  (agent as unknown as { sessionId: string }).sessionId = sessionId;
+
+  return query;
+}
+
+describe("ClaudeAcpAgent.setSessionMode — SDK permission-mode translation", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it.each(CODE_EXECUTION_MODES)(
+    "maps modeId %s to the SDK's permission mode at the setPermissionMode call site",
+    async (modeId) => {
+      const { agent } = makeAgent();
+      const query = installFakeSession(agent, "s-mode");
+
+      await agent.setSessionMode({ sessionId: "s-mode", modeId });
+
+      expect(query.setPermissionMode).toHaveBeenCalledWith(
+        toSdkPermissionMode(modeId),
+      );
+      expect(
+        (agent as unknown as { session: { permissionMode: string } }).session
+          .permissionMode,
+      ).toBe(modeId);
+    },
+  );
+
+  it("reverts session.permissionMode to the previous mode when the SDK rejects", async () => {
+    const { agent } = makeAgent();
+    const query = installFakeSession(agent, "s-mode", "default");
+    vi.mocked(query.setPermissionMode).mockRejectedValueOnce(
+      new Error("sdk rejected"),
+    );
+
+    await expect(
+      agent.setSessionMode({ sessionId: "s-mode", modeId: "auto" }),
+    ).rejects.toThrow("sdk rejected");
+
+    expect(
+      (agent as unknown as { session: { permissionMode: string } }).session
+        .permissionMode,
+    ).toBe("default");
+  });
+
+  it("falls back to a generic error message when the SDK rejection has none", async () => {
+    const { agent } = makeAgent();
+    const query = installFakeSession(agent, "s-mode", "default");
+    vi.mocked(query.setPermissionMode).mockRejectedValueOnce(new Error());
+
+    await expect(
+      agent.setSessionMode({ sessionId: "s-mode", modeId: "auto" }),
+    ).rejects.toThrow("Invalid Mode");
+  });
+
+  it("records modeBeforePlan using the host mode, unaffected by the SDK translation", async () => {
+    const { agent } = makeAgent();
+    installFakeSession(agent, "s-mode", "auto");
+
+    await agent.setSessionMode({ sessionId: "s-mode", modeId: "plan" });
+
+    const session = (
+      agent as unknown as {
+        session: { permissionMode: string; modeBeforePlan?: string };
+      }
+    ).session;
+    expect(session.permissionMode).toBe("plan");
+    expect(session.modeBeforePlan).toBe("auto");
+  });
+});
diff --git a/packages/agent/src/adapters/claude/claude-agent.ts b/packages/agent/src/adapters/claude/claude-agent.ts
index 9e2716888a..2ed43af846 100644
--- a/packages/agent/src/adapters/claude/claude-agent.ts
+++ b/packages/agent/src/adapters/claude/claude-agent.ts
@@ -135,6 +135,7 @@ import {
   CODE_EXECUTION_MODES,
   type CodeExecutionMode,
   getAvailableModes,
+  toSdkPermissionMode,
 } from "./tools";
 import type {
   BackgroundTerminal,
@@ -1778,7 +1779,9 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
       this.session.modeBeforePlan = previousMode;
     }
     try {
-      await this.session.query.setPermissionMode(modeId as CodeExecutionMode);
+      await this.session.query.setPermissionMode(
+        toSdkPermissionMode(modeId as CodeExecutionMode),
+      );
     } catch (error) {
       this.session.permissionMode = previousMode;
       if (error instanceof Error) {
diff --git a/packages/agent/src/adapters/claude/session/options.test.ts b/packages/agent/src/adapters/claude/session/options.test.ts
index b167b5594e..854bc461ff 100644
--- a/packages/agent/src/adapters/claude/session/options.test.ts
+++ b/packages/agent/src/adapters/claude/session/options.test.ts
@@ -57,6 +57,25 @@ describe("buildSessionOptions", () => {
     },
   );
 
+  it("maps the custom auto mode to the SDK's default mode", () => {
+    const options = buildSessionOptions({
+      ...makeParams(),
+      permissionMode: "auto",
+    });
+    expect(options.permissionMode).toBe("default");
+  });
+
+  it.each(["default", "acceptEdits", "plan", "bypassPermissions"] as const)(
+    "passes native SDK mode %s through to options.permissionMode",
+    (mode) => {
+      const options = buildSessionOptions({
+        ...makeParams(),
+        permissionMode: mode,
+      });
+      expect(options.permissionMode).toBe(mode);
+    },
+  );
+
   it("preserves caller-provided agents alongside defaults", () => {
     const params = makeParams();
     const options = buildSessionOptions({
diff --git a/packages/agent/src/adapters/claude/session/options.ts b/packages/agent/src/adapters/claude/session/options.ts
index 25a3817f2b..547f21eda9 100644
--- a/packages/agent/src/adapters/claude/session/options.ts
+++ b/packages/agent/src/adapters/claude/session/options.ts
@@ -24,7 +24,7 @@ import {
   type EnrichedReadCache,
   type OnModeChange,
 } from "../hooks";
-import type { CodeExecutionMode } from "../tools";
+import { type CodeExecutionMode, toSdkPermissionMode } from "../tools";
 import type { EffortLevel } from "../types";
 import { APPENDED_INSTRUCTIONS } from "./instructions";
 import { loadUserClaudeJsonMcpServers } from "./mcp-config";
@@ -445,7 +445,7 @@ export function buildSessionOptions(params: BuildOptionsParams): Options {
     cwd: params.cwd,
     includePartialMessages: true,
     allowDangerouslySkipPermissions: !IS_ROOT || !!process.env.IS_SANDBOX,
-    permissionMode: params.permissionMode,
+    permissionMode: toSdkPermissionMode(params.permissionMode),
     canUseTool: params.canUseTool,
     tools,
     agents,
diff --git a/packages/agent/src/adapters/claude/tools.test.ts b/packages/agent/src/adapters/claude/tools.test.ts
new file mode 100644
index 0000000000..6f87485db4
--- /dev/null
+++ b/packages/agent/src/adapters/claude/tools.test.ts
@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+import type { CodeExecutionMode } from "../../execution-mode";
+import { isToolAllowedForMode, toSdkPermissionMode } from "./tools";
+
+describe("toSdkPermissionMode", () => {
+  it("maps the custom auto mode to the SDK's default mode", () => {
+    expect(toSdkPermissionMode("auto")).toBe("default");
+  });
+
+  it.each<CodeExecutionMode>([
+    "default",
+    "acceptEdits",
+    "plan",
+    "bypassPermissions",
+  ])("passes native SDK mode %s through unchanged", (mode) => {
+    expect(toSdkPermissionMode(mode)).toBe(mode);
+  });
+});
+
+describe("isToolAllowedForMode stays authoritative for auto", () => {
+  it.each(["Bash", "Edit", "Write", "NotebookEdit", "BashOutput", "KillShell"])(
+    "auto-allows %s in auto mode",
+    (tool) => {
+      expect(isToolAllowedForMode(tool, "auto")).toBe(true);
+    },
+  );
+
+  it.each(["Bash", "Edit", "Write"])(
+    "still gates %s in default mode",
+    (tool) => {
+      expect(isToolAllowedForMode(tool, "default")).toBe(false);
+    },
+  );
+});
diff --git a/packages/agent/src/adapters/claude/tools.ts b/packages/agent/src/adapters/claude/tools.ts
index eaccc7d218..1911b037fd 100644
--- a/packages/agent/src/adapters/claude/tools.ts
+++ b/packages/agent/src/adapters/claude/tools.ts
@@ -5,6 +5,7 @@ export {
   type ModeInfo,
 } from "../../execution-mode";
 
+import type { PermissionMode as SdkPermissionMode } from "@anthropic-ai/claude-agent-sdk";
 import type { CodeExecutionMode } from "../../execution-mode";
 import { isMcpToolReadOnly } from "./mcp/tool-metadata";
 
@@ -55,6 +56,12 @@ const AUTO_ALLOWED_TOOLS: Record<string, Set<string>> = {
   plan: new Set(BASE_ALLOWED_TOOLS),
 };
 
+export function toSdkPermissionMode(
+  mode: CodeExecutionMode,
+): SdkPermissionMode {
+  return mode === "auto" ? "default" : mode;
+}
+
 export function isToolAllowedForMode(
   toolName: string,
   mode: CodeExecutionMode,