diff --git a/packages/server/src/server/agent/create-agent-mode.test.ts b/packages/server/src/server/agent/create-agent-mode.test.ts
new file mode 100644
index 000000000..2045c3bd5
--- /dev/null
+++ b/packages/server/src/server/agent/create-agent-mode.test.ts
@@ -0,0 +1,108 @@
+import { describe, expect, it } from "vitest";
+import { resolveAndValidateCreateAgentMode } from "./create-agent-mode.js";
+
+const CLAUDE_MODES = ["default", "acceptEdits", "plan", "bypassPermissions"];
+const OPENCODE_MODES = ["build", "full-access", "plan"];
+const CODEX_MODES = ["auto", "full-access"];
+
+describe("resolveAndValidateCreateAgentMode", () => {
+  it("returns the requested mode when it is valid for the target provider", () => {
+    const resolved = resolveAndValidateCreateAgentMode({
+      requestedMode: "plan",
+      targetProvider: "opencode",
+      parent: null,
+      availableModes: OPENCODE_MODES,
+    });
+    expect(resolved).toBe("plan");
+  });
+
+  it("throws when the requested mode is invalid for the target provider", () => {
+    expect(() =>
+      resolveAndValidateCreateAgentMode({
+        requestedMode: "bypassPermissions",
+        targetProvider: "opencode",
+        parent: null,
+        availableModes: OPENCODE_MODES,
+      }),
+    ).toThrow(
+      "Invalid mode 'bypassPermissions' for provider 'opencode'. Available modes: build, full-access, plan",
+    );
+  });
+
+  it("returns undefined (provider default) when no mode and no caller", () => {
+    const resolved = resolveAndValidateCreateAgentMode({
+      requestedMode: undefined,
+      targetProvider: "claude",
+      parent: null,
+      availableModes: CLAUDE_MODES,
+    });
+    expect(resolved).toBeUndefined();
+  });
+
+  it("inherits the caller mode when caller and target share a provider", () => {
+    const resolved = resolveAndValidateCreateAgentMode({
+      requestedMode: undefined,
+      targetProvider: "claude",
+      parent: { provider: "claude", modeId: "bypassPermissions" },
+      availableModes: CLAUDE_MODES,
+    });
+    expect(resolved).toBe("bypassPermissions");
+  });
+
+  it("returns undefined when same-provider caller has no mode", () => {
+    const resolved = resolveAndValidateCreateAgentMode({
+      requestedMode: undefined,
+      targetProvider: "claude",
+      parent: { provider: "claude", modeId: null },
+      availableModes: CLAUDE_MODES,
+    });
+    expect(resolved).toBeUndefined();
+  });
+
+  it("refuses cross-provider inheritance with the target provider's modes in the message", () => {
+    expect(() =>
+      resolveAndValidateCreateAgentMode({
+        requestedMode: undefined,
+        targetProvider: "opencode",
+        parent: { provider: "claude", modeId: "bypassPermissions" },
+        availableModes: OPENCODE_MODES,
+      }),
+    ).toThrow(
+      "cannot inherit mode 'bypassPermissions' from caller (provider 'claude') for new agent (provider 'opencode'). Pass an explicit mode. Available modes for 'opencode': build, full-access, plan",
+    );
+  });
+
+  it("refuses cross-provider inheritance even when the caller mode is null", () => {
+    expect(() =>
+      resolveAndValidateCreateAgentMode({
+        requestedMode: undefined,
+        targetProvider: "codex",
+        parent: { provider: "opencode", modeId: null },
+        availableModes: CODEX_MODES,
+      }),
+    ).toThrow(
+      "cannot inherit mode '<none>' from caller (provider 'opencode') for new agent (provider 'codex'). Pass an explicit mode. Available modes for 'codex': auto, full-access",
+    );
+  });
+
+  it("passes through an explicit mode when the target provider's modes are unknown", () => {
+    const resolved = resolveAndValidateCreateAgentMode({
+      requestedMode: "default",
+      targetProvider: "zai-custom",
+      parent: null,
+      availableModes: undefined,
+    });
+    expect(resolved).toBe("default");
+  });
+
+  it("renders 'unknown' in cross-provider error when target modes are unknown", () => {
+    expect(() =>
+      resolveAndValidateCreateAgentMode({
+        requestedMode: undefined,
+        targetProvider: "zai-custom",
+        parent: { provider: "claude", modeId: "default" },
+        availableModes: undefined,
+      }),
+    ).toThrow("Available modes for 'zai-custom': unknown");
+  });
+});
diff --git a/packages/server/src/server/agent/create-agent-mode.ts b/packages/server/src/server/agent/create-agent-mode.ts
new file mode 100644
index 000000000..44d08290b
--- /dev/null
+++ b/packages/server/src/server/agent/create-agent-mode.ts
@@ -0,0 +1,49 @@
+import type { AgentProvider } from "./agent-sdk-types.js";
+
+interface CreateAgentModeParent {
+  provider: AgentProvider;
+  modeId: string | null;
+}
+
+export interface ResolveCreateAgentModeInput {
+  requestedMode: string | undefined;
+  targetProvider: AgentProvider;
+  parent: CreateAgentModeParent | null;
+  // `undefined` = target provider's modes unknown: explicit modes pass through
+  // unvalidated, but cross-provider inheritance is still refused.
+  availableModes: string[] | undefined;
+}
+
+function listModes(modes: string[] | undefined): string {
+  if (modes === undefined) {
+    return "unknown";
+  }
+  return modes.length > 0 ? modes.join(", ") : "(none)";
+}
+
+export function resolveAndValidateCreateAgentMode(
+  input: ResolveCreateAgentModeInput,
+): string | undefined {
+  const { requestedMode, targetProvider, parent, availableModes } = input;
+
+  if (requestedMode !== undefined) {
+    if (availableModes !== undefined && !availableModes.includes(requestedMode)) {
+      throw new Error(
+        `Invalid mode '${requestedMode}' for provider '${targetProvider}'. Available modes: ${listModes(availableModes)}`,
+      );
+    }
+    return requestedMode;
+  }
+
+  if (!parent) {
+    return undefined;
+  }
+
+  if (parent.provider === targetProvider) {
+    return parent.modeId ?? undefined;
+  }
+
+  throw new Error(
+    `cannot inherit mode '${parent.modeId ?? "<none>"}' from caller (provider '${parent.provider}') for new agent (provider '${targetProvider}'). Pass an explicit mode. Available modes for '${targetProvider}': ${listModes(availableModes)}`,
+  );
+}
diff --git a/packages/server/src/server/agent/mcp-server.test.ts b/packages/server/src/server/agent/mcp-server.test.ts
index 91963502a..b972937a9 100644
--- a/packages/server/src/server/agent/mcp-server.test.ts
+++ b/packages/server/src/server/agent/mcp-server.test.ts
@@ -593,7 +593,7 @@ describe("create_agent MCP tool", () => {
     await tool.callback({
       cwd: existingCwd,
       title: "Config test",
-      mode: "default",
+      mode: "auto",
       initialPrompt: "Do work",
       provider: "codex/gpt-5.4",
       thinking: "think-hard",
@@ -1169,7 +1169,7 @@ describe("create_agent MCP tool", () => {
     await tool.callback({
       cwd: existingCwd,
       title: "Injected config test",
-      mode: "default",
+      mode: "auto",
       provider: "codex/gpt-5.4",
       initialPrompt: "Do work",
     });
@@ -1183,6 +1183,129 @@ describe("create_agent MCP tool", () => {
     expect(agentIdArg).toBeUndefined();
     expect(optionsArg).toBeUndefined();
   });
+
+  it("rejects an explicit mode that is not valid for the target provider", async () => {
+    const { agentManager, agentStorage, spies } = createTestDeps();
+    const server = await createAgentMcpServer({ agentManager, agentStorage, logger });
+    const tool = registeredTool(server, "create_agent");
+
+    await expect(
+      tool.callback({
+        cwd: existingCwd,
+        title: "Bad mode",
+        provider: "opencode/gpt-5.4",
+        mode: "bypassPermissions",
+        initialPrompt: "Do work",
+      }),
+    ).rejects.toThrow(
+      "Invalid mode 'bypassPermissions' for provider 'opencode'. Available modes: build, full-access, plan",
+    );
+    expect(spies.agentManager.createAgent).not.toHaveBeenCalled();
+  });
+
+  it("inherits the caller mode when the new agent uses the same provider", async () => {
+    const { agentManager, agentStorage, spies } = createTestDeps();
+    spies.agentManager.getAgent.mockReturnValue({
+      id: "parent-agent",
+      cwd: existingCwd,
+      provider: "claude",
+      currentModeId: "bypassPermissions",
+    } as ManagedAgent);
+    spies.agentManager.createAgent.mockResolvedValue({
+      id: "child-agent",
+      cwd: existingCwd,
+      lifecycle: "idle",
+      currentModeId: "bypassPermissions",
+      availableModes: [],
+      config: { title: "Child" },
+    } as ManagedAgent);
+
+    const server = await createAgentMcpServer({
+      agentManager,
+      agentStorage,
+      callerAgentId: "parent-agent",
+      logger,
+    });
+    const tool = registeredTool(server, "create_agent");
+    await tool.callback({
+      title: "Child",
+      provider: "claude/claude-sonnet-4-20250514",
+      initialPrompt: "Do work",
+    });
+
+    expect(spies.agentManager.createAgent).toHaveBeenCalledWith(
+      expect.objectContaining({ modeId: "bypassPermissions" }),
+      undefined,
+      expect.any(Object),
+    );
+  });
+
+  it("refuses cross-provider mode inheritance when no explicit mode is given", async () => {
+    const { agentManager, agentStorage, spies } = createTestDeps();
+    spies.agentManager.getAgent.mockReturnValue({
+      id: "parent-agent",
+      cwd: existingCwd,
+      provider: "claude",
+      currentModeId: "bypassPermissions",
+    } as ManagedAgent);
+
+    const server = await createAgentMcpServer({
+      agentManager,
+      agentStorage,
+      callerAgentId: "parent-agent",
+      logger,
+    });
+    const tool = registeredTool(server, "create_agent");
+
+    await expect(
+      tool.callback({
+        title: "Child",
+        provider: "opencode/gpt-5.4",
+        initialPrompt: "Do work",
+      }),
+    ).rejects.toThrow(
+      "cannot inherit mode 'bypassPermissions' from caller (provider 'claude') for new agent (provider 'opencode'). Pass an explicit mode. Available modes for 'opencode': build, full-access, plan",
+    );
+    expect(spies.agentManager.createAgent).not.toHaveBeenCalled();
+  });
+
+  it("accepts an explicit valid mode across providers", async () => {
+    const { agentManager, agentStorage, spies } = createTestDeps();
+    spies.agentManager.getAgent.mockReturnValue({
+      id: "parent-agent",
+      cwd: existingCwd,
+      provider: "claude",
+      currentModeId: "bypassPermissions",
+    } as ManagedAgent);
+    spies.agentManager.createAgent.mockResolvedValue({
+      id: "child-agent",
+      cwd: existingCwd,
+      lifecycle: "idle",
+      currentModeId: "build",
+      availableModes: [],
+      config: { title: "Child" },
+    } as ManagedAgent);
+
+    const server = await createAgentMcpServer({
+      agentManager,
+      agentStorage,
+      callerAgentId: "parent-agent",
+      logger,
+    });
+    const tool = registeredTool(server, "create_agent");
+    await tool.callback({
+      title: "Child",
+      provider: "opencode/gpt-5.4",
+      mode: "build",
+      initialPrompt: "Do work",
+    });
+
+    expect(spies.agentManager.createAgent).toHaveBeenCalledWith(
+      expect.objectContaining({ modeId: "build" }),
+      undefined,
+      expect.any(Object),
+    );
+  });
 });
 
 describe("create_schedule MCP tool", () => {
diff --git a/packages/server/src/server/agent/mcp-server.ts b/packages/server/src/server/agent/mcp-server.ts
index d479c1c64..a381385b3 100644
--- a/packages/server/src/server/agent/mcp-server.ts
+++ b/packages/server/src/server/agent/mcp-server.ts
@@ -49,6 +49,8 @@ import type {
 import type { ScheduleService } from "../schedule/service.js";
 import { ScheduleSummarySchema, StoredScheduleSchema } from "../schedule/types.js";
 import type { ProviderDefinition } from "./provider-registry.js";
+import { getAgentProviderDefinition } from "./provider-manifest.js";
+import { resolveAndValidateCreateAgentMode } from "./create-agent-mode.js";
 import { resolveSnapshotCwd } from "./provider-snapshot-manager.js";
 import {
   AgentModelSchema,
@@ -492,6 +494,12 @@ export async function createAgentMcpServer(options: AgentMcpServerOptions): Prom
       .trim()
       .min(1, "initialPrompt is required")
       .describe("Required first task to run immediately after creation."),
+    mode: z
+      .string()
+      .optional()
+      .describe(
+        "Optional session mode for the new agent. Required when the new agent uses a different provider than the caller agent.",
+      ),
     background: z
       .boolean()
       .optional()
@@ -628,6 +636,18 @@ export async function createAgentMcpServer(options: AgentMcpServerOptions): Prom
     setupContinuation: AgentWorktreeSetupContinuation | undefined;
   }
 
+  const getAvailableModeIds = (provider: AgentProvider): string[] | undefined => {
+    const fromRegistry = providerRegistry?.[provider];
+    if (fromRegistry) {
+      return fromRegistry.modes.map((mode) => mode.id);
+    }
+    try {
+      return getAgentProviderDefinition(provider).modes.map((mode) => mode.id);
+    } catch {
+      return undefined;
+    }
+  };
+
   const resolveCallerCreateAgentArgs = (
     args: unknown,
     parentAgentId: string,
@@ -645,10 +665,12 @@ export async function createAgentMcpServer(options: AgentMcpServerOptions): Prom
       lockedCwd: callerContext?.lockedCwd,
       allowCustomCwd: callerContext?.allowCustomCwd ?? true,
     });
-    const parentMode = parentAgent.currentModeId;
-    const resolvedMode = parentMode
-      ? mapModeAcrossProviders(parentMode, parentAgent.provider, provider)
-      : undefined;
+    const resolvedMode = resolveAndValidateCreateAgentMode({
+      requestedMode: callerArgs.mode,
+      targetProvider: provider,
+      parent: { provider: parentAgent.provider, modeId: parentAgent.currentModeId },
+      availableModes: getAvailableModeIds(provider),
+    });
     return {
       provider,
       initialPrompt: callerArgs.initialPrompt,
@@ -670,6 +692,12 @@ export async function createAgentMcpServer(options: AgentMcpServerOptions): Prom
     const topLevelArgs = topLevelCreateAgentArgsSchema.parse(args);
     const resolvedProviderModel = resolveRequiredProviderModel(topLevelArgs.provider);
     const { cwd, mode, worktreeName, baseBranch, refName, action, githubPrNumber } = topLevelArgs;
+    const resolvedMode = resolveAndValidateCreateAgentMode({
+      requestedMode: mode,
+      targetProvider: resolvedProviderModel.provider,
+      parent: null,
+      availableModes: getAvailableModeIds(resolvedProviderModel.provider),
+    });
     let resolvedCwd = expandUserPath(cwd);
     let setupContinuation: AgentWorktreeSetupContinuation | undefined;
 
@@ -725,7 +753,7 @@ export async function createAgentMcpServer(options: AgentMcpServerOptions): Prom
       labels: topLevelArgs.labels,
       notifyOnFinish: topLevelArgs.notifyOnFinish ?? false,
       resolvedCwd,
-      resolvedMode: mode,
+      resolvedMode,
       setupContinuation,
     };
   };