fix(self-host): preserve native MCP elicitation mode

.changeset/fuzzy-dragons-elicit.md

@@ -0,0 +1,5 @@
+---
+"executor": patch
+---
+
+Preserve `elicitation_mode=native` when creating self-hosted MCP sessions.

packages/hosts/mcp/src/in-memory-session-store.test.ts

@@ -0,0 +1,54 @@
+import { expect, it } from "@effect/vitest";
+import { Effect } from "effect";
+
+import {
+  makeInMemoryMcpSessionStore,
+  McpEngineBuildError,
+  type McpBuildServerOptions,
+} from "./in-memory-session-store";
+import { defaultMcpResource, type Principal } from "./seams";
+
+const TEST_PRINCIPAL: Principal = {
+  accountId: "acct_test",
+  organizationId: "org_test",
+  organizationName: "Test Org",
+  email: "test@example.com",
+  name: "Test",
+  avatarUrl: null,
+  roles: ["user"],
+};
+
+it("preserves native elicitation mode when creating an in-memory MCP session", async () => {
+  let buildOptions: McpBuildServerOptions | undefined;
+  const sessions = makeInMemoryMcpSessionStore((_principal, options) => {
+    buildOptions = options;
+    return Effect.fail(new McpEngineBuildError({ cause: "stop after capturing options" }));
+  });
+
+  const result = await Effect.runPromise(
+    sessions.store.dispatch({
+      request: new Request("https://executor.test/mcp?elicitation_mode=native", {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({
+          jsonrpc: "2.0",
+          id: 1,
+          method: "initialize",
+          params: {
+            protocolVersion: "2025-06-18",
+            capabilities: { elicitation: { form: {} } },
+            clientInfo: { name: "test-client", version: "1.0.0" },
+          },
+        }),
+      }),
+      principal: TEST_PRINCIPAL,
+      resource: defaultMcpResource,
+      sessionId: null,
+      method: "POST",
+    }),
+  );
+
+  expect(result).toBeInstanceOf(Response);
+  expect((result as Response).status).toBe(500);
+  expect(buildOptions?.elicitationMode).toEqual({ mode: "native" });
+});

packages/hosts/mcp/src/in-memory-session-store.ts

@@ -218,7 +218,8 @@ export const makeInMemoryMcpSessionStore = (
     request: Request,
     sessionId: () => string | null,
   ): McpBuildServerOptions => {
-    if (readElicitationMode(request) !== "browser") return { elicitationMode: { mode: "model" } };
+    const mode = readElicitationMode(request);
+    if (mode !== "browser") return { elicitationMode: { mode } };
     return {
       elicitationMode: {
         mode: "browser",