@@ -0,0 +1,27 @@
+---
+'@objectstack/runtime': patch
+---
+
+fix(runtime): a sandboxed body that throws a plain `{ code, message }` object is answered with its own `message` as the client sentence, never `undefined: MESSAGE` or `[object Object]`; its code is kept (#23718)
+
+**Client sentence change, status and code unchanged.** It applies to `POST /api/v1/actions/:object/:action` on a `type: 'script'` action whose sandboxed body throws a value that is not an `Error`, and to a sandboxed hook body that does the same (its refusal at the action door behind a write, and in the `/data` error classification). The QuickJS runner flattened every thrown value as `NAME: MESSAGE`, else `String(value)`. A plain object has no class name and no `toString` of its own, so the doors served `undefined: MESSAGE`, or `[object Object]` when it had no `message`.
+
+- `throw { code: 'EMAIL_TAKEN', message: 'That address is already on file' }`
+  - FROM `{"success":false,"error":{"code":"VALIDATION_ERROR","message":"undefined: That address is already on file","httpStatus":400,"declaredCode":"EMAIL_TAKEN"}}`
+  - TO `{"success":false,"error":{"code":"VALIDATION_ERROR","message":"That address is already on file","httpStatus":400,"declaredCode":"EMAIL_TAKEN"}}`
+- `throw { code: 'DUPLICATE_RECORD' }`
+  - FROM `{"success":false,"error":{"code":"DUPLICATE_RECORD","message":"[object Object]","httpStatus":400}}`
+  - TO `{"success":false,"error":{"code":"DUPLICATE_RECORD","message":"Internal server error","httpStatus":400}}`
+
+A value with no usable `message` and no `toString` of its own is answered with `Internal server error`, the sentence every door already answers for a fault with no business message. To give the caller words, throw a `message` (or an `Error`).
+
+Two outcomes move with it, both for a thrown plain object only:
+
+- A `name` on a plain object is a field, not a class: `throw { name: 'TypeError', message: 'forged' }` was read as a crash (`500 INTERNAL_ERROR`); it is now a refusal, `400` with `forged`.
+- A plain object whose `String()` is empty (`throw []`) recorded no rejection, so the request waited for the sandbox's wall-clock ceiling and answered `500`; it now answers at once with the no-message sentence.
+
+Control, unchanged: a thrown `Error` (or subclass) keeps `NAME: MESSAGE` in the server log and the client sentence it got before. A thrown string is its own sentence, and an object with a `toString` of its own keeps it.
+
+In-process: the `SandboxError` a caller catches for such a throw carries the value's `message` as `innerMessage`, and its log-only `message` reads `KIND 'NAME' threw: Error: MESSAGE`.
+
+Clause-②: no
@@ -0,0 +1,251 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#23718] A sandboxed body that throws a plain `{ code, message }` object is
+ * answered at the doors with that `message` as the client sentence, and its
+ * code kept, with no change at any door.
+ *
+ * ## What was wrong
+ *
+ * The wrapper's reject handler (`sandbox/quickjs-runner.ts`) flattened every
+ * rejection as `<name>: <message>`, else `String(e)`, and that string became
+ * the `innerMessage` every door serves as the caller's sentence. A plain object
+ * has no class name and no `toString` of its own, so
+ * `POST /api/v1/actions/:object/:action` answered
+ * `400 … "undefined: That address is already on file"` for
+ * `throw { code: 'EMAIL_TAKEN', message: 'That address is already on file' }`
+ * and `400 DUPLICATE_RECORD "[object Object]"` for
+ * `throw { code: 'DUPLICATE_RECORD' }`. A sandboxed HOOK body throwing the same
+ * values broke the same way, at the action door behind a write and in the
+ * `/data` classification.
+ *
+ * ## The rig
+ *
+ * PR #23717's: better-sqlite3 `:memory:` under the real `ObjectQL`, action
+ * bodies bound through `actionBodyRunnerFactory` over a real
+ * `QuickJSScriptRunner`, hook bodies through `hookBodyRunnerFactory`, and the
+ * dispatcher reached through `dispatch()` over a real Fetch `Request`, the
+ * identity step the one stubbed seam. The `/data` classification is read at
+ * function level (`mapDataError`), as PR #23717 read it.
+ *
+ * The sentence IS the contract here (the author's own words, relayed verbatim),
+ * so it is asserted exactly; for a value with no message it is asserted to be
+ * the shared `INTERNAL_ERROR_MESSAGE` constant, never a literal.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { ObjectQL, bindHooksToEngine } from '@objectstack/objectql';
+import { SqlDriver } from '@objectstack/driver-sql';
+import { ObjectStackProtocolImplementation } from '@objectstack/metadata-protocol';
+import { INTERNAL_ERROR_MESSAGE, mapDataError } from '@objectstack/types';
+import { HttpDispatcher } from './http-dispatcher.js';
+import { actionBodyRunnerFactory, hookBodyRunnerFactory } from './sandbox/body-runner.js';
+import { QuickJSScriptRunner } from './sandbox/quickjs-runner.js';
+
+const PREFIX = '/api/v1';
+/** `registry.registerObject` requires a package id (see the sibling real-stack tests). */
+const TEST_PACKAGE_ID = 'objectstack-test';
+const SENTENCE = 'That address is already on file';
+
+/** A `type: 'script'` action whose body is `source`. */
+const scriptAction = (name: string, source: string) => ({
+    name,
+    label: name,
+    type: 'script',
+    body: { language: 'js', source, capabilities: ['api.read', 'api.write'] },
+});
+
+/** A write to `tdoc` whose `name` tells the sandboxed `guard` hook what to throw. */
+const insertTdoc = (name: string) =>
+    `await ctx.api.object('tdoc').insert({ id: 't_' + Math.random().toString(36).slice(2), name: '${name}' });`;
+
+const ACTIONS = [
+    scriptAction('throw_message', `throw { code: 'EMAIL_TAKEN', message: '${SENTENCE}' };`),
+    scriptAction('throw_no_message', `throw { code: 'DUPLICATE_RECORD' };`),
+    // CONTROL — a thrown `Error` answers as before.
+    scriptAction('throw_error', `throw new Error('x');`),
+    // Two VM hops: this body's write fires `guard`'s sandboxed hook, which throws the plain object.
+    scriptAction('write_hooked_message', `${insertTdoc('message')} return { written: true };`),
+];
+
+/** Host-code handlers whose engine write fires `guard`'s sandboxed hook. */
+const HOOKED_BY_HANDLER = ['message', 'no_message', 'error'] as const;
+
+const PDOC = {
+    name: 'pdoc',
+    label: 'Plain-throw doc',
+    fields: {
+        id: { name: 'id', type: 'text' as const, primaryKey: true, label: 'ID' },
+        name: { name: 'name', type: 'text' as const, label: 'Name' },
+    },
+    actions: [
+        ...ACTIONS,
+        ...HOOKED_BY_HANDLER.map((k) => ({ name: `hooked_${k}`, label: `Hooked ${k}`, type: 'script', target: `hooked_${k}` })),
+    ],
+};
+
+const TDOC = {
+    name: 'tdoc',
+    label: 'Hook-guarded doc',
+    fields: {
+        id: { name: 'id', type: 'text' as const, primaryKey: true, label: 'ID' },
+        name: { name: 'name', type: 'text' as const, label: 'Name' },
+    },
+};
+
+/** A sandboxed `beforeInsert` hook body that throws what the record's `name` asks for. */
+const GUARD_HOOK = {
+    name: 'guard',
+    object: 'tdoc',
+    events: ['beforeInsert'],
+    body: {
+        language: 'js',
+        source: `
+            if (ctx.input.name === 'message') throw { code: 'EMAIL_TAKEN', message: '${SENTENCE}' };
+            if (ctx.input.name === 'no_message') throw { code: 'DUPLICATE_RECORD' };
+            if (ctx.input.name === 'error') throw new Error('x');
+        `,
+        capabilities: [],
+    },
+};
+
+const MEMBER = { userId: 'u_member', isSystem: false };
+
+let engine: ObjectQL;
+let dispatcher: HttpDispatcher;
+
+beforeAll(async () => {
+    engine = new ObjectQL();
+    engine.registerDriver(new SqlDriver({
+        client: 'better-sqlite3',
+        connection: { filename: ':memory:' },
+        useNullAsDefault: true,
+    }), true);
+    await engine.init();
+    for (const o of [PDOC, TDOC]) engine.registry.registerObject(o as any, TEST_PACKAGE_ID);
+    await engine.syncSchemas();
+
+    const ql = engine as any;
+    const bindBody = actionBodyRunnerFactory(new QuickJSScriptRunner(), { ql: engine, appId: 'pdoc_app' });
+    for (const action of ACTIONS) ql.registerAction('pdoc', action.name, bindBody({ ...action, object: 'pdoc' }));
+    for (const k of HOOKED_BY_HANDLER) {
+        ql.registerAction('pdoc', `hooked_${k}`, async (ctx: any) => {
+            await ctx.engine.insert('tdoc', { id: 't_' + Math.random().toString(36).slice(2), name: k });
+            return { written: true };
+        });
+    }
+    engine.setDefaultBodyRunner(hookBodyRunnerFactory(new QuickJSScriptRunner(), { ql: engine, appId: 'pdoc_app' }));
+    bindHooksToEngine(engine, [GUARD_HOOK as any], { packageId: TEST_PACKAGE_ID });
+
+    const protocol = new ObjectStackProtocolImplementation(engine as any);
+    const services: Record<string, unknown> = { objectql: engine, data: engine, protocol };
+    const get = (n: string) => services[n] ?? null;
+    const kernel: any = { context: { getService: get }, getService: get, getServiceAsync: async (n: string) => get(n) };
+    dispatcher = new HttpDispatcher(kernel);
+    (dispatcher as any).timedResolveExecutionContext = async () => ({ ...MEMBER });
+}, 60_000);
+
+afterAll(async () => {
+    try { await engine?.destroy(); } catch { /* noop */ }
+});
+
+/** `POST /api/v1/actions/pdoc/<action>` as `@objectstack/hono`'s catch-all hands it to `dispatch()`. */
+async function postAction(action: string) {
+    const path = `/actions/pdoc/${action}`;
+    const request = new Request(`http://localhost${PREFIX}${path}`, {
+        method: 'POST',
+        headers: { 'content-type': 'application/json' },
+        body: '{}',
+    });
+    const res: any = await dispatcher.dispatch('POST', path, {}, {}, { request } as any, PREFIX);
+    return res.response as { status: number; body: any };
+}
+
+/** `{ code: 'EMAIL_TAKEN', message }`: the message verbatim, the unregistered code on `declaredCode` as before. */
+function expectMessageSentence(response: { status: number; body: any }) {
+    expect(response.status, JSON.stringify(response.body)).toBe(400);
+    const error = response.body?.error;
+    expect(error?.message).toBe(SENTENCE);
+    expect(error?.code).toBe('VALIDATION_ERROR');
+    expect(error?.declaredCode).toBe('EMAIL_TAKEN');
+}
+
+/** `{ code: 'DUPLICATE_RECORD' }`: the no-business-message sentence, the registered code kept. */
+function expectNoMessageSentence(response: { status: number; body: any }) {
+    expect(response.status, JSON.stringify(response.body)).toBe(400);
+    const error = response.body?.error;
+    expect(error?.message).toBe(INTERNAL_ERROR_MESSAGE);
+    expect(error?.code).toBe('DUPLICATE_RECORD');
+    expect(JSON.stringify(response.body)).not.toContain('[object Object]');
+}
+
+/** The error `engine.insert('tdoc')` rejects with when `guard` throws for `name`. */
+async function hookRefusal(name: string): Promise<unknown> {
+    const err = await engine.insert('tdoc', { id: 'd_' + Math.random().toString(36).slice(2), name }).then(
+        () => undefined,
+        (e: unknown) => e,
+    );
+    expect(err, `guard did not refuse '${name}'`).toBeDefined();
+    return err;
+}
+
+describe('[#23718] the action door answers a body\'s thrown plain object with its own message', () => {
+    it('{ code, message }: the message is the client sentence, and the code is kept', async () => {
+        expectMessageSentence(await postAction('throw_message'));
+    });
+
+    it('{ code } with no message: not [object Object], and the code is kept', async () => {
+        expectNoMessageSentence(await postAction('throw_no_message'));
+    });
+
+    it('CONTROL — a thrown Error answers as before', async () => {
+        const response = await postAction('throw_error');
+        expect(response.status, JSON.stringify(response.body)).toBe(400);
+        expect(response.body?.error?.code).toBe('VALIDATION_ERROR');
+        expect(response.body?.error?.message).toBe('x');
+    });
+});
+
+describe('[#23718] a sandboxed hook body\'s thrown plain object, at the action door behind a write', () => {
+    it('{ code, message }, behind a host handler\'s write', async () => {
+        expectMessageSentence(await postAction('hooked_message'));
+        expect(await engine.find('tdoc', {})).toHaveLength(0);
+    });
+
+    it('{ code } with no message, behind a host handler\'s write', async () => {
+        expectNoMessageSentence(await postAction('hooked_no_message'));
+    });
+
+    it('{ code, message }, behind a script body\'s write (two VM hops)', async () => {
+        expectMessageSentence(await postAction('write_hooked_message'));
+        expect(await engine.find('tdoc', {})).toHaveLength(0);
+    });
+
+    it('CONTROL — the hook\'s thrown Error answers as before', async () => {
+        const response = await postAction('hooked_error');
+        expect(response.status, JSON.stringify(response.body)).toBe(400);
+        expect(response.body?.error?.message).toBe('x');
+    });
+});
+
+describe('[#23718] the /data classification of the same hook refusals', () => {
+    it('{ code, message }: the message is the sentence, the code on declaredCode', async () => {
+        const { status, body } = mapDataError(await hookRefusal('message'), 'tdoc');
+        expect(status, JSON.stringify(body)).toBe(400);
+        expect(body.error).toBe(SENTENCE);
+        expect(body.declaredCode).toBe('EMAIL_TAKEN');
+    });
+
+    it('{ code } with no message: not [object Object], and the code is kept', async () => {
+        const { status, body } = mapDataError(await hookRefusal('no_message'), 'tdoc');
+        expect(status, JSON.stringify(body)).toBe(400);
+        expect(body.error).toBe(INTERNAL_ERROR_MESSAGE);
+        expect(body.code).toBe('DUPLICATE_RECORD');
+    });
+
+    it('CONTROL — the hook\'s thrown Error is classified as before', async () => {
+        const { status, body } = mapDataError(await hookRefusal('error'), 'tdoc');
+        expect(status, JSON.stringify(body)).toBe(400);
+        expect(body.error).toBe('x');
+    });
+});
@@ -0,0 +1,113 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#23718] A sandboxed body that throws a VALUE which is not an `Error` — the
+ * plain `{ code, message }` object metadata app bodies throw — crosses out of
+ * the VM with its own `message` as `innerMessage`, the field every door serves
+ * as the caller's sentence.
+ *
+ * ## What was wrong
+ *
+ * The wrapper's reject handler flattened every rejection as
+ * `<name>: <message>`, else `String(e)`. A plain object has no class name and
+ * no `toString` of its own, so a body throwing
+ * `{ code: 'EMAIL_TAKEN', message: 'That address is already on file' }` was
+ * answered `undefined: That address is already on file`, and one throwing
+ * `{ code: 'DUPLICATE_RECORD' }` was answered `[object Object]`.
+ *
+ * ## What holds now
+ *
+ * A non-`Error` object is flattened under the default name `Error`, so its
+ * `message` is the sentence byte for byte. With no usable `message` and no
+ * `toString` of its own, the sentence is `INTERNAL_ERROR_MESSAGE`, the one
+ * every door answers for a fault with no business message. `code` rides
+ * `__errorInfo` as before. A thrown `Error` (and subclass), a thrown primitive,
+ * and a value with a `toString` of its own are flattened exactly as before.
+ *
+ * The door rows live in `../sandbox-plain-object-throw-client-sentence.test.ts`.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { INTERNAL_ERROR_MESSAGE } from '@objectstack/types';
+
+import { QuickJSScriptRunner, SandboxError } from './quickjs-runner.js';
+import type { ScriptContext, ScriptRunOptions } from './script-runner.js';
+
+const runner = new QuickJSScriptRunner({ hookTimeoutMs: 10_000 });
+const actionOpts: ScriptRunOptions = { origin: { kind: 'action', name: 'submit' } };
+const hookOpts: ScriptRunOptions = { origin: { kind: 'hook', name: 'guard' } };
+
+const SENTENCE = 'That address is already on file';
+
+/** Run `source` as a body and hand back the error it rejected with. */
+async function thrownBy(source: string, opts: ScriptRunOptions = actionOpts): Promise<SandboxError> {
+    const err = await runner
+        .run({ language: 'js', source, capabilities: [] }, { input: {} } as ScriptContext, opts)
+        .then(
+            () => undefined,
+            (e: unknown) => e,
+        );
+    expect(err).toBeInstanceOf(SandboxError);
+    return err as SandboxError;
+}
+
+describe('[#23718] a thrown plain object crosses with its own message as the sentence', () => {
+    it('{ code, message }: the message is innerMessage, and the code is kept', async () => {
+        const err = await thrownBy(`throw { code: 'EMAIL_TAKEN', message: '${SENTENCE}' };`);
+        expect(err.innerMessage).toBe(SENTENCE);
+        expect(err.code).toBe('EMAIL_TAKEN');
+        expect(err.message).not.toContain('undefined:');
+    });
+
+    it('…on the hook origin too (both wrappers share the handler)', async () => {
+        const err = await thrownBy(`throw { code: 'EMAIL_TAKEN', message: '${SENTENCE}' };`, hookOpts);
+        expect(err.innerMessage).toBe(SENTENCE);
+        expect(err.code).toBe('EMAIL_TAKEN');
+    });
+
+    it('{ code } with no message: the no-business-message sentence, never [object Object]', async () => {
+        const err = await thrownBy(`throw { code: 'DUPLICATE_RECORD' };`);
+        expect(err.innerMessage).toBe(INTERNAL_ERROR_MESSAGE);
+        expect(err.code).toBe('DUPLICATE_RECORD');
+        expect(err.message).not.toContain('[object Object]');
+    });
+
+    it('a value whose String() is empty gets the same sentence (it used to record no rejection)', async () => {
+        const err = await thrownBy(`throw [];`);
+        expect(err.innerMessage).toBe(INTERNAL_ERROR_MESSAGE);
+    });
+
+    it('a message that itself begins "Error: " crosses byte for byte', async () => {
+        const err = await thrownBy(`throw { message: 'Error: lead is incomplete' };`);
+        expect(err.innerMessage).toBe('Error: lead is incomplete');
+    });
+
+    it('a name on a plain object is a field, never a frame: no prefix, and no crash reading', async () => {
+        const err = await thrownBy(`throw { name: 'TypeError', message: 'forged' };`);
+        expect(err.innerMessage).toBe('forged');
+    });
+});
+
+describe('[#23718] CONTROL — everything else is flattened exactly as before', () => {
+    it('a thrown Error still reads `name: message` in the log, and its message is the sentence', async () => {
+        const err = await thrownBy(`throw new Error('x');`);
+        expect(err.message).toBe("action 'submit' threw: Error: x");
+        expect(err.innerMessage).toBe('x');
+    });
+
+    it('a thrown Error subclass keeps its name in both', async () => {
+        const err = await thrownBy(`throw new TypeError('t');`);
+        expect(err.message).toBe("action 'submit' threw: TypeError: t");
+        expect(err.innerMessage).toBe('TypeError: t');
+    });
+
+    it('a thrown string is its own sentence', async () => {
+        const err = await thrownBy(`throw 'Name is required';`);
+        expect(err.innerMessage).toBe('Name is required');
+    });
+
+    it('an object with a toString of its own keeps it', async () => {
+        const err = await thrownBy(`throw { toString() { return 'custom words'; } };`);
+        expect(err.innerMessage).toBe('custom words');
+    });
+});