@@ -0,0 +1,13 @@
+---
+'@objectstack/core': patch
+'@objectstack/runtime': patch
+---
+
+Credential-class field values are now masked on every write response, as on reads.
+
+Clause-②: no
+
+- A `secret` field, and a `password` field on an object that is not `managedBy: 'better-auth'`, already read back as `SECRET_MASK` (`null` when unset) on the generic read path (ADR-0100). Every write response that returns a record (REST, batch and MCP) now answers the same way.
+- The shared write-response helper every write door already calls (`omitInternalFieldsFromWriteResponse`, `@objectstack/core`) now applies the credential mask before it omits `internal: true` fields. New exports beside it: `maskCredentialFieldsInWriteResponse` and `collectCredentialWriteResponseFields`, which read the same `isMaskedOnReadFieldType` declaration as the engine's read mask.
+- `callData`'s fallback create and update arms (`@objectstack/runtime`, used when no protocol service is registered) now pass their response record through the same helper.
+- Unchanged: the engine's own write results still return the stored row whole to privileged server-side callers, and the echoed-mask write guard still treats a `SECRET_MASK` value as "leave unchanged", so a client that saves back a write response does not overwrite the stored credential.
@@ -0,0 +1,76 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { describe, it, expect } from 'vitest';
+import { SECRET_MASK } from '@objectstack/spec/data';
+import {
+  collectCredentialWriteResponseFields,
+  maskCredentialFieldsInWriteResponse,
+  omitInternalFieldsFromWriteResponse,
+} from './internal-write-response.js';
+
+const STORED_PASSWORD = 'stored-password-value-never-returned';
+const STORED_REF = 'secret:handle-never-returned';
+
+const SCHEMA = {
+  name: 'cred_holder',
+  fields: {
+    id: { type: 'text' },
+    name: { type: 'text' },
+    login_password: { type: 'password' },
+    api_token: { type: 'secret' },
+    hidden_hash: { type: 'text', internal: true },
+    both: { type: 'secret', internal: true },
+  },
+};
+
+const row = () => ({
+  id: 'r1',
+  name: 'visible',
+  login_password: STORED_PASSWORD,
+  api_token: STORED_REF,
+  hidden_hash: 'h',
+  both: 'secret:x',
+});
+
+describe('write-response credential mask', () => {
+  it('collects the read-mask set: secret always, password outside the exempt bucket', () => {
+    expect(collectCredentialWriteResponseFields(SCHEMA).sort()).toEqual(['api_token', 'both', 'login_password']);
+    expect(collectCredentialWriteResponseFields({ ...SCHEMA, managedBy: 'better-auth' }).sort())
+      .toEqual(['api_token', 'both']);
+    expect(collectCredentialWriteResponseFields(undefined)).toEqual([]);
+    expect(collectCredentialWriteResponseFields({ name: 'x' })).toEqual([]);
+  });
+
+  it('masks a set credential, keeps an unset one null, never adds a key', () => {
+    const r: Record<string, unknown> = { id: 'r1', login_password: STORED_PASSWORD, api_token: null };
+    maskCredentialFieldsInWriteResponse(SCHEMA, r);
+    expect(r).toEqual({ id: 'r1', login_password: SECRET_MASK, api_token: null });
+    // Idempotent.
+    maskCredentialFieldsInWriteResponse(SCHEMA, r);
+    expect(r).toEqual({ id: 'r1', login_password: SECRET_MASK, api_token: null });
+  });
+
+  it('the shared write-response helper masks credentials and omits internal fields, on every row', () => {
+    const rows = [row(), null, 7, row()];
+    omitInternalFieldsFromWriteResponse(SCHEMA, rows);
+    for (const r of [rows[0], rows[3]] as Array<Record<string, unknown>>) {
+      expect(r).toEqual({ id: 'r1', name: 'visible', login_password: SECRET_MASK, api_token: SECRET_MASK });
+    }
+    const wire = JSON.stringify(rows);
+    expect(wire.includes(STORED_PASSWORD)).toBe(false);
+    expect(wire.includes('secret:')).toBe(false);
+  });
+
+  it('a field removed upstream (field-level security) stays absent', () => {
+    const r: Record<string, unknown> = { id: 'r1', name: 'visible' };
+    omitInternalFieldsFromWriteResponse(SCHEMA, r);
+    expect(r).toEqual({ id: 'r1', name: 'visible' });
+  });
+
+  it('a better-auth object keeps its password column, still masks its secret column', () => {
+    const r = row();
+    omitInternalFieldsFromWriteResponse({ ...SCHEMA, managedBy: 'better-auth' }, r);
+    expect(r.login_password).toBe(STORED_PASSWORD);
+    expect(r.api_token).toBe(SECRET_MASK);
+  });
+});
@@ -89,15 +89,29 @@
  * columns are `required`, so a mask carries zero bits while still shipping a
  * value under a field whose description promises none.
  *
+ * ## The credential-class mask rides the same helper
+ *
+ * The engine masks credential-class fields (`secret`, and `password` outside
+ * the exempt `managedBy` buckets — ADR-0100) on its READ path only, and keeps
+ * its write results whole for the same reason it keeps `internal` columns:
+ * a privileged server-side writer may read the stored value back off the
+ * result. Every external write response therefore owes the credential mask
+ * too, at the same mouths. It is applied by the same helper, ahead of the
+ * omission, so no mouth can hold one guarantee and miss the other, and the
+ * three tripwires named above hold both by one enumeration.
+ *
  * Deletion is IN PLACE and idempotent: records that already lack the field
  * (a re-stripped read result, a fake engine that never returned it) pass
  * through unchanged, and non-record values (`null`, an affected-row count, a
  * driver's boolean delete verdict) are skipped rather than judged.
  */
 
+import { SECRET_MASK, isMaskedOnReadFieldType } from '@objectstack/spec/data';
+
 /** Minimal view of an object schema this module reads — the field map only. */
 interface SchemaWithFields {
-  fields?: Record<string, { internal?: unknown } | undefined> | undefined;
+  fields?: Record<string, { internal?: unknown; type?: unknown } | undefined> | undefined;
+  managedBy?: unknown;
 }
 
 /**
@@ -118,9 +132,65 @@ export function collectInternalWriteResponseFields(schema: unknown): string[] {
 }
 
 /**
- * Drop every `internal: true` field from a write response's record(s), in
- * place. THE single helper every external write mouth goes through — see the
- * module header; the three tripwires enforce the "every".
+ * Collect the names of the credential-class fields the engine masks on read
+ * (ADR-0100: every `secret` field, every `password` field outside the exempt
+ * `managedBy` buckets) — the write-response half of that mask reads the SAME
+ * set, so a write answers what a read of the same row would.
+ *
+ * Not restated: the type set and its per-type `managedBy` exemptions are
+ * declared once in `@objectstack/spec/data` and asked through
+ * `isMaskedOnReadFieldType`, exactly as objectql's `collectMaskedReadFields`
+ * asks it. ⛔ Do not add a `def.type === …` arm here; change the declaration.
+ */
+export function collectCredentialWriteResponseFields(schema: unknown): string[] {
+  const s = schema as SchemaWithFields | null | undefined;
+  const fields = s?.fields;
+  if (!fields || typeof fields !== 'object') return [];
+  const managedBy: unknown = s?.managedBy;
+  const out: string[] = [];
+  for (const [name, def] of Object.entries(fields)) {
+    if (def && isMaskedOnReadFieldType(def.type, managedBy)) out.push(name);
+  }
+  return out;
+}
+
+/**
+ * Replace every credential-class field in a write response's record(s) with
+ * `SECRET_MASK`, in place — the write-response mirror of the engine's read
+ * mask (`maskSecretFields`). A set value becomes the mask; an unset one
+ * (`null` / `undefined`) becomes `null`, so "a credential is set" is the only
+ * bit a response carries. A field the record does not carry (never written, or
+ * already removed by field-level security upstream) stays absent: this mask
+ * never ADDS a key, so it composes with the security plugin's field masking
+ * instead of re-exposing what that removed.
+ *
+ * Idempotent; non-objects are skipped, arrays are walked.
+ */
+export function maskCredentialFieldsInWriteResponse(schema: unknown, records: unknown): void {
+  if (!records) return;
+  const credentialFields = collectCredentialWriteResponseFields(schema);
+  if (credentialFields.length === 0) return;
+  const list = Array.isArray(records) ? records : [records];
+  for (const row of list) {
+    if (!row || typeof row !== 'object') continue;
+    const r = row as Record<string, unknown>;
+    for (const field of credentialFields) {
+      if (!(field in r)) continue;
+      r[field] = r[field] == null ? null : SECRET_MASK;
+    }
+  }
+}
+
+/**
+ * Apply the generic-data-path non-exposure rules to a write response's
+ * record(s), in place: credential-class fields are MASKED
+ * ({@link maskCredentialFieldsInWriteResponse}), then `internal: true` fields
+ * are OMITTED. THE single helper every external write mouth goes through —
+ * see the module header; the three tripwires enforce the "every".
+ *
+ * Mask first, omit second — the engine read path's order, so a field that is
+ * both credential-typed and `internal` ends up omitted (the stricter
+ * disposition wins).
  *
  * @param schema  The registered object schema (`engine.registry.getObject(...)`
  *                / the protocol's own registry view / `metadataService
@@ -133,6 +203,7 @@ export function collectInternalWriteResponseFields(schema: unknown): string[] {
  */
 export function omitInternalFieldsFromWriteResponse(schema: unknown, records: unknown): void {
   if (!records) return;
+  maskCredentialFieldsInWriteResponse(schema, records);
   const internalFields = collectInternalWriteResponseFields(schema);
   if (internalFields.length === 0) return;
   const list = Array.isArray(records) ? records : [records];
 
@@ -52,6 +52,10 @@ import { assertEngineDeleteDispatch, assertEngineUpdateDispatch, assertEngineFin
 const SENTINEL = 'INTERNAL-SENTINEL-8497-NEVER-SERIALIZED';
 /** The value that MUST appear wherever a record was promised (falsifiability). */
 const CONTROL = 'CONTROL-VALUE-8497-RECORD-FLOWED';
+/** Credential-class stored values: a `password` plaintext and a `secret` handle ref. */
+const CREDENTIAL_SENTINELS = ['PASSWORD-SENTINEL-NEVER-SERIALIZED', 'secret:HANDLE-SENTINEL-NEVER-SERIALIZED'];
+const NEVER_SERIALIZED = [SENTINEL, ...CREDENTIAL_SENTINELS];
+const leaks = (wire: string) => NEVER_SERIALIZED.filter((v) => wire.includes(v));
 
 const VAULT = {
   name: 'vault',
@@ -60,6 +64,8 @@ const VAULT = {
     id: { name: 'id', type: 'text' },
     name: { name: 'name', type: 'text' },
     vault_secret: { name: 'vault_secret', type: 'text', internal: true },
+    vault_password: { name: 'vault_password', type: 'password' },
+    vault_token: { name: 'vault_token', type: 'secret' },
   },
   // No `apiEnabled`/`apiMethods` narrowing: the ADR-0049 exposure gate must let
   // every verb through, or a recipe would be measuring a 404 instead of a body.
@@ -78,6 +84,8 @@ function makeSentinelEngine(): IDataEngine {
     id,
     name: (data?.name as string) ?? CONTROL,
     vault_secret: SENTINEL,
+    vault_password: CREDENTIAL_SENTINELS[0],
+    vault_token: CREDENTIAL_SENTINELS[1],
   });
   return {
     find: vi.fn(async () => [storedRow()]),
@@ -157,7 +165,7 @@ const RECIPES: Record<string, Recipe> = {
   remove: { invoke: (b) => b.remove('vault', 'row-1'), writesRecords: false },
 };
 
-describe('#8497 tripwire: no MCP write response carries an `internal: true` value', () => {
+describe('#8497 tripwire: no MCP write response carries an `internal: true` value or a credential-class stored value', () => {
   it('the enumeration is real: it sees the bridge write verbs', () => {
     const faces = enumerateBridgeFaces(makeBridge());
     expect(faces).toEqual(expect.arrayContaining(['create', 'update', 'remove']));
@@ -181,10 +189,10 @@ describe('#8497 tripwire: no MCP write response carries an `internal: true` valu
   });
 
   for (const [name, recipe] of Object.entries(RECIPES)) {
-    it(`${name}: response never carries the internal sentinel${recipe.writesRecords ? ', and really returned a record' : ''}`, async () => {
+    it(`${name}: response never carries the internal or credential sentinels${recipe.writesRecords ? ', and really returned a record' : ''}`, async () => {
       const bridge = makeBridge();
       const wire = JSON.stringify((await recipe.invoke(bridge)) ?? null);
-      expect(wire.includes(SENTINEL), `${name} leaked an internal field: ${wire}`).toBe(false);
+      expect(leaks(wire), `${name} leaked an internal or credential-class field: ${wire}`).toEqual([]);
       if (recipe.writesRecords) {
         expect(
           wire.includes(CONTROL),
@@ -207,6 +215,16 @@ describe('#8497 tripwire: no MCP write response carries an `internal: true` valu
     expect(wire.includes(CONTROL)).toBe(true); // still a real record echo
   });
 
+  it('the caller cannot read their own credential write back in clear from the update echo', async () => {
+    const bridge = makeBridge();
+    const wire = JSON.stringify(await bridge.update('vault', 'row-1', {
+      name: CONTROL,
+      vault_password: 'caller-sent-password',
+    }));
+    expect(wire.includes('caller-sent-password')).toBe(false);
+    expect(wire.includes(CONTROL)).toBe(true);
+  });
+
   it('NEGATIVE CONTROL: the machinery goes red on a write mouth that skips the helper', async () => {
     // Exactly the defect this file was written after: an engine-only mouth that
     // echoes `engine.insert`'s (whole) result. Reintroduce it locally and prove
@@ -221,10 +239,10 @@ describe('#8497 tripwire: no MCP write response carries an `internal: true` valu
     };
 
     const leaked = await leaky.create('vault', { name: CONTROL });
-    expect(JSON.stringify(leaked).includes(SENTINEL)).toBe(true); // the scan bites
+    expect(leaks(JSON.stringify(leaked))).toEqual(NEVER_SERIALIZED); // the scan bites
 
     omitInternalFieldsFromWriteResponse(VAULT, (leaked as any).record);
-    expect(JSON.stringify(leaked).includes(SENTINEL)).toBe(false); // the helper closes it
+    expect(leaks(JSON.stringify(leaked))).toEqual([]); // the helper closes it
     expect(JSON.stringify(leaked).includes(CONTROL)).toBe(true); // …without eating the record
   });
 });
@@ -56,13 +56,24 @@ import {
 const SENTINEL = 'INTERNAL-SENTINEL-7823-NEVER-SERIALIZED';
 /** The value that MUST appear wherever a record was promised (falsifiability). */
 const CONTROL = 'CONTROL-VALUE-7823-RECORD-FLOWED';
+/**
+ * Credential-class stored values (a `password` field's plaintext and a
+ * `secret` field's handle ref) — masked on read by the engine, and owed the
+ * same mask on every write response by the shared helper.
+ */
+const CREDENTIAL_SENTINELS = ['PASSWORD-SENTINEL-NEVER-SERIALIZED', 'secret:HANDLE-SENTINEL-NEVER-SERIALIZED'];
+/** Every stored value no write response may carry. */
+const NEVER_SERIALIZED = [SENTINEL, ...CREDENTIAL_SENTINELS];
+const leaks = (wire: string) => NEVER_SERIALIZED.filter((v) => wire.includes(v));
 
 const VAULT_SCHEMA = {
   name: 'vault',
   fields: {
     id: { name: 'id', type: 'text' },
     name: { name: 'name', type: 'text' },
     vault_secret: { name: 'vault_secret', type: 'text', internal: true },
+    vault_password: { name: 'vault_password', type: 'password' },
+    vault_token: { name: 'vault_token', type: 'secret' },
   },
   enable: { clone: true },
 };
@@ -82,6 +93,8 @@ function makeSentinelEngine() {
     id,
     name: (data as any)?.name ?? CONTROL,
     vault_secret: SENTINEL,
+    vault_password: CREDENTIAL_SENTINELS[0],
+    vault_token: CREDENTIAL_SENTINELS[1],
   });
   let nextId = 1;
   const handle = { id: 'trx-1' };
@@ -241,7 +254,7 @@ const RECIPES: Record<string, Recipe> = {
   runAtomicBatchData: { coveredVia: 'batchData' },
 };
 
-describe('#7823 tripwire: every generic data ingress strips `internal: true` from its write response', () => {
+describe('#7823 tripwire: every generic data ingress strips `internal: true` and masks credential-class fields in its write response', () => {
   const enumerated = enumerateDataMethods(ObjectStackProtocolImplementation.prototype);
 
   it('the enumeration is real: it sees the three ruling-named ingresses', () => {
@@ -276,12 +289,12 @@ describe('#7823 tripwire: every generic data ingress strips `internal: true` fro
   for (const name of enumerated) {
     const recipe = RECIPES[name];
     if (!recipe || 'coveredVia' in recipe) continue;
-    it(`${name}: response never carries the internal sentinel${recipe.expectRecord ? ', and really returned a record' : ''}`, async () => {
+    it(`${name}: response never carries the internal or credential sentinels${recipe.expectRecord ? ', and really returned a record' : ''}`, async () => {
       for (const invoke of recipe.invocations) {
         const p = new ObjectStackProtocolImplementation(makeSentinelEngine());
         const response = await invoke(p);
         const wire = JSON.stringify(response ?? null);
-        expect(wire.includes(SENTINEL), `${name} leaked an internal field: ${wire}`).toBe(false);
+        expect(leaks(wire), `${name} leaked an internal or credential-class field: ${wire}`).toEqual([]);
         if (recipe.expectRecord) {
           expect(wire.includes(CONTROL), `${name} returned no record at all — the probe is blind: ${wire}`).toBe(true);
         }
@@ -305,12 +318,12 @@ describe('#7823 tripwire: every generic data ingress strips `internal: true` fro
 
     const p = new LeakyProtocol(makeSentinelEngine());
     const wire = JSON.stringify(await p.leakyData({ object: 'vault', id: 'row-1', data: { name: 'x' } }));
-    expect(wire.includes(SENTINEL)).toBe(true); // half 2: the scan detects the leak
+    expect(leaks(wire)).toEqual(NEVER_SERIALIZED); // half 2: the scan detects every leak
 
     // And the helper is exactly what closes it — same response, one call.
     const fixed = await p.leakyData({ object: 'vault', id: 'row-1', data: { name: 'x' } });
     omitInternalFieldsFromWriteResponse(VAULT_SCHEMA, (fixed as any).record);
-    expect(JSON.stringify(fixed).includes(SENTINEL)).toBe(false);
+    expect(leaks(JSON.stringify(fixed))).toEqual([]);
   });
 
   it('the collector agrees with the engine rule: strict `internal === true` only', () => {