diff --git a/packages/app-tests/e2e/document-auth/assistant-signing-auth.spec.ts b/packages/app-tests/e2e/document-auth/assistant-signing-auth.spec.ts
new file mode 100644
index 000000000..a4857e6c8
--- /dev/null
+++ b/packages/app-tests/e2e/document-auth/assistant-signing-auth.spec.ts
@@ -0,0 +1,172 @@
+import { type APIRequestContext, expect, test } from '@playwright/test';
+import { FieldType, SigningStatus } from '@prisma/client';
+
+import { NEXT_PUBLIC_WEBAPP_URL } from '@documenso/lib/constants/app';
+import { prisma } from '@documenso/prisma';
+
+import { apiSeedPendingDocument } from '../fixtures/api-seeds';
+
+const WEBAPP_BASE_URL = NEXT_PUBLIC_WEBAPP_URL();
+
+type SeededEnvelopes = {
+  assistantToken: string;
+  otherEnvelopeFieldId: number;
+};
+
+/**
+ * Seeds two unrelated pending envelopes:
+ * - Envelope A has an ASSISTANT (with a token) plus a SIGNER.
+ * - Envelope B is owned by a different user and has a SIGNER with a TEXT field.
+ *
+ * Returns the assistant's token from envelope A and the TEXT field id from
+ * envelope B so callers can exercise signing routes across envelopes.
+ */
+const seedTwoPendingEnvelopes = async (request: APIRequestContext): Promise<SeededEnvelopes> => {
+  const envelopeA = await apiSeedPendingDocument(request, {
+    title: '[TEST] Envelope A',
+    recipients: [
+      {
+        email: `assistant-${Date.now()}@documenso.com`,
+        name: 'Assistant',
+        role: 'ASSISTANT',
+        signingOrder: 1,
+      },
+      {
+        email: `signer-a-${Date.now()}@documenso.com`,
+        name: 'Signer A',
+        role: 'SIGNER',
+        signingOrder: 2,
+      },
+    ],
+    fieldsPerRecipient: [
+      [],
+      // SIGNER needs a SIGNATURE field so distribution succeeds.
+      [{ type: FieldType.SIGNATURE, page: 1, positionX: 5, positionY: 5, width: 5, height: 5 }],
+    ],
+  });
+
+  const assistant = envelopeA.distributeResult.recipients.find((r) => r.role === 'ASSISTANT');
+
+  if (!assistant) {
+    throw new Error('Assistant recipient not found in envelope A');
+  }
+
+  const envelopeB = await apiSeedPendingDocument(request, {
+    title: '[TEST] Envelope B',
+    recipients: [
+      {
+        email: `signer-b-${Date.now()}@documenso.com`,
+        name: 'Signer B',
+        role: 'SIGNER',
+        signingOrder: 1,
+      },
+    ],
+    // A TEXT field is used as the cross-envelope target. The V2 route has a
+    // separate guard that blocks assistants from signing SIGNATURE fields,
+    // which would mask whether the recipient lookup itself was scoped.
+    fieldsPerRecipient: [
+      [
+        { type: FieldType.SIGNATURE, page: 1, positionX: 5, positionY: 5, width: 5, height: 5 },
+        { type: FieldType.TEXT, page: 1, positionX: 5, positionY: 15, width: 5, height: 5 },
+      ],
+    ],
+  });
+
+  const otherEnvelope = await prisma.envelope.findUniqueOrThrow({
+    where: { id: envelopeB.envelope.id },
+    include: { fields: true },
+  });
+
+  const textField = otherEnvelope.fields.find((f) => f.type === FieldType.TEXT);
+
+  if (!textField) {
+    throw new Error('TEXT field not found in envelope B');
+  }
+
+  return {
+    assistantToken: assistant.token,
+    otherEnvelopeFieldId: textField.id,
+  };
+};
+
+const trpcMutation = async (
+  request: APIRequestContext,
+  procedure: string,
+  input: Record<string, unknown>,
+) => {
+  return await request.post(`${WEBAPP_BASE_URL}/api/trpc/${procedure}`, {
+    headers: { 'content-type': 'application/json' },
+    data: JSON.stringify({ json: input }),
+  });
+};
+
+test.describe('[ASSISTANT_SIGNING_AUTH]: cross-envelope field access', () => {
+  test('envelope.field.sign (V2) rejects fieldId from another envelope', async ({ request }) => {
+    const { assistantToken, otherEnvelopeFieldId } = await seedTwoPendingEnvelopes(request);
+
+    const res = await trpcMutation(request, 'envelope.field.sign', {
+      token: assistantToken,
+      fieldId: otherEnvelopeFieldId,
+      fieldValue: { type: FieldType.TEXT, value: 'TEXT' },
+    });
+
+    expect(res.ok()).toBeFalsy();
+
+    const fieldAfter = await prisma.field.findUniqueOrThrow({
+      where: { id: otherEnvelopeFieldId },
+    });
+
+    expect(fieldAfter.inserted).toBe(false);
+    expect(fieldAfter.customText).toBe('');
+  });
+
+  test('field.signFieldWithToken (V1) rejects fieldId from another envelope', async ({
+    request,
+  }) => {
+    const { assistantToken, otherEnvelopeFieldId } = await seedTwoPendingEnvelopes(request);
+
+    const res = await trpcMutation(request, 'field.signFieldWithToken', {
+      token: assistantToken,
+      fieldId: otherEnvelopeFieldId,
+      value: 'TEXT',
+      isBase64: false,
+    });
+
+    expect(res.ok()).toBeFalsy();
+
+    const fieldAfter = await prisma.field.findUniqueOrThrow({
+      where: { id: otherEnvelopeFieldId },
+    });
+
+    expect(fieldAfter.inserted).toBe(false);
+    expect(fieldAfter.customText).toBe('');
+  });
+
+  test('field.removeSignedFieldWithToken (V1) rejects fieldId from another envelope', async ({
+    request,
+  }) => {
+    const { assistantToken, otherEnvelopeFieldId } = await seedTwoPendingEnvelopes(request);
+
+    // Pre-insert the field so a successful (incorrect) uninsert is detectable.
+    await prisma.field.update({
+      where: { id: otherEnvelopeFieldId },
+      data: { inserted: true, customText: 'pre-existing-value' },
+    });
+
+    const res = await trpcMutation(request, 'field.removeSignedFieldWithToken', {
+      token: assistantToken,
+      fieldId: otherEnvelopeFieldId,
+    });
+
+    expect(res.ok()).toBeFalsy();
+
+    const fieldAfter = await prisma.field.findUniqueOrThrow({
+      where: { id: otherEnvelopeFieldId },
+      include: { recipient: true },
+    });
+
+    expect(fieldAfter.inserted).toBe(true);
+    expect(fieldAfter.customText).toBe('pre-existing-value');
+    expect(fieldAfter.recipient.signingStatus).toBe(SigningStatus.NOT_SIGNED);
+  });
+});
diff --git a/packages/lib/server-only/field/get-fields-for-token.ts b/packages/lib/server-only/field/get-fields-for-token.ts
index 0c4534cf8..c34e9771b 100644
--- a/packages/lib/server-only/field/get-fields-for-token.ts
+++ b/packages/lib/server-only/field/get-fields-for-token.ts
@@ -35,6 +35,7 @@ export const getFieldsForToken = async ({ token }: GetFieldsForTokenOptions) =>
               signingOrder: {
                 gte: recipient.signingOrder ?? 0,
               },
+              envelopeId: recipient.envelopeId,
             },
             envelope: {
               id: recipient.envelopeId,
diff --git a/packages/lib/server-only/field/remove-signed-field-with-token.ts b/packages/lib/server-only/field/remove-signed-field-with-token.ts
index 0995ba4a9..1a42f9ca3 100644
--- a/packages/lib/server-only/field/remove-signed-field-with-token.ts
+++ b/packages/lib/server-only/field/remove-signed-field-with-token.ts
@@ -38,6 +38,7 @@ export const removeSignedFieldWithToken = async ({
               signingStatus: {
                 not: SigningStatus.SIGNED,
               },
+              envelopeId: recipient.envelopeId,
             }),
       },
     },
diff --git a/packages/lib/server-only/field/sign-field-with-token.ts b/packages/lib/server-only/field/sign-field-with-token.ts
index f36b02a84..ab7c3a25f 100644
--- a/packages/lib/server-only/field/sign-field-with-token.ts
+++ b/packages/lib/server-only/field/sign-field-with-token.ts
@@ -78,6 +78,7 @@ export const signFieldWithToken = async ({
               signingOrder: {
                 gte: recipient.signingOrder ?? 0,
               },
+              envelopeId: recipient.envelopeId,
             }),
       },
     },
diff --git a/packages/trpc/server/envelope-router/sign-envelope-field.ts b/packages/trpc/server/envelope-router/sign-envelope-field.ts
index fd9695ff7..c48c560a5 100644
--- a/packages/trpc/server/envelope-router/sign-envelope-field.ts
+++ b/packages/trpc/server/envelope-router/sign-envelope-field.ts
@@ -51,6 +51,7 @@ export const signEnvelopeFieldRoute = procedure
                 signingOrder: {
                   gte: recipient.signingOrder ?? 0,
                 },
+                envelopeId: recipient.envelopeId,
               }
             : {
                 id: recipient.id,