diff --git a/app/api/dev-jmap/[...path]/route.ts b/app/api/dev-jmap/[...path]/route.ts
index d2d4ca958..e5cc03c2e 100644
--- a/app/api/dev-jmap/[...path]/route.ts
+++ b/app/api/dev-jmap/[...path]/route.ts
@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server';
+import { pointerTokenValue } from '@/lib/jmap/patch-pointer';
 
 /**
  * Mock JMAP server for local development.
@@ -1725,7 +1726,9 @@ function handleEmailSet(args: MethodArgs, callId: string): MethodResult {
       // Patch-style keyword updates: "keywords/$seen", "keywords/$flagged", etc.
       for (const [key, value] of Object.entries(changes)) {
         if (key.startsWith('keywords/')) {
-          const keyword = key.slice('keywords/'.length);
+          // The key is a JSON Pointer, so a nested tag arrives escaped
+          // (`$label:work~1clients`) and has to be read back to its real name.
+          const keyword = pointerTokenValue(key.slice('keywords/'.length));
           if (value) {
             email.keywords[keyword] = true;
           } else {
diff --git a/components/settings/keyword-settings.tsx b/components/settings/keyword-settings.tsx
index d55317caa..d106d22b8 100644
--- a/components/settings/keyword-settings.tsx
+++ b/components/settings/keyword-settings.tsx
@@ -574,9 +574,15 @@ export function KeywordSettings() {
       try {
         const oldJmapKeyword = `$label:${oldId}`;
         const newJmapKeyword = `$label:${keyword.id}`;
-        await client.migrateKeyword(oldJmapKeyword, newJmapKeyword);
+        const migration = await client.migrateKeyword(oldJmapKeyword, newJmapKeyword);
         renameKeyword(oldId, keyword);
         fetchTagCounts(client);
+        if (migration.refused > 0) {
+          // Some mail kept the old keyword. The definition still follows the
+          // messages that did move; the rest is what the warning is about.
+          const toastModule = await import('sonner');
+          toastModule.toast.warning(t("migration_error"));
+        }
       } catch (error) {
         console.error("Failed to migrate keyword:", error);
         const toastModule = await import('sonner');
diff --git a/lib/__tests__/jmap-keyword-pointer.test.ts b/lib/__tests__/jmap-keyword-pointer.test.ts
new file mode 100644
index 000000000..02b0b483f
--- /dev/null
+++ b/lib/__tests__/jmap-keyword-pointer.test.ts
@@ -0,0 +1,358 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { JMAPClient } from '../jmap/client';
+import { keywordPointer, pointerToken, pointerTokenValue } from '../jmap/patch-pointer';
+
+// A nested tag's id contains a slash (`work/clients`), so its keyword is
+// `$label:work/clients`. PatchObject keys are JSON Pointers (RFC 8620 section
+// 5.3), where an unescaped slash starts a new path segment - the server reads
+// `keywords/$label:work/clients` as "the `clients` member of the `$label:work`
+// keyword", which is not a boolean and is rejected or ignored. Nested tags can
+// therefore only be written with the slash escaped as `~1`.
+
+function createClient(): JMAPClient {
+  const client = new JMAPClient('https://jmap.example.com', 'user@example.com', 'pass');
+  Object.assign(client, {
+    apiUrl: 'https://jmap.example.com/api',
+    accountId: 'primary-account',
+    username: 'user@example.com',
+  });
+  return client;
+}
+
+interface JMAPMethodCall {
+  0: string;
+  1: Record<string, unknown>;
+  2: string;
+}
+
+/**
+ * Answers Email/query with one id and then nothing - the walk ends on an empty
+ * page - and Email/set with a plain success.
+ */
+function mockJmap() {
+  const captured: JMAPMethodCall[] = [];
+  let queried = false;
+  const fetchSpy = vi.spyOn(globalThis, 'fetch');
+  fetchSpy.mockImplementation(async (_url, init) => {
+    const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+    captured.push(...body.methodCalls);
+    const methodResponses = body.methodCalls.map((call) => {
+      if (call[0] !== 'Email/query') return ['Email/set', { updated: { m1: null } }, call[2]];
+      const ids = queried ? [] : ['m1'];
+      queried = true;
+      return ['Email/query', { ids }, call[2]];
+    });
+    return new Response(JSON.stringify({ methodResponses }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    });
+  });
+  return { captured, fetchSpy };
+}
+
+function patchOf(captured: JMAPMethodCall[], emailId = 'm1'): Record<string, unknown> {
+  const set = captured.find((call) => call[0] === 'Email/set');
+  const update = set?.[1].update as Record<string, Record<string, unknown>> | undefined;
+  return update?.[emailId] ?? {};
+}
+
+describe('pointerToken', () => {
+  it('escapes a slash as ~1', () => {
+    expect(pointerToken('work/clients')).toBe('work~1clients');
+  });
+
+  it('escapes a tilde as ~0, and does so before slashes', () => {
+    expect(pointerToken('a~/b')).toBe('a~0~1b');
+    expect(pointerToken('a~1b')).toBe('a~01b');
+  });
+
+  it('leaves a token with nothing to escape alone', () => {
+    expect(keywordPointer('$label:red')).toBe('keywords/$label:red');
+  });
+});
+
+describe('pointerTokenValue', () => {
+  it('reads back what pointerToken wrote', () => {
+    for (const value of ['work/clients', 'a~/b', 'a~1b', '$label:red', '~0/~1']) {
+      expect(pointerTokenValue(pointerToken(value))).toBe(value);
+    }
+  });
+
+  it('reads ~1 before ~0, so an escaped tilde stays literal', () => {
+    expect(pointerTokenValue('~01')).toBe('~1');
+  });
+});
+
+describe('keyword writes escape the JSON Pointer', () => {
+  beforeEach(() => vi.restoreAllMocks());
+  afterEach(() => vi.restoreAllMocks());
+
+  it('setKeyword escapes a nested tag keyword', async () => {
+    const { captured } = mockJmap();
+    await createClient().setKeyword('m1', '$label:work/clients');
+    expect(patchOf(captured)).toEqual({ 'keywords/$label:work~1clients': true });
+  });
+
+  it('removeKeyword escapes a nested tag keyword', async () => {
+    const { captured } = mockJmap();
+    await createClient().removeKeyword('m1', '$label:work/clients');
+    expect(patchOf(captured)).toEqual({ 'keywords/$label:work~1clients': null });
+  });
+
+  it('migrateKeyword escapes both the old and the new keyword', async () => {
+    const { captured } = mockJmap();
+    await createClient().migrateKeyword('$label:work/clients', '$label:work/acme');
+    expect(patchOf(captured)).toEqual({
+      'keywords/$label:work~1clients': null,
+      'keywords/$label:work~1acme': true,
+    });
+  });
+
+  it('leaves a flat keyword unchanged', async () => {
+    const { captured } = mockJmap();
+    await createClient().setKeyword('m1', '$label:red');
+    expect(patchOf(captured)).toEqual({ 'keywords/$label:red': true });
+  });
+});
+
+describe('migrateKeyword reports a server that refused', () => {
+  beforeEach(() => vi.restoreAllMocks());
+  afterEach(() => vi.restoreAllMocks());
+
+  /**
+   * Answers queries from `pages` in order (an empty page ends the walk) and
+   * every Email/set with `setResult`. A page given as an error tuple is
+   * returned as the method response itself.
+   */
+  function mockRun(
+    pages: (string[] | ['error', Record<string, unknown>])[],
+    setResult: Record<string, unknown> | ['error', Record<string, unknown>] = { updated: {} },
+  ) {
+    let page = 0;
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          const next = pages[Math.min(page, pages.length - 1)];
+          page++;
+          if (Array.isArray(next) && next[0] === 'error') {
+            return ['error', next[1] as Record<string, unknown>, call[2]];
+          }
+          return ['Email/query', { ids: next as string[] }, call[2]];
+        }
+        if (Array.isArray(setResult) && setResult[0] === 'error') {
+          return ['error', setResult[1] as Record<string, unknown>, call[2]];
+        }
+        return ['Email/set', setResult as Record<string, unknown>, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+  }
+
+  it('throws when the query for tagged mail fails instead of reading it as none', async () => {
+    mockRun([['error', { type: 'serverFail', description: 'boom' }]]);
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).rejects.toThrow('boom');
+  });
+
+  it('throws when the update fails outright', async () => {
+    mockRun([['m1'], []], ['error', { type: 'serverFail', description: 'no writes today' }]);
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).rejects.toThrow(
+      'no writes today',
+    );
+  });
+
+  it('counts only the messages the server actually updated', async () => {
+    mockRun([['m1', 'm2'], []], { updated: { m1: null }, notUpdated: { m2: { type: 'notFound' } } });
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 1,
+      refused: 0,
+    });
+  });
+
+  it('reports a message refused for a reason other than being gone', async () => {
+    mockRun([['m1', 'm2'], []], { updated: { m1: null }, notUpdated: { m2: { type: 'forbidden' } } });
+    // Throwing would abandon m1, which this same call already migrated.
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 1,
+      refused: 1,
+    });
+  });
+
+  it('keeps what it migrated when a later batch fails outright', async () => {
+    // One message per Email/set, so the first write lands before the second
+    // fails: throwing would strand m1 under the new keyword with the tag
+    // definition still on the old one.
+    const client = createClient();
+    Object.assign(client, { capabilities: { 'urn:ietf:params:jmap:core': { maxObjectsInSet: 1 } } });
+    let set = 0;
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          return ['Email/query', { ids: set === 0 ? ['m1', 'm2'] : [], queryState: 's1' }, call[2]];
+        }
+        set++;
+        return set === 1
+          ? ['Email/set', { updated: { m1: null } }, call[2]]
+          : ['error', { type: 'serverFail', description: 'gave up' }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    await expect(client.migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 1,
+      refused: 1,
+    });
+  });
+
+  it('throws when nothing at all could be migrated', async () => {
+    mockRun([['m1'], []], { notUpdated: { m1: { type: 'forbidden' } } });
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).rejects.toThrow(
+      'forbidden',
+    );
+  });
+
+  it('restarts the walk when the query state changes under it', async () => {
+    // Positions only line up within one queryState. The first page belongs to a
+    // state that is gone by the next request; a walk that kept its position (and
+    // the ids it had collected) would migrate mail the tag is no longer on and
+    // miss the mail it is.
+    const current = ['m1', 'm2'];
+    let queries = 0;
+    const touched: string[] = [];
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          queries++;
+          if (queries === 1) {
+            return ['Email/query', { ids: ['stale1', 'stale2'], queryState: 's1' }, call[2]];
+          }
+          const position = call[1].position as number;
+          const limit = call[1].limit as number;
+          return ['Email/query', { ids: current.slice(position, position + limit), queryState: 's2' }, call[2]];
+        }
+        const update = call[1].update as Record<string, unknown>;
+        touched.push(...Object.keys(update));
+        return ['Email/set', { updated: Object.fromEntries(Object.keys(update).map((id) => [id, null])) }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 2,
+      refused: 0,
+    });
+    expect(touched).toEqual(['m1', 'm2']);
+  });
+
+  it('keeps migrating later batches after one message is refused', async () => {
+    // A refusal is per message: the mail behind it in the queue is unaffected,
+    // and skipping it would leave far more behind than the server refused.
+    const client = createClient();
+    Object.assign(client, { capabilities: { 'urn:ietf:params:jmap:core': { maxObjectsInSet: 1 } } });
+    const touched: string[] = [];
+    let queried = false;
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          const ids = queried ? [] : ['m1', 'm2'];
+          queried = true;
+          return ['Email/query', { ids, queryState: 's1' }, call[2]];
+        }
+        const id = Object.keys(call[1].update as Record<string, unknown>)[0];
+        touched.push(id);
+        return id === 'm1'
+          ? ['Email/set', { notUpdated: { m1: { type: 'forbidden' } } }, call[2]]
+          : ['Email/set', { updated: { [id]: null } }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    await expect(client.migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 1,
+      refused: 1,
+    });
+    expect(touched).toEqual(['m1', 'm2']);
+  });
+
+  it('gives up before writing anything when the mailbox will not hold still', async () => {
+    // Each query reports a new state, so no page ever belongs with the last.
+    // Mixing them would retag mail the tag has since been taken off.
+    let queries = 0;
+    const sets: string[] = [];
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          queries++;
+          return ['Email/query', { ids: [`m${queries}`], queryState: `s${queries}` }, call[2]];
+        }
+        sets.push(call[2]);
+        return ['Email/set', { updated: {} }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).rejects.toThrow(
+      /kept changing/,
+    );
+    expect(sets).toEqual([]);
+  });
+
+  it('ends the walk when a page repeats ids instead of paginating', async () => {
+    // A server that answers every position with the same page would otherwise
+    // be paged forever.
+    mockRun([['m1']]);
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 1,
+      refused: 0,
+    });
+  });
+
+  it('keeps paging when the server returns fewer ids than the limit asked for', async () => {
+    // RFC 8620 section 5.5 lets a server clamp the limit. Treating a short page
+    // as the last one would silently migrate only the first page.
+    const pages = [['m1', 'm2'], ['m3'], []];
+    let query = 0;
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+      const body = JSON.parse((init as { body: string }).body) as { methodCalls: JMAPMethodCall[] };
+      const methodResponses = body.methodCalls.map((call) => {
+        if (call[0] === 'Email/query') {
+          const ids = pages[Math.min(query, pages.length - 1)];
+          query++;
+          return ['Email/query', { ids }, call[2]];
+        }
+        const update = call[1].update as Record<string, unknown>;
+        const updated = Object.fromEntries(Object.keys(update).map((id) => [id, null]));
+        return ['Email/set', { updated }, call[2]];
+      });
+      return new Response(JSON.stringify({ methodResponses }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      });
+    });
+
+    await expect(createClient().migrateKeyword('$label:red', '$label:iso')).resolves.toEqual({
+      migrated: 3,
+      refused: 0,
+    });
+    expect(query).toBe(3);
+  });
+});
diff --git a/lib/demo/demo-client.ts b/lib/demo/demo-client.ts
index 25acd36ac..d9133b5f4 100644
--- a/lib/demo/demo-client.ts
+++ b/lib/demo/demo-client.ts
@@ -1,4 +1,4 @@
-import type { IJMAPClient, KeywordDiscoveryResult } from '@/lib/jmap/client-interface';
+import type { IJMAPClient, KeywordDiscoveryResult, KeywordMigration } from '@/lib/jmap/client-interface';
 import type { Email, Mailbox, StateChange, AccountStates, Thread, Identity, EmailAddress, ContactCard, AddressBook, VacationResponse, Calendar, CalendarEvent, CalendarEventFilter, CalendarTask, FileNode, ScheduledEmail, SendEmailResult, SharedAccount } from '@/lib/jmap/types';
 import type { SieveScript, SieveCapabilities } from '@/lib/jmap/sieve-types';
 import { getDemoData, type DemoData } from './demo-data';
@@ -382,16 +382,16 @@ export class DemoJMAPClient implements IJMAPClient {
     }
   }
 
-  async migrateKeyword(oldKeyword: string, newKeyword: string): Promise<number> {
-    let count = 0;
+  async migrateKeyword(oldKeyword: string, newKeyword: string): Promise<KeywordMigration> {
+    let migrated = 0;
     for (const email of this.data.emails) {
       if (email.keywords[oldKeyword]) {
         delete email.keywords[oldKeyword];
         email.keywords[newKeyword] = true;
-        count++;
+        migrated++;
       }
     }
-    return count;
+    return { migrated, refused: 0 };
   }
 
   async deleteEmail(emailId: string): Promise<void> {
diff --git a/lib/jmap/client-interface.ts b/lib/jmap/client-interface.ts
index d38348f28..961028843 100644
--- a/lib/jmap/client-interface.ts
+++ b/lib/jmap/client-interface.ts
@@ -1,6 +1,17 @@
 import type { Email, Mailbox, StateChange, AccountStates, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, FileNode, FileNodeRights, Principal, PushSubscription, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
 import type { SieveScript, SieveCapabilities } from "./sieve-types";
 
+/** What `migrateKeyword` managed to do. */
+export interface KeywordMigration {
+  /** Messages now carrying the new keyword. */
+  migrated: number;
+  /**
+   * Messages the server refused to update for a reason other than being gone.
+   * Those still carry the old keyword.
+   */
+  refused: number;
+}
+
 export interface KeywordInfo {
   id: string;
   name: string;
@@ -153,7 +164,13 @@ export interface IJMAPClient {
   removeKeyword(emailId: string, keyword: string, accountId?: string): Promise<void>;
   /** Apply one `keywords/<name>` patch fragment (true=add, null=remove) to many messages in a single Email/set. */
   batchUpdateKeywords(emailIds: string[], patch: Record<string, boolean | null>, accountId?: string): Promise<void>;
-  migrateKeyword(oldKeyword: string, newKeyword: string): Promise<number>;
+  /**
+   * Rewrite one keyword to another on every message that carries it, in
+   * batches. Fails outright only when the server refuses the whole call;
+   * per-message refusals are reported in the result, since the messages
+   * migrated alongside them stay migrated.
+   */
+  migrateKeyword(oldKeyword: string, newKeyword: string): Promise<KeywordMigration>;
   deleteEmail(emailId: string, accountId?: string): Promise<void>;
   moveToTrash(emailId: string, trashMailboxId: string, accountId?: string, markAsRead?: boolean): Promise<void>;
   batchDeleteEmails(emailIds: string[], accountId?: string): Promise<void>;
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index ec42ecc7b..a164a0adf 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -1,8 +1,9 @@
 import type { Email, Mailbox, StateChange, AccountStates, Thread, Identity, EmailAddress, ContactCard, AddressBook, AddressBookRights, VacationResponse, Calendar, CalendarRights, CalendarEvent, CalendarEventFilter, CalendarTask, FileNode, FileNodeFilter, FileNodeRights, Principal, PushSubscription, EmailSubmission, ScheduledEmail, SendEmailResult, SharedAccount } from "./types";
 import type { SieveScript, SieveCapabilities } from "./sieve-types";
-import type { IJMAPClient, KeywordDiscoveryResult, KeywordInfo } from "./client-interface";
+import type { IJMAPClient, KeywordDiscoveryResult, KeywordInfo, KeywordMigration } from "./client-interface";
 import { toWildcardQuery } from "./search-utils";
 import { batched, itemsPerRequest } from "./request-limits";
+import { keywordPointer } from "./patch-pointer";
 import { debug } from "@/lib/debug";
 import { normalizeCalendarEventLike } from "@/lib/calendar-event-normalization";
 import { findTasksOnlyCalendarIds, isTaskLikeObject, type ScannedCalendarObject } from "@/lib/calendar-component-detection";
@@ -1756,7 +1757,7 @@ export class JMAPClient implements IJMAPClient {
         accountId: accountId || this.accountId,
         update: {
           [emailId]: {
-            [`keywords/${keyword}`]: true,
+            [keywordPointer(keyword)]: true,
           },
         },
       }, "0"],
@@ -1769,7 +1770,7 @@ export class JMAPClient implements IJMAPClient {
         accountId: accountId || this.accountId,
         update: {
           [emailId]: {
-            [`keywords/${keyword}`]: null,
+            [keywordPointer(keyword)]: null,
           },
         },
       }, "0"],
@@ -1791,11 +1792,18 @@ export class JMAPClient implements IJMAPClient {
     }
   }
 
-  async migrateKeyword(oldKeyword: string, newKeyword: string): Promise<number> {
+  async migrateKeyword(oldKeyword: string, newKeyword: string): Promise<KeywordMigration> {
     // Query all email IDs that have the old keyword
+    const seen = new Set<string>();
     const allIds: string[] = [];
     let position = 0;
     const batchSize = 100;
+    // Positions only line up within one queryState (RFC 8620 section 5.5).
+    // Mail arriving mid-walk shifts every later page, so a changed state
+    // restarts the walk - bounded, since a busy mailbox could change forever.
+    let queryState: string | undefined;
+    let restarts = 0;
+    const maxRestarts = 3;
 
     while (true) {
       const response = await this.request([
@@ -1807,35 +1815,106 @@ export class JMAPClient implements IJMAPClient {
         }, "0"],
       ]);
 
-      const queryResult = response.methodResponses?.[0]?.[1];
-      const ids: string[] = queryResult?.ids || [];
-      allIds.push(...ids);
+      const [queryMethod, queryResult] = response.methodResponses?.[0] || [];
+      if (queryMethod === "error") {
+        // Reading a refusal as "no messages carry the tag" would report a
+        // clean migration of mail that still has the old keyword.
+        throw new Error(queryResult?.description || "Failed to list emails with the keyword");
+      }
+      const state = queryResult?.queryState as string | undefined;
+      if (queryState === undefined) {
+        queryState = state;
+      } else if (state !== undefined && state !== queryState) {
+        if (restarts >= maxRestarts) {
+          // Carrying on would mix ids from states that no longer agree, and
+          // retagging a message the tag has since been taken off is worse than
+          // not retagging at all. Nothing has been written yet, so this is a
+          // clean failure the caller can simply repeat.
+          throw new Error("The mailbox kept changing while listing the messages to retag");
+        }
+        restarts++;
+        queryState = state;
+        seen.clear();
+        allIds.length = 0;
+        position = 0;
+        continue;
+      }
 
-      if (ids.length < batchSize) break;
+      const ids: string[] = queryResult?.ids || [];
+      const fresh = ids.filter(id => !seen.has(id));
+      for (const id of fresh) seen.add(id);
+      allIds.push(...fresh);
+
+      // A server may return fewer ids than the limit asked for (RFC 8620
+      // section 5.5 lets it clamp), so a short page does not end the walk.
+      // A page carrying nothing new under an unchanged state does: the walk
+      // is not advancing, and continuing would loop for as long as the server
+      // keeps answering. Position counts the server's own page, not the ids
+      // this walk had not seen before.
+      if (fresh.length === 0) break;
       position += ids.length;
     }
 
-    if (allIds.length === 0) return 0;
+    if (allIds.length === 0) return { migrated: 0, refused: 0 };
 
     // Batch update: remove old keyword, add new keyword using per-property patches
+    let migrated = 0;
+    let refusals = 0;
+    // A whole batch the server refused to take: it will not take the next one
+    // either. A single message it refused: the rest of the mail is unaffected,
+    // and only the reason is worth keeping.
+    let stopped: Error | null = null;
+    let refusal: Error | null = null;
     for (const batch of batched(allIds, this.getMaxObjectsInSet())) {
+      if (stopped) {
+        refusals += batch.length;
+        continue;
+      }
+
       const update: Record<string, Record<string, boolean | null>> = {};
       for (const id of batch) {
         update[id] = {
-          [`keywords/${oldKeyword}`]: null,
-          [`keywords/${newKeyword}`]: true,
+          [keywordPointer(oldKeyword)]: null,
+          [keywordPointer(newKeyword)]: true,
         };
       }
 
-      await this.request([
+      const response = await this.request([
         ["Email/set", {
           accountId: this.accountId,
           update,
         }, "0"],
       ]);
+
+      const [setMethod, setResult] = response.methodResponses?.[0] || [];
+      if (setMethod === "error") {
+        stopped = new Error(setResult?.description || "Failed to migrate keyword");
+        refusals += batch.length;
+        continue;
+      }
+      // Email/set applies per message, so a refusal is never all-or-nothing:
+      // throwing here would abandon messages this very call already migrated.
+      // Report instead - a message deleted since the query (notFound) is
+      // nothing to report, any other refusal leaves the old keyword in place
+      // on mail the caller would otherwise treat as migrated.
+      const notUpdated = (setResult?.notUpdated || {}) as Record<string, { type?: string; description?: string }>;
+      const failed = Object.entries(notUpdated);
+      const refused = failed.filter(([, error]) => error?.type !== 'notFound');
+      if (refused.length > 0) {
+        debug.warn('keywords', 'Keyword migration refused for', refused.length, 'message(s):', refused[0][1]);
+        refusal = refusal || new Error(refused[0][1]?.description || refused[0][1]?.type || "Failed to migrate keyword");
+      }
+      refusals += refused.length;
+      migrated += batch.length - failed.length;
     }
 
-    return allIds.length;
+    // Nothing moved: the caller must not rename the tag over messages that all
+    // kept the old keyword. Something moved: that is a partial migration the
+    // caller reports, not an error - the messages that did move are migrated
+    // and renaming the definition is what keeps them named.
+    if (migrated === 0 && (stopped || refusal)) throw (stopped || refusal) as Error;
+
+    return { migrated, refused: refusals };
   }
 
   async deleteEmail(emailId: string, accountId?: string): Promise<void> {
diff --git a/lib/jmap/patch-pointer.ts b/lib/jmap/patch-pointer.ts
new file mode 100644
index 000000000..af860631b
--- /dev/null
+++ b/lib/jmap/patch-pointer.ts
@@ -0,0 +1,29 @@
+/**
+ * Naming one member of an object in a JMAP PatchObject.
+ *
+ * A PatchObject key is a JSON Pointer (RFC 8620 section 5.3), so a slash in the
+ * member's own name starts a new path segment unless it is escaped. That bites
+ * keywords: a nested tag is stored as `$label:work/clients`, and the unescaped
+ * pointer `keywords/$label:work/clients` asks for the `clients` member of the
+ * `$label:work` keyword - not a boolean, so the patch is rejected or ignored
+ * and the tag never lands.
+ */
+
+/** One JSON Pointer reference token: `~` -> `~0`, `/` -> `~1` (RFC 6901). */
+export function pointerToken(value: string): string {
+  return value.replace(/~/g, '~0').replace(/\//g, '~1');
+}
+
+/**
+ * The member name a reference token stands for: the inverse of
+ * `pointerToken`. `~1` is read before `~0`, per RFC 6901 - the other order
+ * would turn the escaped `~1` of `~01` back into a slash.
+ */
+export function pointerTokenValue(token: string): string {
+  return token.replace(/~1/g, '/').replace(/~0/g, '~');
+}
+
+/** The PatchObject key that sets or clears the keyword `keyword`. */
+export function keywordPointer(keyword: string): string {
+  return `keywords/${pointerToken(keyword)}`;
+}
diff --git a/stores/message-list-tabs-store.ts b/stores/message-list-tabs-store.ts
index 7b0e74157..8bae86d2f 100644
--- a/stores/message-list-tabs-store.ts
+++ b/stores/message-list-tabs-store.ts
@@ -15,6 +15,7 @@ import type { Email } from '@/lib/jmap/types';
 import type { MessageListTab, MessageListTabsConfig } from '@/lib/plugin-types';
 import { MAX_MESSAGE_LIST_TABS } from '@/lib/plugin-types';
 import { messageListTabHooks } from '@/lib/plugin-hooks';
+import { keywordPointer } from '@/lib/jmap/patch-pointer';
 import { useEmailStore } from './email-store';
 
 // ─── Validation ──────────────────────────────────────────────
@@ -296,8 +297,8 @@ export const useMessageListTabsStore = create<MessageListTabsStore>()((set, get)
     // One PatchObject per message: drop every other category keyword, add the
     // target's (removals of absent keywords are no-ops per RFC 8620 §5.3).
     const patch: Record<string, boolean | null> = {};
-    for (const k of keywordsRemoved) patch[`keywords/${k}`] = null;
-    if (keywordAdded) patch[`keywords/${keywordAdded}`] = true;
+    for (const k of keywordsRemoved) patch[keywordPointer(k)] = null;
+    if (keywordAdded) patch[keywordPointer(keywordAdded)] = true;
 
     // Group by owning account so shared-mailbox messages patch correctly.
     const byAccount = new Map<string | undefined, string[]>();