diff --git a/packages/datastore/src/__tests__/cosmos.test.ts b/packages/datastore/src/__tests__/cosmos.test.ts
new file mode 100644
index 00000000..6bc86e5f
--- /dev/null
+++ b/packages/datastore/src/__tests__/cosmos.test.ts
@@ -0,0 +1,128 @@
+/**
+ * CosmosDatastoreProvider.updateIfMatch — verifies the If-Match (_etag) optimistic
+ * concurrency mapping against a fake @azure/cosmos: a matching etag writes + bumps;
+ * a stale etag surfaces as HTTP 412 -> conflict; a missing doc -> not_found.
+ *
+ * Uses a fake Cosmos client (no live Cosmos) that enforces accessCondition IfMatch.
+ */
+
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+interface StoredRec {
+  doc: Record<string, unknown>;
+  etag: string;
+}
+
+const h = vi.hoisted(() => {
+  const store = new Map<string, StoredRec>();
+  let etagSeq = 0;
+  return {
+    store,
+    seed(id: string, doc: Record<string, unknown>): void {
+      etagSeq += 1;
+      store.set(id, { doc: { ...doc, _etag: `e${etagSeq}` }, etag: `e${etagSeq}` });
+    },
+    nextEtag(): string {
+      etagSeq += 1;
+      return `e${etagSeq}`;
+    },
+    reset(): void {
+      store.clear();
+      etagSeq = 0;
+    },
+  };
+});
+
+vi.mock('@azure/cosmos', () => {
+  type CosmosErr = Error & { code?: number };
+  function err(code: number): CosmosErr {
+    const e = new Error(`cosmos ${code}`) as CosmosErr;
+    e.code = code;
+    return e;
+  }
+  class CosmosClient {
+    constructor(_opts: unknown) {}
+    database(_name: string) {
+      return {
+        container: (_c: string) => ({
+          item: (id: string, _pk: string) => ({
+            read() {
+              const rec = h.store.get(id);
+              if (!rec) throw err(404);
+              return Promise.resolve({ resource: { ...rec.doc, _etag: rec.etag } });
+            },
+            replace(
+              doc: Record<string, unknown>,
+              options?: { accessCondition?: { type: string; condition?: string } }
+            ) {
+              const rec = h.store.get(id);
+              if (!rec) throw err(404);
+              const cond = options?.accessCondition?.condition;
+              if (cond !== undefined && cond !== rec.etag) throw err(412);
+              const etag = h.nextEtag();
+              const saved = { ...doc, _etag: etag };
+              h.store.set(id, { doc: saved, etag });
+              return Promise.resolve({ resource: saved });
+            },
+          }),
+        }),
+      };
+    }
+  }
+  return { CosmosClient };
+});
+
+import { CosmosDatastoreProvider } from '../providers/cosmos.js';
+import type { BaseDocument, DocumentCollection } from '../types.js';
+
+interface RevDoc extends BaseDocument {
+  name: string;
+  rev?: number;
+}
+
+describe('CosmosDatastoreProvider.updateIfMatch', () => {
+  let col: DocumentCollection<RevDoc>;
+  beforeEach(() => {
+    h.reset();
+    const provider = new CosmosDatastoreProvider({
+      endpoint: 'https://fake.documents.azure.com:443/',
+      key: 'fake-key',
+      database: 'test',
+    });
+    col = provider.getCollection<RevDoc>('rev', '/productId');
+    h.seed('1', { id: '1', productId: 'p', name: 'A', rev: 0 });
+  });
+
+  it('matching etag -> writes via If-Match and returns ok', async () => {
+    const cur = await col.findById('1', 'p');
+    const res = await col.updateIfMatch('1', 'p', { etag: cur!._etag }, { name: 'B' });
+    expect(res.ok).toBe(true);
+    if (res.ok) {
+      expect(res.doc.name).toBe('B');
+      expect(res.doc.rev).toBe(1);
+    }
+  });
+
+  it('stale etag -> Cosmos 412 -> conflict, no overwrite', async () => {
+    const cur = await col.findById('1', 'p');
+    const staleEtag = cur!._etag;
+    await col.updateIfMatch('1', 'p', { etag: staleEtag }, { name: 'B' }); // etag rotates
+    const res = await col.updateIfMatch('1', 'p', { etag: staleEtag }, { name: 'C' });
+    expect(res.ok).toBe(false);
+    if (!res.ok) expect(res.reason).toBe('conflict');
+    expect((await col.findById('1', 'p'))?.name).toBe('B');
+  });
+
+  it('stale rev -> conflict before write', async () => {
+    await col.updateIfMatch('1', 'p', { rev: 0 }, { name: 'B' }); // rev -> 1
+    const res = await col.updateIfMatch('1', 'p', { rev: 0 }, { name: 'C' });
+    expect(res.ok).toBe(false);
+    if (!res.ok) expect(res.reason).toBe('conflict');
+  });
+
+  it('missing doc -> not_found', async () => {
+    const res = await col.updateIfMatch('missing', 'p', { rev: 0 }, { name: 'X' });
+    expect(res.ok).toBe(false);
+    if (!res.ok) expect(res.reason).toBe('not_found');
+  });
+});
diff --git a/packages/datastore/src/__tests__/memory.test.ts b/packages/datastore/src/__tests__/memory.test.ts
index 9652d2fc..c831909f 100644
--- a/packages/datastore/src/__tests__/memory.test.ts
+++ b/packages/datastore/src/__tests__/memory.test.ts
@@ -196,6 +196,71 @@ describe('MemoryDatastoreProvider', () => {
       await expect(collection.rawQuery('SELECT * FROM c')).rejects.toThrow('not supported');
     });
   });
+
+  describe('updateIfMatch (optimistic concurrency)', () => {
+    interface RevDoc extends BaseDocument {
+      name: string;
+      rev?: number;
+    }
+    let revCol: DocumentCollection<RevDoc>;
+    beforeEach(async () => {
+      revCol = provider.getCollection<RevDoc>('rev', '/productId');
+      await revCol.create({ id: '1', productId: 'p', name: 'A', rev: 0 });
+    });
+
+    it('matching rev writes the patch and bumps rev + _etag', async () => {
+      const res = await revCol.updateIfMatch('1', 'p', { rev: 0 }, { name: 'B' });
+      expect(res.ok).toBe(true);
+      if (res.ok) {
+        expect(res.doc.name).toBe('B');
+        expect(res.doc.rev).toBe(1);
+        expect(res.doc._etag).toBeDefined();
+      }
+    });
+
+    it('stale rev => conflict and NO write', async () => {
+      await revCol.updateIfMatch('1', 'p', { rev: 0 }, { name: 'B' }); // rev -> 1
+      const res = await revCol.updateIfMatch('1', 'p', { rev: 0 }, { name: 'C' });
+      expect(res.ok).toBe(false);
+      if (!res.ok) expect(res.reason).toBe('conflict');
+      // value unchanged by the losing write
+      expect((await revCol.findById('1', 'p'))?.name).toBe('B');
+    });
+
+    it('matching etag writes; stale etag => conflict', async () => {
+      const first = await revCol.updateIfMatch('1', 'p', { rev: 0 }, { name: 'B' });
+      const goodEtag = first.ok ? first.doc._etag : undefined;
+      const ok = await revCol.updateIfMatch('1', 'p', { etag: goodEtag }, { name: 'C' });
+      expect(ok.ok).toBe(true);
+      const stale = await revCol.updateIfMatch('1', 'p', { etag: goodEtag }, { name: 'D' });
+      expect(stale.ok).toBe(false);
+      if (!stale.ok) expect(stale.reason).toBe('conflict');
+    });
+
+    it('missing doc => not_found', async () => {
+      const res = await revCol.updateIfMatch('nope', 'p', { rev: 0 }, { name: 'X' });
+      expect(res.ok).toBe(false);
+      if (!res.ok) expect(res.reason).toBe('not_found');
+    });
+
+    // THE PROOF: N concurrent compare-and-set on the SAME version — exactly one wins.
+    it('is atomic under TRUE concurrency: N concurrent claims => exactly one winner', async () => {
+      const N = 25;
+      const results = await Promise.all(
+        Array.from({ length: N }, (_, i) =>
+          revCol.updateIfMatch('1', 'p', { rev: 0 }, { name: `claimer-${i}` })
+        )
+      );
+      const winners = results.filter(r => r.ok);
+      const conflicts = results.filter(r => !r.ok);
+      expect(winners).toHaveLength(1);
+      expect(conflicts).toHaveLength(N - 1);
+      expect(conflicts.every(r => !r.ok && r.reason === 'conflict')).toBe(true);
+      // the stored doc reflects exactly one applied write at rev 1
+      const stored = await revCol.findById('1', 'p');
+      expect(stored?.rev).toBe(1);
+    });
+  });
 });
 
 describe('matchesFilter', () => {
diff --git a/packages/datastore/src/index.ts b/packages/datastore/src/index.ts
index e0718b84..6b30e398 100644
--- a/packages/datastore/src/index.ts
+++ b/packages/datastore/src/index.ts
@@ -10,6 +10,8 @@ export type {
   DocumentCollection,
   DatastoreProvider,
   DatastoreProviderType,
+  ConcurrencyToken,
+  UpdateIfMatchResult,
 } from './types.js';
 
 export { getDatastore, createDatastoreProvider, setDatastore, _resetDatastore } from './factory.js';
diff --git a/packages/datastore/src/providers/cosmos.ts b/packages/datastore/src/providers/cosmos.ts
index 059f9d8a..02b8a48f 100644
--- a/packages/datastore/src/providers/cosmos.ts
+++ b/packages/datastore/src/providers/cosmos.ts
@@ -10,9 +10,11 @@ import type {
   AggregateQuery,
   BaseDocument,
   CollectionQuery,
+  ConcurrencyToken,
   DatastoreProvider,
   DocumentCollection,
   FilterMap,
+  UpdateIfMatchResult,
 } from '../types.js';
 
 export interface CosmosProviderConfig {
@@ -145,6 +147,56 @@ class CosmosCollection<T extends BaseDocument> implements DocumentCollection<T>
     return resource as T;
   }
 
+  /**
+   * Atomic compare-and-set via Cosmos optimistic concurrency. Conditions the
+   * replace on the document `_etag` (`accessCondition: { type: 'IfMatch' }`), so
+   * the server rejects the write (HTTP 412) if any other writer committed since
+   * the version this caller read — exactly one concurrent claimer wins. A 412 maps
+   * to `{ ok: false, reason: 'conflict' }`, a 404 to `'not_found'`. `rev` is also
+   * compared (when supplied) and bumped, for parity with the memory provider.
+   */
+  async updateIfMatch(
+    id: string,
+    partitionKey: string,
+    expected: ConcurrencyToken,
+    patch: Partial<T>
+  ): Promise<UpdateIfMatchResult<T>> {
+    const c = await this.container();
+    let existing: T | undefined;
+    try {
+      const read = await c.item(id, partitionKey).read<T>();
+      existing = read.resource;
+    } catch (err: unknown) {
+      if ((err as { code?: number })?.code === 404) return { ok: false, reason: 'not_found' };
+      throw err;
+    }
+    if (!existing) return { ok: false, reason: 'not_found' };
+
+    const cur = existing as T & { rev?: number };
+    // If the caller pinned a rev and it already moved on, a concurrent writer
+    // committed before our read — conflict without attempting the write.
+    if (expected.rev !== undefined && (cur.rev ?? 0) !== expected.rev) {
+      return { ok: false, reason: 'conflict' };
+    }
+    // Condition the write on the caller's etag when given, else the just-read
+    // etag — either way the IfMatch guards the read→replace window server-side.
+    const condition = expected.etag ?? cur._etag;
+    const nextRev = (cur.rev ?? 0) + 1;
+    const merged = { ...existing, ...patch, rev: nextRev } as T;
+
+    try {
+      const { resource } = await c.item(id, partitionKey).replace<T>(merged, {
+        accessCondition: condition ? { type: 'IfMatch', condition } : undefined,
+      });
+      return { ok: true, doc: resource as T };
+    } catch (err: unknown) {
+      const code = (err as { code?: number })?.code;
+      if (code === 412) return { ok: false, reason: 'conflict' };
+      if (code === 404) return { ok: false, reason: 'not_found' };
+      throw err;
+    }
+  }
+
   async upsert(doc: T): Promise<T> {
     const c = await this.container();
     const { resource } = await c.items.upsert<T>(doc);
diff --git a/packages/datastore/src/providers/memory.ts b/packages/datastore/src/providers/memory.ts
index 9d1cc3fa..80ba7ac9 100644
--- a/packages/datastore/src/providers/memory.ts
+++ b/packages/datastore/src/providers/memory.ts
@@ -9,9 +9,11 @@ import type {
   AggregateQuery,
   BaseDocument,
   CollectionQuery,
+  ConcurrencyToken,
   DatastoreProvider,
   DocumentCollection,
   FilterMap,
+  UpdateIfMatchResult,
 } from '../types.js';
 
 function deepClone<T>(obj: T): T {
@@ -123,6 +125,34 @@ class MemoryCollection<T extends BaseDocument> implements DocumentCollection<T>
     return deepClone(merged);
   }
 
+  /**
+   * Atomic compare-and-set. The entire get → compare → set runs SYNCHRONOUSLY in
+   * one block with NO await/yield, so under the single-threaded event loop two
+   * concurrent callers can never interleave between the compare and the set — the
+   * first wins (bumps `rev` + `_etag`), the rest see a mismatch and get `conflict`.
+   */
+  async updateIfMatch(
+    id: string,
+    _partitionKey: string,
+    expected: ConcurrencyToken,
+    patch: Partial<T>
+  ): Promise<UpdateIfMatchResult<T>> {
+    const existing = this.docs.get(id);
+    if (!existing) return { ok: false, reason: 'not_found' };
+    const rec = existing as BaseDocument & { rev?: number };
+    const curRev = typeof rec.rev === 'number' ? rec.rev : 0;
+    if (expected.rev !== undefined && expected.rev !== curRev) {
+      return { ok: false, reason: 'conflict' };
+    }
+    if (expected.etag !== undefined && expected.etag !== rec._etag) {
+      return { ok: false, reason: 'conflict' };
+    }
+    const nextRev = curRev + 1;
+    const merged = { ...existing, ...patch, rev: nextRev, _etag: `m${nextRev}` } as T;
+    this.docs.set(id, deepClone(merged));
+    return { ok: true, doc: deepClone(merged) };
+  }
+
   async upsert(doc: T): Promise<T> {
     const clone = deepClone(doc);
     this.docs.set(doc.id, clone);
diff --git a/packages/datastore/src/types.ts b/packages/datastore/src/types.ts
index 02fd04e9..e16f26f8 100644
--- a/packages/datastore/src/types.ts
+++ b/packages/datastore/src/types.ts
@@ -10,6 +10,11 @@
 export interface BaseDocument {
   id: string;
   productId: string;
+  /**
+   * Optimistic-concurrency tag surfaced by the provider (Cosmos `_etag`). Optional
+   * and provider-managed — callers read it back and pass it to `updateIfMatch`.
+   */
+  _etag?: string;
 }
 
 // ── Filter operators ────────────────────────────────────────────────────────
@@ -57,6 +62,24 @@ export interface AggregationField {
   alias: string;
 }
 
+// ── Optimistic concurrency ──────────────────────────────────────────────────
+
+/**
+ * The version a conditional write expects the stored document to still be at.
+ * `etag` is the provider tag (Cosmos `_etag`); `rev` is a portable monotonic
+ * counter maintained for parity (and used by the memory provider). Provide either
+ * or both — the write succeeds only if every supplied token still matches.
+ */
+export interface ConcurrencyToken {
+  etag?: string;
+  rev?: number;
+}
+
+/** Result of a conditional (compare-and-swap) write. */
+export type UpdateIfMatchResult<T> =
+  | { ok: true; doc: T }
+  | { ok: false; reason: 'conflict' | 'not_found' };
+
 // ── Document collection interface ───────────────────────────────────────────
 
 export interface DocumentCollection<T extends BaseDocument = BaseDocument> {
@@ -78,6 +101,21 @@ export interface DocumentCollection<T extends BaseDocument = BaseDocument> {
   /** Update a document by ID + partition key (merge semantics). */
   update(id: string, partitionKey: string, updates: Partial<T>): Promise<T>;
 
+  /**
+   * Optimistic-concurrency update: merge `patch` into the document only if its
+   * stored version still matches `expected` (Cosmos If-Match on `_etag`; an
+   * atomic compare-and-set on `rev` for the memory provider). Bumps `rev` and
+   * surfaces a fresh `_etag` on success. Never overwrites a doc that changed —
+   * returns `{ ok: false, reason: 'conflict' }` instead, and `'not_found'` when
+   * the document is absent. This is the single-winner primitive for atomic claims.
+   */
+  updateIfMatch(
+    id: string,
+    partitionKey: string,
+    expected: ConcurrencyToken,
+    patch: Partial<T>
+  ): Promise<UpdateIfMatchResult<T>>;
+
   /** Upsert a document (create or replace). */
   upsert(doc: T): Promise<T>;