diff --git a/package.json b/package.json
index e9c7c546..e83e4c2e 100644
--- a/package.json
+++ b/package.json
@@ -50,7 +50,7 @@
     "node": ">=22.0.0"
   },
   "dependencies": {
-    "@salesforce/core": "^9.1.4",
+    "@salesforce/core": "^9.1.6",
     "@salesforce/kit": "^4.0.0",
     "@salesforce/source-deploy-retrieve": "^13.2.0",
     "@salesforce/ts-types": "^3.0.1",
diff --git a/src/shared/remote/fileOperations.ts b/src/shared/remote/fileOperations.ts
index 5c75d3b0..dfa4fdc9 100644
--- a/src/shared/remote/fileOperations.ts
+++ b/src/shared/remote/fileOperations.ts
@@ -86,6 +86,31 @@ export const upgradeFileContents = (contents: ContentsV0): ContentsV1 => ({
   ),
 });
 
+/**
+ * Prevents cross-process stale writes by merging in-memory state with on-disk state.
+ * For each member present in both, takes the higher lastRetrievedFromServer (never regresses).
+ */
+const mergeWithDiskState = async (
+  filePath: string,
+  members: Map<string, MemberRevision>,
+  maxCounter: number
+): Promise<number> => {
+  const diskContents = await readFileContents(filePath);
+  if (!('serverMaxRevisionCounter' in diskContents) || !diskContents.sourceMembers) {
+    return maxCounter;
+  }
+
+  for (const [key, diskMember] of Object.entries(diskContents.sourceMembers)) {
+    const memMember = members.get(key);
+    if (memMember && (diskMember.lastRetrievedFromServer ?? -1) > (memMember.lastRetrievedFromServer ?? -1)) {
+      members.set(key, { ...memMember, lastRetrievedFromServer: diskMember.lastRetrievedFromServer });
+    }
+  }
+
+  const diskMax = diskContents.serverMaxRevisionCounter;
+  return Number.isFinite(diskMax) ? Math.max(maxCounter, diskMax) : maxCounter;
+};
+
 export const writeTrackingFile = async ({
   filePath,
   maxCounter,
@@ -96,6 +121,11 @@ export const writeTrackingFile = async ({
   members: Map<string, MemberRevision>;
 }): Promise<void> => {
   const lockResult = await lockInit(filePath);
+
+  // Merge with on-disk state to prevent cross-process stale overwrites.
+  // readFileContents returns {} for missing/unreadable files, which mergeWithDiskState handles.
+  maxCounter = await mergeWithDiskState(filePath, members, maxCounter);
+
   const CURRENT_FILE_VERSION_ENV = env.getNumber('SF_SOURCE_TRACKING_FILE_VERSION') ?? 0;
   const contents =
     CURRENT_FILE_VERSION_ENV === 1
diff --git a/test/unit/remote/fileOperations.test.ts b/test/unit/remote/fileOperations.test.ts
index 0f3228b0..41f64e2c 100644
--- a/test/unit/remote/fileOperations.test.ts
+++ b/test/unit/remote/fileOperations.test.ts
@@ -78,6 +78,171 @@ describe('writing file version based on env', () => {
   });
 });
 
+describe('cross-process merge on write', () => {
+  const fakeOrgId = '00DFakeFakeFakeMrg';
+  const filePath = getFilePath(fakeOrgId);
+
+  afterEach(async () => {
+    delete process.env.SF_SOURCE_TRACKING_FILE_VERSION;
+    try {
+      await fs.promises.unlink(filePath);
+    } catch {
+      // may not exist
+    }
+  });
+
+  it('preserves higher lastRetrievedFromServer from disk when in-memory is stale', async () => {
+    const diskMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 50,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 50,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 50, members: diskMembers });
+
+    const staleMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 30,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 50,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 50, members: staleMembers });
+
+    const contents = await fs.promises.readFile(filePath, 'utf8');
+    const parsed = parseJsonMap<ContentsV0>(contents, filePath);
+    expect(parsed.sourceMembers['ApexClass__MyClass'].lastRetrievedFromServer).to.equal(50);
+  });
+
+  it('preserves higher lastRetrievedFromServer from memory when disk is stale', async () => {
+    const diskMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 30,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 50,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 30, members: diskMembers });
+
+    const freshMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 50,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 50,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 50, members: freshMembers });
+
+    const contents = await fs.promises.readFile(filePath, 'utf8');
+    const parsed = parseJsonMap<ContentsV0>(contents, filePath);
+    expect(parsed.sourceMembers['ApexClass__MyClass'].lastRetrievedFromServer).to.equal(50);
+  });
+
+  it('does not regress null over a valid lastRetrievedFromServer on disk', async () => {
+    const diskMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 26,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 26,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 26, members: diskMembers });
+
+    const staleMembers = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: undefined,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 26,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 26, members: staleMembers });
+
+    const contents = await fs.promises.readFile(filePath, 'utf8');
+    const parsed = parseJsonMap<ContentsV0>(contents, filePath);
+    expect(parsed.sourceMembers['ApexClass__MyClass'].lastRetrievedFromServer).to.equal(26);
+  });
+
+  it('takes the max serverMaxRevisionCounter between memory and disk', async () => {
+    const members = new Map<string, MemberRevision>([
+      [
+        'ApexClass###MyClass',
+        {
+          MemberIdOrName: '01p000000000001',
+          ChangedBy: 'user1',
+          lastRetrievedFromServer: 10,
+          MemberType: 'ApexClass',
+          IsNameObsolete: false,
+          RevisionCounter: 10,
+          MemberName: 'MyClass',
+          IsNewMember: false,
+          LastModifiedDate: '2026-01-01',
+        },
+      ],
+    ]);
+    await writeTrackingFile({ filePath, maxCounter: 100, members });
+    await writeTrackingFile({ filePath, maxCounter: 80, members });
+
+    const contents = await fs.promises.readFile(filePath, 'utf8');
+    const parsed = parseJsonMap<ContentsV0>(contents, filePath);
+    expect(parsed.serverMaxRevisionCounter).to.equal(100);
+  });
+});
+
 describe('upgrading undefined file version to v1 file', () => {
   it('returns new file version even if file is not versioned', () => {
     const oldFile = {
diff --git a/yarn.lock b/yarn.lock
index 0bbabe9a..96e3942f 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -817,6 +817,31 @@
     ts-retry-promise "^0.8.1"
     zod "^4.1.12"
 
+"@salesforce/core@^9.1.6":
+  version "9.1.6"
+  resolved "https://registry.yarnpkg.com/@salesforce/core/-/core-9.1.6.tgz#cdc951ab573b92f482d0371d29687f9170e4a9d9"
+  integrity sha512-suF9uvuTwuer8zAlN4mRr3Dvd5Z+rM58r//P3JiYEgY4e1cMdiq7LOKA0PLZpbtNJL3rcZZDFTvqD6ypQAU1gA==
+  dependencies:
+    "@jsforce/jsforce-node" "^3.10.22"
+    "@salesforce/kit" "^4.0.0"
+    "@salesforce/ts-types" "^3.0.0"
+    ajv "^8.18.0"
+    change-case "^4.1.2"
+    fast-levenshtein "^3.0.0"
+    faye "^1.4.1"
+    form-data "^4.0.5"
+    js2xmlparser "^4.0.1"
+    jsonwebtoken "9.0.3"
+    jszip "3.10.1"
+    memfs "4.38.1"
+    pino "^9.7.0"
+    pino-abstract-transport "^1.2.0"
+    pino-pretty "^11.3.0"
+    proper-lockfile "^4.1.2"
+    semver "^7.8.0"
+    ts-retry-promise "^0.8.1"
+    zod "^4.1.12"
+
 "@salesforce/dev-config@^4.3.3":
   version "4.3.3"
   resolved "https://registry.yarnpkg.com/@salesforce/dev-config/-/dev-config-4.3.3.tgz#65650087baa542a905ad4e8b38a5c5302f7f009a"