diff --git a/CHANGELOG.md b/CHANGELOG.md
index 208944b..ba78656 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,12 @@ The floating `v1` tag tracks the latest `1.x` release. Consumers pinning
 
 ## [Unreleased]
 
+## [1.9.1] - 2026-05-19
+
+### Fixed
+
+- **Spurious "produced N tarballs; expected 1" warnings in monorepo releases** ([microsoft/apm#1348]). `packPackage` snapshotted the shared `distDir` only after `apm pack` returned and treated any tarball with `mtime >= packStart - 1s` as "fresh by this invocation." In a monorepo loop, sequential per-package packs complete in well under a second, so plugin N saw plugin 1..N-1's tarballs inside the grace window and warned on every pack after the first. Fix: snapshot tarballs and their mtimes before the pack call and accept only files that are new in `after` or whose mtime advanced — the warning now fires only on a genuine producer-side anomaly. Empirically reproduced on [DevExpGbb/zava-agent-config@v6.1.1](https://github.com/DevExpGbb/zava-agent-config/releases/tag/v6.1.1) ([run 26079513903](https://github.com/DevExpGbb/zava-agent-config/actions/runs/26079513903)), which packs 7 plugins from one `dist/` and emitted 6 spurious warnings per release.
+
 ## [1.9.0] - 2026-05-18
 
 ### Added
@@ -228,7 +234,8 @@ Initial public release.
 - **Marketplace name set to "Setup APM"** ([#5]).
 - **Microsoft OSS compliance baseline.** SECURITY.md ([#2]), CODEOWNERS, license, contributing guide, code of conduct, and CI pipeline.
 
-[Unreleased]: https://github.com/microsoft/apm-action/compare/v1.9.0...HEAD
+[Unreleased]: https://github.com/microsoft/apm-action/compare/v1.9.1...HEAD
+[1.9.1]: https://github.com/microsoft/apm-action/compare/v1.9.0...v1.9.1
 [1.9.0]: https://github.com/microsoft/apm-action/compare/v1.8.0...v1.9.0
 [1.8.0]: https://github.com/microsoft/apm-action/compare/v1.7.3...v1.8.0
 [1.7.3]: https://github.com/microsoft/apm-action/compare/v1.7.2...v1.7.3
diff --git a/dist/index.js b/dist/index.js
index a00c964..d4ab7e7 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -41949,14 +41949,24 @@ async function runGate(workingDir) {
  * Pack a single package: `cd <dir> && apm pack --offline --archive -o <dist>`.
  * Returns the absolute path to the produced .tar.gz.
  *
- * Selects the produced tarball by mtime (newest after pack) rather than
- * diffing the directory before/after. This is robust to the case where
- * `apm pack` overwrites an existing tarball of the same name -- the diff
- * approach would see fresh=[] and incorrectly throw despite pack succeeding.
+ * Identifies the produced tarball by snapshotting `distDir` before and after
+ * the pack invocation. A tarball is "produced by this call" if it is new in
+ * `after`, or if it existed in `before` but its mtime advanced. This is
+ * correct under two conditions a naive mtime heuristic gets wrong:
+ *
+ *   1. Monorepo runs share `distDir`. Sequential per-package pack invocations
+ *      complete in <1s each, so prior tarballs from the same run fall inside
+ *      any reasonable "newer than packStart" grace window. The before/after
+ *      diff isolates exactly the tarball this invocation touched.
+ *   2. Re-runs overwrite an existing tarball of the same name. A pure
+ *      set-difference would miss this; mtime advance catches it.
  */
 async function packPackage(dir, distDir) {
     external_node_fs_namespaceObject.mkdirSync(distDir, { recursive: true });
-    const packStartMs = Date.now();
+    const before = new Map();
+    for (const p of listTarballs(distDir)) {
+        before.set(p, external_node_fs_namespaceObject.statSync(p).mtimeMs);
+    }
     const rc = await lib_exec/* exec */.m('apm', [
         'pack',
         '--offline',
@@ -41972,19 +41982,25 @@ async function packPackage(dir, distDir) {
             + `Verify that the package has a 'dependencies:' block or primitives `
             + `to bundle.`);
     }
-    // listTarballs sorts newest-first by mtime. Accept the newest tarball
-    // whose mtime is >= packStart (1s grace for fs mtime granularity).
-    const graceMs = packStartMs - 1000;
-    const fresh = after.filter(p => external_node_fs_namespaceObject.statSync(p).mtimeMs >= graceMs);
-    if (fresh.length === 0) {
-        throw new Error(`apm pack in ${dir} succeeded but no tarball in ${distDir} has an `
-            + `mtime newer than the pack invocation. Filesystem clock skew?`);
-    }
-    if (fresh.length > 1) {
-        lib_core/* warning */.$e(`apm pack in ${dir} produced ${fresh.length} tarballs; expected 1. `
-            + `Using the most recently modified: ${fresh[0]}`);
-    }
-    return fresh[0];
+    const touched = after.filter(p => {
+        const prev = before.get(p);
+        if (prev === undefined)
+            return true;
+        return external_node_fs_namespaceObject.statSync(p).mtimeMs > prev;
+    });
+    if (touched.length === 0) {
+        throw new Error(`apm pack in ${dir} succeeded but no tarball in ${distDir} was added `
+            + `or modified by this invocation. Filesystem clock skew?`);
+    }
+    if (touched.length > 1) {
+        // One pack invocation is expected to produce or modify exactly one
+        // tarball. More than one is a real producer-side anomaly worth surfacing
+        // -- the before/after diff has already filtered out prior packages in
+        // the same monorepo run.
+        lib_core/* warning */.$e(`apm pack in ${dir} produced ${touched.length} tarballs; expected 1. `
+            + `Using the most recently modified: ${touched.sort((a, b) => external_node_fs_namespaceObject.statSync(b).mtimeMs - external_node_fs_namespaceObject.statSync(a).mtimeMs)[0]}`);
+    }
+    return touched.sort((a, b) => external_node_fs_namespaceObject.statSync(b).mtimeMs - external_node_fs_namespaceObject.statSync(a).mtimeMs)[0];
 }
 function listTarballs(dir) {
     if (!external_node_fs_namespaceObject.existsSync(dir))
diff --git a/dist/release.d.ts b/dist/release.d.ts
index 5d61a03..c7d034b 100644
--- a/dist/release.d.ts
+++ b/dist/release.d.ts
@@ -100,10 +100,17 @@ export declare function runGate(workingDir: string): Promise<{
  * Pack a single package: `cd <dir> && apm pack --offline --archive -o <dist>`.
  * Returns the absolute path to the produced .tar.gz.
  *
- * Selects the produced tarball by mtime (newest after pack) rather than
- * diffing the directory before/after. This is robust to the case where
- * `apm pack` overwrites an existing tarball of the same name -- the diff
- * approach would see fresh=[] and incorrectly throw despite pack succeeding.
+ * Identifies the produced tarball by snapshotting `distDir` before and after
+ * the pack invocation. A tarball is "produced by this call" if it is new in
+ * `after`, or if it existed in `before` but its mtime advanced. This is
+ * correct under two conditions a naive mtime heuristic gets wrong:
+ *
+ *   1. Monorepo runs share `distDir`. Sequential per-package pack invocations
+ *      complete in <1s each, so prior tarballs from the same run fall inside
+ *      any reasonable "newer than packStart" grace window. The before/after
+ *      diff isolates exactly the tarball this invocation touched.
+ *   2. Re-runs overwrite an existing tarball of the same name. A pure
+ *      set-difference would miss this; mtime advance catches it.
  */
 export declare function packPackage(dir: string, distDir: string): Promise<string>;
 /**
diff --git a/package-lock.json b/package-lock.json
index 1573a11..3e12f3c 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "apm-action",
-  "version": "1.0.0",
+  "version": "1.9.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "apm-action",
-      "version": "1.0.0",
+      "version": "1.9.1",
       "license": "MIT",
       "dependencies": {
         "@actions/core": "^3.0.0",
diff --git a/package.json b/package.json
index 65e94a9..580bf7f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "apm-action",
-  "version": "1.0.0",
+  "version": "1.9.1",
   "description": "GitHub Action for APM - Agent Package Manager",
   "type": "module",
   "main": "dist/index.js",
diff --git a/src/__tests__/release.test.ts b/src/__tests__/release.test.ts
index 75e8d78..d36f375 100644
--- a/src/__tests__/release.test.ts
+++ b/src/__tests__/release.test.ts
@@ -320,6 +320,30 @@ describe('packPackage', () => {
     expect(result).toBe(tarballPath);
     expect(fs.readFileSync(tarballPath, 'utf8')).toBe('fresh');
   });
+
+  it('returns only the tarball produced by this invocation in a shared distDir (monorepo regression)', async () => {
+    // Simulate a monorepo loop: distDir already contains prior packages'
+    // tarballs from THIS run (mtime just a few ms ago). A mtime-grace-window
+    // heuristic would classify them all as "fresh" and emit a spurious
+    // "produced N tarballs; expected 1" warning. The before/after diff
+    // must isolate only the file this invocation actually touched.
+    fs.mkdirSync(distDir, { recursive: true });
+    const priorA = path.join(distDir, 'sibling-a-1.0.0.tar.gz');
+    const priorB = path.join(distDir, 'sibling-b-1.0.0.tar.gz');
+    fs.writeFileSync(priorA, 'a');
+    fs.writeFileSync(priorB, 'b');
+
+    const newTarball = path.join(distDir, 'mypkg-1.0.0.tar.gz');
+    mockExec.mockImplementationOnce(async () => {
+      fs.writeFileSync(newTarball, 'new');
+      return 0;
+    });
+    const result = await packPackage(tmpDir, distDir);
+    expect(result).toBe(newTarball);
+    // Sibling files must still exist on disk (we did not delete them).
+    expect(fs.existsSync(priorA)).toBe(true);
+    expect(fs.existsSync(priorB)).toBe(true);
+  });
 });
 
 describe('runReleaseMode (integration, mocked exec)', () => {
diff --git a/src/release.ts b/src/release.ts
index 83fd20a..045b911 100644
--- a/src/release.ts
+++ b/src/release.ts
@@ -284,17 +284,27 @@ export async function runGate(
  * Pack a single package: `cd <dir> && apm pack --offline --archive -o <dist>`.
  * Returns the absolute path to the produced .tar.gz.
  *
- * Selects the produced tarball by mtime (newest after pack) rather than
- * diffing the directory before/after. This is robust to the case where
- * `apm pack` overwrites an existing tarball of the same name -- the diff
- * approach would see fresh=[] and incorrectly throw despite pack succeeding.
+ * Identifies the produced tarball by snapshotting `distDir` before and after
+ * the pack invocation. A tarball is "produced by this call" if it is new in
+ * `after`, or if it existed in `before` but its mtime advanced. This is
+ * correct under two conditions a naive mtime heuristic gets wrong:
+ *
+ *   1. Monorepo runs share `distDir`. Sequential per-package pack invocations
+ *      complete in <1s each, so prior tarballs from the same run fall inside
+ *      any reasonable "newer than packStart" grace window. The before/after
+ *      diff isolates exactly the tarball this invocation touched.
+ *   2. Re-runs overwrite an existing tarball of the same name. A pure
+ *      set-difference would miss this; mtime advance catches it.
  */
 export async function packPackage(
   dir: string,
   distDir: string,
 ): Promise<string> {
   fs.mkdirSync(distDir, { recursive: true });
-  const packStartMs = Date.now();
+  const before = new Map<string, number>();
+  for (const p of listTarballs(distDir)) {
+    before.set(p, fs.statSync(p).mtimeMs);
+  }
   const rc = await exec.exec('apm', [
     'pack',
     '--offline',
@@ -312,23 +322,28 @@ export async function packPackage(
       + `to bundle.`,
     );
   }
-  // listTarballs sorts newest-first by mtime. Accept the newest tarball
-  // whose mtime is >= packStart (1s grace for fs mtime granularity).
-  const graceMs = packStartMs - 1000;
-  const fresh = after.filter(p => fs.statSync(p).mtimeMs >= graceMs);
-  if (fresh.length === 0) {
+  const touched = after.filter(p => {
+    const prev = before.get(p);
+    if (prev === undefined) return true;
+    return fs.statSync(p).mtimeMs > prev;
+  });
+  if (touched.length === 0) {
     throw new Error(
-      `apm pack in ${dir} succeeded but no tarball in ${distDir} has an `
-      + `mtime newer than the pack invocation. Filesystem clock skew?`,
+      `apm pack in ${dir} succeeded but no tarball in ${distDir} was added `
+      + `or modified by this invocation. Filesystem clock skew?`,
     );
   }
-  if (fresh.length > 1) {
+  if (touched.length > 1) {
+    // One pack invocation is expected to produce or modify exactly one
+    // tarball. More than one is a real producer-side anomaly worth surfacing
+    // -- the before/after diff has already filtered out prior packages in
+    // the same monorepo run.
     core.warning(
-      `apm pack in ${dir} produced ${fresh.length} tarballs; expected 1. `
-      + `Using the most recently modified: ${fresh[0]}`,
+      `apm pack in ${dir} produced ${touched.length} tarballs; expected 1. `
+      + `Using the most recently modified: ${touched.sort((a, b) => fs.statSync(b).mtimeMs - fs.statSync(a).mtimeMs)[0]}`,
     );
   }
-  return fresh[0];
+  return touched.sort((a, b) => fs.statSync(b).mtimeMs - fs.statSync(a).mtimeMs)[0];
 }
 
 function listTarballs(dir: string): string[] {