diff --git a/scripts/postinstall.js b/scripts/postinstall.js
index cc504933f..6e3abb25f 100644
--- a/scripts/postinstall.js
+++ b/scripts/postinstall.js
@@ -2,6 +2,24 @@ import fs from "node:fs";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 
+function detectPackageManager(ua = process.env.npm_config_user_agent ?? "") {
+  // Examples:
+  // - "pnpm/10.23.0 npm/? node/v22.21.1 darwin arm64"
+  // - "npm/10.9.4 node/v22.12.0 linux x64"
+  // - "bun/1.2.2"
+  const normalized = String(ua).trim();
+  if (normalized.startsWith("pnpm/")) return "pnpm";
+  if (normalized.startsWith("bun/")) return "bun";
+  if (normalized.startsWith("npm/")) return "npm";
+  if (normalized.startsWith("yarn/")) return "yarn";
+  return "unknown";
+}
+
+function shouldApplyPnpmPatchedDependenciesFallback(pm = detectPackageManager()) {
+  // pnpm already applies pnpm.patchedDependencies itself; re-applying would fail.
+  return pm !== "pnpm";
+}
+
 function getRepoRoot() {
   const here = path.dirname(fileURLToPath(import.meta.url));
   return path.resolve(here, "..");
@@ -195,6 +213,10 @@ function main() {
   const repoRoot = getRepoRoot();
   process.chdir(repoRoot);
 
+  if (!shouldApplyPnpmPatchedDependenciesFallback()) {
+    return;
+  }
+
   const pkgPath = path.join(repoRoot, "package.json");
   const pkg = JSON.parse(fs.readFileSync(pkgPath, "utf-8"));
   const patched = pkg?.pnpm?.patchedDependencies ?? {};
@@ -230,5 +252,7 @@ export {
   applyPatchFile,
   applyPatchSet,
   applyPatchToFile,
+  detectPackageManager,
   parsePatch,
+  shouldApplyPnpmPatchedDependenciesFallback,
 };
diff --git a/src/postinstall-patcher.test.ts b/src/postinstall-patcher.test.ts
index 855905eb5..79afd2c51 100644
--- a/src/postinstall-patcher.test.ts
+++ b/src/postinstall-patcher.test.ts
@@ -3,13 +3,32 @@ import os from "node:os";
 import path from "node:path";
 import { describe, expect, it } from "vitest";
 
-import { applyPatchSet } from "../scripts/postinstall.js";
+import {
+  applyPatchSet,
+  detectPackageManager,
+  shouldApplyPnpmPatchedDependenciesFallback,
+} from "../scripts/postinstall.js";
 
 function makeTempDir() {
   return fs.mkdtempSync(path.join(os.tmpdir(), "clawdbot-patch-"));
 }
 
 describe("postinstall patcher", () => {
+  it("detects package manager from user agent", () => {
+    expect(detectPackageManager("pnpm/10.0.0 npm/? node/v22.0.0")).toBe("pnpm");
+    expect(detectPackageManager("npm/10.9.0 node/v22.0.0")).toBe("npm");
+    expect(detectPackageManager("bun/1.2.2")).toBe("bun");
+    expect(detectPackageManager("yarn/4.0.0 npm/? node/v22.0.0")).toBe("yarn");
+    expect(detectPackageManager("")).toBe("unknown");
+  });
+
+  it("skips pnpm.patchedDependencies fallback for pnpm", () => {
+    expect(shouldApplyPnpmPatchedDependenciesFallback("pnpm")).toBe(false);
+    expect(shouldApplyPnpmPatchedDependenciesFallback("npm")).toBe(true);
+    expect(shouldApplyPnpmPatchedDependenciesFallback("bun")).toBe(true);
+    expect(shouldApplyPnpmPatchedDependenciesFallback("unknown")).toBe(true);
+  });
+
   it("applies a simple patch", () => {
     const dir = makeTempDir();
     const target = path.join(dir, "lib");