diff --git a/src/resources/extensions/gsd/commands-handlers.ts b/src/resources/extensions/gsd/commands-handlers.ts
index 25074d634..0272ef289 100644
--- a/src/resources/extensions/gsd/commands-handlers.ts
+++ b/src/resources/extensions/gsd/commands-handlers.ts
@@ -28,6 +28,11 @@ import { loadPrompt } from "./prompt-loader.js";
 const UPDATE_REGISTRY_URL = "https://registry.npmjs.org/gsd-pi/latest";
 const UPDATE_FETCH_TIMEOUT_MS = 5000;
 
+function resolveInstallCommand(pkg: string): string {
+  if ('bun' in process.versions) return `bun add -g ${pkg}`;
+  return `npm install -g ${pkg}`;
+}
+
 async function fetchLatestVersionForCommand(): Promise<string | null> {
   const controller = new AbortController();
   const timeout = setTimeout(() => controller.abort(), UPDATE_FETCH_TIMEOUT_MS);
@@ -431,8 +436,9 @@ export async function handleUpdate(ctx: ExtensionCommandContext): Promise<void>
 
   ctx.ui.notify(`Updating: v${current} → v${latest}...`, "info");
 
+  const installCmd = resolveInstallCommand(`${NPM_PACKAGE}@latest`);
   try {
-    execSync(`npm install -g ${NPM_PACKAGE}@latest`, {
+    execSync(installCmd, {
       stdio: ["ignore", "pipe", "ignore"],
     });
     ctx.ui.notify(
@@ -441,7 +447,7 @@ export async function handleUpdate(ctx: ExtensionCommandContext): Promise<void>
     );
   } catch {
     ctx.ui.notify(
-      `Update failed. Try manually: npm install -g ${NPM_PACKAGE}@latest`,
+      `Update failed. Try manually: ${installCmd}`,
       "error",
     );
   }
diff --git a/src/tests/cross-platform-filesystem-safety.test.ts b/src/tests/cross-platform-filesystem-safety.test.ts
index 84e5b2790..0125c13f5 100644
--- a/src/tests/cross-platform-filesystem-safety.test.ts
+++ b/src/tests/cross-platform-filesystem-safety.test.ts
@@ -77,13 +77,9 @@ const ALLOW_HARDCODED_TMP: Array<[string, string]> = [
 
 /** Pattern 4 — shell commands with interpolated variables */
 const ALLOW_SHELL_INTERPOLATION: Array<[string, string]> = [
-  // NPM_PACKAGE is a compile-time constant ('gsd-pi'), not user input.
-  ["update-cmd.ts", "npm view ${NPM_PACKAGE}"],
-  ["update-cmd.ts", "npm install -g ${NPM_PACKAGE}"],
-  ["update-check.ts", "npm install -g ${NPM_PACKAGE_NAME}"],
-  // Same constant forwarded through commands-handlers.
-  ["resources/extensions/gsd/commands-handlers.ts", "npm view ${NPM_PACKAGE}"],
-  ["resources/extensions/gsd/commands-handlers.ts", "npm install -g ${NPM_PACKAGE}"],
+  // update-cmd.ts, update-check.ts, and commands-handlers.ts all pass a
+  // pre-built variable (installCmd) to execSync — no template literal inside
+  // the execSync call, so no entries are needed here.
 ];
 
 function isAllowlisted(
diff --git a/src/tests/update-cmd-diagnostics.test.ts b/src/tests/update-cmd-diagnostics.test.ts
index 8f3c5c088..21bb2981b 100644
--- a/src/tests/update-cmd-diagnostics.test.ts
+++ b/src/tests/update-cmd-diagnostics.test.ts
@@ -1,6 +1,7 @@
 /**
  * Regression test for #3445: gsd update must print both current and latest
  * versions for diagnostics, and bypass npm cache.
+ * Regression test for #4145: gsd update must use bun when installed via Bun.
  */
 import { test } from "node:test";
 import assert from "node:assert/strict";
@@ -32,3 +33,52 @@ test("update commands use the registry fetch helper instead of npm view (#3806)"
   );
   assert.ok(!handlerSrc.includes("npm view "), "/gsd update should no longer shell out to npm view");
 });
+
+test("update-check exports resolveInstallCommand (#4145)", async () => {
+  const { resolveInstallCommand } = await import("../update-check.js");
+  assert.equal(typeof resolveInstallCommand, "function", "resolveInstallCommand must be exported from update-check");
+});
+
+test("resolveInstallCommand returns bun command when running under Bun (#4145)", async () => {
+  const { resolveInstallCommand } = await import("../update-check.js");
+  const orig = (process.versions as Record<string, string | undefined>).bun;
+  try {
+    (process.versions as Record<string, string | undefined>).bun = "1.0.0";
+    assert.equal(resolveInstallCommand("gsd-pi@latest"), "bun add -g gsd-pi@latest");
+  } finally {
+    if (orig === undefined) {
+      delete (process.versions as Record<string, string | undefined>).bun;
+    } else {
+      (process.versions as Record<string, string | undefined>).bun = orig;
+    }
+  }
+});
+
+test("resolveInstallCommand returns npm command when not running under Bun (#4145)", async () => {
+  const { resolveInstallCommand } = await import("../update-check.js");
+  const orig = (process.versions as Record<string, string | undefined>).bun;
+  try {
+    delete (process.versions as Record<string, string | undefined>).bun;
+    assert.equal(resolveInstallCommand("gsd-pi@latest"), "npm install -g gsd-pi@latest");
+  } finally {
+    if (orig !== undefined) {
+      (process.versions as Record<string, string | undefined>).bun = orig;
+    }
+  }
+});
+
+test("update-cmd uses resolveInstallCommand instead of hardcoded npm (#4145)", () => {
+  const src = readFileSync(join(__dirname, "..", "update-cmd.ts"), "utf-8");
+  assert.ok(
+    src.includes("resolveInstallCommand"),
+    "update-cmd should use resolveInstallCommand for package manager detection",
+  );
+});
+
+test("commands-handlers uses resolveInstallCommand instead of hardcoded npm (#4145)", () => {
+  const handlerSrc = readFileSync(join(__dirname, "..", "resources", "extensions", "gsd", "commands-handlers.ts"), "utf-8");
+  assert.ok(
+    handlerSrc.includes("resolveInstallCommand"),
+    "/gsd update handler should use resolveInstallCommand for package manager detection",
+  );
+});
diff --git a/src/update-check.ts b/src/update-check.ts
index d560c318b..204197c43 100644
--- a/src/update-check.ts
+++ b/src/update-check.ts
@@ -74,10 +74,18 @@ export async function fetchLatestVersionFromRegistry(
   }
 }
 
+export function resolveInstallCommand(pkg: string): string {
+  if ('bun' in process.versions) {
+    return `bun add -g ${pkg}`
+  }
+  return `npm install -g ${pkg}`
+}
+
 function printUpdateBanner(current: string, latest: string): void {
+  const installCmd = resolveInstallCommand('gsd-pi')
   process.stderr.write(
     `  ${chalk.yellow('Update available:')} ${chalk.dim(`v${current}`)} → ${chalk.bold(`v${latest}`)}\n` +
-    `  ${chalk.dim('Run')} npm update -g gsd-pi ${chalk.dim('or')} /gsd update ${chalk.dim('to upgrade')}\n\n`,
+    `  ${chalk.dim('Run')} ${installCmd} ${chalk.dim('or')} /gsd update ${chalk.dim('to upgrade')}\n\n`,
   )
 }
 
@@ -184,7 +192,7 @@ export async function checkAndPromptForUpdates(options: UpdateCheckOptions = {})
 
   const choice = await new Promise<string>((resolve) => {
     process.stderr.write(
-      `  ${chalk.bold('[1]')} Update now   ${chalk.dim(`npm install -g ${NPM_PACKAGE_NAME}@latest`)}\n` +
+      `  ${chalk.bold('[1]')} Update now   ${chalk.dim(resolveInstallCommand(`${NPM_PACKAGE_NAME}@latest`))}\n` +
       `  ${chalk.bold('[2]')} Skip\n\n`,
     )
 
@@ -210,13 +218,14 @@ export async function checkAndPromptForUpdates(options: UpdateCheckOptions = {})
   process.stdin.pause()
 
   if (choice === '1') {
-    process.stderr.write(`\n  ${chalk.dim('Running:')} npm install -g ${NPM_PACKAGE_NAME}@latest\n\n`)
+    const installCmd = resolveInstallCommand(`${NPM_PACKAGE_NAME}@latest`)
+    process.stderr.write(`\n  ${chalk.dim('Running:')} ${installCmd}\n\n`)
     try {
-      execSync(`npm install -g ${NPM_PACKAGE_NAME}@latest`, { stdio: 'inherit' })
+      execSync(installCmd, { stdio: 'inherit' })
       process.stderr.write(`\n  ${chalk.green.bold(`✓ Updated to v${latestVersion}`)}\n\n`)
       return true
     } catch {
-      process.stderr.write(`\n  ${chalk.yellow(`Update failed. You can run: npm install -g ${NPM_PACKAGE_NAME}@latest`)}\n\n`)
+      process.stderr.write(`\n  ${chalk.yellow(`Update failed. You can run: ${installCmd}`)}\n\n`)
     }
   } else {
     process.stderr.write(`  ${chalk.dim('Skipped. Run')} gsd update ${chalk.dim('anytime to upgrade.')}\n\n`)
diff --git a/src/update-cmd.ts b/src/update-cmd.ts
index 18dcd0c48..20d2ba3dc 100644
--- a/src/update-cmd.ts
+++ b/src/update-cmd.ts
@@ -1,5 +1,5 @@
 import { execSync } from 'node:child_process'
-import { compareSemver, fetchLatestVersionFromRegistry } from './update-check.js'
+import { compareSemver, fetchLatestVersionFromRegistry, resolveInstallCommand } from './update-check.js'
 
 const NPM_PACKAGE = 'gsd-pi'
 
@@ -29,13 +29,14 @@ export async function runUpdate(): Promise<void> {
 
   process.stdout.write(`${dim}Updating:${reset} v${current} → ${bold}v${latest}${reset}\n`)
 
+  const installCmd = resolveInstallCommand(`${NPM_PACKAGE}@latest`)
   try {
-    execSync(`npm install -g ${NPM_PACKAGE}@latest`, {
+    execSync(installCmd, {
       stdio: 'inherit',
     })
     process.stdout.write(`\n${green}${bold}Updated to v${latest}${reset}\n`)
   } catch {
-    process.stderr.write(`\n${yellow}Update failed. Try manually: npm install -g ${NPM_PACKAGE}@latest${reset}\n`)
+    process.stderr.write(`\n${yellow}Update failed. Try manually: ${installCmd}${reset}\n`)
     process.exit(1)
   }
 }