bun build ignores import aliases (@/) when marking packages as external

[zohayrslileh]

What version of Bun is running?

1.1.45+196621f25

What platform is your computer?

Linux 6.12.9-200.fc41.x86_64 x86_64 unknown

What steps can reproduce the bug?

📝 Description
When using Bun.build() with packages: "external", import aliases (e.g., @/) are incorrectly treated as external dependencies instead of being bundled. This causes build errors when the alias is expected to be resolved within the project.

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🔄 Steps to Reproduce

1️⃣ Build Configuration (build.ts)

Bun.build({
    entrypoints: ["index.ts"],
    packages: "external", // Mark all packages as external
    outdir: "./dist"
});

2️⃣ Source File Before Build (index.ts)

// Importing a Node.js built-in module (expected to remain external)
import path from "node:path";

// Importing a local module (expected to be bundled)
import notAliasModule from "./notAliasModule";

// Importing a module using an alias (expected to be bundled but incorrectly treated as external)
import aliasModule from "@/aliasModule";

console.log({ path, aliasModule, notAliasModule });

3️⃣ Output After Build (index.js)

// ✅ Node.js module is correctly kept external
import path from "node:path";

// ✅ Local module is correctly bundled
var notAliasModule_default = "Not Alias Module";

// ❌ Alias module is incorrectly treated as an external package instead of being bundled
import aliasModule from "@/aliasModule";

console.log({ path, aliasModule, notAliasModule: notAliasModule_default });

Would appreciate any guidance or confirmation if this is intended behavior! 🚀

What is the expected behavior?

❗ Expected Behavior

• Node.js built-in modules (e.g., path) should remain external ✅
• Local relative imports (e.g., "./notAliasModule") should be bundled ✅
• Aliased imports (@/) should also be bundled, just like relative imports. However, they are being incorrectly marked as external ❌

What do you see instead?

🏷️ Suggested Fix

• Ensure that import aliases defined in tsconfig.json (or equivalent) are correctly resolved and bundled when using Bun.build(), even when packages: "external" is specified.

Additional information

No response