fix: prevent source map references from being resolved

server/utils/docs/client.ts

@@ -130,8 +130,14 @@ function createLoader(): (
  *
  * Handles resolving relative imports and esm.sh redirects.
  */
-function createResolver(): (specifier: string, referrer: string) => string {
+export function createResolver(): (specifier: string, referrer: string) => string {
   return (specifier: string, referrer: string) => {
+    // Source map references are not modules. Resolving bare source map
+    // filenames can incorrectly treat them as package specifiers
+    if (specifier.endsWith('.map')) {
+      return specifier
+    }
+
     // Handle relative imports
     if (specifier.startsWith('.') || specifier.startsWith('/')) {
       return new URL(specifier, referrer).toString()

test/unit/server/utils/docs/client.spec.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest'
+import { createResolver } from '#server/utils/docs/client'
+
+// =============================================================================
+// Issue #2739: sourceMappingURL specifiers resolved as package names
+// https://github.com/npmx-dev/npmx.dev/issues/2739
+// =============================================================================
+
+describe('issue #2739 - source map specifiers', () => {
+  const resolve = createResolver()
+  const referrer = 'https://esm.sh/effect-web-midi@0.2.1/dist-types/index.d.ts'
+
+  it('does not resolve source map references', () => {
+    expect(resolve('index.d.ts.map', referrer)).toBe('index.d.ts.map')
+  })
+
+  it('does not resolve relative source map references', () => {
+    expect(resolve('./index.d.ts.map', referrer)).toBe('./index.d.ts.map')
+  })
+})
+
+describe('createResolver', () => {
+  const resolve = createResolver()
+  const referrer = 'https://esm.sh/effect-web-midi@0.2.1/dist-types/index.d.ts'
+
+  it('resolves relative imports', () => {
+    expect(resolve('./src/index.d.ts', referrer)).toBe(
+      'https://esm.sh/effect-web-midi@0.2.1/dist-types/src/index.d.ts',
+    )
+  })
+
+  it('resolves absolute paths', () => {
+    expect(resolve('/foo.d.ts', referrer)).toBe('https://esm.sh/foo.d.ts')
+  })
+
+  it('resolves bare specifiers', () => {
+    expect(resolve('effect', referrer)).toBe('https://esm.sh/effect')
+  })
+
+  it('leaves absolute URLs unchanged', () => {
+    expect(resolve('https://esm.sh/effect', referrer)).toBe('https://esm.sh/effect')
+  })
+
+  it('leaves node builtins unchanged', () => {
+    expect(resolve('node:fs', referrer)).toBe('node:fs')
+  })
+})