diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 6020101c28fff50e778eca8edcd8bf453438c8b5..3f0eda357297cf8c764c494a931607f18acfedc7 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -4,6 +4,12 @@
 This document provides a summary of all notable changes to Antora Assembler by release.
 For a detailed view of what's changed, refer to the {url-repo}/commits[commit history] of this project.
 
+== Unreleased
+
+=== Fixed
+
+* don't assign build command from result of `getDefaultCommand` if user specified command (#123)
+
 == 1.0.0-beta.7 (2025-09-26)
 
 === Added
diff --git a/packages/assembler/lib/assemble-content.js b/packages/assembler/lib/assemble-content.js
index 5b028cacb55fce37d6c1b83221daf7e8f39382b3..91bf6f5670d92a30f66a3168ccc4fd55cb694316 100644
--- a/packages/assembler/lib/assemble-content.js
+++ b/packages/assembler/lib/assemble-content.js
@@ -65,7 +65,7 @@ async function assembleContent (playbook, contentCatalog, converter, { configSou
     createResolveAssemblyModel(context, contentCatalog, assemblyConfig, intrinsicAttributes, navigationCatalog)
   )
   if (!(assemblyFiles.length && typeof convert === 'function')) return assemblyFiles
-  if (typeof getDefaultCommand === 'function') {
+  if (buildConfig.command == null && typeof getDefaultCommand === 'function') {
     buildConfig.command = await getDefaultCommand(buildConfig.cwd)
   }
   const { publishSite: publishFiles = require('@antora/site-publisher') } = generatorFunctions
diff --git a/packages/assembler/test/assemble-content-test.js b/packages/assembler/test/assemble-content-test.js
index 42414b33c04e513311291e783acb7e36cf7efeaf..9867713baf4dc81d74529e940054f57b75438246 100644
--- a/packages/assembler/test/assemble-content-test.js
+++ b/packages/assembler/test/assemble-content-test.js
@@ -62,7 +62,8 @@ describe('assembleContent()', () => {
       convertCalls.push([doc].concat(rest))
       return doc
     }
-    const converter = { convert, backend: 'pdf', extname: '.pdf', mediaType: 'application/pdf' }
+    const getDefaultCommand = async () => 'the-default-command'
+    const converter = { convert, getDefaultCommand, backend: 'pdf', extname: '.pdf', mediaType: 'application/pdf' }
     const expectedOutdir = ospath.join(buildDir, 'the-component/1.0/_exports')
     const expectedConvertAttributes = {
       'assembler-backend': 'pdf',
@@ -96,6 +97,7 @@ describe('assembleContent()', () => {
     assert.equal(actualDoc.assembler.backend, 'pdf')
     assertx.partialDeepEqual(actualConvertAttributes, expectedConvertAttributes)
     assert.equal(actualBuildConfig.dir, buildDir)
+    assert.equal(actualBuildConfig.command, 'the-default-command')
     assert.equal(contentCatalog.getFiles().length, 1)
   })
 
@@ -436,7 +438,8 @@ describe('assembleContent()', () => {
       const opts = { parse: true, cwd, stdin: doc.contents, stdout: 'print', stderr: 'print' }
       return runCommand(command, args, opts).then(() => fs.createReadStream(outfile))
     }
-    const converter = { convert, extname: '.pdf', mediaType: 'application/pdf' }
+    const getDefaultCommand = async () => 'the-default-command'
+    const converter = { convert, getDefaultCommand, extname: '.pdf', mediaType: 'application/pdf' }
     const actual = await assembleContent(playbook, contentCatalog, converter, { configSource })
     assertx.doesNotHaveProperty(actualConvertAttributes, 'doctype') // TODO make a separate test for this assertion
     assert.equal(actual.length, 1)