diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 743e61b3a92d87c329e3ede6685fb235cc2aa905..bb743d078038b89ededfd85266e2ea49948df958 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
+
+* integrate Asciidoctor log messages properly when jsonl formatter is not enabled (#133)
+
 == 1.0.0-beta.10 (2025-10-17)
 
 === Added
diff --git a/packages/assembler/lib/assemble-content.js b/packages/assembler/lib/assemble-content.js
index 3534117bdaad3614d85eedf8a32df2db8b1a4cc7..6eae90c7ad572959bd1585864b39e326f002735d 100644
--- a/packages/assembler/lib/assemble-content.js
+++ b/packages/assembler/lib/assemble-content.js
@@ -267,10 +267,15 @@ function resolveFileOrContents (convertResult, convertAttributes, buildConfig, l
           if (entry.name) ctx.program = entry.name
           if (entry.file?.line) ctx.line = entry.file.line
           logger[entry.level](ctx, entry.msg)
-        } else if ((match = /^(.+):(\d+): warning: (.+)/.exec(line))) {
+        } else if ((match = /^([^:]+):(\d+): warning: (.+)/.exec(line))) {
           const [, scriptPath, lineno, msg] = match
           ctx.stack = [{ file: { path: scriptPath }, line: parseInt(lineno, 10) }]
           logger.warn(ctx, msg)
+        } else if ((match = /^asciidoctor: ([A-Z]+): (?:[^:]+: line (\d+): )?(.+)/.exec(line))) {
+          // NOTE we don't care about the filename in the message since it can only be docfile
+          const [, level, lineno, msg] = match
+          if (lineno) ctx.line = parseInt(lineno, 10)
+          logger[level === 'WARNING' ? 'warn' : level.toLowerCase()](ctx, msg)
         } else {
           logger.info(ctx, line)
         }
diff --git a/packages/assembler/test/assemble-content-test.js b/packages/assembler/test/assemble-content-test.js
index 4c4d3894cd83c2cb0710ff3644437c5cd64977fd..715d91eadf635a141331b730b52e8809401af7a3 100644
--- a/packages/assembler/test/assemble-content-test.js
+++ b/packages/assembler/test/assemble-content-test.js
@@ -338,7 +338,7 @@ describe('assembleContent()', () => {
     assert.equal(messages.error[0][1], 'section title out of sequence')
   })
 
-  it('should route non-jsonl lines on stderr to Antora logger', async () => {
+  it('should route Ruby warnings to Antora logger', async () => {
     const scenario = 'insert-page'
     const { playbook, contentCatalog, assemblerConfig: configSource } = await loadScenario(scenario, __dirname)
     configSource.build.publish = false
@@ -394,6 +394,67 @@ describe('assembleContent()', () => {
     assertx.empty(messages.error)
   })
 
+  it('should route non-json Asciidoctor log messages on stderr to Antora logger', async () => {
+    const scenario = 'insert-page'
+    const { playbook, contentCatalog, assemblerConfig: configSource } = await loadScenario(scenario, __dirname)
+    configSource.build.publish = false
+    configSource.build.stderr = 'log'
+    configSource.build.command = 'asciidoctor-pdf'
+    let docfile
+    const convert = async (...args) => {
+      docfile = args[1].docfile
+      const contents = Readable.from('converted contents')
+      const stderrLines = [
+        `asciidoctor: WARNING: skipping references to missing attribute: foobar`,
+        `asciidoctor: ERROR: ${ospath.basename(docfile)}: line 3: level 0 sections can only be used when doctype is book`,
+      ]
+      return { status: 0, stderr: stderrLines.join('\n'), contents }
+    }
+    const converter = { convert, extname: '.pdf', mediaType: 'application/pdf' }
+    const messages = { info: [], warn: [], error: [] }
+    const logger = {
+      info () {
+        messages.info.push(Array.from(arguments))
+      },
+      warn () {
+        messages.warn.push(Array.from(arguments))
+      },
+      error () {
+        messages.error.push(Array.from(arguments))
+      },
+    }
+    const actualLoggerNames = []
+    const context = {
+      getFunctions: () => ({}),
+      getLogger: (name) => {
+        actualLoggerNames.push(name)
+        return logger
+      },
+      getVariables: () => ({}),
+    }
+    const actual = await assembleContent.call(context, playbook, contentCatalog, converter, { configSource })
+    assert.equal(actual.length, 1)
+    assert.deepEqual(actualLoggerNames, ['@antora/assembler', '@antora/assembler'])
+    assertx.empty(messages.info)
+    assert.equal(messages.warn.length, 1)
+    assert.deepEqual(messages.warn[0], [
+      {
+        command: 'asciidoctor-pdf',
+        file: { path: docfile },
+      },
+      'skipping references to missing attribute: foobar',
+    ])
+    assert.equal(messages.error.length, 1)
+    assert.deepEqual(messages.error[0], [
+      {
+        command: 'asciidoctor-pdf',
+        file: { path: docfile },
+        line: 3,
+      },
+      'level 0 sections can only be used when doctype is book',
+    ])
+  })
+
   it('should use file as is if already coerced to output identity', async () => {
     const scenario = 'insert-page'
     const { playbook, contentCatalog, assemblerConfig: configSource } = await loadScenario(scenario, __dirname)