diff --git a/lib/maptool/pmtiles_builder.rb b/lib/maptool/pmtiles_builder.rb
index 7abe4bb..52720b9 100644
--- a/lib/maptool/pmtiles_builder.rb
+++ b/lib/maptool/pmtiles_builder.rb
@@ -506,25 +506,33 @@ module Maptool
       ENV.fetch("PLANETILER_JAR", "/usr/local/lib/planetiler.jar")
     end
 
+    # Some CLI tools (go-pmtiles included) write their actual error to
+    # stdout rather than stderr — combine both rather than risk raising a
+    # BuildError with nothing useful in it.
+    def self.command_output(stdout, stderr)
+      [stdout, stderr].map(&:strip).reject(&:empty?).join("\n")
+    end
+    private_class_method :command_output
+
     def self.run_pmtiles!(*args)
-      _stdout, stderr, status = Open3.capture3(pmtiles_bin, *args)
-      raise BuildError, "pmtiles exited with status #{status.exitstatus}: #{stderr.strip}" unless status.success?
+      stdout, stderr, status = Open3.capture3(pmtiles_bin, *args)
+      raise BuildError, "pmtiles exited with status #{status.exitstatus}: #{command_output(stdout, stderr)}" unless status.success?
     rescue Errno::ENOENT, Errno::EACCES => e
       raise BuildError, "could not run '#{pmtiles_bin}' (#{e.message}) — is go-pmtiles installed and on PATH? See docs/development.adoc."
     end
     private_class_method :run_pmtiles!
 
     def self.run_osmium!(*args)
-      _stdout, stderr, status = Open3.capture3(osmium_bin, *args)
-      raise BuildError, "osmium exited with status #{status.exitstatus}: #{stderr.strip}" unless status.success?
+      stdout, stderr, status = Open3.capture3(osmium_bin, *args)
+      raise BuildError, "osmium exited with status #{status.exitstatus}: #{command_output(stdout, stderr)}" unless status.success?
     rescue Errno::ENOENT, Errno::EACCES => e
       raise BuildError, "could not run '#{osmium_bin}' (#{e.message}) — is osmium-tool installed and on PATH? See docs/development.adoc."
     end
     private_class_method :run_osmium!
 
     def self.run_planetiler!(*args)
-      _stdout, stderr, status = Open3.capture3(java_bin, "-jar", planetiler_jar, *args)
-      raise BuildError, "planetiler exited with status #{status.exitstatus}: #{stderr.strip}" unless status.success?
+      stdout, stderr, status = Open3.capture3(java_bin, "-jar", planetiler_jar, *args)
+      raise BuildError, "planetiler exited with status #{status.exitstatus}: #{command_output(stdout, stderr)}" unless status.success?
     rescue Errno::ENOENT, Errno::EACCES => e
       raise BuildError, "could not run '#{java_bin} -jar #{planetiler_jar}' (#{e.message}) — is Java and planetiler.jar installed? See docs/development.adoc."
     end