diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7b9d2be..fa15fbe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,10 @@
 
 # Changelog
 
+## 0.8.10
+
+- Fixed Python serverless transforms failing on Windows. The local subprocess backend hardcoded `python3`, which the python.org Windows installer does not put on PATH (it installs `python` and the `py` launcher), so every Python transform raised "interpreter not available". The interpreter is now resolved from a per-language list of command candidates (`python3`, then `python`, then the `py` launcher) using a cross-platform PATH lookup, leaving POSIX behavior unchanged. (#116)
+
 ## 0.8.9
 
 - Fixed `trmnlp serve` binding to localhost under Podman, which left the dev server unreachable through published ports. Container detection only looked for `/.dockerenv`, which Docker writes but Podman does not, so `serve` fell back to `127.0.0.1`. It now also checks for `/run/.containerenv`, which Podman writes, so the automatic `0.0.0.0` bind works for both runtimes. (#112)
diff --git a/Gemfile.lock b/Gemfile.lock
index 3caf23b..2ae9460 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    trmnl_preview (0.8.9)
+    trmnl_preview (0.8.10)
       activesupport (~> 8.0)
       cgi (~> 0.5)
       faraday (~> 2.1)
diff --git a/lib/trmnlp/transform_backend/subprocess.rb b/lib/trmnlp/transform_backend/subprocess.rb
index 58a576a..150ddc4 100644
--- a/lib/trmnlp/transform_backend/subprocess.rb
+++ b/lib/trmnlp/transform_backend/subprocess.rb
@@ -4,6 +4,7 @@
 require 'tmpdir'
 
 require_relative '../transform_client'
+require_relative 'which'
 require_relative 'wrapper'
 
 module TRMNLP
@@ -19,11 +20,16 @@ class Subprocess
       # Seconds a TERM'd process is given to exit before escalating to KILL.
       GRACE_PERIOD = 0.1
 
+      # Candidate commands per language, highest priority first. Windows is
+      # why a language needs more than one: its python.org installer provides
+      # `python` and the `py` launcher but no `python3`. `py` ranks last yet
+      # is the surest Windows hit — it stays on PATH even when the installer's
+      # optional "Add to PATH" step is skipped.
       INTERPRETERS = {
-        'python' => { cmd: 'python3', ext: 'py' },
-        'ruby' => { cmd: 'ruby',    ext: 'rb' },
-        'node' => { cmd: 'node',    ext: 'js' },
-        'php' => { cmd: 'php', ext: 'php' }
+        'python' => { cmds: %w[python3 python py], ext: 'py' },
+        'ruby' => { cmds: %w[ruby],                ext: 'rb' },
+        'node' => { cmds: %w[node],                ext: 'js' },
+        'php' => { cmds: %w[php],                  ext: 'php' }
       }.freeze
 
       def execute(code:, language:, stdin: '', timeout_seconds: DEFAULT_TIMEOUT)
@@ -40,7 +46,7 @@ def invoke(spec, language, code, stdin, timeout_seconds)
           output_path = File.join(dir, 'output.json')
           src_path = File.join(dir, "transform.#{spec[:ext]}")
           File.write(src_path, Wrapper.for(language, code, sink_for(language, output_path)))
-          run_process(spec[:cmd], src_path, stdin, timeout_seconds, output_path)
+          run_process(Which.resolve(spec[:cmds]), src_path, stdin, timeout_seconds, output_path)
         end
       rescue Errno::ENOENT, Errno::EACCES => e
         failure("interpreter not available: #{e.message}")
diff --git a/lib/trmnlp/transform_backend/which.rb b/lib/trmnlp/transform_backend/which.rb
new file mode 100644
index 0000000..4659bcc
--- /dev/null
+++ b/lib/trmnlp/transform_backend/which.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module TRMNLP
+  module TransformBackend
+    # Resolves a command to the interpreter actually present on PATH, so one
+    # transform runs unchanged on platforms that name an interpreter
+    # differently — most pressingly Windows, which ships no `python3`.
+    module Which
+      module_function
+
+      # Falls back to the first candidate when none resolve, leaving the
+      # caller to surface the usual "interpreter not available" ENOENT.
+      def resolve(candidates, path: ENV.fetch('PATH', ''), pathext: ENV.fetch('PATHEXT', ''))
+        candidates.find { |cmd| locate(cmd, path: path, pathext: pathext) } || candidates.first
+      end
+
+      # Windows marks executables by extension, enumerated in PATHEXT
+      # (.EXE/.BAT/...); POSIX leaves PATHEXT unset and relies on the exec bit.
+      def locate(cmd, path: ENV.fetch('PATH', ''), pathext: ENV.fetch('PATHEXT', ''))
+        suffixes = pathext.split(File::PATH_SEPARATOR)
+        suffixes = [''] if suffixes.empty?
+        path.split(File::PATH_SEPARATOR)
+            .product(suffixes)
+            .map { |dir, suffix| File.join(dir, "#{cmd}#{suffix}") }
+            .find { |candidate| File.file?(candidate) && File.executable?(candidate) }
+      end
+    end
+  end
+end
diff --git a/lib/trmnlp/version.rb b/lib/trmnlp/version.rb
index 0729f2d..90afeb0 100644
--- a/lib/trmnlp/version.rb
+++ b/lib/trmnlp/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module TRMNLP
-  VERSION = '0.8.9'
+  VERSION = '0.8.10'
 end
diff --git a/spec/lib/trmnlp/transform_backend/subprocess_spec.rb b/spec/lib/trmnlp/transform_backend/subprocess_spec.rb
index a21fab3..0e37222 100644
--- a/spec/lib/trmnlp/transform_backend/subprocess_spec.rb
+++ b/spec/lib/trmnlp/transform_backend/subprocess_spec.rb
@@ -94,7 +94,7 @@
 
     it 'reports the interpreter command as failure when it is not on PATH' do
       stub_const("#{described_class}::INTERPRETERS", {
-                   'ghost' => { cmd: 'this-binary-does-not-exist', ext: 'rb', wrapper: :ruby_wrapper }
+                   'ghost' => { cmds: %w[this-binary-does-not-exist], ext: 'rb' }
                  })
 
       result = backend.execute(code: 'noop', language: 'ghost', stdin: '')
diff --git a/spec/lib/trmnlp/transform_backend/which_spec.rb b/spec/lib/trmnlp/transform_backend/which_spec.rb
new file mode 100644
index 0000000..24a1ebf
--- /dev/null
+++ b/spec/lib/trmnlp/transform_backend/which_spec.rb
@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'fileutils'
+require 'tmpdir'
+require 'trmnlp/transform_backend/which'
+
+RSpec.describe TRMNLP::TransformBackend::Which do
+  subject(:which) { described_class }
+
+  let(:bin) { Dir.mktmpdir('trmnlp-which-') }
+  # Joined with the platform separator so the PATHEXT split matches Windows
+  # (`;`) and POSIX (`:`) without the spec caring which it runs on.
+  let(:windows_pathext) { ['.COM', '.BAT'].join(File::PATH_SEPARATOR) }
+
+  after { FileUtils.remove_entry(bin) }
+
+  describe '.locate' do
+    it 'answers the full path of an executable found on PATH' do
+      executable = File.join(bin, 'widget')
+      File.write(executable, '')
+      File.chmod(0o755, executable)
+
+      expect(which.locate('widget', path: bin)).to eq(executable)
+    end
+
+    it 'answers nil when the command is absent from PATH' do
+      expect(which.locate('ghost', path: bin)).to be(nil)
+    end
+
+    it 'ignores a matching file that is not executable' do
+      File.write(File.join(bin, 'widget'), '')
+
+      expect(which.locate('widget', path: bin)).to be(nil)
+    end
+
+    it 'appends a PATHEXT suffix when locating a Windows executable' do
+      executable = File.join(bin, 'widget.BAT')
+      File.write(executable, '')
+      File.chmod(0o755, executable)
+
+      expect(which.locate('widget', path: bin, pathext: windows_pathext)).to eq(executable)
+    end
+  end
+
+  describe '.resolve' do
+    it 'answers the first candidate present on PATH' do
+      %w[python py].each do |name|
+        File.write(File.join(bin, name), '')
+        File.chmod(0o755, File.join(bin, name))
+      end
+
+      expect(which.resolve(%w[python3 python py], path: bin)).to eq('python')
+    end
+
+    it 'answers the highest-priority candidate when several are present' do
+      %w[python3 python].each do |name|
+        File.write(File.join(bin, name), '')
+        File.chmod(0o755, File.join(bin, name))
+      end
+
+      expect(which.resolve(%w[python3 python py], path: bin)).to eq('python3')
+    end
+
+    it 'answers the first candidate name when none resolve, preserving the ENOENT path' do
+      expect(which.resolve(%w[python3 python py], path: bin)).to eq('python3')
+    end
+  end
+end