diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -536,6 +536,27 @@
     raise error.InMemoryMergeConflictsError('in-memory merge does not support '
                                             'external merge tools')
 
+def _describemerge(ui, repo, env, toolpath, args):
+    template = ui.config('ui', 'pre-merge-tool-output-template')
+    if not template:
+        return
+
+    # Remove HG_ prefix from entries in `env` and lowercase them
+    def sanitizeenv(k):
+        if k.startswith('HG_'):
+            return k[3:].lower()
+        return k
+
+    data = {sanitizeenv(k): v for k, v in env.items()}
+
+    # TODO: make all of this something that can be specified on a per-tool basis
+    template = templater.unquotestring(template)
+
+    fm = ui.formatter("extmerge", pycompat.byteskwargs({'template': template}))
+    fm.startitem()
+    fm.data(repo=repo, **data)
+    fm.end()
+
 def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     if fcd.isabsent() or fco.isabsent():
@@ -584,6 +605,7 @@
         if scriptfn is None:
             cmd = toolpath + ' ' + args
             repo.ui.debug('launching merge tool: %s\n' % cmd)
+            _describemerge(ui, repo, env, toolpath, args)
             r = ui.system(cmd, cwd=repo.root, environ=env,
                           blockedtag='mergetool')
         else: