diff --git a/hgext/fix.py b/hgext/fix.py
--- a/hgext/fix.py
+++ b/hgext/fix.py
@@ -102,6 +102,13 @@
     mapping fixer tool names to lists of metadata values returned from
     executions that modified a file. This aggregates the same metadata
     previously passed to the "postfixfile" hook.
+
+Fixer tools are run the in same working directory as the :hg:`fix` command. This
+allows them to read configuration files from the working copy, or even write to
+the working copy. The working copy is not updated to match the revision being
+fixed. In fact, several revisions may be fixed in parallel. Writes to the
+working copy are not amended into the revision being fixed; fixer tools should
+always write fixed file content back to stdout as documented above.
 """
 
 from __future__ import absolute_import
@@ -233,7 +240,7 @@
             for rev, path in items:
                 ctx = repo[rev]
                 olddata = ctx[path].data()
-                metadata, newdata = fixfile(ui, opts, fixers, ctx, path,
+                metadata, newdata = fixfile(ui, repo, opts, fixers, ctx, path,
                                             basectxs[rev])
                 # Don't waste memory/time passing unchanged content back, but
                 # produce one result per item either way.
@@ -530,7 +537,7 @@
                 basectxs[rev].add(pctx)
     return basectxs
 
-def fixfile(ui, opts, fixers, fixctx, path, basectxs):
+def fixfile(ui, repo, opts, fixers, fixctx, path, basectxs):
     """Run any configured fixers that should affect the file in this context
 
     Returns the file content that results from applying the fixers in some order
@@ -539,7 +546,8 @@
     (i.e. they will only avoid lines that are common to all basectxs).
 
     A fixer tool's stdout will become the file's new content if and only if it
-    exits with code zero.
+    exits with code zero. The fixer tool's working directory is inherited from
+    this process.
     """
     metadata = {}
     newdata = fixctx[path].data()
@@ -553,7 +561,7 @@
             proc = subprocess.Popen(
                 procutil.tonativestr(command),
                 shell=True,
-                cwd=procutil.tonativestr(b'/'),
+                cwd=None,
                 stdin=subprocess.PIPE,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.PIPE)
diff --git a/tests/test-fix.t b/tests/test-fix.t
--- a/tests/test-fix.t
+++ b/tests/test-fix.t
@@ -1264,3 +1264,37 @@
 
   $ cd ..
 
+We run fixer tools in the cwd so they can look for config files or other
+important things in the working directory. This does NOT mean we are
+reconstructing a working copy of every revision being fixed; we're just giving
+the tool knowledge of the cwd in case it can do something reasonable with that.
+
+  $ hg init subprocesscwd
+  $ cd subprocesscwd
+
+  $ cat >> .hg/hgrc <<EOF
+  > [fix]
+  > printcwd:command = pwd
+  > printcwd:pattern = path:foo/bar
+  > EOF
+
+  $ mkdir foo
+  $ printf "bar\n" > foo/bar
+  $ hg commit -Aqm blah
+
+  $ hg fix -w -r . foo/bar
+  $ hg cat -r tip foo/bar
+  $TESTTMP/subprocesscwd
+  $ cat foo/bar
+  $TESTTMP/subprocesscwd
+
+  $ cd foo
+
+  $ hg fix -w -r . bar
+  $ hg cat -r tip bar
+  $TESTTMP/subprocesscwd/foo
+  $ cat bar
+  $TESTTMP/subprocesscwd/foo
+
+  $ cd ../..
+