diff --git a/BuildFlags.hs b/BuildFlags.hs
index d273c216e4..8724b94e45 100644
--- a/BuildFlags.hs
+++ b/BuildFlags.hs
@@ -69,6 +69,11 @@ buildFlags = filter (not . null)
 	, "Testsuite"
 	, "S3"
 	, "WebDAV"
+#ifdef WITH_OSPATH
+	, "OsPath"
+#else 
+#warning Building without the OsPath build flag set results in slower filename manipulation and is not recommended.
+#endif
 	]
 
 -- Not a complete list, let alone a listing transitive deps, but only
diff --git a/CHANGELOG b/CHANGELOG
index 379d28a732..2297edecee 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -3,6 +3,10 @@ git-annex (10.20250829) UNRELEASED; urgency=medium
   * drop: --fast support when dropping from a remote.
   * Fix crash operating on filenames that are exactly 21 bytes long
     and begin with a utf-8 character.
+  * git-annex.cabal: Turn on the OsPath build flag by default.
+  * Add build warnings when git-annex is built without the OsPath
+    build flag.
+  * version: Report on whether it was built with the OsPath build flag.
 
  -- Joey Hess <id@joeyh.name>  Fri, 29 Aug 2025 12:34:06 -0400
 
diff --git a/doc/todo/RawFilePath_conversion.mdwn b/doc/todo/RawFilePath_conversion.mdwn
index c10b62cf2e..b488353a60 100644
--- a/doc/todo/RawFilePath_conversion.mdwn
+++ b/doc/todo/RawFilePath_conversion.mdwn
@@ -12,10 +12,11 @@ But this conversion is not yet complete. This is a todo to keep track
 of the status.
 
 * The OsPath build flag makes git-annex build with OsPath. Otherwise,
-  it builds with RawFilePath. The plan is to make that build flag the
-  default where it is not already as time goes on. And then eventually
-  remove the build flag and simplify code in git-annex to not need to
-  support two different build methods.
+  it builds with RawFilePath. That build flag is now on by default,
+  and there is a build warning when it is not set.
+
+* The plan is to eventually remove the build flag and simplify code in
+  git-annex to not need to support two different build methods.
 
 * unix has modules that operate on RawFilePath but no OSPath versions yet.
   See https://github.com/haskell/unix/issues/240
diff --git a/git-annex.cabal b/git-annex.cabal
index dda13f7071..484e94abfd 100644
--- a/git-annex.cabal
+++ b/git-annex.cabal
@@ -177,6 +177,7 @@ Flag Servant
 
 Flag OsPath
   Description: Use the os-string library and related libraries, for faster filename manipulation
+  Default: True
 
 Flag Benchmark
   Description: Enable benchmarking