diff --git a/doc/changes/fixed/16180.md b/doc/changes/fixed/16180.md
new file mode 100644
index 00000000000..03757bbe62f
--- /dev/null
+++ b/doc/changes/fixed/16180.md
@@ -0,0 +1,2 @@
+- Prevent diagnostic source locations at compact-position field boundaries from
+  being truncated (#16180, @rgrinberg)
diff --git a/otherlibs/stdune/src/compact_position.ml b/otherlibs/stdune/src/compact_position.ml
index 8b0700ba824..772cf85808e 100644
--- a/otherlibs/stdune/src/compact_position.ml
+++ b/otherlibs/stdune/src/compact_position.ml
@@ -9,8 +9,7 @@ module Position = struct
   let shift_cnum = 2 * field_size
 
   let small_enough =
-    let max_size = 1 lsl field_size in
-    let test int = int <= max_size in
+    let test int = int >= 0 && int <= field_mask in
     fun[@inline] { Lexing.pos_bol; pos_cnum; pos_lnum; pos_fname = _ } ->
       test pos_bol && test pos_cnum && test pos_lnum
   ;;
@@ -73,10 +72,7 @@ module Same_line_loc = struct
     create ~bol ~lnum ~start_cnum:stop_cnum ~stop_cnum
   ;;
 
-  let small_enough =
-    let max_size = 1 lsl field_size in
-    fun[@inline] int -> int <= max_size
-  ;;
+  let[@inline] small_enough int = int >= 0 && int <= field_mask
 
   let[@inline] to_loc t ~fname:pos_fname =
     let pos_lnum = lnum t in
diff --git a/otherlibs/stdune/test/compact_position_tests.ml b/otherlibs/stdune/test/compact_position_tests.ml
index fa65d6669c4..11869c5786f 100644
--- a/otherlibs/stdune/test/compact_position_tests.ml
+++ b/otherlibs/stdune/test/compact_position_tests.ml
@@ -27,23 +27,9 @@ let%expect_test "round trip tests" =
   test { base with pos_cnum = (1 lsl 21) - 1; pos_lnum = 2_200; pos_bol = 300 };
   [%expect {| [PASS] |}];
   test { base with pos_cnum = 1 lsl 21; pos_lnum = 2_200; pos_bol = 300 };
-  [%expect
-    {|
-    [FAIL]
-    expected:
-    { pos_lnum = 2200; pos_bol = 300; pos_cnum = 2097152 }
-    received:
-    { pos_lnum = 2200; pos_bol = 300; pos_cnum = 0 }
-    |}];
+  [%expect {| position too large |}];
   test { base with pos_cnum = -1; pos_lnum = 2_200; pos_bol = 300 };
-  [%expect
-    {|
-    [FAIL]
-    expected:
-    { pos_lnum = 2200; pos_bol = 300; pos_cnum = -1 }
-    received:
-    { pos_lnum = 2200; pos_bol = 300; pos_cnum = 2097151 }
-    |}];
+  [%expect {| position too large |}];
   test { base with pos_cnum = 1 lsl 32; pos_lnum = 2_200; pos_bol = 300 };
   [%expect {| position too large |}]
 ;;
@@ -55,5 +41,5 @@ let%expect_test "same-line locations at the field boundary" =
    | Same_line _ -> print_endline "same-line encoding overflowed"
    | Loc _ -> print_endline "used the wider encoding"
    | Loc_does_not_fit -> print_endline "location does not fit");
-  [%expect {| same-line encoding overflowed |}]
+  [%expect {| used the wider encoding |}]
 ;;