Use ", " separator when appending to a spaced inline table

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fix appending a key to a parsed inline table that uses `", "` separators emitting a bare `","` for the new entry, leaving the table inconsistently spaced (e.g. `{x = 1, y = 2,z = 3}`). ([#595](https://github.com/python-poetry/tomlkit/pull/595))
+
 ## [0.15.1] - 2026-07-17
 
 ### Changed

tests/test_items.py

@@ -979,6 +979,17 @@ def test_appending_to_parsed_inline_table_preserves_separator() -> None:
     parse(doc.as_string())
 
 
+def test_appending_to_compact_inline_table_uses_spaced_separator() -> None:
+    # A compact inline table separates its existing entries with ", " (comma +
+    # space). A key appended after parsing must use the same spacing, instead of
+    # a bare "," that leaves the table inconsistently spaced ("y = 2,z = 3").
+    doc = parse("a = {x = 1, y = 2}\n")
+    doc["a"]["z"] = 3
+
+    assert doc.as_string() == "a = {x = 1, y = 2, z = 3}\n"
+    assert parse(doc.as_string()) == {"a": {"x": 1, "y": 2, "z": 3}}
+
+
 def test_append_key_after_inline_table_trailing_comment() -> None:
     doc = parse("tbl = {\n    p = { k = 1 },\n    q = { k = 2 }  # comment\n}\n")
     doc["tbl"]["added"] = 3

tomlkit/items.py

@@ -2117,7 +2117,15 @@ def as_string(self) -> str:
                 # Insert the deferred separator right after the previous value,
                 # not after any trailing comment/whitespace -- otherwise the
                 # comma is swallowed by a trailing comment (see #512).
-                buf = f"{buf[:last_value_end]},{buf[last_value_end:]}"
+                # Match the conventional ", " spacing used by the explicit
+                # separators already in the table. Only add the space when the
+                # text that will follow the comma (the previous value's trailing
+                # trivia plus the new key's own indent) does not already start
+                # with one, so a padded table like ``{ a = 1 }`` -- which
+                # contributes the space itself -- does not end up double-spaced.
+                following = buf[last_value_end:] + v.trivia.indent
+                separator = "," if following.startswith(" ") else ", "
+                buf = f"{buf[:last_value_end]}{separator}{buf[last_value_end:]}"
                 needs_separator = False
 
             v_trivia_trail = v.trivia.trail.replace("\n", "")