diff --git a/api/core/rag/splitter/fixed_text_splitter.py b/api/core/rag/splitter/fixed_text_splitter.py
index 7a00e8a886..27c58a5d1a 100644
--- a/api/core/rag/splitter/fixed_text_splitter.py
+++ b/api/core/rag/splitter/fixed_text_splitter.py
@@ -53,7 +53,10 @@ class FixedRecursiveCharacterTextSplitter(EnhanceRecursiveCharacterTextSplitter)
     def __init__(self, fixed_separator: str = "\n\n", separators: list[str] | None = None, **kwargs: Any):
         """Create a new TextSplitter."""
         super().__init__(**kwargs)
-        self._fixed_separator = codecs.decode(fixed_separator, "unicode_escape")
+        if "\\" in fixed_separator:
+            self._fixed_separator = codecs.decode(fixed_separator, "unicode_escape")
+        else:
+            self._fixed_separator = fixed_separator
         self._separators = separators or ["\n\n", "\n", "。", ". ", " ", ""]
 
     def split_text(self, text: str) -> list[str]:
diff --git a/api/tests/unit_tests/core/rag/splitter/test_fixed_text_splitter_invisible_delimiters.py b/api/tests/unit_tests/core/rag/splitter/test_fixed_text_splitter_invisible_delimiters.py
new file mode 100644
index 0000000000..633c567b05
--- /dev/null
+++ b/api/tests/unit_tests/core/rag/splitter/test_fixed_text_splitter_invisible_delimiters.py
@@ -0,0 +1,140 @@
+"""
+Test for invisible Unicode delimiter support in FixedRecursiveCharacterTextSplitter.
+
+Regression test for issue #31672:
+ZWSP and other invisible Unicode characters should work as delimiters.
+"""
+
+import pytest
+
+from core.rag.splitter.fixed_text_splitter import FixedRecursiveCharacterTextSplitter
+
+
+class TestInvisibleDelimiters:
+    """Test invisible Unicode characters as delimiters."""
+
+    @pytest.fixture
+    def base_splitter_kwargs(self):
+        """Common kwargs for creating splitters."""
+
+        def length_function(texts: list[str]) -> list[int]:
+            return [len(text) for text in texts]
+
+        return {
+            "chunk_size": 100,
+            "chunk_overlap": 0,
+            "length_function": length_function,
+        }
+
+    @pytest.mark.parametrize(
+        "delimiter",
+        [
+            pytest.param("\u200b", id="zwsp"),
+            pytest.param("\ufeff", id="zwnbsp"),
+            pytest.param("\u2063", id="invisible_separator"),
+            pytest.param("\u2060", id="word_joiner"),
+            pytest.param("\u200e", id="ltr_mark"),
+        ],
+    )
+    def test_invisible_literal_delimiters(self, base_splitter_kwargs, delimiter):
+        """Test that various invisible Unicode characters work as literal delimiters."""
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator=delimiter,
+            **base_splitter_kwargs,
+        )
+
+        text = f"chunk1{delimiter}chunk2{delimiter}chunk3"
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert result == ["chunk1", "chunk2", "chunk3"]
+
+    @pytest.mark.parametrize(
+        ("escaped_char", "literal_char"),
+        [
+            pytest.param("\\n", "\n", id="newline"),
+            pytest.param("\\t", "\t", id="tab"),
+        ],
+    )
+    def test_escaped_chars_still_work(self, base_splitter_kwargs, escaped_char, literal_char):
+        """Escaped characters should still be decoded properly."""
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator=escaped_char,
+            **base_splitter_kwargs,
+        )
+
+        text = f"chunk1{literal_char}chunk2{literal_char}chunk3"
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert result == ["chunk1", "chunk2", "chunk3"]
+
+    def test_literal_newline_works(self, base_splitter_kwargs):
+        """Literal newline should work without escaping."""
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator="\n",
+            **base_splitter_kwargs,
+        )
+
+        text = "chunk1\nchunk2\nchunk3"
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert result == ["chunk1", "chunk2", "chunk3"]
+
+    def test_chinese_punctuation_literal(self, base_splitter_kwargs):
+        """Chinese punctuation should work as literal delimiters (related to issue)."""
+        # Test Chinese comma
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator="，",
+            **base_splitter_kwargs,
+        )
+
+        text = "chunk1，chunk2，chunk3"
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert result == ["chunk1", "chunk2", "chunk3"]
+
+    def test_mixed_content_with_zwsp(self, base_splitter_kwargs):
+        """Test realistic content with ZWSP delimiters."""
+        zwsp = "\u200b"
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator=zwsp,
+            **base_splitter_kwargs,
+        )
+
+        text = f"First paragraph with some text.{zwsp}Second paragraph with more content.{zwsp}Third paragraph here."
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert "First paragraph" in result[0]
+        assert "Second paragraph" in result[1]
+        assert "Third paragraph" in result[2]
+
+    def test_empty_separator(self, base_splitter_kwargs):
+        """Empty separator should not split."""
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator="",
+            **base_splitter_kwargs,
+        )
+
+        text = "chunk1 chunk2 chunk3"
+        result = splitter.split_text(text)
+
+        # Should not split on empty separator
+        assert len(result) == 1
+
+    def test_escaped_unicode_hex_notation(self, base_splitter_kwargs):
+        """Escaped Unicode hex notation \\u200b should be decoded."""
+        splitter = FixedRecursiveCharacterTextSplitter(
+            fixed_separator="\\u200b",
+            **base_splitter_kwargs,
+        )
+
+        zwsp = "\u200b"
+        text = f"chunk1{zwsp}chunk2{zwsp}chunk3"
+        result = splitter.split_text(text)
+
+        assert len(result) == 3
+        assert result == ["chunk1", "chunk2", "chunk3"]