diff --git a/backend/packages/harness/deerflow/persistence/postgres_schema.py b/backend/packages/harness/deerflow/persistence/postgres_schema.py
index 9d07f81b56..35167cc1a8 100644
--- a/backend/packages/harness/deerflow/persistence/postgres_schema.py
+++ b/backend/packages/harness/deerflow/persistence/postgres_schema.py
@@ -20,7 +20,7 @@ defense-in-depth; connection-argument helpers only assemble driver payloads.
 from __future__ import annotations
 
 import re
-from urllib.parse import parse_qsl, quote, urlencode, urlsplit, urlunsplit
+from urllib.parse import quote, unquote, urlsplit, urlunsplit
 
 from deerflow.utils.file_io import await_drained
 
@@ -200,18 +200,26 @@ def dsn_with_search_path(dsn: str, schema: str) -> str:
     if scheme_base not in {"postgres", "postgresql"}:
         raise ValueError(f"Unsupported PostgreSQL DSN scheme for schema injection: {parts.scheme!r}")
 
+    # Rebuild the query by hand rather than with parse_qsl/urlencode: parse_qsl
+    # decodes '+' as a space (an HTML-form rule libpq does not apply), so
+    # round-tripping an existing parameter would rewrite a literal plus as '%20'
+    # and change the value the server sees. Everything but ``options`` is carried
+    # over exactly as written.
     options_values: list[str] = []
-    query_pairs = []
-    for key, value in parse_qsl(parts.query, keep_blank_values=True):
-        if key == "options":
-            options_values.append(value)
+    preserved: list[str] = []
+    for pair in parts.query.split("&"):
+        if not pair:
+            continue
+        raw_key, _, raw_value = pair.partition("=")
+        if unquote(raw_key) == "options":
+            options_values.append(unquote(raw_value))
         else:
-            query_pairs.append((key, value))
+            preserved.append(pair)
 
     options = _merge_search_path_option(" ".join(options_values), schema)
-    query_pairs.append(("options", options))
-    # quote_via=quote encodes space as %20 (libpq-safe), not + (form-style).
-    query = urlencode(query_pairs, quote_via=quote)
+    # quote() encodes space as %20 (libpq-safe) and a literal '+' as %2B, so the
+    # merged value keeps both distinctions.
+    query = "&".join([*preserved, f"options={quote(options, safe='')}"])
     return urlunsplit((scheme_base, parts.netloc, parts.path, query, parts.fragment))
 
 
diff --git a/backend/tests/test_postgres_schema_helper.py b/backend/tests/test_postgres_schema_helper.py
index bf25ec2290..0ede45ab91 100644
--- a/backend/tests/test_postgres_schema_helper.py
+++ b/backend/tests/test_postgres_schema_helper.py
@@ -71,6 +71,32 @@ class TestDsnWithSearchPath:
         assert query["sslmode"] == ["require"]
         assert query["options"] == ["-c search_path=deerflow"]
 
+    @staticmethod
+    def _raw_query(out: str) -> dict[str, str]:
+        # Read the pairs back undecoded: parse_qs applies the HTML-form rule that
+        # turns '+' into a space, which is exactly the rule libpq does NOT apply.
+        return dict(pair.split("=", 1) for pair in urlsplit(out).query.split("&") if pair)
+
+    def test_preserves_plus_in_existing_query_value(self):
+        # A '+' in a parameter value is a literal plus for libpq, so rewriting it
+        # into '%20' silently changes the value the server receives: 'web+app'
+        # would arrive as 'web app'.
+        dsn = "postgresql://u:p@h:5432/db?application_name=web+app"
+        out = dsn_with_search_path(dsn, "deerflow")
+        assert self._raw_query(out)["application_name"] == "web+app"
+        assert parse_qs(urlsplit(out).query)["options"] == ["-c search_path=deerflow"]
+
+    def test_preserves_plus_in_existing_options_value(self):
+        dsn = "postgresql://u:p@h:5432/db?options=-c%20timezone%3DUTC+8"
+        out = dsn_with_search_path(dsn, "deerflow")
+        assert self._raw_query(out)["options"] == "-c%20timezone%3DUTC%2B8%20-c%20search_path%3Ddeerflow"
+        assert parse_qs(urlsplit(out).query, keep_blank_values=True)["options"] == ["-c timezone=UTC+8 -c search_path=deerflow"]
+
+    def test_does_not_double_encode_an_existing_percent_escape(self):
+        dsn = "postgresql://u:p@h:5432/db?application_name=web%2Bapp"
+        out = dsn_with_search_path(dsn, "deerflow")
+        assert self._raw_query(out)["application_name"] == "web%2Bapp"
+
     def test_replaces_existing_options_query(self):
         dsn = "postgresql://u:p@h:5432/db?options=-c%20search_path%3Dpublic"
         out = dsn_with_search_path(dsn, "deerflow")