diff --git a/.github/workflows/main-ci.yml b/.github/workflows/main-ci.yml
index e66324d200e..d6cc9bf8c72 100644
--- a/.github/workflows/main-ci.yml
+++ b/.github/workflows/main-ci.yml
@@ -66,6 +66,7 @@ jobs:
               - 'scripts/lint_controller_sqlalchemy.py'
               - 'scripts/ast_grep_rules/no_new_getattr.yml'
               - 'scripts/ast_grep_rules/no_new_controller_sqlalchemy.yml'
+              - 'scripts/ast_grep_rules/no_new_session_mock.yml'
               - '.github/workflows/style.yml'
               - '.github/workflows/main-ci.yml'
               - '.github/workflows/api-tests.yml'
@@ -160,9 +161,11 @@ jobs:
               - 'scripts/ast_grep_guard.py'
               - 'scripts/check_no_new_getattr.py'
               - 'scripts/check_no_new_controller_sqlalchemy.py'
+              - 'scripts/check_no_new_session_mock.py'
               - 'scripts/lint_controller_sqlalchemy.py'
               - 'scripts/ast_grep_rules/no_new_getattr.yml'
               - 'scripts/ast_grep_rules/no_new_controller_sqlalchemy.yml'
+              - 'scripts/ast_grep_rules/no_new_session_mock.yml'
               - '.github/workflows/style.yml'
               - '.github/workflows/main-ci.yml'
             dify-agent-style:
diff --git a/.github/workflows/style.yml b/.github/workflows/style.yml
index 232480785e1..7f17e22f48d 100644
--- a/.github/workflows/style.yml
+++ b/.github/workflows/style.yml
@@ -89,6 +89,10 @@ jobs:
         if: inputs.run-python-style
         run: uv run --project api python scripts/check_no_new_controller_sqlalchemy.py --base-rev "${{ inputs.base-rev }}"
 
+      - name: Run No New Session Mock Guard
+        if: inputs.run-python-style
+        run: uv run --project api python scripts/check_no_new_session_mock.py --base-rev "${{ inputs.base-rev }}"
+
       - name: Run Type Checks
         if: inputs.run-python-style
         env:
diff --git a/scripts/ast_grep_rules/no_new_session_mock.yml b/scripts/ast_grep_rules/no_new_session_mock.yml
new file mode 100644
index 00000000000..6237766034c
--- /dev/null
+++ b/scripts/ast_grep_rules/no_new_session_mock.yml
@@ -0,0 +1,11 @@
+id: no-new-session-mock
+language: python
+rule:
+  any:
+    - pattern: $SESSION = MagicMock($$$ARGS)
+    - pattern: $SESSION = Mock($$$ARGS)
+constraints:
+  SESSION:
+    regex: ^(session|db_session|mock_session|read_session|write_session)$
+message: net-new session mock assignment in test code
+severity: error
diff --git a/scripts/check_no_new_session_mock.py b/scripts/check_no_new_session_mock.py
new file mode 100644
index 00000000000..941f576b487
--- /dev/null
+++ b/scripts/check_no_new_session_mock.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Block net-new session mock assignments in unit tests.
+
+This catches net-new top-level session mocks like
+`session = MagicMock()` or `db_session = Mock()` added to
+`tests/unit_tests/**`. Pre-existing matches are ignored via the
+shared baseline diff (`scripts/ast_grep_guard.py`), so historical
+mocks that haven't been cleaned up yet do not block the gate.
+
+A `# guard-ignore: no-new-session-mock -- <reason>` line on the same
+source line suppresses a violation with a human-readable reason.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from ast_grep_guard import Match, has_reasoned_guard_ignore, is_python_source_path, rule_path, run_guard
+
+
+RULE_ID = "no-new-session-mock"
+RULE_PATH = rule_path("no_new_session_mock.yml")
+TEST_ROOT = Path("api/tests/unit_tests")
+VIOLATION_MESSAGE = "no-new-session-mock net-new session mock assignment in test code"
+
+
+def is_test_source_path(path: str) -> bool:
+    if not is_python_source_path(path):
+        return False
+    return Path(path).is_relative_to(TEST_ROOT)
+
+
+def is_reportable_match(match: Match) -> bool:
+    return not has_reasoned_guard_ignore(match.source_line, RULE_ID)
+
+
+def main() -> int:
+    return run_guard(
+        description=__doc__,
+        rule=RULE_PATH,
+        is_scanned_path=is_test_source_path,
+        is_reportable_match=is_reportable_match,
+        violation_message=VIOLATION_MESSAGE,
+    )
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())