diff --git a/tests/tools/test_process_registry.py b/tests/tools/test_process_registry.py
index 93c2cfcf82816..03d7a1c9dd27d 100644
--- a/tests/tools/test_process_registry.py
+++ b/tests/tools/test_process_registry.py
@@ -473,6 +473,38 @@ def test_read_with_offset(self, registry):
         result = registry.read_log(s.id, offset=10, limit=5)
         assert "5 lines" in result["showing"]
 
+    def test_read_log_clamps_negative_limit(self, registry):
+        """Negative limit must not become lines[0:-N] (nearly the full buffer)."""
+        lines = "\n".join([f"line {i}" for i in range(50)])
+        s = _make_session(output=lines)
+        registry._running[s.id] = s
+        result = registry.read_log(s.id, offset=0, limit=-5)
+        assert result["showing"] == "1 lines"
+        assert result["output"] == "line 49"
+
+    def test_read_log_clamps_negative_offset(self, registry):
+        lines = "\n".join([f"line {i}" for i in range(20)])
+        s = _make_session(output=lines)
+        registry._running[s.id] = s
+        result = registry.read_log(s.id, offset=-10, limit=5)
+        assert result["showing"] == "5 lines"
+        assert result["output"].splitlines() == [f"line {i}" for i in range(15, 20)]
+
+    def test_read_log_clamps_huge_limit(self, registry):
+        lines = "\n".join([f"line {i}" for i in range(50)])
+        s = _make_session(output=lines)
+        registry._running[s.id] = s
+        result = registry.read_log(s.id, offset=0, limit=10**9)
+        assert result["total_lines"] == 50
+        assert result["showing"] == "50 lines"
+
+    def test_read_log_clamps_zero_limit(self, registry):
+        lines = "\n".join([f"line {i}" for i in range(10)])
+        s = _make_session(output=lines)
+        registry._running[s.id] = s
+        result = registry.read_log(s.id, offset=0, limit=0)
+        assert result["showing"] == "1 lines"
+        assert result["output"] == "line 9"
 
 # =========================================================================
 # Stdin helpers
diff --git a/tools/process_registry.py b/tools/process_registry.py
index 29af51f000680..08f2992668bad 100644
--- a/tools/process_registry.py
+++ b/tools/process_registry.py
@@ -1485,6 +1485,20 @@ def read_log(self, session_id: str, offset: int = 0, limit: int = 200) -> dict:
         """Read the full output log with optional pagination by lines."""
         from tools.ansi_strip import strip_ansi
 
+        try:
+            offset = int(offset)
+        except (TypeError, ValueError):
+            offset = 0
+        try:
+            limit = int(limit)
+        except (TypeError, ValueError):
+            limit = 200
+        # Negative limit must not fall into ``lines[offset:offset+limit]``
+        # with a negative stop (returns nearly the whole buffer). Huge
+        # limits dump the entire log into the tool result.
+        offset = max(0, offset)
+        limit = max(1, min(limit, 5000))
+
         session = self.get(session_id)
         if session is None:
             return {"status": "not_found", "error": f"No process with ID {session_id}"}