diff --git a/bantam.hpp b/bantam.hpp
index 269e6f9..c03cf06 100644
--- a/bantam.hpp
+++ b/bantam.hpp
@@ -197,8 +197,8 @@ int rune_width(int cp);
 
 void mkdir_p(std::string_view dir);
 std::string project_id();
-std::string bantam_tmp_dir();
-std::string tool_res_dir();
+std::string bantam_tmp_dir(std::string_view sess_id = "");
+std::string tool_res_dir(std::string_view sess_id = "");
 std::string md5_hex(std::string_view data);
 
 /* ------------------------------------------------------------------ */
diff --git a/main.cpp b/main.cpp
index 0337f09..d7527db 100644
--- a/main.cpp
+++ b/main.cpp
@@ -229,6 +229,8 @@ static int AL(Cfg &cfg, Messages &msgs, Usage &out, volatile int *interrupted, s
           else if (!has_line && !has_byte && !has_del_lines && !has_del_bytes) {
             /* No offset/deletion parameters: overwrite the whole file. */
             std::string c = filter_text(cv);
+            /* Ensure the written file ends with a single trailing newline. */
+            if (!c.empty() && c.back() != '\n') c.push_back('\n');
             auto sl = fp.find_last_of('/');
             if (sl != std::string::npos) mkdir_p(std::string_view(fp).substr(0, sl));
             else mkdir_p(".");
@@ -236,6 +238,10 @@ static int AL(Cfg &cfg, Messages &msgs, Usage &out, volatile int *interrupted, s
             else { res = sfmt("Successfully wrote %zu bytes to %s", c.size(), fp.c_str()); }
           } else {
             int werr = 0; std::string c = filter_text(cv);
+            /* Line-based edits keep the text-file trailing-newline convention;
+             * byte-based edits are exact byte operations and must not have a
+             * newline injected. */
+            if (!(has_byte || has_del_bytes) && !c.empty() && c.back() != '\n') c.push_back('\n');
             res = write_file(fp, line_off, byte_off, del_lines, del_bytes, c, &werr);
             if (werr) sty = 31;
           }
diff --git a/tools.cpp b/tools.cpp
index ea5c05c..ffcf10f 100644
--- a/tools.cpp
+++ b/tools.cpp
@@ -291,6 +291,13 @@ std::string write_file(std::string_view path, long line_offset, long byte_offset
   nd.append(data, 0, prefix_len);
   nd.append(content);
   if (suffix_len) nd.append(data, end_del, suffix_len);
+  /* Keep a single trailing newline for line-based / whole-file text writes.
+   * Byte-based edits are exact byte operations and must not be mutated with an
+   * injected newline, so the guard is skipped whenever a byte offset or byte
+   * deletion is actually in effect. */
+  bool byte_based = (line_offset < 0 && byte_offset >= 0) ||
+                   (del_lines < 0 && del_bytes >= 0);
+  if (!byte_based && !nd.empty() && nd.back() != '\n') nd.push_back('\n');
   if (!write_file_str(p, nd)) {
     *err = 1;
     return sfmt("[tool error: write %s: %s]", p.c_str(), strerror(errno));
diff --git a/util.cpp b/util.cpp
index c7bf5e8..da49fa1 100644
--- a/util.cpp
+++ b/util.cpp
@@ -192,15 +192,27 @@ void mkdir_p(std::string_view dir) {
   }
   mkdir(path.c_str(), 0755);
 }
-std::string bantam_tmp_dir() {
+std::string bantam_tmp_dir(std::string_view sess_id) {
   const char *tmp = std::getenv("TMPDIR");
   if (!tmp || !*tmp) tmp = "/tmp";
-  std::string p = std::string(tmp) + "/bantam";
+  std::string base(tmp);
+  /* Drop a single trailing slash so we never build a "//bantam" path (which
+   * some platforms treat as a special "super-root"); keep "/" itself intact. */
+  if (base.size() > 1 && base.back() == '/') base.pop_back();
+  std::string p = base;
+  if (p != "/") p += "/";
+  p += "bantam";
+  /* Segregate temporary artifacts per session so different projects/sessions
+   * never share or collide on scratch files. */
+  std::string sid(sess_id);
+  if (sid.empty()) sid = project_id();
+  if (!sid.empty()) { p += "/"; p += sid; }
   mkdir_p(p);
   return p;
 }
-std::string tool_res_dir() {
-  std::string base = bantam_tmp_dir();
+
+std::string tool_res_dir(std::string_view sess_id) {
+  std::string base = bantam_tmp_dir(sess_id);
   std::string p = base + "/toolres";
   mkdir_p(p);
   return p;