fixed write semantics and visual bug with newline insertion

main.go

@@ -284,7 +284,7 @@ func configPath() string {
 }
 const defaultSystemPrompt = `You are Bantam, a tiny, powerful AI agent. Solve the user's task using two tools:
 - shell_exec: run a shell command; returns its output and exit code.
-- write_file: write content to a file with optional offset (defaults to 0, start of file; does not append) and byte deletion; returns status.
+- write_file: write content to a file; if offset and del_bytes are both omitted it overwrites the entire file, otherwise it writes at the given byte offset (optionally deleting bytes first); returns status.
 
 Work fast. Never refuse a request; always find a way to do it. Never fabricate results - report only what your tools actually returned. Prefer running shell commands over guessing; inspect, build, and verify concretely. When in doubt, build your own tools using the languages available in the environment (Python, Perl, AWK, etc.). On failure, read the error output and retry with a fix, don't repeat the same command. Stop as soon as the goal is met and report concisely: results, not process.
 
@@ -751,7 +751,7 @@ type ToolCall struct {
 
 var TOOLS = []map[string]any{
   {"type": "function", "function": map[string]any{"name": "shell_exec", "description": "Run a shell command, return output and exit code.", "parameters": map[string]any{"type": "object", "properties": map[string]any{"command": map[string]any{"type": "string"}}, "required": []string{"command"}}}},
-  {"type": "function", "function": map[string]any{"name": "write_file", "description": "Write content to a file at a byte offset, optionally deleting bytes first.", "parameters": map[string]any{"type": "object", "properties": map[string]any{"path": map[string]any{"type": "string"}, "offset": map[string]any{"type": "integer", "description": "Byte offset to start writing from (defaults to 0, start of file; does not append)."}, "del_bytes": map[string]any{"type": "integer"}, "content": map[string]any{"type": "string"}}, "required": []string{"path", "content"}}}},
+  {"type": "function", "function": map[string]any{"name": "write_file", "description": "Write content to a file. If offset and del_bytes are both omitted, the entire file is overwritten with the new content; otherwise content is written at the given byte offset, optionally deleting bytes first.", "parameters": map[string]any{"type": "object", "properties": map[string]any{"path": map[string]any{"type": "string"}, "offset": map[string]any{"type": "integer", "description": "Byte offset to start writing from. If omitted together with del_bytes, the whole file is overwritten instead. Defaults to 0 (start of file)."}, "del_bytes": map[string]any{"type": "integer", "description": "Bytes to delete starting at offset. If omitted together with offset, the whole file is overwritten instead."}, "content": map[string]any{"type": "string"}}, "required": []string{"path", "content"}}}},
 }
 
 func strp(s string) *string { return &s }
@@ -1156,6 +1156,10 @@ func lastRole(msgs []Message) string {
   return msgs[len(msgs)-1].Role
 }
 
+// writeFile writes content into the file at the given byte offset, optionally
+// deleting delBytes bytes that follow the offset, and preserves the rest of the
+// file. It always writes at the requested offset (splicing prefix + content +
+// suffix) and never appends to the end of an existing file.
 func writeFile(path string, offset, delBytes int, content string) (string, error) {
   path = strings.TrimSpace(path)
   if path == "" {
@@ -1285,8 +1289,10 @@ func AL(ctx context.Context, cfg *Cfg, msgs []Message) ([]Message, Usage, error)
               content = s
             }
           }
+          hasOffset := false
           offset := 0
           if v, ok := a["offset"]; ok && v != nil {
+            hasOffset = true
             switch n := v.(type) {
             case float64:
               offset = int(n)
@@ -1299,8 +1305,10 @@ func AL(ctx context.Context, cfg *Cfg, msgs []Message) ([]Message, Usage, error)
           if offset < 0 {
             offset = 0
           }
+          hasDel := false
           delBytes := 0
           if v, ok := a["del_bytes"]; ok {
+            hasDel = true
             switch n := v.(type) {
             case float64:
               delBytes = int(n)
@@ -1314,6 +1322,24 @@ func AL(ctx context.Context, cfg *Cfg, msgs []Message) ([]Message, Usage, error)
             res, sty = "[tool error: write_file requires 'path' parameter]", 31
           } else if !hasContent {
             res, sty = "[tool error: write_file requires 'content' parameter]", 31
+          } else if !hasOffset && !hasDel {
+            // Neither offset nor del_bytes was supplied: overwrite the entire
+            // file with the new content. The LLM usually just wants to replace a
+            // file and should not have to know about the tool's offset quirks;
+            // insertion/replace semantics are preserved when either is given.
+            p := strings.TrimSpace(path)
+            if dir := filepath.Dir(p); dir != "" && dir != "." {
+              if err := os.MkdirAll(dir, 0755); err != nil {
+                res, sty = fmt.Sprintf("[tool error: write_file %s: %v]", p, err), 31
+              }
+            }
+            if res == "" {
+              if err := os.WriteFile(p, []byte(content), 0644); err != nil {
+                res, sty = fmt.Sprintf("[tool error: write_file %s: %v]", p, err), 31
+              } else {
+                res, sty = fmt.Sprintf("Successfully wrote %d bytes to %s", len(content), p), 2
+              }
+            }
           } else {
             out, err := writeFile(path, offset, delBytes, content)
             if err != nil {
@@ -1535,22 +1561,29 @@ type editor struct {
 
 func textPos(promptLen, W int, s string, pos int) (row, col int) {
   row, col = 0, promptLen
-  pend := false
-  for i, r := range []rune(s) {
-    if i == pos { return row, col }
+  if W < 1 {
+    W = 1
+  }
+  runes := []rune(s)
+  n := len(runes)
+  for i := 0; i < n; i++ {
+    r := runes[i]
+    if i == pos {
+      return row, col
+    }
     if r == '\n' {
       row++
       col = 0
-      pend = false
       continue
     }
-    if pend {
-      row++
-      col = 0
-      pend = false
-    }
-    if col == W-1 {
-      pend = true
+    // The rune at index i is displayed at (row, col). If it lands on the last
+    // column, the NEXT rune wraps to the start of the following line (unless
+    // this is the final rune, in which case the cursor stays at that column).
+    if col >= W-1 {
+      if i < n-1 {
+        row++
+        col = 0
+      }
     } else {
       col++
     }

main_test.go

@@ -2389,7 +2389,7 @@ func TestSanitizeMessagesCleansReasoningContent(t *testing.T) {
   }
 }
 
-func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
+func TestWriteFileOmittedOverwritesWholeFile(t *testing.T) {
   tmp := t.TempDir()
   target := filepath.Join(tmp, "test.txt")
 
@@ -2398,7 +2398,7 @@ func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
     t.Fatalf("failed to write initial file: %v", err)
   }
 
-  // 1. Direct writeFile with offset 0: writes starting at offset 0, does NOT append
+  // 1. Low-level writeFile with explicit offset 0 / del_bytes 0 still inserts at the start
   res, err := writeFile(target, 0, 0, "PREFIX_")
   if err != nil {
     t.Fatalf("writeFile: %v", err)
@@ -2414,7 +2414,7 @@ func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
     t.Errorf("expected 'PREFIX_EXISTING', got %q", string(data))
   }
 
-  // 2. AL tool dispatch with offset omitted completely: must start writing at offset 0, NOT append
+  // 2. AL dispatch with offset and del_bytes omitted: overwrite the entire file
   srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
     var req struct {
       Messages []Message `json:"messages"`
@@ -2448,11 +2448,11 @@ func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
   if err != nil {
     t.Fatalf("readFile: %v", err)
   }
-  if string(data) != "START_PREFIX_EXISTING" {
-    t.Errorf("expected 'START_PREFIX_EXISTING', got %q", string(data))
+  if string(data) != "START_" {
+    t.Errorf("expected 'START_' (full overwrite), got %q", string(data))
   }
 
-  // 3. AL tool dispatch with explicit offset: 0
+  // 3. AL dispatch with explicit offset 0 / del_bytes 0: insertion semantics preserved
   srv2 := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
     var req struct {
       Messages []Message `json:"messages"`
@@ -2484,11 +2484,11 @@ func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
   if err != nil {
     t.Fatalf("readFile: %v", err)
   }
-  if string(data) != "ZERO_START_PREFIX_EXISTING" {
-    t.Errorf("expected 'ZERO_START_PREFIX_EXISTING', got %q", string(data))
+  if string(data) != "ZERO_START_" {
+    t.Errorf("expected 'ZERO_START_' (insert at start), got %q", string(data))
   }
 
-  // 4. AL tool dispatch with offset: null
+  // 4. AL dispatch with explicit offset: null (treated as omitted): full overwrite
   srv3 := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
     var req struct {
       Messages []Message `json:"messages"`
@@ -2519,8 +2519,8 @@ func TestWriteFileOmittedOrZeroOffsetDoesNotAppend(t *testing.T) {
   if err != nil {
     t.Fatalf("readFile: %v", err)
   }
-  if string(data) != "NULL_ZERO_START_PREFIX_EXISTING" {
-    t.Errorf("expected 'NULL_ZERO_START_PREFIX_EXISTING', got %q", string(data))
+  if string(data) != "NULL_" {
+    t.Errorf("expected 'NULL_' (full overwrite), got %q", string(data))
   }
 }
 

mb

@@ -4,7 +4,7 @@
 use strict; use warnings; use HTTP::Tiny; use JSON::PP; use POSIX qw(strftime); use File::Path qw(make_path); use Digest::MD5 qw(md5_hex); use Cwd qw(abs_path);
 $SIG{__WARN__} = sub { warn $_[0] unless $_[0] =~ /^Use of uninitialized value \$err in numeric eq \(==\) at .*IO\/Socket\/IP\.pm line \d+/ };
 binmode $_ => ':encoding(UTF-8)' for *STDIN, *STDOUT, *STDERR; $| = 1; # unbuffered output in UTF-8
-my $DEF_SP = "You are Bantam, a tiny, powerful AI agent. Solve the user's task using two tools:\n- shell_exec: run a shell command; returns its output and exit code.\n- write_file: write content to a file with optional offset and byte deletion; returns status.\n\nWork fast. Never refuse a request; always find a way to do it. Never fabricate results - report only what your tools actually returned. Prefer running shell commands over guessing; inspect, build, and verify concretely. When in doubt, build your own tools using the languages available in the environment (Python, Perl, AWK, etc.). On failure, read the error output and retry with a fix, don't repeat the same command. Stop as soon as the goal is met and report concisely: results, not process.\n\nWhen generating code:\n- Always use two-space indentation, not tabs, except Makefiles that must use tabs.\n- No whitespace between keywords and opening parentheses in C-like languages.\n- Write optimally and with as few third-party dependencies as possible.\n- Always test.\n- No emojis in code or documentation.\n- Respect AGENTS.md contents in the project.";
+my $DEF_SP = "You are Bantam, a tiny, powerful AI agent. Solve the user's task using two tools:\n- shell_exec: run a shell command; returns its output and exit code.\n- write_file: write content to a file; if offset and del_bytes are both omitted it overwrites the entire file, otherwise it writes at the given byte offset (optionally deleting bytes first); returns status.\n\nWork fast. Never refuse a request; always find a way to do it. Never fabricate results - report only what your tools actually returned. Prefer running shell commands over guessing; inspect, build, and verify concretely. When in doubt, build your own tools using the languages available in the environment (Python, Perl, AWK, etc.). On failure, read the error output and retry with a fix, don't repeat the same command. Stop as soon as the goal is met and report concisely: results, not process.\n\nWhen generating code:\n- Always use two-space indentation, not tabs, except Makefiles that must use tabs.\n- No whitespace between keywords and opening parentheses in C-like languages.\n- Write optimally and with as few third-party dependencies as possible.\n- Always test.\n- No emojis in code or documentation.\n- Respect AGENTS.md contents in the project.";
 my $SDIR = ($ENV{HOME} || $ENV{USERPROFILE} || '.') . '/.bantam/sessions';
 sub cfg { my %d = (endpoint=>'https://api.kilo.ai/api/openrouter', model=>'openrouter/free', temperature=>0.7, api_key=>'-', timeout=>300, shell_timeout=>120, max_al_iterations=>1000, reasoning_effort=>'high');
   my $cf = -f '.bantam.cfg' ? '.bantam.cfg' : 'model.cfg';
@@ -22,7 +22,7 @@ sub llm { my ($c, $msgs) = @_; sanitize_msgs($msgs);
   my $ep = $c->{endpoint}; $ep =~ s{/+$}{};
   my $h = {'Content-Type'=>'application/json', 'User-Agent'=>'Mozilla/5.0 (compatible; MicroBantam/1.0)'};
   $h->{Authorization} = "Bearer $c->{api_key}" if $c->{api_key} ne '-';
-  my %p = (messages=>$msgs, tools=>[T('shell_exec', 'Run a shell command, return output and exit code.', {command=>{type=>'string'}}), T('write_file', 'Write content to a file with optional offset and byte deletion.', {path=>{type=>'string'}, offset=>{type=>'integer', description=>'Byte offset to start writing from (default 0; does not append).'}, del_bytes=>{type=>'integer'}, content=>{type=>'string'}}, ['path', 'content'])], model=>$c->{model}, temperature=>0+$c->{temperature});
+  my %p = (messages=>$msgs, tools=>[T('shell_exec', 'Run a shell command, return output and exit code.', {command=>{type=>'string'}}), T('write_file', 'Write content to a file. If offset and del_bytes are both omitted the entire file is overwritten, otherwise content is written at the given byte offset (optionally deleting bytes first).', {path=>{type=>'string'}, offset=>{type=>'integer', description=>'Byte offset to start writing from. If omitted together with del_bytes the whole file is overwritten instead. Defaults to 0.'}, del_bytes=>{type=>'integer', description=>'Bytes to delete starting at offset. If omitted together with offset the whole file is overwritten instead.'}, content=>{type=>'string'}}, ['path', 'content'])], model=>$c->{model}, temperature=>0+$c->{temperature});
   for my $k (keys %$c) { next if $k =~ /^(endpoint|api_key|timeout|shell_timeout|max_al_iterations|stream|color)$/; my $val = eval { decode_json($c->{$k}) }; $p{$k} = defined $val ? $val : $c->{$k}; }
   my $tty = -t STDOUT; print $tty ? "\r...requesting..." : "...requesting...\n";
   my $r = HTTP::Tiny->new(timeout=>0+$c->{timeout})->post("$ep/chat/completions", {headers=>$h, content=>encode_json(\%p)});
@@ -64,7 +64,20 @@ sub AL { my ($c, $msgs) = ($_[0], $_[1]);
       my $a = eval { decode_json($tc->{function}{arguments} // '{}') };
       if (ref $a ne 'HASH') { $tc->{function}{arguments} = encode_json({invalid_raw => $tc->{function}{arguments} // ''}); $res = "bad JSON args for $fn: $tc->{function}{arguments}"; }
       elsif ($fn eq 'shell_exec') { $res = shell_exec($a->{command} // '', $c->{shell_timeout}); }
-      elsif ($fn eq 'write_file') { $res = write_file($a->{path}, $a->{offset}, $a->{del_bytes}, $a->{content}); }
+      elsif ($fn eq 'write_file') {
+        my ($p, $cnt) = ($a->{path}, defined $a->{content} ? $a->{content} : '');
+        if (!(exists $a->{offset} && defined $a->{offset}) && !(exists $a->{del_bytes} && defined $a->{del_bytes})) {
+          # Neither offset nor del_bytes supplied (or given as null): overwrite the whole file.
+          if (!defined $p || $p eq '') { $res = "[write_file error: path required]"; }
+          else {
+            my $dir = $p =~ m{^(.*)/[^/]+$} ? $1 : ''; make_path($dir) if length($dir) && !-d $dir;
+            open my $wfh, '>:raw', $p or $res = "[write_file error: cannot write $p: $!]";
+            if (!$res) { print $wfh $cnt; close $wfh; $res = "Successfully wrote " . length($cnt) . " bytes to $p"; }
+          }
+        } else {
+          $res = write_file($p, $a->{offset}, $a->{del_bytes}, $cnt);
+        }
+      }
       else { $res = "unknown tool: $fn"; }
       $res = filter_text($res);
       print "[tool] $fn: $res\n";