diff --git a/README.md b/README.md
index 71d2f38..abc0129 100644
--- a/README.md
+++ b/README.md
@@ -679,6 +679,20 @@ var name = m.fields['name'][0];
 var file = m.files['avatar'][0];
 ```
 
+`HttpMultipartWriter` builds the same bodies for client uploads:
+
+```fun
+use 'http-multipart.fun';
+
+var w = HttpMultipartWriter();
+w.field('name', 'alice');
+w.file('avatar', 'a.txt', data, [contentType: 'text/plain']);
+var body = w.body();
+var ct = w.contentType();
+```
+
+Pass `body` and `Content-Type: <ct>` to `HttpClient` to send it.
+
 Environment variables are available without platform-specific FFI:
 
 ```fun
diff --git a/README.zh.md b/README.zh.md
index 8f53f26..eae122e 100644
--- a/README.zh.md
+++ b/README.zh.md
@@ -638,6 +638,20 @@ var name = m.fields['name'][0];
 var file = m.files['avatar'][0];
 ```
 
+客户端上传用 `HttpMultipartWriter` 生成同样的 body：
+
+```fun
+use 'http-multipart.fun';
+
+var w = HttpMultipartWriter();
+w.field('name', 'alice');
+w.file('avatar', 'a.txt', data, [contentType: 'text/plain']);
+var body = w.body();
+var ct = w.contentType();
+```
+
+把 `body` 和 `Content-Type: <ct>` 传给 `HttpClient` 即可发送。
+
 环境变量也可以直接读取，不需要自己写平台 FFI：
 
 ```fun
diff --git a/fun/lib/http-multipart.fun b/fun/lib/http-multipart.fun
index 54aaf59..8ce0036 100644
--- a/fun/lib/http-multipart.fun
+++ b/fun/lib/http-multipart.fun
@@ -1,16 +1,25 @@
 // Copyright (c) 2010-2026 Zhang Weidong <zwd@funlang.org>
 // SPDX-License-Identifier: MIT
 //
-// http-multipart: parse multipart/form-data bodies (RFC 7578 / RFC 2046).
+// http-multipart: parse and build multipart/form-data bodies (RFC 7578 / RFC 2046).
 //
+// Parse:
 //     var m = HttpMultipartParse(req.body, req.header('content-type'));
 //     if not m.ok then raise m.error; end if;
 //     var name = m.fields['name'][0];
 //     var file = m.files['avatar'][0];
 //
-// The body must already be in memory. Fields are strings; files are part
-// objects with filename/contentType/headers/data. Duplicate names stay as
-// arrays. Invalid input returns ok:false/error; it does not raise.
+// Build:
+//     var w = HttpMultipartWriter();
+//     w.field('user', 'alice');
+//     w.file('avatar', 'a.txt', data);
+//     var body = w.body();
+//     var ct = w.contentType();
+//
+// Bodies are built in memory. Fields are strings; files are part objects with
+// filename/contentType/headers/data. Duplicate names stay as arrays. Invalid
+// parse input returns ok:false/error; writer misuse raises.
+use 'uuid.fun';
 
 var _MP_CRLF = 13.toChar() & 10.toChar();
 var _MP_CRLF2 = _MP_CRLF & _MP_CRLF;
@@ -275,3 +284,248 @@ fun HttpMultipartParse(body, ct, opts)
   end do;
   result = new [ok: true, error: nil, parts: parts, fields: fields, files: files];
 end fun;
+
+// --- writer ---------------------------------------------------------------
+
+var _MPW_BCHARS = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz' & "'()+_,-./:=? ";
+var _MPW_TCHARS = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!#$%&' & 39.toChar() & '*+-.^_`|~';
+
+fun _mpwString(v, label)
+  if v.type() <> 8 then raise 'http-multipart: ' & label & ' must be a string'; end if;
+  result = true;
+end fun;
+
+fun _mpwHeaderSafe(s, label)
+  if s.subpos(13.toChar()) >= 0 or s.subpos(10.toChar()) >= 0 or s.subpos(0.toChar()) >= 0 then
+    raise 'http-multipart: ' & label & ' must not contain CR, LF or NUL';
+  end if;
+  result = true;
+end fun;
+
+fun _mpwEscape(s)
+  var out = new [];
+  var i = 0;
+  while i < s.length() do
+    var c = s[i];
+    if c = 92.toChar() or c = 34.toChar() then out.@add(92.toChar()); end if;
+    out.@add(c);
+    i += 1;
+  end do;
+  result = out.@toJson(-1);
+end fun;
+
+fun _mpwBchar(c)
+  result = _MPW_BCHARS.subpos(c) >= 0;
+end fun;
+
+fun _mpwBoundaryValid(b)
+  var n = b.length();
+  if n < 1 or n > 70 then result = false; return; end if;
+  if b[n - 1] = ' ' then result = false; return; end if;
+  var i = 0;
+  while i < n do
+    if not _mpwBchar(b[i]) then result = false; return; end if;
+    i += 1;
+  end do;
+  result = true;
+end fun;
+
+fun _mpwNeedsQuote(b)
+  var i = 0;
+  while i < b.length() do
+    if _MPW_TCHARS.subpos(b[i]) < 0 then result = true; return; end if;
+    i += 1;
+  end do;
+  result = false;
+end fun;
+
+fun _mpwDelimiterAt(s, pos, b)
+  var d = '--' & b;
+  if pos + d.length() > s.length() then result = false; return; end if;
+  if s.substr(pos, d.length()) <> d then result = false; return; end if;
+  var q = _mpSkipHws(s, pos + d.length());
+  // The writer always appends CRLF after data, so a delimiter ending the data
+  // still becomes a real delimiter in the final body.
+  if q >= s.length() then result = true; return; end if;
+  var m = '';
+  if q + 2 <= s.length() then m = s.substr(q, 2); end if;
+  result = m = _MP_CRLF or m = '--';
+end fun;
+
+fun _mpwDataCollides(data, b)
+  if _mpwDelimiterAt(data, 0, b) then result = true; return; end if;
+  var mark = _MP_CRLF & '--' & b;
+  var base = 0;
+  var rest = data;
+  loop
+    var j = rest.subpos(mark);
+    if j < 0 then result = false; return; end if;
+    if _mpwDelimiterAt(data, base + j + _MP_CRLF.length(), b) then result = true; return; end if;
+    base += j + 1;
+    rest = data.substr(base);
+  end loop;
+end fun;
+
+class _MPW(o)
+  var parts = new [];
+  var boundaryValue = nil;
+  var custom = nil;
+  var finished = false;
+  var bodyText = '';
+
+  if o <> nil then
+    var bad = '';
+    var listish = false;
+    try
+      o.@each((v, k){
+        if k = nil or k = '' then
+          listish = true;
+        elsif k <> 'boundary' then
+          bad = k;
+        end if;
+      });
+    except
+      raise 'http-multipart: opts must be a set';
+    end try;
+    if listish then raise 'http-multipart: opts must be a set'; end if;
+    if bad <> '' then raise 'http-multipart: unknown option: ' & bad; end if;
+    if o.@has('boundary') then
+      var b = o['boundary'];
+      _mpwString(b, 'boundary');
+      if not _mpwBoundaryValid(b) then raise 'http-multipart: invalid boundary'; end if;
+      custom = b;
+    end if;
+  end if;
+
+  fun checkFileOpts(opts)
+    if opts = nil then result = true; return; end if;
+    var bad = '';
+    var listish = false;
+    try
+      opts.@each((v, k){
+        if k = nil or k = '' then
+          listish = true;
+        elsif k <> 'contentType' then
+          bad = k;
+        end if;
+      });
+    except
+      raise 'http-multipart: file opts must be a set';
+    end try;
+    if listish then raise 'http-multipart: file opts must be a set'; end if;
+    if bad <> '' then raise 'http-multipart: unknown option: ' & bad; end if;
+    result = true;
+  end fun;
+
+  fun fileContentType(opts)
+    result = 'application/octet-stream';
+    if opts = nil or not opts.@has('contentType') then return; end if;
+    var ct = opts['contentType'];
+    _mpwString(ct, 'contentType');
+    if ct.length() = 0 then raise 'http-multipart: contentType must not be empty'; end if;
+    _mpwHeaderSafe(ct, 'contentType');
+    result = ct;
+  end fun;
+
+  fun field(name, value)
+    if this.finished then raise 'http-multipart: writer is finished'; end if;
+    _mpwString(name, 'name');
+    if name.length() = 0 then raise 'http-multipart: name is required'; end if;
+    _mpwHeaderSafe(name, 'name');
+    _mpwString(value, 'field value');
+    var p = new [];
+    p.name = name;
+    p.isFile = false;
+    p.filename = nil;
+    p.contentType = nil;
+    p.data = value;
+    this.parts.@add(p);
+    result = true;
+  end fun;
+
+  fun file(name, filename, data, opts)
+    if this.finished then raise 'http-multipart: writer is finished'; end if;
+    _mpwString(name, 'name');
+    if name.length() = 0 then raise 'http-multipart: name is required'; end if;
+    _mpwHeaderSafe(name, 'name');
+    _mpwString(filename, 'filename');
+    _mpwHeaderSafe(filename, 'filename');
+    _mpwString(data, 'file data');
+    this.checkFileOpts(opts);
+    var ct = this.fileContentType(opts);
+    var p = new [];
+    p.name = name;
+    p.isFile = true;
+    p.filename = filename;
+    p.contentType = ct;
+    p.data = data;
+    this.parts.@add(p);
+    result = true;
+  end fun;
+
+  fun dataCollides(b)
+    var i = 0;
+    while i < this.parts.@count() do
+      if _mpwDataCollides(this.parts[i].data, b) then result = true; return; end if;
+      i += 1;
+    end do;
+    result = false;
+  end fun;
+
+  fun ensureBoundary()
+    if this.boundaryValue <> nil then result = true; return; end if;
+    if this.custom <> nil then
+      this.boundaryValue = this.custom;
+      result = true;
+      return;
+    end if;
+    var tries = 0;
+    loop
+      var b = 'fun-' & UUID();
+      if not this.dataCollides(b) then
+        this.boundaryValue = b;
+        exit;
+      end if;
+      tries += 1;
+      if tries >= 10 then raise 'http-multipart: cannot choose a boundary'; end if;
+    end loop;
+    result = true;
+  end fun;
+
+  fun contentType()
+    this.ensureBoundary();
+    var b = this.boundaryValue;
+    if _mpwNeedsQuote(b) then b = '"' & b & '"'; end if;
+    result = 'multipart/form-data; boundary=' & b;
+  end fun;
+
+  fun body()
+    if this.finished then result = this.bodyText; return; end if;
+    this.ensureBoundary();
+    if this.dataCollides(this.boundaryValue) then
+      raise 'http-multipart: boundary appears in data';
+    end if;
+    var out = new [];
+    var i = 0;
+    while i < this.parts.@count() do
+      var p = this.parts[i];
+      out.@add('--' & this.boundaryValue & _MP_CRLF);
+      var cd = 'Content-Disposition: form-data; name="' & _mpwEscape(p.name) & '"';
+      if p.isFile then cd &= '; filename="' & _mpwEscape(p.filename) & '"'; end if;
+      out.@add(cd & _MP_CRLF);
+      if p.isFile then out.@add('Content-Type: ' & p.contentType & _MP_CRLF); end if;
+      out.@add(_MP_CRLF);
+      out.@add(p.data);
+      out.@add(_MP_CRLF);
+      i += 1;
+    end do;
+    out.@add('--' & this.boundaryValue & '--' & _MP_CRLF);
+    this.bodyText = out.@toJson(-1);
+    this.finished = true;
+    result = this.bodyText;
+  end fun;
+end class;
+
+fun HttpMultipartWriter(opts)
+  result = _MPW(opts);
+end fun;
diff --git a/fun/test/expected/lib-http-multipart-test.fun.txt b/fun/test/expected/lib-http-multipart-test.fun.txt
index 1e040e4..3c5ee60 100644
--- a/fun/test/expected/lib-http-multipart-test.fun.txt
+++ b/fun/test/expected/lib-http-multipart-test.fun.txt
@@ -141,3 +141,68 @@ True
 True
 True
 True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
diff --git a/fun/test/lib-http-multipart-test.fun b/fun/test/lib-http-multipart-test.fun
index 3f0d312..84915b2 100644
--- a/fun/test/lib-http-multipart-test.fun
+++ b/fun/test/lib-http-multipart-test.fun
@@ -342,3 +342,152 @@ r.body = 'a=1&b=2';
 r.headS['content-type'] = 'application/x-www-form-urlencoded';
 ?. r.form()['a'] = '1';
 ?. r.form()['b'] = '2';
+
+// --- writer: basic fields, file and round-trip ---------------------------
+var w1 = HttpMultipartWriter([boundary: 'fun-w1']);
+?. w1.contentType() = 'multipart/form-data; boundary=fun-w1';
+w1.field('user', 'alice');
+w1.field('tag', 'one');
+w1.field('tag', 'two');
+w1.file('avatar', 'a.txt', 'hello', [contentType: 'text/plain']);
+var wb1 = w1.body();
+?. w1.body() = wb1;
+?. wb1.substr(0, ('--fun-w1' & CRLF).length()) = '--fun-w1' & CRLF;
+?. wb1.subpos('--fun-w1--' & CRLF) = wb1.length() - ('--fun-w1--' & CRLF).length();
+var wm1 = HttpMultipartParse(wb1, w1.contentType());
+?. wm1['ok'];
+?. wm1['parts'].@count() = 4;
+?. wm1['fields']['user'][0] = 'alice';
+?. wm1['fields']['tag'].@count() = 2;
+?. wm1['fields']['tag'][0] = 'one';
+?. wm1['fields']['tag'][1] = 'two';
+?. wm1['files']['avatar'][0]['filename'] = 'a.txt';
+?. wm1['files']['avatar'][0]['contentType'] = 'text/plain';
+?. wm1['files']['avatar'][0]['data'] = 'hello';
+?. wm1['parts'][0]['contentType'] = 'text/plain';
+
+// --- writer: file defaults, empty filename, empty writer -----------------
+var w2 = HttpMultipartWriter([boundary: 'fun-w2']);
+w2.file('f', 'x.bin', 'abc');
+var wm2 = HttpMultipartParse(w2.body(), w2.contentType());
+?. wm2['files']['f'][0]['contentType'] = 'application/octet-stream';
+
+var w3 = HttpMultipartWriter([boundary: 'fun-w3']);
+w3.file('empty', '', '');
+var wb3 = w3.body();
+?. wb3.subpos('filename=""') >= 0;
+var wm3 = HttpMultipartParse(wb3, w3.contentType());
+?. wm3['files']['empty'].@count() = 1;
+?. wm3['files']['empty'][0]['filename'].type() = 8;
+?. wm3['files']['empty'][0]['filename'].length() = 0;
+?. wm3['files']['empty'][0]['data'].length() = 0;
+
+var w4 = HttpMultipartWriter([boundary: 'fun-w4']);
+var wb4 = w4.body();
+?. wb4 = '--fun-w4--' & CRLF;
+var wm4 = HttpMultipartParse(wb4, w4.contentType());
+?. wm4['ok'];
+?. wm4['parts'].@count() = 0;
+
+// --- writer: binary, Unicode, embedded CRLF ------------------------------
+var wbytes = '';
+for i = 0 to 255 do wbytes &= i.toChar(); end do;
+var w5 = HttpMultipartWriter([boundary: 'fun-w5']);
+w5.field('bin', wbytes);
+w5.file('blob', 'b.bin', wbytes);
+var wm5 = HttpMultipartParse(w5.body(), w5.contentType());
+?. wm5['fields']['bin'][0].length() = 256;
+?. wm5['fields']['bin'][0].toByte(0) = 0;
+?. wm5['fields']['bin'][0].toByte(255) = 255;
+?. wm5['files']['blob'][0]['data'].length() = 256;
+?. wm5['files']['blob'][0]['data'].toByte(128) = 128;
+
+var w6 = HttpMultipartWriter([boundary: 'fun-w6']);
+w6.field('名字', '张三');
+w6.file('文件', '报告.txt', '内容');
+var wm6 = HttpMultipartParse(w6.body(), w6.contentType());
+?. wm6['fields']['名字'][0] = '张三';
+?. wm6['files']['文件'][0]['filename'] = '报告.txt';
+?. wm6['files']['文件'][0]['data'] = '内容';
+
+var w7 = HttpMultipartWriter([boundary: 'fun-w7']);
+w7.field('note', 'a' & CRLF & 'b' & CRLF & '--not-the-boundary');
+w7.file('raw', 'r.txt', 'x' & CRLF & '--other' & CRLF & 'y');
+var wm7 = HttpMultipartParse(w7.body(), w7.contentType());
+?. wm7['fields']['note'][0] = 'a' & CRLF & 'b' & CRLF & '--not-the-boundary';
+?. wm7['files']['raw'][0]['data'] = 'x' & CRLF & '--other' & CRLF & 'y';
+
+// --- writer: quoted names and quoted boundary ----------------------------
+var wbs = 92.toChar();
+var wq = 34.toChar();
+var w8 = HttpMultipartWriter([boundary: 'fun-w8']);
+w8.field('a' & wq & 'b', 'v');
+w8.file('c' & wbs & 'd', 'e' & wq & 'f.txt', 'x');
+var wm8 = HttpMultipartParse(w8.body(), w8.contentType());
+?. wm8['fields']['a' & wq & 'b'][0] = 'v';
+?. wm8['files']['c' & wbs & 'd'][0]['filename'] = 'e' & wq & 'f.txt';
+
+var w9 = HttpMultipartWriter([boundary: 'a b(c)']);
+?. w9.contentType() = 'multipart/form-data; boundary="a b(c)"';
+w9.field('x', 'y');
+var wm9 = HttpMultipartParse(w9.body(), w9.contentType());
+?. wm9['ok'];
+?. wm9['fields']['x'][0] = 'y';
+
+// --- writer: default boundary and locked boundary ------------------------
+var wbp = 'multipart/form-data; boundary=';
+var w10 = HttpMultipartWriter();
+var wct10 = w10.contentType();
+?. wct10.substr(0, wbp.length()) = wbp;
+var wbnd = wct10.substr(wbp.length());
+?. wbnd.substr(0, 4) = 'fun-';
+?. wbnd.length() = 40;
+w10.field('k', 'v');
+var wb10 = w10.body();
+?. wb10.substr(0, ('--' & wbnd & CRLF).length()) = '--' & wbnd & CRLF;
+var wm10 = HttpMultipartParse(wb10, wct10);
+?. wm10['ok'];
+?. wm10['fields']['k'][0] = 'v';
+
+var w11 = HttpMultipartWriter();
+var wct11 = w11.contentType();
+w11.field('k', 'v');
+?. w11.contentType() = wct11;
+
+// A boundary-looking prefix that is not a delimiter stays data.
+var w12 = HttpMultipartWriter([boundary: 'hit']);
+w12.field('a', '--hitz' & CRLF & 'ok');
+w12.field('b', 'x' & CRLF & '--hitz' & CRLF & 'y');
+var wm12 = HttpMultipartParse(w12.body(), w12.contentType());
+?. wm12['ok'];
+?. wm12['fields']['a'][0] = '--hitz' & CRLF & 'ok';
+?. wm12['fields']['b'][0] = 'x' & CRLF & '--hitz' & CRLF & 'y';
+
+// --- writer: invalid input raises ----------------------------------------
+fun mpwRaise(fn)
+  var bad = false;
+  try
+    fn();
+  except
+    bad = true;
+  end try;
+  result = bad;
+end fun;
+
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'bad' & wq & 'b']); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'a'.x(71)]); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: '']); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([nope: 1]); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter('x'); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.field('', 'v'); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.field('a' & CRLF, 'v'); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.field('a', 1); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.file('a', 1, 'x'); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.file('a', 'b', 1); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.file('a', 'b', 'c', [contentType: 'text/plain' & CRLF]); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.file('a', 'b', 'c', [nope: 1]); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter(); wx.field('a', 'b'); wx.body(); wx.field('c', 'd'); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'hit']); wx.field('a', 'x' & CRLF & '--hit' & CRLF); wx.body(); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'hit']); wx.field('a', '--hit' & CRLF); wx.body(); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'hit']); wx.field('a', 'x' & CRLF & '--hit'); wx.body(); }));
+?. mpwRaise(((){ var wx = HttpMultipartWriter([boundary: 'hit']); wx.field('a', '--hit'); wx.body(); }));