diff --git a/src/browser/Mime.zig b/src/browser/Mime.zig
index 72268c7fb..6b5d85068 100644
--- a/src/browser/Mime.zig
+++ b/src/browser/Mime.zig
@@ -90,6 +90,70 @@ pub fn contentTypeString(mime: *const Mime) []const u8 {
     };
 }
 
+/// Parses `essence` and parameters of `Content-Type` header value.
+pub const ContentTypeIterator = struct {
+    /// Header value.
+    rest: []const u8,
+    /// Always the first.
+    essence: []const u8,
+
+    /// Initializes a `ContentTypeIterator`.
+    pub fn init(content_type: []const u8) ContentTypeIterator {
+        // Skip whitespace.
+        const trimmed = std.mem.trimStart(u8, content_type, &.{ ' ', '\t' });
+        // Find semicolon delimiter; or just use the end position.
+        const end = std.mem.indexOfScalar(u8, trimmed, ';') orelse trimmed.len;
+        const essence = std.mem.trimEnd(u8, trimmed[0..end], &.{ ' ', '\t' });
+
+        // Rest of the parameters.
+        const rest = trimmed[end..];
+        return .{ .rest = rest, .essence = essence };
+    }
+
+    pub const Parameter = struct {
+        key: []const u8,
+        /// `value` can be an empty string ("").
+        value: []const u8,
+    };
+
+    /// Returns the next parameter or null if there aren't any parameters.
+    pub fn next(self: *ContentTypeIterator) ?Parameter {
+        while (self.rest.len > 0) {
+            // `rest` always sits at the `;` that introduced this parameter.
+            var param = self.rest[1..];
+            const end = std.mem.indexOfScalar(u8, param, ';') orelse param.len;
+            self.rest = param[end..];
+            param = std.mem.trim(u8, param[0..end], " \t");
+
+            // Parameters without `=` are malformed; skip them.
+            const eq = std.mem.indexOfScalar(u8, param, '=') orelse continue;
+            const key = std.mem.trimEnd(u8, param[0..eq], " \t");
+            if (key.len == 0) {
+                continue;
+            }
+
+            var value = std.mem.trimStart(u8, param[eq + 1 ..], " \t");
+            if (value.len >= 2 and value[0] == '"' and value[value.len - 1] == '"') {
+                value = value[1 .. value.len - 1];
+            }
+            return .{ .key = key, .value = value };
+        }
+
+        return null;
+    }
+
+    /// Returns boundary string; consuming other parameters along the way.
+    /// Boundary can be an empty string.
+    pub fn findBoundary(self: *ContentTypeIterator) []const u8 {
+        while (self.next()) |p| {
+            if (std.ascii.eqlIgnoreCase(p.key, "boundary")) {
+                return p.value;
+            }
+        }
+        return "";
+    }
+};
+
 /// Returns the null-terminated charset value.
 pub fn charsetStringZ(mime: *const Mime) [:0]const u8 {
     return mime.charset[0..mime.charset_len :0];
diff --git a/src/browser/tests/net/request.html b/src/browser/tests/net/request.html
index 0bca84d45..0f78a259e 100644
--- a/src/browser/tests/net/request.html
+++ b/src/browser/tests/net/request.html
@@ -259,6 +259,104 @@
   }
 </script>
 
+<script id=form_data type=module>
+  const state = await testing.async();
+
+  // An urlencoded body parses into a FormData.
+  const req1 = new Request('https://example.com', {
+    method: 'POST',
+    body: 'a=1&b=hello+world&c=%26%3D&no_value',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+  });
+  const fd1 = await req1.formData();
+
+  // A FormData body round-trips through the multipart encoding.
+  const src = new FormData();
+  src.append('username', 'alice');
+  src.append('note', 'two\r\nlines');
+  src.append('a\tb', 'tabbed');
+  src.append('username', 'bob');
+  const req2 = new Request('https://example.com', { method: 'POST', body: src });
+  const fd2 = await req2.formData();
+
+  // A URLSearchParams body parses back (urlencoded default Content-Type).
+  const req3 = new Request('https://example.com', {
+    method: 'POST',
+    body: new URLSearchParams({ x: 'y z' }),
+  });
+  const fd3 = await req3.formData();
+
+  // A non-form Content-Type rejects with a TypeError.
+  let rejected4 = false;
+  const req4 = new Request('https://example.com', { method: 'POST', body: 'plain' });
+  try { await req4.formData(); } catch (e) { rejected4 = e instanceof TypeError; }
+
+  // So does re-consuming an already-used body.
+  let rejected5 = false;
+  try { await req1.formData(); } catch (e) { rejected5 = e instanceof TypeError; }
+
+  // A missing Content-Type rejects (never throws synchronously) with a
+  // TypeError.
+  let rejected6 = false;
+  const p6 = new Request('https://example.com', { method: 'POST' }).formData();
+  const promised6 = p6 instanceof Promise;
+  try { await p6; } catch (e) { rejected6 = e instanceof TypeError; }
+
+  // A null body with a form Content-Type acts as an empty byte sequence and
+  // resolves with an empty FormData.
+  const req7 = new Request('https://example.com', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+  });
+  const fd7 = await req7.formData();
+
+  // A multipart Content-Type without a boundary rejects with a TypeError.
+  let rejected8 = false;
+  const req8 = new Request('https://example.com', {
+    method: 'POST',
+    body: 'x',
+    headers: { 'Content-Type': 'multipart/form-data' },
+  });
+  try { await req8.formData(); } catch (e) { rejected8 = e instanceof TypeError; }
+
+  // A part with an unquoted RFC 5987 filename* parameter parses; the
+  // parameter itself is skipped.
+  const req9 = new Request('https://example.com', {
+    method: 'POST',
+    body: '--B\r\nContent-Disposition: form-data; filename*=UTF-8\'\'a.jpg; name="n"\r\n\r\nv\r\n--B--\r\n',
+    headers: { 'Content-Type': 'multipart/form-data; boundary=B' },
+  });
+  const fd9 = await req9.formData();
+
+  state.resolve();
+  await state.done(() => {
+    testing.expectEqual(true, fd1 instanceof FormData);
+    testing.expectEqual('1', fd1.get('a'));
+    testing.expectEqual('hello world', fd1.get('b'));
+    testing.expectEqual('&=', fd1.get('c'));
+    testing.expectEqual('', fd1.get('no_value'));
+
+    testing.expectEqual(true, fd2 instanceof FormData);
+    testing.expectEqual(2, fd2.getAll('username').length);
+    testing.expectEqual('alice', fd2.getAll('username')[0]);
+    testing.expectEqual('bob', fd2.getAll('username')[1]);
+    testing.expectEqual('two\r\nlines', fd2.get('note'));
+    testing.expectEqual('tabbed', fd2.get('a\tb'));
+
+    testing.expectEqual('y z', fd3.get('x'));
+
+    testing.expectEqual(true, rejected4);
+    testing.expectEqual(true, rejected5);
+
+    testing.expectEqual(true, promised6);
+    testing.expectEqual(true, rejected6);
+    testing.expectEqual(true, fd7 instanceof FormData);
+    testing.expectEqual(0, [...fd7].length);
+    testing.expectEqual(true, rejected8);
+    testing.expectEqual('v', fd9.get('n'));
+  });
+</script>
+
 <script id=priority>
   {
     new Request('https://example.com', { priority: 'high' });
diff --git a/src/browser/tests/net/response.html b/src/browser/tests/net/response.html
index eb7acf4aa..6fd33d006 100644
--- a/src/browser/tests/net/response.html
+++ b/src/browser/tests/net/response.html
@@ -86,6 +86,96 @@
   });
 </script>
 
+<script id=form_data type=module>
+  const state = await testing.async();
+
+  // An urlencoded body parses into a FormData.
+  const res1 = new Response('a=1&b=hello+world&c=%26%3D&no_value', {
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+  });
+  const fd1 = await res1.formData();
+
+  // A FormData body round-trips through the multipart encoding.
+  const src = new FormData();
+  src.append('username', 'alice');
+  src.append('note', 'two\r\nlines');
+  src.append('a\tb', 'tabbed');
+  src.append('username', 'bob');
+  const res2 = new Response(src);
+  const fd2 = await res2.formData();
+
+  // A URLSearchParams body parses back (urlencoded default Content-Type).
+  const res3 = new Response(new URLSearchParams({ x: 'y z' }));
+  const fd3 = await res3.formData();
+
+  // A non-form Content-Type rejects with a TypeError.
+  let rejected4 = false;
+  const res4 = new Response('plain');
+  try { await res4.formData(); } catch (e) { rejected4 = e instanceof TypeError; }
+
+  // So does re-consuming an already-used body.
+  let rejected5 = false;
+  try { await res1.formData(); } catch (e) { rejected5 = e instanceof TypeError; }
+
+  // A bodyless Response has no Content-Type: formData() rejects (never
+  // throws synchronously) with a TypeError.
+  let rejected6 = false;
+  const p6 = new Response().formData();
+  const promised6 = p6 instanceof Promise;
+  try { await p6; } catch (e) { rejected6 = e instanceof TypeError; }
+
+  // A null body with a form Content-Type acts as an empty byte sequence and
+  // resolves with an empty FormData.
+  const res7 = new Response(null, {
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+  });
+  const fd7 = await res7.formData();
+
+  // A multipart Content-Type without a boundary rejects with a TypeError.
+  let rejected8 = false;
+  const res8 = new Response('x', {
+    headers: { 'Content-Type': 'multipart/form-data' },
+  });
+  try { await res8.formData(); } catch (e) { rejected8 = e instanceof TypeError; }
+
+  // A part with an unquoted RFC 5987 filename* parameter — which real
+  // servers emit — parses; the parameter itself is skipped.
+  const res9 = new Response(
+    '--B\r\nContent-Disposition: form-data; filename*=UTF-8\'\'a.jpg; name="n"\r\n\r\nv\r\n--B--\r\n',
+    { headers: { 'Content-Type': 'multipart/form-data; boundary=B' } },
+  );
+  const fd9 = await res9.formData();
+
+  state.resolve();
+  await state.done(() => {
+    testing.expectEqual(true, fd1 instanceof FormData);
+    testing.expectEqual('1', fd1.get('a'));
+    testing.expectEqual('hello world', fd1.get('b'));
+    testing.expectEqual('&=', fd1.get('c'));
+    testing.expectEqual('', fd1.get('no_value'));
+
+    testing.expectEqual(true, fd2 instanceof FormData);
+    testing.expectEqual(2, fd2.getAll('username').length);
+    testing.expectEqual('alice', fd2.getAll('username')[0]);
+    testing.expectEqual('bob', fd2.getAll('username')[1]);
+    testing.expectEqual('two\r\nlines', fd2.get('note'));
+    testing.expectEqual('tabbed', fd2.get('a\tb'));
+
+    testing.expectEqual('y z', fd3.get('x'));
+
+    testing.expectEqual(true, rejected4);
+    testing.expectEqual(true, rejected5);
+
+    testing.expectEqual(true, promised6);
+    testing.expectEqual(true, rejected6);
+
+    testing.expectEqual(true, fd7 instanceof FormData);
+    testing.expectEqual(0, [...fd7].length);
+    testing.expectEqual(true, rejected8);
+    testing.expectEqual('v', fd9.get('n'));
+  });
+</script>
+
 <script id=clone type=module>
   const state = await testing.async();
 
diff --git a/src/browser/webapi/net/FormData.zig b/src/browser/webapi/net/FormData.zig
index 6335ba068..04ba28d2d 100644
--- a/src/browser/webapi/net/FormData.zig
+++ b/src/browser/webapi/net/FormData.zig
@@ -1,4 +1,4 @@
-// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+// Copyright (C) 2023-2026 Lightpanda (Selecy SAS)
 //
 // Francis Bouvier <francis@lightpanda.io>
 // Pierre Tachoire <pierre@lightpanda.io>
@@ -24,9 +24,11 @@ const Page = @import("../../Page.zig");
 const Frame = @import("../../Frame.zig");
 const Form = @import("../element/html/Form.zig");
 const Element = @import("../Element.zig");
-const File = @import("../File.zig");
 const Blob = @import("../Blob.zig");
+const File = @import("../File.zig");
+
 const KeyValueList = @import("../KeyValueList.zig");
+const header_parser = @import("../../../network/header_parser.zig");
 
 const log = lp.log;
 const String = lp.String;
@@ -119,6 +121,46 @@ pub fn init(form_: ?*Form, submitter: ?*Element, exec: *const Execution) !*FormD
     return form_data;
 }
 
+// Fetch §6.4 "package data" with type FormData: parse an
+// application/x-www-form-urlencoded body back into a FormData.
+pub fn initFromUrlEncoded(bytes: []const u8, exec: *const Execution) !*FormData {
+    const arena = try exec.getArena(.small, "FormData");
+    errdefer exec.releaseArena(arena);
+
+    const form_data = try arena.create(FormData);
+    form_data.* = .{
+        ._rc = .{},
+        ._arena = arena,
+        ._entries = .empty,
+    };
+    try form_data.parseUrlEncoded(bytes);
+    return form_data;
+}
+
+// Fetch §6.4 "package data" with type FormData: parse a multipart/form-data
+// body back into a FormData. `boundary` is the Content-Type boundary param.
+pub fn initFromMultipart(bytes: []const u8, boundary: []const u8, exec: *const Execution) !*FormData {
+    const arena = try exec.getArena(.small, "FormData");
+    errdefer exec.releaseArena(arena);
+
+    const form_data = try arena.create(FormData);
+    form_data.* = .{
+        ._rc = .{},
+        ._arena = arena,
+        ._entries = .empty,
+    };
+
+    // On failure, drop the refs parseMultipart acquired on the file entries
+    // appended so far (runs before the arena release above frees the list).
+    errdefer for (form_data._entries.items) |entry| switch (entry.value) {
+        .file => |file| file.releaseRef(exec.page),
+        else => {},
+    };
+
+    try form_data.parseMultipart(exec.page, bytes, boundary);
+    return form_data;
+}
+
 pub fn deinit(self: *FormData, page: *Page) void {
     for (self._entries.items) |entry| {
         switch (entry.value) {
@@ -388,6 +430,255 @@ fn writeMultipartName(writer: *std.Io.Writer, name: []const u8) !void {
     }
 }
 
+// Inverse of urlEncode: application/x-www-form-urlencoded parsing per
+// URL §5.1 — '+' decodes to a space, invalid percent sequences pass through
+// verbatim, and a pair without '=' becomes an entry with an empty value.
+pub fn parseUrlEncoded(self: *FormData, bytes: []const u8) !void {
+    var it = std.mem.splitScalar(u8, bytes, '&');
+    while (it.next()) |pair| {
+        if (pair.len == 0) {
+            continue;
+        }
+        if (std.mem.indexOfScalar(u8, pair, '=')) |idx| {
+            try self.appendText(
+                try urlDecode(self._arena, pair[0..idx]),
+                try urlDecode(self._arena, pair[idx + 1 ..]),
+            );
+        } else {
+            const key = try urlDecode(self._arena, pair);
+            // Insert with empty value.
+            try self.appendText(key, "");
+        }
+    }
+}
+
+/// Index of the first byte needing URL-decoding ('%' or '+'), or null if
+/// the slice needs no decoding at all.
+fn indexOfSpecial(slice: []const u8) ?usize {
+    const vector_len = std.simd.suggestVectorLength(u8) orelse {
+        // Non-SIMD path.
+        return std.mem.indexOfAnyPos(u8, slice, 0, "%+");
+    };
+    const Vector = @Vector(vector_len, u8);
+
+    var end: usize = 0;
+    while (end + vector_len <= slice.len) : (end += vector_len) {
+        const percent: Vector = @splat('%');
+        const plus: Vector = @splat('+');
+        const chunk: Vector = slice[end..][0..vector_len].*;
+
+        const mask = @intFromBool(chunk == percent) | @intFromBool(chunk == plus);
+        const mask_int = @as(std.meta.Int(.unsigned, vector_len), @bitCast(mask));
+
+        if (mask_int != 0) {
+            return end + @ctz(mask_int);
+        }
+    }
+
+    return std.mem.indexOfAnyPos(u8, slice, end, "%+");
+}
+
+/// URL-decodes passed `raw` slice; returned value may or may not be heap allocated.
+/// TODO: This can be more efficient I believe.
+fn urlDecode(arena: Allocator, raw: []const u8) ![]const u8 {
+    // Get where to start decoding.
+    var i: usize = indexOfSpecial(raw) orelse return raw;
+
+    var out: std.ArrayList(u8) = try .initCapacity(arena, raw.len);
+    out.appendSliceAssumeCapacity(raw[0..i]);
+
+    while (i < raw.len) {
+        const c = raw[i];
+        switch (c) {
+            '+' => {
+                out.appendAssumeCapacity(' ');
+                i += 1;
+            },
+            '%' => {
+                // Per URL §5.1 percent-decode, an invalid or truncated
+                // escape sequence passes through verbatim.
+                const decoded: ?u8 = blk: {
+                    if (i + 2 >= raw.len) break :blk null;
+                    const hi = std.fmt.charToDigit(raw[i + 1], 16) catch break :blk null;
+                    const lo = std.fmt.charToDigit(raw[i + 2], 16) catch break :blk null;
+                    break :blk hi * 16 + lo;
+                };
+                if (decoded) |b| {
+                    out.appendAssumeCapacity(b);
+                    i += 3;
+                } else {
+                    out.appendAssumeCapacity('%');
+                    i += 1;
+                }
+            },
+            else => {
+                out.appendAssumeCapacity(c);
+                i += 1;
+            },
+        }
+    }
+
+    return out.items;
+}
+
+// Inverse of multipartEncode: strict multipart/form-data parsing (no
+// preamble, CRLF line breaks). Parts carrying a filename become File
+// entries — the FormData holds a ref on each, released in deinit — and the
+// rest become string entries.
+fn parseMultipart(self: *FormData, page: *Page, bytes: []const u8, boundary: []const u8) !void {
+    // The body must open with the dash-boundary: "--" boundary.
+    if (!std.mem.startsWith(u8, bytes, "--") or !std.mem.startsWith(u8, bytes[2..], boundary)) {
+        return error.InvalidFormData;
+    }
+    // Skip-past boundary.
+    const slice = bytes[2 + boundary.len ..];
+
+    var cursor = header_parser.Cursor{
+        .idx = slice.ptr,
+        .end = slice.ptr + slice.len,
+        .start = slice.ptr,
+    };
+
+    while (true) {
+        if (!cursor.hasLength(2)) {
+            return error.InvalidFormData;
+        }
+        // Check if we've reached the end.
+        if (cursor.peek2('-', '-')) {
+            return;
+        }
+        // If we haven't reached the end, CRLF is required.
+        if (!cursor.peek2('\r', '\n')) {
+            return error.InvalidFormData;
+        }
+        // Consume prefix.
+        cursor.advance(2);
+
+        // Content-Disposition can appear once a part, and is required.
+        var disposition: ?header_parser.Disposition = null;
+        // Default Content-Type; can be overwritten while parsing headers.
+        var content_type: []const u8 = "text/plain";
+        // Reused for parsing headers.
+        var header: header_parser.Header = undefined;
+
+        // Parse the whole header block; the content starts only after the
+        // terminating empty line.
+        while (true) {
+            if (cursor.reachedEnd()) {
+                return error.InvalidFormData;
+            }
+
+            // Check if headers part has finished.
+            switch (cursor.char()) {
+                '\n' => {
+                    // End of headers.
+                    cursor.advance(1);
+                    break;
+                },
+                '\r' => {
+                    // We need an LF too.
+                    if (!cursor.hasLength(2) or !cursor.peek2('\r', '\n')) {
+                        return error.InvalidFormData;
+                    }
+                    // End of headers.
+                    cursor.advance(2);
+                    break;
+                },
+                else => {},
+            }
+
+            // Parse an HTTP header.
+            try header.parse(&cursor);
+
+            if (std.ascii.eqlIgnoreCase(header.key, "content-type")) {
+                content_type = header.value;
+            } else if (std.ascii.eqlIgnoreCase(header.key, "content-disposition")) {
+                if (disposition != null) {
+                    // Content-Disposition found twice, report an error.
+                    return error.InvalidFormData;
+                }
+                disposition = try header_parser.parseDisposition(header.value);
+            }
+        }
+
+        const parsed = disposition orelse return error.InvalidFormData;
+        const name = try decodeMultipartName(self._arena, parsed.name orelse return error.InvalidFormData);
+
+        const content_end = indexOfBoundary(cursor.remaining(), boundary) orelse return error.InvalidFormData;
+        const content = cursor.remaining()[0..content_end];
+        cursor.advance(content_end + "\r\n--".len + boundary.len);
+
+        // Got a file.
+        if (parsed.filename) |filename| {
+            const blob = try Blob.initFromBytes(content, content_type, page);
+            errdefer blob.deinit(page);
+
+            const file = try blob._arena.create(File);
+            file.* = .{
+                ._proto = blob,
+                ._name = try blob._arena.dupe(u8, try decodeMultipartName(self._arena, filename)),
+                ._last_modified = @intCast(lp.datetime.milliTimestamp(.real)),
+            };
+            blob._type = .{ .file = file };
+
+            file.acquireRef();
+            try self._entries.append(self._arena, .{
+                .name = try String.init(self._arena, name, .{}),
+                .value = .{ .file = file },
+            });
+        } else {
+            try self.appendText(name, content);
+        }
+    }
+}
+
+// Finds the "\r\n--" ++ boundary delimiter in haystack without materializing
+// the needle. Per RFC 2046 §5.1.1 the delimiter must be followed by CRLF (or
+// "--" for the close delimiter), so a value containing the boundary as a
+// prefix of longer text does not terminate the part.
+fn indexOfBoundary(haystack: []const u8, boundary: []const u8) ?usize {
+    var start: usize = 0;
+    while (std.mem.indexOfPos(u8, haystack, start, "\r\n--")) |i| {
+        const rest = haystack[i + 4 ..];
+        if (std.mem.startsWith(u8, rest, boundary)) {
+            const after = rest[boundary.len..];
+            if (std.mem.startsWith(u8, after, "\r\n") or std.mem.startsWith(u8, after, "--")) {
+                return i;
+            }
+        }
+        start = i + 1;
+    }
+    return null;
+}
+
+// "Parse a multipart/form-data name": undo writeMultipartName's escapes
+// (%0A, %0D, %22); any other percent sequence passes through verbatim.
+fn decodeMultipartName(arena: Allocator, raw: []const u8) ![]const u8 {
+    if (std.mem.indexOfScalar(u8, raw, '%') == null) {
+        return raw;
+    }
+
+    var out: std.ArrayList(u8) = try .initCapacity(arena, raw.len);
+    var i: usize = 0;
+    while (i < raw.len) {
+        const rest = raw[i..];
+        if (std.mem.startsWith(u8, rest, "%22")) {
+            out.appendAssumeCapacity('"');
+            i += 3;
+        } else if (std.mem.startsWith(u8, rest, "%0D")) {
+            out.appendAssumeCapacity('\r');
+            i += 3;
+        } else if (std.mem.startsWith(u8, rest, "%0A")) {
+            out.appendAssumeCapacity('\n');
+            i += 3;
+        } else {
+            out.appendAssumeCapacity(raw[i]);
+            i += 1;
+        }
+    }
+    return out.items;
+}
+
 // Used by URLSearchParams to ingest a FormData; file entries collapse via Value.asString.
 pub fn toKeyValueList(self: *const FormData, arena: Allocator) !KeyValueList {
     var list: KeyValueList = .empty;
@@ -879,3 +1170,172 @@ test "FormData: plaintext empty body" {
 
     try testing.expectString("", buf.written());
 }
+
+test "FormData: urlencoded parse" {
+    const allocator = testing.arena_allocator;
+
+    var fd = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    try fd.parseUrlEncoded("a=1&b=hello+world&c=%26%3D&no_value&&bad=100%zz");
+
+    try testing.expectEqual(5, fd._entries.items.len);
+    try testing.expectString("1", fd.get(.wrap("a")).?);
+    try testing.expectString("hello world", fd.get(.wrap("b")).?);
+    try testing.expectString("&=", fd.get(.wrap("c")).?);
+    try testing.expectString("", fd.get(.wrap("no_value")).?);
+    // An invalid percent sequence passes through verbatim.
+    try testing.expectString("100%zz", fd.get(.wrap("bad")).?);
+}
+
+test "FormData: urlencoded parse exercises the vectorized guard" {
+    const allocator = testing.arena_allocator;
+
+    var fd = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    // Values longer than any SIMD vector length, with the lone special
+    // character early so only the vectorized loop (not the scalar tail)
+    // can spot it — a regression guard for needsDecoding's chunk mask.
+    try fd.parseUrlEncoded("plus=aaa+aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" ++
+        "&pct=aaa%41aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" ++
+        "&clean=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+
+    try testing.expectString("aaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", fd.get(.wrap("plus")).?);
+    try testing.expectString("aaaAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", fd.get(.wrap("pct")).?);
+    try testing.expectString("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", fd.get(.wrap("clean")).?);
+}
+
+test "FormData: multipart parse" {
+    const allocator = testing.arena_allocator;
+    const frame = try testing.createFrame();
+    defer testing.test_session.closeAllPages();
+
+    var fd = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    try fd.parseMultipart(frame._page, "--BOUNDARY\r\n" ++
+        "Content-Disposition: form-data; name=\"name\"\r\n\r\n" ++
+        "John\r\n" ++
+        "--BOUNDARY\r\n" ++
+        "Content-Disposition: form-data; name=\"a%22b%0D%0Ac\"\r\n\r\n" ++
+        "two\r\nlines\r\n" ++
+        "--BOUNDARY\r\n" ++
+        "Content-Disposition: form-data; name=\"tricky\"\r\n\r\n" ++
+        "a\r\n--BOUNDARYx b\r\n" ++
+        "--BOUNDARY--\r\n", "BOUNDARY");
+
+    try testing.expectEqual(3, fd._entries.items.len);
+    try testing.expectString("John", fd.get(.wrap("name")).?);
+    // Escaped name decodes, and a value containing CRLF survives.
+    try testing.expectString("a\"b\r\nc", fd._entries.items[1].name.str());
+    try testing.expectString("two\r\nlines", fd._entries.items[1].value.asString());
+    // The boundary as a prefix of longer text is not a delimiter.
+    try testing.expectString("a\r\n--BOUNDARYx b", fd.get(.wrap("tricky")).?);
+}
+
+test "FormData: multipart parse with file" {
+    const allocator = testing.arena_allocator;
+    const frame = try testing.createFrame();
+    defer testing.test_session.closeAllPages();
+
+    var fd = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    try fd.parseMultipart(frame._page, "--B\r\n" ++
+        "Content-Disposition: form-data; name=\"upload\"; filename=\"hello.txt\"\r\n" ++
+        "Content-Type: text/plain\r\n\r\n" ++
+        "hello\r\n" ++
+        "--B\r\n" ++
+        "Content-Disposition: form-data; name=\"raw\"; filename=\"raw.bin\"\r\n\r\n" ++
+        "bytes\r\n" ++
+        "--B--\r\n", "B");
+    defer for (fd._entries.items) |entry| switch (entry.value) {
+        .file => |file| file.releaseRef(frame._page),
+        else => {},
+    };
+
+    try testing.expectEqual(2, fd._entries.items.len);
+
+    const file = fd._entries.items[0].value.file;
+    try testing.expectString("upload", fd._entries.items[0].name.str());
+    try testing.expectString("hello.txt", file.getName());
+    try testing.expectString("hello", file._proto._slice);
+    try testing.expectString("text/plain", file._proto._mime);
+
+    // A file part without a Content-Type header defaults to text/plain.
+    const raw = fd._entries.items[1].value.file;
+    try testing.expectString("raw.bin", raw.getName());
+    try testing.expectString("bytes", raw._proto._slice);
+    try testing.expectString("text/plain", raw._proto._mime);
+}
+
+test "FormData: multipart parse rejects malformed bodies" {
+    const allocator = testing.arena_allocator;
+    const frame = try testing.createFrame();
+    defer testing.test_session.closeAllPages();
+
+    const cases = [_][]const u8{
+        "", // no dash-boundary
+        "--OTHER\r\n", // wrong boundary
+        "--B\r\nContent-Disposition: form-data; name=\"a\"\r\n\r\nv\r\n", // unterminated
+        "--B\r\n\r\nv\r\n--B--\r\n", // no Content-Disposition
+        "--B\r\nContent-Disposition: inline; name=\"a\"\r\n\r\nv\r\n--B--\r\n", // not form-data
+        "--B\r\nContent-Disposition: form-data\r\n\r\nv\r\n--B--\r\n", // no name
+    };
+    for (cases) |case| {
+        var fd = FormData{
+            ._rc = .{},
+            ._arena = allocator,
+            ._entries = .empty,
+        };
+        try testing.expectError(error.InvalidFormData, fd.parseMultipart(frame._page, case, "B"));
+    }
+}
+
+test "FormData: multipart round-trip" {
+    const allocator = testing.arena_allocator;
+    const frame = try testing.createFrame();
+    defer testing.test_session.closeAllPages();
+
+    var src = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    try src.appendText("username", "alice");
+    try src.appendText("username", "bob");
+    try src.appendText("a\"b\r\nc", "quoted \"value\"");
+    // HTAB in a name survives the trip: the encoder writes it raw into the
+    // Content-Disposition value and the parser accepts it (RFC 9110).
+    try src.appendText("a\tb", "tabbed");
+
+    var buf = std.Io.Writer.Allocating.init(allocator);
+    try src.write(.{
+        .encoding = .{ .formdata = "BOUNDARY" },
+        .allocator = allocator,
+    }, &buf.writer);
+
+    var fd = FormData{
+        ._rc = .{},
+        ._arena = allocator,
+        ._entries = .empty,
+    };
+    try fd.parseMultipart(frame._page, buf.written(), "BOUNDARY");
+
+    try testing.expectEqual(4, fd._entries.items.len);
+    try testing.expectString("username", fd._entries.items[0].name.str());
+    try testing.expectString("alice", fd._entries.items[0].value.asString());
+    try testing.expectString("username", fd._entries.items[1].name.str());
+    try testing.expectString("bob", fd._entries.items[1].value.asString());
+    try testing.expectString("quoted \"value\"", fd.get(.wrap("a\"b\r\nc")).?);
+    try testing.expectString("tabbed", fd.get(.wrap("a\tb")).?);
+}
diff --git a/src/browser/webapi/net/Request.zig b/src/browser/webapi/net/Request.zig
index b9b28c31e..36e713a02 100644
--- a/src/browser/webapi/net/Request.zig
+++ b/src/browser/webapi/net/Request.zig
@@ -1,4 +1,4 @@
-// Copyright (C) 2023-2025  Lightpanda (Selecy SAS)
+// Copyright (C) 2023-2026 Lightpanda (Selecy SAS)
 //
 // Francis Bouvier <francis@lightpanda.io>
 // Pierre Tachoire <pierre@lightpanda.io>
@@ -26,8 +26,10 @@ const URL = @import("../URL.zig");
 const Page = @import("../../Page.zig");
 const Blob = @import("../Blob.zig");
 const AbortSignal = @import("../AbortSignal.zig");
+const ContentTypeIterator = @import("../../Mime.zig").ContentTypeIterator;
 
 const Headers = @import("Headers.zig");
+const FormData = @import("FormData.zig");
 const body_init = @import("body_init.zig");
 const BodyInit = body_init.BodyInit;
 
@@ -298,6 +300,49 @@ pub fn bytes(self: *Request, exec: *const Execution) !js.Promise {
     return local.resolvePromise(js.TypedArray(u8){ .values = self._body orelse "" });
 }
 
+pub fn formData(self: *Request, exec: *const Execution) !js.Promise {
+    const local = exec.js.local.?;
+    if (self.consume(local)) |rejected| {
+        return rejected;
+    }
+
+    // Per Fetch, a null body acts as an empty byte sequence.
+    const body = self._body orelse "";
+
+    const headers = try self.getHeaders(exec);
+    const content_type = try headers.get("content-type", exec) orelse {
+        return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+    };
+    var it = ContentTypeIterator.init(content_type);
+    const essence = it.essence;
+
+    // [RFC7578]
+    // Parse bytes, using the value of the `boundary` parameter from mimeType,
+    // per the rules set forth in Returning Values from Forms: multipart/form-data.
+    if (std.ascii.eqlIgnoreCase(essence, "multipart/form-data")) {
+        const boundary = it.findBoundary();
+        if (boundary.len == 0) {
+            return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+        }
+
+        const form_data = FormData.initFromMultipart(body, boundary, exec) catch |err| switch (err) {
+            error.OutOfMemory => return err,
+            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+        };
+        return local.resolvePromise(form_data);
+    }
+
+    if (std.ascii.eqlIgnoreCase(essence, "application/x-www-form-urlencoded")) {
+        const form_data = FormData.initFromUrlEncoded(body, exec) catch |err| switch (err) {
+            error.OutOfMemory => return err,
+            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+        };
+        return local.resolvePromise(form_data);
+    }
+
+    return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+}
+
 pub fn clone(self: *const Request, exec: *const Execution) !*Request {
     const arena = try exec.getArena(if (self._body) |b| b.len else 512, "Request.clone");
     errdefer exec.releaseArena(arena);
@@ -340,6 +385,7 @@ pub const JsApi = struct {
     pub const json = bridge.function(Request.json, .{});
     pub const arrayBuffer = bridge.function(Request.arrayBuffer, .{});
     pub const bytes = bridge.function(Request.bytes, .{});
+    pub const formData = bridge.function(Request.formData, .{});
     pub const clone = bridge.function(Request.clone, .{});
 };
 
diff --git a/src/browser/webapi/net/Response.zig b/src/browser/webapi/net/Response.zig
index 06efeac55..55858fbf7 100644
--- a/src/browser/webapi/net/Response.zig
+++ b/src/browser/webapi/net/Response.zig
@@ -1,4 +1,4 @@
-// Copyright (C) 2023-2026  Lightpanda (Selecy SAS)
+// Copyright (C) 2023-2026 Lightpanda (Selecy SAS)
 //
 // Francis Bouvier <francis@lightpanda.io>
 // Pierre Tachoire <pierre@lightpanda.io>
@@ -26,10 +26,13 @@ const Transfer = @import("../../../network/HttpClient.zig").Transfer;
 
 const Blob = @import("../Blob.zig");
 const ReadableStream = @import("../streams/ReadableStream.zig");
+const FormData = @import("FormData.zig");
 
 const Headers = @import("Headers.zig");
 const body_init = @import("body_init.zig");
 
+const ContentTypeIterator = @import("../../Mime.zig").ContentTypeIterator;
+
 const Execution = js.Execution;
 const Allocator = std.mem.Allocator;
 
@@ -456,6 +459,48 @@ pub fn bytes(self: *Response, exec: *const Execution) !js.Promise {
     return local.resolvePromise(js.TypedArray(u8){ .values = body });
 }
 
+pub fn formData(self: *Response, exec: *const Execution) !js.Promise {
+    const local = exec.js.local.?;
+    if (self.consume(local)) |rejected| return rejected;
+    const body = switch (self._body) {
+        .bytes => |b| b,
+        .empty => "",
+        .stream => return local.rejectPromise(.{ .type_error = "Cannot read FormData from stream body" }),
+    };
+
+    const content_type = try self._headers.get("content-type", exec) orelse {
+        return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+    };
+    var it = ContentTypeIterator.init(content_type);
+    const essence = it.essence;
+
+    // [RFC7578]
+    // Parse bytes, using the value of the `boundary` parameter from mimeType,
+    // per the rules set forth in Returning Values from Forms: multipart/form-data.
+    if (std.ascii.eqlIgnoreCase(essence, "multipart/form-data")) {
+        const boundary = it.findBoundary();
+        if (boundary.len == 0) {
+            return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+        }
+
+        const form_data = FormData.initFromMultipart(body, boundary, exec) catch |err| switch (err) {
+            error.OutOfMemory => return err,
+            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+        };
+        return local.resolvePromise(form_data);
+    }
+
+    if (std.ascii.eqlIgnoreCase(essence, "application/x-www-form-urlencoded")) {
+        const form_data = FormData.initFromUrlEncoded(body, exec) catch |err| switch (err) {
+            error.OutOfMemory => return err,
+            else => return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" }),
+        };
+        return local.resolvePromise(form_data);
+    }
+
+    return local.rejectPromise(.{ .type_error = "Failed to parse body as FormData" });
+}
+
 pub fn clone(self: *const Response, exec: *const Execution) !*Response {
     const session = exec.session;
     const body_len = switch (self._body) {
@@ -519,6 +564,7 @@ pub const JsApi = struct {
     pub const arrayBuffer = bridge.function(Response.arrayBuffer, .{});
     pub const blob = bridge.function(Response.blob, .{});
     pub const bytes = bridge.function(Response.bytes, .{});
+    pub const formData = bridge.function(Response.formData, .{});
     pub const clone = bridge.function(Response.clone, .{});
 };
 
diff --git a/src/network/header_parser.zig b/src/network/header_parser.zig
new file mode 100644
index 000000000..54e672d03
--- /dev/null
+++ b/src/network/header_parser.zig
@@ -0,0 +1,557 @@
+// Copyright (C) 2023-2026 Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+const std = @import("std");
+const builtin = @import("builtin");
+
+/// Block size of the CPU.
+const block_size = @sizeOf(usize);
+
+/// Whether a byte is allowed in a header key; controls (including space),
+/// the `:` delimiter and DEL are excluded.
+fn isHeaderKeyByte(byte: u8) bool {
+    return switch (byte) {
+        0...' ', ':', 0x7f => false,
+        else => true,
+    };
+}
+
+/// Whether a byte is allowed in a header value; controls other than HTAB,
+/// and DEL, are excluded. HTAB is legal field content per RFC 9110.
+fn isHeaderValueByte(byte: u8) bool {
+    return switch (byte) {
+        0...0x08, 0x0a...0x1f, 0x7f => false,
+        else => true,
+    };
+}
+
+/// Returns a mask with the high bit of each byte set where the corresponding
+/// byte of `v` is zero. Exact per byte: carries never cross byte lanes
+/// (Hacker's Delight zero-byte detection).
+fn zeroBytes(v: usize) usize {
+    const low7 = comptime broadcast(0x7f);
+    return ~(((v & low7) + low7) | v | low7);
+}
+
+/// Returns an integer filled with a given byte.
+fn broadcast(byte: u8) usize {
+    return @as(usize, @intCast(byte)) * 0x01_01_01_01_01_01_01_01;
+}
+
+fn matchHeaderKey(cursor: *Cursor) void {
+    // Pick a good default for relatively small strings.
+    const vec_size = 16;
+    const use_vectors = comptime if (std.simd.suggestVectorLength(u8)) |recommended|
+        recommended >= vec_size
+    else
+        false;
+
+    if (comptime use_vectors) {
+        const Vec = @Vector(vec_size, u8);
+        const Int = @Int(.unsigned, vec_size);
+
+        while (cursor.hasLength(vec_size)) {
+            const spaces: Vec = @splat(' ');
+            const colons: Vec = @splat(':');
+            const deletes: Vec = @splat(0x7f);
+            const chunk = cursor.asVector(vec_size);
+
+            // Per-lane `isHeaderKeyByte`: a lane is 1 when its byte is valid,
+            // above space (which also rules out the controls) and neither the
+            // `:` delimiter nor DEL. Inverting the mask makes invalid lanes
+            // 1s, so `@ctz` yields the index of the first invalid byte, or
+            // `vec_size` when the whole chunk is valid.
+            const mask = @intFromBool(chunk > spaces) & ~(@intFromBool(chunk == colons) | @intFromBool(chunk == deletes));
+            const advance_by = @ctz(~@as(Int, @bitCast(mask)));
+            // Advance.
+            cursor.advance(advance_by);
+            if (advance_by != vec_size) {
+                return;
+            }
+        }
+    }
+
+    // NOTE: SWAR is not preferred here, this might change in the future
+    // but honestly header keys are not so long.
+
+    // Fallback for len < `vec_size`.
+    while (cursor.end - cursor.idx > 0) : (cursor.advance(1)) {
+        if (isHeaderKeyByte(cursor.char()) == false) {
+            return;
+        }
+    }
+}
+
+fn matchHeaderValue(cursor: *Cursor) void {
+    const maybe_vec_size: ?usize = comptime blk: {
+        if (std.simd.suggestVectorLength(u8)) |recommended| {
+            break :blk if (recommended >= 64) 32 else recommended;
+        }
+        break :blk null;
+    };
+
+    if (comptime maybe_vec_size) |vec_size| {
+        const Vec = @Vector(vec_size, u8);
+        const Int = @Int(.unsigned, vec_size);
+
+        while (cursor.hasLength(vec_size)) {
+            // Fill a vector with DEL (127).
+            const deletes: Vec = @splat(0x7f);
+            // Fill a vector with US (31).
+            const full_31: Vec = @splat(0x1f);
+            // Fill a vector with HTAB (9).
+            const tabs: Vec = @splat('\t');
+            // Load the next chunk from the buffer.
+            const chunk = cursor.asVector(vec_size);
+
+            // Per-lane `isHeaderValueByte`: a lane is 1 when its byte is
+            // valid, above US (which rules out the controls but keeps space)
+            // or an HTAB, and not DEL. Inverting the mask makes invalid lanes
+            // 1s, so `@ctz` yields the index of the first invalid byte, or
+            // `vec_size` when the whole chunk is valid.
+            const mask = (@intFromBool(chunk > full_31) | @intFromBool(chunk == tabs)) & ~@intFromBool(chunk == deletes);
+            const advance_by = @ctz(~@as(Int, @bitCast(mask)));
+            cursor.advance(advance_by);
+            if (advance_by != vec_size) {
+                return;
+            }
+        }
+    }
+
+    // SWAR path.
+    while (cursor.hasLength(block_size)) {
+        const tabs = comptime broadcast('\t');
+        const dels = comptime broadcast(0x7f);
+        // A byte is below space (32) exactly when its top three bits are
+        // all zero.
+        const high3 = comptime broadcast(0xe0);
+        const chunk = cursor.asInteger(usize);
+
+        const is_ctl = zeroBytes(chunk & high3);
+        const is_tab = zeroBytes(chunk ^ tabs);
+        const is_del = zeroBytes(chunk ^ dels);
+        const advance_by = @ctz((is_ctl & ~is_tab) | is_del) >> 3;
+
+        cursor.advance(advance_by);
+        if (advance_by != block_size) {
+            return;
+        }
+    }
+
+    while (cursor.end - cursor.idx > 0) : (cursor.advance(1)) {
+        if (isHeaderValueByte(cursor.char()) == false) {
+            return;
+        }
+    }
+}
+
+/// Represents a single HTTP header.
+pub const Header = struct {
+    key: []const u8,
+    value: []const u8,
+
+    pub const ParseError = error{ Incomplete, Invalid };
+
+    pub fn parse(header: *Header, cursor: *Cursor) ParseError!void {
+        const key_start = cursor.current();
+        matchHeaderKey(cursor);
+        const key_end = cursor.current();
+
+        // Buffer has been consumed fully without a delimiter; the caller can
+        // read more data and try to parse again. Checked before `char`, which
+        // reads unchecked.
+        if (cursor.reachedEnd()) {
+            return error.Incomplete;
+        }
+
+        switch (cursor.char()) {
+            ':' => {
+                @branchHint(.likely);
+                // 0 length header key, which is invalid.
+                if (key_end == key_start) {
+                    return error.Invalid;
+                }
+                cursor.advance(1);
+            },
+            // Invalid character, so a malformed header. Can't go further.
+            else => return error.Invalid,
+        }
+
+        // Get rid of leading spaces if there are any.
+        cursor.skipSpaces();
+
+        // Found where header value starts.
+        const val_start = cursor.current();
+        matchHeaderValue(cursor);
+        const val_end = cursor.current();
+
+        // Buffer has been consumed fully without a line ending; the caller
+        // can read more data and try to parse again.
+        if (cursor.reachedEnd()) {
+            return error.Incomplete;
+        }
+
+        // Both LF and CRLF indicate the end of the value part.
+        switch (cursor.char()) {
+            '\n' => cursor.advance(1),
+            '\r' => {
+                // We need an LF too.
+                if (!cursor.hasLength(2)) {
+                    return error.Incomplete;
+                }
+                if (!cursor.peek2('\r', '\n')) {
+                    @branchHint(.unlikely);
+                    return error.Invalid;
+                }
+                cursor.advance(2);
+            },
+            // Any other character is invalid.
+            else => return error.Invalid,
+        }
+
+        header.* = .{
+            .key = key_start[0 .. key_end - key_start],
+            .value = val_start[0 .. val_end - val_start],
+        };
+    }
+};
+
+pub const Disposition = struct {
+    name: ?[]const u8 = null,
+    filename: ?[]const u8 = null,
+};
+
+// Parses a part's Content-Disposition value: `form-data` followed by
+// `; key="value"` params. Values are quoted-strings whose CR/LF/" were
+// percent-escaped by the encoder (writeMultipartName), so the next raw '"'
+// always closes the value; a raw ';' inside quotes is legal and preserved.
+// Parameters that are not `key="quoted"` pairs — e.g. RFC 5987
+// `filename*=UTF-8''...`, which real servers emit — are skipped rather than
+// failing the whole parse, matching browser leniency.
+pub fn parseDisposition(value: []const u8) !Disposition {
+    var rest = std.mem.trim(u8, value, " \t");
+    if (!std.ascii.startsWithIgnoreCase(rest, "form-data")) {
+        return error.InvalidFormData;
+    }
+    rest = rest["form-data".len..];
+
+    var disposition = Disposition{};
+    while (true) {
+        rest = std.mem.trimStart(u8, rest, " \t");
+        if (rest.len == 0) {
+            return disposition;
+        }
+        if (rest[0] != ';') {
+            return error.InvalidFormData;
+        }
+        rest = std.mem.trimStart(u8, rest[1..], " \t");
+
+        // An unquoted value cannot contain a `;`, so the next one (or the end
+        // of the header) bounds this parameter when it has to be skipped.
+        const semi = std.mem.indexOfScalar(u8, rest, ';') orelse rest.len;
+        const eq = std.mem.indexOfScalar(u8, rest, '=') orelse rest.len;
+        if (eq >= semi) {
+            // No `=` in this parameter; skip it.
+            rest = rest[semi..];
+            continue;
+        }
+        const key = rest[0..eq];
+        const after_eq = rest[eq + 1 ..];
+
+        if (after_eq.len == 0 or after_eq[0] != '"') {
+            // Unquoted value; skip it.
+            rest = rest[semi..];
+            continue;
+        }
+        const end = std.mem.indexOfScalarPos(u8, after_eq, 1, '"') orelse {
+            // Unterminated quoted value; ignore the rest of the header.
+            return disposition;
+        };
+        const param_value = after_eq[1..end];
+        rest = after_eq[end + 1 ..];
+
+        if (std.ascii.eqlIgnoreCase(key, "name")) {
+            disposition.name = param_value;
+        } else if (std.ascii.eqlIgnoreCase(key, "filename")) {
+            disposition.filename = param_value;
+        }
+    }
+}
+
+/// Helper for wandering around & parsing things along the way.
+pub const Cursor = struct {
+    /// Pointer to current position of cursor.
+    idx: [*]const u8,
+    /// Pointer to end of the buffer.
+    end: [*]const u8,
+    /// Pointer to start of the buffer.
+    start: [*]const u8,
+
+    /// Returns the current position.
+    pub fn current(cursor: *const Cursor) [*]const u8 {
+        return cursor.idx;
+    }
+
+    /// Returns the current character.
+    pub fn char(cursor: *const Cursor) u8 {
+        return cursor.idx[0];
+    }
+
+    /// Advances the position of the cursor by given value.
+    /// SAFETY: This function doesn't check if out of bounds reachable.
+    pub fn advance(cursor: *Cursor, by: usize) void {
+        cursor.idx += by;
+    }
+
+    /// Checks if buffer has `len` length of characters.
+    /// `(cursor.end - cursor.idx >= len)`
+    pub fn hasLength(cursor: *const Cursor, len: usize) bool {
+        return cursor.end - cursor.idx >= len;
+    }
+
+    /// Loads a `@Vector(len, u8)` from the current position of cursor without advancing.
+    /// SAFETY: This function doesn't check if out of bounds reachable.
+    pub fn asVector(cursor: *const Cursor, len: comptime_int) @Vector(len, u8) {
+        return cursor.idx[0..len].*;
+    }
+
+    /// Creates an integer from the current position of the cursor without advancing.
+    /// SAFETY: This function doesn't check if out of bounds reachable.
+    /// SAFETY: T must be an integer with bit size >= @bitSizeOf(u8).
+    pub fn asInteger(cursor: *const Cursor, comptime T: type) T {
+        return @bitCast(cursor.idx[0 .. @bitSizeOf(T) / @bitSizeOf(u8)].*);
+    }
+
+    /// Peek the current and the next but don't advance.
+    /// SAFETY: This function doesn't check if out of bounds reachable.
+    pub fn peek2(cursor: *const Cursor, c0: u8, c1: u8) bool {
+        return cursor.asInteger(u16) == @as(u16, @bitCast([2]u8{ c0, c1 }));
+    }
+
+    /// Moves the cursor until no leading spaces there are.
+    pub fn skipSpaces(cursor: *Cursor) void {
+        while (cursor.end - cursor.current() > 0 and (cursor.char() == ' ' or cursor.char() == '\t')) : (cursor.advance(1)) {}
+    }
+
+    /// Returns true if `Cursor` reached end.
+    pub fn reachedEnd(cursor: *const Cursor) bool {
+        return cursor.idx == cursor.end;
+    }
+
+    /// Returns the unread portion of the buffer.
+    pub fn remaining(cursor: *const Cursor) []const u8 {
+        return cursor.idx[0 .. cursor.end - cursor.idx];
+    }
+};
+
+const testing = @import("../testing.zig");
+
+fn initCursor(bytes: []const u8) Cursor {
+    return .{ .idx = bytes.ptr, .end = bytes.ptr + bytes.len, .start = bytes.ptr };
+}
+
+fn consumed(cursor: *const Cursor) usize {
+    return cursor.idx - cursor.start;
+}
+
+test "header_parser: parse HTTP header" {
+    const bytes = "Content-Disposition: attachment; filename*=UTF-8''file%20name.jpg\r\nrest";
+    var cursor = initCursor(bytes);
+    var header: Header = undefined;
+    try header.parse(&cursor);
+
+    try testing.expectEqual(bytes.len - "rest".len, consumed(&cursor));
+    try testing.expectString("Content-Disposition", header.key);
+    try testing.expectString("attachment; filename*=UTF-8''file%20name.jpg", header.value);
+
+    // Lone `\n` line endings are accepted too; the cursor is left at the next
+    // header, so parsing can continue from there.
+    cursor = initCursor("Host: abc\nHost: def\n");
+    try header.parse(&cursor);
+    try testing.expectEqual(10, consumed(&cursor));
+    try testing.expectString("Host", header.key);
+    try testing.expectString("abc", header.value);
+    try header.parse(&cursor);
+    try testing.expectString("def", header.value);
+    try testing.expectEqual(true, cursor.reachedEnd());
+
+    // Leading whitespaces of the value — spaces and HTABs — are skipped,
+    // trailing ones are kept.
+    cursor = initCursor("Key:  \t value \t\r\n");
+    try header.parse(&cursor);
+    try testing.expectString("value \t", header.value);
+
+    // 0 length values are fine.
+    cursor = initCursor("Key:\r\n");
+    try header.parse(&cursor);
+    try testing.expectString("Key", header.key);
+    try testing.expectString("", header.value);
+
+    // HTAB is legal field content, including next to spaces.
+    cursor = initCursor("Key: a\tb\t c\r\n");
+    try header.parse(&cursor);
+    try testing.expectString("a\tb\t c", header.value);
+}
+
+test "header_parser: parse HTTP header incomplete" {
+    var header: Header = undefined;
+
+    // Buffer may end anywhere before the line ending is complete.
+    const bytes = "Content-Type: text/plain\r\n";
+    for (0..bytes.len) |len| {
+        var cursor = initCursor(bytes[0..len]);
+        try testing.expectError(error.Incomplete, header.parse(&cursor));
+    }
+    // The lone `\n` variant completes a byte earlier.
+    var cursor = initCursor("Content-Type: text/plain\n");
+    try header.parse(&cursor);
+    try testing.expectEqual(true, cursor.reachedEnd());
+}
+
+test "header_parser: parse HTTP header invalid" {
+    var header: Header = undefined;
+
+    const cases = [_][]const u8{
+        ": value\r\n", // 0 length key
+        "Key\x01: value\r\n", // invalid character in the key
+        "Key name: value\r\n", // space in the key
+        "Key: val\x01ue\r\n", // invalid character in the value
+        "Key: val\x7fue\r\n", // DEL in the value
+        "Key: value\rX\n", // `\r` must be followed by `\n`
+    };
+    for (cases) |case| {
+        var cursor = initCursor(case);
+        try testing.expectError(error.Invalid, header.parse(&cursor));
+    }
+}
+
+test "header_parser: parse Content-Disposition" {
+    var d = try parseDisposition("form-data; name=\"a\"; filename=\"b.txt\"");
+    try testing.expectString("a", d.name.?);
+    try testing.expectString("b.txt", d.filename.?);
+
+    // Type and parameter names are case-insensitive; surrounding whitespace
+    // is tolerated.
+    d = try parseDisposition("  Form-Data; NAME=\"x\" ");
+    try testing.expectString("x", d.name.?);
+    try testing.expectEqual(null, d.filename);
+
+    // No parameters at all is fine; both fields stay null.
+    d = try parseDisposition("form-data");
+    try testing.expectEqual(null, d.name);
+    try testing.expectEqual(null, d.filename);
+
+    // A raw ';' inside a quoted value is preserved; unknown parameters are
+    // skipped.
+    d = try parseDisposition("form-data; foo=\"bar\"; name=\"a;b\"");
+    try testing.expectString("a;b", d.name.?);
+
+    // Parameters without `=` or with an unquoted value — e.g. RFC 5987
+    // `filename*` — are skipped without failing the parse; later well-formed
+    // parameters still apply.
+    d = try parseDisposition("form-data; filename*=UTF-8''file%20name.jpg; name=\"n\"");
+    try testing.expectString("n", d.name.?);
+    try testing.expectEqual(null, d.filename);
+    d = try parseDisposition("form-data; foo; name=bare; filename=\"f.txt\"");
+    try testing.expectEqual(null, d.name);
+    try testing.expectString("f.txt", d.filename.?);
+
+    // An unterminated quoted value ends the parse, keeping what was found.
+    d = try parseDisposition("form-data; name=\"a\"; filename=\"f");
+    try testing.expectString("a", d.name.?);
+    try testing.expectEqual(null, d.filename);
+
+    // Not form-data at all, or a parameter without a `;` separator, is
+    // rejected.
+    try testing.expectError(error.InvalidFormData, parseDisposition("attachment; name=\"a\""));
+    try testing.expectError(error.InvalidFormData, parseDisposition("form-data name=\"a\""));
+}
+
+test "header_parser: cursor" {
+    var cursor = initCursor("  ab\r\ncd");
+    try testing.expectEqual(8, cursor.remaining().len);
+    cursor.skipSpaces();
+    try testing.expectEqual(2, consumed(&cursor));
+    try testing.expectEqual('a', cursor.char());
+    try testing.expectEqual(true, cursor.peek2('a', 'b'));
+    try testing.expectEqual(false, cursor.peek2('a', 'c'));
+    try testing.expectEqual(true, cursor.hasLength(6));
+    try testing.expectEqual(false, cursor.hasLength(7));
+    cursor.advance(6);
+    try testing.expectEqual(true, cursor.reachedEnd());
+    try testing.expectString("", cursor.remaining());
+
+    // Skipping spaces covers HTABs too and stops at the end of the buffer.
+    cursor = initCursor(" \t \t");
+    cursor.skipSpaces();
+    try testing.expectEqual(true, cursor.reachedEnd());
+}
+
+fn expectMatchesReference(bytes: []const u8) !void {
+    var key_expected: usize = 0;
+    while (key_expected < bytes.len and isHeaderKeyByte(bytes[key_expected])) key_expected += 1;
+    var value_expected: usize = 0;
+    while (value_expected < bytes.len and isHeaderValueByte(bytes[value_expected])) value_expected += 1;
+
+    var cursor = initCursor(bytes);
+    matchHeaderKey(&cursor);
+    try testing.expectEqual(key_expected, consumed(&cursor));
+
+    cursor = initCursor(bytes);
+    matchHeaderValue(&cursor);
+    try testing.expectEqual(value_expected, consumed(&cursor));
+}
+
+test "header_parser: match functions against scalar reference" {
+    // Exhaustive: place every possible byte at every position of an otherwise
+    // valid buffer, across lengths covering the vector, SWAR and scalar paths.
+    var buf: [48]u8 = undefined;
+    for (1..buf.len + 1) |len| {
+        for (0..len) |pos| {
+            for (0..256) |c| {
+                @memset(buf[0..len], 'a');
+                buf[pos] = @intCast(c);
+                try expectMatchesReference(buf[0..len]);
+            }
+        }
+    }
+
+    // HTAB is the one valid byte below space; pair it with every byte at
+    // every position to prove it never disturbs its neighbor's verdict
+    // (a cross-lane borrow in the SWAR path would).
+    for (2..buf.len + 1) |len| {
+        for (0..len - 1) |pos| {
+            for (0..256) |c| {
+                @memset(buf[0..len], 'a');
+                buf[pos] = '\t';
+                buf[pos + 1] = @intCast(c);
+                try expectMatchesReference(buf[0..len]);
+            }
+        }
+    }
+
+    // Randomized: fully random buffers to exercise multiple invalid bytes per
+    // chunk at once.
+    var prng = std.Random.DefaultPrng.init(0x5eed);
+    const random = prng.random();
+    for (0..20_000) |_| {
+        const len = random.intRangeAtMost(usize, 0, buf.len);
+        random.bytes(buf[0..len]);
+        try expectMatchesReference(buf[0..len]);
+    }
+}