diff --git a/shared/sx/async_eval.py b/shared/sx/async_eval.py
index a260d4f..4388ecd 100644
--- a/shared/sx/async_eval.py
+++ b/shared/sx/async_eval.py
@@ -25,6 +25,7 @@ from .types import Component, Keyword, Lambda, Macro, NIL, Symbol
 from .evaluator import _expand_macro, EvalError
 from .primitives import _PRIMITIVES
 from .primitives_io import IO_PRIMITIVES, RequestContext, execute_io
+from .parser import SxExpr, serialize
 from .html import (
     HTML_TAGS, VOID_ELEMENTS, BOOLEAN_ATTRS,
     escape_text, escape_attr, _RawHTML, css_class_collector,
@@ -837,3 +838,386 @@ _ASYNC_RENDER_FORMS: dict[str, Any] = {
     "filter": _arsf_filter,
     "for-each": _arsf_for_each,
 }
+
+
+# ---------------------------------------------------------------------------
+# Async serialize — evaluate I/O/control flow, produce sx source (not HTML)
+# ---------------------------------------------------------------------------
+# Used by defhandler execution.  Fragment providers need to return sx wire
+# format (s-expression source) that consuming apps parse and render client-
+# side.  This mirrors the old Python handlers that used sx_call() to build
+# sx strings.
+#
+# _aser ("async serialize") works like _arender but instead of producing
+# HTML for components/tags/<>, it serializes them back to sx source with
+# their arguments evaluated.
+
+
+async def async_eval_to_sx(
+    expr: Any,
+    env: dict[str, Any],
+    ctx: RequestContext | None = None,
+) -> str:
+    """Evaluate *expr* (resolving I/O inline) and produce sx source string.
+
+    Unlike ``async_render`` (which produces HTML), this produces sx wire
+    format suitable for fragment responses that clients render themselves.
+    """
+    if ctx is None:
+        ctx = RequestContext()
+    result = await _aser(expr, env, ctx)
+    if isinstance(result, SxExpr):
+        return result.source
+    if result is None or result is NIL:
+        return ""
+    return serialize(result)
+
+
+async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
+    """Evaluate *expr*, producing SxExpr for rendering forms, raw values
+    for everything else."""
+    if isinstance(expr, (int, float, bool)):
+        return expr
+    if isinstance(expr, str):
+        return expr
+    if isinstance(expr, SxExpr):
+        return expr
+    if expr is None or expr is NIL:
+        return NIL
+
+    if isinstance(expr, Symbol):
+        name = expr.name
+        if name in env:
+            return env[name]
+        if name in _PRIMITIVES:
+            return _PRIMITIVES[name]
+        if name == "true":
+            return True
+        if name == "false":
+            return False
+        if name == "nil":
+            return NIL
+        raise EvalError(f"Undefined symbol: {name}")
+
+    if isinstance(expr, Keyword):
+        return expr.name
+
+    if isinstance(expr, dict):
+        return {k: await _aser(v, env, ctx) for k, v in expr.items()}
+
+    if not isinstance(expr, list):
+        return expr
+    if not expr:
+        return []
+
+    head = expr[0]
+    if not isinstance(head, (Symbol, Lambda, list)):
+        return [await _aser(x, env, ctx) for x in expr]
+
+    if isinstance(head, Symbol):
+        name = head.name
+
+        # I/O primitives — await, return actual data
+        if name in IO_PRIMITIVES:
+            args, kwargs = await _parse_io_args(expr[1:], env, ctx)
+            return await execute_io(name, args, kwargs, ctx)
+
+        # <> — serialize children as sx fragment
+        if name == "<>":
+            return await _aser_fragment(expr[1:], env, ctx)
+
+        # raw! — serialize
+        if name == "raw!":
+            return await _aser_call("raw!", expr[1:], env, ctx)
+
+        # Component call — serialize (don't expand)
+        if name.startswith("~"):
+            return await _aser_call(name, expr[1:], env, ctx)
+
+        # Serialize-mode special/HO forms (checked BEFORE HTML_TAGS
+        # because some names like "map" are both HTML tags and sx forms)
+        sf = _ASER_FORMS.get(name)
+        if sf is not None:
+            return await sf(expr, env, ctx)
+
+        # HTML tag — serialize (don't render to HTML)
+        if name in HTML_TAGS:
+            return await _aser_call(name, expr[1:], env, ctx)
+
+        # Macro expansion
+        if name in env:
+            val = env[name]
+            if isinstance(val, Macro):
+                expanded = _expand_macro(val, expr[1:], env)
+                return await _aser(expanded, env, ctx)
+
+    # Function / lambda call — evaluate (produces data, not rendering)
+    fn = await async_eval(head, env, ctx)
+    args = [await async_eval(a, env, ctx) for a in expr[1:]]
+
+    if callable(fn) and not isinstance(fn, (Lambda, Component)):
+        return fn(*args)
+    if isinstance(fn, Lambda):
+        return await _async_call_lambda(fn, args, env, ctx)
+    if isinstance(fn, Component):
+        # Component invoked as function — serialize the call
+        return await _aser_call(f"~{fn.name}", expr[1:], env, ctx)
+    raise EvalError(f"Not callable: {fn!r}")
+
+
+async def _aser_fragment(children: list, env: dict, ctx: RequestContext) -> SxExpr:
+    """Serialize ``(<> child1 child2 ...)`` to sx source."""
+    parts: list[str] = []
+    for child in children:
+        result = await _aser(child, env, ctx)
+        if isinstance(result, list):
+            # Flatten lists (e.g. from map)
+            for item in result:
+                if item is not NIL and item is not None:
+                    parts.append(serialize(item))
+        elif result is not NIL and result is not None:
+            parts.append(serialize(result))
+    if not parts:
+        return SxExpr("")
+    return SxExpr("(<> " + " ".join(parts) + ")")
+
+
+async def _aser_call(
+    name: str, args: list, env: dict, ctx: RequestContext,
+) -> SxExpr:
+    """Serialize ``(name :key val child ...)`` — evaluate args but keep
+    as sx source instead of rendering to HTML."""
+    parts = [name]
+    i = 0
+    while i < len(args):
+        arg = args[i]
+        if isinstance(arg, Keyword) and i + 1 < len(args):
+            val = await _aser(args[i + 1], env, ctx)
+            if val is not NIL and val is not None:
+                parts.append(f":{arg.name}")
+                parts.append(serialize(val))
+            i += 2
+        else:
+            result = await _aser(arg, env, ctx)
+            if result is not NIL and result is not None:
+                parts.append(serialize(result))
+            i += 1
+    return SxExpr("(" + " ".join(parts) + ")")
+
+
+# ---------------------------------------------------------------------------
+# Serialize-mode special forms
+# ---------------------------------------------------------------------------
+
+async def _assf_if(expr, env, ctx):
+    cond = await async_eval(expr[1], env, ctx)
+    if cond and cond is not NIL:
+        return await _aser(expr[2], env, ctx)
+    if len(expr) > 3:
+        return await _aser(expr[3], env, ctx)
+    return NIL
+
+
+async def _assf_when(expr, env, ctx):
+    cond = await async_eval(expr[1], env, ctx)
+    if cond and cond is not NIL:
+        result: Any = NIL
+        for body_expr in expr[2:]:
+            result = await _aser(body_expr, env, ctx)
+        return result
+    return NIL
+
+
+async def _assf_let(expr, env, ctx):
+    bindings = expr[1]
+    local = dict(env)
+    if isinstance(bindings, list):
+        if bindings and isinstance(bindings[0], list):
+            for binding in bindings:
+                var = binding[0]
+                vname = var.name if isinstance(var, Symbol) else var
+                local[vname] = await _aser(binding[1], local, ctx)
+        elif len(bindings) % 2 == 0:
+            for i in range(0, len(bindings), 2):
+                var = bindings[i]
+                vname = var.name if isinstance(var, Symbol) else var
+                local[vname] = await _aser(bindings[i + 1], local, ctx)
+    result: Any = NIL
+    for body_expr in expr[2:]:
+        result = await _aser(body_expr, local, ctx)
+    return result
+
+
+async def _assf_cond(expr, env, ctx):
+    clauses = expr[1:]
+    if not clauses:
+        return NIL
+    if (isinstance(clauses[0], list) and len(clauses[0]) == 2
+            and not (isinstance(clauses[0][0], Symbol) and clauses[0][0].name in (
+                "=", "<", ">", "<=", ">=", "!=", "and", "or"))):
+        for clause in clauses:
+            test = clause[0]
+            if isinstance(test, Symbol) and test.name in ("else", ":else"):
+                return await _aser(clause[1], env, ctx)
+            if isinstance(test, Keyword) and test.name == "else":
+                return await _aser(clause[1], env, ctx)
+            if await async_eval(test, env, ctx):
+                return await _aser(clause[1], env, ctx)
+    else:
+        i = 0
+        while i < len(clauses) - 1:
+            test = clauses[i]
+            result = clauses[i + 1]
+            if isinstance(test, Keyword) and test.name == "else":
+                return await _aser(result, env, ctx)
+            if isinstance(test, Symbol) and test.name in (":else", "else"):
+                return await _aser(result, env, ctx)
+            if await async_eval(test, env, ctx):
+                return await _aser(result, env, ctx)
+            i += 2
+    return NIL
+
+
+async def _assf_case(expr, env, ctx):
+    match_val = await async_eval(expr[1], env, ctx)
+    clauses = expr[2:]
+    i = 0
+    while i < len(clauses) - 1:
+        test = clauses[i]
+        result = clauses[i + 1]
+        if isinstance(test, Keyword) and test.name == "else":
+            return await _aser(result, env, ctx)
+        if isinstance(test, Symbol) and test.name in (":else", "else"):
+            return await _aser(result, env, ctx)
+        if match_val == await async_eval(test, env, ctx):
+            return await _aser(result, env, ctx)
+        i += 2
+    return NIL
+
+
+async def _assf_begin(expr, env, ctx):
+    result: Any = NIL
+    for sub in expr[1:]:
+        result = await _aser(sub, env, ctx)
+    return result
+
+
+async def _assf_define(expr, env, ctx):
+    await async_eval(expr, env, ctx)
+    return NIL
+
+
+async def _assf_lambda(expr, env, ctx):
+    return await _asf_lambda(expr, env, ctx)
+
+
+async def _assf_and(expr, env, ctx):
+    return await _asf_and(expr, env, ctx)
+
+
+async def _assf_or(expr, env, ctx):
+    return await _asf_or(expr, env, ctx)
+
+
+async def _assf_quote(expr, env, ctx):
+    return expr[1] if len(expr) > 1 else NIL
+
+
+async def _assf_quasiquote(expr, env, ctx):
+    return await _async_qq_expand(expr[1], env, ctx)
+
+
+async def _assf_thread_first(expr, env, ctx):
+    return await _asf_thread_first(expr, env, ctx)
+
+
+async def _assf_set_bang(expr, env, ctx):
+    return await _asf_set_bang(expr, env, ctx)
+
+
+# ---------------------------------------------------------------------------
+# Serialize-mode higher-order forms
+# ---------------------------------------------------------------------------
+
+async def _asho_ser_map(expr, env, ctx):
+    fn = await async_eval(expr[1], env, ctx)
+    coll = await async_eval(expr[2], env, ctx)
+    results = []
+    for item in coll:
+        if isinstance(fn, Lambda):
+            local = dict(fn.closure)
+            local.update(env)
+            for p, v in zip(fn.params, [item]):
+                local[p] = v
+            results.append(await _aser(fn.body, local, ctx))
+        elif callable(fn):
+            results.append(fn(item))
+        else:
+            raise EvalError(f"map requires callable, got {type(fn).__name__}")
+    return results
+
+
+async def _asho_ser_map_indexed(expr, env, ctx):
+    fn = await async_eval(expr[1], env, ctx)
+    coll = await async_eval(expr[2], env, ctx)
+    results = []
+    for i, item in enumerate(coll):
+        if isinstance(fn, Lambda):
+            local = dict(fn.closure)
+            local.update(env)
+            local[fn.params[0]] = i
+            local[fn.params[1]] = item
+            results.append(await _aser(fn.body, local, ctx))
+        elif callable(fn):
+            results.append(fn(i, item))
+        else:
+            raise EvalError(f"map-indexed requires callable, got {type(fn).__name__}")
+    return results
+
+
+async def _asho_ser_filter(expr, env, ctx):
+    # filter is value-producing, delegate to eval
+    return await async_eval(expr, env, ctx)
+
+
+async def _asho_ser_for_each(expr, env, ctx):
+    fn = await async_eval(expr[1], env, ctx)
+    coll = await async_eval(expr[2], env, ctx)
+    results = []
+    for item in coll:
+        if isinstance(fn, Lambda):
+            local = dict(fn.closure)
+            local.update(env)
+            local[fn.params[0]] = item
+            results.append(await _aser(fn.body, local, ctx))
+        elif callable(fn):
+            results.append(fn(item))
+    return results
+
+
+_ASER_FORMS: dict[str, Any] = {
+    "if": _assf_if,
+    "when": _assf_when,
+    "cond": _assf_cond,
+    "case": _assf_case,
+    "and": _assf_and,
+    "or": _assf_or,
+    "let": _assf_let,
+    "let*": _assf_let,
+    "lambda": _assf_lambda,
+    "fn": _assf_lambda,
+    "define": _assf_define,
+    "defcomp": _assf_define,
+    "defmacro": _assf_define,
+    "defhandler": _assf_define,
+    "begin": _assf_begin,
+    "do": _assf_begin,
+    "quote": _assf_quote,
+    "quasiquote": _assf_quasiquote,
+    "->": _assf_thread_first,
+    "set!": _assf_set_bang,
+    "map": _asho_ser_map,
+    "map-indexed": _asho_ser_map_indexed,
+    "filter": _asho_ser_filter,
+    "for-each": _asho_ser_for_each,
+}
diff --git a/shared/sx/handlers.py b/shared/sx/handlers.py
index 9d845bd..6e653f2 100644
--- a/shared/sx/handlers.py
+++ b/shared/sx/handlers.py
@@ -122,7 +122,7 @@ async def execute_handler(
     4. Return rendered string
     """
     from .jinja_bridge import get_component_env, _get_request_context
-    from .async_eval import async_render
+    from .async_eval import async_eval_to_sx
     from .types import NIL
 
     if args is None:
@@ -139,8 +139,9 @@ async def execute_handler(
     # Get request context for I/O primitives
     ctx = _get_request_context()
 
-    # Async eval+render — I/O primitives are awaited inline
-    return await async_render(handler_def.body, env, ctx)
+    # Async eval → sx source — I/O primitives are awaited inline,
+    # but component/tag calls serialize to sx wire format (not HTML).
+    return await async_eval_to_sx(handler_def.body, env, ctx)
 
 
 # ---------------------------------------------------------------------------
diff --git a/shared/sx/primitives.py b/shared/sx/primitives.py
index 29d4fd5..eb4d0b9 100644
--- a/shared/sx/primitives.py
+++ b/shared/sx/primitives.py
@@ -189,7 +189,10 @@ def prim_is_dict(x: Any) -> bool:
 def prim_is_empty(coll: Any) -> bool:
     if coll is None or coll is NIL:
         return True
-    return len(coll) == 0
+    try:
+        return len(coll) == 0
+    except TypeError:
+        return False
 
 @register_primitive("contains?")
 def prim_contains(coll: Any, key: Any) -> bool: