diff --git a/runtime/pb/wire.lua b/runtime/pb/wire.lua
index 6262045a766e3063d3e08a1b790d7b20326b4fc3..8e21add2974bcf573c114e2a3e54985a02ab8890 100644
--- a/runtime/pb/wire.lua
+++ b/runtime/pb/wire.lua
@@ -45,7 +45,15 @@ -- ---------------------------------------------------------------------------
 
 -- encode_varint(n) -> string
 -- Accepts uint64_t/int64_t cdata, Lua number, or boolean.
+--
+-- Fast path: small non-negative Lua numbers (0..127) become a single
+-- string.char(n) call with no cdata allocation, no `out` table, no
+-- table.concat. Covers most length prefixes for short strings, many
+-- enum ordinals, and most small int values in typical RPC payloads.
 local function encode_varint(n)
+    if type(n) == 'number' and n >= 0 and n < 0x80 then
+        return string.char(n)
+    end
     n = to_uint64(n)
     local out = {}
     local i = 1