diff --git a/runtime/pb/wire.lua b/runtime/pb/wire.lua
index 3675156c629b055f0f81e89d416be024d8d74c0a..1de02e9072503dfc3d161a61dfe4f2196d7041ee 100644
--- a/runtime/pb/wire.lua
+++ b/runtime/pb/wire.lua
@@ -131,17 +131,25 @@                 bit.bor(bit.band(bit.rshift(n, 7), 0x7f), 0x80),
                 bit.bor(bit.band(bit.rshift(n, 14), 0x7f), 0x80),
                 bit.rshift(n, 21))
         end
-        -- 2^28 <= n < 2^53: still exact in double, but past uint32.
-        -- Emit one byte via Lua-number bit op, then recurse on the
-        -- residue (which is now < n/128 and may now fit a fast path).
-        if n < 2^53 then
+        -- 2^28 <= n < 2^51: emit one byte via a Lua-number bit op, then
+        -- recurse on the residue (now < n/128, may fit a fast path).
+        --
+        -- The bound is 2^51, not 2^53: `bit.band(n, 0x7f)` routes through
+        -- LuaJIT's number->int32 conversion, which on x64 uses the
+        -- magic-number trick (add 2^52 + 2^51, read the low bits). That is
+        -- exact only while n + 2^52 + 2^51 < 2^53, i.e. n < 2^51; above it
+        -- the addition rounds to an even double and silently drops low
+        -- bits, corrupting the varint. (arm64 uses an exact FP->int op, so
+        -- this only ever bit on x86_64.) Values in [2^51, 2^53) fall
+        -- through to the exact uint64 cdata loop below.
+        if n < 2^51 then
             return string.char(bit.bor(bit.band(n, 0x7f), 0x80))
                 .. encode_varint_slow(math.floor(n / 128))
         end
     end
     -- Fallback: uint64 cdata path. Reached by cdata inputs, negative
-    -- Lua numbers (sign-extended to 10-byte varint), and the (rare)
-    -- case of a Lua number larger than 2^53.
+    -- Lua numbers (sign-extended to 10-byte varint), and Lua numbers in
+    -- [2^51, 2^53) that the fast path above deliberately skips.
     n = to_uint64(n)
     local out = {}
     local i = 1
diff --git a/test/wire_varint_test.lua b/test/wire_varint_test.lua
new file mode 100644
index 0000000000000000000000000000000000000000..77918d2b63b59e1aa0a02c9ba9f6a85f526be9a6
--- /dev/null
+++ b/test/wire_varint_test.lua
@@ -0,0 +1,49 @@
+-- Regression coverage for varint encoding of large Lua-number inputs.
+--
+-- encode_varint had a fast path for Lua numbers in [2^28, 2^53) that emitted
+-- bytes via `bit.band(n, 0x7f)`. On x86_64 LuaJIT's number->int32 conversion
+-- uses the magic-number trick (add 2^52 + 2^51, read the low bits), which is
+-- exact only for n < 2^51; above that it rounds and silently dropped low bits,
+-- corrupting the varint. arm64 uses an exact FP->int op, so the bug was
+-- invisible on Apple-Silicon dev machines and only surfaced on x86_64 CI
+-- (a 64-bit lease ID round-tripped to a different value over gRPC).
+local t = require('luatest')
+local ffi = require('ffi')
+local wire = require('pb.wire')
+
+local g = t.group('wire.varint')
+
+-- Round-trip a value through encode_varint -> decode_varint and assert the
+-- decoded uint64 equals the input. Inputs are given as Lua numbers; the bug
+-- only manifested for the Lua-number encode path, not for cdata inputs.
+local function assert_roundtrip(n)
+    local enc = wire.encode_varint(n)
+    local dec = wire.decode_varint(enc, 1)
+    t.assert_equals(dec, ffi.cast('uint64_t', n),
+        string.format('varint round-trip for %.0f', n))
+end
+
+g.test_large_lua_number_roundtrip = function()
+    -- The exact value that corrupted on x86_64 (a representative lease ID),
+    -- plus the boundaries around the old 2^53 / new 2^51 fast-path cutoff.
+    assert_roundtrip(3041234677171912)   -- corrupted to ...171940 on x64 pre-fix
+    assert_roundtrip(2 ^ 51)             -- first value past the safe fast path
+    assert_roundtrip(2 ^ 51 + 12345)
+    assert_roundtrip(2 ^ 52)
+    assert_roundtrip(2 ^ 53 - 1)         -- largest exact double integer
+end
+
+g.test_fast_path_boundaries_still_exact = function()
+    for _, n in ipairs({0, 1, 127, 128, 16383, 16384, 2 ^ 21, 2 ^ 28,
+                        2 ^ 28 + 1, 2 ^ 51 - 1}) do
+        assert_roundtrip(n)
+    end
+end
+
+g.test_cdata_inputs_unchanged = function()
+    for _, n in ipairs({ffi.cast('uint64_t', 3041234677171912ULL),
+                        ffi.cast('uint64_t', 0xFFFFFFFFFFFFFFFFULL)}) do
+        local dec = wire.decode_varint(wire.encode_varint(n), 1)
+        t.assert_equals(dec, n)
+    end
+end