diff --git a/packages/protons-runtime/package.json b/packages/protons-runtime/package.json
index a3a5cd9..ab669f8 100644
--- a/packages/protons-runtime/package.json
+++ b/packages/protons-runtime/package.json
@@ -128,6 +128,13 @@
     "dep-check": "aegir dep-check",
     "doc-check": "aegir doc-check",
     "build": "aegir build",
+    "test": "aegir test",
+    "test:chrome": "aegir test -t browser --cov",
+    "test:chrome-webworker": "aegir test -t webworker",
+    "test:firefox": "aegir test -t browser -- --browser firefox",
+    "test:firefox-webworker": "aegir test -t webworker -- --browser firefox",
+    "test:node": "aegir test -t node --cov",
+    "test:electron-main": "aegir test -t electron-main",
     "release": "aegir release"
   },
   "dependencies": {
diff --git a/packages/protons-runtime/src/utils/longbits.ts b/packages/protons-runtime/src/utils/longbits.ts
index 31b120f..0682618 100644
--- a/packages/protons-runtime/src/utils/longbits.ts
+++ b/packages/protons-runtime/src/utils/longbits.ts
@@ -122,26 +122,10 @@ export class LongBits {
       return this.fromNumber(Number(value))
     }
 
-    const negative = value < 0n
+    // two's complement in one step, so a zero low word carries into the high word
+    const bits = BigInt.asUintN(64, value)
 
-    if (negative) {
-      value = -value
-    }
-
-    let hi = value >> 32n
-    let lo = value - (hi << 32n)
-
-    if (negative) {
-      hi = ~hi | 0n
-      lo = ~lo | 0n
-
-      if (++lo > TWO_32) {
-        lo = 0n
-        if (++hi > TWO_32) { hi = 0n }
-      }
-    }
-
-    return new LongBits(Number(lo), Number(hi))
+    return new LongBits(Number(bits & 0xffffffffn), Number(bits >> 32n))
   }
 
   /**
@@ -185,5 +169,3 @@ const zero = new LongBits(0, 0)
 zero.toBigInt = function () { return 0n }
 zero.zzEncode = zero.zzDecode = function () { return this }
 zero.length = function () { return 1 }
-
-const TWO_32 = 4294967296n
diff --git a/packages/protons-runtime/test/longbits.spec.ts b/packages/protons-runtime/test/longbits.spec.ts
new file mode 100644
index 0000000..882bd5f
--- /dev/null
+++ b/packages/protons-runtime/test/longbits.spec.ts
@@ -0,0 +1,51 @@
+import { expect } from 'aegir/chai'
+import { reader, writer } from '../src/index.ts'
+
+// negative values below -(2^53 - 1) whose low 32 bits are zero used to lose
+// the two's-complement carry into the high word and encode as value - 2^32
+describe('negative 64-bit values', () => {
+  const cases: Record<'int64' | 'sint64' | 'sfixed64', Array<[bigint, number[]]>> = {
+    int64: [
+      [-(2n ** 53n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0xf0, 0xff, 0x01]],
+      [-(2n ** 53n + 2n ** 32n), [0x80, 0x80, 0x80, 0x80, 0xf0, 0xff, 0xff, 0xef, 0xff, 0x01]],
+      [-(2n ** 60n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0xf0, 0x01]],
+      [-(2n ** 63n - 2n ** 32n), [0x80, 0x80, 0x80, 0x80, 0x90, 0x80, 0x80, 0x80, 0x80, 0x01]],
+      [-(2n ** 63n), [0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01]],
+      [-(2n ** 53n + 1n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xef, 0xff, 0x01]]
+    ],
+    sint64: [
+      [-(2n ** 53n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1f]],
+      [-(2n ** 53n + 2n ** 32n), [0xff, 0xff, 0xff, 0xff, 0x9f, 0x80, 0x80, 0x20]],
+      [-(2n ** 60n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1f]],
+      [-(2n ** 63n - 2n ** 32n), [0xff, 0xff, 0xff, 0xff, 0xdf, 0xff, 0xff, 0xff, 0xff, 0x01]],
+      [-(2n ** 63n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x01]],
+      [-(2n ** 53n + 1n), [0x81, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x20]]
+    ],
+    sfixed64: [
+      [-(2n ** 53n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xe0, 0xff]],
+      [-(2n ** 53n + 2n ** 32n), [0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xdf, 0xff]],
+      [-(2n ** 60n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf0]],
+      [-(2n ** 63n - 2n ** 32n), [0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x80]],
+      [-(2n ** 63n), [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80]],
+      [-(2n ** 53n + 1n), [0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xdf, 0xff]]
+    ]
+  }
+
+  for (const method of ['int64', 'sint64', 'sfixed64'] as const) {
+    for (const [value, bytes] of cases[method]) {
+      it(`should encode ${method} ${value}`, () => {
+        const w = writer()
+        w[method](value)
+
+        expect(w.finish()).to.equalBytes(bytes)
+      })
+
+      it(`should round trip ${method} ${value}`, () => {
+        const w = writer()
+        w[method](value)
+
+        expect(reader(w.finish())[method]()).to.equal(value)
+      })
+    }
+  }
+})