diff --git a/packages/tsp-js/CHANGELOG.md b/packages/tsp-js/CHANGELOG.md
index abfc143..7bee63d 100644
--- a/packages/tsp-js/CHANGELOG.md
+++ b/packages/tsp-js/CHANGELOG.md
@@ -8,6 +8,13 @@ For history before this file, see `git log` on `packages/tsp-js`.
 
 ## [Unreleased]
 
+### Fixed
+
+- An XSCS/XCTL body that is not exactly one Bytes primitive is refused. It was
+  read as its first primitive, silently dropping the rest of the `-A##` stream,
+  and data after the stream was ignored. See
+  [tswg-tsp-specification#77](https://github.com/trustoverip/tswg-tsp-specification/issues/77).
+
 ## [0.3.0] — Trust Spanning Protocol specification Rev 3
 
 **Breaking. This package now packs Rev 3, and a Rev 2 peer cannot read what it
diff --git a/packages/tsp-js/src/rev3/payload.ts b/packages/tsp-js/src/rev3/payload.ts
index 528458f..0569d95 100644
--- a/packages/tsp-js/src/rev3/payload.ts
+++ b/packages/tsp-js/src/rev3/payload.ts
@@ -196,11 +196,17 @@ export function decodePayloadFrame(
     decodePadding(frame, cur);
     const streamQuadlets = wire.decodeCount(wire.TSP_GENERIC_STREAM, frame, cur);
     if (streamQuadlets === undefined) throw new Error("tsp: missing -A payload stream");
+    // The body is an `-A##` stream that ends the frame and holds exactly one
+    // Bytes primitive — the form the spec's vectors and the ToIP reference
+    // use. Anything else is refused, never truncated to its first primitive
+    // (trustoverip/tswg-tsp-specification#77).
     const streamEnd = cur.pos + streamQuadlets * 3;
-    if (streamEnd > frameEnd) throw new Error("tsp: -A stream overruns the payload frame");
+    if (streamEnd !== frameEnd) throw new Error("tsp: -A stream does not end the payload frame");
     const body = wire.decodeVariableData(wire.TSP_PLAINTEXT, frame, cur);
     if (body === undefined) throw new Error("tsp: missing payload body");
-    if (cur.pos > streamEnd) throw new Error("tsp: payload body overruns the -A stream");
+    if (cur.pos !== streamEnd) {
+      throw new Error("tsp: -A stream must hold exactly one Bytes primitive");
+    }
     // `XCTL` carries an upper-layer control payload — opaque to TSP, exactly
     // like `XSCS`. It is not a relationship-forming message and shares nothing
     // with one but the word "control".
diff --git a/packages/tsp-js/tests/payload.app-stream.mjs b/packages/tsp-js/tests/payload.app-stream.mjs
new file mode 100644
index 0000000..0fa167d
--- /dev/null
+++ b/packages/tsp-js/tests/payload.app-stream.mjs
@@ -0,0 +1,43 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import * as wire from "../dist/cesr/wire.js";
+import { decodePayloadFrame } from "../dist/rev3/payload.js";
+
+// An XSCS body is exactly one Bytes primitive (tswg-tsp-specification#77):
+// an -H## group, a second primitive, or data after the stream is refused,
+// never truncated to the first primitive.
+const enc = new TextEncoder();
+const prim = (s) => {
+  const out = [];
+  wire.encodeVariableData(wire.TSP_PLAINTEXT, enc.encode(s), out);
+  return out;
+};
+const frameOf = (stream, trailing = []) => {
+  const body = [...wire.XSCS, ...prim(""), ...prim("")]; // NULL sender, no padding
+  wire.encodeCount(wire.TSP_GENERIC_STREAM, stream.length / 3, body);
+  body.push(...stream, ...trailing);
+  const frame = [];
+  wire.encodeCount(wire.TSP_PAYLOAD, body.length / 3, frame);
+  return Uint8Array.from([...frame, ...body]);
+};
+const open = (f) => decodePayloadFrame(f, "did:example:alice", new Uint8Array());
+
+test("a single Bytes primitive is the application body", () => {
+  assert.equal(new TextDecoder().decode(open(frameOf(prim("hello world"))).body), "hello world");
+});
+
+test("an -H## group is refused", () => {
+  const json = prim('{"hello":"world"}');
+  const group = [];
+  wire.encodeCount(wire.cesrInt("H"), json.length / 3, group);
+  assert.throws(() => open(frameOf([...group, ...json])));
+});
+
+test("a second primitive is refused, not dropped", () => {
+  assert.throws(() => open(frameOf([...prim("one"), ...prim("two")])), /exactly one Bytes primitive/);
+});
+
+test("data after the stream is refused", () => {
+  assert.throws(() => open(frameOf(prim("one"), prim("x"))), /does not end the payload frame/);
+});