diff --git a/src/idea194_attractorforge_verified_attractor/models.py b/src/idea194_attractorforge_verified_attractor/models.py
index f2ae756..d600ec5 100644
--- a/src/idea194_attractorforge_verified_attractor/models.py
+++ b/src/idea194_attractorforge_verified_attractor/models.py
@@ -68,6 +68,33 @@ class BasinSketch:
     attractor_fingerprint: str
     dt: float
 
+    def compact_payload(self) -> bytes:
+        """Return a compact, deterministic payload suitable for DTN exchange.
+
+        The payload is a minified JSON of a small set of stable fields. If the
+        resulting UTF-8 encoding exceeds 128 bytes, return a SHA-256 hex digest
+        (64 ASCII chars) of that JSON instead. This guarantees the payload is
+        deterministic and bounded in size.
+        """
+        import json
+        import hashlib
+
+        payload = {
+            "id": self.attractor_id,
+            "kind": self.kind,
+            "radius": _canonical_float(self.estimated_basin_radius),
+            "conv_rate": _canonical_float(self.convergence_rate_bound),
+            "ctrl_hash": self.controller_hash,
+            "fp": self.attractor_fingerprint,
+            "dt": _canonical_float(self.dt),
+        }
+        j = json.dumps(payload, sort_keys=True, separators=(",", ":"))
+        b = j.encode("utf-8")
+        if len(b) <= 128:
+            return b
+        # fallback to a stable SHA-256 hex representation (64 bytes ASCII)
+        return hashlib.sha256(j.encode("utf-8")).hexdigest().encode("ascii")
+
 
 @dataclass(frozen=True, slots=True)
 class PlanCert:
diff --git a/tests/test_basin_sketch.py b/tests/test_basin_sketch.py
new file mode 100644
index 0000000..d095faa
--- /dev/null
+++ b/tests/test_basin_sketch.py
@@ -0,0 +1,26 @@
+from idea194_attractorforge_verified_attractor.models import BasinSketch
+
+
+def test_basin_sketch_compact_payload_is_bounded_and_deterministic() -> None:
+    sketch = BasinSketch(
+        attractor_id="demo-1",
+        kind="motion_manifold",
+        estimated_basin_radius=1.23456789,
+        convergence_rate_bound=0.3456789,
+        controller_hash="deadbeef" * 8,
+        attractor_fingerprint="f" * 64,
+        dt=0.1,
+    )
+
+    p1 = sketch.compact_payload()
+    p2 = sketch.compact_payload()
+
+    # deterministic
+    assert p1 == p2
+
+    # size bound: either the inline JSON (<=128) or the 64-char hex digest
+    assert len(p1) <= 128
+
+    # if JSON fits, it must start with a brace
+    if p1.startswith(b"{"):
+        assert b'"id":"demo-1"' in p1