diff --git a/cosmic_ledger/delta.py b/cosmic_ledger/delta.py
index 8eb2b65..635454c 100644
--- a/cosmic_ledger/delta.py
+++ b/cosmic_ledger/delta.py
@@ -2,6 +2,7 @@ import hashlib
 import json
 from typing import List
 from .crypto import Signer
+import math
 
 def _hash_bytes(data: bytes) -> str:
     return hashlib.sha256(data).hexdigest()
@@ -115,6 +116,57 @@ def verify_inclusion(leaf_digest: str, merkle_path: List[str], root: str, index:
 
     return node.hex() == (root or "")
 
+
+class BloomFilter:
+    """A minimal Bloom filter implementation used for compact existence indexes.
+
+    This is intentionally small and dependency-free: it stores bits in a bytearray
+    and uses multiple SHA-256-based hash functions with different salts.
+    """
+    def __init__(self, m: int = 1024, k: int = 3):
+        if m <= 0:
+            raise ValueError("m (number of bits) must be > 0")
+        if k <= 0:
+            raise ValueError("k (number of hash functions) must be > 0")
+        self.m = int(m)
+        self.k = int(k)
+        self._bits = bytearray((self.m + 7) // 8)
+
+    def _hashes(self, item: str):
+        if isinstance(item, str):
+            item = item.encode("utf-8")
+        for i in range(self.k):
+            h = hashlib.sha256()
+            # include the index as a single byte salt
+            h.update(i.to_bytes(1, "little"))
+            h.update(item)
+            yield int.from_bytes(h.digest(), "big") % self.m
+
+    def add(self, item: str) -> None:
+        for pos in self._hashes(item):
+            byte_index = pos // 8
+            bit_index = pos % 8
+            self._bits[byte_index] |= (1 << bit_index)
+
+    def __contains__(self, item: str) -> bool:
+        for pos in self._hashes(item):
+            byte_index = pos // 8
+            bit_index = pos % 8
+            if not (self._bits[byte_index] & (1 << bit_index)):
+                return False
+        return True
+
+    def to_dict(self) -> dict:
+        return {"m": self.m, "k": self.k, "bits": self._bits.hex()}
+
+    @staticmethod
+    def from_dict(d: dict) -> "BloomFilter":
+        bf = BloomFilter(m=d.get("m", 1024), k=d.get("k", 3))
+        bits_hex = d.get("bits", "")
+        if bits_hex:
+            bf._bits = bytearray.fromhex(bits_hex)
+        return bf
+
 class DeltaLog:
     def __init__(self):
         self.entries = []  # each entry is a dict with digest and payload
@@ -225,14 +277,26 @@ class DeltaLog:
             # nothing to archive
             cp_digests = []
             cp_root = ""
+            cp_bloom = None
         else:
             cp_digests = [e["digest"] for e in self.entries[:prune_before_index]]
             cp_root = merkle_root(cp_digests)
+            # Build a compact Bloom filter summarizing archived digests to
+            # allow peers to quickly test probable presence before requesting
+            # proofs or shards. Choose conservative defaults for m and k for
+            # typical small checkpoints; these can be tuned per mission.
+            m = max(1024, len(cp_digests) * 16)  # bits
+            k = 4
+            bf = BloomFilter(m=m, k=k)
+            for d in cp_digests:
+                bf.add(d)
+            cp_bloom = bf.to_dict()
 
         checkpoint = {
             "root": cp_root,
             "index": prune_before_index,
             "digests": cp_digests,
+            "bloom": cp_bloom,
         }
         # store checkpoint
         self.checkpoints.append(checkpoint)
diff --git a/tests/test_bloom.py b/tests/test_bloom.py
new file mode 100644
index 0000000..3f9c33b
--- /dev/null
+++ b/tests/test_bloom.py
@@ -0,0 +1,33 @@
+import sys
+import pathlib
+
+# Ensure repository root is on sys.path so tests can import the package
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parents[1]))
+
+from cosmic_ledger.delta import DeltaLog, BloomFilter
+
+
+def test_checkpoint_includes_bloom_and_membership():
+    dl = DeltaLog()
+    # add several payloads
+    for i in range(8):
+        dl.add_entry({"v": i})
+
+    # create checkpoint archiving first 5 entries
+    cp = dl.checkpoint(prune_before_index=5)
+
+    assert "bloom" in cp
+    assert cp["bloom"] is not None
+
+    # load bloom and check that all archived digests are (probably) present
+    bf = BloomFilter.from_dict(cp["bloom"])
+    archived = cp["digests"]
+    # Bloom filters may have false positives but must not have false negatives
+    for d in archived:
+        assert d in bf
+
+    # non-archived digest may or may not be in bloom (allow either)
+    remaining = [e["digest"] for e in dl.entries]
+    if remaining:
+        # ensure at least one remaining digest is not erroneously marked as missing
+        _ = remaining[0]  # presence not asserted