diff --git a/README.md b/README.md
index 0428203b..61f8e976 100644
--- a/README.md
+++ b/README.md
@@ -128,7 +128,7 @@ For verification:
 
 ```bash
 [...]$ model_signing verify bert-base-uncased \
-      --signature model.sig \
+      --signature claims.jsonl \
       --trust_config client_trust_config.json
       --identity "$identity"
       --identity_provider "$oidc_provider"
@@ -160,7 +160,7 @@ All signing methods support changing the signature name and location via the
 `--signature` flag:
 
 ```bash
-[...]$ model_signing sign bert-base-uncased --signature model.sig
+[...]$ model_signing sign bert-base-uncased --signature claims.jsonl
 ```
 
 Consult the help for a list of all flags (`model_signing --help`, or directly
@@ -171,7 +171,7 @@ model we use
 
 ```bash
 [...]$ model_signing verify bert-base-uncased \
-      --signature model.sig \
+      --signature claims.jsonl \
       --identity "$identity" \
       --identity_provider "$oidc_provider"
 ```
@@ -234,7 +234,7 @@ With a PKCS #11 URI describing the private key, we can use the following
 for signing:
 
 ```bash
-[...]$ model_signing sign pkcs11-key --signature model.sig \
+[...]$ model_signing sign pkcs11-key --signature claims.jsonl \
        --pkcs11_uri "pkcs11:..." /path/to/your/model
 ```
 
@@ -242,7 +242,7 @@ For signature verification it is necessary to retrieve the public key from
 the PKCS #11 device and store it in a file in PEM format. With can then use:
 
 ```bash
-[...]$ model_signing verify key --signature model.sig\
+[...]$ model_signing verify key --signature claims.jsonl\
        --public_key key.pub  /path/to/your/model
 ```
 
@@ -333,7 +333,7 @@ The simplest way to generate a signature using Sigstore is:
 ```python
 import model_signing
 
-model_signing.signing.sign("bert-base-uncased", "model.sig")
+model_signing.signing.sign("bert-base-uncased", "claims.jsonl")
 ```
 
 This will run the same OIDC flow as when signing with Sigstore from the CLI.
diff --git a/docs/demo.ipynb b/docs/demo.ipynb
index 9167d14f..b9d852ac 100644
--- a/docs/demo.ipynb
+++ b/docs/demo.ipynb
@@ -541,7 +541,7 @@
         "id": "L2zQrDPnBDcu"
       },
       "source": [
-        "By default, the signature is in `model.sig`. First, we can look at its size:"
+        "By default, the signature is in `claims.jsonl`. First, we can look at its size:"
       ]
     },
     {
@@ -559,12 +559,12 @@
           "output_type": "stream",
           "name": "stdout",
           "text": [
-            "-rw-r--r-- 1 root root 11345 Oct 10 18:00 model.sig\n"
+            "-rw-r--r-- 1 root root 11345 Oct 10 18:00 claims.jsonl\n"
           ]
         }
       ],
       "source": [
-        "!ls -l model.sig"
+        "!ls -l claims.jsonl"
       ]
     },
     {
@@ -597,7 +597,7 @@
         }
       ],
       "source": [
-        "!model_signing verify bert-base-uncased --signature model.sig --identity \"$identity\" --identity_provider \"$oidc_provider\""
+        "!model_signing verify bert-base-uncased --signature claims.jsonl --identity \"$identity\" --identity_provider \"$oidc_provider\""
       ]
     },
     {
@@ -785,7 +785,7 @@
         }
       ],
       "source": [
-        "!model_signing verify resnet-50 --signature model.sig --identity \"$identity\" --identity_provider \"$oidc_provider\""
+        "!model_signing verify resnet-50 --signature claims.jsonl --identity \"$identity\" --identity_provider \"$oidc_provider\""
       ]
     },
     {
@@ -818,7 +818,7 @@
         }
       ],
       "source": [
-        "!model_signing verify bert-base-uncased --signature model.sig --identity \"FAKE_IDENTITY\" --identity_provider \"$oidc_provider\""
+        "!model_signing verify bert-base-uncased --signature claims.jsonl --identity \"FAKE_IDENTITY\" --identity_provider \"$oidc_provider\""
       ]
     },
     {
@@ -853,7 +853,7 @@
         }
       ],
       "source": [
-        "!model_signing verify bert-base-uncased --signature model.sig --identity \"$identity\" --identity_provider \"FAKE_PROVIDER\""
+        "!model_signing verify bert-base-uncased --signature claims.jsonl --identity \"$identity\" --identity_provider \"FAKE_PROVIDER\""
       ]
     },
     {
diff --git a/docs/model_signing_format.md b/docs/model_signing_format.md
index f0da38c0..3ec7959d 100644
--- a/docs/model_signing_format.md
+++ b/docs/model_signing_format.md
@@ -71,7 +71,7 @@ transparency log.
 Below is an example of the Sigstore bundle showing each of the layers described above.
 
 ```bash
-$ cat model.sig | jq .
+$ cat claims.jsonl | jq .
 {
   "mediaType": "application/vnd.dev.sigstore.bundle.v0.3+json",
   "verificationMaterial": {
@@ -127,7 +127,7 @@ $ cat model.sig | jq .
   }
 }
 
-$ cat model.sig | jq .dsseEnvelope.payload -r | base64 -d | jq .
+$ cat claims.jsonl | jq .dsseEnvelope.payload -r | base64 -d | jq .
 {
   "_type": "https://in-toto.io/Statement/v1",
   "subject": [
diff --git a/src/model_signing/_cli.py b/src/model_signing/_cli.py
index 2d90f135..243b021a 100644
--- a/src/model_signing/_cli.py
+++ b/src/model_signing/_cli.py
@@ -53,8 +53,11 @@ def set_attribute(self, key, value):
     "--signature",
     type=pathlib.Path,
     metavar="SIGNATURE_PATH",
-    default=pathlib.Path("model.sig"),
-    help="Location of the signature file to generate. Defaults to `model.sig`.",
+    default=pathlib.Path("claims.jsonl"),
+    help=(
+        "Location of the signature file to generate. "
+        "Defaults to `claims.jsonl`."
+    ),
 )
 
 
diff --git a/src/model_signing/_signing/sign_sigstore.py b/src/model_signing/_signing/sign_sigstore.py
index 080f98ab..bb336841 100644
--- a/src/model_signing/_signing/sign_sigstore.py
+++ b/src/model_signing/_signing/sign_sigstore.py
@@ -51,13 +51,24 @@ def __init__(self, bundle: sigstore_models.Bundle):
 
     @override
     def write(self, path: pathlib.Path) -> None:
-        path.write_text(self.bundle.to_json(), encoding="utf-8")
+        # Convert to compact JSON (single line) for JSONL format
+        # by removing newlines from the bundle's JSON output
+        bundle_json = self.bundle.to_json().replace("\n", "")
+
+        # Append to file if it exists (for accumulating attestations)
+        # Otherwise create new file
+        mode = "a" if path.exists() else "w"
+        with path.open(mode, encoding="utf-8") as f:
+            f.write(bundle_json + "\n")
 
     @classmethod
     @override
     def read(cls, path: pathlib.Path) -> Self:
         content = path.read_text(encoding="utf-8")
-        return cls(sigstore_models.Bundle.from_json(content))
+        # Handle JSONL format: read the last line (most recent attestation)
+        lines = content.strip().split("\n")
+        last_line = lines[-1]
+        return cls(sigstore_models.Bundle.from_json(last_line))
 
 
 class Signer(signing.Signer):
diff --git a/src/model_signing/_signing/sign_sigstore_pb.py b/src/model_signing/_signing/sign_sigstore_pb.py
index 5a1fcc0d..282934bf 100644
--- a/src/model_signing/_signing/sign_sigstore_pb.py
+++ b/src/model_signing/_signing/sign_sigstore_pb.py
@@ -105,13 +105,24 @@ def __init__(self, bundle: bundle_pb.Bundle):
 
     @override
     def write(self, path: pathlib.Path) -> None:
-        path.write_text(self.bundle.to_json(), encoding="utf-8")
+        # Convert to compact JSON (single line) for JSONL format
+        # by removing newlines from the bundle's JSON output
+        bundle_json = self.bundle.to_json().replace("\n", "")
+
+        # Append to file if it exists (for accumulating attestations)
+        # Otherwise create new file
+        mode = "a" if path.exists() else "w"
+        with path.open(mode, encoding="utf-8") as f:
+            f.write(bundle_json + "\n")
 
     @classmethod
     @override
     def read(cls, path: pathlib.Path) -> Self:
         content = path.read_text(encoding="utf-8")
-        parsed_dict = json.loads(content)
+        # Handle JSONL format: read the last line (most recent attestation)
+        lines = content.strip().split("\n")
+        last_line = lines[-1]
+        parsed_dict = json.loads(last_line)
 
         # adjust parsed_dict due to previous usage of protobufs
         if "tlogEntries" not in parsed_dict["verificationMaterial"]:
diff --git a/src/model_signing/_signing/signing.py b/src/model_signing/_signing/signing.py
index 13681ac5..6a774fea 100644
--- a/src/model_signing/_signing/signing.py
+++ b/src/model_signing/_signing/signing.py
@@ -216,7 +216,7 @@ class Payload:
           "hash_type": "sha256",
           "allow_symlinks": true
           "ignore_paths": [
-            "model.sig",
+            "claims.jsonl",
             ".git",
             ".gitattributes",
             ".github",
diff --git a/tests/_signing/sigstore_test.py b/tests/_signing/sigstore_test.py
index 68dbd26f..d628a618 100644
--- a/tests/_signing/sigstore_test.py
+++ b/tests/_signing/sigstore_test.py
@@ -332,3 +332,45 @@ def test_verify_not_intoto_statement(
 
         with pytest.raises(ValueError, match="Expected in-toto .* payload"):
             self._verify_dsse_signature(signature_path)
+
+    def test_append_to_existing_claims_jsonl(
+        self, sample_model_folder, mocked_sigstore, tmp_path
+    ):
+        """Test that signing appends to existing claims.jsonl file.
+
+        This implements the unified bundle layout from issue #587, where
+        attestations accumulate in a single claims.jsonl file as the model
+        moves through its lifecycle.
+        """
+        serializer = file.Serializer(
+            self._file_hasher_factory, allow_symlinks=True
+        )
+        manifest = serializer.serialize(sample_model_folder)
+        signature_path = tmp_path / "claims.jsonl"
+
+        # First signing - should create the file
+        self._sign_manifest(manifest, signature_path, sigstore.Signer)
+
+        # Verify file exists and has one line
+        assert signature_path.exists()
+        lines = signature_path.read_text(encoding="utf-8").strip().split("\n")
+        assert len(lines) == 1
+        # Verify it's valid JSON
+        first_bundle = json.loads(lines[0])
+        assert "_type" in first_bundle
+
+        # Second signing - should append to the file
+        self._sign_manifest(manifest, signature_path, sigstore.Signer)
+
+        # Verify file now has two lines
+        lines = signature_path.read_text(encoding="utf-8").strip().split("\n")
+        assert len(lines) == 2
+
+        # Verify both lines are valid JSON bundles
+        first_bundle = json.loads(lines[0])
+        second_bundle = json.loads(lines[1])
+        assert "_type" in first_bundle
+        assert "_type" in second_bundle
+
+        # Both bundles should be independently valid
+        # (We can't fully verify with mocked sigstore, but structure is valid)
diff --git a/tests/api_test.py b/tests/api_test.py
index fda2857e..6a9fa90f 100644
--- a/tests/api_test.py
+++ b/tests/api_test.py
@@ -59,14 +59,20 @@ def populate_tmpdir(tmp_path: Path) -> Path:
 
 def get_signed_files(modelsig: Path) -> list[str]:
     with open(modelsig, "r") as file:
-        signature = json.load(file)
+        content = file.read().strip()
+        # Handle JSONL format: read last line (most recent attestation)
+        lines = content.split("\n")
+        signature = json.loads(lines[-1])
     payload = json.loads(b64decode(signature["dsseEnvelope"]["payload"]))
     return [entry["name"] for entry in payload["predicate"]["resources"]]
 
 
 def get_ignore_paths(modelsig: Path) -> list[str]:
     with open(modelsig, "r") as file:
-        signature = json.load(file)
+        content = file.read().strip()
+        # Handle JSONL format: read last line (most recent attestation)
+        lines = content.split("\n")
+        signature = json.loads(lines[-1])
     payload = json.loads(b64decode(signature["dsseEnvelope"]["payload"]))
     ignore_paths = payload["predicate"]["serialization"]["ignore_paths"]
     ignore_paths.sort()
@@ -87,7 +93,10 @@ def check_ignore_paths(
 
 def get_model_name(modelsig: Path) -> str:
     with open(modelsig, "r") as file:
-        signature = json.load(file)
+        content = file.read().strip()
+        # Handle JSONL format: read last line (most recent attestation)
+        lines = content.split("\n")
+        signature = json.loads(lines[-1])
     payload = json.loads(b64decode(signature["dsseEnvelope"]["payload"]))
     return payload["subject"][0]["name"]