diff --git a/Scripts/sparkle/sign-appcast-kms.py b/Scripts/sparkle/sign-appcast-kms.py
index a53c211..03d28b3 100755
--- a/Scripts/sparkle/sign-appcast-kms.py
+++ b/Scripts/sparkle/sign-appcast-kms.py
@@ -1,5 +1,10 @@
 #!/usr/bin/env python3
-"""Sign Sparkle appcast enclosures with a Google Cloud KMS Ed25519 key."""
+"""Sign Sparkle appcast enclosures.
+
+Google Cloud KMS Ed25519 signing can only sign small payloads directly. Sparkle
+requires a standard EdDSA signature over the full update archive, so normal
+release artifacts use Sparkle's signing tool with the decrypted release key.
+"""
 
 from __future__ import annotations
 
@@ -20,13 +25,14 @@ DEFAULT_LOCATION = "global"
 DEFAULT_KEYRING = "burrow-identity"
 DEFAULT_KEY = "sparkle-ed25519"
 DEFAULT_VERSION = "1"
+DEFAULT_KMS_MAX_INPUT_BYTES = 65536
 
 
-def run(command: list[str]) -> None:
-    subprocess.run(command, check=True)
+def run(command: list[str]) -> subprocess.CompletedProcess[str]:
+    return subprocess.run(command, check=True, text=True, stdout=subprocess.PIPE)
 
 
-def kms_sign(args: argparse.Namespace, payload: Path) -> bytes:
+def kms_sign(args: argparse.Namespace, payload: Path) -> str:
     with tempfile.TemporaryDirectory(prefix="burrow-sparkle-kms.") as tmp:
         signature_path = Path(tmp) / "signature.bin"
         command = [
@@ -49,7 +55,33 @@ def kms_sign(args: argparse.Namespace, payload: Path) -> bytes:
         if args.gcloud_project:
             command.extend(["--project", args.gcloud_project])
         run(command)
-        return base64.b64decode(signature_path.read_text(encoding="utf-8").strip())
+        return base64.b64encode(signature_path.read_bytes()).decode("ascii")
+
+
+def sign_update(args: argparse.Namespace, payload: Path) -> str:
+    key_path = Path(args.ed_key_path) if args.ed_key_path else None
+    if key_path is None or not key_path.is_file():
+        raise FileNotFoundError(
+            "Sparkle artifact is too large for direct Google KMS Ed25519 signing "
+            "and SPARKLE_EDDSA_KEY_PATH does not point to a key file"
+        )
+    result = run([
+        args.sign_update,
+        "--ed-key-file",
+        str(key_path),
+        "-p",
+        str(payload),
+    ])
+    signature = result.stdout.strip().splitlines()[-1].strip()
+    if not signature:
+        raise ValueError(f"Sparkle sign_update produced no signature for {payload}")
+    return signature
+
+
+def sparkle_signature(args: argparse.Namespace, payload: Path) -> str:
+    if payload.stat().st_size <= args.kms_max_input_bytes:
+        return kms_sign(args, payload)
+    return sign_update(args, payload)
 
 
 def enclosure_path(enclosure: ET.Element, artifact_dir: Path) -> Path:
@@ -78,8 +110,7 @@ def sign_appcast(args: argparse.Namespace) -> None:
 
     for enclosure in enclosures:
         artifact = enclosure_path(enclosure, artifact_dir)
-        signature = kms_sign(args, artifact)
-        enclosure.attrib[f"{{{SPARKLE_NS}}}edSignature"] = base64.b64encode(signature).decode("ascii")
+        enclosure.attrib[f"{{{SPARKLE_NS}}}edSignature"] = sparkle_signature(args, artifact)
 
     tree.write(appcast, encoding="utf-8", xml_declaration=True)
     print(f"signed {len(enclosures)} enclosure(s) in {appcast}")
@@ -94,7 +125,10 @@ def parse_args(argv: list[str]) -> argparse.Namespace:
     parser.add_argument("--kms-keyring", default=os.environ.get("BURROW_KMS_KEYRING", DEFAULT_KEYRING))
     parser.add_argument("--kms-key", default=os.environ.get("BURROW_SPARKLE_KMS_KEY", DEFAULT_KEY))
     parser.add_argument("--kms-version", default=os.environ.get("BURROW_SPARKLE_KMS_VERSION", DEFAULT_VERSION))
+    parser.add_argument("--kms-max-input-bytes", type=int, default=DEFAULT_KMS_MAX_INPUT_BYTES)
     parser.add_argument("--gcloud", default=os.environ.get("GCLOUD", "gcloud"))
+    parser.add_argument("--ed-key-path", default=os.environ.get("SPARKLE_EDDSA_KEY_PATH"))
+    parser.add_argument("--sign-update", default=os.environ.get("SPARKLE_SIGN_UPDATE", "sign_update"))
     return parser.parse_args(argv)
 
 
diff --git a/docs/DISTRIBUTION.md b/docs/DISTRIBUTION.md
index b522e96..9c6b7ae 100644
--- a/docs/DISTRIBUTION.md
+++ b/docs/DISTRIBUTION.md
@@ -30,8 +30,13 @@ pins iOS App Store profiles to that certificate. Existing App Store profiles
 with the same name are deleted and recreated if Apple reports that they contain
 a different distribution certificate.
 
-Sparkle appcast signing uses the non-exportable `sparkle-ed25519` Google KMS
-key. macOS release builds embed public EdDSA key
+Sparkle appcast signing keeps the `sparkle-ed25519` Google KMS key available
+for payloads small enough for direct KMS Ed25519 signing, but normal macOS
+release archives are larger than Google KMS accepts for direct Ed25519
+messages. Those archives are signed with Sparkle's `sign_update` using the
+decrypted release `SPARKLE_EDDSA_KEY_PATH` so Sparkle receives the standard
+full-archive EdDSA signature it verifies at update time. macOS release builds
+embed public EdDSA key
 `Myv9ZNZT6YGKMtMezh52ra4WqaeEKc4VlvVU0evhJeI=` and point Sparkle metadata at
 `https://releases.burrow.net/sparkle/appcast.xml`. The appcast signer runs only
 when `BURROW_SPARKLE_SIGN_WITH_KMS=true` so unsigned local validation artifacts
diff --git a/evolution/proposals/BEP-0009-release-infrastructure-and-store-upload-pipeline.md b/evolution/proposals/BEP-0009-release-infrastructure-and-store-upload-pipeline.md
index 51b55e7..3458875 100644
--- a/evolution/proposals/BEP-0009-release-infrastructure-and-store-upload-pipeline.md
+++ b/evolution/proposals/BEP-0009-release-infrastructure-and-store-upload-pipeline.md
@@ -141,10 +141,14 @@ The same change also makes the forge itself the release authority: release tags
     sign the app/extension bundles with the patched `rcodesign` PKCS#11 path.
   - The signed iOS IPA is uploaded to App Store Connect, and the signed macOS
     ZIP is used as the Sparkle tester artifact.
-- Add a non-exportable Google KMS key named `sparkle-ed25519` for Sparkle
-  appcast signatures. It uses `EC_SIGN_ED25519` with software protection level
-  because Google KMS rejects Ed25519 for HSM protection. macOS builds embed
-  public EdDSA key `Myv9ZNZT6YGKMtMezh52ra4WqaeEKc4VlvVU0evhJeI=`.
+- Add a Google KMS key named `sparkle-ed25519` for small Sparkle signing probes
+  and future signing-service work. It uses `EC_SIGN_ED25519` with software
+  protection level because Google KMS rejects Ed25519 for HSM protection. Google
+  KMS also rejects normal macOS release archives as direct Ed25519 messages, so
+  the tester pipeline signs full-size Sparkle archives with Sparkle's
+  `sign_update` and the decrypted `SPARKLE_EDDSA_KEY_PATH` release secret until
+  a compatible KMS-backed Sparkle signing service is introduced. macOS builds
+  embed public EdDSA key `Myv9ZNZT6YGKMtMezh52ra4WqaeEKc4VlvVU0evhJeI=`.
 - Use Namespace labels for platform lanes:
   - `namespace-profile-linux-medium`
   - `namespace-profile-macos-large`
diff --git a/infra/identity/README.md b/infra/identity/README.md
index cad3fdb..21b74da 100644
--- a/infra/identity/README.md
+++ b/infra/identity/README.md
@@ -81,10 +81,13 @@ Its public key matches KMS key version
 
 ## Sparkle Appcast Signing
 
-Sparkle appcasts use `sparkle-ed25519`, a non-exportable Google KMS key with
-algorithm `EC_SIGN_ED25519` and software protection level. Google KMS rejects
-Ed25519 at HSM protection level, so this key is intentionally separate from the
-HSM RSA keys used for Apple certificate CSRs and repository signing.
+Sparkle appcasts keep `sparkle-ed25519`, a Google KMS key with algorithm
+`EC_SIGN_ED25519` and software protection level, for small signing probes and
+future signing-service work. Google KMS rejects Ed25519 at HSM protection level
+and also rejects full-size macOS release archives as direct Ed25519 messages.
+The tester pipeline therefore signs normal Sparkle archives with Sparkle's
+`sign_update` and the decrypted `SPARKLE_EDDSA_KEY_PATH` release secret so the
+appcast contains the standard full-archive EdDSA signature Sparkle verifies.
 
 The public EdDSA key embedded in macOS release builds is: