diff --git a/CHANGELOG.md b/CHANGELOG.md
index 74fefd5..fc1078c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,13 @@ Note that 0.3.3 was never released; 0.3.4 follows 0.3.2.
   registered replaced the others and, for a while, requests used caches that
   could not see each other's entries. The lazy set-up and the `@cache`
   fallback now happen under one lock. ([#76](https://github.com/allen0099/FastAPI-CacheX/issues/76))
+- A JWT session configured with an asymmetric `jwt_algorithm` (`RS*`, `ES*`,
+  `PS*`, `EdDSA`) and the built-in serializer now fails when the
+  `SessionManager` is built, with a `ValueError` that names the fix. The
+  built-in serializer only has the `secret_key` string, so such a configuration
+  was accepted at startup and then failed inside PyJWT on the first
+  `create_session()`. Only the HMAC algorithms work without a custom
+  `token_serializer`; configurations that pass one are unaffected. ([#86](https://github.com/allen0099/FastAPI-CacheX/issues/86))
 
 ### Documentation
 
diff --git a/docs/JWT_CLAIMS.md b/docs/JWT_CLAIMS.md
index 74f83d8..7fa0b95 100644
--- a/docs/JWT_CLAIMS.md
+++ b/docs/JWT_CLAIMS.md
@@ -37,7 +37,7 @@ The implementation lives in [`fastapi_cachex/session/token_serializers.py`](http
 | `session_ttl` | `3600` | Session lifetime in seconds; used for `exp` when the session has no `expires_at` |
 
 > [!NOTE]
-> **Asymmetric algorithms:** `jwt_algorithm` accepts `HS*`, `RS*`, `ES*`, `PS*` and `EdDSA`, but the built-in serializer signs and verifies with the single `secret_key` string. In practice only the HMAC algorithms (`HS256`, `HS384`, `HS512`) work out of the box; an asymmetric algorithm needs a custom serializer that encodes with a private key and decodes with the matching public key (see [Extension Guide](#extension-guide-adding-custom-claims)).
+> **Asymmetric algorithms:** `jwt_algorithm` accepts `HS*`, `RS*`, `ES*`, `PS*` and `EdDSA`, but the built-in serializer signs and verifies with the single `secret_key` string, so it only supports the HMAC algorithms (`HS256`, `HS384`, `HS512`). Building a `SessionManager` with an asymmetric algorithm and no custom serializer raises `ValueError`. To use one, pass a custom `token_serializer` that encodes with a private key and decodes with the matching public key (see [Extension Guide](#extension-guide-adding-custom-claims)).
 
 ### Standard Claims That Are Not Implemented
 
diff --git a/docs/SESSION.md b/docs/SESSION.md
index 175e9e0..7acc956 100644
--- a/docs/SESSION.md
+++ b/docs/SESSION.md
@@ -464,7 +464,9 @@ config = SessionConfig(
 
 `jwt_algorithm` must be one of `HS256`, `HS384`, `HS512`, `RS256`, `RS384`, `RS512`, `ES256`,
 `ES384`, `ES512`, `PS256`, `PS384`, `PS512` or `EdDSA`; anything else (including `none`) raises a
-`ValidationError`. The same `secret_key` is used to sign and verify tokens.
+`ValidationError`. The built-in serializer signs and verifies with the same `secret_key`, so it
+only supports `HS256`, `HS384` and `HS512`: with an asymmetric algorithm, `SessionManager` raises
+`ValueError` unless you pass a custom `token_serializer` that holds the key pair.
 
 Security notes:
 
diff --git a/fastapi_cachex/session/config.py b/fastapi_cachex/session/config.py
index 52bf4a7..142ee4e 100644
--- a/fastapi_cachex/session/config.py
+++ b/fastapi_cachex/session/config.py
@@ -30,6 +30,12 @@
     }
 )
 
+# The algorithms the built-in `JWTTokenSerializer` can use: it signs and
+# verifies with the single `secret_key` string. The asymmetric ones above need
+# a private key to sign and a public key to verify, so they only work with a
+# custom `token_serializer`.
+JWT_HMAC_ALGORITHMS = frozenset({"HS256", "HS384", "HS512"})
+
 
 class SessionConfig(BaseModel):
     """Session configuration settings."""
diff --git a/fastapi_cachex/session/token_serializers.py b/fastapi_cachex/session/token_serializers.py
index 8f5a854..d32a6b6 100644
--- a/fastapi_cachex/session/token_serializers.py
+++ b/fastapi_cachex/session/token_serializers.py
@@ -18,6 +18,7 @@
 from typing import Any
 from typing import Protocol
 
+from .config import JWT_HMAC_ALGORITHMS
 from .models import SessionToken
 
 if TYPE_CHECKING:  # Import for typing only to avoid circular import concerns
@@ -109,7 +110,24 @@ def __init__(self, config: SessionConfig, jwt_module: Any | None = None) -> None
             config: Session configuration instance.
             jwt_module: Optional JWT-compatible module providing ``encode`` and
                 ``decode``; defaults to importing ``jwt`` (PyJWT).
+
+        Raises:
+            ValueError: If ``config.jwt_algorithm`` is asymmetric. This
+                serializer signs and verifies with the ``secret_key`` string,
+                which only the HMAC algorithms can use; an asymmetric
+                algorithm needs a custom ``token_serializer`` that holds the
+                key pair.
         """
+        if config.jwt_algorithm not in JWT_HMAC_ALGORITHMS:
+            supported = ", ".join(sorted(JWT_HMAC_ALGORITHMS))
+            msg = (
+                f"jwt_algorithm {config.jwt_algorithm!r} needs a private/public "
+                f"key pair, but the built-in JWT serializer signs with secret_key; "
+                f"use one of {supported}, or pass a custom token_serializer to "
+                f"SessionManager"
+            )
+            raise ValueError(msg)
+
         if jwt_module is not None:
             self.jwt_encoder = jwt_module
         else:
diff --git a/tests/session/test_token_serializers.py b/tests/session/test_token_serializers.py
index 49e87e0..63e9963 100644
--- a/tests/session/test_token_serializers.py
+++ b/tests/session/test_token_serializers.py
@@ -5,7 +5,9 @@
 import pytest
 from pydantic import SecretStr
 
+from fastapi_cachex.backends import MemoryBackend
 from fastapi_cachex.session.config import SessionConfig
+from fastapi_cachex.session.manager import SessionManager
 from fastapi_cachex.session.models import SessionToken
 from fastapi_cachex.session.token_serializers import JWTTokenSerializer
 from fastapi_cachex.session.token_serializers import SimpleTokenSerializer
@@ -242,3 +244,66 @@ def decode(self, token_str: str, **kwargs: object) -> dict[str, object]:
 
     assert token.session_id == "session-id"
     assert int(token.issued_at.timestamp()) == 1700000000
+
+
+@pytest.mark.parametrize(
+    "algorithm",
+    ["RS256", "RS512", "ES256", "ES384", "PS256", "EdDSA"],
+)
+def test_jwt_serializer_rejects_asymmetric_algorithms(algorithm: str) -> None:
+    """The built-in serializer only has `secret_key`, so it cannot sign RS/ES/PS/EdDSA.
+
+    The config accepted these, and the first `create_session()` then failed
+    inside PyJWT. They must fail when the manager is built instead.
+    """
+    config = SessionConfig(
+        secret_key=SecretStr("a" * 32),
+        token_format="jwt",
+        jwt_algorithm=algorithm,
+    )
+
+    with pytest.raises(ValueError, match="custom token_serializer"):
+        JWTTokenSerializer(config, jwt_module=StubJWTModule())
+
+
+def test_session_manager_fails_at_startup_for_asymmetric_jwt() -> None:
+    """The error surfaces when `SessionManager` is built, not on first use."""
+    config = SessionConfig(
+        secret_key=SecretStr("a" * 32),
+        token_format="jwt",
+        jwt_algorithm="RS256",
+    )
+
+    with pytest.raises(ValueError, match="RS256"):
+        SessionManager(MemoryBackend(), config)
+
+
+def test_session_manager_accepts_asymmetric_jwt_with_custom_serializer() -> None:
+    """A custom serializer that holds the key pair is still allowed."""
+    config = SessionConfig(
+        secret_key=SecretStr("a" * 32),
+        token_format="jwt",
+        jwt_algorithm="RS256",
+    )
+    serializer = SimpleTokenSerializer()
+
+    manager = SessionManager(MemoryBackend(), config, token_serializer=serializer)
+
+    assert manager._serializer is serializer
+
+
+@pytest.mark.parametrize("algorithm", ["HS256", "HS384", "HS512"])
+def test_jwt_serializer_round_trips_hmac_algorithms(algorithm: str) -> None:
+    """The HMAC algorithms work end to end with real PyJWT."""
+    pytest.importorskip("jwt")
+    config = SessionConfig(
+        secret_key=SecretStr("a" * 32),
+        token_format="jwt",
+        jwt_algorithm=algorithm,
+    )
+    serializer = JWTTokenSerializer(config)
+    token = SessionToken(
+        session_id="sid-1", signature="", issued_at=datetime.now(timezone.utc)
+    )
+
+    assert serializer.from_string(serializer.to_string(token)).session_id == "sid-1"