diff --git a/.gitignore b/.gitignore
index 5b5b47357a..d8b39c1dc6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -39,6 +39,8 @@ docs/_spelling/
 /skyrl-gym/dist
 
 *.log
+nohup.out
+tensorboard_log/
 
 # SQLite database files
 *.db
diff --git a/skyrl-tx/pyproject.toml b/skyrl-tx/pyproject.toml
index 587d9c19ea..c0f9db742c 100644
--- a/skyrl-tx/pyproject.toml
+++ b/skyrl-tx/pyproject.toml
@@ -10,8 +10,6 @@ readme = "README.md"
 requires-python = ">=3.11"
 dependencies = [
     "datasets>=4.0.0",
-    "flax>=0.12.2",
-    "optax>=0.2.5",
     "pillow>=11.3.0",
     "rich>=14.1.0",
     "safetensors>=0.6.2",
@@ -22,7 +20,6 @@ dependencies = [
     "peft",
     "hf_transfer",
     "cloudpathlib>=0.23.0",
-    "jax>=0.8,<1.0",
 ]
 
 [project.optional-dependencies]
@@ -61,7 +58,10 @@ azure = [
 # respectively.
 
 jax = [
+    "jax>=0.8,<1.0",
     "jax[cuda12]>=0.7.2; sys_platform == 'linux'",
+    "flax>=0.12.2",
+    "optax>=0.2.5",
 ]
 
 skyrl_train = [
diff --git a/skyrl-tx/tx/tinker/engine.py b/skyrl-tx/tx/tinker/engine.py
index a0618f68c4..6cdd90d940 100644
--- a/skyrl-tx/tx/tinker/engine.py
+++ b/skyrl-tx/tx/tinker/engine.py
@@ -14,10 +14,8 @@
 from tx.tinker.db_models import FutureDB, RequestStatus, CheckpointDB, CheckpointStatus, ModelDB, SessionDB
 from tx.tinker import types
 from tx.tinker.config import EngineConfig, add_model
-from tx.tinker.backends.jax import JaxBackend, JaxBackendConfig
-from tx.tinker.backends.skyrl_train import SkyRLTrainBackend, SkyRLTrainBackendConfig
 from tx.tinker.backends.utils import log_timing
-from tx.tinker.loss_fns import LOSS_TYPES
+from tx.tinker.types import LOSS_TYPES
 from tx.utils.log import logger
 
 
@@ -130,10 +128,21 @@ def prepare_model_pass_batch(
     )
 
 
-BACKENDS = {
-    "jax": (JaxBackend, JaxBackendConfig),
-    "skyrl_train": (SkyRLTrainBackend, SkyRLTrainBackendConfig),
-}
+def get_backend_classes(backend_name: str):
+    """Lazy import backends to avoid importing unused backend dependencies (e.g., JAX, Ray)."""
+    if backend_name == "jax":
+        from tx.tinker.backends.jax import JaxBackend, JaxBackendConfig
+
+        return JaxBackend, JaxBackendConfig
+    elif backend_name == "skyrl_train":
+        from tx.tinker.backends.skyrl_train import SkyRLTrainBackend, SkyRLTrainBackendConfig
+
+        return SkyRLTrainBackend, SkyRLTrainBackendConfig
+    else:
+        raise ValueError(
+            f"Unknown backend: {backend_name}. Available backends: jax, skyrl_train. "
+            f"Make sure the backend's dependencies are installed (e.g., pip install skyrl-tx[jax])"
+        )
 
 
 class TinkerEngine:
@@ -189,10 +198,7 @@ def __init__(
         self.db_engine = create_engine(config.database_url, echo=False)
 
         # Initialize the backend (handles model state, computation, and adapter management)
-        if config.backend not in BACKENDS:
-            raise ValueError(f"Unknown backend: {config.backend}. Available backends: {list(BACKENDS.keys())}")
-
-        backend_class, backend_config_class = BACKENDS[config.backend]
+        backend_class, backend_config_class = get_backend_classes(config.backend)
         backend_config = backend_config_class(**config.backend_config)
         self.backend = backend_class(config.base_model, backend_config)
 
@@ -312,7 +318,7 @@ def find_batchable_sample(self, session: Session) -> dict[str, tuple[str, types.
 
         # TODO: This leaks the abstraction by accessing backend-specific config.
         # We should find a better way to handle this going forward.
-        if isinstance(self.backend, JaxBackend) and self.backend.config.sample_max_num_sequences > 0:
+        if self.config.backend == "jax" and self.backend.config.sample_max_num_sequences > 0:
             batchable = batchable[: self.backend.config.sample_max_num_sequences]
 
         return {str(f.request_id): (f.model_id, types.SampleInput.model_validate(f.request_data)) for f in batchable}
diff --git a/skyrl-tx/tx/tinker/loss_fns.py b/skyrl-tx/tx/tinker/loss_fns.py
index 6aa7c98db0..ad011df5b3 100644
--- a/skyrl-tx/tx/tinker/loss_fns.py
+++ b/skyrl-tx/tx/tinker/loss_fns.py
@@ -1,4 +1,4 @@
-"""Loss functions for training."""
+"""Loss functions for training (JAX implementations)."""
 
 import jax
 import jax.numpy as jnp
diff --git a/skyrl-tx/tx/tinker/types.py b/skyrl-tx/tx/tinker/types.py
index adbe1a8c2d..392da26605 100644
--- a/skyrl-tx/tx/tinker/types.py
+++ b/skyrl-tx/tx/tinker/types.py
@@ -256,3 +256,12 @@ class PreparedSampleBatch(BaseModel):
 
     # Mapping from samples back to requests: (request_id, model_id, start_idx, end_idx, prompt_logprobs_requested)
     request_batch_slices: list[tuple[str, str, int, int, bool]]
+
+
+# Loss function type mappings (used for validation and backend dispatch)
+# NOTE: Must stay in sync with LOSS_FUNCTION_MAP in loss_fns.py
+LOSS_TYPES = {
+    "cross_entropy": 0,
+    "importance_sampling": 1,
+    "ppo": 2,
+}