diff --git a/src/maxtext/trainers/post_train/rl/utils_rl.py b/src/maxtext/trainers/post_train/rl/utils_rl.py
index 5a29b5fbc2..038c78ee13 100644
--- a/src/maxtext/trainers/post_train/rl/utils_rl.py
+++ b/src/maxtext/trainers/post_train/rl/utils_rl.py
@@ -535,15 +535,25 @@ def check_correctness(extracted_response: str, acceptable_answers: list[str], tm
 def get_optimizer(tmvp_config: Any, max_train_steps: int) -> optax.GradientTransformation:
   """Function to obtain an optax optimizer, currently we use adamw.
 
-  Schedule shape is controlled by `learning_rate_schedule_steps` when set
-  (>0); this decouples warmup/decay shape from training length so the same
-  schedule can be applied across runs of different num_batches. Default
-  (-1) falls back to `max_train_steps` for backward compatibility — matches
-  the documented behavior of base.yml's `learning_rate_schedule_steps: -1`
-  ("By default the length of the schedule is set to the number of steps").
+  The LR schedule length defaults to the actual RL run length
+  (`max_train_steps` = num_batches * num_iterations * train_fraction *
+  num_epoch). RL does not use the top-level `steps` (a pretraining concept) for
+  its run length, so the schedule must track `max_train_steps`, not `steps`.
+
+  `learning_rate_schedule_steps` may be set to decouple the schedule shape from
+  the run length (e.g. to match a fixed schedule across runs with different
+  num_batches). It is honored only as a deliberate override: the config
+  validator (`MaxTextConfig.set_derived_and_validate_values`) rewrites
+  `learning_rate_schedule_steps == -1` to `steps` before this runs, so a value
+  equal to `steps` means "unset" and falls back to `max_train_steps`; only a
+  value that differs from `steps` is treated as an explicit schedule length.
   """
-  schedule_steps = getattr(tmvp_config, "learning_rate_schedule_steps", -1)
-  if schedule_steps is None or schedule_steps <= 0:
+  lr_schedule_steps = getattr(tmvp_config, "learning_rate_schedule_steps", -1)
+  config_steps = getattr(tmvp_config, "steps", -1)
+  if lr_schedule_steps is not None and lr_schedule_steps > 0 and lr_schedule_steps != config_steps:
+    # Deliberate decoupling of the schedule length from the run length.
+    schedule_steps = lr_schedule_steps
+  else:
     schedule_steps = max_train_steps
   schedule = optax.schedules.warmup_cosine_decay_schedule(
       init_value=0.0,
diff --git a/tests/post_training/unit/rl_lr_schedule_test.py b/tests/post_training/unit/rl_lr_schedule_test.py
new file mode 100644
index 0000000000..39f317685f
--- /dev/null
+++ b/tests/post_training/unit/rl_lr_schedule_test.py
@@ -0,0 +1,206 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Regression tests for the RL learning-rate schedule (CPU-only).
+
+These guard the contract that PR #4029 ("get_optimizer: respect
+learning_rate_schedule_steps") was supposed to preserve but silently broke:
+
+  With a default RL config (learning_rate_schedule_steps unset == -1) the LR
+  schedule length must equal the actual training length (max_train_steps), so
+  warmup completes inside the run.
+
+Why the bug shipped (and why the existing get_optimizer tests miss it):
+`tests/post_training/unit/rl_utils_test.py::TestGetOptimizer` builds the config
+from a `SimpleNamespace` that does not even carry `learning_rate_schedule_steps`,
+so `getattr(cfg, "learning_rate_schedule_steps", -1)` returns -1, the
+`schedule_steps <= 0` fallback fires, and the schedule correctly tracks
+max_train_steps. The Pydantic validator never runs.
+
+In a real run it DOES run: `MaxTextConfig.set_derived_and_validate_values`
+rewrites `learning_rate_schedule_steps == -1 -> steps` (base.yml default 150_001)
+*before* get_optimizer is called (configs/types.py). The `<= 0` guard in
+get_optimizer therefore can never fire, warmup becomes
+`0.1 * 150_001 = 15_000` instead of `0.1 * max_train_steps`, and on a 500-step
+run the LR is ~300x too low at the same step.
+
+The only way to catch this is to build the config through the REAL pyconfig
+path so the validator runs. That is what these tests do.
+"""
+
+import sys
+import unittest
+
+import pytest
+
+from maxtext.configs import pyconfig
+from maxtext.trainers.post_train.rl import utils_rl
+from tests.utils.test_helpers import get_test_config_path
+
+pytestmark = [pytest.mark.post_training]
+
+
+# Tiny-model overrides known to build a valid MaxTextConfig on CPU without
+# network access, mirroring tests/post_training/unit/lora_utils_test.py. The
+# model shape is irrelevant here (get_optimizer only reads scalar config
+# fields); these just let initialize_pydantic validate quickly.
+_MODEL_OVERRIDES = {
+    "per_device_batch_size": 1.0,
+    "enable_checkpointing": False,
+    "base_num_decoder_layers": 1,
+    "attention": "dot_product",
+    "max_target_length": 8,
+    "base_emb_dim": 128,
+    "base_num_query_heads": 2,
+    "base_num_kv_heads": 2,
+    "base_mlp_dim": 256,
+    "max_prefill_predict_length": 4,
+    "model_name": "llama2-7b",
+    "enable_nnx": True,
+    "pure_nnx_decoder": True,
+    "override_model_config": True,
+    "weight_dtype": "bfloat16",
+}
+
+
+def _make_config(**overrides):
+  """Build a real MaxTextConfig (runs the Pydantic validator) for RL.
+
+  Using initialize_pydantic (not a SimpleNamespace) is the whole point: it is
+  what promotes learning_rate_schedule_steps == -1 to `steps`.
+  """
+  return pyconfig.initialize_pydantic(
+      [sys.argv[0], get_test_config_path()],
+      run_name="rl_lr_schedule_test",
+      **_MODEL_OVERRIDES,
+      **overrides,
+  )
+
+
+def _max_train_steps(config):
+  """Mirror of train_rl.get_max_train_steps.
+
+  Inlined rather than imported because importing train_rl pulls the heavy RL
+  training stack (tunix/vLLM). The formula itself is part of the contract under
+  test, so a drift here is a meaningful signal.
+  """
+  return int(config.num_batches * config.rl.num_iterations * config.train_fraction * config.num_epoch)
+
+
+def _effective_lr_at_step(opt, step):
+  """Step an inject_hyperparams optimizer `step` times and read the LR it
+  exposes in opt_state.hyperparams. This is exactly the per-step LR that
+  tunix's peft_trainer reads and logs, so it reflects what training actually
+  sees, not a re-derivation of the schedule.
+  """
+  import jax.numpy as jnp  # pylint: disable=import-outside-toplevel
+
+  params = {"w": jnp.zeros((), dtype=jnp.float32)}
+  grads = {"w": jnp.zeros((), dtype=jnp.float32)}
+  state = opt.init(params)
+  for _ in range(step):
+    _, state = opt.update(grads, state, params)
+  return float(state.hyperparams["learning_rate"])
+
+
+class RLLearningRateScheduleTest(unittest.TestCase):
+  """Schedule-shape guards for utils_rl.get_optimizer built on a real config."""
+
+  @pytest.mark.cpu_only
+  def test_default_rl_config_warms_up_within_run(self):
+    """REGRESSION GUARD (FAILS on PR #4029, passes once fixed).
+
+    With learning_rate_schedule_steps unset (-1) and base.yml's default
+    `steps` (150_001), the LR must still reach its configured peak by the end
+    of the intended warmup (0.1 * max_train_steps). On the buggy code the
+    warmup is sized to 150_001, so the LR is stuck near zero for the whole run.
+    """
+    peak = 3e-6
+    config = _make_config(
+        learning_rate=peak,
+        warmup_steps_fraction=0.1,
+        gradient_clipping_threshold=0.0,
+        num_batches=500,
+        num_epoch=1,
+        train_fraction=1.0,
+        steps=150_001,  # base.yml default (a pretraining-sized number)
+        learning_rate_schedule_steps=-1,  # "user did not set it" (base.yml default)
+    )
+    max_train_steps = _max_train_steps(config)
+    # Sanity: we are in the regime where the bug shows (run << base `steps`).
+    self.assertLess(max_train_steps, config.steps)
+
+    opt = utils_rl.get_optimizer(config, max_train_steps)
+    warmup_end = int(config.warmup_steps_fraction * max_train_steps)
+    lr = _effective_lr_at_step(opt, warmup_end + 5)
+    # Correct: lr ~= peak.  Buggy (#4029): lr ~= peak * warmup_end / 15000,
+    # i.e. < 1% of peak.  A 0.5*peak threshold cleanly separates the two.
+    self.assertGreaterEqual(
+        lr,
+        0.5 * peak,
+        msg=(
+            f"LR reached only {lr:.2e} by step {warmup_end + 5} (peak={peak:.2e}). "
+            "Warmup was sized to base.yml `steps`, not max_train_steps "
+            f"(={max_train_steps}). learning_rate_schedule_steps in effect="
+            f"{config.learning_rate_schedule_steps}."
+        ),
+    )
+
+  @pytest.mark.cpu_only
+  def test_explicit_schedule_steps_decouples_from_run_length(self):
+    """FEATURE GUARD (passes before and after the fix).
+
+    An explicit learning_rate_schedule_steps must drive the schedule shape
+    independently of max_train_steps (the capability #4029 was adding). This
+    ensures the regression fix does not simply delete the feature.
+    """
+    peak = 3e-6
+    schedule_len = 1000
+    config = _make_config(
+        learning_rate=peak,
+        warmup_steps_fraction=0.1,
+        gradient_clipping_threshold=0.0,
+        num_batches=50,
+        num_epoch=1,
+        train_fraction=1.0,
+        learning_rate_schedule_steps=schedule_len,  # explicitly set by the user
+    )
+    max_train_steps = _max_train_steps(config)
+
+    opt = utils_rl.get_optimizer(config, max_train_steps)
+    # The warmup length must follow the explicit schedule (0.1 * 1000 = 100),
+    # independent of max_train_steps. Probe at fixed steps so the assertion does
+    # not depend on num_iterations: early in a 1000-step warmup the LR is still
+    # small, and by ~100 steps it has reached peak. A fix that wrongly forced
+    # schedule == max_train_steps (a short warmup) would push LR@20 to ~peak and
+    # trip the first assertion.
+    self.assertLessEqual(_effective_lr_at_step(opt, 20), 0.4 * peak)
+    self.assertGreaterEqual(_effective_lr_at_step(opt, int(0.1 * schedule_len) + 5), 0.9 * peak)
+
+  @pytest.mark.cpu_only
+  def test_validator_overwrites_minus_one_sentinel(self):
+    """ROOT-CAUSE characterization (effective-value assertion).
+
+    The value get_optimizer reads is NOT the -1 the user left: the validator
+    promoted it to `steps`. This is precisely why get_optimizer's `<= 0`
+    fallback is dead code in a real run. (Passes today; documents the seam
+    that makes test_default_rl_config_warms_up_within_run fail.)
+    """
+    config = _make_config(steps=150_001, learning_rate_schedule_steps=-1)
+    self.assertNotEqual(config.learning_rate_schedule_steps, -1)
+    self.assertEqual(config.learning_rate_schedule_steps, config.steps)
+
+
+if __name__ == "__main__":
+  unittest.main()