diff --git a/src/cloudai/configurator/env_params.py b/src/cloudai/configurator/env_params.py
index 13db021d2..36d73bf14 100644
--- a/src/cloudai/configurator/env_params.py
+++ b/src/cloudai/configurator/env_params.py
@@ -32,7 +32,7 @@
 import math
 import random
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, List, Optional
+from typing import TYPE_CHECKING, Any, Dict, List, Literal, Optional, Protocol, runtime_checkable
 
 from pydantic import BaseModel, ConfigDict, Field, model_validator
 from typing_extensions import Self
@@ -76,6 +76,50 @@ def _validate_weights(self) -> Self:
         return self
 
 
+class ObsLeafDescriptor(BaseModel):
+    """
+    Description of one leaf of a structured (named) observation.
+
+    A structured observation maps each observed name to a self-describing leaf
+    so adapters can build the matching subspace without guessing: a ``"box"``
+    leaf becomes a continuous vector of width ``dim`` (e.g. a log-encoded
+    env_param as ``dim=2``); a ``"discrete"`` leaf becomes a categorical of
+    size ``n``. Stateless agents that consume the flat observation ignore this.
+    """
+
+    model_config = ConfigDict(extra="forbid")
+
+    kind: Literal["box", "discrete"]
+    dim: int = 1
+    n: Optional[int] = None
+
+    @model_validator(mode="after")
+    def _validate(self) -> Self:
+        if self.dim < 1:
+            raise ValueError(f"ObsLeafDescriptor dim must be >= 1; got {self.dim}")
+        if self.kind == "discrete" and (self.n is None or self.n < 1):
+            raise ValueError(f"ObsLeafDescriptor(kind='discrete') requires n >= 1; got n={self.n}")
+        return self
+
+
+@runtime_checkable
+class StructuredObservation(Protocol):
+    """
+    Optional env hooks that expose a structured (per-leaf) observation.
+
+    An env opts in by returning per-leaf :class:`ObsLeafDescriptor` from
+    ``structured_observation_descriptors`` (``None`` keeps the flat-vector
+    path) and encoding a raw observation into the matching named leaves via
+    ``encode_observation``. ``GymnasiumAdapter`` consumes these to expose a
+    ``gymnasium.spaces.Dict`` observation; the hooks are duck-typed, so envs
+    need not subclass this Protocol.
+    """
+
+    def structured_observation_descriptors(self) -> Optional[Dict[str, ObsLeafDescriptor]]: ...
+
+    def encode_observation(self, observation: list) -> Dict[str, Any]: ...
+
+
 @dataclasses.dataclass(frozen=True)
 class EnvParam:
     """
diff --git a/src/cloudai/core.py b/src/cloudai/core.py
index 752d24972..aefcd0573 100644
--- a/src/cloudai/core.py
+++ b/src/cloudai/core.py
@@ -51,6 +51,7 @@
 from ._core.test_scenario import METRIC_ERROR, MetricErrorSentinel, MetricValue, TestDependency, TestRun, TestScenario
 from .configurator.base_agent import BaseAgent, BaseAgentConfig, RewardOverrides
 from .configurator.cloudai_gym import CloudAIGymEnv
+from .configurator.env_params import ObsLeafDescriptor, StructuredObservation
 from .configurator.grid_search import GridSearchAgent
 from .models.workload import CmdArgs, NsysConfiguration, PredictorConfig, TestDefinition
 from .parser import Parser
@@ -85,6 +86,7 @@
     "MetricValue",
     "MissingTestError",
     "NsysConfiguration",
+    "ObsLeafDescriptor",
     "Parser",
     "PerTestReporter",
     "PredictorConfig",
@@ -96,6 +98,7 @@
     "RewardOverrides",
     "Runner",
     "StatusReporter",
+    "StructuredObservation",
     "System",
     "SystemConfigParsingError",
     "TarballReporter",
diff --git a/tests/test_env_params.py b/tests/test_env_params.py
index 362a5e5d1..a0341d615 100644
--- a/tests/test_env_params.py
+++ b/tests/test_env_params.py
@@ -43,6 +43,7 @@
     EnvParam,
     EnvParams,
     EnvParamSpec,
+    ObsLeafDescriptor,
     write_env_params,
 )
 from cloudai.core import TestRun
@@ -375,3 +376,31 @@ def test_apply_params_set_accepts_weighted_env_param_draw() -> None:
     new_tr = tr.apply_params_set({}, env_params={"ball_speed": 1})
 
     assert new_tr.test.cmd_args.ball_speed == 1
+
+
+# --- ObsLeafDescriptor: structured-observation leaf schema ---
+
+
+def test_obs_leaf_descriptor_box_defaults() -> None:
+    leaf = ObsLeafDescriptor(kind="box", dim=2)
+    assert leaf.kind == "box"
+    assert leaf.dim == 2
+    assert leaf.n is None
+
+
+def test_obs_leaf_descriptor_discrete_requires_n() -> None:
+    leaf = ObsLeafDescriptor(kind="discrete", dim=1, n=3)
+    assert leaf.n == 3
+    with pytest.raises(ValidationError, match="requires n"):
+        ObsLeafDescriptor(kind="discrete", dim=1)
+    with pytest.raises(ValidationError, match="requires n"):
+        ObsLeafDescriptor(kind="discrete", dim=1, n=0)
+
+
+def test_obs_leaf_descriptor_rejects_bad_dim_and_extra_fields() -> None:
+    with pytest.raises(ValidationError, match="dim must be"):
+        ObsLeafDescriptor(kind="box", dim=0)
+    with pytest.raises(ValidationError):
+        ObsLeafDescriptor(kind="box", dim=1, unexpected=1)  # type: ignore
+    with pytest.raises(ValidationError):
+        ObsLeafDescriptor(kind="categorical", dim=1)  # type: ignore