diff --git a/src/core/run.jl b/src/core/run.jl
index 6d4374c..3ae0d50 100644
--- a/src/core/run.jl
+++ b/src/core/run.jl
@@ -34,7 +34,9 @@ function _run(
         hook(PRE_EPISODE_STAGE, policy, env)
 
         while !is_terminated(env) # one episode
-            action = policy(PRE_ACT_STAGE, env)
+            action = policy(env)
+
+            policy(PRE_ACT_STAGE, env, action)
             hook(PRE_ACT_STAGE, policy, env, action)
 
             env(action)
diff --git a/src/policies/agents/agent.jl b/src/policies/agents/agent.jl
index 817c6c9..8e45d86 100644
--- a/src/policies/agents/agent.jl
+++ b/src/policies/agents/agent.jl
@@ -31,16 +31,25 @@ function check(agent::Agent, env::AbstractEnv)
     check(agent.policy, env)
 end
 
+#####
+# Default behaviors
+#####
+
 """
 Here we extend the definition of `(p::AbstractPolicy)(::AbstractEnv)` in
 `RLBase` to accept an `AbstractStage` as the first argument. Algorithm designers
-may customize these behaviors respectively. The default behaviors are:
+may customize these behaviors respectively by implementing:
+
+- `(p::YourPolicy)(::AbstractStage, ::AbstractEnv)`
+- `(p::YourPolicy)(::PreActStage, ::AbstractEnv, action)`
+
+The default behaviors for `Agent` are:
+
 1. Update the inner `trajectory` given the context of `policy`, `env`, and
    `stage`.
   1. By default we do nothing.
-  2. In `PreActStage`, we `push!` the current **state** of the `env` and the
-     **action** generated by `policy(env)` into the `trajectory`. And the
-     **action** is returned.
+  2. In `PreActStage`, we `push!` the current **state** and the **action** into
+     the `trajectory`.
   3. In `PostActStage`, we query the `reward` and `is_terminated` info from
      `env` and push them into `trajectory`.
   4. For `CircularSARTTrajectory`:
@@ -58,39 +67,33 @@ function (agent::Agent)(stage::AbstractStage, env::AbstractEnv)
     update!(agent.policy, agent.trajectory, env, stage)
 end
 
-function (agent::Agent)(stage::PreActStage, env::AbstractEnv)
-    action = update!(agent.trajectory, agent.policy, env, stage)
+function (agent::Agent)(stage::PreActStage, env::AbstractEnv, action)
+    update!(agent.trajectory, agent.policy, env, stage, action)
     update!(agent.policy, agent.trajectory, env, stage)
-    action
 end
 
-RLBase.update!(::AbstractPolicy, ::AbstractTrajectory, ::AbstractEnv, ::AbstractStage) =
-    nothing
+function RLBase.update!(::AbstractPolicy, ::AbstractTrajectory, ::AbstractEnv, ::AbstractStage)
+end
 
-RLBase.update!(p::AbstractPolicy, t::AbstractTrajectory, ::AbstractEnv, ::PreActStage) =
+function RLBase.update!(p::AbstractPolicy, t::AbstractTrajectory, ::AbstractEnv, ::PreActStage)
     update!(p, t)
+end
 
-RLBase.update!(::AbstractTrajectory, ::AbstractPolicy, ::AbstractEnv, ::AbstractStage) =
-    nothing
+function RLBase.update!(::AbstractTrajectory, ::AbstractPolicy, ::AbstractEnv, ::AbstractStage)
+end
 
-function RLBase.update!(
-    trajectory::Union{
-        CircularArraySARTTrajectory,
-        PrioritizedTrajectory{<:CircularArraySARTTrajectory},
-    },
-    ::AbstractPolicy,
-    ::AbstractEnv,
-    ::PreEpisodeStage,
-)
-    if length(trajectory) > 0
-        pop!(trajectory[:state])
-        pop!(trajectory[:action])
-    end
+function RLBase.update!(::AbstractTrajectory, ::AbstractPolicy, ::AbstractEnv, ::PreActStage, action)
 end
 
+#####
+# Default behaviors for known trajectories
+#####
+
 function RLBase.update!(
     trajectory::Union{
+        CircularArraySARTTrajectory,
         CircularArraySLARTTrajectory,
+        PrioritizedTrajectory{<:CircularArraySARTTrajectory},
         PrioritizedTrajectory{<:CircularArraySLARTTrajectory},
     },
     ::AbstractPolicy,
@@ -100,39 +103,53 @@ function RLBase.update!(
     if length(trajectory) > 0
         pop!(trajectory[:state])
         pop!(trajectory[:action])
-        pop!(trajectory[:legal_actions_mask])
+        if haskey(trajectory, :legal_actions_mask)
+            pop!(trajectory[:legal_actions_mask])
+        end
     end
 end
 
 function RLBase.update!(
     trajectory::Union{
         CircularArraySARTTrajectory,
+        CircularArraySLARTTrajectory,
         PrioritizedTrajectory{<:CircularArraySARTTrajectory},
+        PrioritizedTrajectory{<:CircularArraySLARTTrajectory},
     },
     policy::AbstractPolicy,
     env::AbstractEnv,
-    ::Union{PreActStage,PostEpisodeStage},
+    ::PreActStage,
+    action
 )
-    action = policy(env)
     push!(trajectory[:state], state(env))
     push!(trajectory[:action], action)
-    action
+    if haskey(trajectory, :legal_actions_mask)
+        push!(trajectory[:legal_actions_mask], legal_action_space_mask(env))
+    end
 end
 
 function RLBase.update!(
     trajectory::Union{
+        CircularArraySARTTrajectory,
         CircularArraySLARTTrajectory,
+        PrioritizedTrajectory{<:CircularArraySARTTrajectory},
         PrioritizedTrajectory{<:CircularArraySLARTTrajectory},
     },
     policy::AbstractPolicy,
     env::AbstractEnv,
-    ::Union{PreActStage,PostEpisodeStage},
+    ::PostEpisodeStage,
 )
-    action = policy(env)
+    # Note that for trajectories like `CircularArraySARTTrajectory`, data are
+    # stored in a SARSA format, which means we still need to generate a dummy
+    # action at the end of an episode. Here we simply select a random one using
+    # the global rng. In theory it shouldn't affect the performance of specific algorithm.
+    action = rand(action_space(env))
+
     push!(trajectory[:state], state(env))
     push!(trajectory[:action], action)
-    push!(trajectory[:legal_actions_mask], legal_action_space_mask(env))
-    action
+    if haskey(trajectory, :legal_actions_mask)
+        push!(trajectory[:legal_actions_mask], legal_action_space_mask(env))
+    end
 end
 
 function RLBase.update!(
diff --git a/src/policies/base.jl b/src/policies/base.jl
index c2439b8..618e66c 100644
--- a/src/policies/base.jl
+++ b/src/policies/base.jl
@@ -37,4 +37,4 @@ struct PostActStage <: AbstractStage end
 const POST_ACT_STAGE = PostActStage()
 
 (p::AbstractPolicy)(::AbstractStage, ::AbstractEnv) = nothing
-(p::AbstractPolicy)(::PreActStage, env::AbstractEnv) = p(env)
+(p::AbstractPolicy)(::AbstractStage, ::AbstractEnv, action) = nothing