spinning-up-deep-rl – Coding skill for AI agents

Spinning Up in Deep RL

Author: Joshua Achiam (OpenAI) | Source: spinningup.readthedocs.io, MIT | Chapters: 20 | Generated: 2026-08-25

How to Use This Skill

• No argument — load the core frameworks below
• A topic — ask about advantage function, target networks, entropy regularization;

I resolve it through the Topic Index and read that chapter file

• chNN — I load that chapter's summary
• "what chapters do you have?" — the full index

/cs-spinning-up-deep-rl                      # core frameworks + chapter index
/cs-spinning-up-deep-rl entropy regularization   # topic index -> ch19, read that chapter
/cs-spinning-up-deep-rl ch09                 # one chapter summary

When you ask about something not in Core Frameworks, I read the relevant chapter file before

answering rather than guessing from the index.

---

Core Frameworks & Mental Models

The RL problem

pi* = argmax_pi J(pi), where J(pi) = E_{tau~pi}[R(tau)]. Every algorithm approximates

this; where it substitutes a different objective (a Bellman residual, a surrogate), that

substitution is the source of its failure modes. Four value functions — V^pi, Q^pi, V*,

Q* — all obey Bellman self-consistency, and a*(s) = argmax_a Q*(s,a) is why Q-learning

is a viable family at all. Advantage A^pi(s,a) = Q^pi(s,a) - V^pi(s) is the relative-quality

signal policy gradients run on. (ch07)

The two branching questions

Place any algorithm by asking: does it have or learn a model, and what does it learn

(policy, Q-function, value function, model). That generates the whole landscape. (ch08)

Policy optimization vs Q-learning — the central trade-off

• Policy optimization is *principled: you directly optimize the thing you want.* Stable

and reliable. On-policy, so it cannot reuse data, so it is sample-hungry.

• Q-learning only *indirectly* optimizes performance, by training Q_theta to satisfy a

self-consistency equation. Many failure modes, so less stable. But substantially more

sample efficient when it works, because it reuses everything.

• Satisfying the Bellman equations well carries no guarantee of good policy performance.
• The two are not exclusive — DDPG and SAC live between them deliberately. (ch08)

The policy gradient template

grad J = E[ sum_t grad log pi_theta(a_t|s_t) * Phi_t ]. Five valid choices of Phi_t:

full return, reward-to-go, reward-to-go minus a baseline, Q^pi, and A^pi. All share an

expectation and differ in variance. Two rules get you from the first to the last:

• Don't let the past distract you — drop rewards obtained before the action. Those terms

had zero mean and nonzero variance: pure noise.

• Baselines — by the EGLP lemma, any state-only b(s) can be added or subtracted freely.

The standard choice is V^pi(s_t), learned by MSE regression onto reward-to-go. (ch09)

The policy-gradient loss is not a loss function

Its data distribution depends on the parameters, and it does not measure performance even in

expectation. Only at the current parameters, with data from those parameters, does it have

the negative gradient of performance. **You can send it to negative infinity while performance

craters, and it usually will. Only average return means anything.** (ch09)

Broken RL code almost always fails silently

It runs fine; the agent just never learns. Usually something is computed with the wrong

equation, on the wrong distribution, or piped to the wrong place. **If it doesn't work, assume

there's a bug** before touching hyperparameters. Debug by measuring everything and reading the

code critically. The archetype is one missing squeeze: a [N] vs [N,1] shape mismatch is

broadcast-compatible, raises nothing, and silently turns the Bellman backup into an [N,N]

matrix. (ch10, ch12)

Learn by doing

Write your own implementations, shortest correct version of each, **simplest algorithms

first**. VPG, DQN, A2C, PPO, DDPG, roughly in that order; ~250-300 lines each. Single-threaded

before parallel. **Iterate fast in simple environments — under 5 minutes turnaround at the

debug stage.** Do not attempt Atari or Humanoid before the toy task works. Read papers for

their ablations and supplementary material, but do not overfit to paper details (the

original DDPG's architecture, init scheme and batch norm are not strictly necessary) **or to

existing implementations** (their abstractions serve reuse, not your single use case). (ch10)

Rigor: four standards

1. Fair comparisons — tune the baseline as hard as your method; never handicap it.

2. Remove stochasticity as a confounder — at least 3 seeds, 10 or more to be thorough.

Two seed groups can produce curves that look like different distributions.

3. High-integrity experiments — launch fresh final runs and precommit to reporting them.

Tuning produces hypotheses; final runs produce conclusions.

4. Check each claim separately — ablate every design decision. (ch10)

The safe-step family (on-policy)

VPG takes an unconstrained gradient step, so a single bad step can collapse performance.

TRPO constrains the step in **KL-divergence between policies, not distance in parameter

space, then backtracking-line-searches until the exact constraint holds. PPO** drops the

constraint and instead clips the objective so the policy gains nothing by moving far,

which is first-order, far simpler, and empirically at least as good. (ch14, ch15, ch16)

The overestimation family (off-policy)

DDPG amortizes the intractable continuous max_a Q(s,a) into a learned policy:

max_a Q(s,a) ~= Q(s, mu(s)). It needs a replay buffer (licensed because the Bellman

equation is indifferent to how data was collected) and target networks (because the target

otherwise depends on the parameters being trained). Its failure mode is **Q-value

overestimation, which the policy actively exploits. TD3** answers with clipped double-Q,

delayed policy updates and target policy smoothing. SAC adds entropy regularization,

making the explore-exploit trade-off an explicit coefficient alpha. (ch17, ch18, ch19)

---

Chapter Index

| # | Title | Key Frameworks |

|---|-------|----------------|

| [ch01](chapters/ch01-introduction.md) | Introduction | The missing middle step, Code Design Philosophy |

| [ch02](chapters/ch02-installation.md) | Installation | Install-then-verify, MuJoCo optionality |

| [ch03](chapters/ch03-algorithm-lineup.md) | Algorithms: What's Included and Why | The two lineages, on/off-policy trade-off, code template |

| [ch04](chapters/ch04-running-experiments.md) | Running Experiments | One flag per kwarg, ExperimentGrid, save-dir suffixes |

| [ch05](chapters/ch05-experiment-outputs.md) | Experiment Outputs | Tools not files, watch-then-measure |

| [ch06](chapters/ch06-plotting-results.md) | Plotting Results | Performance alias, prefix autocompletion, seed averaging |

| [ch07](chapters/ch07-key-concepts-in-rl.md) | Part 1: Key Concepts in RL | MDPs, four value functions, Bellman equations, advantage |

| [ch08](chapters/ch08-kinds-of-rl-algorithms.md) | Part 2: Kinds of RL Algorithms | Taxonomy, model bias, policy-opt vs Q-learning |

| [ch09](chapters/ch09-intro-to-policy-optimization.md) | Part 3: Intro to Policy Optimization | Log-derivative trick, EGLP lemma, reward-to-go, baselines |

| [ch10](chapters/ch10-spinning-up-as-a-researcher.md) | Spinning Up as a Deep RL Researcher | Learn by doing, three idea frames, four rigor standards |

| [ch11](chapters/ch11-key-papers-in-deep-rl.md) | Key Papers in Deep RL | 13-section topic map |

| [ch12](chapters/ch12-exercises.md) | Exercises | Problem Set 1 and 2, the silent DDPG bug |

| [ch13](chapters/ch13-benchmarks.md) | Benchmarks | The parity disclosure, family-specific metrics |

| [ch14](chapters/ch14-vpg.md) | Vanilla Policy Gradient | The six-step loop |

| [ch15](chapters/ch15-trpo.md) | Trust Region Policy Optimization | KL trust region, line search, conjugate gradient |

| [ch16](chapters/ch16-ppo.md) | Proximal Policy Optimization | PPO-Clip, KL early stopping |

| [ch17](chapters/ch17-ddpg.md) | Deep Deterministic Policy Gradient | MSBE, replay buffers, target networks, polyak |

| [ch18](chapters/ch18-td3.md) | Twin Delayed DDPG | Clipped double-Q, delayed updates, target smoothing |

| [ch19](chapters/ch19-sac.md) | Soft Actor-Critic | Entropy regularization, reparameterization, squashed Gaussian |

| [ch20](chapters/ch20-logger-and-utilities.md) | Logger, MPI Tools and Run Utils | EpochLogger pattern, MPI PyTorch order |

Topic Index

• Advantage function ch07, ch09, ch14
• Baselines ch09
• Bellman equations ch07, ch17
• Benchmarks / parity ch13, ch01
• Clipped double-Q ch18, ch19
• Continuous action spaces ch07, ch17
• Debugging / silent failure ch10, ch12
• DDPG ch17, ch03, ch08
• Entropy regularization ch19
• Exploration vs exploitation ch14, c

… (full content is in the download package)