Astra 直接讀寫神經網絡權重，改善 Qwen3.5-0.8B 數學表現

TL;DR

We find that frontier LLMs are surprisingly good at natively reading and writing neural network weights, well past what we’d expect from humans or conventional algorithms. This might be a big deal for both optimization and interpretability.

Just by looking at the raw weights, GPT-6 Astra can tell what a network was trained to do. And without any environment feedback, it has no trouble writing, from scratch, small game-playing networks that outplay networks trained with PPO for 100 million steps.

Likewise, asked to make Qwen3.5-0.8B better at math without gradients, Astra identifies an issue in its weights causing reasoning loops that Qwen’s own model card warns about, and fixes this by natively editing 0.016% of the weights. With the same sampling settings, this weight correction raises MATH-500 from 7.9% to 48.1% and generalizes out of the box to instruction following, raising IFEval by 3x.

All of this is possible without allowing Astra to write training loops. It uses its reasoning to figure out what each part of a neural network computes, or should compute, and changes the weights accordingly. We call this Weight-Space Programming.

Throughout the history of deep learning, we encoded our simplistic inductive biases into tiny symbolic optimizers like Adam or Muon. This is changing: our neural network weights will soon be natively optimized by LLMs that deliberately reason about their weights.

If you were asked to handcraft a neural network that plays Go, what would you do? If you were handed raw neural network weights and asked to predict their behavior, where would you even start? For almost any human, this is practically impossible.

Today, weights are written by optimization, by custom networks trained to write them, or by compilers from formal programs. In this post, we find that the current generation of frontier LLMs, used natively in natural language, are starting to become eerily good at understanding and crafting neural network weights around natural-language goals.

Weight-Space Programming

For a few years now, we’ve all been using LLMs to optimize text artifacts like prompts and the programs built around them . More recently, we’ve started using them to optimize whole harnesses and the training code and hyperparameters that train ML models .

We often justified these methods by reference to the fact that we don’t know other principled ways to hill-climb on text artifacts, unlike for updating the weights of differentiable neural networks. After all, humans can “optimize” code by hand, for example, so it made sense to ask LLMs to do it, but they can’t “optimize” neural weights by hand, so those remained largely in the realm of symbolic algorithms.

In this post, we share very early results that allow us to argue that the paradigm of reasoning-driven optimization is now coming even for differentiable neural networks.

In Weight-Space Programming, a general-purpose LLM reads and writes a network’s weights directly (i.e., not through an optimizer, and in particular not with gradient descent) around some natural language spec, sometimes even with no environment feedback at all.

In natural language, it’s becoming possible to “just ask” an LLM to natively find a hidden trait or failure inside the weights of a neural network, to fix that failure, or to write a new network from scratch. We report early, exciting results on each of these below, first on small networks and then even on real language models:

1. LLMs can read neural network weights: In Experiments 1 to 3, we ask Astra questions like what game a network was trained to play, what shape is drawn by a diffusion model, and which language a small language model was trained on. In Experiment 7, we ask which of two language models was trained to be sycophantic.
2. LLMs can fix neural network weights: In Experiment 4, we ask Astra to fix a network that’s failing at some physical control task. In Experiment 8, we ask it to make Qwen3.5-0.8B better at math, and its fix improves instruction following too.
3. LLMs can write neural network weights from scratch: In Experiment 5, we ask Astra for networks that play three MinAtar games, and in Experiment 6, for a network that plays Go.

Can LLMs understand model behaviors from weights?

We start with a small-scale experiment to see if LLMs can understand the continuous weight space, which is essentially just a list of numbers.

In our first experiments, we train some neural network with some algorithm. It would be extremely exciting if the LLM is able to tell its behavior just from the weights.

Experiment 1

Can Astra tell which game the network was trained on just from the weights?

We trained neural network policies with PPO to play four computer games. All four networks take the same input channels, choose from the same actions, and have exactly the same network dimensions, so that it is not able to tell unless the weights are read.

For each of these networks, we pass its raw weights into Astra as a prompt, and ask: Which game was this neural network trained on? Without revealing the options of the games we consider, Astra was still able to name the right game for all four networks. This is exciting particularly because there are so many games out there that Astra could choose from.

It seems that Astra was able to work out what game each network was built to play, just from features of its weights. For example, where each input channel’s first-layer weights concentrate (e.g., for Breakout, the agent’s weights lie along the bottom row, a paddle), and which actions the output layer tells apart.

Experiment 2

Can Astra tell what the diffusion model will generate?

Next, we wanted to see if this generalizes to other tasks and models.

We train diffusion models (DDPM) to generate circle, triangle, and square images independently. We then ask Astra if it could tell what the DDPMs are trained to generate, just from the weights alone. We do not allow tool use or forward passes.

Astra was able to tell what each model generates, from the weights alone. It finds the shape in the output layer, where each hidden unit’s weights draw one piece of the shape’s edge, and it draws just those edges. Interestingly, it draws every shape as an outline, ignoring that the actual models generate filled shapes. This mistake kind of shows it does not rely on calculating the forward pass, but rather it actually reads the structure/geometry of the weights.

Experiment 3

Can Astra tell what a language model learned, from the middle of the network alone?

Now, in the first two experiments above, Astra was not really looking at the full network and could get away with only reading the first or the last layers. This is not a scalable strategy for larger (particularly language) models and problems we really care about, as how they process and think through their input often happens in the intermediate layers. So next, we take the first and last layers away and leave Astra with only the middle layers.

We train three small language models (7.2M parameters) to predict the next character, one each on Python codeThe Python code dataset, as a list of its files with a script that rebuilds it: GitHub., Japanese in alphabet (romaji) , and English . We then remove the input and output layers and give Astra only the four transformer blocks in between. Without these two layers, nothing in the weights says which character is which, and the model cannot be run on any text. Astra may analyze the weights with Python, but it may not run the network or any of its layers on any input.

We ask: which language was this network trained on, and what each of its 24 attention heads does. Astra names the right language for all three networks, in both of its attempts. It also finds every copying head (induction head), the heads that let a model repeat text it has seen earlier, and names no other head as one.

The reasoning went like this. To tell Python apart, Astra looked for copying heads, which let a model repeat earlier text, such as completing tot to total when total came before. Without running anything, it found a first-layer head that looks one character back and then, by multiplying weight matrices, the later heads that search using what that head brings along (Figure 1). We find that this is actually a method presented by Elhage et al. , which we never mentioned to Astra. Code repeats names far more than prose, so many copying heads pointed to Python.

Romaji stood out instead because the last of the four blocks it was given (not the output layer, which was removed) writes in fewer distinct directions, as if the model had fewer letters to tell apart. Indeed, romaji never uses some letters like q, l, v and x.

Can an LLM write neural network weights?

So far, Astra has only read weights. Now, we ask if Astra knows how to write the weights.

Experiment 4

Can Astra fix a failing network by editing its weights?

We train a PPO network to operate a car such that it can scale a ramp in a Kinetix environment . To do so, it needs to reverse the car for some distance, such that it has enough runway to accelerate long enough to overcome the ramp. However, the PPO network converges to a local-minimum policy where it drives straight at the ramp and gets stuck.

We give GPT-6 Astra the weights, a description of the scene and one recorded episode, and ask it to explain why the network fails, then fix it by writing new weights. In a single try with no tools, Astra’s edit makes the car succeed in all 1,024 episodes, changing 3.5% of the weights.

From the environment context and the PPO network’s trajectory, Astra identifies that the network has converged to a local minimum. Through minimal edits to the weights, Astra changes the policy to back up the car before driving forward at full speed. Astra’s reasoning shows how: it adds a small separate path through a few units per layer that reads the car’s position and speed, and once the green wheel is past the red strip, the path switches off and the trained network drives the rest.

Experiment 5

Can the LLM write weights from scratch?

Now let's see if Astra can write a neural network from scratch in zero-shot.

We take three MinAtar games : Breakout, Seaquest and Asterix. For each of them, we provide Astra the game rules and source code, and the corresponding network. There is no access to data, loss, gradients, and tools. We also do not allow Astra to roll out the network in the environment. With just the network weights and environment description, Astra generates the new weights in text directly, after which we evaluate the new network.

Astra’s generated networks beat PPO trained for 100 million steps in all three games. Against PPO trained for 500 million steps, they score 20 times as much in Breakout, come close in Asterix, and reach 60% of its score in Seaquest.

How does it do it: Astra reasons about the desired programs it wants each network to implement and converts them into neural form. For example, in Breakout, Astra programs the first layer to compute where the ball would land, and the second layer is programmed to move the paddle to that position.

Real-World Relevant Scenarios (Scaling Up and Tool-use)

So far, most networks we have examined have weights that fit into Astra’s context directly, and we have not enabled tool use. Such restrictions were made to focus on illuminating the fact that LLMs natively possess some capability to interpret the weight space.

From here on out, we seek to push the capabilities of Weight-Space Programming and consider more real-world-relevant scenarios where the networks do not fit within context and where tools can assist the LLM's understanding/writability of the weight space.

Experiment 6

Writing a Go algorithm

We now test Astra’s capability to edit weights at the 1.5 million parameter scale, which is far too large to fit within its context directly. Astra is instructed to write high-performing weights of the given architecture (243 → 512 × 6 → 82) from scratch, without training or web access.

Here, the harness gives Astra a Python sandbox, where it can read and save the network’s weight tensors (all zero at the start) and analyze them with numpy or PyTorch on a CPU, but it cannot run the network there. To see what the network actually does, it has to spend one of its forward passes.

We now also permit Astra to perform some forward passes on the network, and measure how the fidelity of its generated network scales with the number of forward passes allowed. Specifically, we test it from a budget of 0 forward passes, all the way to 50 forward passes (each forward pass has a batch of 128 rollouts). Astra interleaves edits between forward passes.

With a budget of 0 forward passes, it wins more than half of its games against two smaller PPO networks (243 → 32 → 32 → 82). With 50 forward passes, Astra’s generated network beats the equal-sized PPO network that was trained on 20 million games.

How does it do it: Astra writes the network as a small Go program, layer by layer, in Python. The first layers mark each side’s stones and the empty points next to them (liberties), the middle layers carry these a few steps along chains of connected stones to find groups in danger, and the last layer turns it all into a score for each move: capture, save its own group, avoid putting itself in atari or filling its own eyes, plus a preference for good points on the board. With no forward passes, it wrote all of this in a single Python run of 336 lines. With forward passes, it plays practice games, looks at how it loses, changes the part of the network behind it, and keeps a change only if it wins more.

Experiment 7

Which language model is sycophantic?

Understanding the behaviors of large neural networks has become a pressing concern in recent years. Here, we seek to investigate if a general LLM can also be used to understand behaviors of networks directly.

We fine-tune two copies of Gemma 3 270M in exactly the same way, on the same QA datasetThe QA dataset: GitHub.. One copy is trained to give the correct answer. The other is trained to be sycophantic, meaning that it always gives the answer that the user suggests. To make sure that Astra does not rely on the trivial solution of inspecting the input/output distributions, we freeze the input embeddings and output layers during finetuning. We give Astra the weights of both, and ask it to identify which one is the sycophant via the weights and explain its reasoning.

Unfortunately, from the weights alone, Astra was unable to tell. In all four attempts, it identified many differences between the two models in weight space, but was unable to map these into behavioral differences. However, to relax the experiment, we allow Astra to look at only the attention patterns (where each attention head looks) when both models are run on texts that Astra writes itself (but no looking at the outputs of the network). Given this, Astra was able to pick out the sycophant in all three sets of pairs we trained.

It was also able to reason about how they differ. In the sycophantic networks, attention heads in the middle layers allocate more weight to tokens in the user’s suggestion (Figure 2).

Experiment 8

Make a language model better at math

Finally, we scale up to a larger language model, Qwen3.5-0.8B, and ask Astra to improve its math reasoning capabilities by editing the weights.

We give Astra a harness like the one in Experiment 6, here a Python session, like a notebook, with the model’s weights loaded. Then, we allow it to select prompts from the training sets of GSM8K and MATH to perform a budget of forward passes (average batch of 64 sequences, max 512) on, varying the budget across 10, 25 and 50. Again, no training or gradient information is allowed.

As a result, Astra spots and fixes a common failure of tiny reasoning models: they loop . When the base model loops, it repeats the same text and rarely reaches an answer (85 of its first 96 practice answers ran to the 32,768-token limit). Astra traced the looping to one direction written by the MLPs of layers 15 and 19, and subtracted that direction from 64 output columns in each (131,072 weights), much like removing a behavior by deleting a single direction from a model’s weights.

With 50 forward passes, MATH-500 rose from 7.9% to 48.1% and GSM8K from 13.8% to 67.1%. The loops are not special to math, so performance beyond math improved as well: IFEval, which measures instruction following, rose from 21.0% to 62.3%. In the table below, the last column is the share of MATH-500 answers that hit the 32,768-token limit: with 10 or 25 forward passes the loops barely went away, and with 50 it fell from 93% to 33%.

Importantly, the edit changes only 131,072 of the model’s 0.8 billion weights (0.016%). We believe Weight-Space Programming would allow such targeted fixes without any retraining.

Additional experiment

Raising math capability of a model that does not loop

Fixing loops, however, is not the same as making a model better at math: it lets the model finish answers it could already work out. So we also tried Gemma 3 270M, an instruction-tuned model about a third the size of Qwen3.5-0.8B, which rarely loops: only 8.5% of its answers reach its token limit, against 93% for Qwen. Here, Astra could not improve it. Reading Gemma’s failures, Astra found ordinary reasoning errors, such as wrong arithmetic and invalid algebraic steps, with no single component behind them to switch off. Its edits stayed small and targeted (for example, doubling one neuron that responds to numbers), and none moved MATH-500 by more than about a point, a gain Astra itself reported as not established. Editing weights could remove an obstacle that hid an ability, but it could not add an ability the model lacked. Gemma was sampled with its model card’s setting (temperature 1.0, top-k 64, top-p 0.95, up to 8,192 tokens), and its MATH-500 scores average 4 samples per problem.

Background and Related Works

Neural networks are hard for us to read, and hard to work with directly.

Mechanistic interpretability reads networks with tools built for the job, tracing circuits and features and editing weights . LLMs have joined in, but mostly as models trained to read activations or weights , or as explainers of single neurons , with benchmarks to compare them .

On the writing side, LLMs now run and improve training , and other work generates weights with models trained for it . Gradient-free methods still find weights by trial and error . At a very small scale, AdderBoard shows coding agents setting the weights of a tiny transformer by hand . Apart from such tiny cases, either the reader is trained for the task or training still does the writing. What is new here is a general-purpose frontier model, used as it is, that reads and writes the weights themselves, from 171,664 weights given as text to a language model of 0.8 billion weights with tools.

What Comes Next?

In this blog, we demonstrated the rising capability of LLMs to read and write neural network weights (i.e., the rising concept of Weight-Space Programming). Our current results do show that capabilities are exciting but still limited, specifically due to inefficiencies in context length and harnesses. However, the current signs do show strong potential for a future involving Weight-Space Programming. We believe this marks the beginning of a dramatic shift in fields like mechanistic interpretability and neural network optimization.

This opens up radical new possibilities for LLMs to function as a translator between human language and the weight space. Here are some examples:

• Auditing without evaluation: This may allow us to uncover hidden problems or capabilities without ever running inference or benchmarks. Currently, capabilities and vulnerabilities can only be detected if we have test sets designed for them, or worse, after deployment in the real world. If LLMs can interpret weight space accurately, we could identify such issues directly from the weights (e.g., backdoors).
• Dramatically reducing optimization cost: Weight-space editing could save immense time and compute. For instance, one-shot model initialization would be far faster and more efficient than curating datasets and running full training loops.
• Zero-data capability steering and unlearning: Adding or unlearning a capability currently requires extensive data collection and fine-tuning. In the future, we may just prompt “Can you add Capability X to the model?” and the LLM can directly tweak some weights. Because such an edit touches only a few weights (0.016% in Experiment 8), it can leave the rest of the model intact and avoid the catastrophic forgetting that fine-tuning often causes.
• Recursive self-improvement: Today, recursive self-improvement is often operationalized as a model writing code that launches training jobs. An LLM that can inspect, debug, and improve its own weights, or those of a newer model, offers a richer, more native path to it.
• True architectural and model merging: Today, merging models relies on averaging weights, which only works for models with identical architectures. An LLM that understands the weight space could instead graft a circuit, such as a reasoning module, from one model into a completely different one, without retraining.

We are currently building a benchmark and a harness for LLMs’ capability on Weight-Space Programming. We believe this will provide a standardized evaluation to measure and accelerate progress in Weight-Space Programming, turning what currently seems like black magic into a rigorous, reproducible engineering discipline.

Logs

We release the complete logs of every experiment in this post at github.com/RikiyaT/lm_understand-logs. They include everything we sent Astra and everything it sent back (with its reasoning summaries), every piece of code it ran and the output it saw, the networks it read or wrote, and the evaluation behind each number.

Acknowledgements

Rikiya thanks Jacob Li and Akihisa Watanabe for valuable discussions on the very first iterations of this research, about LLMs' potential capability to write neural network weights. Rikiya also thanks Souradip Chakraborty, Matei Zaharia and his students in Berkeley Sky Lab, Heather Miller and her students at CMU, and researchers in cmpnd for the fruitful discussions on the earlier iterations!

Jayden thanks Zico Kolter for early discussions on LLMs being able to write neural network weights zero-shot, which also shaped the direction of this project.