---
name: mosaic
description: "Multi-objective, gradient-based protein binder design with Mosaic. Use this skill when: (1) Composing several structure or sequence models into one design objective, (2) Optimizing binders against a custom loss rather than a fixed pipeline, (3) Wanting gradient descent over sequence space in the style of ColabDesign, RSO, or BindCraft but with interchangeable predictors, (4) Letting the optimizer choose the epitope instead of fixing hotspots.

For an end-to-end binder pipeline with default filters, use bindcraft. For all-atom diffusion design, use boltzgen. For backbone-only generation, use rfdiffusion."
license: MIT
category: design-tools
tags: [design, gradient-optimization, multi-objective, jax, binder]
---

# Mosaic Multi-Objective Design

[Mosaic](https://github.com/escalante-bio/mosaic) (Escalante Bio) is a JAX framework
for "functional, multi-objective protein design using continuous relaxation." It
optimizes a soft sequence by gradient descent over a continuous relaxation of
sequence space, in the lineage of ColabDesign, RSO, and BindCraft, with one key
difference: it composes multiple learned objectives from different models in a single
differentiable loss.

## When Mosaic fits

Mosaic is a framework for custom objectives, not a one-click method. The README is
explicit: it "may require substantial hand-holding (tuning learning rates, etc),
often produces proteins that fail simple in-silico tests, [and] should be combined
with standard filtering methods." Reach for it when a fixed pipeline cannot express
the objective you need. For a turnkey binder run, use `bindcraft` instead.

## Prerequisites

| Requirement | Minimum | Recommended |
|-------------|---------|-------------|
| Python | 3.11+ | 3.11 |
| Framework | JAX with CUDA or TPU | JAX CUDA 12 |
| GPU VRAM | 24GB | 48GB+ (depends on predictors used) |

JIT compilation makes the first call to any loss slow; later calls are fast.

## Install

Mosaic runs locally on a JAX GPU or TPU build. It has no CLI and no Modal
integration; you drive it through the marimo notebooks or the Python API.

```bash
git clone https://github.com/escalante-bio/mosaic && cd mosaic
uv sync --group jax-cuda      # or --group jax-tpu / --group jax-cpu
uv add jax[cuda12]            # may be needed for a GPU build
uv run marimo edit examples/example_notebook.py
```

Ready-made examples include `esmfold_minibinder.py`, `esmfold_vhh.py`,
`boltzgen_pipeline.py`, and `batched_protenix.py`.

## Core idea

A design objective is built from `LossTerm` objects that you add and scale with plain
Python arithmetic, then hand to an optimizer.

```python
import mosaic.losses.structure_prediction as sp

# Compose a loss from interface, confidence, and inverse-folding terms
design_loss = (
    sp.BinderTargetContact()
    + sp.WithinBinderContact()
    + 0.05 * sp.TargetBinderPAE()
    + 0.05 * sp.BinderTargetPAE()
    + 0.025 * sp.IPTMLoss()
    + 0.1 * sp.PLDDTLoss()
)
```

Loss terms can wrap one model used several ways (for example a structure predictor
scoring both the binder-target complex and the binder as a monomer). Composing
different architectures also lowers the chance of finding adversarial sequences that
fool a single predictor.

### What you can compose

| Category | Options |
|----------|---------|
| Structure predictors | AF2, Boltz-1, Boltz-2, Protenix, OpenFold3, ESMFold2 |
| Generative / design | BoltzGen, Proteina-Complexa |
| Inverse folding | ProteinMPNN, SolubleMPNN, AbMPNN |
| Language models | ESM-2, ESM-C, AbLang, trigram |
| Property heads | Stability (megascale-trained) |

### Optimizers

| Optimizer | Use |
|-----------|-----|
| `simplex_APGM` | Default; proximal gradient / mirror descent on the probability simplex |
| `batched_simplex_APGM` | The same, vmapped over many designs |
| `gradient_MCMC` | Discrete moves for fine-tuning a sequence |

A reasonable `simplex_APGM` step size is about `0.1 * sqrt(binder_length)`.

## Worked example: ranking with ipSAE

The published [Nipah competition recipe](https://blog.escalante.bio/180-lines-of-code-to-win-the-in-silico-portion-of-the-adaptyv-nipah-binding-competition/)
optimizes a design loss on Boltz-2, then ranks candidates with a separate
multi-sample loss built from ipTM and ipSAE. The multi-sample loss is a method on the
Boltz2 model, not a free function:

```python
from mosaic.models.boltz2 import Boltz2

boltz2 = Boltz2()
ranking_loss = boltz2.build_multisample_loss(
    loss=1.00 * sp.IPTMLoss()
    + 0.5 * sp.TargetBinderIPSAE()
    + 0.5 * sp.BinderTargetIPSAE(),
    features=design_features,
    num_samples=6,
    recycling_steps=3,
)
```

On the Adaptyv Nipah de novo target, this recipe produced 8 binders out of 9 tested
designs at nanomolar affinity, the highest hit-rate of any method on that target in the
public results. That is a small, expert-tuned sample on one hard target, not a guarantee
across targets, so treat Mosaic as a high-ceiling option that rewards careful objective
design rather than a turnkey default.

Two practices from that work are worth carrying over:

- **Let the optimizer choose the epitope.** Asking for a binder, without fixing
  hotspots, can find a better interface than a manually chosen one.
- **Match filter stringency to assay throughput.** With high-throughput testing,
  filter lightly to keep diversity rather than applying heavy consensus filters that
  can reject good binders.

## Decision tree

```
Should I use Mosaic?
│
├─ Need a custom objective across multiple models? → Mosaic
├─ Want one-click binders with default filters?    → bindcraft
├─ Want all-atom diffusion design?                  → boltzgen
└─ Want backbone-only diversity?                    → rfdiffusion + proteinmpnn
```

## Cost

Adaptyv's own tests of these models showed Mosaic costing about $0.55 per accepted
design, averaged across 7 targets, among the cheapest per design of the methods tested.
That is compute only; the setup and tuning effort is the real cost of using Mosaic.

## Troubleshooting

| Issue | Cause | Fix |
|-------|-------|-----|
| Designs fail simple in-silico checks | Under-constrained objective | Add inverse-folding and confidence terms; filter with `protein-qc` |
| Optimization unstable | Step size too large | Lower the `simplex_APGM` step size |
| First call very slow | JIT compilation | Expected; reuse the compiled loss across designs |
| OOM with large predictors | Several models in one loss | Use smaller predictors or a larger GPU |

---

**Next**: Validate designs with `boltz` or `chai`, rank with `ipsae`, then filter
with `protein-qc`.