Use the Presto solver backend

The internal presto solver delegates conda’s final-state solve to the loopback-only broker service. Conda still computes the local package transaction and performs any requested installation or removal.

Warning

Use the backend per command while evaluating it. Do not configure it as a global default. The protocol depends on the supported conda and conda-rattler-solver versions and has no direct HTTP compatibility promise.

Start the required service

The solver does not start conda-broker itself:

conda broker start conda-presto.server
conda broker wait conda-presto.server --timeout 180

See Run a broker-managed local service if the provider is not discovered or readiness does not complete.

Preview a create transaction

Use --dry-run first:

conda create \
  --dry-run \
  --solver presto \
  --name presto-demo \
  --channel conda-forge \
  python=3.13 numpy

The service returns the final package state. Conda turns that state into its normal unlink and link plan locally.

Without --dry-run, conda downloads packages and modifies the selected prefix after the delegated solve. The broker service never performs that transaction.

Preview changes to an existing environment

Pass --solver presto to supported install, update, and remove commands:

conda install --dry-run --solver presto --name my-env pandas
conda update --dry-run --solver presto --name my-env numpy
conda remove --dry-run --solver presto --name my-env pandas

The request includes installed records, history, pins, virtual packages, channels, requested specs, and relevant solver settings. It does not include the local prefix path or file inventory.

--force-reinstall remains a local conda transaction choice:

conda install \
  --dry-run \
  --solver presto \
  --force-reinstall \
  --name my-env \
  numpy

Make a repeated request eligible for refresh

Two successful cacheable foreground requests with the same solve state make that request eligible for scheduled refresh:

conda create --dry-run --solver presto --name presto-demo python=3.13
conda create --dry-run --solver presto --name presto-demo python=3.13

A repeated dry-run can reuse a final-state cache entry. Timing alone does not prove a cache hit because repodata checks and local client work still occur. Use Refresh cached solver results to configure regular refresh and Monitor a running service to inspect cycle summaries.

The recorded-request identity also includes the caller’s local repodata TTL. The final-state cache key omits that TTL and applies the captured policy during its marker check.

A completed transaction normally changes installed records, so a later solve for that prefix commonly has a different cache identity.

Recognize unsupported operations

The backend rejects operations that the service cannot reproduce accurately:

• offline mode

• --update-deps

• conda-build caller-provided indexes or repodata-subset callbacks

Use another conda solver for those operations. The Presto client does not fall back silently after it rejects unsupported state.

Stop the service

conda broker stop conda-presto.server

Boundaries

The protocol is private and has no compatibility promise for direct HTTP clients. It accepts only loopback clients and is absent from normal and Docker server deployments. There is no remote solver URL setting.

See Presto solver backend for supported semantics, cache identity, and protocol limits.