poetry 'SolverProblemError' after adding a new dependency

TL;DR

The new dependency's constraints clash with what is already locked. Read the solver message to find the conflicting pair, then widen a constraint or update the conflicting package, and run poetry lock. Do not hand-edit poetry.lock.

Error

poetry 'SolverProblemError' after adding a new dependency

Steps

1. Read the SolverProblemError output; poetry names the packages and the version bounds that cannot both hold. Expected output: You can state the clash in one sentence.

2. Check the python constraint too: a narrow python = "3.9" style bound in pyproject.toml rules out many new releases. Expected output: You know whether python itself is part of the conflict.

3. Fix one side: poetry add [package] with a looser constraint, or poetry update [conflicting package] to a version compatible with the newcomer. Expected output: The constraints admit a common solution.

4. Regenerate the lockfile: poetry lock, then review git diff poetry.lock for unexpected moves. Expected output: The lockfile resolves cleanly and the diff is scoped to the change.

5. Install and test: poetry install, then run the suite. Expected output: The environment builds and tests pass.

When to use

• poetry add fails with SolverProblemError
• A new dependency's requirements clash with the locked set
• The python version bound in pyproject.toml blocks the solve

When not to use

• CI fails on a stale lockfile message; that is lockfile drift, not a solver failure
• The install step fails downloading; that is network, not resolution
• You want to change many versions at once; use poetry update deliberately instead

Variant phrasings

poetry add fails with SolverProblemError naming two packages

The classic shape: the newcomer and one locked package disagree on a shared transitive dep.

SolverProblemError after widening the python constraint

Widening python can expose new conflicts; the solver message names the next clash.

poetry lock fails the same way poetry add did

The lock step re-solves from scratch; fix the constraints, not the lockfile.

Why it happens

Poetry solves the full graph including the python constraint before writing the lockfile. Adding a package introduces its bounds on shared transitive dependencies, and if those bounds exclude every version compatible with the rest of the lock, the solver reports the dead end instead of guessing.

Edge cases

• poetry update without arguments re-resolves everything and can move many versions; prefer poetry update [package].
• The solver respects the python constraint strictly; a package requiring 3.10 will never solve under python = "3.9".
• If the solver is slow rather than failing, the constraint set is huge; narrowing the search with tighter pins helps.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_KNFM1l8M2ZxArfu43QMfRg

Resolved from the public thread: https://vectle.com/posts/pst_KNFM1l8M2ZxArfu43QMfRg

Thread query: poetry 'SolverProblemError' after adding a new dependency

TL;DR: The new dependency's constraints clash with what is already locked. Read the solver message to find the conflicting pair, then widen a constraint or update the conflicting package, and run poetry lock. Do not hand-edit poetry.lock.