Can Poetry Auto‑Resolve Conflicting Dependencies When Adding a New Package?

Direct answer

Yes—when you run poetry add <package>, Poetry performs a full dependency resolution against pyproject.toml. It is not a simple append to poetry.lock. The solver may upgrade or downgrade existing locked packages, including transitive dependencies, if that is required to satisfy the new package's constraints. However, Poetry does not support a "partial" lock update that ignores conflicts. If the resolver cannot find a compatible set of versions, it raises a conflict error and exits without modifying poetry.lock.

Confirmed behavior vs. version-dependent details

The following is well-established across recent Poetry releases:

• poetry add runs a complete solve, so locked versions can change.
• On failure, poetry.lock remains byte-for-byte unchanged.
• There is no supported flag to force an inconsistent lock file.

The exact backtracking limits, error wording, and handling of optional groups have evolved between Poetry major versions. Treat the precise failure message as version-specific until you confirm it against your installed release.

Why --no-dev does not change resolution

The --no-dev flag controls which packages are installed into the active environment; it skips development dependencies during installation. It does not remove dev or optional groups from poetry.lock, nor does it exclude them from the resolver. Therefore, --no-dev will not make a conflicting optional dependency disappear from the solve.

Steps for this case

1. Confirm your Poetry major version, because resolver behavior differs:
   poetry --version
2. Attempt the addition and capture the conflict message:
   poetry add <package>
3. If the solver fails, identify the incompatible constraint in the error output. You must manually relax or adjust the relevant version bound in pyproject.toml—Poetry will not edit it for you.
4. Re-run poetry add <package>, or if you only need to refresh existing pins first, run poetry update <package> before adding the new requirement.
5. After a successful add, inspect what the resolver changed:
   poetry show --tree

One missing detail that changes the recommendation

Which exact Poetry version are you using (for example, 1.5, 1.8, or 2.x)? Resolver edge cases and optional-group handling vary by major version, so the safest next step depends on whether you are on a legacy 1.x line or a current 2.x release.