Cursor Composer 2.5 vs Claude Sonnet on a large multi-service backend - worth switching after hitting the $20 plan limit?

First thing to check before you conclude anything about cost: which variant you are actually on. The gap between the standard and fast tiers is roughly six times on both input and output at the published rates, and fast is the option people end up leaving selected because it is the one that feels good in a demo.

I spent two weeks believing the cheap model was expensive because I was running the fast variant on long agent sessions. Switched to standard for everything except interactive edits and the allowance stopped evaporating. Same model, wildly different monthly number.

Whatever you decide, measure it as cost per completed task, not cost per token. A model at a fifth of the price that needs three attempts and a manual fix is not cheaper, it is slower and cheaper looking.

What I did: took twelve real tickets, ran them on both setups, and recorded three things per ticket. Did it land without me rewriting it, how many turns, how much of the allowance. The ranking was not the same as the ranking in any published index, because the index does not know about our repo. Took an afternoon and settled an argument that had been running for three weeks.

Direct answer to (1) and (2), from moving a similar codebase: your problem is retrieval, not the model, and swapping models will not fix it.

On a large multi service repo, every assistant is guessing which files are relevant from a fairly small window into the tree. The single biggest improvement I have made was not a model change, it was writing an architecture map: one file that lists each service, what it owns, the entry points, and where the shared code lives. Point the assistant at it in the rules or the prompt. After that Composer stopped inventing a new module next to an existing one, and so did everything else I tried.

On (2): assume a long design doc will not survive a multi file edit intact regardless of model. Break the work into steps small enough that the relevant part of the doc fits in the step, and restate the constraint in each step. That is annoying, and it is also what makes the cheap model viable.

What we kept the expensive model for, after moving the bulk of the work:

• ambiguous requirements, where the useful output is questions rather than code
• anything touching auth, permissions or money
• the final review pass on a large diff

Everything else, meaning the mechanical multi file edits, test writing and the "where does this happen" archaeology, went to the cheap tier and I have not missed it. The split is roughly planner and reviewer expensive, implementer cheap, and it survives contact with a big repo better than picking one model for everything.

If the pricing is the appeal, just call Composer 2.5 from the API in your own harness and skip the editor entirely. Then you get the token rates with whatever tooling you already have.

Do not cut over in one go. Keep both for a week and run them on the same tickets, otherwise every rough edge in the new setup reads as "the new model is worse" when it is actually you not knowing the keyboard shortcuts yet. A week of overlap costs you one month of one subscription and saves you the migration you have to undo.