> For the complete documentation index, see [llms.txt](https://soroban-cost-estimator.gitbook.io/soroban-cost_estimator-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://soroban-cost-estimator.gitbook.io/soroban-cost_estimator-docs/faq.md).

# FAQ

## Why do fee numbers drift between runs of the same invocation?

Because `simulateTransaction` applies a margin of roughly 20% as a buffer convention, and the rent component varies slightly with ledger state. The recorded cross-check demonstrates this: the same `increment(step=5)` call reported 18,999 stroops in the original run, 19,001 an earlier run (\~40–50 ledgers prior), 17,606 on a re-check \~46,000 ledgers later, and 17,122 on the most recent re-check. CPU instructions, entry counts, and byte sizes reproduce exactly — the variance is a 1–2 stroop run-to-run rent difference plus slow rate/rent drift on the network, not a math error. If you need number stability, treat the fee as "approximately right within \~20%" and the resource footprint as exact.

## Why does `--fn` require `--id`?

`simulateTransaction` loads the contract instance from the ledger by its contract ID. A zeroed ID is not a valid instance, so the tool refuses to guess: when `--fn` is given, `--id <64-hex>` must name a deployed contract. Without `--fn`, no instance is needed — the tool simulates uploading the WASM itself.

## What happens when a `ConfigSetting*` source can't be fetched?

The affected rate is set to 0 and a warning naming the source is printed to stderr — for example:

```
Warning: fee rate source(s) ContractComputeV0 unavailable — affected rate(s) set to 0 (non-refundable fee understated)
```

The non-refundable fee is then *visibly understated* rather than silently wrong: the CPU fee would compute as 0 stroops while the authoritative total still comes from `simulateTransaction`. Treat a warning as a hard "do not trust this number" signal and re-run once the RPC is healthy.

## Why is `Memory Bytes` 0 on modern RPC endpoints?

CPU instructions and memory were historically reported in a `cost` object in the `simulateTransaction` response. Modern RPC versions dropped that object and carry resource usage inside `transactionData` XDR — which includes CPU instructions and footprint entry/byte counts but not memory bytes. The tool reports what the response actually contains: memory is 0 because modern RPC responses no longer expose it.

## Can the refundable portion really be refunded?

The refundable portion covers events and ledger **rent** — the deposit for keeping your contract's data alive. Whether and how much comes back depends on how long the data lives versus the TTL you pay for. The tool reports it separately so you can reason about it; it does not predict the refund.

## Which networks can I use?

`testnet`, `mainnet`, and `futurenet` are resolved to the well-known endpoints (see [Installation](/soroban-cost_estimator-docs/installation.md)). Any other network name is rejected unless you pass an explicit `--rpc-url`, which overrides resolution for every command.

## Does the tool submit transactions?

No. Every command simulates only — nothing is submitted to the network. The native-CLI cross-check in [Verification](/soroban-cost_estimator-docs/verification.md) runs with `--send=no` for the same reason.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://soroban-cost-estimator.gitbook.io/soroban-cost_estimator-docs/faq.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.