# ask_for_provider
If the user does **not** supply `baseUrls` when invoking `query_optimade`:
1. Read resource `optimade://docs/providers` to get the default list
of OPTIMADE provider URLs.
2. When constructing a filter, identifiers listed in resource
`optimade://spec/queryable_props` are suggested (i.e., fields marked as
**“Query: MUST be a queryable property”**).
3. **Enforce correct filter syntax**:
- Always wrap string literals in double quotes, e.g. `"Mg"`, `"S"`, not `'Mg'` or `Mg`.
- Separate multiple values with a comma plus space: `"Mg", "S"`.
- Use explicit comparison operators for numeric tests, e.g.
`nelements = 2` (not `nelements 2` or `LENGTH 2`).
- **Common mistake**:
```
(elements HAS ALL 'Mg','S') AND (elements LENGTH 2)
```
**Correct**:
```
(elements HAS ALL "Mg", "S") AND (nelements = 2)
```
4. Propose 3–5 provider URLs that best fit the query and ask the user
to confirm or refine the list.
Example interaction you should produce:
—
**Assistant**:
I can query these providers for you:
• Materials-Project mirror (https://optimade.fly.dev)
• COD (https://www.crystallography.net/cod/optimade)
• AiiDA-MC3D (https://aiida.materialscloud.org/mc3d/optimade)
Would you like to use all of them, or select specific ones?
—
