adityamparikh opened a new issue, #182:
URL: https://github.com/apache/solr-mcp/issues/182
## Summary
Any `search` call with `facetFields` that produces **zero facet buckets**
fails with:
```
class java.util.ArrayList cannot be cast to class
org.apache.solr.common.util.NamedList
```
This is not specific to a field type or a collection — the same field
succeeds or fails purely
depending on whether the query matched anything.
## Reproduction
Solr 9 in SolrCloud mode, a collection `shows` with a single-valued `string`
field `platform`
(docValues on), 61 documents indexed.
**A — facet has buckets → works**
```json
{"collection": "shows", "query": "*:*", "facetFields": ["platform"], "rows":
0}
```
```
{"numFound":61,...,"facets":{"platform":{"Netflix":20,"HBO Max":7,...}}}
```
**B — same field, filter matches nothing → throws**
```json
{"collection": "shows", "query": "platform:NoSuchPlatform", "facetFields":
["platform"], "rows": 0}
```
```
class java.util.ArrayList cannot be cast to class
org.apache.solr.common.util.NamedList
```
Verified end-to-end over MCP STDIO against a published container image, and
independently over
the HTTP transport.
## Root cause
Solr serializes an empty facet field as an empty JSON **array**:
```json
"facet_counts": { "facet_fields": { "platform": [] } }
```
`JsonResponseParser.isFlatNamedList` rejects zero-length arrays
([`JsonResponseParser.java:157`](https://github.com/apache/solr-mcp/blob/main/src/main/java/org/apache/solr/mcp/server/config/JsonResponseParser.java#L157)):
```java
int size = arrayNode.size();
if (size == 0 || size % 2 != 0)
return false;
```
so `convertArray` falls through to the plain-list branch and returns an
`ArrayList`. SolrJ's
`QueryResponse.getFacetFields()` then casts that value to `NamedList` and
throws.
The heuristic cannot be fixed by simply allowing `size == 0`: a bare `[]` is
genuinely ambiguous
between an empty facet NamedList and an empty plain list, and the parser has
no context to tell
them apart.
## Suggested fix
Request `json.nl=map` explicitly rather than relying on Solr's `flat`
default. Facet fields then
arrive as JSON objects:
```json
"facet_fields": { "platform": {"Netflix": 20} } // non-empty
"facet_fields": { "platform": {} } // empty
```
Both map unambiguously onto `NamedList` through the existing `convertObject`
path, which removes
the flat-array heuristic and this whole class of ambiguity rather than
patching the symptom.
## Impact
A user asking a perfectly ordinary question — *"break these results down by
platform"* where the
filter happens to match nothing — gets a raw Java exception instead of an
empty result set. Zero
matches is a normal outcome of a search, not an error condition.
## Environment
- `main` @ `a84033b`; `JsonResponseParser.java` unmodified at that commit
- Solr 9 (`solr:9-slim`), SolrCloud mode
- solr-mcp 1.0.0-SNAPSHOT, Spring Boot 3.5.14
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