fix: keep Solr failure text out of MCP resource responses; drop JsonUtils

src/main/java/org/apache/solr/mcp/server/collection/CollectionService.java

@@ -19,7 +19,6 @@
 import static org.apache.solr.mcp.server.collection.CollectionUtils.getFloat;
 import static org.apache.solr.mcp.server.collection.CollectionUtils.getInteger;
 import static org.apache.solr.mcp.server.collection.CollectionUtils.getLong;
-import static org.apache.solr.mcp.server.util.JsonUtils.toJson;
 
 import io.micrometer.observation.annotation.Observed;
 import io.modelcontextprotocol.spec.McpSchema.CompleteRequest;
@@ -128,6 +127,10 @@ public class CollectionService {
 
   private static final Logger logger = LoggerFactory.getLogger(CollectionService.class);
 
+  static final String COLLECTIONS_RESOURCE_ERROR =
+      "Failed to list Solr collections. Retry later; if it persists,"
+          + " ask the operator to check Solr availability and the server logs.";
+
   // ========================================
   // Constants for API Parameters and Paths
   // ========================================
@@ -304,17 +307,25 @@ public SolrParams getParams() {
    * available in the Solr cluster. The returned JSON contains an array of collection names.
    *
    * @return JSON string containing the list of collections
-   * @throws SolrServerException if there are errors communicating with Solr
-   * @throws IOException if there are I/O errors during communication
+   * @throws IllegalStateException with {@link #COLLECTIONS_RESOURCE_ERROR} if the collections
+   *     cannot be listed or serialized
    */
   @PreAuthorize("isAuthenticated()")
   @McpResource(
       uri = "solr://collections",
       name = "solr-collections",
       description = "List of all Solr collections available in the cluster",
       mimeType = "application/json")
-  public String getCollectionsResource() throws SolrServerException, IOException {
-    return toJson(objectMapper, listCollections());
+  public String getCollectionsResource() {
+    try {
+      return objectMapper.writeValueAsString(listCollections());
+    } catch (Exception e) {
+      // Deliberately no cause: MCP reports the root cause's message to the client,
+      // and Solr and SolrJ failure text can carry internal host names, URLs and
+      // paths. The log is the sole record of the cause.
+      logger.error("Failed to list collections for the collections resource", e);
+      throw new IllegalStateException(COLLECTIONS_RESOURCE_ERROR);
+    }
   }
 
   /** Maximum number of completion suggestions returned per request. */

src/main/java/org/apache/solr/mcp/server/schema/SchemaService.java

@@ -16,7 +16,6 @@
  */
 package org.apache.solr.mcp.server.schema;
 
-import static org.apache.solr.mcp.server.util.JsonUtils.toJson;
 import static org.apache.solr.mcp.server.util.PromptText.optionalCodeBlock;
 
 import io.micrometer.observation.annotation.Observed;
@@ -119,6 +118,10 @@ public class SchemaService {
 
   private static final Logger logger = LoggerFactory.getLogger(SchemaService.class);
 
+  static final String SCHEMA_RESOURCE_ERROR =
+      "Failed to retrieve the schema. Hint: call list-collections to check"
+          + " the collection name; if it persists, ask the operator to check the server logs.";
+
   /** SolrJ client for communicating with Solr server */
   private final SolrClient solrClient;
 
@@ -149,6 +152,8 @@ public SchemaService(SolrClient solrClient, ObjectMapper objectMapper) {
    *
    * @param collection the name of the collection to retrieve schema for
    * @return JSON string containing the schema representation
+   * @throws IllegalStateException with {@link #SCHEMA_RESOURCE_ERROR} if the schema cannot be
+   *     retrieved or serialized
    */
   @PreAuthorize("isAuthenticated()")
   @McpResource(
@@ -159,13 +164,13 @@ public SchemaService(SolrClient solrClient, ObjectMapper objectMapper) {
       mimeType = "application/json")
   public String getSchemaResource(String collection) {
     try {
-      return toJson(objectMapper, getSchema(collection));
+      return objectMapper.writeValueAsString(getSchema(collection));
     } catch (Exception e) {
+      // Deliberately no cause: MCP reports the root cause's message to the client,
+      // and Solr and SolrJ failure text can carry internal host names, URLs and
+      // paths. The log is the sole record of the cause.
       logger.error("Failed to get schema for collection: {}", collection, e);
-      // Serialise via Jackson rather than concatenating: an exception message
-      // containing a quote, backslash or newline would otherwise emit invalid
-      // JSON to the MCP client.
-      return toJson(objectMapper, Map.of("error", String.valueOf(e.getMessage())));
+      throw new IllegalStateException(SCHEMA_RESOURCE_ERROR);
     }
   }
 

src/test/java/org/apache/solr/mcp/server/McpClientIntegrationTestBase.java

@@ -19,6 +19,7 @@
 import static org.junit.jupiter.api.Assertions.*;
 
 import io.modelcontextprotocol.client.McpSyncClient;
+import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema.CallToolRequest;
 import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
 import io.modelcontextprotocol.spec.McpSchema.CompleteRequest;
@@ -28,6 +29,7 @@
 import io.modelcontextprotocol.spec.McpSchema.GetPromptResult;
 import io.modelcontextprotocol.spec.McpSchema.PromptMessage;
 import io.modelcontextprotocol.spec.McpSchema.PromptReference;
+import io.modelcontextprotocol.spec.McpSchema.ReadResourceRequest;
 import io.modelcontextprotocol.spec.McpSchema.ResourceReference;
 import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
@@ -1228,6 +1230,23 @@ void searchFailureIsAnActionableMcpToolError() {
     assertFalse(extractText(result).contains("Exception"), extractText(result));
   }
 
+  @Test
+  @Order(45)
+  void schemaResourceFailureIsAnMcpErrorWithoutTheSolrUrl() {
+    // An unknown collection makes SolrJ raise "Error from server at <solr url>:
+    // ...".
+    McpError e =
+        assertThrows(
+            McpError.class,
+            () ->
+                mcpClient.readResource(
+                    new ReadResourceRequest("solr://no_such_collection_xyz/schema")));
+    String text = e.getMessage() + " " + e.getJsonRpcError();
+    assertTrue(text.contains("Failed to retrieve the schema"), text);
+    assertFalse(text.contains("Error from server"), text);
+    assertFalse(text.contains("http://") || text.contains("https://"), text);
+  }
+
   private static String extractFirstMessageText(GetPromptResult result) {
     List<PromptMessage> messages = result.messages();
     assertFalse(messages.isEmpty(), "messages must not be empty");

src/test/java/org/apache/solr/mcp/server/collection/CollectionServiceTest.java

@@ -747,6 +747,29 @@ void listCollections_IOException_Propagates() throws Exception {
     assertThrows(IOException.class, () -> collectionService.listCollections());
   }
 
+  @Test
+  void getCollectionsResource_Success() throws Exception {
+    NamedList<Object> response = new NamedList<>();
+    response.add("collections", Arrays.asList("collection1", "collection2"));
+    when(solrClient.request(any(), any())).thenReturn(response);
+
+    assertEquals("[\"collection1\",\"collection2\"]", collectionService.getCollectionsResource());
+  }
+
+  @Test
+  void getCollectionsResource_SolrFailure_DoesNotLeakCause() throws Exception {
+    when(solrClient.request(any(), any()))
+        .thenThrow(
+            new SolrServerException(
+                "Server refused connection at: http://solr-internal.corp:8983/solr"));
+
+    IllegalStateException e =
+        assertThrows(IllegalStateException.class, () -> collectionService.getCollectionsResource());
+
+    assertEquals(CollectionService.COLLECTIONS_RESOURCE_ERROR, e.getMessage());
+    assertNull(e.getCause());
+  }
+
   // Helper methods — mock the Solr Metrics API response format:
   // response -> "metrics" -> "solr.core.<name>" -> "CACHE.searcher.xxx" /
   // "HANDLER./xxx"

src/test/java/org/apache/solr/mcp/server/schema/SchemaServiceTest.java

@@ -119,6 +119,25 @@ void testGetSchema_IOException() throws Exception {
         });
   }
 
+  @Test
+  void testGetSchemaResource_SolrFailure_DoesNotLeakCause() throws Exception {
+    // Given
+    final String collectionName = "test_collection";
+    when(solrClient.request(any(SchemaRequest.class), eq(collectionName)))
+        .thenThrow(
+            new SolrServerException(
+                "Server refused connection at: http://solr-internal.corp:8983/solr"));
+
+    // When
+    IllegalStateException e =
+        assertThrows(
+            IllegalStateException.class, () -> schemaService.getSchemaResource(collectionName));
+
+    // Then the client gets the fixed message, never the Solr failure text
+    assertEquals(SchemaService.SCHEMA_RESOURCE_ERROR, e.getMessage());
+    assertNull(e.getCause());
+  }
+
   @Test
   void testGetSchema_WithEmptyCollection() {
     // Given an empty collection name