t8y2/dbx · error · java.lang.IllegalArgumentException

The default MongoDB _id_ index cannot be dropped

Error message

The default MongoDB _id_ index cannot be dropped

What it means

MongoDB's default _id_ index is created automatically for every collection and cannot be dropped at the server level. The agent rejects requests that name it — either as the literal name "_id_" or as an equivalent key-pattern specification — before contacting the server.

Source

Thrown at agents/drivers/mongodb/src/main/java/com/dbx/agent/mongodb/MongoAgent.java:1382

    static Object parseDropIndexesValue(String indexesJson, boolean single) {
        if (indexesJson == null || indexesJson.isBlank()) {
            if (single) {
                throw new IllegalArgumentException("dropIndex requires a string index name or JSON document");
            }
            return "*";
        }

        JsonElement value = JsonParser.parseString(indexesJson);
        if (value.isJsonPrimitive() && value.getAsJsonPrimitive().isString()) {
            String name = value.getAsString();
            if (name.isBlank()) {
                throw new IllegalArgumentException("Index name is required");
            }
            if (single && "*".equals(name)) {
                throw new IllegalArgumentException("dropIndex does not accept \"*\"; use dropIndexes() or dropIndexes(\"*\") instead");
            }
            if (DEFAULT_ID_INDEX_NAME.equals(name)) {
                throw new IllegalArgumentException("The default MongoDB _id_ index cannot be dropped");
            }
            return name;
        }
        if (value.isJsonObject()) {
            JsonObject object = value.getAsJsonObject();
            if (object.size() == 0) {
                throw new IllegalArgumentException("Index specification is required");
            }
            Document specification = Document.parse(indexesJson);
            if (isDefaultIdIndexSpecification(specification)) {
                throw new IllegalArgumentException("The default MongoDB _id_ index cannot be dropped");
            }
            return specification;
        }
        if (value.isJsonArray()) {
            if (single) {
                throw new IllegalArgumentException("dropIndex only accepts a string index name or JSON document; arrays are not supported");
            }

View on GitHub (pinned to c0390bff16)

Solutions

1. Skip indexes named "_id_" (or key {"_id": 1}) in your drop loop
2. Only drop secondary indexes you explicitly created
3. To change _id behavior, redesign the schema rather than dropping the index

Example fix

// before
for (Document idx : collection.listIndexes()) {
    drop(idx.getString("name")); // includes "_id_"
}
// after
for (Document idx : collection.listIndexes()) {
    String name = idx.getString("name");
    if (!"_id_".equals(name)) {
        drop(name);
    }
}

if ("_id_".equals(indexName)) {
    return; // skip: _id index is mandatory in MongoDB
}

boolean isDroppableIndex(Document indexInfo) {
    String name = indexInfo.getString("name");
    return name != null && !"_id_".equals(name);
}

try {
    agent.execute("dropIndex", params);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("_id_ index cannot be dropped")) {
        log.info("Skipped mandatory _id index");
    } else { throw e; }
}

When it happens

Trigger: Calling dropIndex or dropIndexes with indexes="_id_", or with a JSON document equivalent to the _id index (e.g. {"_id": 1}) that matches isDefaultIdIndexSpecification.

Common situations: Iterating db.collection.getIndexes() output and dropping every returned index including _id_; trying to rebuild/replace the _id index; cleanup scripts that don't special-case _id_.

Related errors

• dropIndex requires a string index name or JSON document
• Index name is required
• dropIndex does not accept "*"; use dropIndexes() or dropInde
• Index specification is required
• MongoDB aggregate option ${key} must be a non-negative integ

AI-assisted analysis of t8y2/dbx@c0390bff16 (2026-09-05). Data as JSON: /api/errors/806481db7c6295c1. Report an issue: GitHub.