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When an insert produces a DuplicateKey error on a clustered collection whose _id is a Decimal128, the server throws an uncaught KeyString format error, either 50810 for values >=1 or 50825 for values Root cause A clustered collection's RecordId is the KeyString encoding of _id with the TypeBits intentionally discarded, the type information lives only in the document body as mentioned on this comment. When there is a write conflict, to produce a user-friendly duplicate-key message, the insert path decodes the existing RecordId back to BSON. That decode runs with an empty TypeBits that is represents of kInt. When the values carry decimal-format markers then we throw in toBsonValue().

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Steps to Reproduce

const conn = MongoRunner.runMongod({}); const coll = conn.getDB("test").c; assert.commandWorked(conn.getDB("test").createCollection( "c", {clusteredIndex: {key: {_id: 1}, name: "id", unique: true}})); assert.commandWorked(coll.insert({_id: NumberDecimal("1.13")})); // Expected: DuplicateKey (11000). Actual: Location50810. assert.writeErrorWithCode(coll.insert({_id: NumberDecimal("1.13")}), 50810); MongoRunner.stopMongod(conn);