diff --git a/Tests/KumoTests/Fixtures/XML/XMLRoundtripTests.swift b/Tests/KumoTests/Fixtures/XML/XMLRoundtripTests.swift
new file mode 100644
index 0000000..7ac2059
--- /dev/null
+++ b/Tests/KumoTests/Fixtures/XML/XMLRoundtripTests.swift
@@ -0,0 +1,219 @@
+import Foundation
+import XCTest
+@testable import Kumo
+@testable import KumoCoding
+
+/// Tests that verify XMLEncoder and XMLDecoder work together for common
+/// XML patterns: flat structures, nested objects, SOAP envelopes, and
+/// PascalCase key conversion — the typical patterns used by enterprise
+/// healthcare XML services.
+class XMLRoundtripTests: XCTestCase {
+
+    // MARK: - Models (generic, not tied to any specific vendor)
+
+    struct PatientRecord: Codable, Equatable {
+        let recordId: String
+        let firstName: String
+        let lastName: String
+        let dateOfBirth: String
+        let gender: String
+    }
+
+    struct Appointment: Codable, Equatable {
+        let appointmentId: String
+        let recordId: String
+        let provider: String
+        let dateTime: String
+        let status: String
+    }
+
+    struct ResponseEnvelope: Codable, Equatable {
+        let statusCode: String
+        let message: String
+        let patient: PatientRecord
+    }
+
+    // MARK: - Plain XML Encode → Decode Roundtrip
+
+    func testFlatStructureRoundtrip() {
+        let encoder = XMLEncoder()
+        encoder.keyEncodingStrategy = .convertToPascalCase
+        encoder.userInfo[.rootNamespace] = XMLNamespace(prefix: "", uri: "urn:test:record:v1")
+
+        let decoder = XMLDecoder()
+        decoder.keyDecodingStrategy = .convertFromPascalCase
+
+        let original = PatientRecord(
+            recordId: "R12345",
+            firstName: "Jane",
+            lastName: "Smith",
+            dateOfBirth: "1985-12-25",
+            gender: "F"
+        )
+
+        do {
+            let data = try encoder.encode(original)
+            let decoded = try decoder.decode(PatientRecord.self, from: data)
+            XCTAssertEqual(original, decoded)
+        } catch {
+            XCTFail("Flat structure roundtrip failed: \(error)")
+        }
+    }
+
+    func testNestedStructureRoundtrip() {
+        let encoder = XMLEncoder()
+        encoder.keyEncodingStrategy = .convertToPascalCase
+        encoder.userInfo[.rootNamespace] = XMLNamespace(prefix: "", uri: "urn:test:response:v1")
+
+        let decoder = XMLDecoder()
+        decoder.keyDecodingStrategy = .convertFromPascalCase
+
+        let original = ResponseEnvelope(
+            statusCode: "200",
+            message: "OK",
+            patient: PatientRecord(
+                recordId: "R99999",
+                firstName: "John",
+                lastName: "Doe",
+                dateOfBirth: "1990-01-15",
+                gender: "M"
+            )
+        )
+
+        do {
+            let data = try encoder.encode(original)
+            let decoded = try decoder.decode(ResponseEnvelope.self, from: data)
+            XCTAssertEqual(original, decoded)
+        } catch {
+            XCTFail("Nested structure roundtrip failed: \(error)")
+        }
+    }
+
+    // MARK: - Plain XML Decoding (no SOAP)
+
+    func testDecodingFlatXML() {
+        let decoder = XMLDecoder()
+        decoder.keyDecodingStrategy = .convertFromPascalCase
+
+        let xml = """
+        <PatientRecord xmlns="urn:test:record:v1">
+            <RecordId>R12345</RecordId>
+            <FirstName>Jane</FirstName>
+            <LastName>Smith</LastName>
+            <DateOfBirth>1985-12-25</DateOfBirth>
+            <Gender>F</Gender>
+        </PatientRecord>
+        """.data(using: .utf8)!
+
+        do {
+            let record = try decoder.decode(PatientRecord.self, from: xml)
+            XCTAssertEqual(record.recordId, "R12345")
+            XCTAssertEqual(record.firstName, "Jane")
+            XCTAssertEqual(record.lastName, "Smith")
+            XCTAssertEqual(record.dateOfBirth, "1985-12-25")
+            XCTAssertEqual(record.gender, "F")
+        } catch {
+            XCTFail("Flat XML decoding failed: \(error)")
+        }
+    }
+
+    func testDecodingNestedXML() {
+        let decoder = XMLDecoder()
+        decoder.keyDecodingStrategy = .convertFromPascalCase
+
+        let xml = """
+        <ResponseEnvelope xmlns="urn:test:response:v1">
+            <StatusCode>200</StatusCode>
+            <Message>OK</Message>
+            <Patient>
+                <RecordId>R12345</RecordId>
+                <FirstName>Jane</FirstName>
+                <LastName>Smith</LastName>
+                <DateOfBirth>1985-12-25</DateOfBirth>
+                <Gender>F</Gender>
+            </Patient>
+        </ResponseEnvelope>
+        """.data(using: .utf8)!
+
+        do {
+            let response = try decoder.decode(ResponseEnvelope.self, from: xml)
+            XCTAssertEqual(response.statusCode, "200")
+            XCTAssertEqual(response.message, "OK")
+            XCTAssertEqual(response.patient.recordId, "R12345")
+            XCTAssertEqual(response.patient.firstName, "Jane")
+        } catch {
+            XCTFail("Nested XML decoding failed: \(error)")
+        }
+    }
+
+    // MARK: - SOAP Roundtrip
+
+    func testSOAPRoundtrip() {
+        let encoder = SOAPEncoder()
+        encoder.keyEncodingStrategy = .convertToPascalCase
+        encoder.soapNamespaceUsage = .define(
+            using: XMLNamespace(prefix: "soap", uri: "http://www.w3.org/2003/05/soap-envelope/"),
+            including: []
+        )
+        encoder.requestPayloadNamespaceUsage = .defineBeneath(
+            XMLNamespace(prefix: "ns", uri: "urn:test:record:v1")
+        )
+
+        let original = PatientRecord(
+            recordId: "R77777",
+            firstName: "Alice",
+            lastName: "Wong",
+            dateOfBirth: "2000-06-15",
+            gender: "F"
+        )
+
+        do {
+            let data = try encoder.encode(original)
+            let xmlString = String(data: data, encoding: .utf8)!
+
+            // Verify SOAP structure
+            XCTAssertTrue(xmlString.contains("soap:Envelope"))
+            XCTAssertTrue(xmlString.contains("soap:Body"))
+            XCTAssertTrue(xmlString.contains("ns:PatientRecord"))
+            XCTAssertTrue(xmlString.contains("ns:RecordId"))
+            XCTAssertTrue(xmlString.contains(">R77777<"))
+
+            // Decode back via SOAPDecoder
+            let decoder = SOAPDecoder()
+            decoder.keyDecodingStrategy = .convertFromPascalCase
+            let decoded: PatientRecord = try decoder.decode(from: data)
+            XCTAssertEqual(original, decoded)
+        } catch {
+            XCTFail("SOAP roundtrip failed: \(error)")
+        }
+    }
+
+    // MARK: - Encoding Verification
+
+    func testEncodingProducesExpectedXML() {
+        let encoder = XMLEncoder()
+        encoder.keyEncodingStrategy = .convertToPascalCase
+        encoder.userInfo[.rootNamespace] = XMLNamespace(prefix: "", uri: "urn:test:appt:v1")
+
+        let appointment = Appointment(
+            appointmentId: "A001",
+            recordId: "R12345",
+            provider: "Dr. Smith",
+            dateTime: "2026-04-01T10:30:00",
+            status: "Confirmed"
+        )
+
+        do {
+            let data = try encoder.encode(appointment)
+            let xml = String(data: data, encoding: .utf8)!
+            XCTAssertTrue(xml.contains("<AppointmentId>A001</AppointmentId>"))
+            XCTAssertTrue(xml.contains("<RecordId>R12345</RecordId>"))
+            XCTAssertTrue(xml.contains("<Provider>Dr. Smith</Provider>"))
+            XCTAssertTrue(xml.contains("<DateTime>2026-04-01T10:30:00</DateTime>"))
+            XCTAssertTrue(xml.contains("<Status>Confirmed</Status>"))
+            XCTAssertTrue(xml.contains("xmlns=\"urn:test:appt:v1\""))
+        } catch {
+            XCTFail("Encoding failed: \(error)")
+        }
+    }
+}