diff --git a/lib/fhir_client/client.rb b/lib/fhir_client/client.rb
index e46a4744..ab6fa40c 100644
--- a/lib/fhir_client/client.rb
+++ b/lib/fhir_client/client.rb
@@ -289,21 +289,21 @@ def try_conformance_formats(default_format)
         rescue
           @cached_capability_statement = nil
         end
-        if @cached_capability_statement.nil?
+        if @cached_capability_statement.nil? || !@cached_capability_statement.fhirVersion.starts_with?('4.0')
           use_r4b
           begin
             @cached_capability_statement = parse_reply(FHIR::R4B::CapabilityStatement, frmt, reply)
           rescue
             @cached_capability_statement = nil
           end
-          unless @cached_capability_statement
+          if @cached_capability_statement.nil? || !@cached_capability_statement.fhirVersion.starts_with?('4')
             use_r5
             begin
               @cached_capability_statement = parse_reply(FHIR::R5::CapabilityStatement, frmt, reply)
             rescue
               @cached_capability_statement = nil
             end
-            unless @cached_capability_statement
+            if @cached_capability_statement.nil? || !@cached_capability_statement.fhirVersion.starts_with?('5')
               use_stu3
               begin
                 @cached_capability_statement = parse_reply(FHIR::STU3::CapabilityStatement, frmt, reply)
@@ -353,11 +353,11 @@ def parse_reply(klass, format, response)
             else
               FHIR::DSTU2::Json.from_json(response.body)
             end
-          elsif(@fhir_version == :r4 || klass&.ancestors&.include?(FHIR::Model))
+          elsif(@fhir_version == :stu3 || klass&.ancestors&.include?(FHIR::STU3::Model))
             if(format.include?('xml'))
-              FHIR::Xml.from_xml(response.body)
+              FHIR::STU3::Xml.from_xml(response.body)
             else
-              FHIR::Json.from_json(response.body)
+              FHIR::STU3::Json.from_json(response.body)
             end
           elsif(@fhir_version == :r4b || klass&.ancestors&.include?(FHIR::R4B::Model))
             if(format.include?('xml'))
@@ -373,9 +373,9 @@ def parse_reply(klass, format, response)
             end
           else
             if(format.include?('xml'))
-              FHIR::STU3::Xml.from_xml(response.body)
+              FHIR::Xml.from_xml(response.body)
             else
-              FHIR::STU3::Json.from_json(response.body)
+              FHIR::Json.from_json(response.body)
             end
           end
         rescue => e
diff --git a/test/fixtures/r4b_capabilitystatement.json b/test/fixtures/r4b_capabilitystatement.json
new file mode 100644
index 00000000..408bf3d9
--- /dev/null
+++ b/test/fixtures/r4b_capabilitystatement.json
@@ -0,0 +1,148 @@
+{
+  "resourceType" : "CapabilityStatement",
+  "id" : "example",
+  "text" : {
+    "status" : "generated",
+    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\">\n\t\t\t<p>The EHR Server supports the following transactions for the resource Person: read, vread, \n        update, history, search(name,gender), create and updates.</p>\n\t\t\t<p>The EHR System supports the following message: admin-notify::Person.</p>\n\t\t\t<p>The EHR Application has a \n        <a href=\"http://fhir.hl7.org/base/Profilebc054d23-75e1-4dc6-aca5-838b6b1ac81d/_history/b5fdd9fc-b021-4ea1-911a-721a60663796\">general document profile</a>.\n      </p>\n\t\t</div>"
+  },
+  "url" : "urn:uuid:68d043b5-9ecf-4559-a57a-396e0d452311",
+  "version" : "20130510",
+  "name" : "ACME-EHR",
+  "title" : "ACME EHR capability statement",
+  "status" : "draft",
+  "experimental" : true,
+  "date" : "2012-01-04",
+  "publisher" : "ACME Corporation",
+  "contact" : [{
+    "name" : "System Administrator",
+    "telecom" : [{
+      "system" : "email",
+      "value" : "wile@acme.org"
+    }]
+  }],
+  "description" : "This is the FHIR capability statement for the main EHR at ACME for the private interface - it does not describe the public interface",
+  "useContext" : [{
+    "code" : {
+      "system" : "http://terminology.hl7.org/CodeSystem/usage-context-type",
+      "code" : "focus"
+    },
+    "valueCodeableConcept" : {
+      "coding" : [{
+        "system" : "http://terminology.hl7.org/CodeSystem/variant-state",
+        "code" : "positive"
+      }]
+    }
+  }],
+  "jurisdiction" : [{
+    "coding" : [{
+      "system" : "urn:iso:std:iso:3166",
+      "code" : "US",
+      "display" : "United States of America (the)"
+    }]
+  }],
+  "purpose" : "Main EHR capability statement, published for contracting and operational support",
+  "copyright" : "Copyright © Acme Healthcare and GoodCorp EHR Systems",
+  "kind" : "instance",
+  "instantiates" : ["http://ihe.org/fhir/CapabilityStatement/pixm-client"],
+  "software" : {
+    "name" : "EHR",
+    "version" : "0.00.020.2134",
+    "releaseDate" : "2012-01-04"
+  },
+  "implementation" : {
+    "description" : "main EHR at ACME",
+    "url" : "http://10.2.3.4/fhir"
+  },
+  "fhirVersion" : "4.3.0",
+  "format" : ["xml",
+  "json"],
+  "patchFormat" : ["application/xml-patch+xml",
+  "application/json-patch+json"],
+  "implementationGuide" : ["http://hl7.org/fhir/us/lab"],
+  "rest" : [{
+    "mode" : "server",
+    "documentation" : "Main FHIR endpoint for acem health",
+    "security" : {
+      "cors" : true,
+      "service" : [{
+        "coding" : [{
+          "system" : "http://terminology.hl7.org/CodeSystem/restful-security-service",
+          "code" : "SMART-on-FHIR"
+        }]
+      }],
+      "description" : "See Smart on FHIR documentation"
+    },
+    "resource" : [{
+      "type" : "Patient",
+      "profile" : "http://registry.fhir.org/r4/StructureDefinition/7896271d-57f6-4231-89dc-dcc91eab2416",
+      "supportedProfile" : ["http://registry.fhir.org/r4/StructureDefinition/00ab9e7a-06c7-4f77-9234-4154ca1e3347"],
+      "documentation" : "This server does not let the clients create identities.",
+      "interaction" : [{
+        "code" : "read"
+      },
+      {
+        "code" : "vread",
+        "documentation" : "Only supported for patient records since 12-Dec 2012"
+      },
+      {
+        "code" : "update"
+      },
+      {
+        "code" : "history-instance"
+      },
+      {
+        "code" : "create"
+      },
+      {
+        "code" : "history-type"
+      }],
+      "versioning" : "versioned-update",
+      "readHistory" : true,
+      "updateCreate" : false,
+      "conditionalCreate" : true,
+      "conditionalRead" : "full-support",
+      "conditionalUpdate" : false,
+      "conditionalDelete" : "not-supported",
+      "searchInclude" : ["Organization"],
+      "searchRevInclude" : ["Person"],
+      "searchParam" : [{
+        "name" : "identifier",
+        "definition" : "http://hl7.org/fhir/SearchParameter/Patient-identifier",
+        "type" : "token",
+        "documentation" : "Only supports search by institution MRN"
+      },
+      {
+        "name" : "general-practitioner",
+        "definition" : "http://hl7.org/fhir/SearchParameter/Patient-general-practitioner",
+        "type" : "reference"
+      }]
+    }],
+    "interaction" : [{
+      "code" : "transaction"
+    },
+    {
+      "code" : "history-system"
+    }],
+    "compartment" : ["http://hl7.org/fhir/CompartmentDefinition/patient"]
+  }],
+  "messaging" : [{
+    "endpoint" : [{
+      "protocol" : {
+        "system" : "http://terminology.hl7.org/CodeSystem/message-transport",
+        "code" : "mllp"
+      },
+      "address" : "mllp:10.1.1.10:9234"
+    }],
+    "reliableCache" : 30,
+    "documentation" : "ADT A08 equivalent for external system notifications",
+    "supportedMessage" : [{
+      "mode" : "receiver",
+      "definition" : "http://hl7.org/fhir/MessageDefinition/example"
+    }]
+  }],
+  "document" : [{
+    "mode" : "consumer",
+    "documentation" : "Basic rules for all documents in the EHR system",
+    "profile" : "http://fhir.hl7.org/base/Profilebc054d23-75e1-4dc6-aca5-838b6b1ac81d/_history/b5fdd9fc-b021-4ea1-911a-721a60663796"
+  }]
+}
\ No newline at end of file
diff --git a/test/fixtures/r5_capabilitystatement.json b/test/fixtures/r5_capabilitystatement.json
new file mode 100644
index 00000000..79a207cb
--- /dev/null
+++ b/test/fixtures/r5_capabilitystatement.json
@@ -0,0 +1,151 @@
+{
+  "resourceType" : "CapabilityStatement",
+  "id" : "example",
+  "text" : {
+    "status" : "generated",
+    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\">\n\t\t\t\n      <p>The EHR Server supports the following transactions for the resource Person: read, vread, \n        update, history, search(name,gender), create and updates.</p>\n\t\t\t\n      <p>The EHR System supports the following message: admin-notify::Person.</p>\n\t\t\t\n      <p>The EHR Application has a \n        \n        <a href=\"http://fhir.hl7.org/base/Profilebc054d23-75e1-4dc6-aca5-838b6b1ac81d/_history/b5fdd9fc-b021-4ea1-911a-721a60663796\">general document profile</a>.\n      \n      </p>\n\t\t\n    </div>"
+  },
+  "url" : "urn:uuid:68d043b5-9ecf-4559-a57a-396e0d452311",
+  "version" : "20130510",
+  "name" : "ACMEEHR",
+  "title" : "ACME EHR capability statement",
+  "status" : "draft",
+  "experimental" : true,
+  "date" : "2012-01-04",
+  "publisher" : "ACME Corporation",
+  "contact" : [{
+    "name" : "System Administrator",
+    "telecom" : [{
+      "system" : "email",
+      "value" : "wile@acme.org"
+    }]
+  }],
+  "description" : "This is the FHIR capability statement for the main EHR at ACME for the private interface - it does not describe the public interface",
+  "useContext" : [{
+    "code" : {
+      "system" : "http://terminology.hl7.org/CodeSystem/usage-context-type",
+      "code" : "focus"
+    },
+    "valueCodeableConcept" : {
+      "coding" : [{
+        "system" : "http://terminology.hl7.org/CodeSystem/variant-state",
+        "code" : "positive"
+      }]
+    }
+  }],
+  "jurisdiction" : [{
+    "coding" : [{
+      "system" : "urn:iso:std:iso:3166",
+      "code" : "US",
+      "display" : "United States of America (the)"
+    }]
+  }],
+  "purpose" : "Main EHR capability statement, published for contracting and operational support",
+  "copyright" : "Copyright © Acme Healthcare and GoodCorp EHR Systems",
+  "kind" : "instance",
+  "instantiates" : ["http://ihe.org/fhir/CapabilityStatement/pixm-client"],
+  "software" : {
+    "name" : "EHR",
+    "version" : "0.00.020.2134",
+    "releaseDate" : "2012-01-04"
+  },
+  "implementation" : {
+    "description" : "main EHR at ACME",
+    "url" : "http://10.2.3.4/fhir"
+  },
+  "fhirVersion" : "5.0.0",
+  "format" : ["xml",
+  "json"],
+  "patchFormat" : ["application/xml-patch+xml",
+  "application/json-patch+json"],
+  "acceptLanguage" : ["en",
+  "es"],
+  "implementationGuide" : ["http://example.org/fhir/us/lab"],
+  "rest" : [{
+    "mode" : "server",
+    "documentation" : "Main FHIR endpoint for acem health",
+    "security" : {
+      "cors" : true,
+      "service" : [{
+        "coding" : [{
+          "system" : "http://hl7.org/fhir/restful-security-service",
+          "code" : "SMART-on-FHIR"
+        }]
+      }],
+      "description" : "See Smart on FHIR documentation"
+    },
+    "resource" : [{
+      "type" : "Patient",
+      "profile" : "http://registry.fhir.org/r5/StructureDefinition/7896271d-57f6-4231-89dc-dcc91eab2416",
+      "supportedProfile" : ["http://registry.fhir.org/r5/StructureDefinition/00ab9e7a-06c7-4f77-9234-4154ca1e3347"],
+      "documentation" : "This server does not let the clients create identities.",
+      "interaction" : [{
+        "code" : "read"
+      },
+      {
+        "code" : "vread",
+        "documentation" : "Only supported for patient records since 12-Dec 2012"
+      },
+      {
+        "code" : "update"
+      },
+      {
+        "code" : "history-instance"
+      },
+      {
+        "code" : "create"
+      },
+      {
+        "code" : "history-type"
+      }],
+      "versioning" : "versioned-update",
+      "readHistory" : true,
+      "updateCreate" : false,
+      "conditionalCreate" : true,
+      "conditionalRead" : "full-support",
+      "conditionalUpdate" : false,
+      "conditionalPatch" : false,
+      "conditionalDelete" : "not-supported",
+      "searchInclude" : ["Patient:organization"],
+      "searchRevInclude" : ["Person:patient"],
+      "searchParam" : [{
+        "name" : "identifier",
+        "definition" : "http://hl7.org/fhir/SearchParameter/Patient-identifier",
+        "type" : "token",
+        "documentation" : "Only supports search by institution MRN"
+      },
+      {
+        "name" : "general-practitioner",
+        "definition" : "http://hl7.org/fhir/SearchParameter/Patient-general-practitioner",
+        "type" : "reference"
+      }]
+    }],
+    "interaction" : [{
+      "code" : "transaction"
+    },
+    {
+      "code" : "history-system"
+    }],
+    "compartment" : ["http://hl7.org/fhir/CompartmentDefinition/patient"]
+  }],
+  "messaging" : [{
+    "endpoint" : [{
+      "protocol" : {
+        "system" : "http://hl7.org/fhir/message-transport",
+        "code" : "mllp"
+      },
+      "address" : "mllp:10.1.1.10:9234"
+    }],
+    "reliableCache" : 30,
+    "documentation" : "ADT A08 equivalent for external system notifications",
+    "supportedMessage" : [{
+      "mode" : "receiver",
+      "definition" : "http://hl7.org/fhir/MessageDefinition/example"
+    }]
+  }],
+  "document" : [{
+    "mode" : "consumer",
+    "documentation" : "Basic rules for all documents in the EHR system",
+    "profile" : "http://fhir.hl7.org/base/Profilebc054d23-75e1-4dc6-aca5-838b6b1ac81d/_history/b5fdd9fc-b021-4ea1-911a-721a60663796"
+  }]
+}
\ No newline at end of file
diff --git a/test/unit/client_interface_sections/read_test.rb b/test/unit/client_interface_sections/read_test.rb
index 14219503..87d8ea1f 100644
--- a/test/unit/client_interface_sections/read_test.rb
+++ b/test/unit/client_interface_sections/read_test.rb
@@ -15,7 +15,7 @@ def test_read
     patient = FHIR::Patient.new({'gender'=>'female', 'active'=>true, 'deceasedBoolean'=>false})
     stub_request(:get, /read-test/).to_return(status: 200, body: patient.to_json, headers: {'Content-Type'=>'application/fhir+json', 'ETag'=>'W/"foo"', 'Last-Modified'=>Time.now.strftime("%a, %e %b %Y %T %Z")})
     temp = client
-    temp.use_stu3
+    temp.use_r4
     temp.default_json
     reply = temp.read(FHIR::Patient,'foo')
     assert reply.resource.is_a?(FHIR::Patient)
@@ -29,7 +29,7 @@ def test_vread
     patient = FHIR::Patient.new({'gender'=>'female', 'active'=>true, 'deceasedBoolean'=>false})
     stub_request(:get, /read-test\/.*_history\/2/).to_return(status: 200, body: patient.to_json, headers: {'Content-Type'=>'application/fhir+json', 'ETag'=>'W/"foo"', 'Last-Modified'=>Time.now.strftime("%a, %e %b %Y %T %Z")})
     temp = client
-    temp.use_stu3
+    temp.use_r4
     temp.default_json
     reply = temp.vread(FHIR::Patient,'foo', 2)
     assert reply.resource.is_a?(FHIR::Patient)
@@ -46,7 +46,7 @@ def test_conditional_read_since
     patient = FHIR::Patient.new({'gender'=>'female', 'active'=>true, 'deceasedBoolean'=>false})
     stub_request(:get, /read-test/).with(headers: {'If-Modified-Since' => 'Wed, 21 Oct 2015 07:28:00 GMT'}).to_return(status: 200, body: patient.to_json, headers: {'Content-Type'=>'application/fhir+json', 'ETag'=>'W/"foo"', 'Last-Modified'=>Time.now.strftime("%a, %e %b %Y %T %Z")})
     temp = client
-    temp.use_stu3
+    temp.use_r4
     temp.default_json
     reply = temp.conditional_read_since(FHIR::Patient,'foo', 'Wed, 21 Oct 2015 07:28:00 GMT')
     assert reply.resource.is_a?(FHIR::Patient)
@@ -59,7 +59,7 @@ def test_conditional_read_version
     patient = FHIR::Patient.new({'gender'=>'female', 'active'=>true, 'deceasedBoolean'=>false})
     stub_request(:get, /read-test/).with(headers: {'If-None-Match' => 'W/ABC'}).to_return(status: 200, body: patient.to_json, headers: {'Content-Type'=>'application/fhir+json', 'ETag'=>'W/"foo"', 'Last-Modified'=>Time.now.strftime("%a, %e %b %Y %T %Z")})
     temp = client
-    temp.use_stu3
+    temp.use_r4
     temp.default_json
     reply = temp.conditional_read_version(FHIR::Patient,'foo','ABC')
     assert reply.resource.is_a?(FHIR::Patient)
@@ -77,7 +77,7 @@ def test_raw_read
                              'ETag'=>'W/"foo"',
                              'Last-Modified'=>Time.now.strftime("%a, %e %b %Y %T %Z")})
     temp = client
-    temp.use_stu3
+    temp.use_r4
     temp.default_json
     options = {resource: FHIR::Patient, id: 'foo'}
     reply = temp.raw_read(options)
diff --git a/test/unit/multiversion_test.rb b/test/unit/multiversion_test.rb
index eb4134d0..a5ecd7f2 100644
--- a/test/unit/multiversion_test.rb
+++ b/test/unit/multiversion_test.rb
@@ -1,5 +1,5 @@
 require_relative '../test_helper'
-
+require 'pry'
 class MultiversionTest < Test::Unit::TestCase
 
   def test_autodetect_stu3
@@ -47,6 +47,36 @@ def test_autodetect_r4
     assert client.default_format.include? 'json'
   end
 
+  def test_autodetect_r4b
+    root = File.expand_path '..', File.dirname(File.absolute_path(__FILE__))
+    capabilitystatement = File.read(File.join(root, 'fixtures', 'r4b_capabilitystatement.json'))
+    stub_request(:get, /autodetect/).to_return(body: capabilitystatement)
+    client = FHIR::Client.new('autodetect')
+    # Intentionally set the client incorrectly
+    client.default_xml
+    client.use_r4
+    assert client.cached_capability_statement.nil?
+    assert client.detect_version == :r4b, "Expected Version to be r4b, but found #{client.detect_version.to_s}"
+    assert !client.cached_capability_statement.nil?, 'Expected Capability Statement to be cached'
+    assert client.cached_capability_statement.is_a?(FHIR::R4B::CapabilityStatement)
+    assert client.default_format.include? 'json'
+  end
+
+  def test_autodetect_r5
+    root = File.expand_path '..', File.dirname(File.absolute_path(__FILE__))
+    capabilitystatement = File.read(File.join(root, 'fixtures', 'r5_capabilitystatement.json'))
+    stub_request(:get, /autodetect/).to_return(body: capabilitystatement)
+    client = FHIR::Client.new('autodetect')
+    # Intentionally set the client incorrectly
+    client.default_xml
+    client.use_r4
+    assert client.cached_capability_statement.nil?
+    assert client.detect_version == :r5, "Expected Version to be r5, but found #{client.detect_version.to_s}"
+    assert !client.cached_capability_statement.nil?, 'Expected Capability Statement to be cached'
+    assert client.cached_capability_statement.is_a?(FHIR::R5::CapabilityStatement)
+    assert client.default_format.include? 'json'
+  end
+
   def test_stu3_patient_manual
     stub_request(:get, /stu3/).to_return(body: FHIR::STU3::Patient.new.to_json)
     client = FHIR::Client.new('stu3')
@@ -156,7 +186,7 @@ def test_r4b_reply_fhir_version
     client.use_r4b
     FHIR::R4B::Model.client = client
     patient = FHIR::R4B::Patient.read('foo')
-    assert_equal :r4, client.reply.fhir_version
+    assert_equal :r4b, client.reply.fhir_version
   end
 
   def test_r5_reply_fhir_version