diff --git a/docs/odata-protocol/odata-protocol.html b/docs/odata-protocol/odata-protocol.html
index 262a83939..677c02639 100644
--- a/docs/odata-protocol/odata-protocol.html
+++ b/docs/odata-protocol/odata-protocol.html
@@ -512,6 +512,11 @@ <h2 id="11-changes-from-earlier-versions"><a name="ChangesfromEarlierVersions" h
 <td><a href="https://github.com/oasis-tcs/odata-specs/issues/534">534</a></td>
 </tr>
 <tr class="even">
+<td><a href="#Conformance">Section 12</a></td>
+<td>Allow <code>400 Bad Request</code> in addition to <code>501 Not Implemented</code> for unsupported functionality</td>
+<td><a href="https://github.com/oasis-tcs/odata-specs/issues/391">391</a></td>
+</tr>
+<tr class="odd">
 <td><a href="#InteroperableODataClients">Section 12.3</a></td>
 <td>Encoding of plus character in URLs</td>
 <td><a href="https://github.com/oasis-tcs/odata-specs/issues/485">485</a></td>
@@ -947,7 +952,7 @@ <h3 id="926-response-code-424-failed-dependency"><a name="ResponseCode424FailedD
 <h2 id="93-server-error-responses"><a name="ServerErrorResponses" href="#ServerErrorResponses">9.3 Server Error Responses</a></h2>
 <p>As defined in <a href="#rfc9110">RFC9110</a>, error codes in the <code>5xx</code> range indicate service errors.</p>
 <h3 id="931-response-code-501-not-implemented"><a name="ResponseCode501NotImplemented" href="#ResponseCode501NotImplemented">9.3.1 Response Code <code>501 Not Implemented</code></a></h3>
-<p>If the client requests functionality not implemented by the OData Service, the service responds with <code>501 Not Implemented</code> and SHOULD include a response body describing the functionality not implemented.</p>
+<p>If the client requests functionality not implemented by the OData Service, the service MAY respond with <code>501 Not Implemented</code> and include a response body describing the functionality not implemented.</p>
 <h2 id="94-error-response-body"><a name="ErrorResponseBody" href="#ErrorResponseBody">9.4 Error Response Body</a></h2>
 <p>An error response body can be the result of a failure of OData processing or of the underlying infrastructure. An OData-specific error response (which can be recognized by the presence of the <a href="#HeaderODataVersion"><code>OData-Version</code></a> header) is format-specific and consists at least of the following information:</p>
 <ul>
@@ -2682,7 +2687,7 @@ <h3 id="1211-odata-40-minimal-conformance-level"><a name="OData40MinimalConforma
 <li><code>OData-MaxVersion</code> (<a href="#HeaderODataMaxVersion">section 8.2.7</a>)</li>
 </ol></li>
 <li>MUST follow OData guidelines for extensibility (<a href="#Extensibility">section 6</a> and all subsections)</li>
-<li>MUST successfully parse the request according to <a href="#ODataABNF">OData-ABNF</a> for any supported system query options and either follow the specification or return <code>501 Not Implemented</code> for any unsupported functionality (<a href="#ResponseCode501NotImplemented">section 9.3.1</a>)</li>
+<li>MUST successfully parse the request according to <a href="#ODataABNF">OData-ABNF</a> for any supported system query options and follow the specification or fail the request</li>
 <li>MUST expose only data types defined in <a href="#ODataCSDL">OData-CSDLXML</a></li>
 <li>MUST NOT require clients to understand any metadata or instance annotations (<a href="#VocabularyExtensibility">section 6.4</a>), custom headers (<a href="#HeaderFieldExtensibility">section 6.5</a>), or custom content (<a href="#PayloadExtensibility">section 6.2</a>) in the payload in order to correctly consume the service</li>
 <li>MUST NOT violate any OData update semantics (<a href="#DataModification">section 11.4</a> and all subsections)</li>
@@ -2720,7 +2725,7 @@ <h3 id="1212-odata-40-intermediate-conformance-level"><a name="OData40Intermedia
 <p>In order to conform to the OData Intermediate Conformance Level, a service:</p>
 <ol type="1">
 <li>MUST conform to the <a href="#OData40MinimalConformanceLevel">OData 4.0 Minimal Conformance Level</a></li>
-<li>MUST successfully parse the <a href="#ODataABNF">OData-ABNF</a> and either follow the specification or return <code>501 Not Implemented</code> for any unsupported functionality (<a href="#ResponseCode501NotImplemented">section 9.3.1</a>)</li>
+<li>MUST successfully parse the request according to <a href="#ODataABNF">OData-ABNF</a> and follow the specification or fail the request</li>
 <li>MUST support <code>$select</code> (<a href="#SystemQueryOptionselect">section 11.2.5.1</a>)</li>
 <li>MUST support casting to a derived type according to <a href="#ODataURL">OData-URL</a> if derived types are present in the model</li>
 <li>MUST support <code>$top</code> (<a href="#SystemQueryOptiontop">section 11.2.6.3</a>)</li>
@@ -2729,8 +2734,8 @@ <h3 id="1212-odata-40-intermediate-conformance-level"><a name="OData40Intermedia
 <ol type="1">
 <li>MUST support <code>eq</code>, <code>ne</code> filter operations on properties of entities in the requested entity set (<a href="#BuiltinFilterOperations">section 11.2.6.1.1</a>)</li>
 <li>MUST support aliases in <code>$filter</code> expressions (<a href="#ParameterAliases">section 11.2.6.1.3</a>)</li>
-<li>SHOULD support additional filter operations (<a href="#BuiltinFilterOperations">section 11.2.6.1.1</a>) and MUST return <code>501 Not Implemented</code> for any unsupported filter operations (<a href="#ResponseCode501NotImplemented">section 9.3.1</a>)</li>
-<li>SHOULD support the canonical functions (<a href="#BuiltinQueryFunctions">section 11.2.6.1.2</a>) and MUST return <code>501 Not Implemented</code> for any unsupported canonical functions (<a href="#ResponseCode501NotImplemented">section 9.3.1</a>)</li>
+<li>SHOULD support additional filter operations (<a href="#BuiltinFilterOperations">section 11.2.6.1.1</a>) and MUST fail the request for any unsupported filter operations</li>
+<li>SHOULD support the canonical functions (<a href="#BuiltinQueryFunctions">section 11.2.6.1.2</a>) and MUST fail the request for any unsupported canonical functions</li>
 <li>SHOULD support <code>$filter</code> on expanded entities (<a href="#ExpandOptions">section 11.2.5.2.1</a>)</li>
 </ol></li>
 <li>SHOULD publish metadata at <code>$metadata</code> according to <a href="#ODataCSDL">OData-CSDLXML</a> (<a href="#MetadataDocumentRequest">section 11.1.2</a>)</li>
@@ -2893,7 +2898,7 @@ <h2 id="123-interoperable-odata-clients"><a name="InteroperableODataClients" hre
 <li>MUST specify identifiers in payloads and URLs in the case they are specified in <code>$metadata</code></li>
 <li>MUST be prepared to receive any valid 4.01 CSDL</li>
 <li>MUST be prepared to receive any valid 4.01 response according to the requested format</li>
-<li>SHOULD use capabilities (see <a href="#ODataVocCap">OData-VocCap</a>) to determine if a 4.01 feature is supported but MAY attempt syntax and be prepared to handle either <a href="#ResponseCode501NotImplemented"><code>501 Not Implemented</code></a> or <code>400 Bad Request</code></li>
+<li>SHOULD use capabilities (see <a href="#ODataVocCap">OData-VocCap</a>) to determine if a 4.01 feature is supported but MAY attempt syntax and be prepared to handle <code>400 Bad Request</code> or <a href="#ResponseCode501NotImplemented"><code>501 Not Implemented</code></a></li>
 </ol>
 <hr />
 <h1 id="appendix-a-references"><a name="References" href="#References">Appendix A. References</a></h1>
diff --git a/docs/odata-protocol/odata-protocol.md b/docs/odata-protocol/odata-protocol.md
index 9b51479d1..5b2599930 100644
--- a/docs/odata-protocol/odata-protocol.md
+++ b/docs/odata-protocol/odata-protocol.md
@@ -354,6 +354,7 @@ Section | Feature / Change | Issue
 [Section 11.4](#DataModification)| Response code `204 No Content` after successful data modification if requested response could not be constructed| [443](https://github.com/oasis-tcs/odata-specs/issues/443)
 [Section 11.4.4](#UpsertanEntity)|  Upserts to single-valued non-containment navigation properties| [455](https://github.com/oasis-tcs/odata-specs/issues/455)
 [Section 11.4.9.3](#UpdateaComplexProperty)| Setting a complex property to a different type| [534](https://github.com/oasis-tcs/odata-specs/issues/534)
+[Section 12](#Conformance) | Allow `400 Bad Request` in addition to `501 Not Implemented` for unsupported functionality| [391](https://github.com/oasis-tcs/odata-specs/issues/391)
 [Section 12.3](#InteroperableODataClients) | Encoding of plus character in URLs | [485](https://github.com/oasis-tcs/odata-specs/issues/485)
 
 ## <a name="Glossary" href="#Glossary">1.2 Glossary</a>
@@ -1919,7 +1920,7 @@ indicate service errors.
 ### <a name="ResponseCode501NotImplemented" href="#ResponseCode501NotImplemented">9.3.1 Response Code `501 Not Implemented`</a>
 
 If the client requests functionality not implemented by the OData
-Service, the service responds with `501 Not Implemented` and SHOULD
+Service, the service MAY respond with `501 Not Implemented` and
 include a response body describing the functionality not implemented.
 
 ## <a name="ErrorResponseBody" href="#ErrorResponseBody">9.4 Error Response Body</a>
@@ -6437,9 +6438,7 @@ request
 ([section 6](#Extensibility) and all subsections)
 7. MUST successfully parse the request according to
 [OData-ABNF](#ODataABNF) for any supported system query options and
-either follow the specification or return
-`501 Not Implemented` for any
-unsupported functionality ([section 9.3.1](#ResponseCode501NotImplemented))
+follow the specification or fail the request
 8. MUST expose only data types defined in [OData-CSDLXML](#ODataCSDL)
 9. MUST NOT require clients to understand any metadata or instance
 annotations ([section 6.4](#VocabularyExtensibility)), custom headers ([section 6.5](#HeaderFieldExtensibility)), or custom
@@ -6505,9 +6504,8 @@ service:
 
 1. MUST conform to the [OData 4.0 Minimal Conformance
 Level](#OData40MinimalConformanceLevel)
-2. MUST successfully parse the [OData-ABNF](#ODataABNF) and either
-follow the specification or return `501 Not Implemented` for any
-unsupported functionality ([section 9.3.1](#ResponseCode501NotImplemented))
+2. MUST successfully parse the request according to [OData-ABNF](#ODataABNF) and
+follow the specification or fail the request
 3. MUST support `$select` ([section 11.2.5.1](#SystemQueryOptionselect))
 4. MUST support casting to a derived type according to
 [OData-URL](#ODataURL) if derived types are present in the model
@@ -6518,11 +6516,10 @@ unsupported functionality ([section 9.3.1](#ResponseCode501NotImplemented))
 in the requested entity set ([section 11.2.6.1.1](#BuiltinFilterOperations))
    2. MUST support aliases in `$filter` expressions ([section 11.2.6.1.3](#ParameterAliases))
    3. SHOULD support additional filter operations ([section 11.2.6.1.1](#BuiltinFilterOperations))
-and MUST return `501 Not Implemented` for any unsupported filter
-operations ([section 9.3.1](#ResponseCode501NotImplemented))
+and MUST fail the request for any unsupported filter
+operations
    4. SHOULD support the canonical functions ([section 11.2.6.1.2](#BuiltinQueryFunctions)) and
-MUST return `501 Not Implemented` for any unsupported canonical
-functions ([section 9.3.1](#ResponseCode501NotImplemented))
+MUST fail the request for any unsupported canonical functions
    5. SHOULD support `$filter` on expanded entities ([section 11.2.5.2.1](#ExpandOptions))
 8. SHOULD publish metadata at `$metadata` according to
 [OData-CSDLXML](#ODataCSDL) ([section 11.1.2](#MetadataDocumentRequest))
@@ -6789,9 +6786,7 @@ specified in `$metadata`
 the requested format
 21. SHOULD use capabilities (see [OData-VocCap](#ODataVocCap)) to
 determine if a 4.01 feature is supported but MAY attempt syntax and be
-prepared to handle either
-[`501 Not Implemented`](#ResponseCode501NotImplemented) or
-`400 Bad Request`
+prepared to handle `400 Bad Request` or [`501 Not Implemented`](#ResponseCode501NotImplemented)
 
 
 -------
diff --git a/odata-protocol/1 Introduction.md b/odata-protocol/1 Introduction.md
index f5df1f453..65dbb0113 100644
--- a/odata-protocol/1 Introduction.md	
+++ b/odata-protocol/1 Introduction.md	
@@ -35,6 +35,7 @@ Response code `204 No Content` after successful data modification if requested r
 [Section ##UpdateaComplexProperty]| 
 Setting a complex property to a different type| 
 [534](https://github.com/oasis-tcs/odata-specs/issues/534)
+[Section ##Conformance] | Allow `400 Bad Request` in addition to `501 Not Implemented` for unsupported functionality| [391](https://github.com/oasis-tcs/odata-specs/issues/391)
 [Section ##InteroperableODataClients] | Encoding of plus character in URLs | [485](https://github.com/oasis-tcs/odata-specs/issues/485)
 
 ## ##subsec Glossary
diff --git a/odata-protocol/12 Conformance.md b/odata-protocol/12 Conformance.md
index 2e92b71bc..37243ccbf 100644
--- a/odata-protocol/12 Conformance.md	
+++ b/odata-protocol/12 Conformance.md	
@@ -59,9 +59,7 @@ request
 ([section ##Extensibility] and all subsections)
 7. MUST successfully parse the request according to
 [OData-ABNF](#ODataABNF) for any supported system query options and
-either follow the specification or return
-`501 Not Implemented` for any
-unsupported functionality ([section ##ResponseCode501NotImplemented])
+follow the specification or fail the request
 8. MUST expose only data types defined in [OData-CSDLXML](#ODataCSDL)
 9. MUST NOT require clients to understand any metadata or instance
 annotations ([section ##VocabularyExtensibility]), custom headers ([section ##HeaderFieldExtensibility]), or custom
@@ -127,9 +125,8 @@ service:
 
 1. MUST conform to the [OData 4.0 Minimal Conformance
 Level](#OData40MinimalConformanceLevel)
-2. MUST successfully parse the [OData-ABNF](#ODataABNF) and either
-follow the specification or return `501 Not Implemented` for any
-unsupported functionality ([section ##ResponseCode501NotImplemented])
+2. MUST successfully parse the request according to [OData-ABNF](#ODataABNF) and
+follow the specification or fail the request
 3. MUST support `$select` ([section ##SystemQueryOptionselect])
 4. MUST support casting to a derived type according to
 [OData-URL](#ODataURL) if derived types are present in the model
@@ -140,11 +137,10 @@ unsupported functionality ([section ##ResponseCode501NotImplemented])
 in the requested entity set ([section ##BuiltinFilterOperations])
    2. MUST support aliases in `$filter` expressions ([section ##ParameterAliases])
    3. SHOULD support additional filter operations ([section ##BuiltinFilterOperations])
-and MUST return `501 Not Implemented` for any unsupported filter
-operations ([section ##ResponseCode501NotImplemented])
+and MUST fail the request for any unsupported filter
+operations
    4. SHOULD support the canonical functions ([section ##BuiltinQueryFunctions]) and
-MUST return `501 Not Implemented` for any unsupported canonical
-functions ([section ##ResponseCode501NotImplemented])
+MUST fail the request for any unsupported canonical functions
    5. SHOULD support `$filter` on expanded entities ([section ##ExpandOptions])
 8. SHOULD publish metadata at `$metadata` according to
 [OData-CSDLXML](#ODataCSDL) ([section ##MetadataDocumentRequest])
@@ -411,6 +407,4 @@ specified in `$metadata`
 the requested format
 21. SHOULD use capabilities (see [OData-VocCap](#ODataVocCap)) to
 determine if a 4.01 feature is supported but MAY attempt syntax and be
-prepared to handle either
-[`501 Not Implemented`](#ResponseCode501NotImplemented) or
-`400 Bad Request`
+prepared to handle `400 Bad Request` or [`501 Not Implemented`](#ResponseCode501NotImplemented)
diff --git a/odata-protocol/8 Header Fields.md b/odata-protocol/8 Header Fields.md
index fcabed4a8..4a700e9ab 100644
--- a/odata-protocol/8 Header Fields.md	
+++ b/odata-protocol/8 Header Fields.md	
@@ -1032,7 +1032,7 @@ indicate service errors.
 ### ##subsubsec Response Code `501 Not Implemented`
 
 If the client requests functionality not implemented by the OData
-Service, the service responds with `501 Not Implemented` and SHOULD
+Service, the service MAY respond with `501 Not Implemented` and
 include a response body describing the functionality not implemented.
 
 ## ##subsec Error Response Body