I've figured out how to update a Task's basic fields via the OData Rest service using POST (X-HTTP-Method=MERGE):


The post content is a JSON object with each standard field's internal name mapped to a value.

But how do I update custom fields? Attempts to write to their internal name in MSPS 2016 are ignored.

How to write custom fields on projects can be found here. But I'm having trouble finding similar REST API for Task custom fields.

  • I am working on very urgent use case to realize for my customer where I have deployed MS SharePoint 2016 and enabled MS Project Server 2016 on it. Created a few projects and created a few tasks, issues in them. Now, I want to achieve updating a few standard and custom fields but getting 403. ![enter image description here](i.sstatic.net/9kRHX.png) I want to update fields like, Percentage complete, description etc. Can someone please help me urgently.
    – Yuvam Jain
    Commented May 28, 2020 at 16:19

1 Answer 1


This is possible but as far as I can see not documented anywhere by Microsoft. Through some Fiddler tracing of the JSOM API I figured out the following way to do this using the ProcessQuery internal SharePoint endpoint. (That's the one that CSOM/JSOM use internally)

    POST /sites/pwa/_vti_bin/client.svc/ProcessQuery HTTP/1.1
    Host: something.sharepoint.com
    Accept: application/json; odata=verbose
    Content-Type: text/xml
    X-RequestDigest: ......
    Cache-Control: no-cache

    <?xml version="1.0" ?>
    <Request ApplicationName="Javascript Library" LibraryVersion="" SchemaVersion="" xmlns="http://schemas.microsoft.com/sharepoint/clientquery/2009">
        <ObjectPath Id="1" ObjectPathId="0"/>
        <ObjectPath Id="3" ObjectPathId="2"/>
        <ObjectPath Id="5" ObjectPathId="4"/>
        <ObjectPath Id="7" ObjectPathId="6"/>
        <ObjectPath Id="9" ObjectPathId="8"/>
        <ObjectPath Id="11" ObjectPathId="10"/>
        <ObjectPath Id="13" ObjectPathId="12"/>
        <Method Id="14" Name="SetCustomFieldValue" ObjectPathId="12">
                <Parameter Type="String">Custom_976b57c4b836e51180cd000d3a201c32</Parameter>
                <Parameter Type="String">Hi</Parameter>
        <ObjectPath Id="16" ObjectPathId="15"/>
        <Query Id="17" ObjectPathId="15">
            <Query SelectAllProperties="true">
        <Constructor Id="0" TypeId="{3a609e5f-e2a1-497c-87a2-e5e8a781c096}"/>
        <Constructor Id="2" TypeId="{f256f605-bb33-4d62-b206-72a06a765075}">
                <Parameter ObjectPathId="0"/>
        <Property Id="4" Name="Projects" ParentId="0"/>
        <Method Id="6" Name="GetByGuid" ParentId="4">
                <Parameter Type="String">36ead592-ee70-e611-8162-000d3a2402b4</Parameter>
        <Property Id="8" Name="Draft" ParentId="6"/>
        <Property Id="10" Name="Tasks" ParentId="8"/>
        <Method Id="12" Name="GetByGuid" ParentId="10">
                <Parameter Type="String">150aca09-ef70-e611-8162-000d3a2402b4</Parameter>
        <Method Id="15" Name="Update" ParentId="8"/>

The above example updates a task custom field after checking out the project, it was specifically 'discovered' using the 2013 JSOM object model, but from recent experience the syntax has simplified in the current version.

If you want to see read how I found this, and how to determine the syntax for other undocumented endpoints, I blogged about this here: nearbaseline.com

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