Why isn't my transform applying?

Check the operation log — every transform element produces an entry even when nothing matched, explicitly saying 'No match found using locator: ...'. The most common cause is a Match/Condition locator referencing an attribute name or value that doesn't exist on any sibling in the base file, or the transform element's name not matching any element at that point in the base document's structure.

What's the difference between Match and Condition?

Match(attr1,attr2) finds sibling elements with the same name where the listed attribute values are equal to the transform element's own values for those attributes — the common case for 'find the with this name'. Condition(xpath) instead evaluates an arbitrary XPath predicate against each same-named sibling, letting you match on things Match can't express, like a value comparison or checking for the absence of an attribute.

Does this work for appsettings.json?

No — XDT transforms are an XML-only mechanism for web.config (and any other .config file using the same syntax). For appsettings.json's Development/Production split, use the appsettings.json Environment Diff tool to find missing keys between environments, or the appsettings.json ↔ Environment Variables Converter to generate the environment variables ASP.NET Core reads instead of a second JSON file.

Is my web.config content sent to a server?

No — both files are processed in-memory on the same Blazor Server connection that renders this page, held only for your active session, and discarded when the session ends. Web.config files sometimes contain connection strings or other secrets — avoid pasting real production values into any online tool, including this one.

Why does this tool refuse to guess at unsupported transforms?

A wrong guess about what an unfamiliar xdt:Transform value does is worse than an honest 'not supported' — it would produce a merged config that looks plausible but is actually incorrect, and you'd have no reason to double-check it. This tool only implements the transforms and locators it can apply with full confidence; anything else is flagged so you know to verify that part by hand or with the actual Microsoft.Web.Xdt-based build tooling.

web.config XDT Transform Tester

Name: web.config Transform Tester – XDT Transform Preview | DevToolsHub
Author: Pankaj Kumar

Paste a base web.config and a transform file to see the merged result and a per-operation log — covers Replace, Insert, InsertBefore/After, Remove, RemoveAll, RemoveAttributes, and SetAttributes with Match/Condition/XPath locators.

By Pankaj Kumar · DevToolsHub · Last updated Aug 2026

What this tool does

Paste your base web.config and a transform file like web.Release.config, and this tool applies the XDT (XML Document Transform) rules and shows you exactly what changed — the merged result plus a per-operation log naming which transform touched which element and what it did. Everything runs in your Blazor Server session; nothing is sent to an external API.

Supported vs. unsupported — read this before trusting the output

This tool implements XDT with System.Xml.Linq and explicit rules, not Microsoft's own Microsoft.Web.Xdt library. It covers the transforms and locators actually documented and commonly used in web.config transforms — anything outside that list is reported explicitly as unsupported, not applied, never guessed at or silently skipped.

Category	Supported	Not supported
Transform	Replace, Insert, InsertBefore, InsertAfter, Remove, RemoveAll, RemoveAttributes, SetAttributes	Anything else (e.g. custom XDT extensions)
Locator	Match(attributes), Condition(xpath), XPath(xpath)	Custom locator implementations

Worked example

Base web.config:

<configuration>
  <connectionStrings>
    <add name="MyDB" connectionString="dev-server" />
  </connectionStrings>
</configuration>

Transform:

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <connectionStrings>
    <add name="MyDB" connectionString="prod-server"
         xdt:Transform="SetAttributes(connectionString)" xdt:Locator="Match(name)" />
  </connectionStrings>
</configuration>

produces a result with connectionString="prod-server", and an operation log entry showing SetAttributes matched the MyDB node and set connectionString.

How to use this tool

Paste your base web.config into the first editor.
Paste your transform file (web.Release.config, web.Debug.config, etc.) into the second editor.
Click Test Transform (or Ctrl+Enter).
Check the unsupported-constructs warning first, then review the operation log and the merged result.

Use this to test web.Release.config before your next deployment, or as a general web config transform online sanity check any time you're editing XDT by hand — the operation log catches a locator that silently matches nothing, which is the failure mode a plain text diff can't show you.

This tool is built with ASP.NET Core 8, Blazor Server, and a hand-written XDT transform engine using System.Xml.Linq (no Microsoft.Web.Xdt dependency). It runs securely on Microsoft Azure.

This tool requires JavaScript to function. Enable JavaScript to use the web.config XDT Transform Tester.

Your input stays private. All processing happens in-memory on the server and is never stored, logged, or associated with your identity. Sensitive data — tokens, signing keys, and passwords — is safe to use here.

Input Section

Base web.config

web.config

<configuration>
  <connectionStrings>
    <add name="MyDB" connectionString="Server=dev;Database=app;" />
  </connectionStrings>
  <appSettings>
    <add key="Debug" value="true" />
  </appSettings>
  <system.web>
    <compilation debug="true" />
  </system.web>
</configuration>

Transform file (e.g. web.Release.config)

Transform XML

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <connectionStrings>
    <add name="MyDB" connectionString="Server=prod;Database=app;"
         xdt:Transform="SetAttributes(connectionString)" xdt:Locator="Match(name)" />
  </connectionStrings>
  <appSettings>
    <add key="Debug" xdt:Transform="Remove" xdt:Locator="Match(key)" />
  </appSettings>
  <system.web>
    <compilation xdt:Transform="RemoveAttributes(debug)" />
  </system.web>
</configuration>

Output Section

3 applied

Operation log

Category	Transform	Element	Detail
Changed	SetAttributes	<add>	Set attribute(s) [connectionString] on <add>.
Removed	Remove	<add>	Removed <add> matched by Match(key).
Changed	RemoveAttributes	<compilation>	Removed attribute(s) [debug] from <compilation>.

Resulting web.config

Merged output

<configuration>
  <connectionStrings>
    <add name="MyDB" connectionString="Server=prod;Database=app;" />
  </connectionStrings>
  <appSettings />
  <system.web>
    <compilation />
  </system.web>
</configuration>

web.config XDT Transform Tester

What this tool does

Supported vs. unsupported — read this before trusting the output

Worked example

How to use this tool

Feedback or suggestions?

Find the tools useful?

web.config XDT Transform Tester

What this tool does

Supported vs. unsupported — read this before trusting the output

Worked example

How to use this tool

FAQ

Related Tools

Feedback or suggestions?

Find the tools useful?