feat: Add --what-if in MCP server tools (#1697)

Author: Gijsreyn

CHANGELOG.md

@@ -75,6 +75,15 @@ changes since the last release, see the [diff on GitHub][unreleased].
     for the `v3.0.0.0-alpha.5` release. Leave the release links under the release section.
 -->
 
+### Added
+
+- Added what-if support to the MCP server tools. The `invoke_dsc_config` tool now accepts a
+  `what_if` option for the `set` operation, and the `invoke_dsc_resource` tool accepts `what_if`
+  for the `set` and `delete` operations. This mirrors the `--what-if` flag on the `dsc config set`,
+  `dsc resource set`, and `dsc resource delete` commands, enabling AI agents to preview changes
+  before applying them. Passing `what_if` with an operation that doesn't support it returns an
+  invalid parameters error.
+
 ## [v3.2.2][release-v3.2.2] - 2026-06-16
 
 This section includes a summary of changes for the `3.2.2` release. For the full list of changes

docs/concepts/dsc-mcp-server.md

@@ -20,10 +20,12 @@ This enhances the overall authoring experience by providing contextual informati
 local DSC environment directly to AI-powered tools.
 
 > [!IMPORTANT]
-> The DSC MCP server is focused on discovery and information retrieval. It does not
-> directly perform any configuration changes or resource modifications unless requested to do so.
-> The information it provides to AI agents can be used to generate configurations and commands
-> that, when executed, will impact your system. Always review and validate any generated
+> The DSC MCP server is primarily focused on discovery and information retrieval. It only
+> performs configuration changes or resource modifications when an agent explicitly invokes the
+> `invoke_dsc_config` or `invoke_dsc_resource` tools with the `set` or `delete` operation. Those
+> tools support a `what_if` option so agents can preview a change before applying it. The
+> information the server provides to AI agents can be used to generate configurations and
+> commands that, when executed, will impact your system. Always review and validate any generated
 > content before execution.
 
 ## What is Model Context Protocol (MCP)?
@@ -74,6 +76,24 @@ to help solve your specific needs:
 
 This helps agents suggest the most suitable approach using your available DSC capabilities.
 
+### Previewing changes with what-if
+
+Before an agent applies a configuration or resource change, it can simulate the change to show
+you what would happen without modifying your system. The `invoke_dsc_config` tool accepts a
+`what_if` option for the `set` operation, and the `invoke_dsc_resource` tool accepts `what_if`
+for the `set` and `delete` operations. This is the same behavior as the `--what-if` flag on the
+`dsc config set`, `dsc resource set`, and `dsc resource delete` commands:
+
+- **You ask**: "Show me what would change if I applied this configuration"
+- **Agent invokes**: `invoke_dsc_config` with `operation: set` and `what_if: true`
+- **Agent provides**: The projected before and after state for each resource, with the result
+  metadata reporting `executionType` as `whatIf`
+
+Resources that natively support what-if run their simulation directly. For resources that don't,
+DSC generates a synthetic what-if result from the resource's `test` operation. When `what_if` is
+requested with an operation that doesn't support it, such as `get`, the tool returns an error
+instead of silently ignoring the option.
+
 > [!NOTE]
 > Additional MCP tools will become available in future releases to expand the capabilities
 > of the DSC MCP server integration. For the latest updates and feature announcements,
@@ -194,6 +214,7 @@ Example prompts that work well with DSC MCP integration:
 - "What DSC resources are available on this machine?"
 - "Show me the schema for the Microsoft.Windows/Registry resource"
 - "List all available DSC functions I can use in expressions"
+- "Preview what this configuration would change before applying it"
 
 :::image type="complex" source="media/dsc-mcp-server/dsc-mcp-usage-example.png" alt-text="Screenshot showing DSC MCP usage example in VS Code":::
    This screenshot demonstrates the DSC MCP integration in action, showing how AI agents use the MCP tools to provide contextual assistance with DSC-related tasks in VS Code.

dsc/locales/en-us.toml

@@ -102,6 +102,7 @@ invalidParameters = "Invalid parameters"
 failedConvertJson = "Failed to convert to JSON"
 failedSerialize = "Failed to serialize configuration"
 failedSetParameters = "Failed to set parameters"
+whatIfOnlySet = "what_if is only supported for the 'set' operation"
 
 [server.invoke_dsc_expression]
 parserInitializationFailed = "Failed to initialize parser: %{error}"
@@ -113,6 +114,7 @@ functionInvocationFailed = "Function '%{function}' invocation failed: %{error}"
 
 [server.invoke_dsc_resource]
 resourceNotFound = "Resource type '%{resource}' does not exist"
+whatIfNotSupported = "what_if is only supported for the 'set' and 'delete' operations"
 
 [server.list_dsc_functions]
 invalidNamePattern = "Invalid function name pattern '%{pattern}'"

dsc/src/server/invoke_dsc_config.rs

@@ -4,7 +4,7 @@
 use crate::server::mcp_server::McpServer;
 use dsc_lib::{
     configure::{
-        config_doc::Configuration,
+        config_doc::{Configuration, ExecutionKind},
         config_result::{
             ConfigurationExportResult, ConfigurationGetResult, ConfigurationSetResult,
             ConfigurationTestResult,
@@ -52,14 +52,19 @@ pub struct InvokeDscConfigRequest {
         description = "Optional parameters to pass to the configuration as a YAML string"
     )]
     pub parameters: Option<String>,
+    #[schemars(
+        description = "When true and operation is 'set', simulate the change (what-if / dry-run) instead of applying it. The result includes 'metadata.Microsoft.DSC.executionType' = 'whatIf'. Only valid with the 'set' operation."
+    )]
+    #[serde(default)]
+    pub what_if: Option<bool>,
 }
 
 #[tool_router(router = invoke_dsc_config_router, vis = "pub")]
 impl McpServer {
     #[tool(
-        description = "Invoke a DSC configuration operation (Get, Set, Test, Export) with optional parameters",
+        description = "Invoke a DSC configuration operation (Get, Set, Test, Export) with optional parameters. Set 'what_if' to true to preview a Set without applying changes.",
         annotations(
-            title = "Invoke a DSC configuration operation (Get, Set, Test, Export) with optional parameters",
+            title = "Invoke a DSC configuration operation (Get, Set, Test, Export) with optional parameters and what-if support",
             read_only_hint = false,
             destructive_hint = true,
             idempotent_hint = true,
@@ -72,6 +77,7 @@ impl McpServer {
             operation,
             configuration,
             parameters,
+            what_if,
         }): Parameters<InvokeDscConfigRequest>,
     ) -> Result<Json<InvokeDscConfigResponse>, McpError> {
         let result = task::spawn_blocking(move || {
@@ -127,6 +133,16 @@ impl McpServer {
 
             configurator.context.dsc_version = Some(env!("CARGO_PKG_VERSION").to_string());
 
+            if what_if.unwrap_or(false) {
+                if !matches!(operation, ConfigOperation::Set) {
+                    return Err(McpError::invalid_params(
+                        t!("server.invoke_dsc_config.whatIfOnlySet"),
+                        None,
+                    ));
+                }
+                configurator.context.execution_type = ExecutionKind::WhatIf;
+            }
+
             let parameters_value: Option<serde_json::Value> = if let Some(params_str) = parameters {
                 let params_json = match serde_yaml::from_str::<serde_yaml::Value>(&params_str) {
                     Ok(yaml) => match serde_json::to_value(yaml) {

dsc/src/server/invoke_dsc_resource.rs

@@ -8,6 +8,8 @@ use dsc_lib::{
     dscresources::{
         dscresource::Invoke,
         invoke_result::{
+            DeleteResult,
+            DeleteResultKind,
             ExportResult,
             GetResult,
             SetResult,
@@ -39,6 +41,7 @@ pub enum ResourceOperationResult {
     TestResult(TestResult),
     ExportResult(ExportResult),
     DeleteResult { success: bool },
+    DeleteWhatIfResult(DeleteResult),
 }
 
 #[derive(Serialize, JsonSchema)]
@@ -54,22 +57,33 @@ pub struct InvokeDscResourceRequest {
     pub resource_type: FullyQualifiedTypeName,
     #[schemars(description = "The properties to pass to the DSC resource as JSON.  Must match the resource JSON schema from `show_dsc_resource` tool.")]
     pub properties_json: String,
+    #[schemars(description = "When true and operation is 'set' or 'delete', simulate the change (what-if / dry-run) instead of applying it. Resources without native what-if support return a synthetic result derived from 'test'. Only valid with the 'set' and 'delete' operations.")]
+    #[serde(default)]
+    pub what_if: Option<bool>,
 }
 
 #[tool_router(router = invoke_dsc_resource_router, vis = "pub")]
 impl McpServer {
     #[tool(
-        description = "Invoke a DSC resource operation (Get, Set, Test, Export, Delete) with specified properties in JSON format",
+        description = "Invoke a DSC resource operation (Get, Set, Test, Export, Delete) with specified properties in JSON format. Set 'what_if' to true to preview a Set or Delete without applying changes.",
         annotations(
-            title = "Invoke a DSC resource operation (Get, Set, Test, Export, Delete) with specified properties in JSON format",
+            title = "Invoke a DSC resource operation (Get, Set, Test, Export, Delete) with specified properties in JSON format and what-if support",
             read_only_hint = false,
             destructive_hint = true,
             idempotent_hint = true,
             open_world_hint = true,
         )
     )]
-    pub async fn invoke_dsc_resource(&self, Parameters(InvokeDscResourceRequest { operation, resource_type, properties_json }): Parameters<InvokeDscResourceRequest>) -> Result<Json<InvokeDscResourceResponse>, McpError> {
+    pub async fn invoke_dsc_resource(&self, Parameters(InvokeDscResourceRequest { operation, resource_type, properties_json, what_if }): Parameters<InvokeDscResourceRequest>) -> Result<Json<InvokeDscResourceResponse>, McpError> {
         let result = task::spawn_blocking(move || {
+            let execution_kind = if what_if.unwrap_or(false) {
+                if !matches!(operation, DscOperation::Set | DscOperation::Delete) {
+                    return Err(McpError::invalid_params(t!("server.invoke_dsc_resource.whatIfNotSupported"), None));
+                }
+                ExecutionKind::WhatIf
+            } else {
+                ExecutionKind::Actual
+            };
             let mut dsc = DscManager::new();
             let Some(resource) = dsc.find_resource(&DiscoveryFilter::new(&resource_type, None, None)).unwrap_or(None) else {
                 return Err(McpError::invalid_request(t!("server.invoke_dsc_resource.resourceNotFound", resource = resource_type), None));
@@ -83,7 +97,7 @@ impl McpServer {
                     Ok(ResourceOperationResult::GetResult(result))
                 },
                 DscOperation::Set => {
-                    let result = match resource.set(&properties_json, false, &ExecutionKind::Actual) {
+                    let result = match resource.set(&properties_json, false, &execution_kind) {
                         Ok(res) => res,
                         Err(e) => return Err(McpError::internal_error(e.to_string(), None)),
                     };
@@ -97,8 +111,10 @@ impl McpServer {
                     Ok(ResourceOperationResult::TestResult(result))
                 },
                 DscOperation::Delete => {
-                    match resource.delete(&properties_json, &ExecutionKind::Actual) {
-                        Ok(_) => Ok(ResourceOperationResult::DeleteResult { success: true }),
+                    match resource.delete(&properties_json, &execution_kind) {
+                        Ok(DeleteResultKind::ResourceActual) => Ok(ResourceOperationResult::DeleteResult { success: true }),
+                        Ok(DeleteResultKind::ResourceWhatIf(delete_result)) => Ok(ResourceOperationResult::DeleteWhatIfResult(delete_result)),
+                        Ok(DeleteResultKind::SyntheticWhatIf(test_result)) => Ok(ResourceOperationResult::TestResult(test_result)),
                         Err(e) => Err(McpError::internal_error(e.to_string(), None)),
                     }
                 },