Fix reusable prompt handling

Author: mkht

Public/Responses/Request-Response.ps1

@@ -395,7 +395,27 @@ function Request-Response {
         #region Construct parameters for API request
         $Response = $null
         $PostBody = [System.Collections.Specialized.OrderedDictionary]::new()
-        $PostBody.model = $Model
+
+        # Reusable prompts
+        $IsReusablePromptSpecified = $false
+        if ($PromptId) {
+            $PromptOptions = @{id = $PromptId }
+            if ($PSBoundParameters.ContainsKey('PromptVariables')) {
+                $PromptOptions.variables = $PromptVariables
+            }
+            if ($PSBoundParameters.ContainsKey('PromptVersion')) {
+                $PromptOptions.version = $PromptVersion
+            }
+            $PostBody.prompt = $PromptOptions
+
+            ## When specifying a reusable prompt, normally required parameters such as model and input become optional.
+            $IsReusablePromptSpecified = $true
+        }
+
+        # Specify model
+        if (-not $IsReusablePromptSpecified -or $PSBoundParameters.ContainsKey('Model')) {
+            $PostBody.model = $Model
+        }
 
         if ($PreviousResponseId) {
             $PostBody.previous_response_id = $PreviousResponseId
@@ -462,18 +482,6 @@ function Request-Response {
             $PostBody.stream = [bool]$Stream
         }
 
-        # Reusable prompts
-        if ($PromptId) {
-            $PromptOptions = @{id = $PromptId }
-            if ($PSBoundParameters.ContainsKey('PromptVariables')) {
-                $PromptOptions.variables = $PromptVariables
-            }
-            if ($PSBoundParameters.ContainsKey('PromptVersion')) {
-                $PromptOptions.version = $PromptVersion
-            }
-            $PostBody.prompt = $PromptOptions
-        }
-
         # Reasoning
         $ReasoningOptions = @{}
         if ($PSBoundParameters.ContainsKey('ReasoningEffort')) {
@@ -870,7 +878,7 @@ function Request-Response {
         #endregion
 
         # Error if message is empty.
-        if ($Messages.Count -eq 0) {
+        if (-not $IsReusablePromptSpecified -and $Messages.Count -eq 0) {
             Write-Error 'No message is specified. You must specify one or more messages.'
             return
         }

Tests/Responses/Request-Response.tests.ps1

@@ -606,6 +606,27 @@ Describe 'Request-Response' {
             $Result.body.model | Should -Be 'gpt-4o-mini'
             $Result.body.input[0].content[0].text | Should -Be 'Hello!'
         }
+
+        It 'Input Message is required' {
+            Mock -Verifiable -ModuleName $script:ModuleName Invoke-OpenAIAPIRequest {}
+            { Request-Response -Model 'gpt-4o-mini' -ea Stop } | Should -Throw 'No message is specified. You must specify one or more messages.'
+            Should -Not -InvokeVerifiable
+        }
+
+        It 'When specifying a reusable prompt, input message is not required' {
+            Mock -Verifiable -ModuleName $script:ModuleName Invoke-OpenAIAPIRequest { @'
+{
+  "id": "resp_7e5a5",
+  "object": "response",
+  "created_at": 1743938395,
+  "status": "completed",
+  "model": "gpt-4o-mini-2024-07-18"
+}
+'@
+            }
+            { Request-Response -PromptId 'pmpt_123' -PromptVersion 4 -PromptVariables @{'city' = 'Tokyo' } -ea Stop } | Should -Not -Throw
+            Should -InvokeVerifiable
+        }
     }
 
     Context 'Integration tests (online)' -Tag 'Online' {