diff --git a/src/main/LLMExtension.scala b/src/main/LLMExtension.scala
index 54f2cd5..d158727 100644
--- a/src/main/LLMExtension.scala
+++ b/src/main/LLMExtension.scala
@@ -594,6 +594,10 @@ class LLMExtension extends DefaultClassManager {
   }
   
   object ChooseReporter extends Reporter {
+    /** Final line of the choose prompt; names the shape the enum schema enforces. */
+    val ChoiceInstruction: String =
+      s"""Your choice as {"${EnumFormat.ChoiceKey}": "<option>"}:"""
+
     override def getSyntax: Syntax = Syntax.reporterSyntax(
       right = List(Syntax.StringType, Syntax.ListType),
       ret = Syntax.StringType
@@ -613,14 +617,18 @@ class LLMExtension extends DefaultClassManager {
           throw new ExtensionException("Choice list cannot be empty")
         }
 
+        // The wording must describe the same shape as the EnumFormat schema
+        // sent with the request. A provider that validates the reply against
+        // the schema server-side (Groq does) rejects the whole response with
+        // HTTP 400 when the model follows a prompt asking for bare text.
         val systemPrompt = "You are a decision-making assistant. " +
-          "When given options, respond with EXACTLY one option from the list. " +
-          "Rules: Reply with ONLY the option text. " +
-          "No explanation, no numbering, no punctuation, no quotes, no extra words. " +
-          "Copy the option exactly as written."
+          "When given options, choose EXACTLY one option from the list. " +
+          s"""Reply with a JSON object of the form {"${EnumFormat.ChoiceKey}": "<option>"} """ +
+          "where <option> is copied exactly as written. " +
+          "No explanation, no extra keys, no extra text."
 
         val userPrompt = s"$prompt\n\nOptions:\n${choices.mkString("\n")}\n\n" +
-          "Your choice (one option, no other text):"
+          ChooseReporter.ChoiceInstruction
 
         // Build temp history with system prompt — don't mutate permanent history
         val tempHistory = ArrayBuffer.from(snapshotHistory(agent))
diff --git a/src/test/DeterministicTestProvider.scala b/src/test/DeterministicTestProvider.scala
index cca0ecb..cb118e5 100644
--- a/src/test/DeterministicTestProvider.scala
+++ b/src/test/DeterministicTestProvider.scala
@@ -82,6 +82,12 @@ class DeterministicTestProvider(implicit ec: ExecutionContext) extends LLMProvid
     // primitive that stopped constraining its request fails a test instead of
     // silently degrading. The enum echo answers with the LAST choice because
     // the unconstrained prompt-only path answers with the first.
+    // __TEST_ECHO_PROMPT_TAIL answers with the prompt's final line so a test can
+    // pin the instruction the model actually sees next to the schema it is sent.
+    if (lastUserMessage.contains("__TEST_ECHO_PROMPT_TAIL")) {
+      return Future.successful(structured(lastUserMessage.linesIterator.toSeq.last))
+    }
+
     if (lastUserMessage.contains("__TEST_ECHO_FORMAT_ENUM")) {
       return Future.successful(structured(format match {
         case EnumFormat(choices) => choices.last
@@ -162,7 +168,7 @@ class DeterministicTestProvider(implicit ec: ExecutionContext) extends LLMProvid
     } else if (lastUserMessage.contains("__TEST_EMPTY_CONTENT")) {
       // Return empty content (simulates thinking model with no content)
       ""
-    } else if (lastUserMessage.contains("Your choice (one option, no other text):")) {
+    } else if (lastUserMessage.contains("Options:\n") && lastUserMessage.contains("Your choice as {")) {
       // Choose prompt — extract and return the first option from the Options block
       val optionsIdx = lastUserMessage.indexOf("Options:\n")
       if (optionsIdx >= 0) {
diff --git a/tests.txt b/tests.txt
index b2ebac2..b4a1503 100644
--- a/tests.txt
+++ b/tests.txt
@@ -685,6 +685,15 @@ LLMChatWithSchemaSendsSchemaFormat
   llm:get result "name" => "netlogo_schema"
   O> llm:clear-history
 
+LLMChoosePromptMatchesEnumSchema
+  extensions [llm]
+  O> llm:set-api-key "test-key"
+  O> llm:set-provider "openai"
+  O> llm:clear-history
+  llm:choose "__TEST_ECHO_PROMPT_TAIL" ["north" "south"] => ERROR Extension exception: llm:choose: response 'Your choice as {"choice": "<option>"}:' did not match any choice. Choices: north, south
+  llm:history => []
+  O> llm:clear-history
+
 LLMChooseDegradesWhenFormatIgnored
   extensions [llm]
   globals [choice]