diff --git a/src/adapters/openai-chat/messages.ts b/src/adapters/openai-chat/messages.ts
index 8e88dabc6b0..6226e4bed2d 100644
--- a/src/adapters/openai-chat/messages.ts
+++ b/src/adapters/openai-chat/messages.ts
@@ -123,20 +123,15 @@ export function messagesToChatFormat(parsed: OcxParsedRequest, provider: OcxProv
   const nativeOpenAI = isNativeOpenAIChatTarget(provider);
   // Hoisting a newly appended reminder rewrites the reusable prompt prefix.
   // Keep this compatibility exception on the destination/model tested with OCG.
-  const chronologicalSystem = parsed.modelId === "deepseek-v4.1-flash"
-    && registryEntryForProviderDestination(provider)?.id === "opencode-go";
-  const toolCatalogNudge = shouldInjectNonOpenAIToolCatalogNudge(provider)
-    ? buildNonOpenAIToolCatalogNudgeForTools(context.tools, options.toolChoice)
-    : undefined;
-  const developerSystemParts = nativeOpenAI || chronologicalSystem
-    ? []
-    : context.messages
-      .map(developerSystemText)
-      .filter((part): part is string => part !== undefined && part.length > 0);
-  const systemParts = [
-    ...(context.systemPrompt ?? []),
-    ...developerSystemParts,
-    ...(toolCatalogNudge ? [toolCatalogNudge] : []),
+    // Preserving chronological order on the Chat Completions wire prevents in-conversation
+    // instructions from being moved out of the timeline into the prompt preamble (#5213).
+    const toolCatalogNudge = shouldInjectNonOpenAIToolCatalogNudge(provider)
+      ? buildNonOpenAIToolCatalogNudgeForTools(context.tools, options.toolChoice)
+      : undefined;
+    const systemParts = [
+      ...(context.systemPrompt ?? []),
+      ...(toolCatalogNudge ? [toolCatalogNudge] : []),
+    ];
   ];
   if (systemParts.length > 0) {
     const wireModelId = provider.modelSuffixBracketStrip
@@ -154,7 +149,6 @@ export function messagesToChatFormat(parsed: OcxParsedRequest, provider: OcxProv
         const hasImages = parts?.some(p => p.type === "image") ?? false;
         let chatMsg: Record<string, unknown>;
         if (msg.role === "developer" && !hasImages) {
-          if (!nativeOpenAI && !chronologicalSystem) break;
           const text = typeof msg.content === "string"
             ? msg.content
             : parts!.map(p => (p as OcxTextContent).text).join("");
@@ -344,3 +338,4 @@ export function safeToolName(name: string | undefined): string {
 export function emptyAssistantContent(provider: OcxProviderConfig): string | { type: "text"; text: string }[] {
   return isVolcengineArkPaygChatTarget(provider) ? [{ type: "text", text: "" }] : "";
 }
+
diff --git a/tests/adapters/openai/openai-chat-system-order.test.ts b/tests/adapters/openai/openai-chat-system-order.test.ts
index def3179d814..d96e27f0874 100644
--- a/tests/adapters/openai/openai-chat-system-order.test.ts
+++ b/tests/adapters/openai/openai-chat-system-order.test.ts
@@ -21,7 +21,7 @@ function buildMessages(context: OcxParsedRequest["context"]): Array<Record<strin
 }
 
 describe("openai-chat system message ordering", () => {
-  test("folds interleaved developer reminders into one leading system message", () => {
+  test("preserves developer reminder position as chronological system message on non-native targets", () => {
     const messages = buildMessages({
       systemPrompt: ["base instructions"],
       messages: [
@@ -44,13 +44,14 @@ describe("openai-chat system message ordering", () => {
 
     expect(messages[0]).toEqual({
       role: "system",
-      content: "base instructions\n\nfirst reminder\n\nsecond reminder",
+      content: "base instructions",
     });
-    expect(messages.slice(1).map(message => message.role)).toEqual(["user", "assistant", "user"]);
-    expect(messages.slice(1).some(message => message.role === "system")).toBe(false);
+    expect(messages.map(message => message.role)).toEqual(["system", "user", "system", "assistant", "system", "user"]);
+    expect(messages[2]).toEqual({ role: "system", content: "first reminder" });
+    expect(messages[4]).toEqual({ role: "system", content: "second reminder" });
   });
 
-  test("keeps tool calls and results adjacent when a developer reminder follows the call", () => {
+  test("preserves developer reminders in chronological order around tool calls on non-native targets", () => {
     const messages = buildMessages({
       messages: [
         { role: "user", content: "inspect", timestamp: 0 },
@@ -72,8 +73,8 @@ describe("openai-chat system message ordering", () => {
       ],
     });
 
-    expect(messages[0]).toEqual({ role: "system", content: "remember the policy" });
-    expect(messages.map(message => message.role)).toEqual(["system", "user", "assistant", "tool"]);
+    expect(messages.map(message => message.role)).toEqual(["user", "assistant", "system", "tool"]);
+    expect(messages[2]).toEqual({ role: "system", content: "remember the policy" });
     expect(messages[3]).toMatchObject({ role: "tool", tool_call_id: "call_1" });
   });
 
@@ -216,3 +217,4 @@ describe("OpenCode Go DeepSeek chronological system messages", () => {
       .toEqual([{ role: "user", content: "Inspect the synthetic project." }]);
   });
 });
+