From ffd90ce7fb605a13a7d027f7b1c4dc8e1af1d88a Mon Sep 17 00:00:00 2001
From: Gahow Wang <gahow.wang@gmail.com>
Date: Sun, 31 May 2026 00:56:39 +0800
Subject: [PATCH] server: emit harmony developer instructions block for gpt-oss

Route caller-supplied system messages into a harmony 'developer'
instructions block (<|start|>developer<|message|># Instructions...),
keeping the fixed system/meta block for the channel declaration. Harmony
puts user instructions on the developer role, not system.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 crates/xserv-server/src/api.rs | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/crates/xserv-server/src/api.rs b/crates/xserv-server/src/api.rs
index 00e3557..6a17d33 100644
--- a/crates/xserv-server/src/api.rs
+++ b/crates/xserv-server/src/api.rs
@@ -350,10 +350,23 @@ fn build_prompt(messages: &[Message], model_type: &str) -> String {
 
 fn build_prompt_gpt_oss(messages: &[Message]) -> String {
     let mut prompt = String::new();
-    // System prompt
+    // System (meta) block: channel declaration required by harmony.
     prompt.push_str("<|start|>system<|message|>");
     prompt.push_str("You are a helpful assistant.\n\n# Valid channels: analysis, commentary, final. Channel must be included for every message.");
     prompt.push_str("<|end|>");
+    // Caller-supplied system prompt(s) go in a developer instructions block
+    // (harmony puts user instructions on the `developer` role, not `system`).
+    let dev_instructions: String = messages
+        .iter()
+        .filter(|m| m.role == "system")
+        .map(|m| m.content.as_str())
+        .collect::<Vec<_>>()
+        .join("\n\n");
+    if !dev_instructions.is_empty() {
+        prompt.push_str("<|start|>developer<|message|># Instructions\n\n");
+        prompt.push_str(&dev_instructions);
+        prompt.push_str("<|end|>");
+    }
     for msg in messages {
         match msg.role.as_str() {
             "user" => {