From 8e5747ff43790b219e77ca61aba8d90090515884 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kent.overstreet@linux.dev>
Date: Wed, 8 Apr 2026 18:42:47 -0400
Subject: [PATCH] Fix tool result format: Qwen expects <tool_response> in user
 role

Qwen's chat template renders tool results as:
  <|im_start|>user\n<tool_response>\n{content}\n</tool_response><|im_end|>

We were rendering as:
  <|im_start|>tool\n{content}<|im_end|>

The model never saw <|im_start|>tool in training, so it ignored our
tool results and looped retrying the same call. Found by comparing
our tokenization against vLLM's /tokenize endpoint with chat messages.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>
---
 src/agent/context.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/agent/context.rs b/src/agent/context.rs
index 05ad935..47e6c7a 100644
--- a/src/agent/context.rs
+++ b/src/agent/context.rs
@@ -146,9 +146,9 @@ impl NodeBody {
                 out.push_str("\n</tool_call>\n");
             }
             Self::ToolResult(text) => {
-                out.push_str("<|im_start|>tool\n");
+                out.push_str("<|im_start|>user\n<tool_response>\n");
                 out.push_str(text);
-                out.push_str("<|im_end|>\n");
+                out.push_str("\n</tool_response><|im_end|>\n");
             }
             Self::Memory { text, .. } => {
                 out.push_str("<|im_start|>memory\n");
@@ -1035,7 +1035,7 @@ mod tests {
     #[test]
     fn test_render_tool_result() {
         let node = AstNode::tool_result("output here");
-        assert_eq!(node.render(), "<|im_start|>tool\noutput here<|im_end|>\n");
+        assert_eq!(node.render(), "<|im_start|>user\n<tool_response>\noutput here\n</tool_response><|im_end|>\n");
     }
 
     #[test]