From f4a9b2bd45ada333e0316d34a178232b76fb628f Mon Sep 17 00:00:00 2001
From: "exe.dev user" <exedev@ropeway-swimming.exe.xyz>
Date: Wed, 9 Sep 2026 07:52:13 +0000
Subject: [PATCH] fix(server): tell agents how to nest fenced code inside code
 blocks

A code block whose body contains triple backticks was rendered broken in
chat: the inner closing fence ended the outer block, and the rest spilled
out as prose. That is correct CommonMark, so the fix is to tell every
provider up front to use a longer fence in that case.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 apps/server/src/provider/RuntimeInstructions.test.ts | 6 ++++++
 apps/server/src/provider/RuntimeInstructions.ts      | 2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/apps/server/src/provider/RuntimeInstructions.test.ts b/apps/server/src/provider/RuntimeInstructions.test.ts
index 320aa332c937..81f205028400 100644
--- a/apps/server/src/provider/RuntimeInstructions.test.ts
+++ b/apps/server/src/provider/RuntimeInstructions.test.ts
@@ -17,4 +17,10 @@ describe("buildRuntimeInstructions", () => {
     expect(instructions).toContain("through the Cursor harness.");
     expect(instructions).not.toContain("reasoning effort");
   });
+
+  it("tells the agent how to nest fenced code inside a code block", () => {
+    expect(buildRuntimeInstructions({ harness: "Claude Code" })).toContain(
+      "must be fenced with a longer fence (four backticks or a ~~~ fence)",
+    );
+  });
 });
diff --git a/apps/server/src/provider/RuntimeInstructions.ts b/apps/server/src/provider/RuntimeInstructions.ts
index 102c49c78a9e..d762836279b6 100644
--- a/apps/server/src/provider/RuntimeInstructions.ts
+++ b/apps/server/src/provider/RuntimeInstructions.ts
@@ -9,7 +9,7 @@ export function buildRuntimeInstructions(runtime: {
   const effort = toSingleLine(runtime.reasoningEffort ?? "");
   const modelInfo = model && model !== "auto" && model !== "default" ? `, as ${model}` : "";
   const effortInfo = effort ? ` with ${effort} reasoning effort` : "";
-  return `<runtime_info>In case you're asked: you are running in T3 Code through the ${harness} harness${modelInfo}${effortInfo}. No need to mention this otherwise. You can embed images and videos in your response using Markdown with absolute file paths.</runtime_info>`;
+  return `<runtime_info>In case you're asked: you are running in T3 Code through the ${harness} harness${modelInfo}${effortInfo}. No need to mention this otherwise. You can embed images and videos in your response using Markdown with absolute file paths. Responses render as CommonMark: a code block that itself contains triple backticks must be fenced with a longer fence (four backticks or a ~~~ fence), otherwise the inner closing fence ends the block early.</runtime_info>`;
 }
 
 function toSingleLine(value: string): string {