diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9f53040..532e9ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+- Fix `experimental.chat.system.transform` to merge the goal continuation block into the primary system entry instead of pushing a separate one. Prevents `"System message must be at the beginning."` errors on strict-template backends (Qwen on vLLM, several Llama.cpp/Mistral templates). See issue #1.
+
 ## 0.1.9 — 2026-05-18
 
 > This release makes the goal plugin much more reliable for real unattended use. Goals now persist across restarts, recover in a safe paused state, expose better status/history visibility, and use smarter no-progress detection to avoid premature stalls. It also hardens persistence with atomic writes, stricter file permissions, and regression tests around corrupt or missing state.
diff --git a/src/goal-plugin.js b/src/goal-plugin.js
index 30c66f6..59cc5f8 100644
--- a/src/goal-plugin.js
+++ b/src/goal-plugin.js
@@ -1059,15 +1059,19 @@ export const GoalPlugin = async ({ client }, pluginOptions = {}) => {
       if (goal.stopped) return
       if (output.system.some((block) => block.includes("<goal_objective>"))) return
 
-      output.system.push(
-        [
-          buildGoalBlock(goal),
-          "Keep working until the goal is fully satisfied.",
-          "When fully satisfied, end the response with `[goal:complete]`.",
-          "If user input is required, explain the blocker in the line immediately before `[goal:blocked]`.",
-          buildLimitWarning(goal),
-        ].filter(Boolean).join("\n"),
-      )
+      const goalBlock = [
+        buildGoalBlock(goal),
+        "Keep working until the goal is fully satisfied.",
+        "When fully satisfied, end the response with `[goal:complete]`.",
+        "If user input is required, explain the blocker in the line immediately before `[goal:blocked]`.",
+        buildLimitWarning(goal),
+      ].filter(Boolean).join("\n")
+
+      if (output.system.length === 0) {
+        output.system.push(goalBlock)
+      } else {
+        output.system[0] = `${output.system[0]}\n\n${goalBlock}`
+      }
     },
   }
 }
diff --git a/test/goal-plugin.test.js b/test/goal-plugin.test.js
index 066d8c5..97dd765 100644
--- a/test/goal-plugin.test.js
+++ b/test/goal-plugin.test.js
@@ -185,6 +185,39 @@ test("system transform is idempotent", async () => {
   assert.match(output.system[0], /<goal_objective>\nship it\n<\/goal_objective>/)
 })
 
+test("system transform merges into existing system block instead of adding a second one", async () => {
+  const { hooks } = await createHooks()
+  await hooks["command.execute.before"](
+    { command: "goal", sessionID: "session-1", arguments: "ship it" },
+    { parts: [] },
+  )
+
+  const basePrompt = "You are opencode, a coding assistant."
+  const output = { system: [basePrompt] }
+  await hooks["experimental.chat.system.transform"]({ sessionID: "session-1" }, output)
+
+  // Strict-template backends (e.g. Qwen on vLLM) reject any request with more
+  // than one role:"system" message. The goal block must be merged into the
+  // existing primary system entry, not pushed as a second array entry.
+  assert.equal(output.system.length, 1)
+  assert.ok(output.system[0].startsWith(basePrompt))
+  assert.match(output.system[0], /<goal_objective>\nship it\n<\/goal_objective>/)
+})
+
+test("system transform pushes a new block when system array is empty", async () => {
+  const { hooks } = await createHooks()
+  await hooks["command.execute.before"](
+    { command: "goal", sessionID: "session-1", arguments: "ship it" },
+    { parts: [] },
+  )
+
+  const output = { system: [] }
+  await hooks["experimental.chat.system.transform"]({ sessionID: "session-1" }, output)
+
+  assert.equal(output.system.length, 1)
+  assert.match(output.system[0], /<goal_objective>\nship it\n<\/goal_objective>/)
+})
+
 test("session.status idle auto-continues once", async () => {
   const { calls, hooks } = await createHooks({ options: { minDelayMs: 1 } })
   await hooks["command.execute.before"](