diff --git a/apps/opencode-plugin/index.ts b/apps/opencode-plugin/index.ts
index 907cbc489..75601ebf6 100644
--- a/apps/opencode-plugin/index.ts
+++ b/apps/opencode-plugin/index.ts
@@ -24,6 +24,7 @@ import { loadConfig, resolveSharingEnabled } from "@plannotator/shared/config";
 import { readImprovementHook } from "@plannotator/shared/improvement-hooks";
 import { composeImproveContext } from "@plannotator/shared/pfm-reminder";
 import {
+  composeSystemPrompt,
   stripConflictingPlanModeRules,
 } from "./plan-mode";
 import {
@@ -406,32 +407,32 @@ tools (except writing markdown files), or otherwise make changes to the system.
 
       if (shouldInjectFullPlanningPrompt(lastUserAgent, workflowOptions)) {
         const stripped = stripConflictingPlanModeRules(output.system);
-        output.system.length = 0;
-        output.system.push(...stripped);
-        output.system.push(getPlanningPrompt());
-
         const hook = readImprovementHook("enterplanmode-improve");
         const pfmEnabled = loadConfig().pfmReminder === true;
         const improveContext = composeImproveContext({
           pfmEnabled,
           improvementHookContent: hook?.content ?? null,
         });
-        if (improveContext) {
-          output.system.push(improveContext);
-        }
+        const parts = [...stripped, getPlanningPrompt()];
+        if (improveContext) parts.push(improveContext);
+        output.system.length = 0;
+        output.system.push(...composeSystemPrompt([], parts));
 
         return;
       }
 
       if (!shouldInjectGenericPlanReminder(lastUserAgent, isSubagent, workflowOptions)) return;
 
-      output.system.push(`## Plan Submission
+      const planSubmissionReminder = `## Plan Submission
 
 When you have completed your plan, call the \`submit_plan\` tool to submit it for user review. Pass your full plan as a single edit: \`{ "edits": [{ "start": 1, "content": "..." }] }\`.
 
 The user will review your plan in a visual UI where they can annotate, approve, or request changes. If rejected, the response includes your plan with line numbers; use targeted edits to revise specific sections.
 
-Do NOT proceed with implementation until your plan is approved.`);
+Do NOT proceed with implementation until your plan is approved.`;
+      const composed = composeSystemPrompt(output.system, [planSubmissionReminder]);
+      output.system.length = 0;
+      output.system.push(...composed);
     },
 
     // Intercept plannotator commands before the agent sees them.
diff --git a/apps/opencode-plugin/plan-mode.test.ts b/apps/opencode-plugin/plan-mode.test.ts
index d96eddfcd..4abc1a02d 100644
--- a/apps/opencode-plugin/plan-mode.test.ts
+++ b/apps/opencode-plugin/plan-mode.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import {
+  composeSystemPrompt,
   normalizeEditPermission,
   stripConflictingPlanModeRules,
 } from "./plan-mode";
@@ -72,3 +73,17 @@ describe("stripConflictingPlanModeRules", () => {
     ).toEqual(["Keep the plan concise."]);
   });
 });
+
+describe("composeSystemPrompt", () => {
+  test.each([
+    ["always one element", ["a"], ["b"], ["a\n\nb"]],
+    ["empty strip case", [], ["prompt"], ["prompt"]],
+    ["multi-element order", ["base", "extra"], ["add1", "add2"], ["base\n\nextra\n\nadd1\n\nadd2"]],
+    ["content order (plan path)", ["strip1"], ["plan", "improve"], ["strip1\n\nplan\n\nimprove"]],
+    ["trailing newlines trimmed", ["base\n"], ["add\n\n"], ["base\n\nadd"]],
+    ["empty string entry collapses", ["", "a"], ["b"], ["a\n\nb"]],
+    ["empty inputs", [], [], [""]],
+  ])("%s", (_name, system, additions, expected) => {
+    expect(composeSystemPrompt(system, additions)).toEqual(expected);
+  });
+});
diff --git a/apps/opencode-plugin/plan-mode.ts b/apps/opencode-plugin/plan-mode.ts
index 09a8d1ef9..234df2350 100644
--- a/apps/opencode-plugin/plan-mode.ts
+++ b/apps/opencode-plugin/plan-mode.ts
@@ -54,3 +54,7 @@ export function stripConflictingPlanModeRules(systemEntries: string[]): string[]
     )
     .filter(Boolean);
 }
+
+export function composeSystemPrompt(system: string[], additions: string[]): string[] {
+  return [system.concat(additions).map((s) => s.trim()).filter(Boolean).join("\n\n")];
+}