diff --git a/src/sync/json-config.ts b/src/sync/json-config.ts
index 180993b..547ca3b 100644
--- a/src/sync/json-config.ts
+++ b/src/sync/json-config.ts
@@ -18,8 +18,8 @@ export async function mergeJsonConfigAtKey(options: {
   const merged = {
     ...existing,
     [key]: {
-      ...existingEntries,
       ...incoming,
+      ...existingEntries, // existing user entries win on conflict
     },
   }
 
diff --git a/src/targets/opencode.ts b/src/targets/opencode.ts
index e0e89ff..b4bf53e 100644
--- a/src/targets/opencode.ts
+++ b/src/targets/opencode.ts
@@ -58,12 +58,16 @@ export async function writeOpenCodeBundle(outputRoot: string, bundle: OpenCodeBu
   const openCodePaths = resolveOpenCodePaths(outputRoot)
   await ensureDir(openCodePaths.root)
 
+  const hadExistingConfig = await pathExists(openCodePaths.configPath)
   const backupPath = await backupFile(openCodePaths.configPath)
   if (backupPath) {
     console.log(`Backed up existing config to ${backupPath}`)
   }
   const merged = await mergeOpenCodeConfig(openCodePaths.configPath, bundle.config)
   await writeJson(openCodePaths.configPath, merged)
+  if (hadExistingConfig) {
+    console.log("Merged plugin config into existing opencode.json (user settings preserved)")
+  }
 
   const agentsDir = openCodePaths.agentsDir
   for (const agent of bundle.agents) {
diff --git a/tests/opencode-writer.test.ts b/tests/opencode-writer.test.ts
index 5c02cc1..f0aa976 100644
--- a/tests/opencode-writer.test.ts
+++ b/tests/opencode-writer.test.ts
@@ -3,6 +3,7 @@ import { promises as fs } from "fs"
 import path from "path"
 import os from "os"
 import { writeOpenCodeBundle } from "../src/targets/opencode"
+import { mergeJsonConfigAtKey } from "../src/sync/json-config"
 import type { OpenCodeBundle } from "../src/types/opencode"
 
 async function exists(filePath: string): Promise<boolean> {
@@ -254,3 +255,38 @@ describe("writeOpenCodeBundle", () => {
     expect(backupContent).toBe("old content\n")
   })
 })
+
+describe("mergeJsonConfigAtKey", () => {
+  test("preserves existing user entries on conflict", async () => {
+    const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "json-merge-"))
+    const configPath = path.join(tempDir, "opencode.json")
+
+    // User has an existing MCP server config
+    const existingConfig = {
+      model: "my-model",
+      mcp: {
+        "user-server": { type: "local", command: ["uvx", "user-srv"] },
+      },
+    }
+    await fs.writeFile(configPath, JSON.stringify(existingConfig, null, 2))
+
+    // Plugin tries to add its own server and override user-server
+    await mergeJsonConfigAtKey({
+      configPath,
+      key: "mcp",
+      incoming: {
+        "plugin-server": { type: "local", command: ["uvx", "plugin-srv"] },
+        "user-server": { type: "local", command: ["uvx", "plugin-override"] },
+      },
+    })
+
+    const merged = JSON.parse(await fs.readFile(configPath, "utf8"))
+
+    // User's top-level keys preserved
+    expect(merged.model).toBe("my-model")
+    // Plugin server added
+    expect(merged.mcp["plugin-server"]).toBeDefined()
+    // User's server NOT overwritten by plugin
+    expect(merged.mcp["user-server"].command[1]).toBe("user-srv")
+  })
+})