diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index ed1b155003da..39ddde426c07 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -1355,12 +1355,22 @@ export namespace Config {
   }
 
   export async function update(config: Info) {
-    const filepath = path.join(Instance.directory, "config.json")
+    const filepath = projectConfigFile()
     const existing = await loadFile(filepath)
     await Bun.write(filepath, JSON.stringify(mergeDeep(existing, config), null, 2))
     await Instance.dispose()
   }
 
+  function projectConfigFile() {
+    const candidates = ["opencode.jsonc", "opencode.json"].map((file) =>
+      path.join(Instance.directory, file),
+    )
+    for (const file of candidates) {
+      if (existsSync(file)) return file
+    }
+    return candidates[1]
+  }
+
   function globalConfigFile() {
     const candidates = ["opencode.jsonc", "opencode.json", "config.json"].map((file) =>
       path.join(Global.Path.config, file),
diff --git a/packages/opencode/test/config/config.test.ts b/packages/opencode/test/config/config.test.ts
index dee633110604..7e4687105903 100644
--- a/packages/opencode/test/config/config.test.ts
+++ b/packages/opencode/test/config/config.test.ts
@@ -549,7 +549,7 @@ test("updates config and writes to file", async () => {
       const newConfig = { model: "updated/model" }
       await Config.update(newConfig as any)
 
-      const writtenConfig = JSON.parse(await Bun.file(path.join(tmp.path, "config.json")).text())
+      const writtenConfig = JSON.parse(await Bun.file(path.join(tmp.path, "opencode.json")).text())
       expect(writtenConfig.model).toBe("updated/model")
     },
   })