diff --git a/packages/opencode/test/effect/config-service.test.ts b/packages/opencode/test/effect/config-service.test.ts
new file mode 100644
index 0000000000..be6f977363
--- /dev/null
+++ b/packages/opencode/test/effect/config-service.test.ts
@@ -0,0 +1,65 @@
+import { describe, expect } from "bun:test"
+import { Config, ConfigProvider, Context, Effect, Layer, Option } from "effect"
+import { ConfigService } from "../../src/effect/config-service"
+import { it } from "../lib/effect"
+
+class TestConfig extends ConfigService.Service<TestConfig>()("@test/ConfigService", {
+  name: Config.string("NAME"),
+  token: Config.string("TOKEN").pipe(Config.option),
+  port: Config.number("PORT").pipe(Config.withDefault(3000)),
+}) {}
+
+const fromConfig = (input: Record<string, unknown>) =>
+  TestConfig.defaultLayer.pipe(Layer.provide(ConfigProvider.layer(ConfigProvider.fromUnknown(input))))
+
+const readConfig = TestConfig.useSync((config) => config)
+
+describe("ConfigService", () => {
+  it.effect("defaultLayer parses values from the active ConfigProvider", () =>
+    Effect.gen(function* () {
+      const config = yield* readConfig.pipe(
+        Effect.provide(
+          fromConfig({
+            NAME: "kit",
+            TOKEN: "secret",
+            PORT: "4096",
+          }),
+        ),
+      )
+
+      expect(config.name).toBe("kit")
+      expect(config.token).toEqual(Option.some("secret"))
+      expect(config.port).toBe(4096)
+    }),
+  )
+
+  it.effect("defaultLayer applies Effect Config defaults", () =>
+    Effect.gen(function* () {
+      const config = yield* readConfig.pipe(Effect.provide(fromConfig({ NAME: "kit" })))
+
+      expect(config.name).toBe("kit")
+      expect(config.token).toEqual(Option.none())
+      expect(config.port).toBe(3000)
+    }),
+  )
+
+  it.effect("layer provides an already parsed service value", () =>
+    Effect.gen(function* () {
+      const config = yield* readConfig.pipe(
+        Effect.provide(
+          TestConfig.layer({
+            name: "direct",
+            token: Option.some("parsed"),
+            port: 9000,
+          }),
+        ),
+      )
+
+      expect(config).toEqual({
+        name: "direct",
+        token: Option.some("parsed"),
+        port: 9000,
+      } satisfies Context.Service.Shape<typeof TestConfig>)
+    }),
+  )
+})