diff --git a/internal/plugin/host.js b/internal/plugin/host.js
index 16c764f7..3d211cf4 100644
--- a/internal/plugin/host.js
+++ b/internal/plugin/host.js
@@ -709,6 +709,19 @@ function sdkHeaders(npm, key) {
   return { authorization: `Bearer ${key}` }
 }
 
+// bodyOf is the body as OpenCode hands it to a plugin's fetch: the string
+// the AI SDK built, as the plugins reshape only a string body. One that
+// isn't UTF-8 text stays bytes.
+function bodyOf(b64) {
+  if (!b64) return undefined
+  const b = Buffer.from(b64, "base64")
+  try {
+    return new TextDecoder("utf-8", { fatal: true, ignoreBOM: true }).decode(b)
+  } catch {
+    return b
+  }
+}
+
 async function doFetch(id, params) {
   const key = accountKey(params.provider, params.account)
   return inScope(params.provider, key, () => fetchAs(id, key, params))
@@ -750,7 +763,7 @@ async function fetchAs(id, key, { provider, model, npm, url, method, headers, bo
     const res = await f(url, {
       method: method ?? "POST",
       headers: h,
-      body: body ? Buffer.from(body, "base64") : undefined,
+      body: bodyOf(body),
       signal: ctl.signal,
     })
     const rh = {}
diff --git a/internal/plugin/plugin_test.go b/internal/plugin/plugin_test.go
index 9c1ac5d2..cc2e1f19 100644
--- a/internal/plugin/plugin_test.go
+++ b/internal/plugin/plugin_test.go
@@ -145,6 +145,10 @@ func TestFakePlugin(t *testing.T) {
 	if h.Get("Authorization") != "Bearer fresh-r-blue" || h.Get("X-Plugin-Model") != "fake-1" || h.Get("X-Models") != "fake-1,fake-claude,fake-gemini" {
 		t.Fatalf("headers = %v", h)
 	}
+	// the plugin's fetch got the body as a string, as OpenCode hands it
+	if h.Get("X-Body-Type") != "string" {
+		t.Fatalf("the plugin got a %s body", h.Get("X-Body-Type"))
+	}
 	b, _ = os.ReadFile(AuthPath())
 	json.Unmarshal(b, &saved)
 	if saved["fakeco"]["access"] != "fresh-r-blue" {
diff --git a/internal/plugin/testdata/fake/index.js b/internal/plugin/testdata/fake/index.js
index 59bbc7e3..8640ec12 100644
--- a/internal/plugin/testdata/fake/index.js
+++ b/internal/plugin/testdata/fake/index.js
@@ -50,6 +50,7 @@ export const FakePlugin = async ({ client }) => ({
         const h = new Headers(init.headers)
         h.set("authorization", "Bearer " + (a.type === "oauth" ? a.access : a.key))
         h.set("x-models", Object.keys(provider.models).sort().join(","))
+        h.set("x-body-type", typeof init.body)
         return fetch(url, { ...init, headers: h })
       },
     }),