diff --git a/packages/provider-opencode/src/server/OpenCodeRuntime.cliParsers.test.ts b/packages/provider-opencode/src/server/OpenCodeRuntime.cliParsers.test.ts
index 499d7d5fee7368c24d2e415479bd4775cf6582fc..8394d8023d7639040f956cbd820c4054b6fdf513 100644
--- a/packages/provider-opencode/src/server/OpenCodeRuntime.cliParsers.test.ts
+++ b/packages/provider-opencode/src/server/OpenCodeRuntime.cliParsers.test.ts
@@ -293,11 +293,15 @@ describe("toOpenCodeFileParts", () => {
     const parts = OpenCodeRuntime.toOpenCodeFileParts({
       attachments: [
         attachment("application/pdf"),
-        attachment("text/markdown"),
+        attachment("text/plain"),
         attachment("image/png"),
         // A ZIP file part makes OpenCode's Anthropic path throw before the
         // turn starts; it must ride only as the prompt's file path line.
         attachment("application/zip"),
+        // OpenCode inlines only text/plain itself. Other text types reach the
+        // model API as raw file parts, which Anthropic and OpenAI reject.
+        attachment("text/html"),
+        attachment("text/markdown"),
         attachment("application/octet-stream"),
         // Image formats the model APIs reject stay on the fallback path too.
         attachment("image/bmp"),
@@ -310,7 +314,7 @@ describe("toOpenCodeFileParts", () => {
 
     NodeAssert.deepEqual(
       parts.map((part) => part.mime),
-      ["application/pdf", "text/markdown", "image/png"],
+      ["application/pdf", "text/plain", "image/png"],
     );
   });
 
diff --git a/packages/provider-opencode/src/server/OpenCodeRuntime.ts b/packages/provider-opencode/src/server/OpenCodeRuntime.ts
index b6651d48a721b975db526ef34cdf8a6983a2a43f..d88a0321867479c7424a7983388b929e8c21beb7 100644
--- a/packages/provider-opencode/src/server/OpenCodeRuntime.ts
+++ b/packages/provider-opencode/src/server/OpenCodeRuntime.ts
@@ -469,7 +469,9 @@ export function openCodeQuestionId(
  * else (ZIP, binaries, image formats like BMP/AVIF/SVG that model APIs
  * reject, or files over the direct-attachment size limit) would make the turn
  * fail before it starts, so those ride only as the file path ProviderService
- * puts in the prompt.
+ * puts in the prompt. Text is native only as text/plain, which OpenCode reads
+ * itself; other text types (HTML, Markdown, CSV) reach the model API as raw
+ * file parts, which Anthropic and OpenAI reject.
  */
 const OPENCODE_NATIVE_IMAGE_MIMES = new Set(["image/png", "image/jpeg", "image/gif", "image/webp"]);
 const OPENCODE_NATIVE_FILE_PART_MAX_BYTES = 20 * 1024 * 1024;
@@ -484,7 +486,7 @@ function isOpenCodeNativeFilePart(input: {
   const normalized = input.mimeType.trim().toLowerCase();
   return (
     OPENCODE_NATIVE_IMAGE_MIMES.has(normalized) ||
-    normalized.startsWith("text/") ||
+    normalized === "text/plain" ||
     normalized === "application/pdf"
   );
 }