fix(opencode): normalize MCP text MIME types

packages/opencode/src/session/message-v2.ts

@@ -222,7 +222,7 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
         // Text, directories, and MCP references are resolved before provider conversion.
         if (
           part.type === "file" &&
-          part.mime !== "text/plain" &&
+          part.mime.split(";")[0]?.trim().toLowerCase() !== "text/plain" &&
           part.mime !== "application/x-directory" &&
           part.source?.type !== "resource"
         ) {

packages/opencode/src/session/prompt.ts

@@ -742,7 +742,7 @@ export const layer = Layer.effect(
                 } else if ("blob" in c && c.blob) {
                   const mime = ("mimeType" in c ? c.mimeType : undefined) ?? part.mime
                   const url = `data:${mime};base64,${c.blob}`
-                  if (mime === "text/plain") {
+                  if (mime.split(";")[0]?.trim().toLowerCase() === "text/plain") {
                     pieces.push({
                       messageID: info.id,
                       sessionID: input.sessionID,

packages/opencode/test/session/message-v2.test.ts

@@ -344,6 +344,13 @@ describe("session.message-v2.toModelMessage", () => {
               text: { value: "@fixture-guide", start: 0, end: 14 },
             },
           },
+          {
+            ...basePart(messageID, "p3"),
+            type: "file",
+            mime: "text/plain; charset=utf-8",
+            filename: "guide.txt",
+            url: "data:text/plain; charset=utf-8;base64,IyBSZXNvdXJjZSBjb250ZW50cw==",
+          },
         ] as SessionV1.Part[],
       },
     ]

packages/opencode/test/session/prompt.test.ts

@@ -262,7 +262,7 @@ const resourceNoLLMServer = testEffect(
               ? [
                   {
                     uri,
-                    mimeType: "text/plain",
+                    mimeType: "text/plain; charset=utf-8",
                     blob: Buffer.from("MCP text blob fixture").toString("base64"),
                   },
                 ]
@@ -2117,8 +2117,8 @@ resourceNoLLMServer.instance(
           (part) =>
             part.type === "file" &&
             !part.source &&
-            part.mime === "text/plain" &&
-            part.url.startsWith("data:text/plain;base64,"),
+            part.mime === "text/plain; charset=utf-8" &&
+            part.url.startsWith("data:text/plain; charset=utf-8;base64,"),
         ),
       ).toBe(true)
       expect(