fix: add charset to text file responses

src/api/routes/surface.ts

@@ -29,6 +29,14 @@ import {
   SHARED_SKILL_TRIGGER_REFUSAL,
 } from "../artifact-share.ts";
 
+function contentTypeForFile(mimetype?: string): string {
+  const contentType = mimetype || "application/octet-stream";
+  if (/^(?:text\/|application\/(?:json|xml)(?:;|$))/i.test(contentType) && !/;\s*charset\s*=/i.test(contentType)) {
+    return `${contentType}; charset=utf-8`;
+  }
+  return contentType;
+}
+
 function isGrant(b: unknown): b is Grant {
   return (
     isObj(b) &&
@@ -252,7 +260,7 @@ async function getFileContent(ctx: ApiCtx): Promise<void> {
   const opened = await app.openFileForViewer(id, viewer);
   if (!opened) return sendJson(res, 404, { error: "not_found" });
   res.writeHead(200, {
-    "content-type": opened.mimetype || "application/octet-stream",
+    "content-type": contentTypeForFile(opened.mimetype),
     "content-length": String(opened.sizeBytes),
     "content-disposition": `inline; filename*=UTF-8''${encodeURIComponent(opened.name)}`,
   });

test/agent-files-route.test.ts

@@ -82,6 +82,27 @@ describe("agent files self-API", async () => {
     assert.deepEqual(page.shared, []);
   });
 
+  it("adds UTF-8 to text file content responses", async () => {
+    const res = await get(`/v1/files/${mineId}/content`, await capFor("U1"));
+    assert.equal(res.status, 200);
+    assert.equal(res.headers.get("content-type"), "text/plain; charset=utf-8");
+    assert.equal(Buffer.from(await res.arrayBuffer()).toString("utf8"), "mine");
+
+    await built.files.put({
+      id: "parameterized-text",
+      ownerScopeId: scopeId("personal", "U1"),
+      createdBy: "U1",
+      name: "parameterized.txt",
+      path: "parameterized.txt",
+      mimetype: "text/plain; charset=iso-8859-1",
+      data: Buffer.from("bytes"),
+      direction: "out",
+    });
+    const parameterized = await get("/v1/files/parameterized-text/content", await capFor("U1"));
+    assert.equal(parameterized.status, 200);
+    assert.equal(parameterized.headers.get("content-type"), "text/plain; charset=iso-8859-1");
+  });
+
   it("returns 404 for another principal's file content", async () => {
     const res = await get(`/v1/files/${theirsId}/content`, await capFor("U1"));
     assert.equal(res.status, 404);