diff --git a/src/lib/x12-detect.test.ts b/src/lib/x12-detect.test.ts
new file mode 100644
index 0000000..653f311
--- /dev/null
+++ b/src/lib/x12-detect.test.ts
@@ -0,0 +1,148 @@
+import { describe, expect, it } from "vitest";
+import {
+  detectKindFromText,
+  detectKindFromFile,
+  detectedKindToParsedBatchKind,
+} from "./x12-detect";
+
+// Minimal X12 envelopes — just enough to exercise the ST01 match.
+// Real fixtures live in backend/tests/fixtures; we use synthetic ones
+// here because the frontend doesn't need the full file content, just
+// the first ~4KB.
+
+const ISA_837P = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260617*1937*^*00501*000000001*0*P*:~" +
+  "GS*HC*SENDER*RECEIVER*20260706*1937*1*X*005010X222A1~" +
+  "ST*837*0001*005010X222A1~"
+);
+const ISA_835 = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260617*1937*^*00501*000000001*0*P*:~" +
+  "GS*HP*SENDER*RECEIVER*20260706*1937*1*X*005010X221A1~" +
+  "ST*835*0001~"
+);
+const ISA_999 = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260617*1937*^*00501*000000001*0*P*:~" +
+  "GS*FA*SENDER*RECEIVER*20260706*1937*1*X*005010X231A1~" +
+  "ST*999*0001~"
+);
+const ISA_277CA = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260617*1937*^*00501*000000001*0*P*:~" +
+  "GS*HN*SENDER*RECEIVER*20260706*1937*1*X*005010X214~" +
+  "ST*277*0001*005010X214~"
+);
+const ISA_277CA_QUALIFIED = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260617*1937*^*00501*000000001*0*P*:~" +
+  "GS*HN*SENDER*RECEIVER*20260706*1937*1*X*005010X214~" +
+  "ST*277CA*0001*005010X214~"
+);
+const TA1_FILE = (
+  "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       " +
+  "*260520*1750*^*00501*000000001*0*P*:~" +
+  "TA1*000000001*20260520*1750*A*000*20260520~" +
+  "IEA*1*000000001~"
+);
+
+describe("detectKindFromText", () => {
+  it("detects ST*837* as 837p", () => {
+    expect(detectKindFromText(ISA_837P)).toBe("837p");
+  });
+
+  it("detects ST*837P* as 837p (professional qualifier)", () => {
+    const withQualifier = ISA_837P.replace("ST*837*", "ST*837P*");
+    expect(detectKindFromText(withQualifier)).toBe("837p");
+  });
+
+  it("detects ST*835* as 835", () => {
+    expect(detectKindFromText(ISA_835)).toBe("835");
+  });
+
+  it("detects ST*999* as 999", () => {
+    expect(detectKindFromText(ISA_999)).toBe("999");
+  });
+
+  it("detects ST*277* as 277ca", () => {
+    expect(detectKindFromText(ISA_277CA)).toBe("277ca");
+  });
+
+  it("detects ST*277CA* as 277ca (qualified form)", () => {
+    expect(detectKindFromText(ISA_277CA_QUALIFIED)).toBe("277ca");
+  });
+
+  it("detects TA1 envelope as ta1 (no ST, bare TA1 segment)", () => {
+    expect(detectKindFromText(TA1_FILE)).toBe("ta1");
+  });
+
+  it("returns unknown for garbage input", () => {
+    expect(detectKindFromText("not edi at all")).toBe("unknown");
+  });
+
+  it("returns unknown for an empty string", () => {
+    expect(detectKindFromText("")).toBe("unknown");
+  });
+
+  it("is case-insensitive on ST segment", () => {
+    // Real X12 is uppercase, but be lenient — files from various tools
+    // sometimes have mixed case.
+    expect(detectKindFromText(ISA_835.toLowerCase())).toBe("835");
+  });
+
+  it("does not false-positive on ST*8370 (5 chars after ST*)", () => {
+    // Catches a regression where the matcher would substring-match too
+    // eagerly (e.g. matching ST*837 against ST*8370*). The require is
+    // that the char after the digits is `*`, not another digit.
+    const noise = "ISA*00*...*ST*8370*0001~";
+    expect(detectKindFromText(noise)).toBe("unknown");
+  });
+});
+
+describe("detectedKindToParsedBatchKind", () => {
+  it("maps 837p → 837p", () => {
+    expect(detectedKindToParsedBatchKind("837p")).toBe("837p");
+  });
+
+  it("maps 835 → 835", () => {
+    expect(detectedKindToParsedBatchKind("835")).toBe("835");
+  });
+
+  it("returns null for 999 (Upload page doesn't ingest 999s)", () => {
+    expect(detectedKindToParsedBatchKind("999")).toBeNull();
+  });
+
+  it("returns null for 277ca", () => {
+    expect(detectedKindToParsedBatchKind("277ca")).toBeNull();
+  });
+
+  it("returns null for ta1", () => {
+    expect(detectedKindToParsedBatchKind("ta1")).toBeNull();
+  });
+
+  it("returns null for unknown", () => {
+    expect(detectedKindToParsedBatchKind("unknown")).toBeNull();
+  });
+});
+
+describe("detectKindFromFile", () => {
+  it("detects 837p from a File blob", async () => {
+    const file = new File([ISA_837P], "test.x12", { type: "text/plain" });
+    expect(await detectKindFromFile(file)).toBe("837p");
+  });
+
+  it("detects 835 from a File blob (the original SP35 repro)", async () => {
+    // The exact scenario the user hit: drop an 835 file on the Upload
+    // page while the dropdown still says "837p". The helper must
+    // return "835" so the UI can switch the dropdown before the user
+    // hits Parse.
+    const file = new File([ISA_835], "tp11525703-835.x12", { type: "text/plain" });
+    expect(await detectKindFromFile(file)).toBe("835");
+  });
+
+  it("returns unknown on an empty file (no crash)", async () => {
+    const file = new File([""], "empty.x12", { type: "text/plain" });
+    expect(await detectKindFromFile(file)).toBe("unknown");
+  });
+});
\ No newline at end of file
diff --git a/src/lib/x12-detect.ts b/src/lib/x12-detect.ts
new file mode 100644
index 0000000..2ed0bb3
--- /dev/null
+++ b/src/lib/x12-detect.ts
@@ -0,0 +1,98 @@
+/**
+ * X12 file-kind auto-detection helper.
+ *
+ * SP35: the Upload page used to default to "837p" silently — dropping an 835
+ * file on the page while the dropdown still said "837p" routed the file to
+ * /api/parse-837, which silently persisted an empty batch. Layer A of the
+ * defense-in-depth fix is to detect the kind from the file's first few KB
+ * and switch the dropdown automatically before the user clicks Parse.
+ *
+ * Layer B is the server-side envelope check (cyclone.api._transaction_set_id_from_segments).
+ * This helper is purely advisory: if it returns null, the UI keeps whatever
+ * the user picked and the server guard rejects with a clean 400 if it's wrong.
+ *
+ * Detection is intentionally cheap: read the first ~4KB and look for the ST
+ * segment (or the bare TA1 segment, which has no ST envelope). The full file
+ * is never loaded — X12 envelopes are always within the first ISA segment.
+ */
+
+import type { ParsedBatchKind } from "@/types";
+
+/** Number of bytes to read from the start of the file for detection. */
+const DETECT_BYTES = 4096;
+
+export type DetectedKind = ParsedBatchKind | "999" | "277ca" | "ta1" | "unknown";
+
+export function detectKindFromText(text: string): DetectedKind {
+  // Strip whitespace and look for the ST segment.
+  // X12 envelopes are always within the first segment, so we don't need
+  // to scan far. The segment terminator is `~` (per the ISA segment,
+  // which we also don't need to parse here — we only care which kind of
+  // payload follows).
+  //
+  // We do a simple substring search rather than full tokenization because:
+  //   - Detection must run before the user even knows if their file is
+  //     valid X12. If we ran `tokenize()` and the file has a malformed
+  //     ISA, we'd throw and the UI couldn't show a helpful message.
+  //   - A simple prefix match on `ST*<kind>*` is enough to disambiguate
+  //     every kind the Upload page might receive. False positives are
+  //     caught by the server-side guard.
+  const upper = text.toUpperCase();
+
+  // 277CA uses ST*277* or ST*277CA* (per parse_277ca.py line 13 comment).
+  // Check 277CA before 277 because ST*277CA contains "277".
+  if (upper.includes("ST*277CA*")) return "277ca";
+  if (upper.includes("ST*277*")) return "277ca";
+
+  // 837P accepts ST*837* or ST*837P* (the trailing P is the professional
+  // claim qualifier, but the ISA envelope alone tells you it's a
+  // professional file). Check 837P before 837 for the same reason.
+  if (upper.includes("ST*837P*")) return "837p";
+  if (upper.includes("ST*837*")) return "837p";
+
+  // 835 has ST*835* — no other qualifier in common use.
+  if (upper.includes("ST*835*")) return "835";
+
+  // 999 ACK has ST*999* — no qualifier.
+  if (upper.includes("ST*999*")) return "999";
+
+  // TA1 has no ST envelope. The interchange-ack segment is the bare TA1*
+  // immediately after ISA/IEA. Match the segment header.
+  if (/\bTA1\*/.test(upper)) return "ta1";
+
+  return "unknown";
+}
+
+/**
+ * Browser-side helper: read the first DETECT_BYTES from a File and detect
+ * its kind. Returns a Promise so it composes naturally with FileReader /
+ * Blob.slice. Returns "unknown" on read error so the caller can keep the
+ * user's current selection and let the server guard surface a clean 400.
+ */
+export async function detectKindFromFile(file: File): Promise<DetectedKind> {
+  try {
+    const blob = file.slice(0, DETECT_BYTES);
+    const text = await blob.text();
+    return detectKindFromText(text);
+  } catch {
+    return "unknown";
+  }
+}
+
+/**
+ * Map a DetectedKind to the ParsedBatchKind the Upload dropdown supports.
+ * Returns null for kinds the Upload page can't ingest (999, 277CA, TA1) —
+ * the UI then surfaces a "this file isn't supported here" hint instead of
+ * silently misrouting it. The server-side guards in cyclone.api still
+ * catch any escape.
+ */
+export function detectedKindToParsedBatchKind(kind: DetectedKind): ParsedBatchKind | null {
+  switch (kind) {
+    case "837p":
+      return "837p";
+    case "835":
+      return "835";
+    default:
+      return null;
+  }
+}
\ No newline at end of file
diff --git a/src/pages/Upload.tsx b/src/pages/Upload.tsx
index 64e82f3..9ec8c11 100644
--- a/src/pages/Upload.tsx
+++ b/src/pages/Upload.tsx
@@ -33,6 +33,10 @@ import { StatPill, ValidationDot } from "@/components/ClaimCard/shared";
 import { api, ApiError, type BatchSummary, type ParseProgress } from "@/lib/api";
 import { downloadBlob } from "@/lib/download";
 import { fmt, toNum } from "@/lib/format";
+import {
+  detectKindFromFile,
+  detectedKindToParsedBatchKind,
+} from "@/lib/x12-detect";
 import { useAppStore } from "@/store";
 import { useParse } from "@/hooks/useParse";
 import { useBatchExport } from "@/hooks/useBatchExport";
@@ -437,9 +441,37 @@ export function Upload() {
   // hook call above. Keeping the comment as a breadcrumb for future
   // readers who grep "Auto-select".)
 
-  function pickFile(f: File | null) {
+  async function pickFile(f: File | null) {
     setFile(f);
     setStream({ items: [], expectedTotal: null, passed: 0, failed: 0 });
+    if (!f) return;
+    // SP35: auto-detect the kind from the file's first few KB and switch
+    // the dropdown before the user hits Parse. This is layer A of the
+    // defense-in-depth fix (layer B is the server-side envelope guard
+    // in cyclone.api). Before SP35 the dropdown defaulted to "837p"
+    // silently — dropping an 835 file on the page routed it to
+    // /api/parse-837 and produced a bogus empty batch. Detection runs
+    // asynchronously on a 4KB slice; if it can't determine the kind
+    // (unknown file, read error) we keep the user's current selection
+    // and the server guard will surface a clean 400 if it's wrong.
+    const detected = await detectKindFromFile(f);
+    const matched = detectedKindToParsedBatchKind(detected);
+    if (matched && matched !== kind) {
+      setKind(matched);
+      setPayer(matched === "837p" ? PAYERS_837[0]!.value : PAYERS_835[0]!.value);
+      toast.message(
+        `Detected ${matched === "837p" ? "837P" : "835"} file — switched the dropdown.`,
+        { description: f.name },
+      );
+    } else if (detected === "999" || detected === "277ca" || detected === "ta1") {
+      // The Upload page only supports 837P and 835; for the other X12
+      // kinds (which have their own endpoints), surface a hint instead
+      // of silently leaving the dropdown on whatever the user picked.
+      toast.error(
+        `Detected ${detected.toUpperCase()} file — the Upload page only accepts 837P or 835.`,
+        { description: "Use the matching endpoint from the History tab or the API." },
+      );
+    }
   }
 
   async function onParse() {