diff --git a/backend/tests/test_api_277ca.py b/backend/tests/test_api_277ca.py
index 79124b4..d5c649d 100644
--- a/backend/tests/test_api_277ca.py
+++ b/backend/tests/test_api_277ca.py
@@ -167,3 +167,48 @@ class TestInboxPayerRejectedLane:
         # The rejected lane (999 envelope) must be empty — we haven't
         # uploaded a 999, so this claim isn't there.
         assert "c1" not in [c["id"] for c in lanes["rejected"]]
+
+
+# --------------------------------------------------------------------------- #
+# SP35: parse-277ca envelope regression lock
+# --------------------------------------------------------------------------- #
+#
+# The 277CA parser already raises CycloneParseError("Expected ST*277 or
+# ST*277CA, got ST*<other>") when fed a file with the wrong ST envelope
+# (parse_277ca.py line 298). This regression lock confirms the HTTP
+# surface converts that error into a 400 (never 200, never 500).
+
+
+def test_parse_277ca_endpoint_rejects_835_input(client: TestClient):
+    """Posting an 835 file to /api/parse-277ca must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_835.txt"
+    text = wrong_kind.read_text()
+    assert "ST*835" in text  # sanity check on the fixture
+
+    resp = client.post(
+        "/api/parse-277ca",
+        files={"file": ("co_medicaid_835.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert "error" in body
+    # The parser-level message must survive (mentions "ST" and the
+    # expected vs actual set id).
+    detail = body.get("detail", "")
+    assert "ST" in detail and ("277" in detail or body["error"] == "Parse error"), body
+
+
+def test_parse_277ca_endpoint_rejects_837_input(client: TestClient):
+    """Posting an 837P file to /api/parse-277ca must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_837p.txt"
+    text = wrong_kind.read_text()
+    assert "ST*837" in text  # sanity check
+
+    resp = client.post(
+        "/api/parse-277ca",
+        files={"file": ("co_medicaid_837p.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    assert "error" in resp.json()
diff --git a/backend/tests/test_api_999.py b/backend/tests/test_api_999.py
index e3d69d5..f37929f 100644
--- a/backend/tests/test_api_999.py
+++ b/backend/tests/test_api_999.py
@@ -140,3 +140,54 @@ def test_get_ack_404_for_missing(client: TestClient) -> None:
     """GET /api/acks/{id} returns 404 for a missing id (not 500)."""
     resp = client.get("/api/acks/9999", headers={"Accept": "application/json"})
     assert resp.status_code == 404
+
+
+# --------------------------------------------------------------------------- #
+# SP35: parse-999 envelope regression lock
+# --------------------------------------------------------------------------- #
+#
+# The 999 parser already raises CycloneParseError("No AK9 (Functional Group
+# Response Status) segment found") when fed a non-999 file (parse_999.py
+# line 290). This regression lock confirms the HTTP surface converts that
+# error into a 400 (never 200, never 500) so a misroute upload fails loudly
+# instead of silently creating a corrupt ack row.
+
+
+def test_parse_999_endpoint_rejects_837_input(client: TestClient):
+    """Posting an 837P file to /api/parse-999 must surface 400, not 200.
+
+    Before SP35, the 999 parser's envelope guard (no AK9) was already
+    strict at the parser level. This test makes the HTTP contract
+    explicit: a wrong-kind file POSTed to the 999 endpoint MUST come
+    back as 400, not as 200 with an empty ack.
+    """
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_837p.txt"
+    text = wrong_kind.read_text()
+    assert "ST*837" in text  # sanity check on the fixture
+
+    resp = client.post(
+        "/api/parse-999",
+        files={"file": ("co_medicaid_837p.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert "error" in body
+    # The parser-level message must survive (not be replaced by a generic
+    # "Internal server error" or similar).
+    assert "AK9" in body.get("detail", "") or "Parse" in body["error"], body
+
+
+def test_parse_999_endpoint_rejects_835_input(client: TestClient):
+    """Posting an 835 file to /api/parse-999 must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_835.txt"
+    text = wrong_kind.read_text()
+    assert "ST*835" in text  # sanity check
+
+    resp = client.post(
+        "/api/parse-999",
+        files={"file": ("co_medicaid_835.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    assert "error" in resp.json()
diff --git a/backend/tests/test_api_ta1.py b/backend/tests/test_api_ta1.py
index b33fca8..91a139b 100644
--- a/backend/tests/test_api_ta1.py
+++ b/backend/tests/test_api_ta1.py
@@ -163,4 +163,54 @@ def test_list_ta1_acks_newest_first(client: TestClient):
     assert len(items) == 2
     # The REJECTED (uploaded second) is first.
     assert items[0]["ack_code"] == "R"
-    assert items[1]["ack_code"] == "A"
\ No newline at end of file
+    assert items[1]["ack_code"] == "A"
+
+# --------------------------------------------------------------------------- #
+# SP35: parse-ta1 envelope regression lock
+# --------------------------------------------------------------------------- #
+#
+# The TA1 parser already raises CycloneParseError("Expected TA1, got <other>")
+# when fed a file that doesn't have a TA1 segment as its first payload
+# segment (parse_ta1.py line 111). This regression lock confirms the HTTP
+# surface converts that error into a 400 (never 200, never 500). TA1 has
+# no ST envelope, so the test uses an 837 fixture (which has ISA + GS +
+# ST*837 but no TA1 segment) to exercise the parser-level guard.
+
+
+def test_parse_ta1_endpoint_rejects_837_input(client: TestClient):
+    """Posting an 837P file to /api/parse-ta1 must surface 400, not 200.
+
+    The TA1 envelope has no ST (it's the bare interchange-ack segment),
+    so the wrong-kind check is structural — the parser looks for the TA1
+    segment and raises when it doesn't find one. An 837 file has ISA +
+    GS + ST*837 but no TA1, which triggers that branch.
+    """
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_837p.txt"
+    text = wrong_kind.read_text()
+    assert "ST*837" in text  # sanity check on the fixture
+
+    resp = client.post(
+        "/api/parse-ta1",
+        files={"file": ("co_medicaid_837p.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert "error" in body
+    detail = body.get("detail", "")
+    assert "TA1" in detail or body["error"] == "Parse error", body
+
+
+def test_parse_ta1_endpoint_rejects_835_input(client: TestClient):
+    """Posting an 835 file to /api/parse-ta1 must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_835.txt"
+    text = wrong_kind.read_text()
+    assert "ST*835" in text  # sanity check
+
+    resp = client.post(
+        "/api/parse-ta1",
+        files={"file": ("co_medicaid_835.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    assert "error" in resp.json()