diff --git a/tests/test_issue3103_sse_no_connection_close.py b/tests/test_issue3103_sse_no_connection_close.py
new file mode 100644
index 00000000..1fcd86cc
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+++ b/tests/test_issue3103_sse_no_connection_close.py
@@ -0,0 +1,60 @@
+"""Regression test for #3103 / PR #3128.
+
+Commit 598fd4ff added `Connection: close` to the two long-lived SSE handlers
+(`_handle_gateway_sse_stream`, `_handle_session_events_stream`). Browsers treat
+`Connection: close` on an SSE response as a signal that the EventSource lifecycle
+has ended and auto-reconnect, producing a reconnect storm that thrashes the
+session list (#3103). These handlers must NOT emit `Connection: close`.
+
+Finite responses (the on-the-fly ZIP download, terminal SSE, etc.) still need the
+header for unambiguous HTTP/1.1 message boundaries — this test only pins the two
+long-lived session/gateway event streams.
+"""
+
+import re
+from pathlib import Path
+
+ROUTES = (Path(__file__).resolve().parents[1] / "api" / "routes.py").read_text(encoding="utf-8")
+
+
+def _handler_body(name: str) -> str:
+    """Return the source of a handler function from its `def` to the next top-level `def`."""
+    start = ROUTES.index(f"def {name}(")
+    # find the next top-level "def " after the function start
+    nxt = ROUTES.find("\ndef ", start + 1)
+    return ROUTES[start: nxt if nxt != -1 else len(ROUTES)]
+
+
+def _emits_connection_close(body: str) -> bool:
+    """True if the handler actually CALLS send_header to emit Connection: close
+    (ignores mentions in comments)."""
+    return bool(
+        re.search(r"send_header\(\s*['\"]Connection['\"]\s*,\s*['\"]close['\"]", body)
+    )
+
+
+def test_session_events_stream_does_not_emit_connection_close():
+    body = _handler_body("_handle_session_events_stream")
+    assert "text/event-stream" in body, "sanity: this is the SSE handler"
+    assert not _emits_connection_close(body), (
+        "_handle_session_events_stream must not emit `Connection: close` — it triggers "
+        "browser EventSource reconnect storms on this long-lived stream (#3103)"
+    )
+
+
+def test_gateway_sse_stream_does_not_emit_connection_close():
+    body = _handler_body("_handle_gateway_sse_stream")
+    assert "text/event-stream" in body, "sanity: this is the SSE handler"
+    assert not _emits_connection_close(body), (
+        "_handle_gateway_sse_stream must not emit `Connection: close` — it triggers "
+        "browser EventSource reconnect storms on this long-lived stream (#3103)"
+    )
+
+
+def test_connection_close_still_present_for_finite_zip_download():
+    """The on-the-fly ZIP download (no Content-Length) still needs Connection: close
+    for unambiguous HTTP/1.1 framing (#2836). Guard against an over-broad removal."""
+    assert ROUTES.count('"Connection", "close"') + ROUTES.count("'Connection', 'close'") >= 1, (
+        "at least the finite-response handlers (ZIP download, etc.) must keep "
+        "`Connection: close` for HTTP/1.1 message-boundary correctness (#2836)"
+    )