diff --git a/packages/cli/src/lablink_cli/commands/doctor.py b/packages/cli/src/lablink_cli/commands/doctor.py
index 5e9e30bef..d37e5d148 100644
--- a/packages/cli/src/lablink_cli/commands/doctor.py
+++ b/packages/cli/src/lablink_cli/commands/doctor.py
@@ -289,6 +289,44 @@ def _check_default_client_ami(region: str, published: str | None) -> dict:
     return result
 
 
+def _check_viewer_streaming(cfg) -> dict:
+    """Warn when the viewer will be served over plain HTTP.
+
+    KasmVNC's H.264 path needs the browser's WebCodecs VideoDecoder, which Chrome
+    exposes only on secure origins. With ssl.provider "none" the viewer is served
+    over http://, the codec probe reports "WebCodecs API not available", and every
+    session falls back to JPEG/WebP stills — re-encoding damaged regions from
+    scratch every frame instead of encoding inter-frame change.
+
+    Nothing else surfaces this. The deploy is green, sessions work, and the
+    server-side encoder is fine; it simply never gets asked for video mode. The
+    symptom reaches an operator as "the desktop feels laggy", which is a long way
+    from "ssl.provider is none".
+    """
+    result = {"check": "Viewer streaming", "status": "pass"}
+
+    if cfg is None:
+        result["status"] = "warn"
+        result["detail"] = "Skipped (no valid config)"
+        return result
+
+    provider = (getattr(getattr(cfg, "ssl", None), "provider", "") or "none").lower()
+
+    if provider == "none":
+        result["status"] = "warn"
+        result["detail"] = (
+            "ssl.provider is 'none', so the viewer is served over HTTP and H.264 "
+            "streaming cannot engage — sessions fall back to JPEG/WebP. Configure an "
+            "SSL provider, or to test as-is port-forward the allocator "
+            "(ssh -L 8443:localhost:5000 ...) and open http://localhost:8443, since "
+            "localhost counts as a secure origin"
+        )
+        return result
+
+    result["detail"] = f"H.264 available — viewer served over HTTPS ({provider})"
+    return result
+
+
 def _check_ami(cfg) -> dict:
     """Check that the configured client AMI exists in the configured region.
 
@@ -378,6 +416,9 @@ def _check_aws_prereqs() -> None:
     # 6. Client AMI
     checks.append(_check_ami(cfg))
 
+    # 7. Viewer streaming
+    checks.append(_check_viewer_streaming(cfg))
+
     _render_checks(
         checks,
         pass_message=(
diff --git a/packages/cli/tests/test_doctor.py b/packages/cli/tests/test_doctor.py
index 644b73d62..7c27f6f42 100644
--- a/packages/cli/tests/test_doctor.py
+++ b/packages/cli/tests/test_doctor.py
@@ -5,8 +5,11 @@
 import json
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 
 from lablink_cli.commands.doctor import (
+    _check_viewer_streaming,
     _check_ami,
     _check_opentofu,
 )
@@ -438,3 +441,42 @@ def test_renders_all_three_checks(self, capsys):
         assert "Registered" in out
         assert "Log shipper" in out
         assert "All checks passed" in out
+
+
+# ------------------------------------------------------------------
+# _check_viewer_streaming
+# ------------------------------------------------------------------
+def _ssl_cfg(provider):
+    cfg = MagicMock()
+    cfg.ssl.provider = provider
+    return cfg
+
+
+class TestCheckViewerStreaming:
+    def test_no_config(self):
+        result = _check_viewer_streaming(None)
+        assert result["status"] == "warn"
+
+    def test_http_deployment_warns_with_a_way_to_test(self):
+        """The whole point: an HTTP deployment silently loses H.264, and the
+        operator sees 'laggy desktop', not 'ssl.provider is none'."""
+        result = _check_viewer_streaming(_ssl_cfg("none"))
+        assert result["status"] == "warn"
+        assert "JPEG/WebP" in result["detail"]
+        assert "localhost" in result["detail"], "must name the port-forward workaround"
+
+    def test_warns_rather_than_fails(self):
+        """HTTP is a supported deployment, just a slower one — failing preflight
+        over a performance cliff would block a legitimate config."""
+        assert _check_viewer_streaming(_ssl_cfg("none"))["status"] != "fail"
+
+    @pytest.mark.parametrize("provider", ["letsencrypt", "cloudflare", "acm"])
+    def test_https_providers_pass(self, provider):
+        result = _check_viewer_streaming(_ssl_cfg(provider))
+        assert result["status"] == "pass"
+        assert provider in result["detail"]
+
+    def test_missing_ssl_section_is_treated_as_http(self):
+        cfg = MagicMock()
+        cfg.ssl.provider = None
+        assert _check_viewer_streaming(cfg)["status"] == "warn"