diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6096ce6..5bf2577 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -90,8 +90,11 @@ jobs:
       - name: Pin toolchain
         run: echo "${{ matrix.toolchain }}" > lean-toolchain
 
+      - name: Install uv (for the leanclient LSP round-trip test)
+        uses: astral-sh/setup-uv@v8.2.0
+
       - name: Build linter library
         run: lake build LeanExtensions
 
-      - name: Behavioral test suite (both registration paths + --wfail gate)
+      - name: Behavioral test suite (registration paths, --wfail gate, LSP round-trip)
         run: bash test/run.sh
diff --git a/test/lsp_diagnostics_test.py b/test/lsp_diagnostics_test.py
new file mode 100755
index 0000000..6cfe80f
--- /dev/null
+++ b/test/lsp_diagnostics_test.py
@@ -0,0 +1,86 @@
+#!/usr/bin/env -S uv run --script
+# /// script
+# requires-python = ">=3.11"
+# dependencies = ["leanclient>=0.10,<0.11"]
+# ///
+"""LSP round-trip test for the axiom guard.
+
+Verifies that the extension's diagnostics actually reach a language-server client: it points
+`leanclient` at the existing `test/consumer` project (which loads the guard via `plugins :=` with
+no import) and asserts the guard's "non-standard axiom" warning is published for the declaration
+that depends on the custom `evil` axiom — exactly what downstream tooling (e.g. lean4-toolkit's
+lsp_utils) parses instead of `#print axioms` injection.
+
+Reuses the consumer fixtures already tested by run.sh — no new project. The LSP server elaborates
+*source* itself, but it cannot load a *plugin* whose shared library has not been built yet (the
+dynlib is a build artifact); without it the file fatals before any diagnostic is published. So this
+test ensures the consumer is built once (which fetches the dep and compiles the guard's dynlib)
+before opening the client.
+
+Run:  test/lsp_diagnostics_test.py     (PEP-723 inline deps; `uv run --script` provides leanclient)
+Exit: 0 on success, 1 on assertion failure.
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+from leanclient import LeanLSPClient
+
+HERE = Path(__file__).resolve().parent
+CONSUMER = HERE / "consumer"
+
+# Distinctive substrings emitted by LeanExtensions/AxiomGuard.lean.
+GUARD_MARKER = "non-standard axiom"
+# The consumer's fixtures (test/consumer/Consumer/Defs.lean + Consumer.lean): `evil`/`benign` are
+# declared axioms; `uses_evil` depends on `evil`.
+EXPECT_AXIOM = "evil"
+
+
+def diagnostics_for(client: LeanLSPClient, rel: str):
+    # Drain initial elaboration, then read the settled diagnostic set.
+    client.get_diagnostics(rel, inactivity_timeout=10)
+    return client.get_diagnostics(rel, inactivity_timeout=15).diagnostics
+
+
+def main() -> int:
+    if not (CONSUMER / "lakefile.lean").exists():
+        print(f"FAIL: consumer project missing at {CONSUMER}")
+        return 1
+
+    # Plugin loading needs the guard's shared library compiled; build once if absent. This also
+    # fetches the lean_extensions dependency. (The LSP elaborates source on its own, but cannot
+    # load a not-yet-built plugin.)
+    print("Building consumer (fetches dep + compiles the guard plugin)...")
+    subprocess.run(["lake", "build"], cwd=CONSUMER, check=True)
+
+    client = LeanLSPClient(str(CONSUMER), max_opened_files=4)
+    failures = []
+
+    # Consumer.lean depends on `evil` (no import of the guard — loaded via plugins:=).
+    consumer_diags = diagnostics_for(client, "Consumer.lean")
+    consumer_msgs = [d.get("message", "") for d in consumer_diags]
+    guard_hits = [m for m in consumer_msgs if GUARD_MARKER in m]
+
+    if not guard_hits:
+        failures.append(
+            f"expected a '{GUARD_MARKER}' diagnostic in Consumer.lean (guard loaded via plugins:=); "
+            f"got messages: {consumer_msgs}"
+        )
+    elif not any(EXPECT_AXIOM in m for m in guard_hits):
+        failures.append(
+            f"guard fired but did not name '{EXPECT_AXIOM}'; guard diagnostics: {guard_hits}"
+        )
+
+    if failures:
+        print("FAIL: LSP round-trip assertions failed:")
+        for f in failures:
+            print("  -", f)
+        return 1
+
+    print(f"PASS: guard '{GUARD_MARKER}' diagnostic published via LSP for Consumer.lean: {guard_hits}")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/test/run.sh b/test/run.sh
index ec04f88..aa96dc4 100755
--- a/test/run.sh
+++ b/test/run.sh
@@ -74,6 +74,20 @@ else
   ok "consumer under --wfail fails the build"
 fi
 
+note ""
+note "== LSP round-trip (guard diagnostics reach a language-server client) =="
+# Requires `uv` (for leanclient). Skip with a notice if uv is unavailable rather than failing the
+# whole suite on a missing optional tool.
+if command -v uv >/dev/null 2>&1; then
+  if test/lsp_diagnostics_test.py; then
+    ok "LSP round-trip: guard diagnostic published to leanclient"
+  else
+    bad "LSP round-trip test failed"
+  fi
+else
+  note "SKIP  LSP round-trip (uv not installed)"
+fi
+
 note ""
 note "== $pass passed, $fail failed =="
 [ "$fail" -eq 0 ]