diff --git a/scripts/ci/current_head_run_coalescer.py b/scripts/ci/current_head_run_coalescer.py
index 778ba4d8c3..ae40b85ac4 100644
--- a/scripts/ci/current_head_run_coalescer.py
+++ b/scripts/ci/current_head_run_coalescer.py
@@ -15,6 +15,7 @@
 import os
 import re
 import subprocess
+import time
 from typing import Any, Iterable, Mapping, Sequence
 from urllib.parse import urlsplit
 
@@ -26,6 +27,8 @@
 PR_EVENTS = frozenset({"pull_request", "pull_request_target"})
 ACTIVE_STATUSES = ("queued", "in_progress")
 API_TIMEOUT_SECONDS = 30
+CANCELLATION_POLL_ATTEMPTS = 6
+CANCELLATION_POLL_INTERVAL_SECONDS = 1.0
 
 
 class CoalescingRefused(RuntimeError):
@@ -369,8 +372,15 @@ def _fetch_run(repo: str, run_id: int) -> dict[str, Any]:
 
 
 def _cancel_run(repo: str, run_id: int) -> None:
-    """Request ordinary cancellation using the same explicit token/timeout contract."""
+    """Cancel one run and prove GitHub reached its terminal cancelled state."""
     _run_json(["gh", "api", "-X", "POST", f"repos/{repo}/actions/runs/{run_id}/cancel"])
+    for attempt in range(CANCELLATION_POLL_ATTEMPTS):
+        run_data = _fetch_run(repo, run_id)
+        if run_data.get("status") == "completed" and run_data.get("conclusion") == "cancelled":
+            return
+        if attempt + 1 < CANCELLATION_POLL_ATTEMPTS:
+            time.sleep(CANCELLATION_POLL_INTERVAL_SECONDS)
+    raise RuntimeError(f"workflow run {run_id} did not reach completed/cancelled")
 
 
 def _associated_prs(
diff --git a/tests/test_current_head_run_coalescer.py b/tests/test_current_head_run_coalescer.py
index eaece3f4f3..571368677f 100644
--- a/tests/test_current_head_run_coalescer.py
+++ b/tests/test_current_head_run_coalescer.py
@@ -454,13 +454,38 @@ def pages(args):
 
 
 def test_cancel_run_uses_explicit_transport_and_ordinary_endpoint(monkeypatch) -> None:
-    """Cancellation shares the token/timeout transport and never uses force-cancel."""
+    """Cancellation uses the ordinary endpoint and proves terminal state."""
     module = load_module()
     calls: list[list[str]] = []
     monkeypatch.setattr(module, "_run_json", lambda args: calls.append(list(args)))
+    states = iter(
+        [
+            {"status": "in_progress", "conclusion": None},
+            {"status": "completed", "conclusion": "cancelled"},
+        ]
+    )
+    monkeypatch.setattr(module, "_fetch_run", lambda _repo, _run_id: next(states))
+    sleeps: list[float] = []
+    monkeypatch.setattr(module.time, "sleep", sleeps.append)
     module._cancel_run("o/r", 123)
     assert calls == [["gh", "api", "-X", "POST", "repos/o/r/actions/runs/123/cancel"]]
     assert "force-cancel" not in " ".join(calls[0])
+    assert sleeps == [module.CANCELLATION_POLL_INTERVAL_SECONDS]
+
+
+def test_cancel_run_fails_when_terminal_cancellation_is_unproven(monkeypatch) -> None:
+    """An accepted cancellation is not reported complete while GitHub stays active."""
+    module = load_module()
+    monkeypatch.setattr(module, "_run_json", lambda _args: None)
+    monkeypatch.setattr(
+        module,
+        "_fetch_run",
+        lambda _repo, _run_id: {"status": "in_progress", "conclusion": None},
+    )
+    monkeypatch.setattr(module.time, "sleep", lambda _seconds: None)
+
+    with pytest.raises(RuntimeError, match="did not reach completed/cancelled"):
+        module._cancel_run("o/r", 123)
 
 
 def test_associated_pr_fetches_only_same_head_noncurrent_numbers(monkeypatch) -> None:
@@ -568,7 +593,11 @@ def test_coalesce_cancels_only_revalidated_redundant_candidates(monkeypatch, cap
     sibling = run_record(101, 10)
     monkeypatch.setattr(module, "_fetch_pr", lambda *_args: live_pr())
     monkeypatch.setattr(module, "_active_runs", lambda *_args: [candidate, sibling])
-    monkeypatch.setattr(module, "_fetch_run", lambda _repo, run_id: sibling if run_id == 101 else candidate)
+    monkeypatch.setattr(
+        module,
+        "_fetch_run",
+        lambda _repo, run_id: sibling if run_id == 101 else candidate,
+    )
     cancelled: list[int] = []
     monkeypatch.setattr(module, "_cancel_run", lambda _repo, run_id: cancelled.append(run_id))
     assert module.coalesce("ContextualWisdomLab/.github", 1, "ContextualWisdomLab/.github", "feature/current", "a" * 40) == [100]
@@ -576,6 +605,33 @@ def test_coalesce_cancels_only_revalidated_redundant_candidates(monkeypatch, cap
     assert "Cancelled redundant queued current-head run 100" in capsys.readouterr().out
 
 
+def test_coalesce_fails_before_reporting_unproven_cancellation(monkeypatch, capsys) -> None:
+    """A cancellation that never reaches terminal state must not be reported."""
+    module = load_module()
+    candidate = run_record(100, 10)
+    sibling = run_record(101, 10)
+    monkeypatch.setattr(module, "_fetch_pr", lambda *_args: live_pr())
+    monkeypatch.setattr(module, "_active_runs", lambda *_args: [candidate, sibling])
+    monkeypatch.setattr(module, "_fetch_run", lambda _repo, run_id: sibling if run_id == 101 else candidate)
+    monkeypatch.setattr(
+        module,
+        "_cancel_run",
+        lambda _repo, _run_id: (_ for _ in ()).throw(
+            RuntimeError("terminal cancellation unproven")
+        ),
+    )
+
+    with pytest.raises(RuntimeError, match="terminal cancellation unproven"):
+        module.coalesce(
+            "ContextualWisdomLab/.github",
+            1,
+            "ContextualWisdomLab/.github",
+            "feature/current",
+            "a" * 40,
+        )
+    assert "Cancelled redundant" not in capsys.readouterr().out
+
+
 def test_parse_args_main_and_script_help(monkeypatch) -> None:
     """CLI parsing forwards exact identity and the executable entrypoint is reachable."""
     module = load_module()
diff --git a/tests/test_current_head_run_coalescer_review_regressions.py b/tests/test_current_head_run_coalescer_review_regressions.py
index 477dbabba0..3f10ab3165 100644
--- a/tests/test_current_head_run_coalescer_review_regressions.py
+++ b/tests/test_current_head_run_coalescer_review_regressions.py
@@ -289,13 +289,19 @@ def test_transport_is_token_bound_and_individually_timeout_bounded(monkeypatch)
 
     def success(args, **kwargs):
         calls.append((list(args), dict(kwargs)))
-        stdout = "{}" if "/cancel" not in " ".join(args) else ""
+        command = " ".join(args)
+        if "/cancel" in command:
+            stdout = ""
+        elif command.endswith("actions/runs/123"):
+            stdout = '{"status":"completed","conclusion":"cancelled"}'
+        else:
+            stdout = "{}"
         return SimpleNamespace(returncode=0, stdout=stdout, stderr="")
 
     monkeypatch.setattr(module.subprocess, "run", success)
     assert module._run_json(["gh", "api", "repos/owner/repo"]) == {}
     module._cancel_run("owner/repo", 123)
-    assert len(calls) == 2
+    assert len(calls) == 3
     assert all(call_kwargs.get("timeout") == module.API_TIMEOUT_SECONDS for _, call_kwargs in calls)