diff --git a/tests/test_checkpoint_integration.py b/tests/test_checkpoint_integration.py
new file mode 100644
index 000000000..09520d936
--- /dev/null
+++ b/tests/test_checkpoint_integration.py
@@ -0,0 +1,82 @@
+"""End-to-end test: write -> compress -> resume cycle."""
+import json
+import pytest
+import yaml
+from datetime import datetime, timedelta
+from unittest.mock import MagicMock
+from agent.checkpoint_store import CheckpointStore
+from agent.checkpoint_injection import build_checkpoint_system_prompt
+from tools.checkpoint_tool import checkpoint_tool
+
+
+@pytest.fixture
+def store(tmp_path):
+    return CheckpointStore(checkpoints_dir=tmp_path / "checkpoints")
+
+
+def test_full_cycle_write_compress_resume(store):
+    """Simulate: agent writes checkpoint -> new session gets injection."""
+    old_agent = MagicMock()
+    old_agent.session_id = "20260422_225800_abc123"
+    old_agent._todo_store = MagicMock()
+    old_agent._todo_store.format_for_injection.return_value = ""
+
+    checkpoint_tool(
+        action="write",
+        task="Red team QA on PR #14303",
+        progress=[
+            {"step": "Read code", "status": "completed"},
+            {"step": "Write tests", "status": "in_progress"},
+        ],
+        state={
+            "active_branch": "fix/delegation",
+            "tests_status": "19/19 passing",
+            "last_commit": "a90557d",
+            "pushed": True,
+            "working_directory": "/tmp/repo",
+        },
+        decisions=["SSRF is low-severity, doc note only"],
+        blocked=[],
+        unresolved=[],
+        store=store,
+        agent=old_agent,
+    )
+
+    # New session starts, checkpoint gets injected
+    prompt = build_checkpoint_system_prompt(
+        store=store,
+        parent_session_id="20260422_225800_abc123",
+    )
+
+    assert "Red team QA on PR #14303" in prompt
+    assert "Write tests" in prompt
+    assert "fix/delegation" in prompt
+    assert "SSRF is low-severity" in prompt
+
+
+def test_gc_removes_stale_checkpoints(store):
+    """GC removes checkpoints older than 7 days."""
+    mock_agent = MagicMock()
+    mock_agent.session_id = "old_sess"
+    mock_agent._todo_store = MagicMock()
+    mock_agent._todo_store.format_for_injection.return_value = ""
+
+    checkpoint_tool(
+        action="write",
+        task="Old task",
+        progress=[],
+        state={},
+        decisions=[],
+        store=store,
+        agent=mock_agent,
+    )
+    # Age the checkpoint by writing directly with an old timestamp
+    data = store.read("old_sess")
+    data["updated"] = (datetime.now() - timedelta(days=10)).isoformat()
+    store._path_for("old_sess").write_text(
+        yaml.dump(data, default_flow_style=False, allow_unicode=True, sort_keys=False),
+        encoding="utf-8",
+    )
+
+    removed = store.garbage_collect(max_age_days=7)
+    assert "old_sess" in removed
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