diff --git a/src/fenris/collector.py b/src/fenris/collector.py
index 0317c61..591789b 100644
--- a/src/fenris/collector.py
+++ b/src/fenris/collector.py
@@ -294,6 +294,13 @@ def run_collection(
         # Initialize store if needed
         store_path = get_store_path(config)
         conn = init_store(store_path)
+
+        # Run legacy import if needed (idempotent)
+        from .legacy import import_legacy_history
+        history_path = Path(config.get("data_dir", ".")) / "history.jsonl"
+        if history_path.exists():
+            import_legacy_history(conn, history_path, clock=clock)
+
         
         try:
             # Validate invariants
diff --git a/src/fenris/legacy.py b/src/fenris/legacy.py
new file mode 100644
index 0000000..2fbd3dd
--- /dev/null
+++ b/src/fenris/legacy.py
@@ -0,0 +1,440 @@
+"""Legacy migration: import history.jsonl into the observation store.
+
+Spec §3.5, ADR 0001 §6. Idempotent and interruption-safe.
+
+Entry points:
+- Installer import detection at ./data/history.jsonl (§10.1)
+- fenris import <path> (§8.8)
+- Collector's first new-version run (ADR 0001 §6)
+
+The import is a single transaction — a scripted kill mid-import leaves the
+store fully pre- or fully post-migration. history.jsonl is the sole authority;
+hourly.jsonl is diffed and logged but never trusted. Malformed lines are
+quarantined with a logged count, never silently dropped. Legacy files are
+renamed *.migrated only after commit and never deleted.
+"""
+import json
+import logging
+import sqlite3
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
+
+from .hour_classify import classify_hour
+from .segment import open_segment, normalize_identity
+from .monitoring_periods import close_period
+
+logger = logging.getLogger(__name__)
+
+# Legacy import marker — stored in a metadata table
+LEGACY_IMPORT_MARKER = "legacy_imported"
+
+
+def _ensure_metadata_table(conn: sqlite3.Connection) -> None:
+    """Create the metadata table if it doesn't exist."""
+    conn.execute("""
+        CREATE TABLE IF NOT EXISTS store_metadata (
+            key TEXT PRIMARY KEY,
+            value TEXT NOT NULL
+        )
+    """)
+
+
+def is_legacy_imported(conn: sqlite3.Connection) -> bool:
+    """Check if legacy history has already been imported.
+    
+    Spec §3.5.1: If the store already carries the legacy-import marker, do nothing.
+    """
+    _ensure_metadata_table(conn)
+    cursor = conn.execute(
+        "SELECT value FROM store_metadata WHERE key = ?",
+        (LEGACY_IMPORT_MARKER,),
+    )
+    row = cursor.fetchone()
+    return row is not None and row[0] == "true"
+
+
+def _parse_history_line(line: str, line_num: int) -> Optional[Dict[str, Any]]:
+    """Parse a single line from history.jsonl.
+    
+    Returns None for malformed lines (quarantined, not silently dropped).
+    """
+    line = line.strip()
+    if not line:
+        return None
+    
+    try:
+        record = json.loads(line)
+    except json.JSONDecodeError as e:
+        logger.warning("Malformed JSON at line %d: %s", line_num, e)
+        return None
+    
+    # Validate required fields
+    required_fields = [
+        "timestamp", "model", "serial", "firmware_version",
+        "data_units_written", "data_units_read", "percentage_used",
+        "power_on_hours", "temperature",
+    ]
+    
+    for field in required_fields:
+        if field not in record:
+            logger.warning("Missing field '%s' at line %d", field, line_num)
+            return None
+    
+    return record
+
+
+def _record_to_sample(record: Dict[str, Any]) -> Dict[str, Any]:
+    """Convert a legacy history.jsonl record to a sample dict."""
+    # Legacy records use different field names
+    duw = record["data_units_written"]
+    dur = record["data_units_read"]
+    
+    return {
+        "ts": record["timestamp"],
+        "device": record.get("device", "/dev/nvme0"),
+        "subnqn": record.get("subsystem_nqn", ""),
+        "sn": record["serial"],
+        "mn": record["model"],
+        "fr": record["firmware_version"],
+        "capacity_bytes": record.get("capacity_bytes", 0),
+        "percentage_used": record["percentage_used"],
+        "available_spare": record.get("available_spare"),
+        "media_errors": record.get("media_errors", 0),
+        "power_on_hours": record["power_on_hours"],
+        "power_cycles": record.get("power_cycles"),
+        "unsafe_shutdowns": record.get("unsafe_shutdowns"),
+        "temperature_c": record["temperature"],
+        "data_units_written": duw,
+        "data_units_read": dur,
+        "bytes_written": duw * 512000,
+        "bytes_read": dur * 512000,
+        "critical_warning": record.get("critical_warning", 0),
+    }
+
+
+def _derive_hour_observation(
+    samples: List[Dict[str, Any]],
+    hour_start: datetime,
+) -> Dict[str, Any]:
+    """Derive a single hour observation from samples in that hour.
+    
+    Pre-migration hours carry an unknown activity split except directly
+    evidenced facts — a sample present means powered on; a DUW delta means
+    writes occurred (§3.5.4).
+    """
+    hour_end = hour_start.replace(hour=hour_start.hour + 1) if hour_start.hour < 23 else                hour_start.replace(hour=0, day=hour_start.day + 1)
+    
+    # Filter samples in this hour
+    hour_samples = []
+    for s in samples:
+        ts = datetime.fromisoformat(s["ts"])
+        if hour_start <= ts < hour_end:
+            hour_samples.append(s)
+    
+    if not hour_samples:
+        return None
+    
+    # Sort by timestamp
+    hour_samples.sort(key=lambda x: x["ts"])
+    
+    # Compute deltas from first to last sample in the hour
+    first = hour_samples[0]
+    last = hour_samples[-1]
+    
+    duw_delta = last["bytes_written"] - first["bytes_written"]
+    dur_delta = last["bytes_read"] - first["bytes_read"]
+    poh_delta = (last["power_on_hours"] - first["power_on_hours"]) * 3600
+    
+    # Temperature stats
+    temps = [s["temperature_c"] for s in hour_samples]
+    
+    # Classify the hour
+    split = classify_hour(
+        wall_clock_seconds=3600,
+        poh_delta=poh_delta,
+        duw_delta=duw_delta,
+        dur_delta=dur_delta,
+        sampled_seconds=3600,  # Legacy samples cover the full hour
+    )
+    
+    return {
+        "hour": hour_start.strftime("%Y-%m-%dT%H:00:00Z"),
+        "active_seconds": split.seconds_active,
+        "idle_seconds": split.seconds_idle,
+        "powered_off_seconds": split.seconds_powered_off,
+        "unknown_seconds": split.seconds_unknown,
+        "bytes_written_delta": duw_delta,
+        "bytes_read_delta": dur_delta,
+        "temperature_min": min(temps),
+        "temperature_avg": sum(temps) / len(temps),
+        "temperature_max": max(temps),
+        "sample_count": len(hour_samples),
+        "coverage": 1.0 if split.seconds_unknown == 0 else (3600 - split.seconds_unknown) / 3600,
+    }
+
+
+def _diff_hourly_jsonl(
+    hourly_path: Path,
+    derived_hours: Dict[str, Dict[str, Any]],
+) -> None:
+    """Diff hourly.jsonl against derived data and log mismatches.
+    
+    Spec §3.5.3: hourly.jsonl is never trusted; mismatches are diffed and logged.
+    """
+    if not hourly_path.exists():
+        logger.info("No hourly.jsonl found for diffing")
+        return
+    
+    try:
+        with open(hourly_path, "r") as f:
+            for line_num, line in enumerate(f, 1):
+                line = line.strip()
+                if not line:
+                    continue
+                
+                try:
+                    record = json.loads(line)
+                except json.JSONDecodeError:
+                    logger.warning("Malformed hourly.jsonl at line %d", line_num)
+                    continue
+                
+                hour_key = record.get("hour")
+                if hour_key not in derived_hours:
+                    logger.info("Hourly.jsonl has hour %s not in derived data", hour_key)
+                    continue
+                
+                derived = derived_hours[hour_key]
+                mismatches = []
+                
+                for field in ["bytes_written_delta", "bytes_read_delta", "sample_count"]:
+                    if field in record and record[field] != derived.get(field):
+                        mismatches.append(
+                            f"{field}: hourly={record[field]} derived={derived.get(field)}"
+                        )
+                
+                if mismatches:
+                    logger.info(
+                        "Hourly.jsonl mismatch for %s: %s",
+                        hour_key,
+                        "; ".join(mismatches),
+                    )
+    except Exception as e:
+        logger.warning("Failed to diff hourly.jsonl: %s", e)
+
+
+def import_legacy_history(
+    conn: sqlite3.Connection,
+    history_path: Path,
+    hourly_path: Optional[Path] = None,
+    clock=None,
+) -> Dict[str, Any]:
+    """Import legacy history.jsonl into the observation store.
+    
+    This is the main entry point for legacy migration. It is:
+    - Idempotent: second run no-ops on the legacy-import marker
+    - Interruption-safe: single transaction
+    - Never creates synthetic baselines
+    
+    Args:
+        conn: Connection to the observation store
+        history_path: Path to history.jsonl
+        hourly_path: Optional path to hourly.jsonl for diffing
+        clock: Injected clock (for testing)
+    
+    Returns:
+        Dict with migration outcome
+    """
+    # Check idempotency
+    if is_legacy_imported(conn):
+        return {"ok": True, "skipped": True, "reason": "already_imported"}
+    
+    # Read and parse history.jsonl
+    samples = []
+    malformed_count = 0
+    
+    if not history_path.exists():
+        return {"ok": False, "error": f"History file not found: {history_path}"}
+    
+    with open(history_path, "r") as f:
+        for line_num, line in enumerate(f, 1):
+            record = _parse_history_line(line, line_num)
+            if record is None:
+                malformed_count += 1
+                continue
+            samples.append(_record_to_sample(record))
+    
+    if malformed_count > 0:
+        logger.warning("Quarantined %d malformed lines from history.jsonl", malformed_count)
+    
+    if not samples:
+        return {"ok": False, "error": "No valid samples found in history.jsonl"}
+    
+    # Sort samples by timestamp
+    samples.sort(key=lambda x: x["ts"])
+    
+    # Derive hour observations
+    hour_observations = {}
+    for sample in samples:
+        ts = datetime.fromisoformat(sample["ts"])
+        hour_start = ts.replace(minute=0, second=0, microsecond=0)
+        hour_key = hour_start.strftime("%Y-%m-%dT%H:00:00Z")
+        
+        if hour_key not in hour_observations:
+            hour_observations[hour_key] = {
+                "hour": hour_start,
+                "samples": [],
+            }
+        hour_observations[hour_key]["samples"].append(sample)
+    
+    derived_hours = {}
+    for hour_key, hour_data in hour_observations.items():
+        obs = _derive_hour_observation(hour_data["samples"], hour_data["hour"])
+        if obs is not None:
+            derived_hours[hour_key] = obs
+    
+    # Diff against hourly.jsonl if provided
+    if hourly_path:
+        _diff_hourly_jsonl(hourly_path, derived_hours)
+    
+    # Single transaction for the entire import
+    try:
+        # Begin transaction
+        conn.execute("BEGIN IMMEDIATE")
+        
+        # 1. Insert raw samples
+        for sample in samples:
+            conn.execute(
+                """
+                INSERT INTO samples (
+                    ts, device, subnqn, sn, mn, fr, capacity_bytes,
+                    percentage_used, available_spare, media_errors, power_on_hours,
+                    power_cycles, unsafe_shutdowns, temperature_c,
+                    data_units_written, data_units_read, bytes_written, bytes_read,
+                    critical_warning
+                ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+                """,
+                (
+                    sample["ts"],
+                    sample["device"],
+                    sample["subnqn"],
+                    sample["sn"],
+                    sample["mn"],
+                    sample["fr"],
+                    sample["capacity_bytes"],
+                    sample["percentage_used"],
+                    sample["available_spare"],
+                    sample["media_errors"],
+                    sample["power_on_hours"],
+                    sample["power_cycles"],
+                    sample["unsafe_shutdowns"],
+                    sample["temperature_c"],
+                    sample["data_units_written"],
+                    sample["data_units_read"],
+                    sample["bytes_written"],
+                    sample["bytes_read"],
+                    sample["critical_warning"],
+                ),
+            )
+        
+        # 2. Insert derived hour observations
+        for hour_key, obs in sorted(derived_hours.items()):
+            conn.execute(
+                """
+                INSERT OR REPLACE INTO hour_observations (
+                    hour, active_seconds, idle_seconds, powered_off_seconds,
+                    unknown_seconds, bytes_written_delta, bytes_read_delta,
+                    temperature_min, temperature_avg, temperature_max,
+                    sample_count, coverage
+                ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+                """,
+                (
+                    obs["hour"],
+                    obs["active_seconds"],
+                    obs["idle_seconds"],
+                    obs["powered_off_seconds"],
+                    obs["unknown_seconds"],
+                    obs["bytes_written_delta"],
+                    obs["bytes_read_delta"],
+                    obs["temperature_min"],
+                    obs["temperature_avg"],
+                    obs["temperature_max"],
+                    obs["sample_count"],
+                    obs["coverage"],
+                ),
+            )
+        
+        # 3. Open implicit monitoring period at first legacy sample
+        first_sample_ts = datetime.fromisoformat(samples[0]["ts"])
+        conn.execute(
+            "INSERT INTO monitoring_periods (started_at) VALUES (?)",
+            (first_sample_ts.isoformat(),),
+        )
+        
+        # 4. Close period with end_cause = migrated at migration moment
+        if clock:
+            migration_time = clock.utcnow()
+        else:
+            migration_time = datetime.now(timezone.utc)
+        
+        conn.execute(
+            "UPDATE monitoring_periods SET ended_at = ?, end_cause = ? WHERE ended_at IS NULL",
+            (migration_time.isoformat(), "migrated"),
+        )
+        
+        # 5. Open legacy controller segment (mn-only)
+        # Legacy identity is model-scoped only (§4.4)
+        first_sample = samples[0]
+        legacy_identity = {
+            "mn": first_sample["mn"],
+            "sn": "",  # Legacy segments are mn-only
+            "subnqn": "",
+            "fr": "",
+        }
+        legacy_identity_key = f"legacy|{first_sample['mn']}"
+        open_segment(
+            conn,
+            migration_time,
+            legacy_identity,
+            legacy_identity_key,
+            identity_degraded=False,
+        )
+        
+        # 6. Set legacy import marker
+        _ensure_metadata_table(conn)
+        conn.execute(
+            "INSERT OR REPLACE INTO store_metadata (key, value) VALUES (?, ?)",
+            (LEGACY_IMPORT_MARKER, "true"),
+        )
+        
+        # Commit
+        conn.commit()
+        
+    except Exception as e:
+        conn.rollback()
+        raise RuntimeError(f"Migration failed: {e}") from e
+    
+    # 7. Rename legacy files to *.migrated (only after commit)
+    try:
+        migrated_path = history_path.with_suffix(history_path.suffix + ".migrated")
+        history_path.rename(migrated_path)
+        logger.info("Renamed %s to %s", history_path, migrated_path)
+        
+        if hourly_path and hourly_path.exists():
+            hourly_migrated = hourly_path.with_suffix(hourly_path.suffix + ".migrated")
+            hourly_path.rename(hourly_migrated)
+            logger.info("Renamed %s to %s", hourly_path, hourly_migrated)
+    except Exception as e:
+        # Non-fatal: files weren't renamed but migration succeeded
+        logger.warning("Failed to rename legacy files: %s", e)
+    
+    return {
+        "ok": True,
+        "skipped": False,
+        "samples_imported": len(samples),
+        "hours_imported": len(derived_hours),
+        "malformed_lines": malformed_count,
+        "first_sample": samples[0]["ts"],
+        "last_sample": samples[-1]["ts"],
+        "legacy_identity_key": legacy_identity_key,
+    }
diff --git a/src/fenris/store.py b/src/fenris/store.py
index 3c3b4a0..e80cb78 100644
--- a/src/fenris/store.py
+++ b/src/fenris/store.py
@@ -166,6 +166,15 @@ def _create_schema(conn: sqlite3.Connection):
     """)
 
 
+    # Metadata table for store state (e.g., legacy import marker)
+    conn.execute("""
+        CREATE TABLE IF NOT EXISTS store_metadata (
+            key TEXT PRIMARY KEY,
+            value TEXT NOT NULL
+        )
+    """)
+
+
 def _apply_migrations(conn: sqlite3.Connection, current_version: int):
     """Apply forward-only migrations from current_version to SCHEMA_VERSION."""
     # Future migrations will go here
diff --git a/tests/test_collector_tracer.py b/tests/test_collector_tracer.py
index f0c758d..a42566d 100644
--- a/tests/test_collector_tracer.py
+++ b/tests/test_collector_tracer.py
@@ -247,6 +247,7 @@ def test_store_initialization(config_fixture: Dict[str, Any]):
     
     # sqlite_sequence is a system table created by AUTOINCREMENT
     expected_tables.add("sqlite_sequence")
+    expected_tables.add("store_metadata")
     assert expected_tables == tables
     
     conn.close()
diff --git a/tests/test_legacy_migration.py b/tests/test_legacy_migration.py
new file mode 100644
index 0000000..037b2bc
--- /dev/null
+++ b/tests/test_legacy_migration.py
@@ -0,0 +1,404 @@
+"""Legacy migration tests.
+
+Tests the idempotent, interruption-safe import of history.jsonl into the
+observation store.
+"""
+import json
+import os
+import sqlite3
+import tempfile
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Any, Dict
+
+import pytest
+
+# Add src to path for imports
+import sys
+sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
+
+from fenris.legacy import import_legacy_history, is_legacy_imported, _parse_history_line
+from fenris.store import init_store
+
+
+# Fixtures
+
+@pytest.fixture
+def history_fixture() -> str:
+    """Minimal history.jsonl content with two samples."""
+    samples = [
+        {
+            "timestamp": "2026-08-30T10:00:00Z",
+            "device": "/dev/nvme0",
+            "model": "Samsung SSD 970 EVO Plus 1TB",
+            "serial": "S4EWNX0N123456",
+            "firmware_version": "2B2QEXM7",
+            "capacity_bytes": 1024000000000,
+            "data_units_written": 1000000,
+            "data_units_read": 500000,
+            "percentage_used": 5,
+            "power_on_hours": 8765,
+            "temperature": 35,
+            "available_spare": 100,
+            "media_errors": 0,
+            "power_cycles": 1234,
+            "unsafe_shutdowns": 5,
+            "critical_warning": 0,
+        },
+        {
+            "timestamp": "2026-08-30T11:00:00Z",
+            "device": "/dev/nvme0",
+            "model": "Samsung SSD 970 EVO Plus 1TB",
+            "serial": "S4EWNX0N123456",
+            "firmware_version": "2B2QEXM7",
+            "capacity_bytes": 1024000000000,
+            "data_units_written": 1001000,
+            "data_units_read": 501000,
+            "percentage_used": 5,
+            "power_on_hours": 8766,
+            "temperature": 36,
+            "available_spare": 100,
+            "media_errors": 0,
+            "power_cycles": 1234,
+            "unsafe_shutdowns": 5,
+            "critical_warning": 0,
+        },
+    ]
+    return "\n".join(json.dumps(s) for s in samples)
+
+
+@pytest.fixture
+def hourly_fixture() -> str:
+    """Minimal hourly.jsonl content for diffing."""
+    hours = [
+        {
+            "hour": "2026-08-30T10:00:00Z",
+            "bytes_written_delta": 512000000,
+            "bytes_read_delta": 256000000,
+            "sample_count": 1,
+        },
+        {
+            "hour": "2026-08-30T11:00:00Z",
+            "bytes_written_delta": 513000000,
+            "bytes_read_delta": 257000000,
+            "sample_count": 1,
+        },
+    ]
+    return "\n".join(json.dumps(h) for h in hours)
+
+
+@pytest.fixture
+def config_fixture(tmp_path: Path) -> Dict[str, Any]:
+    """Configuration fixture."""
+    return {
+        "device": "/dev/nvme0",
+        "store_path": str(tmp_path / "observations.db"),
+        "data_dir": str(tmp_path),
+    }
+
+
+@pytest.fixture
+def clock_fixture():
+    """Injected clock returning fixed time."""
+    class FakeClock:
+        def __init__(self):
+            self.now = datetime(2026, 9, 1, 12, 0, 0, tzinfo=timezone.utc)
+        
+        def utcnow(self):
+            return self.now
+    
+    return FakeClock()
+
+
+# Test: Idempotency - second run no-ops
+
+def test_import_idempotent(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given a store with legacy import marker,
+    when import is called again,
+    then it no-ops."""
+    # Create history file
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # First import
+    result1 = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result1["ok"] is True
+    assert result1["skipped"] is False
+    
+    # Second import (should no-op)
+    history_path2 = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path2.write_text(history_fixture)
+    result2 = import_legacy_history(conn, history_path2, clock=clock_fixture)
+    assert result2["ok"] is True
+    assert result2["skipped"] is True
+    
+    conn.close()
+
+
+# Test: Interruption safety - single transaction
+
+def test_import_single_transaction(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl,
+    when import runs,
+    then the entire import is a single transaction."""
+    # Create history file
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result["ok"] is True
+    
+    # Verify all data was imported atomically
+    cursor = conn.execute("SELECT COUNT(*) FROM samples")
+    assert cursor.fetchone()[0] == 2
+    
+    cursor = conn.execute("SELECT COUNT(*) FROM hour_observations")
+    assert cursor.fetchone()[0] == 2
+    
+    cursor = conn.execute("SELECT COUNT(*) FROM monitoring_periods")
+    assert cursor.fetchone()[0] == 1
+    
+    conn.close()
+
+
+# Test: Legacy files renamed to *.migrated after commit
+
+def test_legacy_files_renamed(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl and hourly.jsonl,
+    when import commits,
+    then files are renamed to *.migrated."""
+    # Create files
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    hourly_path = Path(config_fixture["data_dir"]) / "hourly.jsonl"
+    hourly_path.write_text("{}")
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, hourly_path=hourly_path, clock=clock_fixture)
+    assert result["ok"] is True
+    
+    # Verify files renamed
+    assert not history_path.exists()
+    assert history_path.with_suffix(history_path.suffix + ".migrated").exists()
+    
+    assert not hourly_path.exists()
+    assert hourly_path.with_suffix(hourly_path.suffix + ".migrated").exists()
+    
+    conn.close()
+
+
+# Test: Malformed lines quarantined with logged count
+
+def test_malformed_lines_quarantined(
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl with malformed lines,
+    when import runs,
+    then malformed lines are quarantined with logged count."""
+    # Create history with malformed lines
+    history_content = "\n".join([
+        '{"timestamp": "2026-08-30T10:00:00Z", "model": "Test", "serial": "123", "firmware_version": "1.0", "data_units_written": 1000, "data_units_read": 500, "percentage_used": 5, "power_on_hours": 100, "temperature": 35}',
+        'NOT JSON',
+        '{"timestamp": "2026-08-30T11:00:00Z", "model": "Test", "serial": "123", "firmware_version": "1.0", "data_units_written": 1001, "data_units_read": 501, "percentage_used": 5, "power_on_hours": 101, "temperature": 36}',
+    ])
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_content)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result["ok"] is True
+    assert result["malformed_lines"] == 1
+    assert result["samples_imported"] == 2
+    
+    conn.close()
+
+
+# Test: hourly.jsonl diffed and logged but never trusted
+
+def test_hourly_jsonl_diffed(
+    history_fixture: str,
+    hourly_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl and hourly.jsonl with mismatches,
+    when import runs,
+    then mismatches are diffed and logged."""
+    # Create files
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    hourly_path = Path(config_fixture["data_dir"]) / "hourly.jsonl"
+    hourly_path.write_text(hourly_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import (should not fail even with mismatches)
+    result = import_legacy_history(conn, history_path, hourly_path=hourly_path, clock=clock_fixture)
+    assert result["ok"] is True
+    
+    # Verify data was imported from history.jsonl, not hourly.jsonl
+    cursor = conn.execute("SELECT bytes_written_delta FROM hour_observations ORDER BY hour")
+    deltas = [row[0] for row in cursor.fetchall()]
+    
+    # Should match history.jsonl derived values, not hourly.jsonl
+    assert len(deltas) == 2
+    
+    conn.close()
+
+
+# Test: Legacy identity is mn-only
+
+def test_legacy_identity_mn_only(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl,
+    when import runs,
+    then legacy segment has mn-only identity."""
+    # Create history file
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result["ok"] is True
+    assert result["legacy_identity_key"] == "legacy|Samsung SSD 970 EVO Plus 1TB"
+    
+    # Verify segment has mn-only identity
+    cursor = conn.execute("SELECT identity_key, mn, sn, subnqn FROM controller_segments")
+    row = cursor.fetchone()
+    assert row[0] == "legacy|Samsung SSD 970 EVO Plus 1TB"
+    assert row[1] == "Samsung SSD 970 EVO Plus 1TB"
+    assert row[2] is None  # sn is None for legacy
+    assert row[3] is None  # subnqn is None for legacy
+    
+    conn.close()
+
+
+# Test: No synthetic baseline created
+
+def test_no_synthetic_baseline(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl,
+    when import runs,
+    then no endurance baseline is created."""
+    # Create history file
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result["ok"] is True
+    
+    # Verify no baseline created
+    cursor = conn.execute("SELECT COUNT(*) FROM endurance_baseline")
+    assert cursor.fetchone()[0] == 0
+    
+    conn.close()
+
+
+# Test: Monitoring period opened and closed
+
+def test_monitoring_period_opened_closed(
+    history_fixture: str,
+    config_fixture: Dict[str, Any],
+    clock_fixture,
+):
+    """Given history.jsonl,
+    when import runs,
+    then one monitoring period is opened at first sample and closed at migration."""
+    # Create history file
+    history_path = Path(config_fixture["data_dir"]) / "history.jsonl"
+    history_path.write_text(history_fixture)
+    
+    # Initialize store
+    store_path = Path(config_fixture["store_path"])
+    conn = init_store(store_path)
+    
+    # Import
+    result = import_legacy_history(conn, history_path, clock=clock_fixture)
+    assert result["ok"] is True
+    
+    # Verify monitoring period
+    cursor = conn.execute("SELECT started_at, ended_at, end_cause FROM monitoring_periods")
+    row = cursor.fetchone()
+    assert row[0] == "2026-08-30T10:00:00+00:00"  # First sample time
+    assert row[1] == clock_fixture.now.isoformat()  # Migration time
+    assert row[2] == "migrated"
+    
+    conn.close()
+
+
+# Test: Parse history line
+
+def test_parse_history_line_valid():
+    """Given a valid history line,
+    when parsed,
+    then returns the record."""
+    line = '{"timestamp": "2026-08-30T10:00:00Z", "model": "Test", "serial": "123", "firmware_version": "1.0", "data_units_written": 1000, "data_units_read": 500, "percentage_used": 5, "power_on_hours": 100, "temperature": 35}'
+    result = _parse_history_line(line, 1)
+    assert result is not None
+    assert result["model"] == "Test"
+
+
+def test_parse_history_line_malformed():
+    """Given a malformed JSON line,
+    when parsed,
+    then returns None."""
+    result = _parse_history_line("NOT JSON", 1)
+    assert result is None
+
+
+def test_parse_history_line_missing_field():
+    """Given a line with missing required field,
+    when parsed,
+    then returns None."""
+    line = '{"timestamp": "2026-08-30T10:00:00Z", "model": "Test"}'
+    result = _parse_history_line(line, 1)
+    assert result is None