diff --git a/src/kitty_workbench/project.py b/src/kitty_workbench/project.py
new file mode 100644
index 0000000..032f899
--- /dev/null
+++ b/src/kitty_workbench/project.py
@@ -0,0 +1,111 @@
+"""Project directory management — create, log, read, list."""
+
+from __future__ import annotations
+
+import json
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Optional
+
+DEFAULT_WORKBENCH_DIR = Path.home() / "Kitty-Workbench"
+
+
+def _now_iso() -> str:
+    return datetime.now(timezone.utc).astimezone().isoformat(timespec="seconds")
+
+
+def project_path(name: str, workbench_dir: Path = DEFAULT_WORKBENCH_DIR) -> Path:
+    return workbench_dir / name
+
+
+def project_exists(name: str, workbench_dir: Path = DEFAULT_WORKBENCH_DIR) -> bool:
+    return project_path(name, workbench_dir).is_dir()
+
+
+def create_project(
+    name: str, title: str, workbench_dir: Path = DEFAULT_WORKBENCH_DIR
+) -> Path:
+    """Create a project directory with log files. Idempotent — won't overwrite existing logs."""
+    pdir = project_path(name, workbench_dir)
+    pdir.mkdir(parents=True, exist_ok=True)
+    (pdir / "assets").mkdir(exist_ok=True)
+
+    md_path = pdir / "session.md"
+    if not md_path.exists():
+        md_path.write_text(f"# {title} — Session Log\n\nStarted: {_now_iso()}\n\n")
+
+    jsonl_path = pdir / "session.jsonl"
+    if not jsonl_path.exists():
+        jsonl_path.write_text("")
+
+    cost_path = pdir / "cost-log.jsonl"
+    if not cost_path.exists():
+        cost_path.write_text("")
+
+    return pdir
+
+
+def append_log(
+    name: str,
+    entry: str,
+    data: Optional[dict] = None,
+    level: str = "info",
+    workbench_dir: Path = DEFAULT_WORKBENCH_DIR,
+) -> dict:
+    """Append a log entry to session.md and session.jsonl. Returns the entry dict."""
+    pdir = project_path(name, workbench_dir)
+    ts = _now_iso()
+
+    with open(pdir / "session.md", "a") as f:
+        f.write(f"\n### {ts}\n{entry}\n")
+
+    obj = {"ts": ts, "entry": entry, "level": level}
+    if data:
+        obj.update(data)
+    with open(pdir / "session.jsonl", "a") as f:
+        f.write(json.dumps(obj) + "\n")
+
+    return obj
+
+
+def read_log(
+    name: str, tail: int = 20, workbench_dir: Path = DEFAULT_WORKBENCH_DIR
+) -> list[dict]:
+    """Read recent log entries from session.jsonl."""
+    jsonl_path = project_path(name, workbench_dir) / "session.jsonl"
+    if not jsonl_path.exists():
+        return []
+
+    lines = jsonl_path.read_text().strip().split("\n")
+    lines = [l for l in lines if l.strip()]
+    recent = lines[-tail:] if len(lines) > tail else lines
+
+    entries = []
+    for line in recent:
+        try:
+            entries.append(json.loads(line))
+        except json.JSONDecodeError:
+            entries.append({"raw": line})
+    return entries
+
+
+def list_projects(workbench_dir: Path = DEFAULT_WORKBENCH_DIR) -> list[dict]:
+    """List all project directories."""
+    if not workbench_dir.exists():
+        return []
+    projects = []
+    for d in sorted(workbench_dir.iterdir()):
+        if d.is_dir() and not d.name.startswith(".") and d.name != "START.md":
+            projects.append({"name": d.name})
+    return projects
+
+
+def log_session_event(
+    name: str, event: str, workbench_dir: Path = DEFAULT_WORKBENCH_DIR, **extra
+) -> None:
+    """Append to cost-log.jsonl."""
+    pdir = project_path(name, workbench_dir)
+    obj = {"ts": _now_iso(), "event": event, "project": name}
+    obj.update(extra)
+    with open(pdir / "cost-log.jsonl", "a") as f:
+        f.write(json.dumps(obj) + "\n")
diff --git a/tests/test_project.py b/tests/test_project.py
new file mode 100644
index 0000000..8d7b8f8
--- /dev/null
+++ b/tests/test_project.py
@@ -0,0 +1,67 @@
+import json
+from kitty_workbench.project import (
+    create_project, project_exists, project_path,
+    append_log, read_log, list_projects, log_session_event,
+)
+
+
+def test_create_project(tmp_workbench):
+    path = create_project("io102", "Heathkit IO-102", workbench_dir=tmp_workbench)
+    assert path.exists()
+    assert (path / "session.md").exists()
+    assert (path / "session.jsonl").exists()
+    assert (path / "cost-log.jsonl").exists()
+    assert (path / "assets").is_dir()
+    md = (path / "session.md").read_text()
+    assert "Heathkit IO-102" in md
+
+
+def test_create_project_idempotent(tmp_workbench):
+    create_project("io102", "First", workbench_dir=tmp_workbench)
+    append_log("io102", "test entry", workbench_dir=tmp_workbench)
+    create_project("io102", "Second", workbench_dir=tmp_workbench)
+    entries = read_log("io102", workbench_dir=tmp_workbench)
+    assert len(entries) == 1
+
+
+def test_project_exists(tmp_workbench):
+    assert not project_exists("io102", workbench_dir=tmp_workbench)
+    create_project("io102", "Test", workbench_dir=tmp_workbench)
+    assert project_exists("io102", workbench_dir=tmp_workbench)
+
+
+def test_append_and_read_log(tmp_workbench):
+    create_project("io102", "Test", workbench_dir=tmp_workbench)
+    append_log("io102", "R412 measured 1.05M", data={"ohms": 1050000}, workbench_dir=tmp_workbench)
+    append_log("io102", "R413 OK", workbench_dir=tmp_workbench)
+    entries = read_log("io102", tail=10, workbench_dir=tmp_workbench)
+    assert len(entries) == 2
+    assert entries[0]["entry"] == "R412 measured 1.05M"
+    assert entries[0]["ohms"] == 1050000
+    assert entries[1]["entry"] == "R413 OK"
+
+
+def test_read_log_tail(tmp_workbench):
+    create_project("io102", "Test", workbench_dir=tmp_workbench)
+    for i in range(30):
+        append_log("io102", f"Entry {i}", workbench_dir=tmp_workbench)
+    entries = read_log("io102", tail=5, workbench_dir=tmp_workbench)
+    assert len(entries) == 5
+    assert entries[0]["entry"] == "Entry 25"
+
+
+def test_list_projects(tmp_workbench):
+    assert list_projects(workbench_dir=tmp_workbench) == []
+    create_project("io102", "First", workbench_dir=tmp_workbench)
+    create_project("psu-rebuild", "Second", workbench_dir=tmp_workbench)
+    projects = list_projects(workbench_dir=tmp_workbench)
+    assert [p["name"] for p in projects] == ["io102", "psu-rebuild"]
+
+
+def test_log_session_event(tmp_workbench):
+    create_project("io102", "Test", workbench_dir=tmp_workbench)
+    log_session_event("io102", "session_start", workbench_dir=tmp_workbench)
+    cost_log = (tmp_workbench / "io102" / "cost-log.jsonl").read_text().strip()
+    entry = json.loads(cost_log.split("\n")[-1])
+    assert entry["event"] == "session_start"
+    assert entry["project"] == "io102"