diff --git a/src/scripts/memlog.py b/src/scripts/memlog.py
new file mode 100644
index 000000000..709870b32
--- /dev/null
+++ b/src/scripts/memlog.py
@@ -0,0 +1,222 @@
+#!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.10"
+# ///
+"""memlog — an append-only memory log: LLM-optimal working memory for a skill.
+
+A memlog is the dense, chronological record of everything that mattered in a piece of
+work — every item the user generated or accepted — kept minimal like human memory: only
+what's important, never bloated. It persists ACROSS sessions, so a fresh session can
+load it and continue. It is NOT a deliverable; downstream artifacts (a brief, a PRD, a
+deck, a report) are *derived* from it on demand. The host skill supplies the vocabulary
+by how it calls `append` — the tool stays neutral.
+
+It is a FLAT log: there are no sections or grouping. Every entry is one line, recorded
+at the END in the order it happened. The chronology itself is the structure — an event
+like "started technique X" is just another entry, same as an idea or an insight.
+
+Three invariants make it trustworthy:
+
+  1. Append-only, chronological. Entries land at the end, in the order they happen.
+     Nothing is ever inserted backward, reordered, edited, or removed. There is no
+     edit or delete subcommand by design; history is never rewritten.
+  2. Write-only / blind. Every command is an atomic, context-free write and echoes the
+     new state as one line of JSON, so the caller never re-reads the file mid-session.
+     The one time the file is read is on resume — and the caller reads it itself, not
+     via this script.
+  3. No lifecycle status. A memory log has no "complete" flag. Whether the work is done,
+     blocked, or paused is itself a fact that happened, so it is recorded as an entry
+     (e.g. `append --type event --text "session complete"`), never as frontmatter the
+     log would have to mutate. The chronology stays the single source of truth, and a
+     resume learns the state by reading the last entries — the same way it learns
+     everything else.
+
+Atomicity: every write goes to a temp file, is flushed and fsync'd, then atomically
+renamed over the target, so a crash never leaves a half-written entry.
+
+The file shape (.memlog.md):
+
+    ---
+    topic: Onboarding flow for a budgeting app
+    goal: lift week-1 retention
+    updated: 2026-06-07T14:22
+    ---
+
+    - (note) user picked techniques: SCAMPER, then Six Thinking Hats
+    - (technique) started SCAMPER
+    - (idea) skip the signup wall: let people try with sample data first
+    - (idea) auto-import one bank account so the first screen shows real numbers
+    - (question) is open-banking consent too heavy for step one?
+    - (insight) the "scary numbers" risk and the "real numbers" idea are one lever: show real data, pre-categorized
+    - (direction) optimize for the anxious first-timer, not the power user
+    - (decision) lead with one pre-categorized account; defer multi-account import
+    - (event) session complete
+
+Each entry may carry an optional `--type` — what KIND it is (idea, insight, question,
+decision, direction, assumption, gap, note, event, …) — and an optional `--by` naming
+who it came from (e.g. `user`, `coach`), for sessions where authorship matters. Both
+render into one short inline tag: `(idea)`, `(idea by user)`, `(by coach)`. Omit them
+for a plain note. The host skill names the vocabulary; the script does not enforce one.
+
+Commands:
+  init   (--workspace DIR | --path FILE) [--field k=v ...]    create the memlog (errors if it exists)
+  append (--workspace DIR | --path FILE) --text STR [--type T] [--by W]  append one entry at the end
+  set    (--workspace DIR | --path FILE) --key K --value V    set/replace a descriptive frontmatter field
+
+Addressing: `--workspace` is the run folder, and the memlog is always {workspace}/.memlog.md.
+`--path` points straight at the memlog file instead, for callers that already hold the path.
+"""
+import argparse
+import json
+import os
+import sys
+from datetime import datetime
+from pathlib import Path
+
+MEMLOG = ".memlog.md"
+
+
+def now() -> str:
+    return datetime.now().strftime("%Y-%m-%dT%H:%M")
+
+
+def resolve(args) -> Path:
+    """The memlog file, from either addressing mode: {workspace}/.memlog.md or an explicit --path."""
+    return Path(args.path) if args.path else Path(args.workspace) / MEMLOG
+
+
+def split(text: str) -> tuple[dict, str]:
+    """Return (frontmatter dict in source order, body str). Frontmatter is plain key: value.
+
+    The closing fence is the first line that is *exactly* `---`, so a `---` inside a
+    field value (topic/goal are free user text) never truncates the frontmatter.
+    """
+    lines = text.splitlines()
+    if not lines or lines[0] != "---":
+        raise ValueError(".memlog.md has no frontmatter")
+    end = next((i for i in range(1, len(lines)) if lines[i] == "---"), None)
+    if end is None:
+        raise ValueError(".memlog.md frontmatter is not terminated")
+    meta: dict[str, str] = {}
+    for line in lines[1:end]:
+        if ":" in line:
+            k, v = line.split(":", 1)
+            meta[k.strip()] = v.strip()
+    return meta, "\n".join(lines[end + 1:]).lstrip("\n")
+
+
+def render(meta: dict, body: str) -> str:
+    # Neutralize newlines in values so a multi-line field can't break the fence on re-read.
+    fm = "\n".join(f"{k}: {' '.join(str(v).splitlines())}" for k, v in meta.items())
+    return "---\n" + fm + "\n---\n\n" + body.rstrip("\n") + "\n"
+
+
+def touch(meta: dict) -> None:
+    """Stamp `updated` and keep it last so the field order stays predictable."""
+    meta.pop("updated", None)
+    meta["updated"] = now()
+
+
+def write_atomic(path: Path, text: str) -> None:
+    """Temp + flush + fsync + atomic rename, so a crash never half-writes an entry."""
+    tmp = path.with_suffix(path.suffix + ".tmp")
+    with open(tmp, "w", encoding="utf-8") as f:
+        f.write(text)
+        f.flush()
+        os.fsync(f.fileno())
+    os.replace(tmp, path)
+
+
+def entry_count(body: str) -> int:
+    return sum(1 for ln in body.splitlines() if ln.startswith("- "))
+
+
+def ack(path: Path, body: str) -> None:
+    """Echo new state so the caller never re-reads the file to know where it stands."""
+    print(json.dumps({
+        "ok": True,
+        "memlog": str(path),
+        "entries": entry_count(body),
+    }))
+
+
+def cmd_init(args) -> int:
+    path = resolve(args)
+    if path.exists():
+        print(f"error: {path} already exists; use append/set to update it", file=sys.stderr)
+        return 2
+    path.parent.mkdir(parents=True, exist_ok=True)
+    meta: dict[str, str] = {}
+    for pair in args.field or []:
+        if "=" not in pair:
+            print(f"error: --field expects key=value, got {pair!r}", file=sys.stderr)
+            return 2
+        k, v = pair.split("=", 1)
+        meta[k.strip()] = v.strip()
+    touch(meta)
+    write_atomic(path, render(meta, ""))
+    ack(path, "")
+    return 0
+
+
+def cmd_append(args) -> int:
+    path = resolve(args)
+    meta, body = split(path.read_text(encoding="utf-8"))
+    text = " ".join(args.text.split())  # collapse newlines/runs → one-line entry, no prose bloat
+    label = args.type or ""
+    if args.by:
+        label = f"{label} by {args.by}".strip()  # attribution: "(idea by user)" / "(by coach)"
+    tag = f"({label}) " if label else ""
+    entry = f"- {tag}{text}"
+    body = (body.rstrip("\n") + "\n" + entry) if body.strip() else entry  # always at the end
+    touch(meta)
+    write_atomic(path, render(meta, body))
+    ack(path, body)
+    return 0
+
+
+def cmd_set(args) -> int:
+    path = resolve(args)
+    meta, body = split(path.read_text(encoding="utf-8"))
+    meta[args.key] = args.value
+    touch(meta)
+    write_atomic(path, render(meta, body))
+    ack(path, body)
+    return 0
+
+
+def add_target(sp) -> None:
+    """Every command addresses the memlog the same way: a run folder or an explicit path."""
+    g = sp.add_mutually_exclusive_group(required=True)
+    g.add_argument("--workspace", help="run folder; the memlog is {workspace}/.memlog.md")
+    g.add_argument("--path", help="explicit memlog file path (alternative to --workspace)")
+
+
+def main(argv: list[str] | None = None) -> int:
+    p = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    sub = p.add_subparsers(dest="cmd", required=True)
+
+    pi = sub.add_parser("init", help="create the memlog")
+    add_target(pi)
+    pi.add_argument("--field", action="append", metavar="KEY=VALUE", help="frontmatter field (repeatable)")
+    pi.set_defaults(func=cmd_init)
+
+    pa = sub.add_parser("append", help="append one entry at the end")
+    add_target(pa)
+    pa.add_argument("--text", required=True)
+    pa.add_argument("--type", help="entry kind, rendered as an inline tag")
+    pa.add_argument("--by", help="who the entry came from (e.g. user, coach); rendered into the tag")
+    pa.set_defaults(func=cmd_append)
+
+    pset = sub.add_parser("set", help="set a descriptive frontmatter field")
+    add_target(pset)
+    pset.add_argument("--key", required=True)
+    pset.add_argument("--value", required=True)
+    pset.set_defaults(func=cmd_set)
+
+    args = p.parse_args(argv)
+    return args.func(args)
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/src/scripts/tests/test_memlog.py b/src/scripts/tests/test_memlog.py
new file mode 100644
index 000000000..b756e7b2d
--- /dev/null
+++ b/src/scripts/tests/test_memlog.py
@@ -0,0 +1,306 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = ["pytest>=8.0"]
+# ///
+"""Tests for memlog.py. Run: uv run --with pytest pytest scripts/tests/test_memlog.py
+
+The spine under test is the flat, append-only, chronological invariant: every entry is
+one line recorded at the end in the order it happened — no sections, no grouping, and no
+lifecycle status the log would have to mutate.
+"""
+import json
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+import memlog  # noqa: E402
+
+MEMLOG = ".memlog.md"
+
+
+@pytest.fixture
+def ws(tmp_path):
+    return str(tmp_path)
+
+
+def read(ws):
+    return (Path(ws) / MEMLOG).read_text(encoding="utf-8")
+
+
+def body_of(ws):
+    return memlog.split(read(ws))[1]
+
+
+def entries(ws):
+    return [ln for ln in body_of(ws).splitlines() if ln.startswith("- ")]
+
+
+def init(ws, **fields):
+    fields = fields or {"topic": "Reinvent the lunchbox", "goal": "ideas for a pitch"}
+    argv = ["init", "--workspace", ws]
+    for k, v in fields.items():
+        argv += ["--field", f"{k}={v}"]
+    assert memlog.main(argv) == 0
+
+
+def append(ws, text, entry_type=None, by=None):
+    argv = ["append", "--workspace", ws, "--text", text]
+    if entry_type:
+        argv += ["--type", entry_type]
+    if by:
+        argv += ["--by", by]
+    assert memlog.main(argv) == 0
+
+
+# --- init ---------------------------------------------------------------
+
+def test_init_writes_frontmatter_fields(ws):
+    init(ws)
+    meta, body = memlog.split(read(ws))
+    assert meta["topic"] == "Reinvent the lunchbox"
+    assert meta["goal"] == "ideas for a pitch"
+    assert "updated" in meta
+    assert body.strip() == ""
+
+
+def test_init_has_no_lifecycle_status(ws):
+    # A memory log carries no "status" flag; completion is an appended entry, not frontmatter.
+    init(ws)
+    meta, _ = memlog.split(read(ws))
+    assert "status" not in meta
+
+
+def test_init_arbitrary_fields(ws):
+    init(ws, topic="T", audience="board")
+    meta, _ = memlog.split(read(ws))
+    assert meta["audience"] == "board"
+
+
+def test_init_refuses_overwrite(ws):
+    init(ws)
+    assert memlog.main(["init", "--workspace", ws, "--field", "topic=other"]) == 2
+
+
+def test_init_creates_missing_workspace(tmp_path):
+    nested = str(tmp_path / "a" / "b")
+    assert memlog.main(["init", "--workspace", nested, "--field", "topic=T"]) == 0
+    assert (Path(nested) / MEMLOG).is_file()
+
+
+def test_init_rejects_malformed_field(ws):
+    assert memlog.main(["init", "--workspace", ws, "--field", "noequals"]) == 2
+
+
+# --- addressing: --workspace and --path are interchangeable --------------
+
+def test_path_addressing_targets_the_file_directly(tmp_path):
+    target = tmp_path / "run" / ".memlog.md"
+    assert memlog.main(["init", "--path", str(target), "--field", "topic=T"]) == 0
+    assert target.is_file()
+    assert memlog.main(["append", "--path", str(target), "--text", "an idea", "--type", "idea"]) == 0
+    body = memlog.split(target.read_text(encoding="utf-8"))[1]
+    assert "- (idea) an idea" in body
+
+
+def test_workspace_and_path_resolve_to_same_file(ws):
+    init(ws)
+    via_path = str(Path(ws) / MEMLOG)
+    assert memlog.main(["append", "--path", via_path, "--text", "from path"]) == 0
+    assert memlog.main(["append", "--workspace", ws, "--text", "from workspace"]) == 0
+    assert entries(ws) == ["- from path", "- from workspace"]
+
+
+def test_target_is_required(ws):
+    with pytest.raises(SystemExit):
+        memlog.main(["append", "--text", "orphan"])  # neither --workspace nor --path
+
+
+# --- append: flat chronological order is the whole point -----------------
+
+def test_append_lands_at_end_in_order(ws):
+    init(ws)
+    append(ws, "first")
+    append(ws, "second")
+    append(ws, "third")
+    assert entries(ws) == ["- first", "- second", "- third"]
+
+
+def test_no_sections_or_headings_ever(ws):
+    init(ws)
+    append(ws, "started foo", entry_type="technique")
+    append(ws, "an idea", entry_type="idea")
+    append(ws, "started bar", entry_type="technique")
+    assert "## " not in body_of(ws)  # the flat log never grows headings
+
+
+def test_type_renders_as_inline_tag(ws):
+    init(ws)
+    append(ws, "the earth revolves around the sun", entry_type="idea")
+    append(ws, "how do we handle stampede?", entry_type="question")
+    body = body_of(ws)
+    assert "- (idea) the earth revolves around the sun" in body
+    assert "- (question) how do we handle stampede?" in body
+
+
+def test_append_without_type_is_plain_note(ws):
+    init(ws)
+    append(ws, "bare entry")
+    assert entries(ws) == ["- bare entry"]
+
+
+def test_completion_is_an_entry_not_a_status(ws):
+    # The documented way to mark a session done: append it. Frontmatter never gains a status.
+    init(ws)
+    append(ws, "session complete", entry_type="event")
+    meta, _ = memlog.split(read(ws))
+    assert "status" not in meta
+    assert entries(ws)[-1] == "- (event) session complete"
+
+
+def test_append_collapses_newlines_into_one_line(ws):
+    init(ws)
+    append(ws, "line one\nline two\n  spaced   out")
+    assert entries(ws) == ["- line one line two spaced out"]
+
+
+def test_revisited_technique_is_just_a_later_entry(ws):
+    # the user's model: switching techniques is an entry, not a section to return to
+    init(ws)
+    append(ws, "started SCAMPER", entry_type="technique")
+    append(ws, "magnetic latch", entry_type="idea")
+    append(ws, "started Six Hats", entry_type="technique")
+    append(ws, "stale data risk", entry_type="idea")
+    append(ws, "started SCAMPER", entry_type="technique")  # back to SCAMPER — just appended again
+    append(ws, "stackable tiers", entry_type="idea")
+    assert entries(ws) == [
+        "- (technique) started SCAMPER",
+        "- (idea) magnetic latch",
+        "- (technique) started Six Hats",
+        "- (idea) stale data risk",
+        "- (technique) started SCAMPER",
+        "- (idea) stackable tiers",
+    ]
+
+
+def test_by_renders_attribution_in_tag(ws):
+    # Creative Partner mode must record whose idea each one was
+    init(ws)
+    append(ws, "magnetic latch lid", entry_type="idea", by="user")
+    append(ws, "lid doubles as a plate", entry_type="idea", by="coach")
+    body = body_of(ws)
+    assert "- (idea by user) magnetic latch lid" in body
+    assert "- (idea by coach) lid doubles as a plate" in body
+
+
+def test_by_without_type_renders_alone(ws):
+    init(ws)
+    append(ws, "off-the-cuff thought", by="coach")
+    assert entries(ws) == ["- (by coach) off-the-cuff thought"]
+
+
+def test_heterogeneous_entry_types_coexist(ws):
+    init(ws)
+    append(ws, "an idea", entry_type="idea")
+    append(ws, "an open question", entry_type="question")
+    append(ws, "a decision we made", entry_type="decision")
+    append(ws, "user wants mobile-first", entry_type="direction")
+    body = body_of(ws)
+    for tag in ("(idea)", "(question)", "(decision)", "(direction)"):
+        assert tag in body
+
+
+def test_free_vocabulary_is_not_enforced(ws):
+    # The tool is neutral: any --type the host skill names renders verbatim.
+    init(ws)
+    append(ws, "a custom kind", entry_type="crack")
+    append(ws, "another", entry_type="lock")
+    body = body_of(ws)
+    assert "- (crack) a custom kind" in body
+    assert "- (lock) another" in body
+
+
+# --- set: generic descriptive frontmatter, no lifecycle semantics --------
+
+def test_set_adds_field(ws):
+    init(ws)
+    memlog.main(["set", "--workspace", ws, "--key", "mode", "--value", "partner"])
+    assert memlog.split(read(ws))[0]["mode"] == "partner"
+
+
+def test_set_replaces_field(ws):
+    init(ws, topic="T", mode="facilitator")
+    memlog.main(["set", "--workspace", ws, "--key", "mode", "--value", "partner"])
+    assert memlog.split(read(ws))[0]["mode"] == "partner"
+
+
+def test_set_preserves_body(ws):
+    init(ws)
+    append(ws, "keep me", entry_type="idea")
+    memlog.main(["set", "--workspace", ws, "--key", "mode", "--value", "partner"])
+    meta, body = memlog.split(read(ws))
+    assert meta["mode"] == "partner"
+    assert "- (idea) keep me" in body
+
+
+def test_updated_stays_last(ws):
+    init(ws)
+    memlog.main(["set", "--workspace", ws, "--key", "owner", "--value", "BMad"])
+    meta = memlog.split(read(ws))[0]
+    assert list(meta)[-1] == "updated"
+
+
+# --- robustness ---------------------------------------------------------
+
+def test_roundtrip_render_is_stable(ws):
+    init(ws)
+    append(ws, "one", entry_type="idea")
+    first = read(ws)
+    meta, body = memlog.split(first)
+    assert memlog.render(meta, body) == first
+
+
+def test_commas_in_field_survive(ws):
+    init(ws, topic="cars, trains, and planes")
+    append(ws, "z", entry_type="idea")
+    meta, _ = memlog.split(read(ws))
+    assert meta["topic"] == "cars, trains, and planes"
+
+
+def test_triple_dash_in_field_does_not_corrupt_frontmatter(ws):
+    # A `---` inside a value must NOT be read as the closing fence: topic stays intact
+    # and the body never leaks frontmatter text.
+    init(ws, topic="Pricing --- tiers --- and add-ons")
+    append(ws, "an idea", entry_type="idea")
+    meta, body = memlog.split(read(ws))
+    assert meta["topic"] == "Pricing --- tiers --- and add-ons"
+    assert entries(ws) == ["- (idea) an idea"]
+    assert "topic:" not in body  # frontmatter never bled into the body
+
+
+def test_newline_in_field_is_neutralized(ws):
+    # A value carrying a newline can't break the fence on the next round-trip.
+    memlog.main(["init", "--workspace", ws, "--field", "topic=line one\nline two"])
+    append(ws, "x", entry_type="idea")
+    meta, _ = memlog.split(read(ws))
+    assert "\n" not in meta["topic"]
+
+
+def test_append_emits_json_ack(ws, capsys):
+    init(ws)
+    append(ws, "x", entry_type="idea")
+    out = json.loads(capsys.readouterr().out.strip().splitlines()[-1])
+    assert out["ok"] is True
+    assert out["entries"] == 1
+    assert out["memlog"].endswith(MEMLOG)
+    assert "status" not in out  # no lifecycle status
+    assert "section" not in out  # sections are gone
+
+
+def test_ack_entry_count_climbs(ws, capsys):
+    init(ws)
+    append(ws, "a")
+    append(ws, "b")
+    out = json.loads(capsys.readouterr().out.strip().splitlines()[-1])
+    assert out["entries"] == 2