Mnemolith

Shared memory for AI coding tools that stays true to your code.

Mnemolith (NEE-mo-lith): "memory stone." What your AI learns, carved where every tool can read it. (Why the name)

Formerly Aether Cache. If you installed aether-cache, see Moving from Aether Cache: your memories come with you.

Website · PyPI

Claude Code, Cursor, and other AI tools each forget your project between sessions, and when they do remember things, nothing checks whether those memories are still correct. Mnemolith gives every tool one shared, local memory, and keeps it honest:

• Memories go stale when the code changes. Each memory is linked to the files it's about, with a snapshot of their git content hash. When docker-compose.yml changes, the memory about it is retired automatically instead of being fed to your AI forever.
• Contradictions get replaced, not hoarded. "We migrated to MySQL" retires "we use Postgres" instead of sitting next to it and confusing the model. Near-identical repeats are merged.
• Every memory can explain itself. Which tool and session saved it, which file versions it was based on, and every change since: created, seen again, went stale, confirmed, replaced.
• It works with the files you already use. Current memories sync into a managed section of AGENTS.md and CLAUDE.md, so they're committed, reviewed in PRs, and visible to teammates who don't run Mnemolith.

It runs entirely on your machine, on top of a database engine I wrote from scratch.

Not a Redis clone

Mnemolith started as a from-scratch rebuild of Redis in Java, then kept going. Every layer here is written from scratch, and each one does something the layer beneath it can't:

The bottom layer alone is a working Redis: redis-cli, redis-benchmark, and redis-py talk to it unchanged. The memory layer, git awareness, and MCP server are things Redis doesn't do at all.

Claude Code ─┐                       ┌─────────────────────────────────────────────┐
Cursor ──────┼─ MCP ─► mnemolith ─┼─ RESP ─► Java engine (Main.java)            │
other tools ─┘   (Python: embeddings,│          memories, HNSW vector index,       │
                  git hashing, sync) │          history, append-only file on disk  │
                                     └─────────────────────────────────────────────┘

What it looks like

The dashboard (mnemolith ui): memories whose files changed wait for a decision, with the file versions side by side.

And from your AI tool's side, a memory linked to a file, the file changes, and the next lookup catches it:

> remember("The database runs Postgres via docker compose", files=["docker-compose.yml"])
Saved as m1.

        ... someone edits docker-compose.yml ...

> recall("database setup")
These memories were just retired because files they depend on changed. If one is still true,
call confirm_memory; if it's wrong, save the corrected fact with remember:
[m1] "The database runs Postgres via docker compose" is now marked outdated: docker-compose.yml changed.

> confirm_memory("m1")        # still true, so re-link it to the new version of the file
Confirmed m1; it's active again and linked to the current file versions.

> explain_memory("m1")
m1: "The database runs Postgres via docker compose"
status: active, project: demo
saved by: claude-code (session eea54b5c)
linked files: docker-compose.yml@02ea346...
history:
  19:16  created: by claude-code (session eea54b5c), linked to docker-compose.yml@87da641...
  19:16  active -> stale: docker-compose.yml changed since this was saved
  19:16  re-linked: to docker-compose.yml@02ea346... (confirmed still true)
  19:16  stale -> active: confirmed still true

A newer fact replacing an older one:

> remember("The API timeout was raised to 60 seconds")
Saved as m3.
Related existing memories:
[m1] (similarity 0.58) The API timeout is 30 seconds
If m3 makes any of these no longer true, call replace_memory(old_id, "m3"). Otherwise do nothing.

> replace_memory("m1", "m3")
m1 retired; m3 replaces it.

Quick start

One line installs everything and runs setup. On macOS or Linux:

curl -fsSL https://mnemolith.dev/install.sh | sh

On Windows (PowerShell):

powershell -ExecutionPolicy ByPass -c "irm https://mnemolith.dev/install.ps1 | iex"

The installer (install.sh, install.ps1) gets uv if you don't have it (uv brings Python), installs Mnemolith, and runs mnemolith setup. Prefer your own tools? Any of these works, followed by mnemolith setup:

uv tool install mnemolith
pipx install mnemolith
pip install mnemolith        # Python 3.10+

setup starts the engine in the background, downloads the embedding model once (about 90 MB), and connects every supported AI tool it finds on your machine: Claude Code, Cursor, and Codex. That's it. Open your AI tool in any git repo and try:

Remember that our tests need DOCKER_HOST set. It depends on docker-compose.yml.

Then start a new session and ask how to run the tests.

The engine runs on Java 11+. If you have it, it's used. If you don't, Mnemolith downloads a private copy (about 50 MB) into ~/.mnemolith/java, checks it against its published checksum, and uses it only for Mnemolith; nothing is installed system-wide, and mnemolith uninstall removes it.

Already keep notes for your AI in CLAUDE.md, AGENTS.md, or Cursor rules? Run mnemolith import in the repo (or use Import notes in the dashboard) to turn them into memories, linked to the files they mention.

Memories are kept per project (the git repo's folder name), so projects never mix. Everything stays on your machine: the engine only accepts connections from it, and your memories live in ~/.mnemolith.

If mnemolith isn't found after installing, run uv tool update-shell and open a new terminal. Port 7380 taken? Set MNEMOLITH_PORT and run setup again. To uninstall: mnemolith uninstall, then uv tool uninstall mnemolith.

Moving from Aether Cache

Mnemolith used to be called Aether Cache. If you installed aether-cache, run the installer above. It installs Mnemolith, removes aether-cache, moves your memories from ~/.aether-cache to ~/.mnemolith, and reconnects your AI tools. Nothing is lost, and memories synced into CLAUDE.md / AGENTS.md are picked up where they are.

Installed some other way? Install mnemolith the same way, run mnemolith setup, then remove aether-cache (uv tool uninstall aether-cache, pipx uninstall aether-cache, or pip uninstall aether-cache).

Tools the AI can call

Design decisions, and what the measurements said

Similarity can't detect contradictions, so the AI judges

The first version flagged a new memory as a possible contradiction when it was similar to an existing one. Calibrating with real all-MiniLM-L6-v2 embeddings showed that can't work:

Duplicates scored 0.73 to 0.92 and contradictions 0.30 to 0.94, so no threshold separates them. A threshold low enough to merge real duplicates would silently merge "timeout is 30s" with "timeout is 60s", the worst bug this product could have. So the engine only auto-merges near-identical text (similarity ≥ 0.97). For everything else, embeddings find the related memories and the AI, which already understands the sentences, decides whether one replaces another.

Staleness compares file contents, not commits

Each linked file is stored as path@<git blob hash>, computed with git hash-object. Comparing contents rather than commit ids catches uncommitted edits, ignores commits that didn't touch the file, and needs no git hooks: recall re-hashes the handful of linked files before every search.

Memories never mutate silently

Every change is a logged event with a timestamp and a reason. The engine logs each memory with the id and timestamp it assigned, so replaying the log after a restart rebuilds the exact same memories and histories, down to the millisecond, instead of minting new ones.

The engine underneath

A Redis-compatible server written from scratch in Java. It speaks RESP, so redis-cli, redis-benchmark, and redis-py work with it unchanged. The core was built by completing every stage and extension of the CodeCrafters "Build Your Own Redis" challenge; the vector index, persistence hardening, and the memory layer are my own additions.

Throughput vs. official Redis

redis-benchmark -t set,get, 100K requests, 50 parallel clients, 3-byte values, same machine:

Typical latency is on par with Redis. The gap is in worst-case latency, from JVM garbage collection and JIT warm-up, which Redis (written in C) doesn't have.

HNSW vector search vs. exact search

100,000 AG News headlines embedded with all-MiniLM-L6-v2 (384 dimensions, cosine similarity), 500 held-out queries, k = 10. Recall is measured against exact brute-force search on the same data.

At ef=100, HNSW returns 99.4% of the true top-10 about 22× faster than exact search. Building the index for 100K vectors took 136 seconds (M=16, M0=32, efConstruction=200). Latencies are measured from the Python client, so they include network and client overhead. Full setup and raw numbers: results.json.

How it works: each vector links to its nearest neighbors in a layered graph. A search starts on a sparse top layer of long-range links, hops greedily toward the query, drops down a layer at a time, and finishes with a best-first search on the bottom layer that keeps the ef best candidates. Neighbor selection uses the diversity heuristic from the HNSW paper (prefer neighbors in different directions over near-duplicates), which is what keeps recall high.

Filtered search: HNSW doesn't know about memory status, so searches over-fetch candidates, drop the ones that don't match (stale or superseded), widen the search if too few survive, and fall back to an exact scan if needed. In a test with 2,000 memories where only 2% were active, filtered searches returned exactly the correct top 10 on 30/30 queries. The catch: at that selectivity, the exact fallback runs most of the time, which is the known weak spot of post-filtering on graph indexes.

Crash safety

Every write goes to an append-only file (AOF) that is replayed on startup.

kill -9 crash test (crash_test.py): a client writes as fast as it can, the server is killed at a random moment, restarted, and every write the server acknowledged is checked.

Recovery took 0.6 seconds per restart, replaying up to ~185K writes.

Torn writes: a machine crash can cut the last write off halfway. The first version refused to start on such a file; now replay stops at the last complete command, trims the partial one, and starts normally with all earlier data intact.

Cost of each fsync policy (SET throughput, 50 clients):

everysec is 2.7× faster than always and within 4% of no, which is why it's the sensible default. On macOS, fsync doesn't force the SSD's own cache to flush, so always looks cheaper here than it would on a Linux server.

Full command list

• Memory:
  • MEM.ADD, MEM.GET, MEM.SEARCH (with STATUS filter), MEM.STATUS, MEM.WHY, MEM.LIST
  • MEM.FILES, MEM.SEEN, MEM.CONFLICTS, MEM.RESOLVE
• Vectors: VADD, VSEARCH (HNSW, tunable EF, EXACT mode)
• Strings: GET, SET (EX / PX), INCR, STRLEN, TYPE, KEYS *
• Lists: RPUSH, LPUSH, LRANGE, LLEN, LPOP, blocking BLPOP
• Streams: XADD (explicit, partial, and auto IDs), XRANGE, XREAD (with BLOCK and $)
• Sorted sets: ZADD, ZRANK, ZRANGE, ZCARD, ZSCORE, ZREM
• Bitmaps: SETBIT, GETBIT, BITCOUNT, BITOP AND / OR (binary-safe strings)
• Geospatial: GEOADD, GEOPOS, GEODIST, GEOSEARCH ... FROMLONLAT ... BYRADIUS
• Transactions: MULTI, EXEC, DISCARD, WATCH, UNWATCH (optimistic locking via per-key write versions)
• Pub/Sub: SUBSCRIBE, UNSUBSCRIBE, PUBLISH
• Replication:
  • Master–replica handshake (PING, REPLCONF, PSYNC) and command propagation
  • REPLCONF GETACK offset tracking and WAIT
• Persistence: RDB loading; AOF with a manifest, appendfsync always / everysec / no, and torn-write recovery
• Auth: AUTH, ACL WHOAMI / GETUSER / SETUSER (SHA-256 password hashes)

Running the engine on its own

java src/main/java/Main.java                                   # port 6379
java src/main/java/Main.java --bind 127.0.0.1                  # accept connections from this machine only
java src/main/java/Main.java --port 6380 --replicaof "localhost 6379"
java src/main/java/Main.java --dir /tmp/data --appendonly yes --appendfsync everysec

Reproducing the benchmarks

Each benchmark runs against the engine you've started:

redis-benchmark -p 6379 -t set,get                 # throughput; also run against redis-server to compare
python vector_bench.py                             # HNSW recall and latency (start the engine with -Xmx4g)
python crash_test.py everysec 5                    # kill -9 crash test
python memory_mcp/calibrate.py                     # embedding similarity calibration

Related work

Mnemolith is an early, open-source project. These are the production systems in the same space:

• Vector search in Redis-style stores:
  • Redis 8 vector sets and valkey-search add HNSW vector search to Redis and Valkey.
  • hnswlib is the reference HNSW implementation.
  • Malkov & Yashunin (2016) is the paper this index follows.
• Memory for AI agents: Mem0, Zep, Letta, and the MCP reference memory server.
• Instruction files: AGENTS.md and CLAUDE.md. Mnemolith syncs into these rather than replacing them.

Status and limitations

Mnemolith is v1 and early. It works end to end in automated tests and is now being tried in real Claude Code sessions.

Not done yet:

• A with-memory vs. without-memory evaluation on real coding tasks, to measure whether it actually reduces re-explaining.
• AOF compaction: the log only grows, and every status change adds a line.

Engine limitations:

• Threading: one OS thread per connection, not an event loop. That's fine for a local tool, but it won't scale to thousands of clients like Redis.
• Transactions: EXEC isn't isolated, so other clients' commands can interleave with a transaction's commands.
• Vector index: re-adding a vector updates it without rewiring its graph links, and there's no delete. Retired memories are filtered out, not removed.
• Replication: full resync only, and a new replica assumes the master's dataset is empty.
• Smaller gaps:
  • KEYS * lists only string keys.
  • GEOSEARCH scans every member.
  • ACL supports only the default user.
  • HELLO (RESP3) isn't supported, so clients must use RESP2.

Project layout

src/main/java/Main.java      the engine (single file, from the CodeCrafters setup)
mnemolith/                   the Python package behind the `mnemolith` command
  server.py                  MCP server: embeddings, git staleness, AGENTS.md/CLAUDE.md sync
  engine.py                  finds Java, starts and stops the engine in the background
  javart.py                  downloads a private copy of Java when the computer has none
  connect.py                 registers Mnemolith with Claude Code, Cursor, and Codex
  cli.py                     the `mnemolith` command
  ui.py, ui/index.html       the dashboard (`mnemolith ui`): a local server, its API, and the page
  portability.py             import from notes files, export to JSON
  engine/mnemolith-engine.jar the prebuilt engine (rebuild with `python scripts/build_engine.py`)
memory_mcp/calibrate.py      similarity calibration for duplicate/contradiction thresholds
vector_bench.py              HNSW benchmark (writes results.json)
crash_test.py                kill -9 crash-safety test

Releasing

Bump the version in pyproject.toml, mnemolith/__init__.py, and server.json, commit, then:

git tag v0.2.0 && git push --tags

The release workflow checks the versions match, builds and smoke-tests the package, publishes it to PyPI and the MCP Registry, and creates the GitHub Release.

License

MIT. See LICENSE.