factstore

factstore

A fact store for AI agents, on Postgres. Agents record what a company's systems and documents say as facts: a value for a registered attribute, in a transaction stamped with who wrote it and the document it came from. Nothing is overwritten, so the store can say what it knew on any date. Agents use it through an MCP server, and scripts through a Python SDK.

It comes with skills, procedures an agent follows: one indexes a company's systems from their exports, and one reads documents and mail into facts. The design says why it is built this way, and the evals what it was tested on. MIT-licensed.

Install

You need Postgres 16 or later, Python 3.12 or later, and uv for uvx.

1. Postgres. Use your own, or start one. factstore init needs a superuser.

docker run -d --name factstore-pg -e POSTGRES_PASSWORD=postgres -p 5432:5432 --shm-size=1g postgres:17

2. A store, and a credential for your agent.

pip install factstoreexport FACTSTORE_ADMIN_DSN=postgresql://postgres:postgres@localhost:5432/postgresfactstore init acme                        # a store, with factstore-corefactstore install acme factstore-skills    # the vocabulary the skills usefactstore actor acme "ingest agent"        # prints the actor's ID and its credential

3. Connect your agent. In Claude Code, the plugin brings the MCP server and the skills, and asks for the credential:

/plugin marketplace add Victor-EU/factstore/plugin install factstore@factstore

Any other MCP client runs the server with the credential in its environment:

{"mcpServers": {"factstore": {"command": "uvx", "args": ["factstore", "mcp"],                              "env": {"FACTSTORE_DSN": "<credential>"}}}}

factstore skills DIR copies the skills into your agent's skills directory, such as .claude/skills. The catalogue and ingestion skills also write in bulk with the SDK, so the agent's shell needs pip install factstore.

4. Ask. "Read the invoices in ./invoices into the store" uses factstore-ingest. "Index our Shopify and QuickBooks exports" uses factstore-catalogue. "What kinds of thing does the store hold?" uses factstore-ontology.

What the release carries

factstore install STORE NAME installs a package by name, with the packages it depends on.

Personal data. The server's rules keep people's names, addresses and phone numbers out of the store: they stay in the documents. A business that wants an attribute to hold them allows it with core/personal.

Develop

git clone https://github.com/Victor-EU/factstore && cd factstoredocker compose up -d                      # Postgres 17 on port 54329cd factstorepython3 -m venv .venv && .venv/bin/pip install -e ".[dev]".venv/bin/pytest

The tests create and drop a store per test. From a source checkout, factstore install also takes a package's directory, such as ../packages/ecom-ops. packages/ describes the package format and the packages. A new version of a package registers what it adds. It may also make an attribute many or identity, the two changes the kernel allows. factstore-skills/ holds the skills.

import factstore
store = factstore.connect("<credential from above>")store.register_attribute({"ident": "shopify/order_id", "type": "string", "cardinality": "one",                          "unique": "identity", "doc": "Shopify's ID for an order."})store.transact([{"e": ["shopify/order_id", "1234"], "a": "shopify/order_id", "v": "1234"}])store.query('select e, v from "shopify/order_id"').rows

With FACTSTORE_DSN set, the same calls work at module level: factstore.transact([...]), factstore.query(...).

The MCP server

factstore mcp, or factstore-mcp, serves the calls as tools over stdio. The credential comes from its environment (FACTSTORE_DSN), never from a tool call. excise is listed only when FACTSTORE_EXCISE_DSN holds an excision credential as well. The tool descriptions in tools.py are the documentation models read.

Layout

How the invariants are enforced

Postgres enforces them, not just the Python code:

• Immutability. Writers have INSERT and SELECT on the fact table, and nothing else. Triggers refuse UPDATE, TRUNCATE, and any DELETE outside fs_excise(), even from the owner.
• The kernel says who wrote. A credential is a Postgres login. A trigger sets each transaction's actor from session_user and its time from the clock, overwriting anything the client sends. Only the kernel's own functions can write fs/actor, fs/at and fs/excised_* facts.
• Registered, typed attributes. Every fact must name a registered attribute (enforced by a foreign key), and a trigger checks its value has that attribute's type. Another trigger stops attributes changing in the wrong direction (a type change, many to one, removing uniqueness).
• Commit order. Each write takes one advisory lock per store and allocates its transaction ID inside it. The trigger takes the same lock again and refuses an ID that isn't after the latest.

Reading

query takes one SQL statement over views (design open question 1, decided by the spike):

• Every attribute is a view named after it: "po/status"(e, v, tx) in schema current, and history."po/status"(e, v, tx, op) over the whole log.
• Registration creates the views with a trigger.
• An as-of read sets factstore.as_of, which every view honours.

A model writes the SQL, so read.py runs it on a separate connection:

• inside a READ ONLY transaction that is always rolled back, which undoes anything the statement changes, SET ROLE included;
• prepared, so Postgres refuses a second statement;
• with a 10 s timeout and a 1,000-row cap.

Advisory locks survive a rollback, so they are released after every read; a query can never hold the writer lock.

stats groups entities by signature, the exact set of attributes each one carries, and counts refs between signatures. That is co-occurrence in the form the ontology skill needs: a kind of thing is usually one signature, or a few that differ by optional attributes.

Decisions made while building

• Cardinality one is enforced at write time. Asserting a new value writes an explicit retraction of the old one to the log. Current state is then a plain fold over assertions and retractions, whatever an attribute's cardinality was at the time.
• Redundant writes are dropped. Asserting a value the entity already has, or retracting one it doesn't have, is reported as unchanged and not logged. A call that changes nothing writes no transaction, so re-running an ingestion leaves the log alone.
• Lookups create only from assertions. A lookup that finds nothing creates the entity when the call asserts something. If the lookup is used only to retract, the call is an error.
• String equality has a hash index. The btree on string values indexes their first 200 characters, so long values never break an insert. Equality goes through a hash index on the whole value, which v = 'x' on any view can use.
• Kernel tests run on bare stores. init installs factstore-core, but the kernel's tests pass core=False: the kernel knows no names, so its tests shouldn't depend on core's. The packages have their own tests.
• Near-match thresholds. These are a first calibration on sample attributes, set at the top of store.py; tests/test_register.py records the cases they were checked against. The "attributes registered" measure in M5 is what tunes them.