Factum — Auditable Memory for AI Agents

Status: v0.1.3 — Early stage, seeking early collaborators. Core write/query/retract pipeline works. Not production-ready. Architectural decisions are still open to change.

Factum MCP Demo

Interactive docs — animated syntax parsing, 7-tuple explorer, query pipeline, token efficiency chart, and MCP architecture diagram.

Every fact an agent writes carries mandatory provenance. When a source is retracted, everything derived from it is invalidated automatically — cascade retraction. When facts conflict, Factum returns Ambiguous instead of guessing.

A structured knowledge language (S-expression based), Rust implementation, stdio MCP server — works with Claude Code, Cursor, and any MCP client.

Status: v0.1.3, early stage. Core write/query/retract pipeline works; no semantic search or memory consolidation yet — Factum handles verified structured facts, not conversation context. Best suited for compliance-sensitive agents, multi-agent shared knowledge bases, and anywhere "why did the agent believe X" needs an answer. See the multi-agent usage guide for shared knowledge base patterns.

How is this different from Mem0 / Zep / Letta?

Unique to Factum: grammar-enforced provenance · cascade retraction · conflict refusal.

Not (yet) in Factum: embedding-based semantic retrieval · memory consolidation · HTTP transport.

Complementary: Mem0/Letta store and retrieve context; Factum stores auditable structured facts. They can run side by side via MCP.

What this means in practice

Agent memory pain point	Factum mechanism
Can't tell "user said" from "LLM inferred"	5-level provenance + grammar-enforced model name on Extracted nodes
Stale memory used as current fact	Soft delete + cascade retraction via reverse dependency graph
Conflicting memories silently pick one	`Ambiguous` — refuses to answer rather than guess
Memory pollution (prompt injection)	Provenance chain makes contamination traceable and retractable
Enterprise can't let agents store sensitive data	Index-level permission filtering — no post-query leakage

Academic context: The STALE benchmark (2025) shows that even the best LLM agents achieve only 55.2% accuracy at detecting when their own memories are outdated — confirming that memory staleness is an unsolved problem in agent systems.

Key Features (Implemented)

Node 7-tuple: Every knowledge node carries id, predicate, validity, provenance, confidence, authority, and permissions — plus optional :note for human-readable context
Auto node IDs: Use "auto" as the node ID in factum_insert or factum_assert to auto-generate content-based IDs — no manual ID management
5-level provenance: Verbatim / Summary / Extracted / Derived / Asserted — full audit chain
Grammar-enforced model reference: Extracted nodes MUST carry model + version — the parser rejects them if missing (not just a documentation convention)
Cascade retraction: Derived nodes auto-invalidate when upstream sources are retracted (via deps_rev reverse dependency graph); configurable node limit prevents cascade explosion in large knowledge bases
Corroboration detection: When a second agent asserts the same fact, factum_assert returns corroborated (not an error) — independent agreement is the most valuable multi-agent signal
Conflict arbitration: LatestWins / HighestAuthority / Unanimous / WeightedVote — pattern-aware grouping ensures value conflicts trigger arbitration; returns Ambiguous when it cannot uniquely resolve
Index-level permissions: No post-query filtering — prevents aggregate leakage
Lossless numerics: All numbers use Dec(i128, u8) — zero floating-point error
MCP bridge: JSON-RPC 2.0 tools/resources; stdio transport verified with real MCP clients (Claude Code, Cursor)

What's Implemented vs. What's Not

Module	Status	Notes
factum-core (types, lexer, parser, serialize)	✅ Implemented	100% syntactic round-trip
factum-rt (store, query, arbitration, permissions, verifiers)	✅ Implemented	In-memory store (default) + RocksDB persistence (`--features rocksdb`); 5 column families, WriteBatch atomic writes
factum-mcp (JSON-RPC bridge)	✅ Protocol + handler	stdio transport verified end-to-end; HTTP transport not yet implemented (remote deployment requires custom wrapper)
factum-bench (benchmarks)	✅ Implemented	Syntax round-trip + token efficiency + query perf
factum-l (latent space projection)	❌ Not started	Research item — see ROADMAP.md
Wikidata/Mathlib corpus converters	❌ Not started	M2 milestone
Lean/Z3 verifiers	❌ Not started	Only Schema + DecimalRange verifiers implemented
Embedding-based semantic search	❌ Not started	Not on roadmap — consider using Mem0 alongside Factum
Memory consolidation/summarization	❌ Not started	Not on roadmap — consider using Letta alongside Factum
Inspector (visual debugger)	❌ Not started

Quick Start

bash

# Build
cargo build

# Run tests
cargo test

# Run demo
cargo run -p factum-demo

# Build with RocksDB persistence backend
cargo build --features rocksdb
cargo test --features rocksdb

# Build MCP server with RocksDB persistence
cargo build --release -p factum-mcp --features rocksdb --bin factum-mcp-server

# Fuzz (requires nightly)
cargo +nightly fuzz run fuzz_parser -- -max_total_time=600

Use as an MCP server (Claude Code / Cursor)

bash

# Build the MCP server binary (in-memory, default)
cargo build --release -p factum-mcp --bin factum-mcp-server

# Or build with RocksDB persistence (survives restarts)
cargo build --release -p factum-mcp --features rocksdb --bin factum-mcp-server

# Register with Claude Code (in-memory)
claude mcp add --transport stdio --scope local factum -- "$(pwd)/target/release/factum-mcp-server"

# Register with Claude Code (persistent — recommended for agent memory)
claude mcp add --transport stdio --scope local factum -- \
  "$(pwd)/target/release/factum-mcp-server" --db-path ~/.factum/store

# Verify connection
claude mcp get factum

See Getting Started with Factum MCP for the full guide.

Factum-F Syntax Example

scheme

; Acme Corp knowledge graph
(node n001
  :pred (instance-of @ACME-CORP organization)
  :conf 0.99 :auth 0.95 :perm public
  :src (asserted "wikidata"))

(node n004
  :pred (shareholder-major @ACME-CORP @FOUNDER-1 0.73 :since #date(2001-03-15))
  :conf 0.85 :auth 0.8 :perm confidential
  :src (extracted "doc002" [100 200] (model "claude-sonnet-4" "2025-01")))

(node n006
  :pred (subsidiary-of @ACME-SUB @ACME-CORP :since #date(2001-03-15))
  :src (derived n001 "rule-subsidiary-merge")
  :deps [n001])

When n001 is retracted, n006 is automatically invalidated — the agent knows it can no longer trust the subsidiary relationship.

Three-Layer Architecture

Layer	What It Means	Status
Agent Read	Agent receives Factum-F as context via MCP — lower token overhead than verbose JSON	✅ Token efficiency measured (real o200k_base: canonical −62%, compact −53% vs JSON)
Agent Write	Agent generates Factum-F nodes — parse uniqueness guarantees one valid interpretation, error classes enable self-correction	✅ See authoring guide
Latent Reasoning	factum-l: encode Factum-F into continuous thought vector, reason in latent space, decode back for audit	🔬 Research item — not started, not blocking layers 1-2

A structured knowledge language underpins the memory layer — S-expression based for parse uniqueness, with Dec(i128, u8) for lossless numerics and mandatory model references for LLM self-auditing. Full design rationale in docs/design-rationale.md.

Token Efficiency — The LLM-Native Metric

TL;DR: Factum canonical form saves ~62% tokens vs verbose JSON with the same metadata. All numbers measured with real o200k_base (GPT-4o) tokenizer via tiktoken-rs.

Format	Real tokens (5 nodes)	vs verbose JSON	What it includes
Factum canonical	238	−62%	Full 7-tuple: provenance + confidence + validity + permissions
Factum compact (JSON)	290	−53%	Same 7-tuple, JSON with numeric tags
Markdown	181	−71%	Assertion text only — no provenance, no confidence, no permissions
JSON (pretty)	623	baseline	Same 7-tuple metadata in verbose JSON encoding

Run cargo test -p factum-bench test_token_efficiency_real_tokenizer -- --nocapture to reproduce.

Key finding: The canonical S-expression form (238 tokens) is more token-efficient than the compact JSON form (290 tokens) — BPE tokenizers split JSON delimiters but merge S-expression parentheses. The form designed for correctness is also the most token-efficient.

Round-Trip Fidelity — What Exactly Is 100%?

Syntactic round-trip ✅ — parse(serialize(parse(x))) == parse(x). 100% and verified by the test suite + fuzzing. This is the trust foundation.
Semantic round-trip ❌ Not yet measured — requires factum-l (latent space projection, not implemented).

Morpheme Vocabulary — Current State

Seed morphemes: 200+ across 9 kinds (Entity, Relation, Quantifier, Modal, Temporal, Status, Action, Attribute, Classification) — covering organizational, people, spatial, financial, product/project, version control, document/knowledge, agent memory, permission, and cause/effect domains
Design target: 200–500 (to be loaded from morphemes.toml via build.rs)
Status: Sufficient for agent memory use cases. morphemes.toml codegen is a future M2 item for external contributions.

Relationship to Other Formats

Format	How Factum Differs
RDF / JSON-LD	RDF triples carry no per-node provenance, confidence, or permissions. Factum makes these first-class and non-optional.
Markdown	Markdown has zero metadata. Factum trades human readability for machine verifiability.
JSON	JSON has no schema, no provenance, no temporal validity. Factum compact form uses JSON as transport but adds structure and audit chain.
Mem0 / Zep / Letta	These store and retrieve agent memory. Factum adds grammar-enforced provenance, cascade retraction, and conflict refusal. Complementary, not competitive.

Project Structure

Code

factum/
├── crates/
│   ├── factum-core/     # Data model, lexer, parser, serialization
│   ├── factum-rt/       # Runtime: store (InMemory + RocksDB), query, arbitration, permissions, verifiers
│   ├── factum-mcp/      # MCP bridge: JSON-RPC tools/resources, stdio transport
│   ├── factum-bench/    # Benchmarks: round-trip, token efficiency, query perf
│   └── factum-demo/     # End-to-end demonstration
├── fuzz/              # cargo-fuzz targets (parser, serialize round-trip, lexer)
├── spec/              # Conformance test vectors (JSON, language-agnostic)
├── docs/site/         # Interactive visualization (GitHub Pages)
├── .github/workflows/ # CI: test + fuzz + gitleaks + Pages deploy
├── Cargo.toml         # Workspace root
├── ROADMAP.md         # What's planned and in what order
├── SECURITY.md        # Vulnerability disclosure
├── CONTRIBUTING.md    # How to contribute
├── CHANGELOG.md       # Version history
├── docs/design-rationale.md     # Why each architectural decision was made
├── docs/authoring-for-llms.md   # LLM guide for generating Factum-F
└── docs/getting-started-mcp.md  # MCP setup guide (Claude Code / Cursor)

Test Results

Run cargo test to see the current count. Tests cover: types, lexer, parser, serialization, morphemes, depth-limit/DoS protection, conformance vectors, store, query, arbitration, permission, verifier, subscription, MCP protocol/tools/handler, and benchmarks.

CI badge at the top of this README reflects the latest build status.

Security

Parser has a depth limit (128 levels) to prevent stack overflow DoS on adversarial input
Lexer has a token count limit (1M tokens) to prevent OOM
cargo-fuzz targets for parser, serialize round-trip, and lexer
gitleaks runs in CI to scan for leaked secrets

See SECURITY.md for vulnerability disclosure.

License

MIT

Trademark

"Factum" and the Factum-F language specification are project names. This MIT license covers code only; the specification may be governed by a separate process in the future.

Contributing

Contributions require DCO sign-off (git commit -s). See CONTRIBUTING.md.

MCP Registry name: mcp-name: io.github.factum-project/factum