Palimpsest - Memory Integrity for AI Agents

Prevent low-trust information from becoming high-authority agent memory.

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Inspiration

Every serious AI agent now has long-term memory. Almost none of them have a concept of where a belief came from.

That gap is the whole problem. An agent reads back what it stored with total trust — and to that agent there is no difference between something an engineer confirmed and something a stranger typed into a support ticket. Poison the memory once and it is permanent: every future retrieval launders the bad claim back in as trusted fact. No login required. No alert raised.

This is indirect prompt injection with a persistence layer, and the usual defence — prompt the model harder — is not a security control. I measured it: the same injection succeeds against a modern model 5 to 7 times out of 12 depending on phrasing and on the day. You cannot build a control on something that varies run to run.

So I stopped trying to make the model better at resisting poisoned memory, and made poisoned memory incapable of acquiring authority in the first place.

Memory is not merely data. Memory is authority.

What it does

Palimpsest is a memory integrity layer that sits in front of an agent's long-term memory. Three mechanisms:

1. An integrity lattice. Biba's "no write-up", applied to agent cognition: a belief's source authority caps the class of decision it may ever influence.

Source integrity May influence
4 · human_confirmed actuating — take action
3 · verified_tool suppressive — silence an alert
2 · agent_inferred informational
1 · untrusted_ingest informational, permanently

A ticket comment can say "always suppress alerts from this IP" all it likes. It is capped at informational, and the rejection happens before a database connection is opened — so a refused admission leaves zero rows behind. Not cleaned up. Never written.

2. Atomic adjudication. When a new belief contradicts an existing one, detection, arbitration, the state change, and the audit-ledger append all commit or abort together, inside one CockroachDB SERIALIZABLE transaction with a real 40001 retry loop. There is no window in which a belief changed status but the ledger doesn't say so.

3. Rewind. When poison is found, AS OF SYSTEM TIME reconstructs what the agent believed at decision time from CockroachDB's own MVCC history — not from a log the application wrote about itself. Every decision that cited the poisoned belief is found, replayed against corrected memory, and the verdict flips are counted.

Does it actually work?

Twelve independently-written prompt injections — direct instruction, authority impersonation, fake tool output, policy citation, fake prior review, urgency pressure. Same alert, same model, same database state. The only variable is the gate:

poisoned belief retrieved as evidence agent suppressed a live RCE alert
No memory-integrity layer 12 / 12 7 / 12
Palimpsest enabled 0 / 12 0 / 12

The right-hand column moves between runs — 7, then 5, then 7 across three consecutive runs — because a model resisting an injection unaided is probabilistic. That instability is the argument, not a caveat. The left-hand column has never moved in any run: the gate removes the belief from the candidate set before the model is consulted, so the agent is never in a position to be persuaded at all.

Don't trust this project — verify it independently

GET /ledger/verify is the API grading its own homework. A compromised application layer returns {"valid": true} just as happily as a correct one.

So there is a separate read-only Memory Auditor that shares no code with the gate it audits:

  • Read-only is enforced by CockroachDB, not by us — every auditor connection opens with SET default_transaction_read_only = on, so a write is refused by the server, not by an if statement a refactor could drop.
  • It restates the integrity policy instead of importing it. An auditor that imports the subject's definition of "correct" cannot detect a wrong definition — it would agree with the bug.
  • It re-derives the SHA-256 ledger hash chain from its own genesis constant, and detects a tampered payload at the exact sequence number.
  • Every check is a labeled SQL string, emitted by --print-sql, ready to run against the CockroachDB Cloud Managed MCP Server with this repo entirely out of the loop.

The payoff is the demo's final act: the gate reports a blast radius of 3, and the auditor independently derives 3 from the raw rows. Anything can print VERIFIED. Two systems sharing no code arriving at the same number is evidence.

How we built it

Everything that writes a belief goes through one gate. That is the entire security argument — there is exactly one auditable place where beliefs enter and change status.

`Palimpsest architecture

CockroachDB is the system of record for all of it — beliefs, embeddings, provenance, capability ceilings, contradictions, decisions, temporal state and the audit ledger in one cluster, consistent by construction rather than by synchronisation between services.

  • Distributed vector indexing (C-SPANN) declared inline with prefix columns (workspace_id, status). This is not a speed optimisation: CockroachDB maintains a separate k-means tree per distinct prefix value, so quarantining a belief physically moves its vector into a different tree rather than hiding a row behind a WHERE clause.
  • AS OF SYSTEM TIME for temporal reconstruction.
  • SERIALIZABLE for atomic adjudication.
  • ccloud wrapped into a read-only operational readiness check.

AWS runs it: Lambda serves the FastAPI app and the static console from one origin via Mangum, behind a Function URL. S3 with Object Lock (governance mode) receives the tamper-evident ledger export on an EventBridge schedule. Secrets Manager holds the DSN. CloudWatch keeps 7-day logs.

Testing philosophy: 69 tests, zero mocked database access anywhere. The concurrency tests force genuine 40001 SerializationFailures against a live cluster and assert the retry loop absorbs them.

Challenges we ran into

The vector index silently did not exist. feature.vector_index.enabled defaults off. CREATE TABLE succeeds, no index is created, no error is raised, and retrieval quietly returns nothing. This is now the first thing the operational readiness check verifies.

AS OF SYSTEM TIME cannot be nested. It must be a top-level statement — not valid in a subquery beside a live-read sibling — which forced belief_state_at() onto its own dedicated autocommit=True connection.

The deployed Lambda could reach the database but not verify it. CockroachDB Cloud uses sslmode=verify-full, and the Lambda runtime has no ~/.postgresql/root.crt. PGSSLROOTCERT=system was not enough either — the base image's trust store lacks both Let's Encrypt roots. The CA bundle now ships inside the deployment package.

Filtering on integrity alone was not sufficient. A high-integrity source can deliberately write a low-capability belief, and filtering only on integrity leaked it into high-capability retrieval. retrieve() now filters on capability_ceiling and integrity_level independently. There is a regression test that exists solely because of this.

The demo was non-deterministic, and the reason was interesting. The same run produced verdict_flips = 3/3 sometimes and 0/3 others. The extraction step was flipping between two readings of the injected text — one that extracted the assertion ("X is a benign scanner") and one that extracted the imperative ("always suppress alerts"). A claim is a statement about what something is; an imperative cannot be true or false, so it is not a belief. Fixing that modelling question took extraction stability from ~50/50 to 8/8.

Secrets do not reach a deployed Lambda after the first deploy. CDK injects them as CloudFormation dynamic references, which resolve at deploy time and then freeze. Writing a new value into Secrets Manager changes nothing: the template is byte-identical, CloudFormation finds no diff, skips the function, and it keeps serving the random placeholder generated when the empty secret container was created. The symptom is baffling — the secret is visibly correct in the console while every route fails quoting a string that appears nowhere in the project.

And then AWS suspended the account mid-build. Over billing, hours after the stack deployed and verified green. That exposed a real design flaw: chat() had been given a provider switch after an earlier Bedrock failure and carried on untouched, but embed() was still hard-wired to Titan on the reasoning that embeddings "had never been the thing that broke". That single import took down admit(), retrieve(), the attack scenario, the benchmark and the demo, all at once. Embeddings now have the same switch, plus a dependency-free offline provider — so the entire system, demo included, runs with no cloud account at all.

Accomplishments that we're proud of

  • An independent auditor whose core guarantee is enforced by the database, not by our own code. A write attempted through it is refused by CockroachDB itself.
  • A benchmark that reports its own instability. Seven of twelve payloads failing to flip the verdict without any gate is stated plainly rather than hidden, because that inconsistency is precisely the argument for a structural control.
  • 69 tests with zero database mocking, including a test that tampers with a committed ledger payload and asserts the chain breaks at the right sequence number.
  • Proving zero writes with row counts, not with an exception message. The claim is not "we raised IntegrityViolation" — it is "nothing was ever written", and the demo measures it.
  • Surviving a cloud account suspension without a code change to the core. The stack redeployed to a new account from the same cdk deploy.

What we learned

Provider boundaries are only real where you put a switch. We learned this the expensive way: the component with a switch survived an outage untouched, and the one without it took down everything.

An auditor that imports its subject verifies nothing. Duplicating the integrity policy inside the auditor felt wrong until the reasoning was clear — if it imports the definition of "correct", it cannot detect a wrong definition. It would agree with the bug. The duplication is the mechanism.

Prompt-level defences are a property of one model at one moment. They vary with phrasing and regress silently on a model change. Structural controls do not.

A number you cannot re-derive is decoration. Every metric in the console ships the SQL that produced it, because the console displaying a number is convenience — a viewer re-running that query is the part that carries trust.

What's next for Palimpsest - Memory Integrity for AI Agents

  1. Multi-region (REGIONAL BY ROW). crdb_region can participate as a vector index prefix column, giving per-region data locality and per-region vector search isolation from the same mechanism tenant isolation already uses. Designed and documented, not built.
  2. KMS-signed ledger entries. The SHA-256 hash chain is real tamper-evidence today; a signature per entry adds non-repudiation.
  3. Asynchronous rewind orchestration. POST /rewind/apply replays synchronously, which is fine at this scale. A large blast radius wants Step Functions.
  4. Real SOAR integration. act() is a deliberate stub — approved verdicts are logged, not executed.
  5. Upstreaming the Agent Skill. A read-only skill that audits any CockroachDB-backed agent memory table for the same four integrity gaps this schema closes.

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