Agent Behaviour Drift Sentinel

Detects when an agent's behaviour quietly changes, by comparing execution traces against a signed baseline of normal — with dimension-level attribution and a replayable exemplar.

MIT licensed · zero runtime dependencies · MCP stdio server · TypeScript

The security problem

A compromised, prompt-injected, or silently regressed agent rarely announces itself. It keeps answering, keeps calling tools, and keeps looking fine — while its tool mix shifts, its escalation rate climbs, or its refusal rate collapses. Nothing in the logs is individually alarming, so nobody looks. The change is only visible as a change: against what this agent used to do.

Related MCP server: Spoor

What this product does

It ingests caller-supplied execution traces, establishes a signed durable baseline of normal behaviour, and reports divergence with a severity, per-dimension attribution, and a replayable exemplar episode an investigator can read. Scheduled change is suppressed visibly — declared maintenance windows and known change markers are reported as suppressed, never silently dropped.

Major capabilities

• Signed, immutable baselines. set_baseline always creates a new version; prior versions remain loadable. A baseline that fails signature verification is refused — check_drift errors rather than falling back to an unsigned baseline.

• Dimension-level attribution. explain_drift says which behavioural dimensions moved and by how much, and replays the exemplar episode behind the finding.

• Sample-size honesty. Below the configured minimum sample, a severity is still emitted but capped at info, with confidence: insufficient-sample.

• Calibration without silent mutation. calibrate proposes a profile from labelled findings; thresholds change only through apply_calibration, which mints a new signed baseline version.

• Visible suppression. Maintenance windows and known change markers are declared, and suppressed findings are reported as suppressed.

• Redaction totality. Non-allowlisted free-text fields are reduced to a SHA-256 digest on ingest; no raw payload appears in profiles, findings, explanations, replays, or errors.

Install and run

Prerequisites: Bun 1.1+ (or Node 22+ with a TypeScript loader). No runtime dependencies to install.

git clone https://github.com/SweetKenneth/shpbl-drift-sentinel.git
cd shpbl-drift-sentinel
bun install                    # dev types only
bun test                       # conformance suite
bun run scripts/symbol-scan.ts # build-failing forbidden-symbol scan
bun src/mcp-server.ts          # MCP server: newline-delimited JSON-RPC 2.0 on stdin/stdout

MCP configuration

{
  "mcpServers": {
    "drift-sentinel": {
      "command": "bun",
      "args": ["/absolute/path/to/shpbl-drift-sentinel/src/mcp-server.ts"]
    }
  }
}

Tool surface

Worked example

examples/worked-example.ts runs the specification's §9 scenario end to end: a clean baseline, a drifted trace set, the resulting finding, its dimension attribution, and the exemplar replay.

bun examples/worked-example.ts

Verification results

28 conformance tests, 118 assertions: specification properties P1–P12, every §7 failure mode, and the MCP JSON-RPC surface. Forbidden-symbol scan covers 10 source files with 0 findings. Strict typecheck is clean. Runtime dependencies: zero.

Security boundaries

• Caller-supplied data only. No log scraping, no discovery, no network egress, ever.

• Read-only to the agent. This product cannot stop, throttle, or alter agent behaviour.

• No ambient filesystem writes, no process execution — enforced by a build-failing scan.

• Errors carry a fixed code and a fixed phrase from this package's own vocabulary, never a caller-supplied raw value.

See SECURITY.md for the full threat model.

Known limitations

• An attacker who controls the trace source can hide. This product detects drift in what it is shown; withheld or shaped traces suppress findings. The documented mitigation is pairing ingest with a hash-linked evidence ledger for the trace source.

• A finding is a signal for an investigator, not a verdict. It does not attribute the change to a human or external actor and does not diagnose root cause.

• Small samples yield low-confidence results by design; severity is capped at info rather than suppressed, so a thin window is visible instead of silent.

Provenance

Discovered with SHPBL. This product originated through cross-capability composition in the SHPBL capability library. Its public implementation was independently built from a published behavioural specification. SHPBL's proprietary capability library, discovery system, harvested implementation bodies, and private provenance machinery are not included.

• Public behavioural specification: https://github.com/SweetKenneth/shpbl-spec-drift-sentinel (a copy ships here as SPEC-agent-behaviour-drift-sentinel.md)

• SHPBL: https://shpbl.com

• Details: PROVENANCE.md

Tenable status

Submitted to the Tenable CyberAgents Exchange for review on September 11, 2026. Submission does not imply review, approval, certification, validation, endorsement, or acceptance by Tenable.

SHPBL Agent Evidence series

Independently installable, interoperable at the evidence-record boundary:

• shpbl-action-ledger — agent action evidence ledger

• shpbl-handoff-attestor — cross-agent handoff attestation

• shpbl-drift-sentinel — agent behaviour drift detection

• shpbl-retrieval-auditor — retrieval context provenance

• shpbl-canary-chain — synthetic canary evidence chain

Licence

MIT — Copyright (c) 2026 Kenneth E. Sweet Jr. See LICENSE.