title: Measuring Watermark-Detector Interoperability
subtitle: A Measurement Protocol for the 2 Feb 2027 Code-of-Practice Commitment
author: Council for the Safety of AI (CSOAI) · POAI axis
date: August 5, 2026
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Measuring Watermark-Detector Interoperability
A Measurement Protocol for the 2 Feb 2027 Code-of-Practice Commitment
CSOAI / POAI · v0.1 SPEC · 2026-08-05 Tagged [SOURCED] / [SPEC] / [CAVEAT]. Nothing asserted.
Abstract
By 2 February 2027, the ~190 signatories of the EU Code of Practice on Transparency of AI-generated Content (Anthropic, Google, Meta, Microsoft, OpenAI, Mistral, and others) commit that watermark detection mechanisms be interoperable and meet quality criteria of effectiveness, reliability, robustness, and interoperability [SOURCED]. The Code itself concedes that "common evaluation standards have yet to emerge." A binding date, ~190 obligated parties, and no agreed way to measure compliance is the exact condition under which a signed, independent measurement standard defines the market. This document specifies a deterministic protocol for the four criteria. It reuses the provenance axis (POAI-Bench) machinery — which already measures marking survival — and extends it to cross-detector interoperability.
1. The four criteria as measurements
[SPEC] Each Code criterion maps to a measurable quantity, graded deterministically (a detector either returns present/absent/uncertain on a given asset — no LLM judge):
| Criterion | Measured as |
|---|---|
| Effectiveness | True-positive rate: fraction of marked assets a detector correctly reports as AI-generated. |
| Reliability | False-positive rate: fraction of unmarked (human-origin) assets wrongly reported as AI-generated. Reported separately — a detector that flags everything is not reliable. |
| Robustness | Effectiveness re-measured after standard real-world transforms (re-encode, resize, crop, screenshot). This is POAI-Bench's existing marking-survival measurement, applied to the detector. |
| Interoperability | The cross term: does detector D_j correctly read a mark written by producer P_i, for every (producer, detector) pair? Reported as an N×M interoperability matrix, not a single number. |
2. The interoperability matrix (the part nobody measures)
[SPEC] Effectiveness/reliability/robustness are per-detector. Interoperability is inherently cross-party and is where "common standards have yet to emerge." The protocol:
- Collect a set of assets marked by each producer P_1 … P_N (the signatories' own marking tools).
- Run each detector D_1 … D_M over all assets.
- Report the N×M matrix M[i][j] = effectiveness of detector j on producer i's marks (and its robustness under transforms).
- The diagonal is self-detection; the off-diagonal is interoperability. A near-diagonal matrix means the ecosystem is not interoperable — each detector only reads its own marks. That is the exact finding the Code fears and cannot currently quantify.
[SPEC] The single headline number is the off-diagonal mean effectiveness — cross-party detection rate. Publishing this, signed and recomputable, is the standard the Code lacks.
3. What ships, and the boundary
- Ships (public): the protocol, the deterministic scoring code, the transform set (identical to POAI-Bench's), and a signed interoperability matrix per evaluation round — bound into the same ML-DSA-65 attestation chain as every GSPC result. No producer is ranked "best"; the matrix is the finding.
- CSOAI's role: the independent measurer. It does not build a marking scheme or a detector (that would make it a competitor to the signatories it measures). It runs the matrix and signs it.
4. Why it is the market-defining move
[SOURCED] A binding 2 Feb 2027 date, ~190 obligated parties, an explicit admission that no evaluation standard exists, and no incumbent measuring it. [SPEC] The provenance machinery already exists (POAI-Bench measures marking survival); the extension to a cross-detector matrix is additive, not a rebuild. Publishing the matrix before Q4 2026 — signed, recomputable, independent — is how a measurement body defines a market rather than competes in one.
5. Limitations, stated
[CAVEAT] v0.1 SPEC — the protocol, not a measured matrix; a real matrix needs the signatories' marking tools + detectors, which is an access question (some are proprietary). [CAVEAT] The transform set defines "robustness"; an adversarial transform outside the set is out of scope and must be declared. [CAVEAT] "Interoperable enough" is ultimately a policy threshold, not a measurement — CSOAI reports the matrix; the threshold is the AI Office's to set.
CSOAI is an independent measurement body — it attests results, never conformity, and builds neither the marking schemes nor the detectors it measures. POAI-Bench is the provenance axis of the GSPC family. — csoai.org