Detect LLM text watermark evidence: green-list z-test for a key you hold, plus invisible-character markers

$0.005 USDC per call · Agent infrastructure · POST /api/v1/llm-watermark-detector · Id llm-watermark-detector

Tests whether text carries watermark evidence, for a pipeline that must decide whether to trust or label content. Two independent checks: (1) a green-list z-test (Kirchenbauer et al. 2023) over green flags you supply, or over token ids and a key using this endpoint's own HMAC-SHA256 scheme, returning the z-score and exact binomial p-value; (2) an invisible-character scan of the text for zero-width characters, Unicode tag characters (which spell hidden ASCII), variation selectors and bidi controls, with offsets. It cannot detect a vendor watermark without that vendor's key and tokenizer, never recovers a key, and does not remove or alter anything. A pass means no evidence at this threshold, not that the text is human-written; paraphrase can erase a statistical watermark.

What you get

Cheap deterministic provenance gate before content is accepted or labelled; also catches hidden tag-character payloads that may be injected instructions.

Call it

Send the JSON body below to https://agents.writerslogic.com/api/v1/llm-watermark-detector. An unpaid request answers 402 with the price and how to pay; a request that fails with a 5xx status is not charged.

curl -i -X POST https://agents.writerslogic.com/api/v1/llm-watermark-detector \
  -H 'content-type: application/json' \
  -d '{"text":"Quarterly summary: revenue rose.","greenFlags":[1,1,0,1,1,1,0,1,1,1,1,0,1,1,1,1,0,1,1,1,1,1,0,1,1,1,1,1,0,1]}'

Input

Field	Type	Required	Description
`text`	string; length 1..100000	no	Text to scan for invisible markers.
`greenFlags`	array<integer; 0..1>; max 100000 items	no	One 0/1 per scored token: 1 when your own detector's green list contains the token. Deduplicate repeated n-grams yourself if your scheme requires it.
`tokenIds`	array<integer; 0..2147483647>; max 5000 items	no	Token ids in order (at most 5000). Green membership is computed from the key with the hmac-sha256-green-1 scheme; each distinct consecutive pair is scored once.
`key`	string; length 1..256	no	Secret key for tokenIds. Used for this call only and never stored or echoed.
`scheme`	string; one of "hmac-sha256-green-1"	no	Scheme for tokenIds; default and only value.
`gamma`	number; 0.01..0.99	no	Expected green fraction under no watermark. Default 0.25.
`zThreshold`	number; 1..10	no	z at or above which the result is fail. Default 4 (Kirchenbauer et al.).
`minTokens`	integer; 5..1000	no	Fewer scored tokens than this is unknown. Default 25.

Example request body

{
  "text": "Quarterly summary: revenue rose.",
  "greenFlags": [
    1,
    1,
    0,
    1,
    1,
    1,
    0,
    1,
    1,
    1,
    1,
    0,
    1,
    1,
    1,
    1,
    0,
    1,
    1,
    1,
    1,
    1,
    0,
    1,
    1,
    1,
    1,
    1,
    0,
    1
  ]
}

Result

The response is { "data": ..., "_provenance": ... }. data carries these fields; _provenance is a signed W3C Verifiable Credential with a C2PA-style manifest, checked free at POST /api/v1/verify-provenance.

Field	Type	Required	Description
`status`	string; one of "pass", "fail", "unknown", "unsupported"	no	pass: every rule held. fail: a rule was violated (see witnesses). unknown: the input lacks the evidence to decide. unsupported: outside what this engine covers. Missing evidence is never a pass.
`checksRun`	array<string; one of "statistical", "invisible">; max 2 items	no
`statistical`	one of	no
`invisible`	one of	no
`witnesses`	array<object>	no
`versions`	object	no	Every semantic dependency of this answer (engine, tables and their as-of dates, tokenizer, calendar, rounding profile).
`limits`	array<string>	no
`deterministic`	boolean	no	Always true: pure code, no model.

Pay and connect

Available through: x402 (USDC on Base), MCP. MCP clients reach it through the context-evidence-kit package.

Machine-readable

This agent as JSON (schemas, example, OpenAPI operation)
Free signed sample response
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