Stroop Probes

Stroop probes are the second layer of Cerno’s verification pipeline. Where the maze tests motor control and path planning, Stroop probes test cognitive presence with server-authored timing.

Why Stroop probes

AI agents can solve mazes. They can also replay cursor traces. What they still do badly is handle a surprise task under time pressure while staying bound to the actual solve state.

The Stroop effect: naming the ink color of a word is slower when the word names a different color. Cerno uses that interference by pausing the solve at a trigger cell and asking the user to tap the correct color under a short deadline.

How it works

1. During challenge generation, generateStroopProbe() selects a trigger cell 40-60% through the maze solution path.
2. It places 3-4 colored cells near that trigger point.
3. When the user’s trace reaches the trigger cell, the client calls POST /probe/arm with the trace prefix through that cell.
4. The server validates the prefix, stores an arm session, and returns a signed probe_ticket with armed_at and deadline_at.
5. The StroopOverlay renders with the server-armed probe. When the user taps a cell, the client calls POST /probe/complete with the probe_ticket.
6. The server computes authoritative reaction timing, validates the tap, and returns a signed completion_token.
7. Final /verify accepts probe_completion_tokens, not raw reaction times.

Client integration

The StroopOverlay component from @cernosh/react renders automatically when the user’s trace hits a trigger cell. You do not need to mount it yourself when using the stock Cerno component.

StroopOverlay props

Prop	Type	Description
probe	StroopProbe	The probe definition (cells, instruction, target color)
mazeWidth	number	Maze grid width in cells
mazeHeight	number	Maze grid height in cells
cellSize	number	Pixel size of each maze cell
theme	’light’ | ‘dark’	Adapts overlay background and text colors
onComplete	(response: ProbeResponse) => void	Callback with tap result before the client exchanges it for a completion token

Client-side callback shape

interface ProbeResponse {
  probe_id: string
  tapped_cell: { x: number; y: number }
  reaction_time_ms: number
}

That callback is not authoritative. The shipping flow exchanges it for a signed completion token.

Server validation

Import the probe flow helpers from @cernosh/server:

import {
  armProbe,
  completeProbe,
  verifyProbeCompletionTokens,
} from '@cernosh/server'

armProbe(config, request)

const result = await armProbe(config, {
  challenge_id,
  site_key,
  session_id,
  probe_id,
  events: tracePrefix,
})

This validates that the trace actually reached the trigger cell and returns:

{
  success: true,
  probe_ticket: string,
  armed_at: number,
  deadline_at: number,
}

completeProbe(config, request)

const result = await completeProbe(config, {
  challenge_id,
  session_id,
  probe_ticket,
  tapped_cell,
})

This returns a signed completion_token when the probe response was valid and timely.

verifyProbeCompletionTokens(config, challenge, sessionId, tokens)

const result = await verifyProbeCompletionTokens(
  config,
  challenge,
  sessionId,
  submission.probe_completion_tokens,
)

Validation rules:

• token count must match the number of required probes
• the token must be signed by the server and bound to the challenge and session
• completion can happen only once
• server-recorded timing must fall within the allowed bounds
• all probes must be answered correctly for the probe stage to pass

Reaction time expectations

Range	Interpretation
Below 150ms	Rejected. Superhuman / automated.
150-300ms	Valid but penalized. Unusually fast for a Stroop task.
300-2000ms	Ideal range. Full timing score.
2000-5000ms	Valid but mild penalty. Suggests hesitation or distraction.
Above 5000ms	Rejected. The server deadline elapsed.

Configuration

Stroop probes activate when the challenge type is maze_stroop. The current implementation injects at most one probe and falls back to a plain maze if the maze is too small.

Key parameters in the generation logic:

Parameter	Value	Notes
Trigger position	40-60% through solution	Ensures user is engaged before probe fires
Distractor count	2-3	Randomly chosen per probe
Auto-timeout	5000ms	Server rejects completion after deadline
Min solution length	6 cells	Below this, no probe is generated