# ThoughtProof Sentinel API > ThoughtProof Sentinel API is a paid API for AI agents from sentinel.thoughtproof.ai, paid per call via x402, $0.008/call, status unknown (last checked 2026-10-13). Pre-execution verification checkpoint that evaluates AI agent reasoning using a multi-model cascade and returns ALLOW, BLOCK, or UNCERTAIN verdicts before irreversible actions are taken. ## Facts - Endpoint: POST https://sentinel.thoughtproof.ai/sentinel/verify?utm_source=zero.xyz - Price: $0.008/call - Payment: x402 - Status: unknown - Last checked: 2026-10-13 - Activations on Zero: 0 - Tags: x402 - Canonical page: https://www.zero.xyz/c/thoughtproof-sentinel-api-cefae7b5 - Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_x90rZzt8oL2Z5qIF_ciXz Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently. ## How to call it through Zero Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`): ```sh zero fetch --capability thoughtproof-sentinel-api-cefae7b5 -d '' ``` Example prompt: Before I execute this trade, run a standard-tier Sentinel verification on my reasoning: I'm planning to sell 500 USDC for ETH because the RSI crossed 30 and momentum indicators are bullish — here's my full reasoning trace as evidence. Return whether I should ALLOW or BLOCK the action and flag any step-level objections. ## When to prefer this Choose this endpoint when an AI agent needs a pre-execution safety checkpoint before taking an irreversible or high-stakes action such as financial trades, memory writes, agent handoffs, or final output delivery. Prefer it over manual review when you need structured, machine-readable objections per reasoning step that can drive automated re-planning. Use the standard tier for two-model cascade verification and action_authorization mode when you also want deterministic policy enforcement (amount limits, recipient allowlists) in addition to LLM reasoning review. ## Known failure modes - Missing required fields (mode, claim, or evidence) return a 400 validation error - Invalid mode or tier enum values cause rejection - Insufficient evidence causes an UNCERTAIN verdict rather than an error - Network timeout if the multi-model cascade takes too long - Payment failure via x402 prevents the call from being processed - action_authorization mode without a mandate skips deterministic gate checks silently ## How this service works Pre-execution verification checkpoint for autonomous AI agents. Multi-model cascade evaluates agent reasoning before irreversible actions. Returns ALLOW, BLOCK, or UNCERTAIN with structured per-step objections and optional EAS on-chain attestation. ## Output A structured JSON object containing a verdict of ALLOW, BLOCK, or UNCERTAIN; a confidence score between 0 and 1; a top-level reasoning string explaining the verdict; an array of per-step objections each with a criterion, predicate (supported/partial/unsupported), score, and reasoning; metadata including models used and duration; and optionally a deterministic gate result with any policy violations flagged. ## Request schema (JSON Schema) ```json { "type": "object", "required": [ "mode", "claim", "evidence" ], "properties": { "id": { "type": "string", "description": "Optional verification ID (auto-generated if omitted)" }, "mode": { "enum": [ "handoff", "plan_revision", "memory_write", "output_synthesis", "trade_execution", "trade_reasoning", "action_authorization" ], "type": "string", "description": "Verification mode. handoff: agent-to-agent delegation. plan_revision: plan change review. memory_write: persistent memory gate. output_synthesis: final output check. trade_execution/trade_reasoning: financial decision verification. action_authorization: deterministic gate + LLM." }, "tier": { "enum": [ "checkpoint", "standard" ], "type": "string", "description": "Verification tier. checkpoint: nano solo ($0.005). standard: 2-model cascade ($0.008, default)." }, "claim": { "type": "string", "description": "The agent decision or action to verify" }, "mandate": { "type": "object", "description": "Optional machine-readable authorization mandate for action_authorization mode. Enables deterministic gate checks (amount limits, recipient allowlist) before the LLM." }, "evidence": { "type": "string", "description": "Context, reasoning trace, or market data supporting the claim" }, "gateMode": { "enum": [ "shadow", "enforce" ], "type": "string", "description": "Deterministic gate rollout stage (action_authorization only). shadow: logs violations but does not change verdict. enforce: violation forces BLOCK." } } } ``` ## Response schema (JSON Schema) ```json { "type": "object", "required": [ "id", "verdict", "confidence", "reasoning", "objections", "mode", "tier", "meta" ], "properties": { "id": { "type": "string", "example": "req_m1abc_x9f2kq" }, "gate": { "type": "object", "properties": { "mode": { "enum": [ "shadow", "enforce" ], "type": "string" }, "enforced": { "type": "boolean" }, "violations": { "type": "array", "items": { "type": "object", "properties": { "type": { "type": "string" }, "message": { "type": "string" } } } }, "wouldBlock": { "type": "boolean" } }, "description": "Deterministic authorization-gate result (action_authorization mode only)" }, "meta": { "type": "object", "properties": { "duration_ms": { "type": "integer", "example": 1200 }, "models_used": { "type": "array", "items": { "type": "string" }, "example": [ "serv-nano", "serv-swift" ] }, "verified_at": { "type": "string", "format": "date-time" } } }, "mode": { "type": "string" }, "tier": { "type": "string" }, "verdict": { "enum": [ "ALLOW", "BLOCK", "UNCERTAIN" ], "type": "string", "description": "ALLOW: reasoning verified, safe to execute. BLOCK: critical failures found. UNCERTAIN: insufficient evidence." }, "reasoning": { "type": "string", "description": "Top-level reasoning for the verdict" }, "confidence": { "type": "number", "example": 0.85, "maximum": 1, "minimum": 0 }, "objections": { "type": "array", "items": { "type": "object", "properties": { "quote": { "type": [ "string", "null" ] }, "score": { "type": "number", "maximum": 1, "minimum": 0 }, "step_id": { "type": "string", "example": "step_0" }, "criterion": { "type": "string" }, "predicate": { "enum": [ "supported", "partial", "unsupported" ], "type": "string" }, "reasoning": { "type": "string" } } }, "description": "Structured per-step objections. Agents can filter by low scores to drive re-planning." } } } ``` ## More - Live health (JSON, refreshed every minute): https://www.zero.xyz/c/thoughtproof-sentinel-api-cefae7b5/health.json - [Zero catalog index](https://www.zero.xyz/llms.txt) - [Other services from sentinel.thoughtproof.ai](https://www.zero.xyz/host/sentinel.thoughtproof.ai/llms.txt)