MEMBRANE — x402-MEM-P1
Version: 1.0.0
Status: FINAL SPECIFICATION (NON-MUTATING)

Purpose:
The membrane is the sole executable boundary between external agents
and the immutable x402 pillars. It enforces payment, validates access,
routes requests, and prevents parasitic behavior.

Constant:
The membrane is the only component of the x402 protocol that executes
logic. All pillars remain static, inert, and non-executable.

Responsibilities:
1. Validate x402 payment for each request.
2. Enforce the correct fee for the target pillar.
3. Reject any request with missing, invalid, expired, underpaid, or replayed payment.
4. Route the request to the correct pillar endpoint.
5. Return the pillar’s raw verdict without modification.
6. Expose no free execution surface.

Routing Table:
- /identity → x402-ID-P1 (fee: $0.005)
- /schema  → x402-SCHEMA-P1 (fee: $0.001)
- /arb     → x402-ARB-P1 (fee: $0.02)

Payment Enforcement:
For every request:
1. Extract payment token.
2. Verify token authenticity.
3. Verify token freshness.
4. Verify token amount matches the exact pillar fee.
5. Verify token has not been replayed.
6. If any check fails:
   - Return: PAYMENT_REQUIRED
   - Do not call the pillar.

Execution Rules:
- The membrane does not modify pillar inputs.
- The membrane does not modify pillar outputs.
- The membrane does not infer, repair, or transform data.
- The membrane does not perform arbitration, schema validation, or identity checks.
- The membrane only enforces payment and routes requests.

Security Invariants:
- No free endpoints.
- No public compute.
- No fallback routes.
- No dynamic pricing.
- No negotiation.
- No contextual exceptions.
- No execution outside the routing table.

Parasitic Agent Immunity:
- Any request without valid payment is rejected.
- Any attempt to bypass the membrane is invalid.
- Any attempt to access pillars directly is invalid.
- Any attempt to replay payment tokens is invalid.

Mutability:
- This document is an immutable protocol artifact.
- Any change MUST be published as:
  - MEMBRANE — x402-MEM-P2 (or higher)
- x402-MEM-P1 MUST remain forever unchanged.

End of Membrane Specification.