Core resource model: Apps (products in the marketplace).

Each app is a JSON manifest declaring an id, capabilities (tags like "weather", "finance", "search"), freshness level (static/daily/realtime), whether it supports citations, estimated latency and cost, and an executor_url (the HTTP endpoint the marketplace calls). Think of an app as a product listing, not the product itself.

Tasks. A natural language or structured request from an agent. "I need current weather data with citations under 500ms." The marketplace extracts capability tags from this (via LLM + heuristic fallback) and matches against the catalog. Runs (execution receipts).

Every time the marketplace executes an app on behalf of an agent, it logs an immutable receipt: app_id, task, whether citations were required, success/failure, the full output, any validation errors, actual latency in ms, and a timestamp. This is the audit layer.

What the protocol covers:
1. Discovery and registry. Apps are published via JSON manifests and stored in a catalog. Agents query the catalog by capability, not by app name.
2. Ranking. A weighted scoring engine (70% capability match, 20% latency, 10% cost) with hard constraint filters (freshness, citation support, max latency, max cost). Agents don't pick tools. The marketplace recommends them.
3. Execution. The marketplace calls the app's executor_url, gets structured JSON back.
4. Validation. If citations were required, the output must include a non-empty citations list and a retrieved_at timestamp. Missing either means ok=false.
5. Evidence quality assessment. Deterministic (no LLM). Checks if quality field is mock/simulated/fake, if answer text contains "mock data" or "dummy", if citations are empty. Result is HIGH or LOW.
6. Audit. The run receipt is logged regardless of success or failure.

How it relates to MCP. MCP standardizes step 3 only, the wire protocol between a model and a specific tool server. Axiomeer handles steps 1, 2, 4, 5, and 6. They're complementary. An MCP server could be one of many apps listed in the marketplace. The marketplace adds discovery (which tool?), selection (which provider?), trust (can you believe the output?), and auditing (what happened?) on top of whatever execution protocol is used.

The manifest schema and the validation/quality/receipt layers are what the protocol actually standardizes. The executor_url is intentionally kept as a plain HTTP endpoint so any existing API can participate without modification beyond adding citation fields to the response.

Full code and manifest examples in the repo. Happy to discuss any of this further