Status: v0.1-draft — pre-review. Open for feedback; do not build production dependencies on a draft. See GOVERNANCE.md.
MCP standardized how agents call tools. The Open Context Protocol standardizes the layer underneath: how the context that feeds a model is represented, governed and proven — independent of which engine produced it.
Every context engine (retrieval layer, compression proxy, memory system, agent harness) answers the same questions with private formats today:
- What exactly entered the model's context, and where did it come from?
- What is this pipeline allowed to read, execute and remember?
- Which governance rules were active, and can a third party verify any of this after the fact?
Private answers don't compose. An auditor cannot verify five vendors' bespoke logs; a platform team cannot enforce one policy across three engines; an agent cannot hand context to another agent without a shared representation. OCP defines the exchange formats for exactly these five surfaces:
| Part | Surface | One line |
|---|---|---|
| 1 | Context-IR | What entered the context: items, sources, tokens, safety flags, verification hashes |
| 2 | Capability Vocabulary | What a pipeline may do: namespaced capability identifiers and their semantics |
| 3 | Policy Packs | Declarative governance rules: redaction patterns, tool allow/deny, budgets, retention |
| 4 | Evidence Chain | Tamper-evident audit records: hash chains, signatures, offline verification |
| 5 | Event Vocabulary | Governance-relevant runtime events: tool calls, violations, budget signals |
Deliberately out of scope: compression algorithms, retrieval ranking, routing heuristics, ML models — everything that makes one implementation better than another. OCP defines what is exchanged, never how it is produced well. (See ADR-0001.)
| MCP | OCP | |
|---|---|---|
| Standardizes | Tool/resource invocation between model hosts and servers | Context representation, governance and evidence |
| Layer | Agent ↔ tool transport | Context pipeline ↔ auditor / policy / other pipelines |
| Typical question | "How do I call this tool?" | "What entered the context, was it allowed, can you prove it?" |
An agent stack typically speaks both: MCP to reach tools, OCP to represent, govern and prove the context those tools produce. OCP is not a transport — its objects travel over MCP results, HTTP bodies, files or queues unchanged.
spec/ocp-v0.1-draft.md The specification (normative)
spec/schemas/*.schema.json JSON Schemas (draft 2020-12) for every wire object
adr/ Architecture decision records (the protocol/product cut)
GOVERNANCE.md Versioning, RFC process, path to shared governance
A conformance suite (test vectors + harness) is planned as a standalone package; schema validity is the floor, not the ceiling. The reference implementation is LeanCTX — its CI validates real engine output against these schemas on every commit.
- Specification text: CC-BY 4.0
- JSON Schemas: Apache-2.0
Attribution: "Open Context Protocol, © LeanCTX maintainers & contributors."