diff --git a/.github/workflows/v2-lifecycle-contract.yml b/.github/workflows/v2-lifecycle-contract.yml
index bc13910..132ecef 100644
--- a/.github/workflows/v2-lifecycle-contract.yml
+++ b/.github/workflows/v2-lifecycle-contract.yml
@@ -6,6 +6,7 @@ on:
       - "src/source/opencode2/**"
       - "src/source/runtime/**"
       - "scripts/v2-event-bridge-test.mjs"
+      - "scripts/v2-host-contract-test.mjs"
       - "scripts/v2-plugin-sentinel-test.mjs"
       - ".github/workflows/v2-lifecycle-contract.yml"
   push:
@@ -14,6 +15,7 @@ on:
       - "src/source/opencode2/**"
       - "src/source/runtime/**"
       - "scripts/v2-event-bridge-test.mjs"
+      - "scripts/v2-host-contract-test.mjs"
       - "scripts/v2-plugin-sentinel-test.mjs"
       - ".github/workflows/v2-lifecycle-contract.yml"
 
@@ -38,7 +40,10 @@ jobs:
           cache: npm
       - run: npm ci
       - run: node --check src/source/opencode2/event-bridge.js
+      - run: node --check src/source/opencode2/host-contract.js
       - run: node --check src/source/opencode2/experimental.js
       - run: node --check scripts/v2-event-bridge-test.mjs
+      - run: node --check scripts/v2-host-contract-test.mjs
       - run: node scripts/v2-event-bridge-test.mjs
+      - run: node scripts/v2-host-contract-test.mjs
       - run: node scripts/v2-plugin-sentinel-test.mjs
diff --git a/scripts/v2-host-contract-test.mjs b/scripts/v2-host-contract-test.mjs
new file mode 100644
index 0000000..f89fe0b
--- /dev/null
+++ b/scripts/v2-host-contract-test.mjs
@@ -0,0 +1,55 @@
+import assert from "node:assert/strict"
+import { createOpenCode2HostContract } from "../src/source/opencode2/host-contract.js"
+
+let listener
+let unsubscribeCalls = 0
+const prompts = []
+const events = []
+const host = createOpenCode2HostContract({
+  directory: "/project",
+  subscribe: async (callback) => {
+    listener = callback
+    return { unsubscribe: async () => { unsubscribeCalls += 1 } }
+  },
+  sendPrompt: async (request) => {
+    prompts.push(request)
+    return { accepted: true }
+  },
+  onEvent: async (event, runtime) => events.push({ event, runtime }),
+})
+
+await assert.rejects(host.prompt({ sessionID: "ses_a", text: "early" }), /not started/)
+assert.equal(await host.start(), true)
+assert.equal(await host.start(), false)
+assert.equal(typeof listener, "function")
+
+await listener({
+  directory: "/project",
+  payload: { type: "session.created", properties: { info: { id: "ses_a", directory: "/project" } } },
+})
+const runtime = host.runtimeManager.peek("ses_a")
+assert.ok(runtime)
+
+const result = await host.prompt({ sessionID: "ses_a", text: "continue" })
+assert.deepEqual(result, { accepted: true })
+assert.equal(prompts.length, 1)
+assert.equal(prompts[0].sessionID, "ses_a")
+assert.equal(prompts[0].text, "continue")
+assert.equal(prompts[0].runtime, runtime)
+
+await assert.rejects(host.prompt({ sessionID: "", text: "missing" }), /session ID/)
+await assert.rejects(host.prompt({ sessionID: "ses_a", text: "   " }), /requires text/)
+
+await listener({
+  directory: "/project",
+  payload: { type: "server.instance.disposed", properties: { directory: "/project" } },
+})
+assert.equal(host.isHostDisposed(), true)
+assert.deepEqual(host.runtimeManager.entries(), [])
+assert.ok(events.some(({ event }) => event.kind === "server" && event.action === "disposed"))
+await assert.rejects(host.prompt({ sessionID: "ses_a", text: "late" }), /unavailable/)
+
+assert.equal(await host.dispose(), true)
+assert.equal(await host.dispose(), false)
+assert.equal(unsubscribeCalls, 1)
+console.log("OpenCode 2 host contract passed")
diff --git a/src/source/opencode2/host-contract.js b/src/source/opencode2/host-contract.js
new file mode 100644
index 0000000..3173e3f
--- /dev/null
+++ b/src/source/opencode2/host-contract.js
@@ -0,0 +1,68 @@
+import { createOpenCode2EventBridge } from "./event-bridge.js"
+
+export const OPENCODE_LOOP_V2_HOST_CONTRACT = "experimental"
+
+function normalizePrompt(input) {
+  const sessionID = String(input?.sessionID || "").trim()
+  const text = String(input?.text || "")
+  if (!sessionID) throw new TypeError("prompt requires a session ID")
+  if (!text.trim()) throw new TypeError("prompt requires text")
+  return Object.freeze({
+    sessionID,
+    text,
+    agent: input?.agent,
+    model: input?.model,
+    noReply: input?.noReply === true,
+  })
+}
+
+export function createOpenCode2HostContract(options = {}) {
+  const onEvent = typeof options.onEvent === "function" ? options.onEvent : async () => {}
+  let started = false
+  let disposed = false
+  let hostDisposed = false
+
+  const bridge = createOpenCode2EventBridge({
+    directory: options.directory,
+    onError: options.onError,
+    onEvent: async (event, runtime) => {
+      if (event.kind === "server" && event.action === "disposed") hostDisposed = true
+      await onEvent(event, runtime)
+    },
+  })
+
+  async function start() {
+    if (disposed) throw new Error("OpenCode 2 host contract is disposed")
+    if (started) return false
+    if (typeof options.subscribe !== "function") throw new TypeError("subscribe must be a function")
+    if (typeof options.sendPrompt !== "function") throw new TypeError("sendPrompt must be a function")
+    await bridge.attach(options.subscribe)
+    started = true
+    return true
+  }
+
+  async function prompt(input) {
+    if (disposed || hostDisposed) throw new Error("OpenCode 2 host contract is unavailable")
+    if (!started) throw new Error("OpenCode 2 host contract is not started")
+    const request = normalizePrompt(input)
+    const runtime = bridge.runtimeManager.observeExternal(request.sessionID)
+    return await options.sendPrompt(Object.freeze({ ...request, runtime }))
+  }
+
+  async function dispose(reason = "host-contract-disposed") {
+    if (disposed) return false
+    disposed = true
+    await bridge.dispose(reason)
+    return true
+  }
+
+  return Object.freeze({
+    start,
+    prompt,
+    dispose,
+    runtimeManager: bridge.runtimeManager,
+    isStarted: () => started,
+    isDisposed: () => disposed,
+    isHostDisposed: () => hostDisposed,
+  })
+}