diff --git a/packages/opencode/src/acp/service.ts b/packages/opencode/src/acp/service.ts
index 39840c0910..b8c700ea06 100644
--- a/packages/opencode/src/acp/service.ts
+++ b/packages/opencode/src/acp/service.ts
@@ -330,25 +330,34 @@ export function make(input: {
     }
   })
 
+  const abortBackingSession = Effect.fn("ACP.abortBackingSession")(function* (current: ACPSession.Info) {
+    yield* request(
+      () => input.sdk.session.abort({ directory: current.cwd, sessionID: current.id }, { throwOnError: true }),
+      "session",
+    ).pipe(
+      Effect.catch((error) =>
+        Effect.sync(() => {
+          log.error("failed to abort ACP backing session", { error, sessionID: current.id })
+        }),
+      ),
+    )
+  })
+
   const closeSession = Effect.fn("ACP.closeSession")(function* (params: CloseSessionRequest) {
     const removed = yield* session.remove(params.sessionId)
     registeredMcp.delete(params.sessionId)
     sessionSnapshots.delete(params.sessionId)
     if (!removed) return {}
 
-    yield* request(
-      () => input.sdk.session.abort({ directory: removed.cwd, sessionID: params.sessionId }, { throwOnError: true }),
-      "session",
-    ).pipe(
-      Effect.catch((error) =>
-        Effect.sync(() => {
-          log.error("failed to abort session while closing ACP session", { error, sessionID: params.sessionId })
-        }),
-      ),
-    )
+    yield* abortBackingSession(removed)
     return {}
   })
 
+  const cancel = Effect.fn("ACP.cancel")(function* (params: CancelNotification) {
+    const current = yield* session.get(params.sessionId)
+    yield* abortBackingSession(current)
+  })
+
   const forkSession = Effect.fn("ACP.forkSession")(function* (params: ForkSessionRequest) {
     const snapshot = yield* directorySnapshot(params.cwd)
     const forked = yield* request(
@@ -563,9 +572,7 @@ export function make(input: {
       yield* sendUsageUpdate(input.usage, input.sdk, input.connection, current.id, current.cwd)
       return promptResponse(undefined, params.messageId)
     }),
-    cancel: Effect.fn("ACP.cancel")(function* (_input: CancelNotification) {
-      return yield* new ACPError.UnsupportedOperationError({ method: "session/cancel" })
-    }),
+    cancel,
   }
 }
 
diff --git a/packages/opencode/test/acp/service-session.test.ts b/packages/opencode/test/acp/service-session.test.ts
index 878d331c44..e5f1bc64c6 100644
--- a/packages/opencode/test/acp/service-session.test.ts
+++ b/packages/opencode/test/acp/service-session.test.ts
@@ -12,10 +12,9 @@ import type {
 } from "@agentclientprotocol/sdk"
 import type { OpencodeClient } from "@opencode-ai/sdk/v2"
 import { ProviderV2 } from "@opencode-ai/core/provider"
-import { Effect, ManagedRuntime } from "effect"
+import { Effect } from "effect"
 import * as ACPService from "@/acp/service"
 import * as ACPError from "@/acp/error"
-import { ACPSession } from "@/acp/session"
 import { UsageService } from "@/acp/usage"
 import type { Provider } from "@/provider/provider"
 
@@ -142,7 +141,10 @@ const provider: Provider.Info = {
 }
 
 describe("ACP service sessions", () => {
-  const makeService = (messages: readonly { info: unknown; parts: readonly unknown[] }[] = []) => {
+  const makeService = (
+    messages: readonly { info: unknown; parts: readonly unknown[] }[] = [],
+    options?: { abort?: (input: { sessionID: string }) => Promise<{ data: boolean }> },
+  ) => {
     const updates: SessionNotification[] = []
     const mcpAdds: string[] = []
     const aborts: string[] = []
@@ -220,10 +222,12 @@ describe("ACP service sessions", () => {
           summarizes.push(input)
           return Promise.resolve({ data: true })
         },
-        abort: (input: { sessionID: string }) => {
-          aborts.push(input.sessionID)
-          return Promise.resolve({ data: true })
-        },
+        abort:
+          options?.abort ??
+          ((input: { sessionID: string }) => {
+            aborts.push(input.sessionID)
+            return Promise.resolve({ data: true })
+          }),
         fork: (input: { sessionID: string }) => {
           forks.push(input.sessionID)
           return Promise.resolve({ data: { id: `fork_${input.sessionID}` } })
@@ -381,34 +385,28 @@ describe("ACP service sessions", () => {
     expect(await Effect.runPromise(service.closeSession({ sessionId: "missing" }))).toEqual({})
   })
 
-  it("does not fail close when backing abort fails", async () => {
-    const sessionService = ManagedRuntime.make(ACPSession.defaultLayer).runSync(
-      ACPSession.Service.use((service) => Effect.succeed(service)),
-    )
-    const { service } = makeService()
-    const sdk = {
-      config: {
-        providers: () => Promise.resolve({ data: { providers: [provider], default: { test: modelID } } }),
-        get: () => Promise.resolve({ data: {} }),
-      },
-      app: {
-        agents: () => Promise.resolve({ data: [{ name: "build", mode: "primary", permission: [], options: {} }] }),
-        skills: () => Promise.resolve({ data: [] }),
-      },
-      command: {
-        list: () => Promise.resolve({ data: [] }),
-      },
-      session: {
-        abort: () => Promise.reject(new Error("nope")),
-      },
-      mcp: {
-        add: () => Promise.resolve({ data: {} }),
-      },
-    } as unknown as OpencodeClient
-    const closing = ACPService.make({ sdk, session: sessionService })
-    await Effect.runPromise(sessionService.create({ id: "ses_close", cwd: "/workspace" }))
+  it("cancel aborts the backing session and keeps the ACP session", async () => {
+    const { service, aborts } = makeService()
+    const created = await Effect.runPromise(service.newSession({ cwd: "/workspace", mcpServers: [] }))
 
-    expect(await Effect.runPromise(closing.closeSession({ sessionId: "ses_close" }))).toEqual({})
+    await Effect.runPromise(service.cancel({ sessionId: created.sessionId }))
+
+    // The running turn was aborted via the core session API.
+    expect(aborts).toEqual([created.sessionId])
+    // Unlike closeSession, the ACP session is still present afterwards so
+    // the client can keep prompting.
+    const stillUsable = await Effect.runPromise(
+      service.setSessionConfigOption({ sessionId: created.sessionId, configId: "effort", value: "high" }),
+    )
+    expect(stillUsable).toBeDefined()
+  })
+
+  it("does not fail cancel or close when the backing abort fails", async () => {
+    const { service } = makeService([], { abort: () => Promise.reject(new Error("nope")) })
+    const created = await Effect.runPromise(service.newSession({ cwd: "/workspace", mcpServers: [] }))
+
+    await Effect.runPromise(service.cancel({ sessionId: created.sessionId }))
+    expect(await Effect.runPromise(service.closeSession({ sessionId: created.sessionId }))).toEqual({})
     expect(await Effect.runPromise(service.closeSession({ sessionId: "missing" }))).toEqual({})
   })