diff --git a/packages/provider-opencode/src/server/driver.test.ts b/packages/provider-opencode/src/server/driver.test.ts
index 71ab041def471..2281365ea23e8 100644
--- a/packages/provider-opencode/src/server/driver.test.ts
+++ b/packages/provider-opencode/src/server/driver.test.ts
@@ -6,7 +6,12 @@ import * as NodePath from "node:path";
 import * as NodeCrypto from "@effect/platform-node/NodeCrypto";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, it } from "@effect/vitest";
-import { DEFAULT_SERVER_SETTINGS, ProviderInstanceId } from "@t3tools/contracts";
+import {
+  DEFAULT_SERVER_SETTINGS,
+  ProviderInstanceId,
+  ProviderSessionId,
+  ThreadId,
+} from "@t3tools/contracts";
 import type { OpenCodeSettings } from "../settings.ts";
 import * as HostProcess from "@t3tools/shared/HostProcess";
 import * as Deferred from "effect/Deferred";
@@ -575,4 +580,39 @@ it.layer(layerUpdate)("OpenCodeDriver updates", (it) => {
         assert.isNull(v2.update);
       }).pipe(Effect.scoped),
   );
+
+  it.effect(
+    "opens sessions through OpenCode 2 after the binary is upgraded in place from 1.x",
+    () =>
+      Effect.gen(function* () {
+        versionOutput.current = "1.18.18\n";
+        const instance = yield* create({}, noHttp);
+        // Let the startup status check claim the refresh permit, then wait behind it so it
+        // cannot probe after the upgrade below.
+        yield* Effect.yieldNow;
+        yield* instance.snapshot.refresh;
+        serverStarts.length = 0;
+        const openSession = instance.orchestrationAdapter
+          .openSession({
+            threadId: ThreadId.make("thread-upgraded"),
+            providerSessionId: ProviderSessionId.make("session-upgraded"),
+            modelSelection: { instanceId: instance.instanceId, model: "opencode/big-pickle" },
+            runtimePolicy: { runtimeMode: "full-access", interactionMode: "default", cwd: null },
+          })
+          .pipe(Effect.scoped, Effect.flip);
+
+        yield* openSession;
+        assert.deepStrictEqual(serverStarts, ["connect"], "1.x opens through the 1.x adapter");
+
+        versionOutput.current = "opencode v2.2.4\n";
+        serverStarts.length = 0;
+        yield* openSession;
+        assert.notInclude(
+          serverStarts,
+          "connect",
+          "after upgrading to 2.x, the next session must not open through the 1.x adapter",
+        );
+        assert.include(serverStarts, "start", "the OpenCode 2 adapter starts its server");
+      }).pipe(Effect.scoped),
+  );
 });
diff --git a/packages/provider-opencode/src/server/versionProbe.test.ts b/packages/provider-opencode/src/server/versionProbe.test.ts
index d698bbcb8785b..c6a9536f774f3 100644
--- a/packages/provider-opencode/src/server/versionProbe.test.ts
+++ b/packages/provider-opencode/src/server/versionProbe.test.ts
@@ -160,9 +160,9 @@ describe("OpenCode version probe", () => {
     }),
   );
 
-  it.effect("remembers a probed runtime but never a failure", () =>
+  it.effect("remembers a probed 2.x runtime but re-probes 1.x and failures", () =>
     Effect.gen(function* () {
-      const outputs = ["", "opencode v2.0.18\n", "1.18.32\n"];
+      const outputs = ["", "opencode v2.0.18\n", "1.18.32\n", "1.18.32\n"];
       let calls = 0;
       const runtime = {
         runOpenCodeCommand: () => {
@@ -190,8 +190,9 @@ describe("OpenCode version probe", () => {
       assert.strictEqual((yield* probe.get).generation, "v2");
       assert.strictEqual(calls, 2);
       assert.strictEqual((yield* probe.refresh).generation, "v1");
-      assert.strictEqual((yield* probe.get).generation, "v1");
       assert.strictEqual(calls, 3);
+      assert.strictEqual((yield* probe.get).generation, "v1");
+      assert.strictEqual(calls, 4);
     }),
   );
 });
diff --git a/packages/provider-opencode/src/server/versionProbe.ts b/packages/provider-opencode/src/server/versionProbe.ts
index 5f0f871d5213f..ab40142de845a 100644
--- a/packages/provider-opencode/src/server/versionProbe.ts
+++ b/packages/provider-opencode/src/server/versionProbe.ts
@@ -172,11 +172,14 @@ export const probeOpenCodeRuntime = (
     ? probeOpenCodeServer(settings.serverUrl, settings.serverPassword)
     : probeOpenCodeBinary(settings.binaryPath, environment);
 
+// Only 2.x is kept: 1.x can be upgraded in place while T3 runs, and 2.x converts the shared
+// database, so going back is unsupported.
 export const makeOpenCodeRuntimeProbe = <E>(probe: Effect.Effect<ProbedOpenCode, E>) =>
   Effect.map(
     Cache.makeWith(() => probe, {
       capacity: 1,
-      timeToLive: (exit) => (Exit.isSuccess(exit) ? Duration.infinity : Duration.zero),
+      timeToLive: (exit) =>
+        Exit.isSuccess(exit) && exit.value.generation === "v2" ? Duration.infinity : Duration.zero,
     }),
     (cache) => ({
       get: Cache.get(cache, undefined),