diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
index d0475e25284..b227ff1ab66 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
@@ -248,6 +248,38 @@ it.layer(OpenCodeAdapterTestLayer)("OpenCodeAdapterLive", (it) => {
     }),
   );
 
+  it.effect("fails sendTurn for missing sessions through the typed error channel", () =>
+    Effect.gen(function* () {
+      const adapter = yield* OpenCodeAdapter;
+      const result = yield* adapter
+        .sendTurn({
+          threadId: asThreadId("thread-opencode-missing-send"),
+          input: "hello",
+          attachments: [],
+        })
+        .pipe(Effect.result);
+
+      NodeAssert.equal(result._tag, "Failure");
+      NodeAssert.equal(result.failure._tag, "ProviderAdapterSessionNotFoundError");
+      NodeAssert.equal(result.failure.provider, "opencode");
+      NodeAssert.equal(result.failure.threadId, "thread-opencode-missing-send");
+    }),
+  );
+
+  it.effect("fails stopSession for missing sessions through the typed error channel", () =>
+    Effect.gen(function* () {
+      const adapter = yield* OpenCodeAdapter;
+      const result = yield* adapter
+        .stopSession(asThreadId("thread-opencode-missing-stop"))
+        .pipe(Effect.result);
+
+      NodeAssert.equal(result._tag, "Failure");
+      NodeAssert.equal(result.failure._tag, "ProviderAdapterSessionNotFoundError");
+      NodeAssert.equal(result.failure.provider, "opencode");
+      NodeAssert.equal(result.failure.threadId, "thread-opencode-missing-stop");
+    }),
+  );
+
   it.effect("stops a configured-server session without trying to own server lifecycle", () =>
     Effect.gen(function* () {
       const adapter = yield* OpenCodeAdapter;
diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
index 1eb6e47bc19..956905e3a3c 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
@@ -228,28 +228,25 @@ function appendTurnItem(
   resolveTurnSnapshot(context, turnId).items.push(item);
 }
 
-function ensureSessionContext(
+const ensureSessionContext = Effect.fn("ensureSessionContext")(function* (
   sessions: ReadonlyMap<ThreadId, OpenCodeSessionContext>,
   threadId: ThreadId,
-): OpenCodeSessionContext {
+) {
   const session = sessions.get(threadId);
   if (!session) {
-    throw new ProviderAdapterSessionNotFoundError({
+    return yield* new ProviderAdapterSessionNotFoundError({
       provider: PROVIDER,
       threadId,
     });
   }
-  // `ensureSessionContext` is a sync gate used from both sync helpers and
-  // Effect bodies. `Ref.getUnsafe` is an atomic read of the backing cell —
-  // no fiber suspension required, which keeps this callable everywhere.
-  if (Ref.getUnsafe(session.stopped)) {
-    throw new ProviderAdapterSessionClosedError({
+  if (yield* Ref.get(session.stopped)) {
+    return yield* new ProviderAdapterSessionClosedError({
       provider: PROVIDER,
       threadId,
     });
   }
   return session;
-}
+});
 
 function normalizeQuestionRequest(request: QuestionRequest): ReadonlyArray<UserInputQuestion> {
   return request.questions.map((question, index) => ({
@@ -1167,7 +1164,7 @@ export function makeOpenCodeAdapter(
     );
 
     const sendTurn: OpenCodeAdapterShape["sendTurn"] = Effect.fn("sendTurn")(function* (input) {
-      const context = ensureSessionContext(sessions, input.threadId);
+      const context = yield* ensureSessionContext(sessions, input.threadId);
       // A sendTurn while a turn is active is a steer: OpenCode queues the
       // prompt into the busy session and the work continues as one turn, so
       // the active turn id is reused instead of opening a new turn.
@@ -1291,7 +1288,7 @@ export function makeOpenCodeAdapter(
 
     const interruptTurn: OpenCodeAdapterShape["interruptTurn"] = Effect.fn("interruptTurn")(
       function* (threadId, turnId) {
-        const context = ensureSessionContext(sessions, threadId);
+        const context = yield* ensureSessionContext(sessions, threadId);
         yield* runOpenCodeSdk("session.abort", () =>
           context.client.session.abort({ sessionID: context.openCodeSessionId }),
         ).pipe(Effect.mapError(toRequestError));
@@ -1313,7 +1310,7 @@ export function makeOpenCodeAdapter(
     const respondToRequest: OpenCodeAdapterShape["respondToRequest"] = Effect.fn(
       "respondToRequest",
     )(function* (threadId, requestId, decision) {
-      const context = ensureSessionContext(sessions, threadId);
+      const context = yield* ensureSessionContext(sessions, threadId);
       if (!context.pendingPermissions.has(requestId)) {
         return yield* new ProviderAdapterRequestError({
           provider: PROVIDER,
@@ -1333,7 +1330,7 @@ export function makeOpenCodeAdapter(
     const respondToUserInput: OpenCodeAdapterShape["respondToUserInput"] = Effect.fn(
       "respondToUserInput",
     )(function* (threadId, requestId, answers) {
-      const context = ensureSessionContext(sessions, threadId);
+      const context = yield* ensureSessionContext(sessions, threadId);
       const request = context.pendingQuestions.get(requestId);
       if (!request) {
         return yield* new ProviderAdapterRequestError({
@@ -1355,7 +1352,7 @@ export function makeOpenCodeAdapter(
       function* (threadId) {
         const context = sessions.get(threadId);
         if (!context) {
-          throw new ProviderAdapterSessionNotFoundError({
+          return yield* new ProviderAdapterSessionNotFoundError({
             provider: PROVIDER,
             threadId,
           });
@@ -1385,7 +1382,7 @@ export function makeOpenCodeAdapter(
 
     const readThread: OpenCodeAdapterShape["readThread"] = Effect.fn("readThread")(
       function* (threadId) {
-        const context = ensureSessionContext(sessions, threadId);
+        const context = yield* ensureSessionContext(sessions, threadId);
         const messages = yield* runOpenCodeSdk("session.messages", () =>
           context.client.session.messages({
             sessionID: context.openCodeSessionId,
@@ -1411,7 +1408,7 @@ export function makeOpenCodeAdapter(
 
     const rollbackThread: OpenCodeAdapterShape["rollbackThread"] = Effect.fn("rollbackThread")(
       function* (threadId, numTurns) {
-        const context = ensureSessionContext(sessions, threadId);
+        const context = yield* ensureSessionContext(sessions, threadId);
         const messages = yield* runOpenCodeSdk("session.messages", () =>
           context.client.session.messages({
             sessionID: context.openCodeSessionId,