fix(claude): avoid resetting the Claude model on every turn

apps/server/src/provider/Layers/ClaudeAdapter.test.ts

@@ -2384,6 +2384,85 @@ describe("ClaudeAdapterLive", () => {
     );
   });
 
+  it.effect(
+    "does not re-set the Claude model when the session already uses the same effective API model",
+    () => {
+      const harness = makeHarness();
+      return Effect.gen(function* () {
+        const adapter = yield* ClaudeAdapter;
+        const modelSelection = {
+          provider: "claudeAgent" as const,
+          model: "claude-opus-4-6",
+        };
+
+        const session = yield* adapter.startSession({
+          threadId: THREAD_ID,
+          provider: "claudeAgent",
+          modelSelection,
+          runtimeMode: "full-access",
+        });
+
+        yield* adapter.sendTurn({
+          threadId: session.threadId,
+          input: "hello",
+          modelSelection,
+          attachments: [],
+        });
+        yield* adapter.sendTurn({
+          threadId: session.threadId,
+          input: "hello again",
+          modelSelection,
+          attachments: [],
+        });
+
+        assert.deepEqual(harness.query.setModelCalls, []);
+      }).pipe(
+        Effect.provideService(Random.Random, makeDeterministicRandomService()),
+        Effect.provide(harness.layer),
+      );
+    },
+  );
+
+  it.effect("re-sets the Claude model when the effective API model changes", () => {
+    const harness = makeHarness();
+    return Effect.gen(function* () {
+      const adapter = yield* ClaudeAdapter;
+
+      const session = yield* adapter.startSession({
+        threadId: THREAD_ID,
+        provider: "claudeAgent",
+        runtimeMode: "full-access",
+      });
+
+      yield* adapter.sendTurn({
+        threadId: session.threadId,
+        input: "hello",
+        modelSelection: {
+          provider: "claudeAgent",
+          model: "claude-opus-4-6",
+          options: {
+            contextWindow: "1m",
+          },
+        },
+        attachments: [],
+      });
+      yield* adapter.sendTurn({
+        threadId: session.threadId,
+        input: "hello again",
+        modelSelection: {
+          provider: "claudeAgent",
+          model: "claude-opus-4-6",
+        },
+        attachments: [],
+      });
+
+      assert.deepEqual(harness.query.setModelCalls, ["claude-opus-4-6[1m]", "claude-opus-4-6"]);
+    }).pipe(
+      Effect.provideService(Random.Random, makeDeterministicRandomService()),
+      Effect.provide(harness.layer),
+    );
+  });
+
   it.effect("sets plan permission mode on sendTurn when interactionMode is plan", () => {
     const harness = makeHarness();
     return Effect.gen(function* () {

apps/server/src/provider/Layers/ClaudeAdapter.ts

@@ -148,6 +148,7 @@ interface ClaudeSessionContext {
   streamFiber: Fiber.Fiber<void, Error> | undefined;
   readonly startedAt: string;
   readonly basePermissionMode: PermissionMode | undefined;
+  currentApiModelId: string | undefined;
   resumeSessionId: string | undefined;
   readonly pendingApprovals: Map<ApprovalRequestId, PendingApproval>;
   readonly pendingUserInputs: Map<ApprovalRequestId, PendingUserInput>;
@@ -2809,6 +2810,7 @@ function makeClaudeAdapter(options?: ClaudeAdapterLiveOptions) {
           streamFiber: undefined,
           startedAt,
           basePermissionMode: permissionMode,
+          currentApiModelId: apiModelId,
           resumeSessionId: sessionId,
           pendingApprovals,
           pendingUserInputs,
@@ -2898,10 +2900,17 @@ function makeClaudeAdapter(options?: ClaudeAdapterLiveOptions) {
 
         if (modelSelection?.model) {
           const apiModelId = resolveApiModelId(modelSelection);
-          yield* Effect.tryPromise({
-            try: () => context.query.setModel(apiModelId),
-            catch: (cause) => toRequestError(input.threadId, "turn/setModel", cause),
-          });
+          if (context.currentApiModelId !== apiModelId) {
+            yield* Effect.tryPromise({
+              try: () => context.query.setModel(apiModelId),
+              catch: (cause) => toRequestError(input.threadId, "turn/setModel", cause),
+            });
+            context.currentApiModelId = apiModelId;
+          }
+          context.session = {
+            ...context.session,
+            model: modelSelection.model,
+          };
         }
 
         // Apply interaction mode by switching the SDK's permission mode.