diff --git a/apps/server/src/provider/Layers/ClaudeAdapter.test.ts b/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
index 191bf8e27db..c3c4dfb2dbe 100644
--- a/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
+++ b/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
@@ -3042,7 +3042,7 @@ describe("ClaudeAdapterLive", () => {
         attachments: [],
       });
 
-      assert.deepEqual(harness.query.setModelCalls, ["claude-opus-4-6"]);
+      assert.deepEqual(harness.query.setModelCalls, ["claude-opus-4-6[1m]"]);
     }).pipe(
       Effect.provideService(Random.Random, makeDeterministicRandomService()),
       Effect.provide(harness.layer),
@@ -3140,10 +3140,11 @@ describe("ClaudeAdapterLive", () => {
       yield* adapter.sendTurn({
         threadId: session.threadId,
         input: "hello again",
-        modelSelection: {
-          instanceId: ProviderInstanceId.make("claudeAgent"),
-          model: "claude-opus-4-6",
-        },
+        modelSelection: createModelSelection(
+          ProviderInstanceId.make("claudeAgent"),
+          "claude-opus-4-6",
+          [{ id: "contextWindow", value: "200k" }],
+        ),
         attachments: [],
       });
 
diff --git a/apps/server/src/provider/Layers/ClaudeAdapter.ts b/apps/server/src/provider/Layers/ClaudeAdapter.ts
index 97a93f85829..a8e6cfda230 100644
--- a/apps/server/src/provider/Layers/ClaudeAdapter.ts
+++ b/apps/server/src/provider/Layers/ClaudeAdapter.ts
@@ -76,6 +76,7 @@ import {
   isClaudeUltracodeEffort,
   normalizeClaudeCliEffort,
   resolveClaudeApiModelId,
+  resolveClaudeContextWindow,
   resolveClaudeEffort,
 } from "./ClaudeProvider.ts";
 import {
@@ -340,24 +341,18 @@ function selectedClaudeContextWindow(
   switch (modelSelection?.model) {
     case "claude-opus-4-8":
     case "claude-opus-4-7":
+      // Always 1M at the API; these models expose no contextWindow option.
       return 1_000_000;
   }
 
-  const optionValue = getModelSelectionStringOptionValue(modelSelection, "contextWindow");
-  if (optionValue === "1m") {
-    return 1_000_000;
-  }
-  if (optionValue === "200k") {
-    return 200_000;
-  }
-  const caps = getClaudeModelCapabilities(modelSelection?.model);
-  const hasContextWindowOption = getProviderOptionDescriptors({ caps }).some(
-    (descriptor) => descriptor.type === "select" && descriptor.id === "contextWindow",
-  );
-  if (hasContextWindowOption) {
-    return 200_000;
+  switch (resolveClaudeContextWindow(modelSelection)) {
+    case "1m":
+      return 1_000_000;
+    case "200k":
+      return 200_000;
+    default:
+      return undefined;
   }
-  return undefined;
 }
 
 function finiteNonNegativeInteger(value: unknown): number | undefined {
diff --git a/apps/server/src/provider/Layers/ClaudeProvider.ts b/apps/server/src/provider/Layers/ClaudeProvider.ts
index 59b7d0464de..5471c710543 100644
--- a/apps/server/src/provider/Layers/ClaudeProvider.ts
+++ b/apps/server/src/provider/Layers/ClaudeProvider.ts
@@ -77,8 +77,8 @@ const BUILT_IN_MODELS: ReadonlyArray<ServerProviderModel> = [
           id: "contextWindow",
           label: "Context Window",
           options: [
-            { value: "200k", label: "200k", isDefault: true },
-            { value: "1m", label: "1M" },
+            { value: "200k", label: "200k" },
+            { value: "1m", label: "1M", isDefault: true },
           ],
         }),
       ],
@@ -163,8 +163,8 @@ const BUILT_IN_MODELS: ReadonlyArray<ServerProviderModel> = [
           id: "contextWindow",
           label: "Context Window",
           options: [
-            { value: "200k", label: "200k", isDefault: true },
-            { value: "1m", label: "1M" },
+            { value: "200k", label: "200k" },
+            { value: "1m", label: "1M", isDefault: true },
           ],
         }),
       ],
@@ -216,8 +216,8 @@ const BUILT_IN_MODELS: ReadonlyArray<ServerProviderModel> = [
           id: "contextWindow",
           label: "Context Window",
           options: [
-            { value: "200k", label: "200k", isDefault: true },
-            { value: "1m", label: "1M" },
+            { value: "200k", label: "200k" },
+            { value: "1m", label: "1M", isDefault: true },
           ],
         }),
       ],
@@ -245,8 +245,8 @@ const BUILT_IN_MODELS: ReadonlyArray<ServerProviderModel> = [
           id: "contextWindow",
           label: "Context Window",
           options: [
-            { value: "200k", label: "200k", isDefault: true },
-            { value: "1m", label: "1M" },
+            { value: "200k", label: "200k" },
+            { value: "1m", label: "1M", isDefault: true },
           ],
         }),
       ],
@@ -370,8 +370,22 @@ export function isClaudeUltracodeEffort(effort: string | null | undefined): bool
   return effort === "ultracode";
 }
 
+export function resolveClaudeContextWindow(
+  modelSelection: ModelSelection | undefined,
+): string | undefined {
+  const caps = getClaudeModelCapabilities(modelSelection?.model);
+  const raw = getModelSelectionStringOptionValue(modelSelection, "contextWindow");
+  const descriptors = getProviderOptionDescriptors({
+    caps,
+    ...(raw ? { selections: [{ id: "contextWindow", value: raw }] } : {}),
+  });
+  const descriptor = descriptors.find((candidate) => candidate.id === "contextWindow");
+  const value = getProviderOptionCurrentValue(descriptor);
+  return typeof value === "string" ? value : undefined;
+}
+
 export function resolveClaudeApiModelId(modelSelection: ModelSelection): string {
-  switch (getModelSelectionStringOptionValue(modelSelection, "contextWindow")) {
+  switch (resolveClaudeContextWindow(modelSelection)) {
     case "1m":
       return `${modelSelection.model}[1m]`;
     default: