diff --git a/apps/web/src/composerDraftStore.ts b/apps/web/src/composerDraftStore.ts
index e823d2ddf33a3..0f441b26e4fb1 100644
--- a/apps/web/src/composerDraftStore.ts
+++ b/apps/web/src/composerDraftStore.ts
@@ -1161,9 +1161,11 @@ export function deriveEffectiveComposerModelState(input: {
     : baseModel;
   const modelOptions =
     modelSelectionByProviderToOptions(input.draft?.modelSelectionByProvider) ??
-    providerSelectionsFromModelSelection(input.threadModelSelection) ??
-    providerSelectionsFromModelSelection(input.projectModelSelection) ??
-    null;
+    // A thread saved without options keeps provider defaults; only threads with
+    // no selection of their own inherit the project/settings default options.
+    (input.threadModelSelection
+      ? providerSelectionsFromModelSelection(input.threadModelSelection)
+      : providerSelectionsFromModelSelection(input.projectModelSelection));
 
   return {
     selectedModel,
diff --git a/apps/web/src/modelSelection.test.ts b/apps/web/src/modelSelection.test.ts
index 05a7e76b6b25b..301323af3bbbd 100644
--- a/apps/web/src/modelSelection.test.ts
+++ b/apps/web/src/modelSelection.test.ts
@@ -361,6 +361,25 @@ describe("instance-scoped model selection", () => {
     expect(dispatch.modelOptionsForDispatch).toBeUndefined();
   });
 
+  it("does not apply default options to an existing thread saved without options", () => {
+    const instanceId = ProviderInstanceId.make("claudeAgent");
+    const driver = ProviderDriverKind.make("claudeAgent");
+    const providers = [provider({ provider: driver, instanceId, models: ["claude-opus-5-5"] })];
+    const state = deriveEffectiveComposerModelState({
+      draft: null,
+      providers,
+      selectedProvider: driver,
+      selectedInstanceId: instanceId,
+      threadModelSelection: createModelSelection(instanceId, "claude-opus-5-5"),
+      projectModelSelection: createModelSelection(instanceId, "claude-opus-5-5", [
+        { id: "effort", value: "high" },
+      ]),
+      settings: settingsWithProviderInstances(),
+    });
+
+    expect(state.modelOptions?.[instanceId]).toBeUndefined();
+  });
+
   it("preserves an explicit draft OpenCode selection while the catalog is empty", () => {
     const instanceId = ProviderInstanceId.make("opencode_work");
     const driver = ProviderDriverKind.make("opencode");
diff --git a/knip.jsonc b/knip.jsonc
index c2093737fd942..08d9109510a45 100644
--- a/knip.jsonc
+++ b/knip.jsonc
@@ -27,6 +27,8 @@
       "entry": [
         // These fixture scripts are injected into a browser page by path.
         "fixtures/*/*.js",
+        // Manual UI proofs import these fixtures by path.
+        "fixtures/*.mjs",
         "knip-schemas.ts",
         "mobile-behavior/run.ts",
         "mobile-native-client.ts",
diff --git a/scripts/fixtures/composer-option-fallback.mjs b/scripts/fixtures/composer-option-fallback.mjs
new file mode 100644
index 0000000000000..79038e46a9486
--- /dev/null
+++ b/scripts/fixtures/composer-option-fallback.mjs
@@ -0,0 +1,42 @@
+export const selection = { instanceId: "claudeAgent", model: "claude-opus-5-5" };
+
+export async function seedComposerOptionFallback(rpc, workspaceRoot) {
+  const call = async (tag, payload) => {
+    const exit = await rpc(tag, payload);
+    if (exit._tag !== "Success") throw new Error(JSON.stringify(exit));
+    return exit.value;
+  };
+  await call("server.updateSettings", {
+    patch: { defaultModelSelection: { ...selection, options: [{ id: "effort", value: "high" }] } },
+  });
+  await call("projects.mutate", {
+    type: "project.create",
+    commandId: "proof-project-create",
+    projectId: "proof-project",
+    title: "Synthetic proof",
+    workspaceRoot,
+  });
+  for (const row of ["V1", "V2", "V3", "V4", "A2", "B"]) {
+    await call("orchestration.dispatchCommand", {
+      type: "thread.create",
+      commandId: `proof-create-${row}`,
+      createdBy: "agent",
+      creationSource: "mcp",
+      threadId: `proof-${row}`,
+      projectId: "proof-project",
+      title: "Option fallback proof",
+      modelSelection: {
+        ...selection,
+        ...(row === "A2"
+          ? { options: [] }
+          : row === "B"
+            ? { options: [{ id: "effort", value: "low" }] }
+            : {}),
+      },
+      runtimeMode: "full-access",
+      interactionMode: "default",
+      branch: null,
+      worktreePath: null,
+    });
+  }
+}