fix(opencode): read v2's native default variant as the Default option

packages/adapters/opencode/src/v2/state.ts

@@ -140,12 +140,25 @@ export function v2State(
           availableThinkingOptions: catalog.thinkingOptions.filter((v) =>
             entry.supportedThinkingOptionIds?.includes(v.id),
           ),
-          effectiveThinkingOptionId: encodeOpenCodeVariant(session.model?.variant),
+          effectiveThinkingOptionId: effectiveVariant(session.model?.variant, entry),
         }
       : {}),
     effectivePermissionModeId: decodeMode(mode?.effect),
   };
 }
+/**
+ * OpenCode v2 reads its default variant back as the string "default"; it is
+ * the catalog's Default option unless the model names a variant "default".
+ */
+function effectiveVariant(
+  variant: string | undefined,
+  entry: HarnessModelCatalog["models"][number],
+) {
+  const id = encodeOpenCodeVariant(variant);
+  return variant === "default" && !entry.supportedThinkingOptionIds?.includes(id)
+    ? encodeOpenCodeVariant(undefined)
+    : id;
+}
 function decodeMode(effect?: string) {
   return effect === "allow" || effect === "ask"
     ? harnessPermissionModeIdSchema.parse(effect)

packages/adapters/opencode/test/v2.test.ts

@@ -1,5 +1,6 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 import type {
+  ModelInfo,
   OpenCodeClient,
   OpenCodeEvent,
   SessionInfo,
@@ -14,9 +15,10 @@ import {
 } from "@codexhost/harness-adapter";
 import { CodexTurnProjector } from "@codexhost/protocol-core";
 import { harnessModelCatalogSchema, hostTurnIdSchema } from "@codexhost/shared-contracts";
+import { encodeOpenCodeVariant } from "../src/model-catalog.js";
 import { openCodeMajor } from "../src/version.js";
 import { assistantItems, projectHistory, readMessages } from "../src/v2/history.js";
-import { v2Locator, v2Permissions, v2Ref } from "../src/v2/state.js";
+import { v2Catalog, v2Locator, v2Permissions, v2Ref, v2State } from "../src/v2/state.js";
 import { V2Session } from "../src/v2/session.js";
 import { v2UsageRequest } from "../src/v2/usage.js";
 import { formInteraction, replyInteraction } from "../src/v2/interactions.js";
@@ -160,6 +162,42 @@ describe("OpenCode protocol versions", () => {
   });
 });
 
+describe("OpenCode v2 Thinking state", () => {
+  const model = (id: string, variants: string[]) =>
+    ({
+      id,
+      providerID: "test",
+      name: id,
+      enabled: true,
+      variants: variants.map((variant) => ({ id: variant })),
+    }) as unknown as ModelInfo;
+  const stateFor = (variants: string[], variant: string | undefined) =>
+    v2State(
+      { ...info, model: { providerID: "test", id: "model", ...(variant ? { variant } : {}) } },
+      v2Catalog([model("model", variants)], null),
+      "default",
+    );
+
+  it("reads the native default variant as the catalog's Default option", () => {
+    const state = stateFor(["high", "low"], "default");
+    expect(state.effectiveThinkingOptionId).toBe(encodeOpenCodeVariant(undefined));
+    expect(state.availableThinkingOptions?.map((option) => option.id)).toContain(
+      state.effectiveThinkingOptionId,
+    );
+  });
+
+  it("keeps a variant the model itself names default", () => {
+    const state = stateFor(["default", "high"], "default");
+    expect(state.effectiveThinkingOptionId).toBe(encodeOpenCodeVariant("default"));
+  });
+
+  it("keeps an explicit variant's own option", () => {
+    expect(stateFor(["high", "low"], "high").effectiveThinkingOptionId).toBe(
+      encodeOpenCodeVariant("high"),
+    );
+  });
+});
+
 describe("OpenCode v2 native history", () => {
   it("groups steered prompts by durable idle and never guesses a terminal", () => {
     const snapshot = projectHistory(