diff --git a/packages/server/src/server/agent/providers/opencode-agent.test.ts b/packages/server/src/server/agent/providers/opencode-agent.test.ts
index 3a480f7b9..3931fe035 100644
--- a/packages/server/src/server/agent/providers/opencode-agent.test.ts
+++ b/packages/server/src/server/agent/providers/opencode-agent.test.ts
@@ -480,6 +480,7 @@ describe("OpenCodeAgentClient adapter smoke tests", () => {
             models: {
               "big-pickle": {
                 name: "Big Pickle",
+                variants: { max: { reasoningEffort: "max" } },
                 limit: {
                   context: 200_000,
                 },
@@ -529,6 +530,11 @@ describe("OpenCodeAgentClient adapter smoke tests", () => {
     expect(catalog.models[0]).toMatchObject({
       id: TEST_MODEL,
       label: "Big Pickle",
+      thinkingOptions: [
+        { id: "default", label: "Default", isDefault: true },
+        { id: "max", label: "max" },
+      ],
+      defaultThinkingOptionId: "default",
       metadata: {
         providerId: "opencode",
         modelId: "big-pickle",
@@ -824,7 +830,40 @@ describe("OpenCodeAgentClient adapter smoke tests", () => {
   }, 180_000);
 });
 
-describe("OpenCode adapter context-window normalization", () => {
+describe("OpenCode adapter normalization", () => {
+  test("omits OpenCode's implicit default variant from new and updated sessions", async () => {
+    const runtime = new TestOpenCodeHarness();
+    const openCode = new TestOpenCodeClient();
+    openCode.sessionCreateResponse = { data: { id: "ses_default_variant" } };
+    openCode.sessionPromptAsyncEvents = [
+      { type: "session.idle", properties: { sessionID: "ses_default_variant" } },
+    ];
+    runtime.enqueueClient(openCode);
+    const client = new OpenCodeAgentClient(createTestLogger(), undefined, {
+      serverManager: runtime,
+      createClient: runtime.createClient,
+    });
+    const session = await client.createSession({
+      provider: "opencode",
+      cwd: "/workspace/repo",
+      model: "catalog-provider/single-variant-model",
+      thinkingOptionId: "default",
+    });
+
+    await collectTurnEvents(streamSession(session, "Use the model default"));
+    await session.setThinkingOption?.("max");
+    await collectTurnEvents(streamSession(session, "Use max"));
+    await session.setThinkingOption?.("default");
+    await collectTurnEvents(streamSession(session, "Return to the model default"));
+
+    expect(openCode.calls.sessionPromptAsync).toEqual([
+      expect.not.objectContaining({ variant: expect.anything() }),
+      expect.objectContaining({ variant: "max" }),
+      expect.not.objectContaining({ variant: expect.anything() }),
+    ]);
+    await session.close();
+  });
+
   test("builds OpenCode file parts for image prompt blocks", () => {
     expect(
       __openCodeInternals.buildOpenCodePromptParts([
diff --git a/packages/server/src/server/agent/providers/opencode-agent.ts b/packages/server/src/server/agent/providers/opencode-agent.ts
index 3ea0baab7..7dbe5d3b7 100644
--- a/packages/server/src/server/agent/providers/opencode-agent.ts
+++ b/packages/server/src/server/agent/providers/opencode-agent.ts
@@ -111,6 +111,7 @@ const OPENCODE_CAPABILITIES: AgentCapabilityFlags = {
 
 const OPENCODE_BUILD_MODE_ID = "build";
 const OPENCODE_LEGACY_FULL_ACCESS_MODE_ID = "full-access";
+const OPENCODE_DEFAULT_VARIANT_ID = "default";
 const OPENCODE_AUTO_ACCEPT_FEATURE_ID = "auto_accept";
 const OPENCODE_PERSISTED_SESSION_LIMIT = 200;
 const OPENCODE_PENDING_ABORT_START_TIMEOUT_MS = 10_000;
@@ -580,6 +581,14 @@ function normalizeOpenCodeModeId(modeId: string | null | undefined): string | nu
   return trimmed;
 }
 
+function normalizeOpenCodeVariantId(variantId: string | null | undefined): string | null {
+  const trimmed = typeof variantId === "string" ? variantId.trim() : "";
+  if (!trimmed || trimmed === OPENCODE_DEFAULT_VARIANT_ID) {
+    return null;
+  }
+  return trimmed;
+}
+
 function resolveOpenCodeRuntimeAgentId(modeId: string | null | undefined): string | undefined {
   const normalizedModeId = normalizeOpenCodeModeId(modeId);
   if (normalizedModeId === null) {
@@ -591,15 +600,19 @@ function resolveOpenCodeRuntimeAgentId(modeId: string | null | undefined): strin
 }
 
 function normalizeOpenCodeConfig(config: OpenCodeAgentConfig): OpenCodeAgentConfig {
-  if (normalizeOpenCodeModeId(config.modeId) !== OPENCODE_LEGACY_FULL_ACCESS_MODE_ID) {
-    return { ...config };
+  const normalized = {
+    ...config,
+    thinkingOptionId: normalizeOpenCodeVariantId(config.thinkingOptionId) ?? undefined,
+  };
+  if (normalizeOpenCodeModeId(normalized.modeId) !== OPENCODE_LEGACY_FULL_ACCESS_MODE_ID) {
+    return normalized;
   }
 
   return {
-    ...config,
+    ...normalized,
     modeId: OPENCODE_BUILD_MODE_ID,
     featureValues: {
-      ...config.featureValues,
+      ...normalized.featureValues,
       [OPENCODE_AUTO_ACCEPT_FEATURE_ID]: true,
     },
   };
@@ -723,11 +736,13 @@ function buildOpenCodeModelDefinition(
   },
 ): AgentModelDefinition {
   const rawVariants = model.variants ? Object.keys(model.variants) : [];
-  const thinkingOptions = rawVariants.map((id, index) => ({
-    id,
-    label: id,
-    isDefault: index === 0,
-  }));
+  // OpenCode lists only overrides; its base model behavior is selected by omitting `variant`.
+  const thinkingOptions = rawVariants.length
+    ? [
+        { id: OPENCODE_DEFAULT_VARIANT_ID, label: "Default", isDefault: true },
+        ...rawVariants.map((id) => ({ id, label: id })),
+      ]
+    : [];
 
   return {
     provider: "opencode",
@@ -3195,10 +3210,7 @@ class OpenCodeAgentSession implements AgentSession {
   }
 
   async setThinkingOption(thinkingOptionId: string | null): Promise<void> {
-    const normalizedThinkingOptionId =
-      typeof thinkingOptionId === "string" && thinkingOptionId.trim().length > 0
-        ? thinkingOptionId
-        : null;
+    const normalizedThinkingOptionId = normalizeOpenCodeVariantId(thinkingOptionId);
     this.config.thinkingOptionId = normalizedThinkingOptionId ?? undefined;
   }