diff --git a/src/agents/sessions/sdk.test.ts b/src/agents/sessions/sdk.test.ts
index c12646a4912e..e2da2ebb40cb 100644
--- a/src/agents/sessions/sdk.test.ts
+++ b/src/agents/sessions/sdk.test.ts
@@ -1,7 +1,15 @@
 // Agent session SDK tests cover default tool wiring, prompt preservation, and
 // session write-lock behavior.
 import { Type } from "typebox";
-import { describe, expect, it } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const thinkingMocks = vi.hoisted(() => ({
+  resolveThinkingDefaultForModel: vi.fn(() => "medium"),
+}));
+
+vi.mock("../../auto-reply/thinking.js", () => ({
+  resolveThinkingDefaultForModel: thinkingMocks.resolveThinkingDefaultForModel,
+}));
 import type { Model } from "../../llm/types.js";
 import { AuthStorage } from "./auth-storage.js";
 import { createExtensionRuntime } from "./extensions/loader.js";
@@ -316,3 +324,117 @@ describe("createAgentSession tool defaults", () => {
     expect(events).toEqual(["lock:start", "lock:end"]);
   });
 });
+
+describe("createAgentSession thinking level defaults", () => {
+  beforeEach(() => {
+    thinkingMocks.resolveThinkingDefaultForModel.mockReset();
+    thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue("medium");
+  });
+
+  it("uses the provider-specific thinking default for new sessions", async () => {
+    thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue("off");
+
+    const ollamaModel = {
+      ...testModel,
+      provider: "ollama",
+      reasoning: true,
+      params: { canonicalModelId: "qwen3:8b" },
+      compat: { thinkingFormat: "ollama" },
+    } satisfies Model;
+    const { session } = await createAgentSession({
+      model: ollamaModel,
+      resourceLoader: createEmptyResourceLoader(),
+      sessionManager: SessionManager.inMemory(),
+      settingsManager: SettingsManager.inMemory(),
+      modelRegistry: ModelRegistry.inMemory(AuthStorage.inMemory()),
+    });
+
+    expect(session.thinkingLevel).toBe("off");
+    expect(thinkingMocks.resolveThinkingDefaultForModel).toHaveBeenCalledWith({
+      provider: "ollama",
+      model: testModel.id,
+      catalog: [ollamaModel],
+    });
+  });
+
+  it("settings default overrides provider thinking default", async () => {
+    thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue("off");
+
+    const { session } = await createAgentSession({
+      model: { ...testModel, provider: "ollama", reasoning: true },
+      resourceLoader: createEmptyResourceLoader(),
+      sessionManager: SessionManager.inMemory(),
+      settingsManager: SettingsManager.inMemory({ defaultThinkingLevel: "low" }),
+      modelRegistry: ModelRegistry.inMemory(AuthStorage.inMemory()),
+    });
+
+    // User-configured settings default beats provider default
+    expect(session.thinkingLevel).toBe("low");
+  });
+
+  it("uses Ollama policy for custom providers backed by the Ollama API", async () => {
+    thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue("off");
+    const customOllamaModel = {
+      ...testModel,
+      provider: "ollama-spark",
+      api: "ollama",
+      reasoning: true,
+    } satisfies Model;
+
+    const { session } = await createAgentSession({
+      model: customOllamaModel,
+      resourceLoader: createEmptyResourceLoader(),
+      sessionManager: SessionManager.inMemory(),
+      settingsManager: SettingsManager.inMemory(),
+      modelRegistry: ModelRegistry.inMemory(AuthStorage.inMemory()),
+    });
+
+    expect(session.thinkingLevel).toBe("off");
+    expect(thinkingMocks.resolveThinkingDefaultForModel).toHaveBeenCalledWith({
+      provider: "ollama",
+      model: testModel.id,
+      catalog: [customOllamaModel],
+    });
+  });
+
+  it("falls back to DEFAULT_THINKING_LEVEL for non-off provider defaults", async () => {
+    // Non-off provider defaults (adaptive, high, low) preserve prior SDK behaviour
+    // to avoid silent cost changes for DeepSeek, OpenRouter, xAI, and Anthropic users.
+    for (const nonOffDefault of ["adaptive", "high", "low"] as const) {
+      thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue(nonOffDefault);
+
+      const { session } = await createAgentSession({
+        model: { ...testModel, reasoning: true },
+        resourceLoader: createEmptyResourceLoader(),
+        sessionManager: SessionManager.inMemory(),
+        settingsManager: SettingsManager.inMemory(),
+        modelRegistry: ModelRegistry.inMemory(AuthStorage.inMemory()),
+      });
+
+      expect(session.thinkingLevel).toBe("medium");
+    }
+  });
+
+  it("uses provider default for legacy sessions that have no thinking entry", async () => {
+    // Sessions created before thinking-level tracking (no thinking_level_change entry)
+    // should inherit the provider default, not the hard-coded global "medium".
+    thinkingMocks.resolveThinkingDefaultForModel.mockReturnValue("off");
+
+    const sessionManager = SessionManager.inMemory();
+    sessionManager.appendMessage({
+      role: "user",
+      content: "hello",
+      timestamp: Date.now(),
+    });
+
+    const { session } = await createAgentSession({
+      model: { ...testModel, provider: "ollama", reasoning: true },
+      resourceLoader: createEmptyResourceLoader(),
+      sessionManager,
+      settingsManager: SettingsManager.inMemory(),
+      modelRegistry: ModelRegistry.inMemory(AuthStorage.inMemory()),
+    });
+
+    expect(session.thinkingLevel).toBe("off");
+  });
+});
diff --git a/src/agents/sessions/sdk.ts b/src/agents/sessions/sdk.ts
index 5ab6170c7e58..3033f713de6c 100644
--- a/src/agents/sessions/sdk.ts
+++ b/src/agents/sessions/sdk.ts
@@ -4,6 +4,7 @@
  * Selects models, wires built-in/custom tools, loads resources, and creates AgentSession instances.
  */
 import { join } from "node:path";
+import { resolveThinkingDefaultForModel } from "../../auto-reply/thinking.js";
 import { clampThinkingLevel } from "../../llm/model-utils.js";
 import { streamSimple } from "../../llm/stream.js";
 import type { Message, Model } from "../../llm/types.js";
@@ -268,16 +269,29 @@ export async function createAgentSession(
 
   let thinkingLevel = options.thinkingLevel;
 
+  // Use "off" when a provider explicitly opts out of thinking (e.g. Ollama). Non-off
+  // provider defaults (high, low, adaptive) fall back to DEFAULT_THINKING_LEVEL to avoid
+  // silent cost changes for DeepSeek, OpenRouter, xAI, and other providers.
+  const resolvedProviderDefault = model
+    ? resolveThinkingDefaultForModel({
+        provider: model.api === "ollama" ? "ollama" : model.provider,
+        model: model.id,
+        catalog: [model],
+      })
+    : undefined;
+  const modelThinkingDefault: ThinkingLevel =
+    resolvedProviderDefault === "off" ? "off" : DEFAULT_THINKING_LEVEL;
+
   // If session has data, restore thinking level from it
   if (thinkingLevel === undefined && hasExistingSession) {
     thinkingLevel = hasThinkingEntry
       ? (existingSession.thinkingLevel as ThinkingLevel)
-      : (settingsManager.getDefaultThinkingLevel() ?? DEFAULT_THINKING_LEVEL);
+      : (settingsManager.getDefaultThinkingLevel() ?? modelThinkingDefault);
   }
 
   // Fall back to settings default
   if (thinkingLevel === undefined) {
-    thinkingLevel = settingsManager.getDefaultThinkingLevel() ?? DEFAULT_THINKING_LEVEL;
+    thinkingLevel = settingsManager.getDefaultThinkingLevel() ?? modelThinkingDefault;
   }
 
   // Clamp to model capabilities
diff --git a/src/auto-reply/thinking.test.ts b/src/auto-reply/thinking.test.ts
index 3761adab0fa1..1273ebb3f7ba 100644
--- a/src/auto-reply/thinking.test.ts
+++ b/src/auto-reply/thinking.test.ts
@@ -768,6 +768,27 @@ describe("resolveThinkingDefaultForModel", () => {
       }),
     ).toBe("off");
   });
+
+  it("respects provider-declared 'off' default for reasoning-capable models", () => {
+    // Providers like Ollama declare defaultLevel:"off" even for reasoning=true models
+    // because thinking must be explicitly opted in, not activated by the global default.
+    providerRuntimeMocks.resolveProviderThinkingProfile.mockImplementation(({ provider }) =>
+      provider === "ollama"
+        ? {
+            levels: [{ id: "off" }, { id: "low" }, { id: "medium" }, { id: "high" }, { id: "max" }],
+            defaultLevel: "off",
+          }
+        : undefined,
+    );
+
+    expect(
+      resolveThinkingDefaultForModel({
+        provider: "ollama",
+        model: "gemma4",
+        catalog: [{ provider: "ollama", id: "gemma4", reasoning: true }],
+      }),
+    ).toBe("off");
+  });
 });
 
 describe("normalizeReasoningLevel", () => {