diff --git a/crates/agent-gui/src/lib/providers/deepSeekProviderAdapter.ts b/crates/agent-gui/src/lib/providers/deepSeekProviderAdapter.ts
index b5c42ddba..f064b4e43 100644
--- a/crates/agent-gui/src/lib/providers/deepSeekProviderAdapter.ts
+++ b/crates/agent-gui/src/lib/providers/deepSeekProviderAdapter.ts
@@ -20,6 +20,7 @@ export const DEEPSEEK_THINKING_LEVEL_MAP: Model<"openai-completions">["thinkingL
   low: "high",
   medium: "high",
   high: "high",
+  max: "max",
   xhigh: "max",
 };
 
@@ -150,7 +151,7 @@ export function mapDeepSeekReasoningEffort(
   reasoning: SimpleStreamOptions["reasoning"] | undefined,
 ) {
   if (!reasoning) return undefined;
-  return reasoning === "xhigh" ? "max" : "high";
+  return reasoning === "xhigh" || reasoning === "max" ? "max" : "high";
 }
 
 function sanitizeTextValue(value: string) {
diff --git a/crates/agent-gui/test/providers/request-options.test.mjs b/crates/agent-gui/test/providers/request-options.test.mjs
index f64305120..39f2499e1 100644
--- a/crates/agent-gui/test/providers/request-options.test.mjs
+++ b/crates/agent-gui/test/providers/request-options.test.mjs
@@ -532,6 +532,67 @@ test("Codex Chat Completions streams forward reasoning effort", async () => {
   assert.equal(captured.options.toolChoice, "auto");
 });
 
+test("DeepSeek OpenAI payload adapter maps reasoning=max to reasoning_effort=max (regression)", async () => {
+  let captured;
+  const localLoader = createTsModuleLoader({
+    mocks: {
+      "@earendil-works/pi-ai/api/openai-completions": {
+        stream(model, context, options) {
+          captured = { model, context, options };
+          return createMockAssistantStream();
+        },
+      },
+    },
+  });
+  const localProviders = localLoader.loadModule("src/lib/providers/llm.ts");
+  const model = localProviders.createModelFromConfig(
+    "codex",
+    "deepseek-v4-pro",
+    "https://api.deepseek.com",
+    "openai-completions",
+  );
+
+  const result = localProviders.streamSimpleByApi(
+    model,
+    { messages: [] },
+    { reasoning: "max", toolChoice: "auto" },
+  );
+  assert.equal(typeof result.result, "function");
+  assert.equal(typeof captured.options.onPayload, "function");
+  assert.equal(
+    captured.options.reasoningEffort,
+    "max",
+    "clampOpenAIReasoningEffort should preserve max for deepseek-v4-pro",
+  );
+
+  const adapted = await captured.options.onPayload(
+    {
+      messages: [
+        {
+          role: "assistant",
+          content: null,
+          tool_calls: [
+            {
+              id: "call_1",
+              type: "function",
+              function: { name: "Read", arguments: "{}" },
+            },
+          ],
+        },
+      ],
+    },
+    model,
+  );
+
+  assert.deepEqual(adapted.thinking, { type: "enabled" });
+  assert.equal(
+    adapted.reasoning_effort,
+    "max",
+    "wire reasoning_effort should be max when UI selects max",
+  );
+  assert.equal(adapted.messages[0].reasoning_content, "");
+});
+
 test("DeepSeek Anthropic streamSimpleByApi strips aborted tool calls before conversion", () => {
   const { localProviders, state } = loadProvidersWithCapturedAnthropicStream();
   const model = createDeepSeekAnthropicModel();