diff --git a/src/shared/model-capability-heuristics.ts b/src/shared/model-capability-heuristics.ts
index fd0b9e7a8..61d9e5cbc 100644
--- a/src/shared/model-capability-heuristics.ts
+++ b/src/shared/model-capability-heuristics.ts
@@ -73,6 +73,12 @@ export const HEURISTIC_MODEL_FAMILY_REGISTRY: ReadonlyArray<HeuristicModelFamily
     family: "deepseek",
     includes: ["deepseek"],
     variants: ["low", "medium", "high"],
+    reasoningEfforts: ["high", "max"],
+    reasoningEffortAliases: {
+      low: "high",
+      medium: "high",
+      xhigh: "max",
+    },
   },
   {
     family: "mistral",
diff --git a/src/shared/model-settings-compatibility.test.ts b/src/shared/model-settings-compatibility.test.ts
index 725b2a54c..134bb9e71 100644
--- a/src/shared/model-settings-compatibility.test.ts
+++ b/src/shared/model-settings-compatibility.test.ts
@@ -257,7 +257,7 @@ describe("resolveCompatibleModelSettings", () => {
       { name: "Kimi (k2)", modelID: "k2-v2", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
       { name: "GLM", modelID: "glm-5", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
       { name: "Minimax", modelID: "minimax-m2.5", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
-      { name: "DeepSeek", modelID: "deepseek-r2", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
+      { name: "DeepSeek", modelID: "deepseek-r2", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: true },
       { name: "Mistral", modelID: "mistral-large-next", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
       { name: "Codestral → Mistral", modelID: "codestral-2506", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
       { name: "Llama", modelID: "llama-4-maverick", expectedVariants: ["low", "medium", "high"], hasReasoningEffort: false },
@@ -320,6 +320,45 @@ describe("resolveCompatibleModelSettings", () => {
     })
   })
 
+  test("DeepSeek keeps canonical high and max reasoningEffort values", () => {
+    for (const reasoningEffort of ["high", "max"]) {
+      const result = resolveCompatibleModelSettings({
+        providerID: "openai-compatible",
+        modelID: "deepseek-v4-pro",
+        desired: { reasoningEffort },
+      })
+
+      expect(result.reasoningEffort).toBe(reasoningEffort)
+      expect(result.changes).toEqual([])
+    }
+  })
+
+  test("DeepSeek maps generic reasoningEffort levels to canonical API values", () => {
+    const cases = [
+      { requested: "low", expected: "high" },
+      { requested: "medium", expected: "high" },
+      { requested: "xhigh", expected: "max" },
+    ]
+
+    for (const { requested, expected } of cases) {
+      const result = resolveCompatibleModelSettings({
+        providerID: "openai-compatible",
+        modelID: "deepseek-v4-pro",
+        desired: { reasoningEffort: requested },
+      })
+
+      expect(result.reasoningEffort).toBe(expected)
+      expect(result.changes).toEqual([
+        {
+          field: "reasoningEffort",
+          from: requested,
+          to: expected,
+          reason: "unsupported-by-model-family",
+        },
+      ])
+    }
+  })
+
   test("GPT-5 downgrades unsupported max variant to xhigh", () => {
     const result = resolveCompatibleModelSettings({
       providerID: "openai",