diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 05b9f031fe6..6be81d5e6af 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -135,7 +135,17 @@ export namespace ProviderTransform {
 
     if (typeof model.capabilities.interleaved === "object" && model.capabilities.interleaved.field) {
       const field = model.capabilities.interleaved.field
-      return msgs.map((msg) => {
+      const isDeepSeek = model.providerID === "deepseek" || model.api.url?.includes("api.deepseek.com")
+      // Check if thinking mode is enabled for DeepSeek
+      const thinking = options.thinking as any
+      const isThinkingEnabled =
+        isDeepSeek &&
+        (model.capabilities.reasoning ||
+          model.api.id === "deepseek-reasoner" ||
+          model.id.includes("deepseek-reasoner") ||
+          (thinking && thinking.type === "enabled"))
+      const lastUserIndex = isThinkingEnabled ? msgs.findLastIndex((msg) => msg.role === "user") : -1
+      return msgs.map((msg, index) => {
         if (msg.role === "assistant" && Array.isArray(msg.content)) {
           const reasoningParts = msg.content.filter((part: any) => part.type === "reasoning")
           const reasoningText = reasoningParts.map((part: any) => part.text).join("")
@@ -144,7 +154,8 @@ export namespace ProviderTransform {
           const filteredContent = msg.content.filter((part: any) => part.type !== "reasoning")
 
           // Include reasoning_content | reasoning_details directly on the message for all assistant messages
-          if (reasoningText) {
+          const shouldIncludeReasoning = reasoningText || (isThinkingEnabled && index > lastUserIndex)
+          if (shouldIncludeReasoning) {
             return {
               ...msg,
               content: filteredContent,
@@ -152,7 +163,7 @@ export namespace ProviderTransform {
                 ...msg.providerOptions,
                 openaiCompatible: {
                   ...(msg.providerOptions as any)?.openaiCompatible,
-                  [field]: reasoningText,
+                  [field]: reasoningText || "",
                 },
               },
             }
@@ -1009,4 +1020,6 @@ export namespace ProviderTransform {
 
     return schema as JSONSchema7
   }
+
+  // DeepSeek thinking mode fix: ensures empty reasoning_content for assistant messages after last user message
 }
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 917d357eafa..5d4b0a55661 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -773,7 +773,7 @@ describe("ProviderTransform.message - DeepSeek reasoning content", () => {
         options: {},
         headers: {},
         release_date: "2023-04-01",
-      },
+      } as any,
       {},
     )
 
@@ -789,6 +789,72 @@ describe("ProviderTransform.message - DeepSeek reasoning content", () => {
     expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBe("Let me think about this...")
   })
 
+  test("DeepSeek sets empty reasoning_content for assistant messages after last user message", () => {
+    const msgs = [
+      {
+        role: "user",
+        content: "First question",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "First answer" }],
+      },
+      {
+        role: "user",
+        content: "Second question",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Second answer" }],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(
+      msgs,
+      {
+        id: "deepseek/deepseek-chat",
+        providerID: "deepseek",
+        api: {
+          id: "deepseek-chat",
+          url: "https://api.deepseek.com",
+          npm: "@ai-sdk/openai-compatible",
+        },
+        name: "DeepSeek Chat",
+        capabilities: {
+          temperature: true,
+          reasoning: true,
+          attachment: false,
+          toolcall: true,
+          input: { text: true, audio: false, image: false, video: false, pdf: false },
+          output: { text: true, audio: false, image: false, video: false, pdf: false },
+          interleaved: {
+            field: "reasoning_content",
+          },
+        },
+        cost: {
+          input: 0.001,
+          output: 0.002,
+          cache: { read: 0.0001, write: 0.0002 },
+        },
+        limit: {
+          context: 128000,
+          output: 8192,
+        },
+        status: "active",
+        options: {},
+        headers: {},
+        release_date: "2023-04-01",
+      } as any,
+      {},
+    )
+
+    expect(result).toHaveLength(4)
+    // First assistant (before last user) should NOT have reasoning_content
+    expect(result[1].providerOptions?.openaiCompatible?.reasoning_content).toBeUndefined()
+    // Second assistant (after last user) should have empty reasoning_content
+    expect(result[3].providerOptions?.openaiCompatible?.reasoning_content).toBe("")
+  })
+
   test("Non-DeepSeek providers leave reasoning content unchanged", () => {
     const msgs = [
       {
@@ -833,7 +899,7 @@ describe("ProviderTransform.message - DeepSeek reasoning content", () => {
         options: {},
         headers: {},
         release_date: "2023-04-01",
-      },
+      } as any,
       {},
     )
 
@@ -843,6 +909,176 @@ describe("ProviderTransform.message - DeepSeek reasoning content", () => {
     ])
     expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBeUndefined()
   })
+
+  test("DeepSeek with reasoner model ID sets empty reasoning_content", () => {
+    const msgs = [
+      {
+        role: "user",
+        content: "Question",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Answer" }],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(
+      msgs,
+      {
+        id: "deepseek/deepseek-reasoner",
+        providerID: "deepseek",
+        api: {
+          id: "deepseek-reasoner",
+          url: "https://api.deepseek.com",
+          npm: "@ai-sdk/openai-compatible",
+        },
+        name: "DeepSeek Reasoner",
+        capabilities: {
+          temperature: true,
+          reasoning: false, // reasoning false but model is reasoner
+          attachment: false,
+          toolcall: true,
+          input: { text: true, audio: false, image: false, video: false, pdf: false },
+          output: { text: true, audio: false, image: false, video: false, pdf: false },
+          interleaved: {
+            field: "reasoning_content",
+          },
+        },
+        cost: {
+          input: 0.001,
+          output: 0.002,
+          cache: { read: 0.0001, write: 0.0002 },
+        },
+        limit: {
+          context: 128000,
+          output: 8192,
+        },
+        status: "active",
+        options: {},
+        headers: {},
+        release_date: "2023-04-01",
+      } as any,
+      {},
+    )
+
+    expect(result).toHaveLength(2)
+    // Assistant after last user should have empty reasoning_content
+    expect(result[1].providerOptions?.openaiCompatible?.reasoning_content).toBe("")
+  })
+
+  test("DeepSeek with thinking option enabled sets empty reasoning_content", () => {
+    const msgs = [
+      {
+        role: "user",
+        content: "Question",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Answer" }],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(
+      msgs,
+      {
+        id: "deepseek/deepseek-chat",
+        providerID: "deepseek",
+        api: {
+          id: "deepseek-chat",
+          url: "https://api.deepseek.com",
+          npm: "@ai-sdk/openai-compatible",
+        },
+        name: "DeepSeek Chat",
+        capabilities: {
+          temperature: true,
+          reasoning: false, // reasoning false
+          attachment: false,
+          toolcall: true,
+          input: { text: true, audio: false, image: false, video: false, pdf: false },
+          output: { text: true, audio: false, image: false, video: false, pdf: false },
+          interleaved: {
+            field: "reasoning_content",
+          },
+        },
+        cost: {
+          input: 0.001,
+          output: 0.002,
+          cache: { read: 0.0001, write: 0.0002 },
+        },
+        limit: {
+          context: 128000,
+          output: 8192,
+        },
+        status: "active",
+        options: {},
+        headers: {},
+        release_date: "2023-04-01",
+      } as any,
+      {
+        thinking: { type: "enabled" },
+      },
+    )
+
+    expect(result).toHaveLength(2)
+    // Assistant after last user should have empty reasoning_content
+    expect(result[1].providerOptions?.openaiCompatible?.reasoning_content).toBe("")
+  })
+
+  test("DeepSeek without thinking mode does not set empty reasoning_content", () => {
+    const msgs = [
+      {
+        role: "user",
+        content: "Question",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Answer" }],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(
+      msgs,
+      {
+        id: "deepseek/deepseek-chat",
+        providerID: "deepseek",
+        api: {
+          id: "deepseek-chat",
+          url: "https://api.deepseek.com",
+          npm: "@ai-sdk/openai-compatible",
+        },
+        name: "DeepSeek Chat",
+        capabilities: {
+          temperature: true,
+          reasoning: false, // reasoning false
+          attachment: false,
+          toolcall: true,
+          input: { text: true, audio: false, image: false, video: false, pdf: false },
+          output: { text: true, audio: false, image: false, video: false, pdf: false },
+          interleaved: {
+            field: "reasoning_content",
+          },
+        },
+        cost: {
+          input: 0.001,
+          output: 0.002,
+          cache: { read: 0.0001, write: 0.0002 },
+        },
+        limit: {
+          context: 128000,
+          output: 8192,
+        },
+        status: "active",
+        options: {},
+        headers: {},
+        release_date: "2023-04-01",
+      } as any,
+      {}, // no thinking option
+    )
+
+    expect(result).toHaveLength(2)
+    // Assistant should NOT have reasoning_content because thinking mode not enabled
+    expect(result[1].providerOptions?.openaiCompatible?.reasoning_content).toBeUndefined()
+  })
 })
 
 describe("ProviderTransform.message - empty image handling", () => {