diff --git a/.changeset/deepseek-reasoning-complete.md b/.changeset/deepseek-reasoning-complete.md
new file mode 100644
index 00000000000..738c7129ba6
--- /dev/null
+++ b/.changeset/deepseek-reasoning-complete.md
@@ -0,0 +1,5 @@
+---
+"kilo-code": patch
+---
+
+Fix DeepSeek reasoning_content error in thinking mode for multi-turn tool calls. Applies to all provider paths including OpenRouter and custom OpenAI-compatible. Ensures empty reasoning_content is always preserved for historical messages and dynamic SDK key selection for OpenRouter models.
diff --git a/packages/kilo-gateway/src/provider.ts b/packages/kilo-gateway/src/provider.ts
index 62bd6716d1e..00b9c2db33f 100644
--- a/packages/kilo-gateway/src/provider.ts
+++ b/packages/kilo-gateway/src/provider.ts
@@ -79,8 +79,12 @@ export function createKilo(options: KiloProviderOptions = {}): KiloProvider {
   const openrouter = createOpenRouter(sdkOptions)
   const alibaba = createAlibaba(sdkOptions)
   const anthropic = createAnthropic(sdkOptions)
-  const openai = createOpenAI(sdkOptions)
-  const openaiCompatible = createOpenAICompatible({ ...sdkOptions, name: "openaiCompatible" })
+  const openai = createOpenAI({ ...sdkOptions, extractReasoning: true } as any)
+  const openaiCompatible = createOpenAICompatible({
+    ...sdkOptions,
+    name: "openaiCompatible",
+    extractReasoning: true,
+  } as any)
 
   return {
     languageModel(modelId) {
diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index 060c55055df..c556a0b9c9b 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -1012,7 +1012,7 @@ function fromModelsDevModel(provider: ModelsDev.Provider, model: ModelsDev.Model
         video: model.modalities?.output?.includes("video") ?? false,
         pdf: model.modalities?.output?.includes("pdf") ?? false,
       },
-      interleaved: model.interleaved ?? false,
+      interleaved: model.interleaved ?? (model.reasoning && model.id.includes("deepseek") ? { field: "reasoning_content" } : false), // kilocode_change
     },
     release_date: model.release_date ?? "",
     variants: {},
@@ -1185,7 +1185,8 @@ const layer: Layer.Layer<
                     model.modalities?.output?.includes("video") ?? existingModel?.capabilities.output.video ?? false,
                   pdf: model.modalities?.output?.includes("pdf") ?? existingModel?.capabilities.output.pdf ?? false,
                 },
-                interleaved: model.interleaved ?? false,
+                interleaved: model.interleaved ?? existingModel?.capabilities.interleaved ??
+                  (model.reasoning && (model.id?.includes("deepseek") || modelID.includes("deepseek")) ? { field: "reasoning_content" } : false), // kilocode_change
               },
               cost: {
                 input: model?.cost?.input ?? existingModel?.cost?.input ?? 0,
@@ -1413,6 +1414,10 @@ const layer: Layer.Layer<
           options["includeUsage"] = true
         }
 
+        if (model.capabilities.reasoning && options["extractReasoning"] === undefined) {
+          options["extractReasoning"] = true // kilocode_change
+        }
+
         const baseURL = iife(() => {
           let url =
             typeof options["baseURL"] === "string" && options["baseURL"] !== "" ? options["baseURL"] : model.api.url
diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index f3dee7e6ec4..83365ee4155 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -185,37 +185,94 @@ function normalizeMessages(
 
   if (typeof model.capabilities.interleaved === "object" && model.capabilities.interleaved.field) {
     const field = model.capabilities.interleaved.field
-    return msgs.map((msg) => {
-      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("")
+    const sdk = sdkKey(model.api.npm) ?? "openaiCompatible"
+    msgs = msgs.map((msg) => {
+      if (msg.role === "assistant") {
+        let reasoningText = ""
 
-        // Filter out reasoning parts from content
-        const filteredContent = msg.content.filter((part: any) => part.type !== "reasoning")
+        if (Array.isArray(msg.content)) {
+          const reasoningParts = msg.content.filter((part: any) => part.type === "reasoning")
+          reasoningText = reasoningParts.map((part: any) => part.text).join("")
 
-        // kilocode_change start - cherry-picked from anomalyco/opencode#24146;
-        // will be reverted on the next wholesale upstream merge.
-        // Include reasoning_content | reasoning_details directly on the message for all assistant messages.
-        // Always set the field even when empty — some providers (e.g. DeepSeek) may return empty
-        // reasoning_content which still needs to be sent back in subsequent requests.
-        return {
-          ...msg,
-          content: filteredContent,
-          providerOptions: {
-            ...msg.providerOptions,
-            openaiCompatible: {
-              ...msg.providerOptions?.openaiCompatible,
-              [field]: reasoningText,
+          // Filter out reasoning parts from content for interleaved messages
+          const filteredContent = msg.content.filter((part: any) => part.type !== "reasoning")
+
+          return {
+            ...msg,
+            content: filteredContent,
+            providerOptions: {
+              ...msg.providerOptions,
+              [sdk]: {
+                ...(msg.providerOptions as any)?.[sdk],
+                [field]: reasoningText,
+              },
             },
-          },
+          }
+        } else if (typeof msg.content === "string") {
+          // Handle string content messages - inject empty reasoning_content if missing
+          const existing = (msg.providerOptions as any)?.[sdk]?.[field]
+          if (existing === undefined || existing === null) {
+            return {
+              ...msg,
+              providerOptions: {
+                ...msg.providerOptions,
+                [sdk]: {
+                  ...(msg.providerOptions as any)?.[sdk],
+                  [field]: "",
+                },
+              },
+            }
+          }
+          return msg
         }
-        // kilocode_change end
       }
 
       return msg
     })
   }
 
+  // kilocode_change start - cherry-picked from anomalyco/opencode#24250;
+  // inject empty reasoning_content for all assistant messages on reasoning models.
+  // This handles historical messages saved before the fix and tool-only responses
+  // where the SDK produced no reasoning part but the API still requires the field.
+  if (model.capabilities.reasoning) {
+    const sdk = sdkKey(model.api.npm) ?? "openaiCompatible"
+    const field =
+      (typeof model.capabilities.interleaved === "object" &&
+        model.capabilities.interleaved.field) ||
+      "reasoning_content"
+
+    msgs = msgs.map((msg) => {
+      if (msg.role !== "assistant") return msg
+
+      // Don't overwrite already-set reasoning content
+      const existing = (msg.providerOptions as any)?.[sdk]?.[field]
+      if (existing !== undefined && existing !== null) return msg
+
+      // Extract reasoning from content parts if present
+      let reasoningText = ""
+      if (Array.isArray(msg.content)) {
+        const parts = msg.content.filter((p: any) => p.type === "reasoning")
+        reasoningText = parts.map((p: any) => p.text).join("")
+      }
+
+      // Don't double-set if already handled by interleaved block (would have stripped reasoning parts)
+      if (reasoningText) return msg
+
+      return {
+        ...msg,
+        providerOptions: {
+          ...msg.providerOptions,
+          [sdk]: {
+            ...(msg.providerOptions as any)?.[sdk],
+            [field]: reasoningText, // empty string for historical/no-reasoning messages
+          },
+        },
+      }
+    })
+  }
+  // kilocode_change end
+
   return msgs
 }
 
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index ccaf1460427..21f66f8c0ad 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -977,6 +977,122 @@ describe("ProviderTransform.message - DeepSeek reasoning content", () => {
     ])
     expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBeUndefined()
   })
+
+  const baseCapabilities = {
+    temperature: true,
+    reasoning: true,
+    attachment: true,
+    toolcall: true,
+    input: { text: true, audio: false, image: true, video: false, pdf: false },
+    output: { text: true, audio: false, image: false, video: false, pdf: false },
+  }
+
+  const createModel = (overrides: Partial<any> = {}) =>
+    ({
+      id: "test/test-model",
+      providerID: "test",
+      api: {
+        id: "test-model",
+        url: "https://api.test.com",
+        npm: "@ai-sdk/openai",
+      },
+      name: "Test Model",
+      capabilities: {
+        ...baseCapabilities,
+        interleaved: false,
+      },
+      cost: {
+        input: 0.001,
+        output: 0.002,
+        cache: { read: 0.0001, write: 0.0002 },
+      },
+      limit: {
+        context: 200_000,
+        output: 64_000,
+      },
+      status: "active",
+      options: {},
+      headers: {},
+      release_date: "2024-01-01",
+      ...overrides,
+    }) as any
+
+  test("OpenRouter DeepSeek uses 'openrouter' key for reasoning_content", () => {
+    const model = createModel({
+      id: "deepseek-ai/DeepSeek-R1-0528",
+      providerID: "openrouter",
+      api: { id: "deepseek-deepseek-r1", url: "https://api.openrouter.ai", npm: "@openrouter/ai-sdk-provider" },
+      capabilities: { ...baseCapabilities, reasoning: true, interleaved: { field: "reasoning_content" } },
+    })
+    const msgs = [{
+      role: "assistant",
+      content: [
+        { type: "reasoning", text: "thinking step" },
+        { type: "tool-call", toolCallId: "t1", toolName: "bash", input: { cmd: "ls" } },
+      ],
+    }] as any[]
+    const result = ProviderTransform.message(msgs, model, {})
+    expect(result[0].providerOptions?.openrouter?.reasoning_content).toBe("thinking step")
+    expect(result[0].providerOptions?.openaiCompatible).toBeUndefined()
+  })
+
+  test("assistant message without reasoning part gets empty reasoning_content", () => {
+    const model = createModel({
+      id: "deepseek/deepseek-reasoner",
+      providerID: "deepseek",
+      api: { id: "deepseek-reasoner", url: "https://api.deepseek.com", npm: "@ai-sdk/openai-compatible" },
+      capabilities: { ...baseCapabilities, reasoning: true, interleaved: false },
+    })
+    const msgs = [{
+      role: "assistant",
+      content: [{ type: "text", text: "Hello" }],
+    }] as any[]
+    const result = ProviderTransform.message(msgs, model, {})
+    expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBe("")
+  })
+
+  test("model with reasoning:true and no interleaved gets auto-injected", () => {
+    const model = createModel({
+      id: "openrouter/deepseek-r1",
+      providerID: "openrouter",
+      api: { id: "deepseek-ai/DeepSeek-R1", url: "https://api.openrouter.ai", npm: "@openrouter/ai-sdk-provider" },
+      capabilities: { ...baseCapabilities, reasoning: true, interleaved: false },
+    })
+    const msgs = [{
+      role: "assistant",
+      content: [{ type: "text", text: "No reasoning here" }],
+    }] as any[]
+    const result = ProviderTransform.message(msgs, model, {})
+    expect(result[0].providerOptions?.openrouter?.reasoning_content).toBe("")
+  })
+
+  test("non-reasoning model does not add reasoning_content", () => {
+    const model = createModel({
+      id: "openai/gpt-4",
+      providerID: "openai",
+      api: { id: "gpt-4", url: "https://api.openai.com", npm: "@ai-sdk/openai" },
+      capabilities: { ...baseCapabilities, reasoning: false, interleaved: false },
+    })
+    const msgs = [{ role: "assistant", content: [{ type: "text", text: "Hello" }] }] as any[]
+    const result = ProviderTransform.message(msgs, model, {})
+    expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBeUndefined()
+  })
+
+  test("interleaved block handles string content assistant messages", () => {
+    const model = createModel({
+      id: "deepseek/deepseek-reasoner",
+      providerID: "deepseek",
+      api: { id: "deepseek-reasoner", url: "https://api.deepseek.com", npm: "@ai-sdk/openai-compatible" },
+      capabilities: { ...baseCapabilities, reasoning: true, interleaved: { field: "reasoning_content" } },
+    })
+    const msgs = [{
+      role: "assistant",
+      content: "This is a string content message"
+    }] as any[]
+    const result = ProviderTransform.message(msgs, model, {})
+    expect(result[0].content).toBe("This is a string content message")
+    expect(result[0].providerOptions?.openaiCompatible?.reasoning_content).toBe("")
+  })
 })
 
 describe("ProviderTransform.message - empty image handling", () => {