diff --git a/.changeset/fix-deepseek-tool-message.md b/.changeset/fix-deepseek-tool-message.md
new file mode 100644
index 0000000..69e6496
--- /dev/null
+++ b/.changeset/fix-deepseek-tool-message.md
@@ -0,0 +1,5 @@
+---
+"opencode-bridge-copilot-chat": patch
+---
+
+Serialize tool-only Chat Completions messages with provider-compatible empty content instead of `null`.
diff --git a/src/provider/content.test.ts b/src/provider/content.test.ts
new file mode 100644
index 0000000..1b0007c
--- /dev/null
+++ b/src/provider/content.test.ts
@@ -0,0 +1,15 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { chatContent } from "./content";
+
+test("uses an empty string for textless Chat Completions content", () => {
+  assert.equal(chatContent("", []), "");
+});
+
+test("preserves text and image content parts", () => {
+  const image = { type: "image_url" as const, image_url: { url: "data:image/png;base64,AA==" } };
+  assert.deepEqual(chatContent("Describe this image", [image]), [
+    { type: "text", text: "Describe this image" },
+    image,
+  ]);
+});
diff --git a/src/provider/content.ts b/src/provider/content.ts
new file mode 100644
index 0000000..49bb28b
--- /dev/null
+++ b/src/provider/content.ts
@@ -0,0 +1,13 @@
+export interface ContentPart {
+  type: "text" | "image_url";
+  text?: string;
+  image_url?: { url: string };
+}
+
+export type ChatContent = string | ContentPart[];
+
+export function chatContent(value: string, images: readonly ContentPart[]): ChatContent {
+  return images.length
+    ? [...(value ? [{ type: "text" as const, text: value }] : []), ...images]
+    : value;
+}
diff --git a/src/provider/messages.ts b/src/provider/messages.ts
index 6ab5efd..c19b0b1 100644
--- a/src/provider/messages.ts
+++ b/src/provider/messages.ts
@@ -1,16 +1,13 @@
 import * as vscode from "vscode";
+import { chatContent, type ChatContent, type ContentPart } from "./content";
 
-const MAX_INLINE_IMAGE_BYTES = 3_750_000;
+export type { ChatContent, ContentPart } from "./content";
 
-export interface ContentPart {
-  type: "text" | "image_url";
-  text?: string;
-  image_url?: { url: string };
-}
+const MAX_INLINE_IMAGE_BYTES = 3_750_000;
 
 export interface ChatMessage {
   role: "user" | "assistant" | "tool";
-  content: string | ContentPart[] | null;
+  content: ChatContent;
   tool_calls?: Array<{ id: string; type: "function"; function: { name: string; arguments: string } }>;
   tool_call_id?: string;
   reasoning_content?: string;
@@ -47,9 +44,7 @@ export function convertChatMessages(messages: readonly vscode.LanguageModelChatR
       }
     }
     const value = text.join("\n");
-    const content: string | ContentPart[] | null = images.length
-      ? [...(value ? [{ type: "text" as const, text: value }] : []), ...images]
-      : value || null;
+    const content = chatContent(value, images);
     const current: ChatMessage = {
       role,
       content,
diff --git a/src/provider/request.test.ts b/src/provider/request.test.ts
index 61415d9..e481eb6 100644
--- a/src/provider/request.test.ts
+++ b/src/provider/request.test.ts
@@ -66,6 +66,31 @@ test("builds a Chat Completions request with usage and optional tools", () => {
   });
 });
 
+test("keeps tool-only assistant content provider-compatible", () => {
+  const chatModel = { ...model, endpoint: "chat-completions" as const };
+  const body = buildRequestBody(
+    chatModel,
+    [{
+      role: "assistant",
+      content: "",
+      tool_calls: [{ id: "call-1", type: "function", function: { name: "lookup", arguments: "{}" } }],
+    }],
+    [],
+    [],
+    [],
+    undefined,
+    1_024,
+    "auto",
+  );
+
+  assert.deepEqual(body.messages, [{
+    role: "assistant",
+    content: "",
+    tool_calls: [{ id: "call-1", type: "function", function: { name: "lookup", arguments: "{}" } }],
+  }]);
+  assert.equal(JSON.stringify(body).includes('"content":null'), false);
+});
+
 test("extension request fields override catalog request options", () => {
   const body = buildRequestBody(model, [], [{ type: "message", role: "user", content: "Hello" }], [], [], undefined, 512, "auto");
   const merged = mergeRequestBody({