fix: replace empty text in reasoning messages to preserve thinking block positions

packages/opencode/src/provider/transform.ts

@@ -60,14 +60,28 @@ function normalizeMessages(
           return msg
         }
         if (!Array.isArray(msg.content)) return msg
-        const filtered = msg.content.filter((part) => {
-          if (part.type === "text" || part.type === "reasoning") {
-            return part.text !== ""
+          const hasReasoning = msg.role === "assistant" && msg.content.some((p) => p.type === "reasoning" && p.providerOptions !== undefined)
+          const filtered = msg.content
+            .filter((part) => {
+              if (part.type === "reasoning") {
+                return part.text !== "" || part.providerOptions !== undefined
+              }
+              if (part.type === "text" && !hasReasoning) {
+                return part.text !== ""
+              }
+              return true
+            })
+            .map((part) => {
+              if (hasReasoning && part.type === "text" && part.text === "") {
+                return { ...part, text: "..." } as typeof part
+              }
+              return part
+            })
+          if (filtered.length === 0) return undefined
+          if (hasReasoning && filtered.length > 0 && filtered[filtered.length - 1].type === "reasoning") {
+            filtered.push({ type: "text", text: "..." } as (typeof filtered)[number])
           }
-          return true
-        })
-        if (filtered.length === 0) return undefined
-        return { ...msg, content: filtered }
+          return { ...msg, content: filtered }
       })
       .filter((msg): msg is ModelMessage => msg !== undefined && msg.content !== "")
   }

packages/opencode/test/provider/transform.test.ts

@@ -1212,6 +1212,103 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     expect(result[0].content[1]).toEqual({ type: "text", text: "Result" })
   })
 
+  test("replaces empty text with placeholder in assistant messages with reasoning", () => {
+    const msgs = [
+      {
+        role: "assistant",
+        content: [
+          { type: "reasoning", text: "thinking...", providerOptions: { anthropic: { signature: "sig_abc" } } },
+          { type: "text", text: "" },
+          { type: "reasoning", text: "more thinking", providerOptions: { anthropic: { signature: "sig_xyz" } } },
+          { type: "text", text: "Answer" },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    expect(result[0].content).toHaveLength(4)
+    expect(result[0].content[0]).toEqual({ type: "reasoning", text: "thinking...", providerOptions: { anthropic: { signature: "sig_abc" } } })
+    expect(result[0].content[1]).toEqual({ type: "text", text: "..." })
+    expect(result[0].content[2]).toEqual({ type: "reasoning", text: "more thinking", providerOptions: { anthropic: { signature: "sig_xyz" } } })
+    expect(result[0].content[3]).toEqual({ type: "text", text: "Answer" })
+  })
+
+  test("replaces empty text and appends fallback when only reasoning remains", () => {
+    const msgs = [
+      {
+        role: "assistant",
+        content: [
+          { type: "reasoning", text: "thinking...", providerOptions: { anthropic: { signature: "sig_abc" } } },
+          { type: "text", text: "" },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    expect(result[0].content).toHaveLength(2)
+    expect((result[0].content as any[])[0].type).toBe("reasoning")
+    expect(result[0].content[1]).toEqual({ type: "text", text: "..." })
+  })
+
+  test("appends fallback text when assistant has only reasoning with signature", () => {
+    const msgs = [
+      {
+        role: "assistant",
+        content: [
+          { type: "reasoning", text: "deep thought", providerOptions: { anthropic: { signature: "sig_xyz" } } },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    expect(result[0].content).toHaveLength(2)
+    expect((result[0].content as any[])[0].type).toBe("reasoning")
+    expect(result[0].content[1]).toEqual({ type: "text", text: "..." })
+  })
+
+  test("does not replace text in assistant messages without reasoning", () => {
+    const msgs = [
+      {
+        role: "assistant",
+        content: [
+          { type: "text", text: "" },
+          { type: "text", text: "Hello" },
+          { type: "text", text: "" },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    expect(result[0].content).toHaveLength(1)
+    expect(result[0].content[0]).toEqual({ type: "text", text: "Hello" })
+  })
+
+  test("does not replace empty text in user messages with reasoning", () => {
+    const msgs = [
+      {
+        role: "user",
+        content: [
+          { type: "reasoning", text: "user reasoning", providerOptions: { anthropic: { signature: "sig_abc" } } },
+          { type: "text", text: "" },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    expect(result[0].content).toHaveLength(1)
+    expect((result[0].content as any[])[0].type).toBe("reasoning")
+  })
+
   test("filters empty content for bedrock provider", () => {
     const bedrockModel = {
       ...anthropicModel,