fix(provider): preserve assistant message content when reasoning blocks present

packages/opencode/src/provider/transform.ts

@@ -52,7 +52,10 @@ export namespace ProviderTransform {
     options: Record<string, unknown>,
   ): ModelMessage[] {
     // Anthropic rejects messages with empty content - filter out empty string messages
-    // and remove empty text/reasoning parts from array content
+    // and remove empty text/reasoning parts from array content.
+    // Assistant messages with reasoning blocks are excluded from filtering because
+    // thinking block signatures encode positional context — removing an empty text
+    // part between reasoning blocks invalidates the signatures.
     if (model.api.npm === "@ai-sdk/anthropic" || model.api.npm === "@ai-sdk/amazon-bedrock") {
       msgs = msgs
         .map((msg) => {
@@ -61,6 +64,7 @@ export namespace ProviderTransform {
             return msg
           }
           if (!Array.isArray(msg.content)) return msg
+          if (msg.role === "assistant" && msg.content.some((part) => part.type === "reasoning")) return msg
           const filtered = msg.content.filter((part) => {
             if (part.type === "text" || part.type === "reasoning") {
               return part.text !== ""

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

@@ -1080,7 +1080,7 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     expect(result[0].content[0]).toEqual({ type: "text", text: "Hello" })
   })
 
-  test("filters out empty reasoning parts from array content", () => {
+  test("preserves assistant message verbatim when reasoning blocks present", () => {
     const msgs = [
       {
         role: "assistant",
@@ -1094,12 +1094,16 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
 
     const result = ProviderTransform.message(msgs, anthropicModel, {})
 
+    // Assistant messages with reasoning blocks are preserved verbatim
+    // because thinking block signatures encode positional context
     expect(result).toHaveLength(1)
-    expect(result[0].content).toHaveLength(1)
-    expect(result[0].content[0]).toEqual({ type: "text", text: "Answer" })
+    expect(result[0].content).toHaveLength(3)
+    expect(result[0].content[0]).toEqual({ type: "reasoning", text: "" })
+    expect(result[0].content[1]).toEqual({ type: "text", text: "Answer" })
+    expect(result[0].content[2]).toEqual({ type: "reasoning", text: "" })
   })
 
-  test("removes entire message when all parts are empty", () => {
+  test("preserves assistant message with all-empty parts when reasoning present", () => {
     const msgs = [
       { role: "user", content: "Hello" },
       {
@@ -1114,6 +1118,25 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
 
     const result = ProviderTransform.message(msgs, anthropicModel, {})
 
+    // Preserved because reasoning blocks are present
+    expect(result).toHaveLength(3)
+    expect(result[0].content).toBe("Hello")
+    expect(result[1].content).toHaveLength(2)
+    expect(result[2].content).toBe("World")
+  })
+
+  test("removes entire non-assistant message when all parts are empty", () => {
+    const msgs = [
+      { role: "user", content: "Hello" },
+      {
+        role: "user",
+        content: [{ type: "text", text: "" }],
+      },
+      { role: "user", content: "World" },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
     expect(result).toHaveLength(2)
     expect(result[0].content).toBe("Hello")
     expect(result[1].content).toBe("World")
@@ -1142,7 +1165,7 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     })
   })
 
-  test("keeps messages with valid text alongside empty parts", () => {
+  test("preserves all parts in assistant message with reasoning alongside empty text", () => {
     const msgs = [
       {
         role: "assistant",
@@ -1156,10 +1179,55 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
 
     const result = ProviderTransform.message(msgs, anthropicModel, {})
 
+    // All 3 parts preserved — reasoning present means no filtering
     expect(result).toHaveLength(1)
-    expect(result[0].content).toHaveLength(2)
+    expect(result[0].content).toHaveLength(3)
     expect(result[0].content[0]).toEqual({ type: "reasoning", text: "Thinking..." })
-    expect(result[0].content[1]).toEqual({ type: "text", text: "Result" })
+    expect(result[0].content[1]).toEqual({ type: "text", text: "" })
+    expect(result[0].content[2]).toEqual({ type: "text", text: "Result" })
+  })
+
+  test("preserves empty text between reasoning blocks in assistant messages", () => {
+    // Anthropic adaptive thinking (Opus 4.6) emits:
+    //   reasoning(sig1) → text("") → reasoning(sig2) → text("answer") → tool_use
+    // The empty text is positionally significant — thinking block signatures
+    // encode block arrangement. Removing it invalidates signatures.
+    const msgs = [
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "reasoning",
+            text: "analyzing...",
+            providerOptions: { anthropic: { signature: "sig1" } },
+          },
+          { type: "text", text: "" },
+          {
+            type: "reasoning",
+            text: "checking auth...",
+            providerOptions: { anthropic: { signature: "sig2" } },
+          },
+          { type: "text", text: "Let me check." },
+          {
+            type: "tool-call",
+            toolCallId: "toolu_01ABC",
+            toolName: "grep",
+            input: { pattern: "endpoint" },
+          },
+        ],
+      },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(1)
+    const parts = result[0].content as any[]
+    expect(parts).toHaveLength(5)
+    expect(parts[0].type).toBe("reasoning")
+    expect(parts[1]).toEqual({ type: "text", text: "" })
+    expect(parts[2].type).toBe("reasoning")
+    expect(parts[3].text).toBe("Let me check.")
+    expect(parts[4].type).toBe("tool-call")
   })
 
   test("filters empty content for bedrock provider", () => {