fix: strip trailing text-only assistant when thinking rejects prefill

Some models (Opus 4.6) reject assistant-last messages as unsupported
prefill when thinking is enabled. During tool-call loops, multi-step
assistant messages can produce a trailing text-only model message after
toModelMessages conversion. The loop continues (finish=tool-calls) but
the next request ends with assistant.

Strip trailing text-only assistant messages at the end of
ProviderTransform.message() when options.thinking is set.

Author: chan1103

packages/opencode/src/provider/transform.ts

@@ -322,6 +322,17 @@ export namespace ProviderTransform {
       })
     }
 
+    if (options.thinking) {
+      while (msgs.length > 0) {
+        const last = msgs[msgs.length - 1]
+        if (last.role !== "assistant") break
+        if (typeof last.content === "string") { msgs = msgs.slice(0, -1); continue }
+        if (!Array.isArray(last.content)) break
+        if (!last.content.every((p) => p.type === "text")) break
+        msgs = msgs.slice(0, -1)
+      }
+    }
+
     return msgs
   }
 

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

@@ -1214,6 +1214,69 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     expect(result[0].content[1]).toEqual({ type: "text", text: "Result" })
   })
 
+  test("removes trailing text-only assistant when thinking is active", () => {
+    const msgs = [
+      { role: "user", content: [{ type: "text", text: "Run ls" }] },
+      { role: "assistant", content: [{ type: "tool-call", toolCallId: "t1", toolName: "bash", args: {} }] },
+      { role: "tool", content: [{ type: "tool-result", toolCallId: "t1", result: "file.txt" }] },
+      { role: "assistant", content: [{ type: "text", text: "Now I will edit." }] },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, { thinking: { type: "enabled", budgetTokens: 10000 } })
+
+    expect(result).toHaveLength(3)
+    expect(result[result.length - 1].role).toBe("tool")
+  })
+
+  test("keeps trailing text-only assistant when thinking is NOT active", () => {
+    const msgs = [
+      { role: "user", content: [{ type: "text", text: "Run ls" }] },
+      { role: "assistant", content: [{ type: "text", text: "Planning..." }] },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, {})
+
+    expect(result).toHaveLength(2)
+    expect(result[result.length - 1].role).toBe("assistant")
+  })
+
+  test("does not remove assistant with tool calls even when thinking is active", () => {
+    const msgs = [
+      { role: "user", content: [{ type: "text", text: "Run ls" }] },
+      { role: "assistant", content: [{ type: "text", text: "Let me check" }, { type: "tool-call", toolCallId: "t1", toolName: "bash", args: {} }] },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, { thinking: { type: "enabled", budgetTokens: 10000 } })
+
+    expect(result).toHaveLength(2)
+    expect(result[result.length - 1].role).toBe("assistant")
+  })
+
+  test("removes consecutive trailing text-only assistants when thinking is active", () => {
+    const msgs = [
+      { role: "user", content: [{ type: "text", text: "Hello" }] },
+      { role: "assistant", content: [{ type: "text", text: "Step 1" }] },
+      { role: "assistant", content: [{ type: "text", text: "Step 2" }] },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, { thinking: { type: "adaptive" } })
+
+    expect(result).toHaveLength(1)
+    expect(result[0].role).toBe("user")
+  })
+
+  test("does not remove assistant with reasoning blocks even when thinking is active", () => {
+    const msgs = [
+      { role: "user", content: [{ type: "text", text: "Hello" }] },
+      { role: "assistant", content: [{ type: "reasoning", text: "thinking...", providerOptions: { anthropic: { signature: "sig" } } }, { type: "text", text: "Answer" }] },
+    ] as any[]
+
+    const result = ProviderTransform.message(msgs, anthropicModel, { thinking: { type: "enabled", budgetTokens: 10000 } })
+
+    expect(result).toHaveLength(2)
+    expect(result[result.length - 1].role).toBe("assistant")
+  })
+
   test("filters empty content for bedrock provider", () => {
     const bedrockModel = {
       ...anthropicModel,