diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 9952762d85..c791aebf97 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -213,66 +213,29 @@ function normalizeMessages(
     })
   }
   if (["@ai-sdk/anthropic", "@ai-sdk/google-vertex/anthropic"].includes(model.api.npm)) {
-    // A tool result must immediately follow its tool call, but signed thinking must
-    // not move. Split merged steps around their matching tool results instead.
-    const result: ModelMessage[] = []
-    for (let i = 0; i < msgs.length; i++) {
-      const msg = msgs[i]
-      const next = msgs[i + 1]
-      if (msg.role !== "assistant" || !Array.isArray(msg.content) || next?.role !== "tool") {
-        result.push(msg)
-        continue
-      }
+    // Anthropic rejects assistant turns where tool_use blocks are followed by non-tool
+    // content, e.g. [tool_use, tool_use, text], with:
+    // `tool_use` ids were found without `tool_result` blocks immediately after...
+    //
+    // Reorder that invalid shape into [text] + [tool_use, tool_use]. Consecutive
+    // assistant messages are later merged by the provider/SDK, so preserving the
+    // original [tool_use...] then [text] order still produces the invalid payload.
+    //
+    // The root cause appears to be somewhere upstream where the stream is originally
+    // processed. We were unable to locate an exact narrower reproduction elsewhere,
+    // so we keep this transform in place for the time being.
+    msgs = msgs.flatMap((msg) => {
+      if (msg.role !== "assistant" || !Array.isArray(msg.content)) return [msg]
 
-      const clientToolIDs = new Set(
-        msg.content.flatMap((part) =>
-          part.type === "tool-call" && part.providerExecuted !== true ? [part.toolCallId] : [],
-        ),
-      )
-      if (clientToolIDs.size === 0) {
-        result.push(msg)
-        continue
-      }
-
-      const chunks = msg.content.reduce<Array<typeof msg.content>>(
-        (chunks, part) => {
-          const current = chunks.at(-1)!
-          const hasToolCall = current.some(
-            (item) => item.type === "tool-call" && clientToolIDs.has(item.toolCallId),
-          )
-          const isClientToolPart =
-            (part.type === "tool-call" || part.type === "tool-result") && clientToolIDs.has(part.toolCallId)
-          if (hasToolCall && !isClientToolPart) chunks.push([])
-          chunks.at(-1)!.push(part)
-          return chunks
-        },
-        [[]],
-      )
-      if (chunks.length === 1) {
-        result.push(msg)
-        continue
-      }
-
-      const used = new Set<string>()
-      for (const chunk of chunks) {
-        result.push({ ...msg, content: chunk })
-        const calls = new Set(
-          chunk.flatMap((part) =>
-            part.type === "tool-call" && clientToolIDs.has(part.toolCallId) ? [part.toolCallId] : [],
-          ),
-        )
-        if (calls.size === 0) continue
-        const output = next.content.filter((part) => part.type === "tool-result" && calls.has(part.toolCallId))
-        if (output.length === 0) continue
-        output.forEach((part) => used.add(part.toolCallId))
-        result.push({ ...next, content: output })
-      }
-
-      const remaining = next.content.filter((part) => part.type !== "tool-result" || !used.has(part.toolCallId))
-      if (remaining.length > 0) result.push({ ...next, content: remaining })
-      i++
-    }
-    msgs = result
+      const parts = msg.content
+      const first = parts.findIndex((part) => part.type === "tool-call")
+      if (first === -1) return [msg]
+      if (!parts.slice(first).some((part) => part.type !== "tool-call")) return [msg]
+      return [
+        { ...msg, content: parts.filter((part) => part.type !== "tool-call") },
+        { ...msg, content: parts.filter((part) => part.type === "tool-call") },
+      ]
+    })
   }
   if (
     model.providerID === "mistral" ||
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 862e06fc21..3c43b419b8 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -285,7 +285,13 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
         if (part.type !== "reasoning") return false
         return part.metadata?.anthropic?.signature != null
       })
+      const splitAnthropicSteps = ["@ai-sdk/anthropic", "@ai-sdk/google-vertex/anthropic"].includes(model.api.npm)
+      let hasClientToolCall = false
       for (const part of msg.parts) {
+        if (splitAnthropicSteps && hasClientToolCall && (part.type === "text" || part.type === "reasoning")) {
+          assistantMessage.parts.push({ type: "step-start" })
+          hasClientToolCall = false
+        }
         if (part.type === "text") {
           const text = part.text === "" && hasSignedReasoning ? " " : part.text
           assistantMessage.parts.push({
@@ -294,11 +300,14 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
             ...(differentModel ? {} : { providerMetadata: part.metadata }),
           })
         }
-        if (part.type === "step-start")
+        if (part.type === "step-start") {
           assistantMessage.parts.push({
             type: "step-start",
           })
+          hasClientToolCall = false
+        }
         if (part.type === "tool") {
+          if (part.metadata?.providerExecuted !== true) hasClientToolCall = true
           toolNames.add(part.tool)
           if (part.state.status === "completed") {
             const outputText = part.state.time.compacted
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index aba5952884..7fb22ddf57 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -1614,7 +1614,7 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     expect(result[1].content).toHaveLength(1)
   })
 
-  test("places anthropic tool results before trailing text", () => {
+  test("splits anthropic assistant messages when text trails tool calls", () => {
     const msgs = [
       {
         role: "user",
@@ -1646,70 +1646,16 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
 
     expect(result).toHaveLength(4)
     expect(result[1]).toMatchObject({
-      role: "assistant",
-      content: [
-        { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
-        { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
-      ],
-    })
-    expect(result[2]).toMatchObject({ role: "tool" })
-    expect(result[3]).toMatchObject({
       role: "assistant",
       content: [{ type: "text", text: "I checked your home directory and looked for PDF files." }],
     })
-  })
-
-  test("keeps anthropic signed reasoning in order around tool results", () => {
-    const msgs = [
-      {
-        role: "user",
-        content: [{ type: "text", text: "Check my home directory for PDFs" }],
-      },
-      {
-        role: "assistant",
-        content: [
-          { type: "reasoning", text: "First thought", providerOptions: { anthropic: { signature: "sig-1" } } },
-          { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
-          { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
-          { type: "reasoning", text: "Second thought", providerOptions: { anthropic: { signature: "sig-2" } } },
-          { type: "tool-call", toolCallId: "toolu_3", toolName: "bash", input: { command: "pwd" } },
-        ],
-      },
-      {
-        role: "tool",
-        content: [
-          { type: "tool-result", toolCallId: "toolu_1", toolName: "read", output: { type: "text", value: "ok" } },
-          {
-            type: "tool-result",
-            toolCallId: "toolu_2",
-            toolName: "glob",
-            output: { type: "text", value: "No files found" },
-          },
-          { type: "tool-result", toolCallId: "toolu_3", toolName: "bash", output: { type: "text", value: "/root" } },
-        ],
-      },
-    ] as any[]
-
-    const result = ProviderTransform.message(msgs, anthropicModel, {}) as any[]
-
-    expect(result).toHaveLength(5)
-    expect(result[1]).toMatchObject({
+    expect(result[2]).toMatchObject({
       role: "assistant",
       content: [
-        { type: "reasoning", text: "First thought", providerOptions: { anthropic: { signature: "sig-1" } } },
         { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
         { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
       ],
     })
-    expect(result[2]).toMatchObject({ role: "tool" })
-    expect(result[3]).toMatchObject({
-      role: "assistant",
-      content: [
-        { type: "reasoning", text: "Second thought", providerOptions: { anthropic: { signature: "sig-2" } } },
-        { type: "tool-call", toolCallId: "toolu_3", toolName: "bash", input: { command: "pwd" } },
-      ],
-    })
-    expect(result[4]).toMatchObject({ role: "tool", content: [{ type: "tool-result", toolCallId: "toolu_3" }] })
   })
 
   test("leaves valid anthropic assistant tool ordering unchanged", () => {
@@ -1734,53 +1680,7 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
     ])
   })
 
-  test("keeps vertex anthropic signed reasoning in order around tool results", () => {
-    const model = {
-      ...anthropicModel,
-      providerID: "google-vertex-anthropic",
-      api: {
-        id: "claude-sonnet-4@20250514",
-        url: "https://us-central1-aiplatform.googleapis.com",
-        npm: "@ai-sdk/google-vertex/anthropic",
-      },
-    }
-
-    const msgs = [
-      {
-        role: "assistant",
-        content: [
-          { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
-          { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
-          { type: "reasoning", text: "Second thought", providerOptions: { anthropic: { signature: "sig-2" } } },
-        ],
-      },
-      {
-        role: "tool",
-        content: [
-          { type: "tool-result", toolCallId: "toolu_1", toolName: "read", output: { type: "text", value: "ok" } },
-          { type: "tool-result", toolCallId: "toolu_2", toolName: "glob", output: { type: "text", value: "ok" } },
-        ],
-      },
-    ] as any[]
-
-    const result = ProviderTransform.message(msgs, model, {}) as any[]
-
-    expect(result).toHaveLength(3)
-    expect(result[0]).toMatchObject({
-      role: "assistant",
-      content: [
-        { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
-        { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
-      ],
-    })
-    expect(result[1]).toMatchObject({ role: "tool" })
-    expect(result[2]).toMatchObject({
-      role: "assistant",
-      content: [{ type: "reasoning", text: "Second thought", providerOptions: { anthropic: { signature: "sig-2" } } }],
-    })
-  })
-
-  test("places vertex anthropic tool results before trailing text", () => {
+  test("splits vertex anthropic assistant messages when text trails tool calls", () => {
     const model = {
       ...anthropicModel,
       providerID: "google-vertex-anthropic",
@@ -1800,30 +1700,22 @@ describe("ProviderTransform.message - anthropic empty content filtering", () =>
           { type: "text", text: "I checked your home directory and looked for PDF files." },
         ],
       },
-      {
-        role: "tool",
-        content: [
-          { type: "tool-result", toolCallId: "toolu_1", toolName: "read", output: { type: "text", value: "ok" } },
-          { type: "tool-result", toolCallId: "toolu_2", toolName: "glob", output: { type: "text", value: "ok" } },
-        ],
-      },
     ] as any[]
 
     const result = ProviderTransform.message(msgs, model, {}) as any[]
 
-    expect(result).toHaveLength(3)
+    expect(result).toHaveLength(2)
     expect(result[0]).toMatchObject({
+      role: "assistant",
+      content: [{ type: "text", text: "I checked your home directory and looked for PDF files." }],
+    })
+    expect(result[1]).toMatchObject({
       role: "assistant",
       content: [
         { type: "tool-call", toolCallId: "toolu_1", toolName: "read", input: { filePath: "/root" } },
         { type: "tool-call", toolCallId: "toolu_2", toolName: "glob", input: { pattern: "**/*.pdf" } },
       ],
     })
-    expect(result[1]).toMatchObject({ role: "tool" })
-    expect(result[2]).toMatchObject({
-      role: "assistant",
-      content: [{ type: "text", text: "I checked your home directory and looked for PDF files." }],
-    })
   })
 })
 
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index 75850e59fb..6556dfe8a6 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -1150,6 +1150,113 @@ describe("session.message-v2.toModelMessage", () => {
     ])
   })
 
+  test("splits anthropic replay when text follows a completed tool call", async () => {
+    const anthropicModel: Provider.Model = {
+      ...model,
+      id: ProviderV2.ModelID.make("claude-opus-4-8"),
+      providerID: ProviderV2.ID.make("anthropic"),
+      api: { id: "claude-opus-4-8", url: "https://api.anthropic.com", npm: "@ai-sdk/anthropic" },
+    }
+    const assistantID = "m-assistant"
+
+    const result = await MessageV2.toModelMessages(
+      [
+        {
+          info: assistantInfo(assistantID, "m-parent", undefined, {
+            providerID: anthropicModel.providerID,
+            modelID: anthropicModel.id,
+          }),
+          parts: [
+            { ...basePart(assistantID, "p1"), type: "step-start" },
+            {
+              ...basePart(assistantID, "p2"),
+              type: "tool",
+              callID: "toolu_1",
+              tool: "read",
+              state: {
+                status: "completed",
+                input: { filePath: "/root" },
+                output: "ok",
+                title: "Read",
+                metadata: {},
+                time: { start: 0, end: 1 },
+              },
+            },
+            { ...basePart(assistantID, "p3"), type: "text", text: "done" },
+          ] as SessionLegacy.Part[],
+        },
+      ],
+      anthropicModel,
+    )
+
+    expect(result.map((message) => message.role)).toEqual(["assistant", "tool", "assistant"])
+    expect(result[0].content).toMatchObject([{ type: "tool-call", toolCallId: "toolu_1" }])
+    expect(result[2].content).toMatchObject([{ type: "text", text: "done" }])
+  })
+
+  test("splits anthropic replay without moving signed reasoning", async () => {
+    const anthropicModel: Provider.Model = {
+      ...model,
+      id: ProviderV2.ModelID.make("claude-opus-4-8"),
+      providerID: ProviderV2.ID.make("anthropic"),
+      api: { id: "claude-opus-4-8", url: "https://api.anthropic.com", npm: "@ai-sdk/anthropic" },
+    }
+    const assistantID = "m-assistant"
+    const tool = (id: string, callID: string) => ({
+      ...basePart(assistantID, id),
+      type: "tool" as const,
+      callID,
+      tool: "bash",
+      state: {
+        status: "completed" as const,
+        input: { command: "pwd" },
+        output: "ok",
+        title: "Bash",
+        metadata: {},
+        time: { start: 0, end: 1 },
+      },
+    })
+
+    const result = await MessageV2.toModelMessages(
+      [
+        {
+          info: assistantInfo(assistantID, "m-parent", undefined, {
+            providerID: anthropicModel.providerID,
+            modelID: anthropicModel.id,
+          }),
+          parts: [
+            { ...basePart(assistantID, "p1"), type: "step-start" },
+            {
+              ...basePart(assistantID, "p2"),
+              type: "reasoning",
+              text: "one",
+              metadata: { anthropic: { signature: "sig-1" } },
+            },
+            tool("p3", "toolu_1"),
+            {
+              ...basePart(assistantID, "p4"),
+              type: "reasoning",
+              text: "two",
+              metadata: { anthropic: { signature: "sig-2" } },
+            },
+            tool("p5", "toolu_2"),
+          ] as SessionLegacy.Part[],
+        },
+      ],
+      anthropicModel,
+    )
+
+    expect(result.map((message) => message.role)).toEqual(["assistant", "tool", "assistant", "tool"])
+    expect(result[0].content).toMatchObject([
+      { type: "reasoning", text: "one", providerOptions: { anthropic: { signature: "sig-1" } } },
+      { type: "tool-call", toolCallId: "toolu_1" },
+    ])
+    expect(result[2].content).toMatchObject([
+      { type: "reasoning", text: "two", providerOptions: { anthropic: { signature: "sig-2" } } },
+      { type: "tool-call", toolCallId: "toolu_2" },
+    ])
+  })
+
   test("drops messages that only contain step-start parts", async () => {
     const assistantID = "m-assistant"