diff --git a/src/index.ts b/src/index.ts
index 3edda7f..dcad089 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -260,6 +260,7 @@ const plugin: Plugin = async () => {
 
         return {
           apiKey: "",
+          baseURL: "https://api.anthropic.com/v1",
           async fetch(input: RequestInfo | URL, init?: RequestInit) {
             const latest = getCachedCredentials()
             if (!latest) {
diff --git a/src/transforms.test.ts b/src/transforms.test.ts
index 763a5b5..bde16b9 100644
--- a/src/transforms.test.ts
+++ b/src/transforms.test.ts
@@ -8,7 +8,7 @@ import {
 } from "./transforms.ts"
 
 describe("transforms", () => {
-  it("transformBody moves non-core system text to user message and prefixes tool names", () => {
+  it("transformBody moves non-core system text to user message and does not prefix tool names", () => {
     const input = JSON.stringify({
       system: [{ type: "text", text: "OpenCode and opencode" }],
       tools: [{ name: "search" }],
@@ -36,8 +36,9 @@ describe("transforms", () => {
     // The original system text should now be prepended to the first user message
     assert.equal(parsed.messages[0].content[0].type, "text")
     assert.equal(parsed.messages[0].content[0].text, "OpenCode and opencode")
-    assert.equal(parsed.tools[0].name, "mcp_search")
-    assert.equal(parsed.messages[0].content[1].name, "mcp_lookup")
+    // Tool names should NOT be prefixed with mcp_
+    assert.equal(parsed.tools[0].name, "search")
+    assert.equal(parsed.messages[0].content[1].name, "lookup")
   })
 
   it("transformBody relocates non-core system text to user message", () => {
@@ -454,6 +455,66 @@ describe("transforms", () => {
     assert.equal(stripToolPrefix(input), '{"name": "search","type":"tool_use"}')
   })
 
+  it("stripToolPrefix reverses TodoWrite back to todowrite", () => {
+    const input = '{"name":"TodoWrite","type":"tool_use"}'
+    assert.equal(stripToolPrefix(input), '{"name": "todowrite","type":"tool_use"}')
+  })
+
+  it("stripToolPrefix reverses backgroundOutput and backgroundCancel", () => {
+    const input1 = '{"name":"backgroundOutput","type":"tool_use"}'
+    assert.equal(stripToolPrefix(input1), '{"name": "background_output","type":"tool_use"}')
+    const input2 = '{"name":"backgroundCancel","type":"tool_use"}'
+    assert.equal(stripToolPrefix(input2), '{"name": "background_cancel","type":"tool_use"}')
+  })
+
+  it("transformBody renames blocked tool names in tools and messages", () => {
+    const input = JSON.stringify({
+      system: [],
+      tools: [
+        { name: "todowrite" },
+        { name: "background_output" },
+        { name: "background_cancel" },
+        { name: "search" },
+      ],
+      messages: [
+        {
+          role: "assistant",
+          content: [
+            { type: "tool_use", id: "t1", name: "todowrite" },
+            { type: "tool_use", id: "t2", name: "background_output" },
+            { type: "tool_use", id: "t3", name: "search" },
+          ],
+        },
+        {
+          role: "user",
+          content: [
+            { type: "tool_result", tool_use_id: "t1", content: "ok" },
+            { type: "tool_result", tool_use_id: "t2", content: "ok" },
+            { type: "tool_result", tool_use_id: "t3", content: "ok" },
+          ],
+        },
+      ],
+    })
+
+    const output = transformBody(input)
+    const parsed = JSON.parse(output as string) as {
+      tools: Array<{ name: string }>
+      messages: Array<{
+        content: Array<{ type: string; name?: string }>
+      }>
+    }
+
+    assert.equal(parsed.tools[0].name, "TodoWrite")
+    assert.equal(parsed.tools[1].name, "backgroundOutput")
+    assert.equal(parsed.tools[2].name, "backgroundCancel")
+    assert.equal(parsed.tools[3].name, "search")
+
+    const assistantContent = parsed.messages[0].content
+    assert.equal(assistantContent[0].name, "TodoWrite")
+    assert.equal(assistantContent[1].name, "backgroundOutput")
+    assert.equal(assistantContent[2].name, "search")
+  })
+
   it("transformResponseStream passes error responses through without SSE parsing", async () => {
     const errorBody = JSON.stringify({
       type: "error",
diff --git a/src/transforms.ts b/src/transforms.ts
index acf5b00..91e9fad 100644
--- a/src/transforms.ts
+++ b/src/transforms.ts
@@ -1,8 +1,6 @@
 import { buildBillingHeaderValue } from "./signing.ts"
 import { config, getModelOverride } from "./model-config.ts"
 
-const TOOL_PREFIX = "mcp_"
-
 const SYSTEM_IDENTITY =
   "You are Claude Code, Anthropic's official CLI for Claude."
 
@@ -206,30 +204,44 @@ export function transformBody(
       }
     }
 
+    // Anthropic's OAuth billing validation rejects tool names with the
+    // mcp_ prefix when multiple tools are present. Skip prefixing and
+    // rename only the specific tool names that the validator blocks.
+    const BLOCKED_TOOL_NAMES: Record<string, string> = {
+      "todowrite": "TodoWrite",
+      "background_output": "backgroundOutput",
+      "background_cancel": "backgroundCancel",
+    }
     if (Array.isArray(parsed.tools)) {
-      parsed.tools = parsed.tools.map((tool) => ({
-        ...tool,
-        name: tool.name ? `${TOOL_PREFIX}${tool.name}` : tool.name,
-      }))
+      parsed.tools = parsed.tools.map((tool) => {
+        if (typeof tool.name === "string" && BLOCKED_TOOL_NAMES[tool.name]) {
+          return { ...tool, name: BLOCKED_TOOL_NAMES[tool.name] }
+        }
+        return tool
+      })
     }
 
     if (Array.isArray(parsed.messages)) {
       parsed.messages = parsed.messages.map((message) => {
-        if (!Array.isArray(message.content)) {
-          return message
-        }
-
+        if (!Array.isArray(message.content)) return message
+        const hasBlocked = message.content.some(
+          (block) =>
+            block.type === "tool_use" &&
+            typeof block.name === "string" &&
+            BLOCKED_TOOL_NAMES[block.name],
+        )
+        if (!hasBlocked) return message
         return {
           ...message,
           content: message.content.map((block) => {
-            if (block.type !== "tool_use" || typeof block.name !== "string") {
-              return block
-            }
-
-            return {
-              ...block,
-              name: `${TOOL_PREFIX}${block.name}`,
+            if (
+              block.type === "tool_use" &&
+              typeof block.name === "string" &&
+              BLOCKED_TOOL_NAMES[block.name]
+            ) {
+              return { ...block, name: BLOCKED_TOOL_NAMES[block.name] }
             }
+            return block
           }),
         }
       })
@@ -246,7 +258,11 @@ export function transformBody(
 }
 
 export function stripToolPrefix(text: string): string {
-  return text.replace(/"name"\s*:\s*"mcp_([^"]+)"/g, '"name": "$1"')
+  return text
+    .replace(/"name"\s*:\s*"mcp_([^"]+)"/g, '"name": "$1"')
+    .replace(/"name"\s*:\s*"TodoWrite"/g, '"name": "todowrite"')
+    .replace(/"name"\s*:\s*"backgroundOutput"/g, '"name": "background_output"')
+    .replace(/"name"\s*:\s*"backgroundCancel"/g, '"name": "background_cancel"')
 }
 
 export function transformResponseStream(response: Response): Response {