fix: PascalCase tool names after mcp_ prefix to match Claude Code convention (#191)

## Summary

- PascalCase tool names after `mcp_` prefix (e.g. `bash` → `mcp_Bash`,
`read` → `mcp_Read`) to match Claude Code's actual naming convention
- Lowercase `mcp_` names (e.g. `mcp_bash`) are flagged as
non-Claude-Code clients by Anthropic's billing validator, causing
spurious 400 "out of extra usage" errors
- Reverse mapping in `stripToolPrefix` restores original tool names by
lowercasing the first char after stripping `mcp_`
- Add explicit `baseURL` to ensure correct API endpoint construction

## Context

Anthropic's billing validator rejects requests with lowercase
`mcp_`-prefixed tool names when multiple tools are present. Binary
search testing with isolated curl requests confirmed specific tool names
like `background_output` trigger 400 errors, while PascalCase
equivalents (`mcp_Background_output`) pass.

Approach aligned with ex-machina-co/opencode-anthropic-auth#81.

Fixes #190 #188 

## Test plan

- [x] All 39 tests pass
- [x] Verify Claude Max users no longer receive spurious "out of extra
usage" errors
- [x] Verify all tools remain functional with PascalCase mcp_ prefix
- [x] Verify responses correctly restore original tool names

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: SeaL773 <SeaL773@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: SeaL773

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) {

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 PascalCase-prefixes tool names", () => {
     const input = JSON.stringify({
       system: [{ type: "text", text: "OpenCode and opencode" }],
       tools: [{ name: "search" }],
@@ -36,8 +36,8 @@ 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")
+    assert.equal(parsed.tools[0].name, "mcp_Search")
+    assert.equal(parsed.messages[0].content[1].name, "mcp_Lookup")
   })
 
   it("transformBody relocates non-core system text to user message", () => {
@@ -449,6 +449,51 @@ describe("transforms", () => {
     assert.equal(parsed.thinking, undefined)
   })
 
+  it("transformBody PascalCase-prefixes tool names with mcp_", () => {
+    const input = JSON.stringify({
+      system: [],
+      tools: [
+        { name: "bash" },
+        { name: "read" },
+        { name: "background_output" },
+      ],
+      messages: [
+        {
+          role: "user",
+          content: [
+            { type: "tool_use", name: "bash" },
+            { type: "tool_use", name: "background_output" },
+          ],
+        },
+      ],
+    })
+
+    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, "mcp_Bash")
+    assert.equal(parsed.tools[1].name, "mcp_Read")
+    assert.equal(parsed.tools[2].name, "mcp_Background_output")
+    assert.equal(parsed.messages[0].content[0].name, "mcp_Bash")
+    assert.equal(parsed.messages[0].content[1].name, "mcp_Background_output")
+  })
+
+  it("stripToolPrefix reverses PascalCase mcp_ prefix", () => {
+    assert.equal(
+      stripToolPrefix('{"name": "mcp_Bash"}'),
+      '{"name": "bash"}',
+    )
+    assert.equal(
+      stripToolPrefix('{"name": "mcp_Background_output"}'),
+      '{"name": "background_output"}',
+    )
+  })
+
   it("stripToolPrefix removes mcp_ from response payload names", () => {
     const input = '{"name":"mcp_search","type":"tool_use"}'
     assert.equal(stripToolPrefix(input), '{"name": "search","type":"tool_use"}')

src/transforms.ts

@@ -3,6 +3,22 @@ import { config, getModelOverride } from "./model-config.ts"
 
 const TOOL_PREFIX = "mcp_"
 
+/**
+ * Prefix a tool name with TOOL_PREFIX and uppercase the first character.
+ * Claude Code uses PascalCase tool names (e.g. mcp_Bash, mcp_Read);
+ * lowercase names (mcp_bash, mcp_read) are flagged as non-Claude-Code clients.
+ */
+function prefixName(name: string): string {
+  return `${TOOL_PREFIX}${name.charAt(0).toUpperCase()}${name.slice(1)}`
+}
+
+/**
+ * Reverse prefixName: strip TOOL_PREFIX and restore the original leading case.
+ */
+function unprefixName(name: string): string {
+  return `${name.charAt(0).toLowerCase()}${name.slice(1)}`
+}
+
 const SYSTEM_IDENTITY =
   "You are Claude Code, Anthropic's official CLI for Claude."
 
@@ -206,10 +222,13 @@ export function transformBody(
       }
     }
 
+    // Anthropic's OAuth billing validation rejects lowercase tool names
+    // when multiple tools are present. Claude Code uses PascalCase after
+    // the mcp_ prefix (e.g. mcp_Bash, mcp_Read). Apply the same convention.
     if (Array.isArray(parsed.tools)) {
       parsed.tools = parsed.tools.map((tool) => ({
         ...tool,
-        name: tool.name ? `${TOOL_PREFIX}${tool.name}` : tool.name,
+        name: tool.name ? prefixName(tool.name) : tool.name,
       }))
     }
 
@@ -226,10 +245,7 @@ export function transformBody(
               return block
             }
 
-            return {
-              ...block,
-              name: `${TOOL_PREFIX}${block.name}`,
-            }
+            return { ...block, name: prefixName(block.name) }
           }),
         }
       })
@@ -246,7 +262,10 @@ export function transformBody(
 }
 
 export function stripToolPrefix(text: string): string {
-  return text.replace(/"name"\s*:\s*"mcp_([^"]+)"/g, '"name": "$1"')
+  return text.replace(
+    /"name"\s*:\s*"mcp_([^"]+)"/g,
+    (_match, name: string) => `"name": "${unprefixName(name)}"`,
+  )
 }
 
 export function transformResponseStream(response: Response): Response {