API Error: 400 x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt

[dogo425f]

API Error: 400 {"error":{"message":"{"error":{"message":"x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt.","code":"invalid_request_body"}}\n","type":"error"}}

--> POST /v1/messages?beta=true 400 2s

ERROR Failed to create chat completions Response { status: 400, 6:04:12 PM
statusText: 'Bad Request',
headers:
Headers { 'cache-control': 'no-cache',
'content-security-policy': "default-src 'none'; sandbox",
'content-type': 'text/plain; charset=utf-8',
'copilot-edits-session': '1769076252.80ecf760d7de5c851eed8baded0e83e0e925765543914b4d7325cd4e90775a29',
'strict-transport-security': 'max-age=31536000',
'x-content-type-options': 'nosniff',
'x-request-id': '9d4415c5-df09-4eed-b751-dce29ceb29b5',
date: 'Thu, 22 Jan 2026 10:04:13 GMT',
'transfer-encoding': 'chunked',
'x-github-backend': 'Kubernetes',
'x-github-request-id': 'BA1E:66B94:21A2874:26E58ED:6971F61B' },
body: ReadableStream { locked: false, state: 'readable', supportsBYOB: true },
bodyUsed: false,
ok: false,
redirected: false,
type: 'basic',
url: 'https://api.githubcopilot.com/chat/completions' }

ERROR Error occurred: Failed to create chat completions 6:04:12 PM

at createChatCompletions (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/dist/main.js:783:9)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
at async handleCompletion (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/dist/main.js:1227:19)
at async .nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/dist/main.js:1265:10
at async dispatch (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/compose.js:22:17)
at async cors2 (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/middleware/cors/index.js:79:5)
at async dispatch (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/compose.js:22:17)
at async logger2 (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/middleware/logger/index.js:38:5)
at async dispatch (.nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/compose.js:22:17)
at async .nvm/versions/node/v22.16.0/lib/node_modules/copilot-api/node_modules/hono/dist/hono-base.js:301:25

ERROR HTTP error: { error: 6:04:12 PM
{ message:
'x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt.',
code: 'invalid_request_body' } }

[dogo425f]

Has anyone ever encountered this situation when use Claude Code

[rexxiang]

I encountered the same problem.

[mrhanhan]

I had the same problem, but I found that if I removed the content starting with x-anthropic-billing-header from the system message, it would work normally.

Although the method is rather crude, it is a feasible solution at present.

[FlorianBruniaux]

same issue here

[caozhiyuan]

try use claude code 2.1.7 @dogo425f

claude code is the software with the most bugs I've ever encountered, terrible.

[FlorianBruniaux]

Complete Fix Documentation: copilot-api Issue #174

PR for copilot-api Repository

Title

Fix #174: Filter x-anthropic-billing-header from system prompt

Description

Problem

Claude Code v2.1.15+ injects x-anthropic-billing-header into the system prompt for internal billing tracking. When this prompt is forwarded through copilot-api to the Anthropic API, it's rejected with:

400 Bad Request: x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt

This breaks all Claude Code CLI users relying on copilot-api as a GitHub Copilot proxy.

Error details

• HTTP Status → 400 Bad Request
• Error Code → invalid_request_body
• Error Message → x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt

Root Cause

Anthropic reserves certain keywords (like x-anthropic-billing-header) for internal infrastructure use. The API accepts this header when sent directly from official clients, but explicitly rejects it when coming from third-party proxies to prevent abuse and maintain billing accuracy.

Format injected by Claude Code

x-anthropic-billing-header: ?cc_version=2.1.15; ?cc_entrypoint=\+

./

<actual system prompt content>

Solution

Filter out the reserved header from the system prompt in translateAnthropicMessagesToOpenAI() before forwarding requests to the Anthropic API.

Implementation

• File → dist/main.js (or src/main.ts if modifying source)
• Function → translateAnthropicMessagesToOpenAI (line ~897)

Changes Made

• Changed const to let → Allow reassignment of systemMessages
• Added filtering logic → Map over system messages to remove billing header
• Added informative logging → Log character count before/after filtering for debugging

Code Diff

function translateAnthropicMessagesToOpenAI(anthropicMessages, system) {
-   const systemMessages = handleSystemPrompt(system);
+   let systemMessages = handleSystemPrompt(system);
+   
+   // FIX #174: Filter x-anthropic-billing-header from system prompt
+   systemMessages = systemMessages.map((it) => {
+       if (typeof it.content === "string" && it.content.startsWith("x-anthropic-billing-header")) {
+           const before = it.content.length;
+           it.content = it.content.replace(/^x-anthropic-billing-header:[^\n]*\n+/, "");
+           const after = it.content.length;
+           console.info(`Filtered x-anthropic-billing-header: ${before} → ${after} chars`);
+       }
+       return it;
+   });
+   
    const otherMessages = anthropicMessages.flatMap((message) => 
        message.role === "user" ? handleUserMessage(message) : handleAssistantMessage(message)
    );
    return [...systemMessages, ...otherMessages];
}

Regex Explanation

Original Proposal (From Issue Comments) - Doesn't Work

/x-anthropic-billing-header: \?cc_version=.+; \?cc_entrypoint=\\+\n{0,2}\./

Problems

• Too specific to a particular header format
• Doesn't match actual header structure sent by Claude Code
• Will break if Claude Code changes version format

Improved Regex - Works Reliably

/^x-anthropic-billing-header:[^\n]*\n+/

Pattern breakdown

• ^ → Start of string (ensures we match from beginning)
• x-anthropic-billing-header: → Literal keyword match
• [^\n]* → Any characters EXCEPT newline (captures rest of header line)
• \n+ → One or more newlines (removes trailing blank lines)

Why this regex is better

• Simple and robust → Works regardless of header value format
• Future-proof → Doesn't depend on Claude Code version specifics
• Complete removal → Removes entire header line + trailing newlines
• Maintainable → Easy to understand and verify

Examples

// Test 1: Real Claude Code format
Input:  "x-anthropic-billing-header: ?cc_version=2.1.15; ?cc_entrypoint=\\+\n\n./\n\nYou are helpful."
Output: "./\n\nYou are helpful."

// Test 2: Simple format
Input:  "x-anthropic-billing-header: test\n\nYou are helpful."
Output: "You are helpful."

// Test 3: Generic format
Input:  "x-anthropic-billing-header: anything here\n\nPrompt content"
Output: "Prompt content"

Testing

Automated Test Script

Created test-billing-header-fix.sh for validation:

Test Coverage

• System prompt WITH x-anthropic-billing-header → Should be accepted (filtered)
• Normal system prompt without header → Should be accepted (control)

Usage

# Requires: copilot-api running on port 4141
./test-billing-header-fix.sh

Expected Output

Test du fix pour issue copilot-api #174

1️⃣  Verification copilot-api...
✅ copilot-api active on :4141

2️⃣  Test 1: System prompt with x-anthropic-billing-header...
✅ SUCCESS: Request accepted without error
   Response: Hello! How can I help you today?...

3️⃣  Test 2: Normal system prompt (control)...
✅ SUCCESS: Normal request also works

✅ All tests pass - Fix works!

Manual Testing with Claude Code CLI

# Launch Claude Code via copilot-api
ccc
❯ 1+1

# Expected: Normal response, no 400 error
# Before fix: "400 Bad Request: x-anthropic-billing-header is a reserved keyword"
# After fix: "2"

Log Verification

Terminal running copilot-api shows:

Filtered x-anthropic-billing-header: 127 → 25 chars

Confirms:

• Header detected (startsWith check passes)
• Header removed (character count reduction)
• Filter executed successfully

Impact Assessment

Compatibility

• Tested with → copilot-api v0.7.0, Claude Code CLI v2.1.15
• Breaking changes → None
• Existing functionality → Unaffected (only filters reserved keyword)
• Performance → Negligible (simple string operation on system messages only)

Security

• No security impact → Filter is defensive, doesn't expose data
• Maintains proxy integrity → Properly handles reserved keywords
• No data leakage → Billing header stripped before forwarding

Providers Affected

• Anthropic Direct → Not affected (Native API handles header internally)
• Copilot via copilot-api → Affected (Proxy rejected reserved keyword)
• Ollama Local → Not affected (Doesn't use Anthropic API)

Related Issues

• Fixes API Error: 400 x-anthropic-billing-header is a reserved keyword and may not be used in the system prompt #174
• Original workaround proposed by @mrhanhan
• Improved regex implementation tested in production by cc-copilot-bridge project

Checklist

• Code follows project style guidelines
• Added filtering logic to translateAnthropicMessagesToOpenAI
• Added informative logging for debugging
• Tested with real Claude Code CLI requests
• Verified no breaking changes to existing functionality
• Created automated test script
• Documented regex choice and rationale
• Tested edge cases (empty prompts, various header formats)

Additional Notes

Why Not Block at API Level?

Anthropic's API validation intentionally rejects this header from proxies to:

• Prevent billing fraud/manipulation
• Ensure accurate usage tracking
• Maintain infrastructure security

Filtering at the proxy level (copilot-api) is the correct approach.

Future Considerations

If Anthropic adds more reserved keywords, the filter can be extended:

const RESERVED_PREFIXES = [
    'x-anthropic-billing-header',
    'x-anthropic-internal-',
    // Add more as needed
];

systemMessages = systemMessages.map((it) => {
    if (typeof it.content === "string") {
        for (const prefix of RESERVED_PREFIXES) {
            if (it.content.startsWith(prefix)) {
                it.content = it.content.replace(
                    new RegExp(`^${prefix}:[^\\n]*\\n+`),
                    ""
                );
                console.info(`Filtered reserved keyword: ${prefix}`);
            }
        }
    }
    return it;
});

This maintains extensibility while keeping the current fix minimal and focused.

Test Script (Include in PR)

Create tests/test-billing-header-fix.sh:

#!/bin/bash
# Test for issue #174 fix - x-anthropic-billing-header filtering
# Verifies that copilot-api correctly filters the reserved billing header

set -euo pipefail

echo "Test for copilot-api issue #174 fix"
echo ""

# Verify copilot-api is running
echo "1️⃣  Checking copilot-api..."
if ! nc -z localhost 4141 2>/dev/null; then
    echo "❌ copilot-api not running on port 4141"
    echo "   Start it with: npm start"
    exit 1
fi
echo "✅ copilot-api active on :4141"
echo ""

# Test 1: Request with billing header in system prompt
echo "2️⃣  Test 1: System prompt with x-anthropic-billing-header..."
RESPONSE=$(curl -s -X POST http://localhost:4141/v1/messages \
  -H "Content-Type: application/json" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet-4.5",
    "max_tokens": 100,
    "system": "x-anthropic-billing-header: ?cc_version=2.1.15; ?cc_entrypoint=\\+\n./\n\nYou are a helpful assistant.",
    "messages": [{"role": "user", "content": "Say hello"}]
  }')

# Check for 400 error
if echo "$RESPONSE" | jq -e '.error.code == "invalid_request_body"' >/dev/null 2>&1; then
    echo "❌ FAIL: API still returns invalid_request_body error"
    echo "$RESPONSE" | jq '.error'
    exit 1
fi

# Check for valid response
if echo "$RESPONSE" | jq -e '.content[0].text' >/dev/null 2>&1; then
    echo "✅ SUCCESS: Request accepted without error"
    echo "   Response: $(echo "$RESPONSE" | jq -r '.content[0].text' | head -c 50)..."
else
    echo "⚠️  WARNING: Unexpected response"
    echo "$RESPONSE" | jq '.' 2>/dev/null || echo "$RESPONSE"
fi
echo ""

# Test 2: Normal request without billing header (control)
echo "3️⃣  Test 2: Normal system prompt (control)..."
RESPONSE2=$(curl -s -X POST http://localhost:4141/v1/messages \
  -H "Content-Type: application/json" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet-4.5",
    "max_tokens": 100,
    "system": "You are a helpful assistant.",
    "messages": [{"role": "user", "content": "Say hello"}]
  }')

if echo "$RESPONSE2" | jq -e '.content[0].text' >/dev/null 2>&1; then
    echo "✅ SUCCESS: Normal request also works"
else
    echo "❌ FAIL: Even normal requests fail"
    echo "$RESPONSE2" | jq '.' 2>/dev/null || echo "$RESPONSE2"
    exit 1
fi
echo ""

echo "✅ All tests pass - Fix works correctly!"
echo ""
echo "📋 Summary:"
echo "   • System prompt with billing header : ✅ Accepted (filtered)"
echo "   • Normal system prompt              : ✅ Accepted"
echo ""
echo "🎯 Next: Test with Claude Code CLI"
echo "   Start copilot-api and run: claude"

Files to Create/Modify for copilot-api PR

• Modify src/main.ts (or dist/main.js if no TypeScript source)

  • Function: translateAnthropicMessagesToOpenAI
  • Add filtering logic with regex /^x-anthropic-billing-header:[^\n]*\n+/

• Create (optional) tests/test-billing-header-fix.sh

  • Automated validation script

• Update (if exists) README.md or CHANGELOG.md

  • Document the fix

Key Technical Details

• Regex → /^x-anthropic-billing-header:[^\n]*\n+/
• Log message → Filtered x-anthropic-billing-header: ${before} → ${after} chars
• Impact → Minimal, defensive filtering only
• Performance → Negligible (one regex replace per system message)

Testing Commands

# Run automated test
./tests/test-billing-header-fix.sh

# Manual test with Claude Code
ccc
❯ 1+1

# Expected logs in copilot-api terminal
Filtered x-anthropic-billing-header: 127 → 25 chars

Issue: #174

[FlorianBruniaux]

it works for me now

[FlorianBruniaux]

I created a repo using this copilot api repo including the fix : https://github.com/FlorianBruniaux/cc-copilot-bridge

[zhujian0805]

I'm having the same issue, can this patched with Florian's fix?

[caozhiyuan]

set env "CLAUDE_CODE_ATTRIBUTION_HEADER": "0" in claude settings.json @zhujian0805 @dogo425f @FlorianBruniaux

[caozhiyuan]

@mrhanhan

[caozhiyuan]

There are too many bugs in the latest version, especially when using Claude Code in Windows environments. It is recommended to use version 2.1.7