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Grok 4.5: 351 tools fail with HTTP 400 instead of reporting the 350-tool limit #4836
Description
Type: Bug
Copilot CLI Version: 1.0.84-5
OS Version: Windows 11 x64
Model: grok-4.5
Summary
When the total number of advertised tools exceeds 350, requests to grok-4.5 fail with HTTP 400 Bad Request.
Copilot CLI does not enforce or report the tool count limit before making the API call.
Steps to Reproduce
- Use a test MCP configuration with no other user/plugin MCP servers enabled, so they do not add to the count.
- Save
schema-repro.mjs,grok-350.json, andgrok-351.jsonfrom the Reproducer files section below in the same directory. Node.js must be available. - From that directory, run these native Copilot commands:
copilot --model grok-4.5 --no-custom-instructions --disable-builtin-mcps --disable-mcp-server computer-use --additional-mcp-config "@grok-350.json" --available-tools "mcp:*" --allow-all-tools -p "Reply with exactly OK. Do not call tools."
copilot --model grok-4.5 --no-custom-instructions --disable-builtin-mcps --disable-mcp-server computer-use --additional-mcp-config "@grok-351.json" --available-tools "mcp:*" --allow-all-tools -p "Reply with exactly OK. Do not call tools."The fixture tools have unique names and identical simple schemas. mcp:* excludes native CLI tools; the MCP-only precondition isolates the fixture's 350/351 count. There is no SDK dependency or shell helper.
Actual Behaviour
With 350 tools:
- Request succeeds with exit code 0.
With 351 tools:
- Request immediately fails with:
Execution failed: 400 Bad Request (Request ID: ...)
Comparison:
| Tool Count | Model | Result |
|---|---|---|
| 350 tools | grok-4.5 | OK (exit 0) |
| 351 tools | grok-4.5 | HTTP 400 (exit 1) |
The debug tool-surface records confirm exactly 350 and 351 advertised tools for these two runs.
Expected Behaviour
Copilot CLI should track provider-specific limits:
- Enforce a tool count budget before sending the request.
- If tool count exceeds 350 for Grok, report an actionable error explaining that the active tool count (351) exceeds Grok's maximum limit of 350 tools.
- Alternatively, defer tools via tool search rather than sending the entire catalog upfront.
Notes
- Tested with inert synthetic tools with identical simple schemas (
{"type":"object","properties":{"value":{"type":"string"}}}). - The issue is purely triggered by the total number of advertised tools, regardless of whether tools are actually invoked by the model.
Impact
Users with multiple MCP servers easily exceed 350 tools in aggregate (native + MCP tools), causing Grok to become completely unusable across the entire session with an unhelpful 400 error.
Reproducer files
schema-repro.mjs
import { createInterface } from 'node:readline';
const mode = process.argv[2];
const count = Number(process.argv[3] ?? 1);
const allowedModes = ['array-enum-bad', 'array-enum-fixed', 'count'];
if (!allowedModes.includes(mode) ||
!Number.isInteger(count) ||
count < 1 ||
count > 400) {
throw new Error('Usage: node schema-repro.mjs array-enum-bad|array-enum-fixed|count [1..400]');
}
const levels = { type: ['array', 'null'], items: { type: 'integer' } };
if (mode === 'array-enum-bad') {
levels.enum = [0, 1, 2, 3, 4, 25];
} else {
levels.items.enum = [0, 1, 2, 3, 4, 25];
}
const tools = Array.from({ length: mode === 'count' ? count : 1 }, (_, index) => ({
name: `probe_${String(index).padStart(3, '0')}`,
description: 'Schema diagnostic only; performs no operation.',
inputSchema: {
type: 'object',
properties: mode === 'count' ? { value: { type: 'string' } } : { levels },
},
}));
for await (const line of createInterface({ input: process.stdin, crlfDelay: Infinity })) {
if (!line.trim()) {
continue;
}
const request = JSON.parse(line);
if (request.id === undefined) {
continue;
}
let result;
if (request.method === 'initialize') {
result = {
protocolVersion: request.params.protocolVersion,
capabilities: { tools: {} },
serverInfo: { name: 'schema-repro', version: '1.0.0' },
};
} else if (request.method === 'tools/list') {
result = { tools };
} else if (request.method === 'tools/call') {
result = {
isError: true,
content: [{ type: 'text', text: 'This diagnostic never executes an operation.' }],
};
} else if (request.method === 'ping') {
result = {};
} else {
process.stdout.write(`${JSON.stringify({
jsonrpc: '2.0',
id: request.id,
error: { code: -32601, message: 'Method not found' },
})}\n`);
continue;
}
process.stdout.write(`${JSON.stringify({ jsonrpc: '2.0', id: request.id, result })}\n`);
}grok-350.json
{
"mcpServers": {
"schema_repro": {
"type": "stdio",
"command": "node",
"args": ["schema-repro.mjs", "count", "350"],
"tools": ["*"]
}
}
}grok-351.json
{
"mcpServers": {
"schema_repro": {
"type": "stdio",
"command": "node",
"args": ["schema-repro.mjs", "count", "351"],
"tools": ["*"]
}
}
}
Activity
patniko commentedon Oct 26, 2026
Thanks for the report. We've shipped fixes in this area since this was filed (most recently in Copilot CLI v1.0.91), and we believe this may now be resolved. Could you retest on the latest release (Copilot CLI v1.0.103) and let us know whether you still see it? If we don't hear back in a couple of weeks, we may close this issue — you can always reopen it.
Triaged as part of a Copilot SDK backlog review, with help from GitHub Copilot.
kondv commentedon Oct 28, 2026
Thanks @patniko. I retested on 1.0.104 with the same reproducer and it no longer fails.
grok-4.5isn't available anymore, so I ran it ongrok-4.6andgrok-4.7, and the 351-tool fixture returns OK on both. The CLI now sends onlytool_search_toolup front and defers the MCP tools, so the request doesn't go over the 350 limit. Fine to close from my side.Retest on 1.0.104
Same
schema-repro.mjs,grok-350.jsonandgrok-351.jsonas in the issue. All user and built-in MCP servers were disabled, so only the fixture's tools count:copilot --model grok-4.7 --no-custom-instructions --disable-builtin-mcps --disable-mcp-server computer-use --disable-mcp-server <each user MCP server> --additional-mcp-config "@grok-351.json" --available-tools "mcp:*" --allow-all-tools -p "Reply with exactly OK. Do not call tools." --log-level debug --log-dir <dir>catalog_tool_countmcp:*grok-4.7tool_search_tool)mcp:*grok-4.7tool_search_tool)mcp:*grok-4.6tool_search_tool)--available-toolsgrok-4.7tool_search_tool)--available-toolsgrok-4.6tool_search_tool)In the debug log of the 351-tool run,
turn tool surface resolvedshows"catalog_tool_count":351,"tool_count":1, and the request'scopilotToolsFingerprintlists onlytool_search_tool.— kondv's Copilot 🤖
copilot-cli-bot commentedon Oct 28, 2026
This appears to be resolved as of GitHub Copilot CLI v1.0.105. Please update to that release or a newer stable version. We're closing this as completed. If it still happens, please comment with your version and reproduction steps.