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Setup: Cursor

Connect the l6e-budget MCP server to Cursor for session-scoped budget enforcement.

The agent gates calls using pre-call token estimates. Out of the box, budgets are directionally accurate — calibration makes them billing-accurate. Call l6e_record_usage if you want to feed actual token counts back into the ledger for exact accounting.

Install​

No separate pip install is required if you use uvx (recommended). uvx runs l6e-mcp in an isolated environment on first use.

If you prefer a manual install:

pip install l6e-mcp

Configure​

Add the following to your MCP configuration file.

Global (applies to all projects): ~/.cursor/mcp.json

Project-level (checked into git, shared with team): .cursor/mcp.json in your project root

{
  "mcpServers": {
    "l6e-budget": {
      "command": "uvx",
      "args": ["l6e-mcp"],
      "env": {
        "L6E_LOG_PATH": "${HOME}/.l6e/runs.jsonl",
        "L6E_API_KEY": "sk-l6e-...",
        "L6E_CLOUD_SYNC": "1"
      }
    }
  }
}

L6E_LOG_PATH should always be an absolute path. Cursor spawns MCP servers as child processes, so without it runs.jsonl will be written relative to wherever the Cursor process started — which is not always your project directory, particularly with a global config.

L6E_API_KEY and L6E_CLOUD_SYNC are optional — omit them to run fully local. When set, session run logs sync to the l6e cloud and gate decisions use your personal calibration factor. See l6e.ai Integration for what cloud sync enables.

If you installed l6e-mcp manually instead of using uvx:

{
  "mcpServers": {
    "l6e-budget": {
      "command": "l6e-mcp",
      "env": {
        "L6E_LOG_PATH": "${HOME}/.l6e/runs.jsonl",
        "L6E_API_KEY": "sk-l6e-...",
        "L6E_CLOUD_SYNC": "1"
      }
    }
  }
}

Restart Cursor completely (Cmd+Q → reopen) after editing this file. MCP server processes are spawned at startup.

Verify​

Open Cursor Settings → Features → MCP. The l6e-budget server should appear with a green dot and four tools listed:

  • l6e_run_start
  • l6e_authorize_call
  • l6e_record_usage
  • l6e_run_end

The "No MCP resources available" message in Cursor chat is expected and harmless — l6e-budget exposes tools, not resources. If the server dot is red, check that uvx is on your PATH (which uvx) or that l6e-mcp is installed (pip show l6e-mcp).

Rules for AI​

Set up a Cursor rule — either globally or per-project. The rule from .cursor/rules/l6e-budget-enforcement.mdc on the latest release tag is always the best source.

Example conversation starter​

Use this at the start of a new chat to test the full flow:

Using the l6e-budget MCP tools, call l6e_run_start with budget_usd=1.00,
model="gpt-4o", client="cursor". Show me the full JSON response including
session_id. Then add a one-line docstring to any function in this project.
Call l6e_authorize_call before the edit and l6e_run_end when done.

Reading your run log​

After a session ends:

# Most recent session
tail -1 ~/.l6e/runs.jsonl | python -m json.tool

# All sessions — cost summary
cat ~/.l6e/runs.jsonl | python -c "
import sys, json
for line in sys.stdin:
    r = json.loads(line)
    print(f\"{r['run_id']}  \${r['total_cost']:.4f}  {r['calls_made']} calls  {r['reroutes']} reroutes  source={r['source']}\")
"

Verify the log path is correct​

Run a minimal session to confirm runs.jsonl lands in ~/.l6e/ and not somewhere else:

Call l6e_run_start with budget_usd=0.10, model="gpt-4o",
client="cursor". Then immediately call l6e_run_end with the session_id.

Then check:

tail -1 ~/.l6e/runs.jsonl | python -m json.tool

If the file doesn't exist or is empty, L6E_LOG_PATH is not being passed to the server process. Re-check the env block in your config and restart Cursor.

Known limitations​

  • Always call l6e_run_end. If the Cursor window closes before l6e_run_end is called, the run log for that session is not written.
  • Never import l6e_mcp directly. The session registry lives only in the MCP server process. Importing l6e_mcp.server in a subprocess will always return "Unknown session".
  • Rerouting is advisory only. When l6e_authorize_call returns "action": "reroute", the agent stops work and tells you to switch to a cheaper model. The MCP protocol has no mechanism for forcing a model switch — the response is a signal to you, not an automatic redirect.