GitHub Copilot CLI

Copilot CLI integrates via an MCP (Model Context Protocol) server that Isartor registers as a stdio subprocess. Isartor also exposes the same MCP tools over Streamable HTTP at http://localhost:8080/mcp/ for editors and web agents that prefer HTTP/SSE transport. Both transports expose two tools:

• isartor_chat — cache lookup only. Returns the cached answer on hit (L1a exact or L1b semantic), or an empty string on miss. On a miss, Copilot uses its own LLM to answer — Isartor never routes through its configured L3 provider for Copilot traffic.
• isartor_cache_store — stores a prompt/response pair in Isartor's cache so future identical or similar prompts are deflected locally.

This design means Copilot still owns the conversation loop, while Isartor acts as a transparent cache layer that reduces redundant cloud calls. On a cache hit, Isartor returns the cached text and does not call its own Layer 3 provider. Copilot CLI may still emit its normal final-answer event after the tool result, but that is a Copilot-side render step rather than an Isartor L3 forward.

Prerequisites

• Isartor installed (curl -fsSL https://raw.githubusercontent.com/isartor-ai/Isartor/main/install.sh | sh)
• GitHub Copilot CLI installed

Step-by-step setup

# 1. Start Isartor
isartor up --detach

# 2. Register the MCP server with Copilot CLI
isartor connect copilot

# 3. Start Copilot normally — plain chat prompts will use Isartor cache first
copilot

How it works

1. isartor connect copilot adds an isartor entry to ~/.copilot/mcp-config.json
2. isartor connect copilot also installs a managed instruction block in ~/.copilot/copilot-instructions.md
3. When Copilot CLI starts, it launches isartor mcp as a stdio subprocess and loads the Isartor instruction block
4. The MCP server exposes isartor_chat (cache lookup) and isartor_cache_store (cache write)
5. For plain conversational prompts, Copilot now prefers this flow:
   • Call isartor_chat with the user's prompt
   • Cache hit: return the cached answer immediately, verbatim
   • Cache miss: answer with Copilot's own model, then call isartor_cache_store
6. When Copilot calls isartor_chat:
   • Cache hit (L1a exact or L1b semantic): returns the cached answer instantly
   • Cache miss: returns empty → Copilot uses its own LLM
7. After Copilot gets an answer from its LLM, it can call isartor_cache_store to populate the cache for future requests

HTTP/SSE MCP endpoint

Isartor now exposes the same MCP tool surface at /mcp/ using Streamable HTTP:

• POST /mcp/ — client → server JSON-RPC
• GET /mcp/ — server → client SSE stream
• DELETE /mcp/ — explicit session teardown

The HTTP transport uses the MCP Mcp-Session-Id header after initialize, and supports both JSON responses and SSE responses for POST requests. A minimal editor config looks like:

{"servers":{"isartor":{"type":"http","url":"http://localhost:8080/mcp/"}}}

Important note about "still going to L3"

If you inspect Copilot CLI JSON traces, you may still see a normal final_answer event after isartor_chat returns a cache hit. That does not mean Isartor forwarded the prompt to its own Layer 3 provider. The important signal is Isartor's own log and headers:

• Cache lookup: L1a exact hit or Cache lookup: L1b semantic hit
• no new Layer 3: Forwarding to LLM via Rig entry for that prompt

In other words:

• Isartor L3 call = bad for a cache hit
• Copilot final-answer render after a tool hit = expected CLI behavior

Isartor now installs stricter Copilot instructions that tell Copilot to emit the cached tool result verbatim on cache hits, without paraphrasing or extra tool calls.

Cache endpoints (used by MCP internally)

The MCP server calls these HTTP endpoints on the Isartor gateway:

# Cache lookup — returns cached response or 204 No Content
curl -X POST http://localhost:8080/api/v1/cache/lookup \
  -H "Content-Type: application/json" \
  -d '{"prompt": "capital of France"}'

# Cache store — saves a prompt/response pair
curl -X POST http://localhost:8080/api/v1/cache/store \
  -H "Content-Type: application/json" \
  -d '{"prompt": "capital of France", "response": "The capital of France is Paris."}'

Custom gateway URL

# If Isartor runs on a non-default port
isartor connect copilot --gateway-url http://localhost:18080

Disconnecting

isartor connect copilot --disconnect

This removes the isartor entry from ~/.copilot/mcp-config.json. It also removes the managed Isartor block from ~/.copilot/copilot-instructions.md.

Troubleshooting