NousResearch/hermes-agent · error · RuntimeError

Copilot ACP did not return a sessionId.

Error message

Copilot ACP did not return a sessionId.

What it means

The Copilot ACP 'session/new' request succeeded at the JSON-RPC level but the result object had no usable sessionId (empty or whitespace after str()/strip()). Without a session id no 'session/prompt' can be sent, so Hermes aborts immediately after initialize succeeds.

Source

Thrown at agent/copilot_acp_client.py:647

                        }
                    },
                    "clientInfo": {
                        "name": "hermes-agent",
                        "title": "Hermes Agent",
                        "version": "0.0.0",
                    },
                },
            )
            session = _request(
                "session/new",
                {
                    "cwd": self._acp_cwd,
                    "mcpServers": [],
                },
            ) or {}
            session_id = str(session.get("sessionId") or "").strip()
            if not session_id:
                raise RuntimeError("Copilot ACP did not return a sessionId.")

            text_parts: list[str] = []
            reasoning_parts: list[str] = []
            _request(
                "session/prompt",
                {
                    "sessionId": session_id,
                    "prompt": [
                        {
                            "type": "text",
                            "text": prompt_text,
                        }
                    ],
                },
                text_parts=text_parts,
                reasoning_parts=reasoning_parts,
            )
            return "".join(text_parts), "".join(reasoning_parts)

View on GitHub (pinned to c896c09c42)

Solutions

1. Update the Copilot CLI to the current stable (npm install -g @github/copilot).
2. If on a bleeding-edge CLI, downgrade to the last stable version.
3. Report to Hermes maintainers with CLI version if a current stable build still omits sessionId.

try:
    client = CopilotACPClient(...)
except RuntimeError as e:
    if 'did not return a sessionId' in str(e):
        # CLI version skew — update or downgrade CLI, then retry
        ...

When it happens

Trigger: session/new returns {}, null, or {'sessionId': ''} — typically a Copilot CLI version that changed the session result shape, or a CLI build that requires an extra handshake step before issuing sessions.

Common situations: Version skew between Hermes' ACP client expectations and the installed Copilot CLI; pre-release/beta CLI builds with a different session payload.

Related errors

• ACP file-system paths must be absolute.
• Copilot ACP {method} failed: {err.get('message') or err}
• patch content required
• Path '{resolved}' is outside the session cwd '{root}'.
• Could not start Copilot ACP command '{self._acp_command}'. I

AI-assisted analysis of NousResearch/hermes-agent@c896c09c42 (2026-08-14). Data as JSON: /api/errors/2e4000a1c6a22381. Report an issue: GitHub.