Devin CLI (experimental)

Happier runs the locally installed Devin CLI through its ACP server for normal sessions and execution runs. For execution runs, Happier lazily resolves one run-scoped set of Happier MCP servers and supplies it through Devin's native process-scoped MCP configuration when the Devin process starts.

Requirements and authentication

Install Devin CLI using the official Devin CLI guide, make sure devin is available on PATH, and authenticate on the machine where Happier runs:

devin auth login
devin auth status

You can also open the native login flow and refresh its status from Settings → AI & Agents → AI backends → Devin.

Start and resume sessions

happier devin

Happier supports new sessions, vendor resume, follow-up messages, cancellation, live model changes through Devin's ACP model config option, and execution runs. When Devin advertises a family through reversible combined identifiers such as swe-2-medium, swe-2-high, and swe-2-max, Happier presents one model with a structured Reasoning effort control. If Devin also advertises every matching standard and fast variant, Happier adds a Speed control with Standard and Fast choices. Incomplete or ambiguous families remain as separate model choices rather than being guessed.

Permissions

Happier maps explicit choices to Devin's advertised modes:

• Default sends no override and preserves your Devin configuration.
• Read only uses ask.
• Auto (the canonical safe-yolo intent) uses smart.
• YOLO uses bypass.
• Plan uses plan.

MCP tools

Devin supports MCP natively through its layered mcp_config.json files. Its ACP server currently ignores ACP's standard per-session mcpServers input and does not expose a direct MCP launch flag. Happier therefore merges the selected session tools—or the lazily resolved, run-scoped tools for an execution run—with your user-level Devin MCP servers in a protected temporary config root, starts Devin against that root, and removes it when the process ends. Untouched Devin and Cognition config entries are projected into that root with symlinks (directory junctions on Windows); only the merged mcp_config.json is process-private. If those links cannot be created, Happier stops the launch instead of copying provider state into a stale snapshot. Your normal config files are not modified by Happier.

Project and project-local Devin MCP servers still load. If one uses the same name as a Happier-selected session server, Happier stops before launch instead of allowing the higher-priority project entry to silently replace it.

For shared MCP setup and session selection, see MCP servers.

Troubleshooting

• Run devin version and devin auth status on the agent machine.
• If a model is unavailable, run devin models list under the same account.
• If an MCP tool is missing, check for a same-named entry in .devin/mcp_config.json or .devin/mcp_config.local.json.