# Reasonix Engineering Spec > Reasonix is a coding agent: a thin harness driving multiple models, with **all > capabilities supplied by configuration and plugins**. This document is the > contract — code follows it. Change the contract first, then the code. ## 1. Design Principles 1. **Config- and plugin-driven core.** The core knows only interfaces. Concrete models and tools are resolved by name from registries, declared in config, or injected by plugins. No hardcoded `switch model`. 2. **Single static binary.** `CGO_ENABLED=0`; cross-compile with one command; CLI works out of the box. 3. **Lean dependencies.** Standard library by default. A third-party dependency must be pure-Go, lightweight, and must not compromise the single-binary / cross-platform / distribution story. TOML parsing is the one accepted dependency. 4. **Two extension tiers.** Compile-time built-ins (self-register via `init()`), and runtime external plugins (stdio JSON-RPC subprocesses, MCP-compatible). 5. **Interface-first & registry-based.** `Provider` and `Tool` are interfaces. 6. **Evolve, don't over-engineer.** Language: **English is the primary language for all code** — comments, user-facing strings, tool descriptions, system prompts, and this spec. The README is bilingual (`README.md` English + `README.zh-CN.md`). ## 2. Layout ``` reasonix/ ├── go.mod / go.sum # module reasonix; require BurntSushi/toml ├── Makefile # build / cross / vet / fmt / test ├── README.md / README.zh-CN.md ├── reasonix.example.toml # sample config ├── docs/SPEC.md # this file ├── cmd/reasonix/main.go # entry; blank-imports built-in providers/tools ├── cmd/reasonix-plugin-example/ # reference MCP stdio plugin (a runnable example) └── internal/ ├── cli/ # subcommand routing, flags, assembly, exit codes ├── config/ # TOML loading (flag > project > user > defaults) ├── provider/ # Provider interface + types + kind→factory registry │ └── openai/ # OpenAI-compatible impl; init() registers "openai" ├── tool/ # Tool interface + Registry │ └── builtin/ # read_file/write_file/edit_file/bash/ls/glob/grep ├── permission/ # per-call Policy: allow/ask/deny rules → Decision ├── command/ # custom slash commands loaded from .reasonix/commands/*.md ├── plugin/ # stdio JSON-RPC (MCP) client; adapts remote tools └── agent/ # Session + harness loop ``` Dependency direction (acyclic): `cli → {agent, plugin, config} → {tool, provider}`. Built-in subpackages (`provider/openai`, `tool/builtin`) import their parent to self-register; parents never import children. ## 3. Core Abstractions ### 3.1 Provider + registry (`internal/provider`) ```go type Provider interface { Name() string Stream(ctx context.Context, req Request) (<-chan Chunk, error) } // Factory builds a Provider from a resolved config instance. type Factory func(cfg Config) (Provider, error) // Register adds a factory under a kind (e.g. "openai"). Called from init(). func Register(kind string, f Factory) // New instantiates the provider of the given kind. func New(kind string, cfg Config) (Provider, error) type Config struct { Name string // instance name, e.g. "deepseek" BaseURL string Model string APIKey string Extra map[string]any // kind-specific options } ``` - The `openai` kind is an OpenAI-compatible `/chat/completions` implementation. - **DeepSeek and MiMo are not code — they are config instances** of `kind = "openai"`, differing only in `base_url` / `model` / `api_key_env`. Adding another OpenAI- compatible model is a config edit, not a code change. - **A provider is a vendor endpoint** (one `base_url` + `api_key_env`) that offers one or more models. An entry declares either a single `model = "..."` or a `models = ["...", "..."]` list (with an optional `default`); the list form lets one vendor expose several models without re-declaring the endpoint/key — picking a model reuses the same connection. A **model reference** (`default_model`, the `--model` flag, the desktop switcher) resolves via `Config.ResolveModel`, which accepts a provider name (→ its default model), a bare model name, or an explicit `provider/model`. `context_window` / `price` are per-provider, so models that need distinct values stay separate single-`model` entries. - Streaming tool-call deltas are accumulated by index inside the provider; only complete `ToolCall`s are emitted. ### 3.2 Tool + registry (`internal/tool`) ```go type Tool interface { Name() string Description() string Schema() json.RawMessage // JSON Schema for parameters Execute(ctx context.Context, args json.RawMessage) (string, error) } ``` - Built-in tools self-register into a process-global builtin set via `init()` (`tool.RegisterBuiltin(t)`); `tool.Builtins()` lists them. - A runtime `*Registry` is assembled per run: enabled built-ins (filtered by config) **plus** plugin-provided tools. The agent only sees the `*Registry`. - `Execute` parses raw JSON args itself. Errors are returned, not fatal — the agent feeds them back so the model can self-correct. ### 3.3 Plugins (`internal/plugin`) — MCP client An external plugin is an MCP server declared in config. The wire protocol is **JSON-RPC 2.0** in every case; only the transport differs. A `transport` interface (`call` / `notify` / `close`) abstracts that, so the MCP-level logic (handshake, `tools/list`, `tools/call`, …) is written once. - **Transports** (config `type`): - `stdio` (default) — a local subprocess; one JSON message per line over the child's stdin/stdout (the MCP stdio convention). Declared with `command` / `args` / `env`; terminated on ctx cancel / shutdown. - `http` (a.k.a. `streamable-http`) — a remote server at `url`. Each request is an HTTP POST; the server replies with either `application/json` (one response) or `text/event-stream` (an SSE stream carrying the response plus any server notifications). The `Mcp-Session-Id` response header, once seen, is echoed on subsequent requests. Static `headers` (e.g. a bearer token) are sent on every request. OAuth is out of scope for now (see §9). - `sse` — the legacy 2024-11-05 HTTP+SSE transport; recognised but deferred (deprecated upstream — use `http`). Configuring it returns a clear error. - `${VAR}` / `${VAR:-default}` are expanded in `command`, `args`, `env`, `url`, and `headers` so secrets come from the environment, not the config file. - Lifecycle: `initialize` → `notifications/initialized` → `tools/list`; invocation via `tools/call {name, arguments}`. - Each remote tool is adapted to the `Tool` interface and injected into the run registry, namespaced `mcp____` (spaces normalised to `_`) to match Claude Code and avoid clashes. - A tool's MCP `annotations.readOnlyHint` maps to `Tool.ReadOnly()`. It defaults to false (a remote tool is opaque — we can't see its side effects), so a plugin opts a tool into parallel-batch dispatch and the permission layer's reader-default by declaring `readOnlyHint: true` in `tools/list`. - `prompts/list` + `prompts/get` surface as `/mcp____` slash commands; `resources/list` + `resources/read` are referenced as `@:` in chat. `/mcp` shows connected servers and their counts. - `cmd/reasonix-plugin-example` is a runnable reference stdio server (`echo`, `wordcount`), driven by an end-to-end test that builds the real binary. ### 3.4 Agent (`internal/agent`) - `Session` holds `[]Message`. - `Run(ctx, input)` loop: build `Request` (with tool schemas) → `provider.Stream` → print text deltas live, collect complete tool calls → if none, done; else execute each tool (built-in or plugin) and append results → repeat, bounded by `maxSteps`. `ctx` threads throughout (Ctrl-C aborts in-flight requests). - A `Runner` is anything with `Run(ctx, input) error`; both `Agent` and `Coordinator` satisfy it, so the CLI is agnostic to single- vs two-model mode. ### 3.5 Two-model collaboration (`Coordinator`) When `agent.planner_model` names a provider different from the executor, a `Coordinator` runs two models in **separate sessions** to keep each one's prompt prefix cache-stable: - The **planner** (low-frequency) runs in its own session with no tools and produces a concise plan. - The plan is handed off as structured text to the **executor** — a full tool-using `Agent` in its own session — which carries it out. - The sessions never mix, so neither model's prefix is disturbed by the other's turns; both grow prepend-only and stay cache-friendly. This reconciles "cache-first" with "two-model collaboration": switching models *inside one shared conversation* would break the prefix and tank cache hits, so we don't. ### 3.6 Context management (compaction) Long tasks eventually fill the model's context window. Reasonix manages this with **low-frequency compaction** that respects the cache-first design: - Each provider declares its `context_window` (tokens). When a turn's reported `prompt_tokens` reach `compactRatio` (default `0.8`) of that window, the executor compacts **once** before the next turn. - Compaction summarizes the older middle of the session into a single briefing — using the executor's own provider, no tools — and replaces it in place: the session becomes `system + summary + recentKeep` (default `8`) verbatim messages. The boundary is aligned backward off any tool result so the recent tail never begins with an orphan tool message whose `tool_calls` were summarized away. - The dropped originals are archived to `~/.config/reasonix/archive/.jsonl` (one message per line), so the full history stays traceable. This is the **only** point where the prompt prefix changes — a deliberate, rare "cache-reset point". Between compactions the session grows prepend-only and stays cache-friendly, so cache hit rate (the key observability signal) stays high. `context_window = 0` disables compaction for an instance. ### 3.7 Permissions (`internal/permission`) — per-call gating A coding agent runs shell commands and edits files autonomously. The permission layer decides, **per tool call**, whether to allow it, deny it, or ask the user first. It is independent of the model and of the CLI — the agent consults a `Gate` interface at execute time; the gate is built from a static `Policy` plus an optional interactive `Approver`. ```go type Decision int // permission package const (Allow Decision = iota; Ask; Deny) // Policy evaluates static rules against a tool call. Pure, no I/O. type Policy struct { Mode Decision; Allow, Ask, Deny []Rule } func (p Policy) Decide(toolName string, readOnly bool, args json.RawMessage) Decision ``` - **Rule syntax.** A rule is `ToolName` (matches any call to that tool) or `ToolName(glob)` (matches when the call's *subject* matches the glob, via `path.Match`). The subject is extracted generically from the call's JSON args by a small set of known keys — `command` (bash), `path` / `file_path` (file tools), `pattern` (grep/glob) — so tools need not change. A rule whose subject the args don't expose only matches in its bare `ToolName` form. - **Precedence.** `deny` > `ask` > `allow` > fallback. Fallback is `Allow` for read-only tools and `Mode` (default `Ask`) for writers. `deny` always wins, so a broad `allow = ["bash"]` can still be carved by `deny = ["bash(rm -rf*)"]`; conversely `ask` overrides a broad `allow` to force a prompt on a risky subset. - **Resolving `Ask`.** The interactive front-end (the chat TUI) prompts the user — allow once / always allow / deny — via an `Approver`. A non-interactive run (`reasonix run`, a sub-agent, anything with no TTY / no approver) cannot prompt, so it resolves `Ask` to **allow** — preserving autonomous behaviour. A `Deny` is a hard block in *every* mode: the tool never executes and the model receives a "blocked" result it can adapt to (the same shape as a plan-mode refusal). - **Relationship to plan mode.** Plan mode (§3.4) is an orthogonal, coarser gate that refuses *all* writers regardless of policy; it is checked first. The permission layer is the fine-grained, always-on gate underneath it. Out of the box (`mode = "ask"`, no rules) `reasonix run` behaves exactly as before (writers resolve `Ask`→allow with no TTY), while `reasonix chat` now prompts before each writer/bash call. `deny` rules harden both modes. ### 3.8 Slash commands (`internal/command`) The chat TUI accepts `/command` input. Three kinds share one dispatch: - **Built-in actions** (`/compact`, `/new`, `/mcp`, `/help`) manipulate session state locally and never reach the model. - **Custom commands** are Markdown files under `.reasonix/commands/` (project) and `~/.config/reasonix/commands/` (user); the project dir overrides the user dir on a name clash. A file `review.md` becomes `/review`; a subdirectory namespaces it (`git/commit.md` → `/git:commit`). Invoking one renders its body and sends the result as the next user turn. - **MCP prompts** (§3.3) appear as `/mcp____`. ```markdown --- description: Review the staged diff argument-hint: [focus-area] --- Review the staged diff. Focus on $ARGUMENTS, list bugs with file:line. ``` - Frontmatter is an optional `---`-fenced block of simple `key: value` lines; `description` and `argument-hint` are recognised (no YAML dependency — Reasonix stays lean). The remainder is the body template. - Substitution in the body: `$ARGUMENTS` (all args, space-joined), `$1`…`$N` (positional, empty when absent), `$$` (a literal `$`). Arguments are the space-separated tokens after the command. - Loading is pure (`command.Load(dirs...)`) and tested; a malformed file is skipped, not fatal. Custom and MCP-prompt commands both resolve to text and reuse the same "start a turn" path as a typed message. ### 3.9 Chat references (`@`) A chat message may embed `@` references; before the turn is sent, each is resolved and prepended to the message as a tagged block the model can read. - `@:` where `` is a connected MCP server → an MCP resource (`resources/read`), wrapped `…`. - `@` otherwise → a **local file or directory**, but only when the path actually exists on disk. This existence gate is the disambiguator: an ordinary `@mention` or an email address resolves to no file and stays literal text. A file is wrapped `…` (size-capped, binary files noted not dumped); a directory becomes a one-level listing. - Resolution is asynchronous (off the TUI event loop); a fetch failure surfaces as a notice but doesn't block the turn. Reads are user-initiated and read-only — they do **not** pass the permission gate (§3.7). - Typing `/` or `@` opens an autocomplete menu above the input. The `@` menu navigates **one directory level at a time** (`os.ReadDir`, never a recursive walk — bounded for huge directories): a directory entry descends, a file completes, and MCP resources appear alongside top-level entries. The bottom-region menu changes height only on these discrete actions, never per streamed token, so scrollback stays clean (§ rendering). ## 4. Data Types (`internal/provider`) ```go type Role string const (RoleSystem Role = "system"; RoleUser Role = "user" RoleAssistant Role = "assistant"; RoleTool Role = "tool") type Message struct { Role Role `json:"role"` Content string `json:"content,omitempty"` ToolCalls []ToolCall `json:"tool_calls,omitempty"` ToolCallID string `json:"tool_call_id,omitempty"` Name string `json:"name,omitempty"` } type ToolCall struct { ID, Name, Arguments string } // Arguments: raw JSON type ToolSchema struct { Name, Description string; Parameters json.RawMessage } type Request struct { Messages []Message; Tools []ToolSchema; Temperature float64; MaxTokens int } type ChunkType int const (ChunkText ChunkType = iota; ChunkToolCall; ChunkDone; ChunkError) type Chunk struct { Type ChunkType Text string // ChunkText ToolCall *ToolCall // ChunkToolCall Err error // ChunkError } ``` ## 5. Configuration (TOML) Resolution order: **flag > project `./reasonix.toml` > user `~/.config/reasonix/config.toml` > built-in defaults**. Secrets come from the environment via `api_key_env` and are never stored in config files. A `.env` in the working directory is loaded if present. ```toml default_model = "deepseek" # provider name (→ its default model) or "provider/model" # language = "zh" # ui language tag; empty = auto-detect from $LANG / $REASONIX_LANG [agent] system_prompt = "You are Reasonix, a coding agent..." # or system_prompt_file = "..." max_steps = 25 temperature = 0.0 # planner_model = "mimo" # optional: two-model collaboration (low-frequency planner) # subagent_model = "deepseek-pro" # optional default for runAs=subagent skills # subagent_models = { review = "deepseek-pro", security_review = "deepseek-pro" } # A vendor endpoint exposing several models under one base_url/key. [[providers]] name = "deepseek" kind = "openai" base_url = "https://api.deepseek.com" models = ["deepseek-v4-flash", "deepseek-v4-pro"] default = "deepseek-v4-flash" # optional; defaults to models[0] api_key_env = "DEEPSEEK_API_KEY" context_window = 1000000 # tokens; harness compacts older history near this limit (0 disables) # A single-model entry (use when a model needs its own base_url/context_window/price). [[providers]] name = "mimo-pro" kind = "openai" base_url = "https://api.xiaomimimo.com/v1" model = "mimo-v2.5-pro" api_key_env = "MIMO_API_KEY" [[providers]] name = "mimo-flash" kind = "openai" base_url = "https://api.xiaomimimo.com/v1" model = "mimo-v2-flash" api_key_env = "MIMO_API_KEY" [tools] enabled = [] # omit/empty = all built-ins [permissions] mode = "ask" # writer fallback when no rule matches: ask|allow|deny deny = ["bash(rm -rf*)", "bash(git push*)"] # hard-blocked in every mode allow = ["bash(go test*)", "bash(git status*)"] # never prompted ask = [] # force a prompt even if otherwise allowed [sandbox] # workspace_root = "" # file-writers confined here; empty = cwd (writes stay in-project) # allow_write = ["/tmp"] # extra dirs write_file/edit_file/multi_edit may modify [[plugins]] name = "example" # type defaults to "stdio" command = "reasonix-plugin-example" args = [] # env = { FOO = "bar" } # [[plugins]] # a remote MCP server over Streamable HTTP # name = "stripe" # type = "http" # "stdio" (default) | "http" | "sse" # url = "https://mcp.stripe.com" # headers = { Authorization = "Bearer ${STRIPE_KEY}" } # ${VAR} / ${VAR:-default} expanded ``` `reasonix setup` writes this default config so the CLI is usable out of the box. MCP servers may also be declared in a project-root `.mcp.json` using Claude Code's exact `mcpServers` schema (`command`/`args`/`env`, `type`/`url`/`headers`, `${VAR}` expansion). It is read after the TOML files and merged into `[[plugins]]`; on a name collision `reasonix.toml` wins (it is the more explicit, Reasonix-specific source). This lets a server already configured for Claude work in Reasonix unchanged. ```json { "mcpServers": { "stripe": { "type": "http", "url": "https://mcp.stripe.com", "headers": { "Authorization": "Bearer ${STRIPE_KEY}" } } } } ``` `[sandbox]` is the *enforcement* layer beneath permissions (which are *policy*). Phase 0 confines the file-writing built-ins (`write_file`, `edit_file`, `multi_edit`) to `workspace_root` (default cwd) plus `allow_write`: a write whose target — resolved to an absolute, symlink-free path so a symlinked dir or `..` cannot tunnel out — falls outside every root is refused, and the error is fed back to the model. Confinement is on by default (root = cwd), so edits stay in the project; reads are unrestricted. `bash` is itself jailed on macOS by default (`[sandbox] bash = "enforce"`, Seatbelt): each command runs under sandbox-exec allowed to write only the same roots (+ temp and toolchain caches) and to reach the network only when `network = true`. Unsupported platforms fall back to running unconfined. The escape-prompt and Linux support are Phase 1's remainder (§9). ## 6. Error Handling - Library code wraps with `fmt.Errorf("...: %w", err)` and returns; it never prints or calls `os.Exit`. - Only `cli` / `main` decide exit codes and user-facing messages. - Tool execution errors are fed back to the model, not fatal. - Network layer should apply bounded exponential backoff on 429 / 5xx (interface reserved; implementation may follow). ## 7. Code Style - `gofmt` + `go vet` must be clean; package names lowercase; exported identifiers documented; comments explain *why*, not *what*. - No premature generalization. Prefer clear and direct. ## 8. Distribution - Build: `CGO_ENABLED=0 go build -ldflags "-s -w -X main.version=$(VERSION)" -o reasonix ./cmd/reasonix` - Cross matrix: `darwin|linux|windows` × `amd64|arm64`. - Version injected via ldflags (`git describe --tags --always`). - Install: prebuilt binary / `go install` / future `brew tap`. ## 9. Roadmap (not in current scope) - Sandbox Phase 1: an OS-level jail for `bash` so commands — not just the file-writer built-ins (Phase 0) — are confined to the workspace. **macOS (Seatbelt via `sandbox-exec`) ships, on by default** (see §5). Remaining: (a) the escape-prompt — detect a sandbox-denied failure and offer to re-run the command unconfined via the permission gate (in `reasonix run`, the command just fails and the model adapts), which completes the "allow inside the box, prompt at its edge" model; (b) Linux (bubblewrap / landlock). Shells out to OS tooling so the binary stays dependency-free; Windows is out of scope. With this in place, "always allow" rule persistence becomes optional rather than load-bearing. - MCP long tail (deferred deliberately — no consumer / no foundation yet): OAuth 2.0 + `headersHelper` auth for remote servers; the remaining `.mcp.json` scopes (local / user — project scope shipped, see §5); tool-search deferral; `list_changed` live updates; channels / elicitation / roots; plugins that provide *providers*, not just tools. - An Anthropic-native provider `kind` (native prompt-cache control), proving the registry generalises beyond one wire format. - "Always allow" persistence writing learned rules back to project config; a per-session permission override flag for `reasonix run`.