shiro-neko/docs/architecture.md

Architecture

The loop

One turn is a streamText call whose stream is translated into UI events.

user prompt
  → messages.push({ role: 'user', ... })
  → streamText({ model, system, messages, tools, activeTools, reasoning, toolApproval })
      → for each stream part → yield an AgentEvent
      → if any tool needs approval, the stream ends suspended
          → collect decisions from the UI
          → push a tool message with the approval responses
          → loop
      → otherwise done

src/session.ts is an async generator. The UI consumes events; it never touches the SDK. That is what lets the same session drive the Ink app, the headless printer, and the tests.

Why approval goes through the SDK

An obvious design is a promise inside each tool's execute, resolved when the user answers. That was rejected: it makes "denied" a convention the tool must remember to honour, and one tool forgetting it is a silent security hole.

Instead the SDK's toolApproval is used. A denied call provably never executes — the SDK never reaches execute. The tool cannot opt out because the tool is not consulted.

toolApproval: async ({ toolCall }) => {
  const blocked = await plugins?.guard({ toolName: toolCall.toolName, input: toolCall.input, cwd });
  if (blocked) return { type: 'denied', reason: blocked };   // --yolo cannot reach this
  if (yolo) return undefined;
  if (!needsApproval(toolCall.toolName)) return undefined;
  return 'user-approval';
}

Guards are checked first, so --yolo skips prompts but not refusals.

One subtlety: when this function denies, the SDK emits tool-approval-request with isAutomatic: true and answers it itself. Queueing that would prompt the user for a call that is already settled, so automatic requests are skipped and denial is surfaced from tool-approval-response instead.

Where state lives

The system prompt is rebuilt on every step, not once per turn:

prepareStep: ({ messages }) => {
  const instructions = this.systemFor();          // task list, memory, skills, agent
  if (estimateTokens(messages) <= threshold) return { instructions };
  return { instructions, messages: prunePreservingItems({ messages, reasoning: 'all', ... }) };
}

That is not an optimisation. A todo_write on step one must be visible to step two, and system: on streamText is bound once for the whole run. Returning instructions from prepareStep is the only place per-step state can enter.

The prompt also describes only the tools actually offered this turn. A prompt that mentions a withheld tool teaches the model to attempt impossible calls. Two things narrow that set: a read-only agent variant, and toolSets in config. Both go through activeTools(), so a withheld tool is absent from the wire and from the prompt together.

Rendering

Ink re-renders the whole tree on every setState. At 50 tokens a second that is 50 full renders and a visibly flickering terminal.

Two things fix it:

• Finished lines go into <Static>, rendered once and never redrawn.
• Token deltas accumulate in a ref and flush on a 60 ms interval, not per token.

Answer text and reasoning text are separate refs on the same interval. Reasoning is shown collapsed as a token estimate, expandable with ctrl-r, and dropped when the turn ends: it is progress, not the answer, and keeping it would bury the reply it was leading up to.

Markdown is parsed on every flush. An unclosed fence renders as a code block that grows, which is what a reader expects while text is still arriving.

Input

ink-text-input was replaced. It discards up and down before its own handler, so history recall is impossible, and it only ever shrinks its internal cursor offset, so an externally set value leaves the cursor stranded mid-string.

src/ui/PromptInput.tsx owns the cursor and reports it with every change, which is what makes @path completion possible at all. That also gives home, end, and ctrl-a/e/k/u/w for free. It hands up, down, tab, and escape to a parent callback first, so the file picker, the command menu, and open panels can claim them before the input treats them as editing keys.

The file picker claims those keys ahead of the command menu. While an @ token is open, up and down mean "move in the list", not "recall an earlier prompt".

src/complete.ts holds the token extraction, ranking, and insertion as pure functions, so the rules are testable without a terminal. Two of them are decisions rather than mechanics:

• The @ must start a word, or user@host opens a file picker.
• Prefix matches rank above substring matches, because @src/ means "under src/" and a substring hit on vendor/src/ would bury what the user pointed at.

The prompt queue

The input stays mounted while the model works. A prompt submitted mid-turn is pushed onto a queue and drained in order when the turn ends, going back through submit so a queued slash command behaves exactly as if it were typed at that moment.

The queue is a ref as well as state. The drain runs synchronously as the turn ends, between renders, and a closure over a stale array would silently lose a prompt. busy is mirrored into a ref for the same reason.

esc clears the queue as well as aborting. Interrupting and then watching two more prompts fire anyway is not what anyone means by interrupt.

Interrupting one command

esc aborts the whole turn. That is the wrong tool for a runaway command, because it throws away the conversation to stop a sleep.

ctrl-c kills the command in flight and leaves the turn alive. src/tools.ts keeps the running processes by tool call id, and interruptBash() kills them and returns what it killed. The call then throws rather than returning:

The user interrupted this command. It did not finish, so its effects are unknown.

Throwing is the point. A returned exit: 1 reads to the model as a command that ran and failed on its own terms, which is a different fact from a command that was stopped partway. The model gets a tool error, and the loop continues to the next step.

The kill has to take the whole process tree. cmd /c and bash -lc run the real command as a child, and killing the shell alone leaves that child holding both pipes open, so the read never returns — measured at 19 seconds for a ping -n 20 that should have died instantly. On Windows that means taskkill /T /F. The kill is also awaited before the tool returns, because a surviving grandchild keeps the working directory locked.

Ink's own exitOnCtrlC is turned off in cli.tsx so the key reaches the app; with nothing running, the handler exits as usual.

Subagents

task runs a nested streamText and returns one message. The subagent kinds hold different tool sets: explore and review the read-only tools, worker those plus every write tool.

The consequences follow from the tool set, not from policy:

• explore and review can never need approval, because they hold no gated tool.
• worker needs approval for exactly the calls a direct one would, so the parent owns the gate: the subagent's toolApproval callback routes back through the parent's permission rules, guard plugins, and prompt. A subagent with its own approval would be a way to launder a tool call past the user.
• The parent's context holds the findings, not the search transcript.

Progress is reported through a callback, wired to a bus the panel subscribes to. Without the bus the panel would need a reference to the tool, and the tool would need one to React.

Provider differences

Two are handled explicitly.

Thinking levels. off/low/medium/high/max become reasoning_effort on OpenAI and a thinking token budget on Anthropic. The SDK does the mapping; src/agents.ts only picks the level.

Endpoint fallback. Newer OpenAI models reject function tools on /v1/chat/completions and require /v1/responses. src/fallback.ts presents both as one model and switches when the first rejects the request shape — 400, 404, 405, 415, 422, 501 with isRetryable false. Retryable failures are left to the SDK's backoff.

The switch is sticky. Once an endpoint rejects the shape it will reject every later step too, so re-probing it each turn would waste a round trip per step.

Only api.openai.com gets the chain. Third-party endpoints do not implement /v1/responses.

Compaction and its repair

Pruning breaks two provider invariants, and src/prune.ts repairs both.

A message detached from its reasoning item. pruneMessages({ reasoning: 'all' }) strips a reasoning item and keeps the message item from the same response. A part carrying a provider itemId is not sent inline: the responses provider serialises it as { type: 'item_reference', id }, pointing at an item stored on their side, and that stored item depends on the reasoning item that pruning just removed. The result is a 400.

The two carry different ids, so they cannot be matched by id. What links them is the assistant message they arrived in: one message is one response, and its reasoning item covers every other item in it. detachOrphanedItems strips the itemId from those parts, which is what sends the same content inline instead — verified against the provider's own serialiser, where a text part with an itemId goes out as item_reference and the identical part without one goes out as output_text.

Dropping the parts was the first attempt and it broke the loop. On a reasoning model every tool call carries an itemId, so after the first compaction the model could not see what it had already run, and re-ran the same tools until the step limit ended the turn. Compaction may shorten the history; it must not blank it.

A tool result without its tool call. Tool pruning counts messages, so a cut can land between an assistant tool-call and the tool message answering it. What reaches the wire is a function_call_output with no function_call:

400 No tool call found for function call output with call_id call_…

dropOrphanedResults collects the surviving call ids and drops any result that has none. The reverse pairing is deliberately left alone: a call still awaiting its result is exactly what a suspended approval looks like, and dropping it would break resume.

An item the provider no longer holds. A reference resolves only while the item is still in provider storage, which a resumed session or an endpoint fallback cannot count on:

404 Item with id 'msg_…' not found.

Nothing about the same history can succeed on retry, so pruneToFit strips every provider itemId from what it sends, and Session.run answers that 404 by rewriting its own history inline and running the request again — once per turn, and only when the rejection arrived before any output, since delivered text cannot be unsent.

The pruning ladder drops reasoning first and then keeps the widest recent tool tail that fits. The SDK carries that returned message view into later steps, and the session reports compaction once per turn rather than once per step.

Registry

/registry fetches an index of external skills and plugins over https. Skills are prompt text and are shown in full before install; plugins are a JSON manifest of refusal rules, never code. The guard evaluating those rules is compiled, identical for every installed plugin, so an entry from a registry cannot execute anything. See registry for the validation and the reasoning.

src/registry.ts has no UI and no side effects until install() is called, which is what lets stage() show a body before it becomes part of every future prompt.

Module map

Module	Responsibility
session.ts	the loop, approvals, compaction, event stream
tools.ts	file and shell tools, tool sets, ripgrep bridge, bash streaming and interrupt
tools-git.ts	read-only git tools, spawned with a fixed argv
commit.ts	git_commit_message, a nested model call over the staged diff
tools-net.ts	web_fetch, web_search, private-address and redirect checks
ignore.ts	gitignore-aware walker, path jail
complete.ts	@path token extraction, ranking, insertion
registry.ts	external index, validation, install and removal
prompt.ts	system prompt assembly from live state
agents.ts	variants, thinking levels
skills.ts	discovery, catalogue, skill tool
memory.ts	durable notes, search, model compaction
notebook.ts	session task list
plugins.ts	host, hooks, guard chain
subagent.ts	task tool and progress events
ask.ts	the ask tool
mcp.ts	MCP clients and namespacing
fallback.ts	endpoint chain
prune.ts	provider-item and tool-pairing repair
markdown.ts	parser, no dependency
store.ts	sessions, prompt history
farewell.ts	the exit message and its resume commands
config.ts	resolution, model construction
providers.ts	presets, /models fetch
pricing.ts	USD rates
commands.ts	slash registry, parsing, menu matching
headless.ts	-p mode
cli.tsx	argv, wiring, lifecycle
ui/App.tsx	state, the turn loop, slash-command routing
ui/transcript.ts	line types, tool argument and result formatting
ui/buses.ts	notice and subagent channels, subagent view folding
ui/Approval.tsx	the approval bridge and its prompt
ui/Pickers.tsx	command menu, shared list picker, install confirm
ui/panel-bodies.ts	/tools, /cost, /context, /todos bodies
ui/Panels.tsx	presentational panels and the status bar
ui/*	remaining Ink components

Every module is pure of the UI except ui/, and ui/ never touches the SDK. The seam is the AgentEvent stream.

Testing

538 tests became 713 as the suites grew; no mocking framework. MockLanguageModelV4 from ai/test drives the loop; ink-testing-library drives the UI with real keystrokes; MCP is tested against a real stdio server subprocess; provider wire formats and the registry are tested against a local HTTP server; the interrupt path spawns a real subprocess and asserts it died early rather than ran out.

The pattern throughout is to assert on what actually crossed a boundary — what went on the wire, what is on screen, what is on disk — rather than on internal calls.

Compaction is asserted on behaviour, not shape: the loop must terminate because the model chose to, and every call after the first must still carry the earlier exchange. A shape assertion would have passed while the model was losing its memory.