Source inventory

This inventory freezes the migration baseline at Zig commit fc124be (0.0.3). The source contains 541 Zig modules, 674,840 lines, 8,251 in-source unit tests, and 871 root end-to-end tests across 58 files. Counts include large test fixtures and generated Unicode tables, so line count is a scope signal, not an architectural target for the Rust rewrite.

Runtime domains

Domain	Files	Lines	Tests	Product responsibility	Rust boundary
agent	40	52,895	746	model loop, stream recovery, parallel tool batches, interruption, finalization	fx-core orchestration
app	35	64,633	865	native composition, lifecycle, input actions, worker/UI coordination	ACP-reachable composition only
auth	9	7,354	104	API keys, OAuth, login, secret stores	fx-provider, fx-auth
background	10	9,219	84	supervised persistent background processes	ACP-reachable process capability; internal diagnostics only otherwise
cli	6	16,107	200	top-level parsing and typed text/JSON snapshots	internal test/diagnostic harness only
config	10	9,175	148	layered settings, capabilities, prompt policy	fx-config
execution	7	2,170	23	local/devbox routing and process effects	fx-process
gateway core	5	3,014	50	provider ports, catalog metadata, failure diagnostics	fx-core contracts
gateway adapter	7	8,216	130	HTTP streaming, usage, web search, JS host adapters	built-in modules in fx-provider
hooks	6	1,621	16	lifecycle/prompt/tool hook contracts	fx-core plus host adapters
hosts	11	2,686	40	native/WASM/keychain/URL host capabilities	native capabilities required by ACP only
images	2	4,368	87	attachment validation, snapshots, commands	fx-media candidate
input	27	8,758	111	editor state, Unicode navigation, undo, selection, paste	out of scope
MCP	36	47,986	361	stdio/HTTP/SSE, OAuth, schema, resources/prompts/tools, lifecycle	fx-mcp
output	6	7,180	102	typed snapshots, transcript projection, diffs, activity	fx-core snapshots plus frontends
permissions	13	15,995	231	rules, grants, auto review, sandbox admission	fx-core policy plus fx-process sandbox
sessions	39	61,715	576	event log, projections, migration/recovery, usage, catalogs	fx-store
shared	17	18,014	179	domain types, Unicode/display helpers, diagnostics	split by owning crate
shell command	3	2,689	66	lexical command classification and effects	fx-process policy
skills	4	6,082	92	discovery, invocation, commands, trust boundaries	fx-tools::skills
slash commands	2	2,995	80	registry, help, completion, routing	out of scope
subagents	21	53,372	338	durable child identity, authority, communication, manager	fx-core plus fx-store
terminal host	16	36,209	274	terminal sessions, tmux/native backends, recovery	fx-terminal candidate
tooling	31	30,856	397	registry, validation, admission, MCP bridge, result limits	fx-core contracts
workspace	19	15,524	246	path authority, indexes, search, context, metrics	fx-workspace, fx-tools, and future fx-context
UI	85	128,560	1,790	terminal rendering, footer surfaces, transcript, resize	out of scope
ACP	8	9,777	118	JSON-RPC server and editor session projection	fx-acp
built-ins	13	16,281	291	composition catalogs for commands, tools, gateway, MCP, modes	relevant composition crates

Smaller domains cover GitHub publishing, notifications, modes, upgrades, feedback, tasks, and generic registries. They remain separate feature slices rather than becoming layers of fx-core.

Source behavior surface

The top-level CLI has help, ask, ACP, PR/issue, authentication, status, permissions, models, doctor, background, teams, session inspection/listing, resume, credits, usage, upgrade, replay, and workspace management. Interactive mode adds 42 registered slash-command entries, including nested background actions, MCP and skills management, model/appearance/security settings, images, compaction, diagnostics, and session lifecycle.

Built-in tool families are:

• filesystem observation: read_file, list_files, glob_files, grep_files, file_info, semantic_search, open_file;
• filesystem mutation: write_file, edit_file, delete_file, rename_file, copy_file, create_folder;
• execution: terminal sessions and background processes;
• agent: user questions, vision, and subagents;
• web: search and fetch;
• extensions/state: skills, skill installation, memory, and stored tool-result reads;
• dynamic MCP tools, prompts, resources, and completion.

This inventory describes the source, not the Rust product boundary. The rewrite uses ACP as its sole agent protocol and ports other behavior only when ACP exposes or depends on it. The Rust TUI is an ACP client rather than a second composition root.

Cross-cutting invariants found in source and tests

• The Zig main.zig is a composition root, not the owner of leaf behavior.
• Fast CLI paths run before configuration, credentials, network, threads, or terminal initialization.
• Text and JSON output are rendered from the same typed snapshots.
• Explicit absolute, home-relative, or lexically escaping paths express external intent; workspace-relative paths may not escape through symlinks.
• Permission denies precede session grants. Yolo bypasses fx policy and uses no effective sandbox without rewriting the configured sandbox.
• Agent-owned model attempts cannot also be retried by the transport after possible delivery.
• Provider-executed tool results are projected but never locally re-executed.
• Current sessions use an append-only, sequenced event log with atomic state replacement; legacy schema 1 and 2 snapshots migrate into schema 3 storage.
• Child agents are ordinary durable sessions. Parent/child authority and create operation identity survive interruption and resume.
• MCP transport callbacks do not run while reader/state locks are held, and a final precommit guard closes authorization races.
• TUI tests cover rendering as a state machine under resize, interruption, terminal takeover, and partial gateway streams; a widget-by-widget port is therefore insufficient.

Migration order

1. Freeze domain contracts, config precedence, permission order, path intent, and the ACP product boundary.
2. Port read-only and mutation tools behind the registry, including read-before- write freshness evidence.
3. Add streaming gateway transport and the minimal noninteractive agent loop.
4. Add event-log sessions and recovery before interactive resume or subagents.
5. Add MCP and ACP as protocol adapters over the established core.
6. Port the terminal and extension capabilities reachable through ACP; omit WASM, N-API, and standalone SDK surfaces.
7. Add the TUI as an isolated ACP client after the protocol semantics stabilize.
8. Qualify ACP interoperability, semantic E2E parity, cold start, binary size, and failure recovery.