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+# RFC: File Decomposition for v0.9.0
+
+## Problem
+
+Six files exceed 5,000 lines. The worst offenders accumulate provider-specific
+logic, test code, and UI rendering in single translation units. This makes
+provider additions touch 15+ files and makes code review fragile.
+
+### Current state (lines)
+
+| File | Lines | Contents |
+|------|-------|----------|
+| `crates/tui/src/config.rs` | 10,046 | Provider resolution, env handling, model aliases, capability matrix, 2,000+ lines of tests |
+| `crates/tui/src/tui/ui.rs` | 9,400 | TUI render loop, input handling, command dispatch, /logout clearing |
+| `crates/tui/src/tui/ui/tests.rs` | 8,360 | Tests for ui.rs |
+| `crates/tui/src/main.rs` | 7,998 | CLI arg parsing, mode selection, startup |
+| `crates/tui/src/tui/app.rs` | 7,256 | Application state struct and lifecycle |
+| `crates/tui/src/runtime_threads.rs` | 5,454 | Async runtime orchestration |
+
+## Proposed decomposition
+
+### 1. `config.rs` → provider module tree
+
+Split `crates/tui/src/config.rs` into:
+
+```
+crates/tui/src/config/
+├── mod.rs              # Re-exports, Config struct, load/save
+├── provider.rs         # ApiProvider enum, parse/as_str/display_name/all
+├── capability.rs       # ProviderCapability, provider_capability()
+├── model_resolution.rs # wire_model_for_provider, normalize_model_name_for_provider
+├── env.rs              # EnvGuard, env var precedence, per-provider env handling
+├── constants.rs        # All DEFAULT_*_MODEL and DEFAULT_*_BASE_URL constants
+└── tests/              # Test module
+    ├── mod.rs
+    ├── provider.rs
+    ├── capability.rs
+    ├── model_resolution.rs
+    └── env.rs
+```
+
+**Why:** Every new provider currently requires edits to ~20 match arms scattered
+across one 10K-line file. With constants in their own module and resolution
+logic isolated, adding a provider becomes: add constants, add enum variant, add
+one match arm per function. The drift check script can validate each sub-module
+independently.
+
+### 2. `ui.rs` → view modules
+
+Split `crates/tui/src/tui/ui.rs` into:
+
+```
+crates/tui/src/tui/
+├── ui.rs               # Core render loop, frame dispatch (keep under 2,000 lines)
+├── input.rs            # Keyboard/mouse input handling
+├── command_dispatch.rs # /command routing, /logout, /config
+└── status_bar.rs       # Status bar rendering
+```
+
+**Why:** The /logout clearing logic, command dispatch, and render loop are
+independent concerns. `ui.rs` currently has a 6,200-line function body for
+`execute_command_input` that mixes input parsing, command routing, and state
+mutation.
+
+### 3. `main.rs` → CLI module
+
+Split `crates/tui/src/main.rs` into:
+
+```
+crates/tui/src/cli/
+├── mod.rs              # Cli struct, arg parsing
+├── args.rs             # Argument definitions
+└── startup.rs          # Mode selection, config loading, resume logic
+```
+
+**Why:** `main.rs` at 8K lines suggests the CLI definition has outgrown a
+single file. Separating arg definitions from startup logic makes the entry
+point readable.
+
+### 4. Provider additions should be data-driven
+
+The current provider pattern requires touching:
+- `config.rs`: 20+ match arms
+- `cli/src/lib.rs`: 4+ match arms
+- `agent/src/lib.rs`: static registry
+- `tui/provider_picker.rs`: picker list
+- `docs/PROVIDERS.md`: registry table
+- `config.example.toml`: example section
+- `README.md`: env vars table
+- `scripts/check-provider-registry.py`: drift check
+
+A data-driven approach would define each provider as a struct with its
+constants, env vars, capability metadata, and display name — then derive the
+match arms from the data. This is a larger refactor but would reduce provider
+additions to a single file change.
+
+## Priority
+
+1. **config.rs decomposition** — highest impact, most provider churn happens here
+2. **ui.rs decomposition** — second highest, /logout and command dispatch are independent
+3. **Data-driven providers** — aspirational for v0.9.0, requires trait design
+
+## Migration strategy
+
+Each decomposition should be a standalone PR that:
+1. Creates the new module tree
+2. Moves code with `git mv` (preserves history)
+3. Adds `pub use` re-exports in the old file location (zero API change)
+4. Runs the full test suite
+5. Removes the re-exports in a follow-up PR once consumers are updated
+
+No functional changes in decomposition PRs. Keep them boring.