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+# The C library compatibility layer
+
+A design note and milestone plan for **libdanos-c** — the mini C library that lets
+`zig cc` cross-compile C programs for danos. It is milestone **P0** of
+[python-on-danos-milestones.md](python-on-danos-milestones.md), expanded here the
+way [character-devices-and-tty.md](character-devices-and-tty.md) expands P1.
+CPython is the driving consumer, but the layer is general: any portable C program
+within its surface should build.
+
+## What it is — and the three things it is not
+
+The deliverable is a **sysroot**: a set of C headers plus a static `libdanos-c.a`,
+handed to `zig cc -target x86_64-freestanding-none` via `-isystem` and linked into
+every C binary. Three explicit non-goals keep it small:
+
+- **Not a musl port.** Whole-musl assumes Linux syscall semantics at its bottom
+  (the door the Zig roadmap deferred, twice now). We *lift* musl's pure-computation
+  source files and *write* a danos-native bottom — see the layer split below.
+- **Not full POSIX — *yet*.** Stage 1's surface is "what CPython's minimal
+  configuration and ordinary portable C need" — roughly 100–150 functions — and
+  at that stage absence is a *feature*: configure scripts probe and adapt, and a
+  linker error is honest. But the end state is a **full C compatibility layer**
+  (see "The road to full coverage" below); the absence table is a schedule of
+  arrivals, not a wall.
+- **Not a second runtime.** The library is a thin C-ABI re-spelling of the same
+  danos-native surface `runtime` already provides. It contains no policy of its
+  own; when the Zig track's `runtime.os` seam is authored, the libc bottom
+  re-targets it near-mechanically — the fourth appearance of the roadmap's "same
+  surface" symmetry.
+
+One scoping rule sits above all three — the **size doctrine**: this layer serves
+**applications only**. The kernel and the system services never link libdanos-c;
+they stay danos-native Zig over `runtime`, small and static, because leanness is
+an operating-system property. Applications have their own budget and may be as
+big as they need to be. The libc is how big software *lands on* danos, never how
+danos itself is built.
+
+## The layer split: lift the mathematics, write the plumbing
+
+The realization that makes 100–150 functions tractable: a libc is two very
+different kinds of code, and the hard kind is portable.
+
+| Layer | Contents | Source |
+|-------|----------|--------|
+| **Pure computation** | `string.h`/`memcpy` family, all of libm, `strtod`/`dtoa`, `strtol`, `qsort`, `ctype` tables, `gmtime` calendar math, the `printf`/`scanf` engines, `setjmp` (a dozen instructions of x86-64 asm) | **Lift from musl**, vendored under `library/c/third-party/musl/` (MIT; files compile standalone) |
+| **OS plumbing** | fds (`open`/`read`/`write`/`close`/`lseek`/`stat`/`getcwd`/`chdir`/`isatty`), `mmap`/`munmap`, clocks, `exit`, `getenv`, `getentropy` | **Write in Zig**, exporting C ABI over the `runtime` syscall + VFS client surface |
+| **The middle** | `malloc` over danos `mmap` (simple free-list; CPython's arenas sit above), `FILE*` buffering, `errno` | **Write in Zig** (small, danos-shaped) |
+| **Entry** | `crt0`: the existing danos entry shim ([sysv.md](os-development/sysv.md)) bridged to C `main(argc, argv, envp)`, `environ` initialised, `exit` flushing stdio | **Write** |
+
+Two liftings deserve their own line because getting them wrong is silent
+corruption rather than a linker error:
+
+- **`strtod`/float formatting.** Python's float `repr` guarantees shortest
+  round-trip; that property lives entirely in these routines. musl's are correct;
+  an improvised one would be subtly wrong for years. Lift, never write.
+- **The stdio engines.** musl's `vfprintf`/`vfscanf` are self-contained around
+  its `FILE` abstraction (function-pointer read/write slots), so the whole
+  formatted-I/O engine lifts too — we implement only the fd-backed slots
+  (`__stdio_write`-shaped) and the buffering glue.
+
+## Header policy
+
+Hand-write the headers as danos's own minimal set rather than importing musl's
+(musl's are entangled with Linux ABI details), borrowing declarations freely.
+Freestanding compiler headers (`stdint.h`, `stddef.h`, `stdarg.h`, `stdbool.h`,
+`float.h`, `limits.h`) come from clang via `zig cc` — do not duplicate them.
+`errno.h` values are the danos errno enum re-spelled with POSIX names; there is no
+Linux numbering to be compatible with, so the enum is the truth.
+
+Deliberate absences, and their planned arrivals — this table is the
+compatibility matrix, and "the road to full coverage" below is the schedule
+that empties it:
+
+| Absent | Arrives with |
+|--------|--------------|
+| `pthread.h` | the post-P5 pthread subset over `thread_spawn`/futex — but see the risk below |
+| real `signal.h` (beyond no-op `signal()`/`raise` stubs) | M17 signals-over-IPC in the libc |
+| `dlfcn.h` | [dynamic-libraries.md](dynamic-libraries.md) D1 |
+| `fork`/`exec*`/`wait*` | P5 exposes danos spawn as `posix_spawn`; `fork` itself never (see below) |
+| `socket.h` | a future networking track |
+| locale beyond `"C"` | stage 3 evaluation (CPython is UTF-8-mode happy without it) |
+| pipes (`pipe()`) | P5 process-control cluster |
+
+## The road to full coverage
+
+The layer grows in three stages; only stage 1 is a current milestone (P0), but
+the stages exist so stage-1 decisions never have to be unmade:
+
+- **Stage 1 — CPython-minimal** (P0, the slicing below): ~100–150 functions,
+  static-only, absences honest.
+- **Stage 2 — the danos-complete layer**: the full hosted C11 standard library,
+  plus every POSIX facility danos semantics support, landing as its enabling
+  milestone lands — pipes and `posix_spawn` at P5, real signals at M17, the
+  pthread subset after P5, `dlfcn.h` at
+  [dynamic-libraries](dynamic-libraries.md) D1, sockets with networking. Stage 2
+  is not one milestone but the standing rule that **every system capability
+  gets its C spelling when it ships**, so the matrix above drains as the OS
+  grows.
+- **Stage 3 — ecosystem grade**: the point where "portable C program" generally
+  means "builds on danos" (autotools-style probing included). Reaching it is
+  mostly stage 2 compounding, plus the long tail (locale, wide-char,
+  `fnmatch`/`glob`/`regex` — the last three lift from musl like the rest). At
+  this stage, re-evaluate hand-grown-vs-musl-port once with real data; the
+  standing recommendation remains danos-native — musl's bottom assumes Linux
+  syscall semantics, and by stage 3 the danos bottom exists and is tested —
+  with musl continuing as the quarry for computation code.
+
+Two boundaries are permanent and worth stating at every stage: **`fork` never
+comes** — danos is a spawn-shaped OS, and `fork`'s address-space-duplication
+semantics are hostile to everything from capabilities to threads; software that
+hard-requires `fork` (not `posix_spawn`) stays off the platform. And the
+**public ABI stays the vDSO + IPC protocols** — a full libc is a compatibility
+*layer*, not a second stable system ABI.
+
+## Milestone slicing
+
+1. **sysroot-skeleton** — layout under `library/c/` (a build package:
+   `include/`, Zig sources, vendored musl subtree); `crt0`; string/mem +
+   `ctype` lifted; a `build.zig` step making C binaries first-class targets.
+   *Test:* a C program using only computation links and runs in QEMU
+   (`c-hello` printing via a raw `write` extern to `debug_write`).
+2. **fd-plumbing** — `errno`; open/read/write/close/lseek/stat/unlink/mkdir/
+   rename over the `runtime` VFS client; `getcwd`/`chdir`/`getenv`/
+   `getentropy` arriving as P1 lands them (stubbed truthfully until then:
+   `getenv` empty, `getentropy` `ENOSYS`). *Test:* QEMU `c-file-io` — create,
+   write, reopen, read back, stat size + mtime through FAT.
+3. **malloc** — free-list allocator over danos `mmap`; `calloc`/`realloc`/
+   `free`; alignment guarantees documented. *Test:* host + QEMU allocator
+   torture (interleaved sizes, realloc growth, alignment asserts).
+4. **stdio** — `FILE*`, buffering modes, the lifted printf/scanf engines wired
+   to the fd slots; `snprintf` family; stdin/stdout/stderr over fd 0/1/2.
+   *Test:* host round-trip suite for format engines (especially `%.17g`
+   float round-trip); QEMU `c-stdio` cooked-line echo once P1's console exists.
+5. **mathematics-and-time** — libm lifted wholesale; `strtod`/`strtol`;
+   `clock_gettime` (monotonic + realtime over `clock`/`wall_clock`);
+   `gmtime`/`mktime`/`strftime` (UTC only — no timezone database);
+   `setjmp`/`longjmp`; `qsort`/`bsearch`; `abort`/`assert`. *Test:* host
+   `strtod`/`dtoa` vectors against known-hard cases; QEMU `c-time` sanity
+   against the wall clock.
+
+Slices 1, 3, 4-host, and 5-host have **no dependency on P1** and can start
+immediately; slice 2 and the QEMU halves interleave with P1 as it lands.
+
+**Exit for the layer as a whole** (= P0's exit): `c-hello` and `c-file-io` green
+in the QEMU suite, and the host-side computation tests green — at which point P2
+(CPython configure) becomes the layer's real integration test.
+
+## Testing strategy: two targets, on purpose
+
+The computation layer is target-independent, so it is unit-tested **on the host**
+(built for the host triple, compared against the host libc's answers —
+thousands of cheap oracle checks for `strtod`, `printf`, libm edge cases). The
+plumbing layer only means anything **on danos**, so it is tested in the QEMU
+suite like every other subsystem. Keeping the split explicit stops the slow-QEMU
+suite from absorbing tests that a host `zig test` runs in milliseconds.
+
+## Risks and gotchas
+
+- **CPython's configure may insist on pthreads.** WASI-class targets build
+  threadless, but verify this *first* in P2 bring-up; the fallback is a
+  truthfully-single-threaded `pthread.h` stub set (create returns `EAGAIN`,
+  mutexes are no-ops — valid when only one thread can exist). Decide from
+  evidence, not assumption.
+- **`long double` is x87 80-bit on x86-64.** musl's libm handles it, but keep
+  CPython away from it (`configure` uses `double` throughout by default);
+  don't hand-write anything touching x87.
+- **errno is a contract, not a convention.** The Zig plumbing must map every
+  `runtime` error to a POSIX name consistently — CPython turns errno into
+  exception types (`FileNotFoundError` is `ENOENT`). One table, tested.
+- **`malloc` alignment**: 16-byte minimum on x86-64 (SSE spills in
+  compiled C). The free-list must guarantee it from day one; retrofitting
+  alignment bugs out of an allocator is misery.
+- **Vendoring discipline.** The musl subtree is lift-only — never edited in
+  place (patches live beside it if ever needed), pinned to one musl release,
+  with the file list documented so a version bump is a re-copy, not an
+  archaeology dig.
+- **stdio buffering vs. crashes.** Buffered stdout + a crashing program eats
+  output — the classic debugging trap. `stderr` stays unbuffered (per C
+  standard) and `exit`/`abort` flush; document that `_exit` does not.
+
+## Decisions needing sign-off
+
+- **Lift-from-musl for all pure computation** (vendored, pinned, unedited) rather
+  than writing or porting whole-musl.
+- **Hand-written danos-native headers**; danos errno values are the numbering.
+- **`library/c/` as a build package** producing both the sysroot and the
+  first-class C-binary build step.
+- The **deliberate-absence table** as the living compatibility matrix, drained
+  by the three-stage road above — with exactly one permanent "never": `fork`.
+- **Full coverage as the end state** (stage 3), reached by the standing rule
+  that every system capability ships with its C spelling — not by a musl port.
+
+## Related
+
+- [python-on-danos-milestones.md](python-on-danos-milestones.md) — this is P0.
+- [dynamic-libraries.md](dynamic-libraries.md) — ships in this sysroot
+  (`dlfcn.h` + the loader) once its D1 lands.
+- [python-on-danos.md](python-on-danos.md) — the design note that scoped the
+  layer.
+- [character-devices-and-tty.md](character-devices-and-tty.md) — P1; supplies
+  the console that makes stdio interactive.
+- [zig-self-hosting.md](zig-self-hosting.md) — the `runtime.os` seam the
+  plumbing layer will re-target when it exists.
+- [os-development/sysv.md](os-development/sysv.md) — the entry stack `crt0`
+  bridges.
diff --git a/docs/character-devices-and-tty.md b/docs/character-devices-and-tty.md
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+# Character devices, the console, and the tty question
+
+A design note for the **stream** half of the device world. danos has block devices
+(the USB storage service behind the FAT mount) but no character devices — and three
+tracks now need them at once: the terminal application, Zig self-hosting Phase 1
+("wire fd 0/1/2 to a console byte stream"), and [Python on danos](python-on-danos.md)
+Phase 1. This note settles what a character device *is* on danos before any of those
+tracks build one.
+
+## The Unix picture, briefly
+
+Unix splits devices in two: **block devices** are seekable arrays of fixed-size
+sectors (disks); **character devices** are unseekable byte streams (keyboards,
+serial ports, terminals, `/dev/null`, entropy). A **tty** is the canonical
+character device — a byte stream plus a *line discipline* (echo, line buffering,
+erase handling, Ctrl-C-to-signal) that lives in the kernel. A **pty** is a pair of
+character devices (master/slave) that exists so a *userspace* program — a terminal
+emulator — can impersonate terminal hardware to the kernel's in-kernel line
+discipline.
+
+The identification asked for and confirmed: yes, tty and pty are character
+devices in this taxonomy.
+
+## The realization that shapes everything: danos already has the mechanism
+
+A Unix character device is an in-kernel dispatch table: major/minor numbers route
+`read()`/`write()` to a driver. danos already has exactly that dispatch — the VFS:
+`fs_resolve` routes a path to a mounted backend service, and `Operation.mount`
+attaches a backend *endpoint* at a prefix. What is missing is not a device model;
+it is **one node kind with stream semantics**. And the protocol already reserved
+it: `NodeKind.character_device = 2` sits unimplemented in
+[vfs-protocol.zig](../library/protocol/vfs/vfs-protocol.zig), exactly like
+`symbolic_link`.
+
+So the design is small:
+
+**A character device on danos is a VFS node, served by an ordinary service over
+the existing VFS wire protocol, whose read/write have stream semantics.**
+
+No device numbers, no `/dev` special casing, no new syscalls, no new protocol —
+a service mounts itself at a path (per the FSH, e.g. `/device/console`), clients
+open it with `runtime.fs` like any file, and `FileStatus.kind` says what it is.
+
+### Stream semantics (the actual contract change)
+
+For a node whose kind is `character_device`:
+
+- **`offset` is ignored** on read and write; there is no seek position. (`lseek`,
+  when the C layer exists, returns `ESPIPE`.)
+- **Reads block** until at least one byte is available, then return what is there —
+  **short reads are normal**, not EOF. A zero-length read reply means the stream
+  is closed (hangup), not end-of-file-at-size.
+- **`FileStatus.size` is 0** and means nothing; `mtime` may be 0.
+- Writes may be short if the service's buffer is full; the client loops as it
+  already must for the 256-byte message cap.
+
+This is a semantics note on existing operations, not a wire change — the `Request`
+and `Reply` structs are untouched. The one true protocol addition is a **`control`
+operation** (appended to `Operation`, values stable): a typed request the stream's
+service interprets. Deliberately *not* an `ioctl` grab-bag — the control payloads
+are enumerated per protocol, starting with the terminal set below.
+
+## The first character device is a pseudo-device
+
+The first device is deliberately **not hardware**: an in-memory **loopback** — a
+byte queue served over the stream contract, where bytes written to one end are
+read from the other. It is the reference implementation of the semantics above
+(blocking reads, short reads, hangup on close, the `control` round-trip), it
+tests deterministically with no QEMU serial scripting, and it keeps hardware off
+the critical path entirely. `null` and `zero` come along nearly for free as
+degenerate cases. This is a decision, not a convenience: the dead-COM1 boot bug
+on real hardware already proved serial cannot be assumed present or alive, so
+**nothing in this milestone writes to COM1**. (A serial-backed stream node can
+exist *later* as one more optional backend for headless debugging; it is on
+nobody's critical path.)
+
+The loopback is also not throwaway — it is the seed of P5's `pipe()`, which is
+the same object with two fds.
+
+## The console service
+
+A `console` service owns the line discipline — **in userspace**, where a
+microkernel wants it, not in the kernel as Unix has it:
+
+- **The discipline is a pure library first**: bytes and key events in, bytes
+  out, no I/O of its own — developed and host-tested against in-memory buffers,
+  then shared verbatim between the console and the future terminal application.
+- **Input**: subscribes to keyboard `InputEvent` IPC (the structured events that
+  exist today) and cooks them into bytes. Cooked mode is the default: echo, line
+  buffering, backspace/erase, so a line is delivered on Enter. Raw mode delivers
+  bytes as they come (the REPL's line editor and any full-screen program need it).
+- **Output is a pluggable sink**, and the stream contract is independent of it:
+  the bring-up sink is in-memory (readable back by tests, mirrored to the boot
+  log), and the real one is the framebuffer text renderer when the display
+  track's font work lands.
+- **Control set** (the `control` payloads): mode raw/cooked, echo on/off, and
+  window-size query — the minimal termios. Ctrl-C-to-signal joins when M17
+  signals-over-IPC lands; until then Ctrl-C is just a byte.
+- Mounts itself at `/device/console` as a `character_device` node.
+
+**fd 0/1/2** then stop being special: spawn hands the child three open handles
+(console by default; anything else if the parent chooses), and `runtime`'s fd
+table maps 0/1/2 to them. `isatty` is simply "does `status` say
+`character_device`" — no side channel needed.
+
+## The pty answer: there is no pty
+
+The pty exists in Unix *because the line discipline is in the kernel* — userspace
+terminal emulators need a kernel gadget to impersonate hardware. On danos the
+terminal emulator is already a userspace server, so the pair collapses:
+
+**The graphical terminal application serves the VFS stream protocol itself and
+hands its own endpoints to the children it spawns as their fd 0/1/2.**
+
+The terminal *is* the console service for its children — same protocol, same
+control set, same line discipline code (shared as a library with the boot
+console). No master/slave device pair, no `/dev/pts`, no new kernel object. When
+CPython arrives, the libc's `isatty`/read/write see a character device and are
+none the wiser; when xonsh eventually wants job control, that lands as control
+messages + M17 signals, still with no pty object.
+
+What this costs: programs that *specifically* manipulate Unix ptys
+(`os.openpty()`, `pexpect`-style tools) have no direct equivalent — the danos
+answer is "spawn the child yourself with your own stream endpoints," which is the
+same capability with less machinery. Accepted.
+
+## Milestone slicing
+
+1. **pseudo-devices** — VFS honors `character_device` semantics end to end;
+   `Operation.control` added; the in-memory **loopback** (plus `null`/`zero`)
+   as the first device. QEMU test: one client writes, another reads — open,
+   offsetless read/write, blocking read, short read, hangup on close, control
+   round-trip. No hardware anywhere.
+2. **console-service** — the line-discipline library (host-tested, pure) plus
+   the console composing keyboard `InputEvent`s with an in-memory output sink;
+   mounted at `/device/console`. QEMU test injects key events and reads cooked
+   lines and raw bytes back through the sink.
+3. **fd-inheritance** — spawn passes 0/1/2 handles; `runtime` fd table; `isatty`
+   via `status`; existing binaries' stdout migrates from `debug_write` to fd 1
+   (the logger keeps its own path).
+4. **terminal-as-server** — deferred to the terminal application milestone
+   (Python track P3): the terminal reuses the discipline library and serves its
+   children directly.
+
+Steps 1–3 are exactly the shared seam that Zig self-hosting Phase 1 and Python
+Phase 1 both list; neither track repeats them.
+
+## Decisions needing sign-off
+
+- **No pty object; the terminal serves its children directly** (the section
+  above) — the load-bearing simplification.
+- **`control` as an enumerated, typed operation** rather than an ioctl-style
+  opaque pass-through.
+- **Line discipline in userspace services** (console + terminal, shared library),
+  never in the kernel.
+
+## Related
+
+- [python-on-danos.md](python-on-danos.md) — consumes this as its Phase 1.
+- [zig-self-hosting.md](zig-self-hosting.md) — ditto ("stdio as fds").
+- [file-system-development/vfs-protocol.md](file-system-development/vfs-protocol.md) —
+  the wire protocol this note extends.
+- [file-system-development/danos-file-system-hierarchy-FSH.md](file-system-development/danos-file-system-hierarchy-FSH.md)
+  — where `/device/console` lives.
+- [device-driver-development/input.md](device-driver-development/input.md) — the
+  `InputEvent` stream the console cooks.
diff --git a/docs/dynamic-libraries.md b/docs/dynamic-libraries.md
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+# Dynamic libraries on danos
+
+A design note and milestone plan for shared objects: building them, loading them
+with `dlopen`, and — the part that needs kernel work — actually *sharing* them
+between processes. Directional, post-P5 of
+[python-on-danos-milestones.md](python-on-danos-milestones.md); nothing on the
+CPython bring-up path depends on it.
+
+## Reconciling the earlier "rejected"
+
+Dynamic libraries were evaluated once before and rejected — but as an answer to a
+*different question*: whether they could claw back ReleaseSafe's measured ~2×
+code size. They cannot (the safety checks inline at every call site; no library
+scheme dedups them), and that verdict stands for that question. The reasons to
+build them now are the ones that investigation never weighed:
+
+- **`ctypes` and runtime FFI** — Python calling into a danos library without
+  rebuilding the interpreter. This is the piece that makes Python prototyping
+  self-serve: drop a `.so` on the image, `ctypes.CDLL` it, iterate.
+- **Loadable CPython extension modules** — today every C extension means
+  relinking the interpreter (`Modules/Setup`); with `dlopen`, an extension is a
+  file.
+- **One interpreter image, many Python services** — a statically-linked CPython
+  is tens of megabytes *per process*. A shared `libpython` mapped read-only once
+  (milestone D3 below) makes Python services cheap enough to be the default way
+  to prototype one.
+- **Plugin-shaped applications** — the UI toolkit and the terminal will want
+  them eventually.
+
+The scoping that dissolves the apparent contradiction is the **size doctrine**:
+leanness is an *operating-system* property — the kernel and system services stay
+small and statically linked, and none of them ever link the loader — while
+*applications* have their own budget and may be big. Dynamic libraries are an
+**application-layer facility**, full stop.
+
+What also does **not** change: the public ABI stays the vDSO + the IPC
+protocols. Shared objects are artifacts *within* one system image, versioned by
+the build — not a new stable ABI surface for the OS.
+
+## Design
+
+- **Format and codegen are free.** ELF shared objects with position-independent
+  code; `zig cc -fPIC -shared` against the [libdanos-c](c-library-compatibility.md)
+  sysroot already emits them. The work is entirely on the loading side.
+- **The loader lives in userspace, inside the libc.** `dlopen` reads the `.so`
+  through the VFS, maps its segments, applies relocations, resolves symbols
+  against the process and the `DT_NEEDED` dependency graph, runs constructors,
+  returns a handle. No kernel loader changes in v1 — segments land in anonymous
+  `mmap` as private copies.
+- **Bind-now, always.** All relocations resolved at `dlopen` time
+  (`RTLD_NOW` semantics only). Lazy PLT binding buys startup latency danos does
+  not care about, at the price of a writable GOT dance and a much subtler
+  loader. Not worth it; keep it out permanently.
+- **W^X from day one.** Map, relocate, then flip text pages read-execute —
+  which requires memory-protection change (`mprotect`-shaped) in the danos
+  `mmap` surface if it is not already there. No page is ever writable and
+  executable at once.
+- **TLS in shared objects is deferred.** Thread-local storage models
+  (initial-exec vs. general-dynamic) are the deep end of every dynamic linker.
+  v1 refuses a `.so` with a TLS segment; revisit alongside the post-P5 pthread
+  subset, which is when it could matter.
+- **Executables stay static until D4.** v1 is "a static binary that can
+  `dlopen`" — no `PT_INTERP`, no program interpreter, no dynamically-linked
+  `main` binaries. That keeps process startup untouched.
+
+## Milestones
+
+1. **D1 — dlopen in-process.** The `.so` build target; the loader in libdanos-c:
+   map, relocate (`RELATIVE`/`GLOB_DAT`/`JUMP_SLOT`), resolve, constructors;
+   `dlopen`/`dlsym`/`dlerror`/`dlclose`; private anonymous mappings; no TLS.
+   *Test:* QEMU `dlopen-hello` — load a `.so`, call a symbol, unload, reload.
+2. **D2 — the FFI payoff.** `DT_NEEDED` dependency graphs; a **libffi port**
+   (x86-64 SysV assembly is upstream; the port is its closure-allocation paths,
+   which must respect W^X); CPython's `ctypes` enabled; extension modules
+   loadable from file. *Test:* QEMU — a Python script `ctypes.CDLL`s a danos
+   `.so` and round-trips a call; a `.so` extension module imports.
+3. **D3 — actual sharing (the kernel milestone).** Shared read-only file-backed
+   mappings — a page-cache-shaped facility so N processes mapping `libpython`
+   hold one physical copy. This is the memory-win milestone and the only one
+   touching the kernel; design it with the existing shm machinery in view
+   (the shared-fate walks already locked the relevant paths). *Test:* N Python
+   services up; measure physical pages against N× the static baseline.
+4. **D4 — dynamically-linked executables** (optional, evaluate after D3):
+   `PT_INTERP`, a danos program interpreter, and the spawn path teaching the
+   loader about it. Only worth it if the image-size or update story demands it.
+
+## Risks and gotchas
+
+- **Scope creep is the failure mode.** Every dynamic linker grows toward glibc.
+  The fences: bind-now only, no lazy binding ever, no TLS until pthreads demand
+  it, no dlopen-from-memory, no versioned symbols. Each fence removed is a
+  design discussion, not a patch.
+- **Code loading is a security event.** `dlopen` turns file bytes into executable
+  code, so W^X discipline is table stakes and *what may be dlopened* is a
+  capability question — the natural danos answer is that loadability follows VFS
+  readability of the `.so`, and services' images are supervised like any other
+  artifact. Revisit explicitly at D3 when mappings become shared.
+- **`dlclose` is where loaders go to die.** Constructors/destructors,
+  dangling function pointers, re-open identity. Keep v1 semantics honest and
+  simple: `dlclose` runs destructors and unmaps; holding pointers past it is
+  undefined; no reference-counted deferral cleverness.
+- **The ReleaseSafe fact still applies to `.so`s** — a ReleaseSafe shared object
+  carries its inlined checks like any static code; D3's sharing saves *copies*,
+  not check overhead. Size expectations should be set accordingly.
+
+## Decisions needing sign-off
+
+- Dynamic libraries join the roadmap at all (this note exists because the
+  earlier size-motivated rejection was re-opened for ABI/sharing reasons).
+- **Bind-now only; no lazy binding, permanently.**
+- **Loader in userspace libc; kernel involvement only at D3** (shared read-only
+  mappings).
+- **Static executables until D4**, and D4 only on demonstrated need.
+
+## Related
+
+- [c-library-compatibility.md](c-library-compatibility.md) — the sysroot the
+  loader ships in; its absence table gains `dlfcn.h` at D1.
+- [python-on-danos.md](python-on-danos.md) — the `ctypes` story this unlocks.
+- [python-on-danos-milestones.md](python-on-danos-milestones.md) — sequencing;
+  this work is post-P5.
+- [os-development/memory-map.md](os-development/memory-map.md) /
+  [os-development/paging.md](os-development/paging.md) — where W^X and shared
+  mappings land.
diff --git a/docs/python-on-danos-milestones.md b/docs/python-on-danos-milestones.md
new file mode 100644
index 0000000..3a02886
--- /dev/null
+++ b/docs/python-on-danos-milestones.md
@@ -0,0 +1,192 @@
+# Python on danos: the milestone plan
+
+The execution plan for [python-on-danos.md](python-on-danos.md). That note holds
+the *why* and the design decisions; this one slices the work into milestones with
+concrete deliverables, tests, and exit criteria. Milestones are numbered **P0–P5**
+(track-local — the global M-series stays with the driver/lifecycle tracks).
+
+Dependencies at a glance:
+
+```
+P0 toolchain + mini-libc ──┐
+P1 streams + console + seam ─┴─→ P2 CPython minimal ─→ P3 terminal + REPL
+                                        │                    │
+                                        └─→ P4 danos module  │
+                                             + Python service│
+P5 process control + shell ←─────────────────────────────────┘
+```
+
+P0 and P1 are independent of each other and can proceed in parallel. P1 is shared
+work — it is also Zig self-hosting Phase 1 and the first three slices of
+[character-devices-and-tty.md](character-devices-and-tty.md).
+
+## P0 — Toolchain + the C library compatibility layer
+
+**Goal:** a C hello-world, cross-compiled on the host with `zig cc`, runs on danos.
+
+Design and slicing live in
+[c-library-compatibility.md](c-library-compatibility.md): the **libdanos-c**
+sysroot (hand-written danos-native headers + `libc.a`) as a `library/c/` build
+package — pure computation (string, libm, `strtod`, the printf/scanf engines)
+lifted from a vendored, pinned musl subtree; the OS plumbing written in Zig over
+the `runtime` surface (re-targeting `runtime.os` when the Zig track authors it);
+`malloc` over danos `mmap`; a `crt0` bridging the danos entry shim to C `main`.
+Driven by `zig cc -target x86_64-freestanding-none -isystem` (the triple becomes
+`x86_64-danos` if the Zig fork lands first; nothing else changes).
+
+Its five slices (sysroot-skeleton, fd-plumbing, malloc, stdio,
+mathematics-and-time) carry their own tests — host-side oracle suites for the
+computation layer, QEMU cases (`c-hello`, `c-file-io`, `c-stdio`, `c-time`) for
+the plumbing.
+
+**Exit:** `c-hello` and `c-file-io` green in the QEMU suite; host computation
+tests green.
+
+## P1 — Stream nodes, console, and the seam pieces
+
+**Goal:** the shared Phase-1 surface exists: byte-stream stdio, cwd, environment,
+entropy. Design and slicing live in
+[character-devices-and-tty.md](character-devices-and-tty.md); this milestone is
+its slices 1–3 plus three small seam pieces:
+
+- **cwd/chdir** — per-process current directory used by path resolution (the
+  kernel already anchors a VFS root per `fs_resolve`; the cwd is the same idea,
+  process-scoped, with `getcwd`/`chdir` exposed through `runtime` and the libc).
+- **Environment** — spawn carries an environment block; the SysV entry stack's
+  `envp` slot ([sysv.md](os-development/sysv.md)) stops being empty; `getenv`
+  reads it. An empty block stays valid.
+- **Entropy** — a kernel `entropy` syscall (RDSEED/RDRAND with a jitter fallback,
+  mirroring the TSC-reliability posture of not trusting one CPU feature blindly);
+  the libc exposes `getentropy`.
+
+- **Tests.** QEMU: the character-device tests from the tty note (offsetless
+  read/write, blocking read, cooked/raw control round-trip), plus `cwd-basics`
+  (chdir + relative open), `env-roundtrip` (spawn with env, child reads it),
+  `entropy-sane` (nonzero, changing, correct length).
+
+**Exit:** a C program reads a cooked line from fd 0 and echoes it to fd 1 —
+injected key events in, bytes read back through the console's in-memory sink,
+all under QEMU with no hardware involved — and `getcwd`/`getenv`/`getentropy`
+return real answers.
+
+## P2 — CPython, minimal configuration
+
+**Goal:** `python -c 'print(2**100)'` runs on danos under QEMU.
+
+- Pin **CPython 3.13.x**; vendor as `third-party/cpython/` or fetch via the build
+  (decide with the build-packages conventions).
+- Host build-Python of the same version (`--with-build-python`).
+- `config.site` cache for the cross answers; `config.sub` patch so
+  `x86_64-unknown-danos` parses; a small `configure`/`pyconfig` patch set kept as
+  rebasable diffs, WASI-style.
+- `--disable-shared`; static `Modules/Setup`: `posix errno _io _codecs _weakref
+  time math _stat _collections itertools _functools _locale _sre` plus what the
+  interpreter core insists on; threadless build (WASI precedent).
+- `Lib/` on the FAT image under the FSH (e.g. `/system/python/lib`);
+  `PYTHONHOME` set accordingly; `.pyc` written with **checked-hash
+  invalidation** (FAT's 2-second mtime granularity makes mtime-based validation
+  lie during fast edit-run cycles).
+- `PYTHONHASHSEED` pinned only if P1's entropy slipped — otherwise real
+  hash randomization from day one.
+- **Tests.** QEMU: `python-expr` (the exit criterion), `python-file` (run a
+  script from FAT, write a file, read it back), then a curated slice of CPython's
+  own suite (`test_int`, `test_float`, `test_io`, `test_dict`) as a
+  longer-running target — the suite is the porting harness.
+
+**Exit:** the four QEMU cases green; the CPython test slice green or with a
+short, documented skip list.
+
+## P3 — Terminal + REPL: the first real application
+
+**Goal:** an interactive `python` REPL in a graphical danos terminal — the
+milestone demo for the OS.
+
+- Depends on the display track's font rendering (its stated next step) — until
+  that lands, the REPL is exercised end-to-end through the pseudo-device
+  harness from P1+P2 (scripted input in, output read back), so P2's exit is
+  never blocked on graphics; the graphical terminal is the *interactive* debut.
+- The terminal application: draws with the UI toolkit / display client, consumes
+  keyboard `InputEvent`s, and — per the tty note's load-bearing decision —
+  **serves the VFS stream protocol itself** to its children, reusing the console's
+  line-discipline library. Spawns `python` with its endpoints as fd 0/1/2.
+- Raw mode + the control set give the REPL line editing; window-size control
+  gives it wrapping.
+- **Tests.** QEMU: scripted terminal session (inject key events, assert rendered
+  or captured output). Real-hardware smoke on the Intel box joins the existing
+  checklist.
+
+**Exit:** typing `2+2` into the terminal on the QEMU GPU target prints `4`.
+
+## P4 — The `danos` extension module + a Python service
+
+**Goal:** Python can speak danos: IPC, capabilities, spawn.
+
+- The `danos` module, **written in Zig against `Python.h`**, statically linked
+  via `Modules/Setup`: endpoints (create/send/receive), capability passing,
+  spawn + exit-notification, and the service bootstrap (announce, supervision
+  handshake) — the same surface Zig services use, re-exposed.
+- UI-toolkit bindings as a second module once the toolkit's API settles.
+- Prototype **one real service in Python** — policy-shaped, not data-plane
+  (candidates: hot-plug policy, a settings service) — speaking an existing wire
+  protocol, supervised by the device manager like any service.
+- **Tests.** QEMU: `python-ipc-echo` (Python service echoes over an endpoint, a
+  Zig client asserts), plus the prototype service's own protocol test.
+
+**Exit:** a Python process runs as a supervised danos service exchanging IPC
+with Zig peers.
+
+## P5 — Process control, then the shell
+
+**Goal:** danos can spawn arbitrary programs with arguments and pipes; a small
+Python shell uses it.
+
+The kernel/VFS cluster a shell forces (any shell, any language):
+
+- **exec-of-path** — spawn an arbitrary VFS path, not a named ramdisk binary;
+- **argv/envp** — carried through spawn onto the child's entry stack (env from
+  P1, argv new);
+- **numeric exit status** — extend the exit record beyond the categorical
+  `ExitReason` (the gotcha the Zig roadmap flagged: `WEXITSTATUS` must be real);
+- **fd inheritance + pipes** — a kernel or service pipe (a character device by
+  the tty note's definition) and spawn-time fd mapping.
+
+Then, in order: `subprocess` enabled in CPython (maps onto spawn + the
+exit-notification endpoint — no fork, Windows-style); a **small Python shell** (a
+few hundred lines over `subprocess` + the console: prompt, argv parsing, pipes,
+cwd) as the forcing function that reveals what job control actually needs.
+
+**Explicitly deferred past P5:** the pthread subset over `thread_spawn`/futex,
+signals-in-libc via M17, termios job control (Ctrl-C to foreground child), and
+**xonsh** — which wants all three and is the arc's endpoint, not a milestone.
+
+**Tests.** QEMU: `spawn-argv-exit` (child echoes argv, exits 42, parent sees
+42), `pipe-through` (parent → child → parent), `python-subprocess`, and a
+scripted shell session.
+
+**Exit:** the Python shell runs `program | program` typed at the terminal and
+reports the exit status.
+
+## Post-P5 outlook
+
+Two tracks continue past this plan, each with its own design doc rather than a
+P-number here:
+
+- **Dynamic libraries** ([dynamic-libraries.md](dynamic-libraries.md), D1–D4) —
+  an application-layer facility (the OS stays static and lean): `dlopen` in the
+  libc, then libffi + `ctypes` + loadable extension modules, then shared
+  read-only mappings so N Python services hold one physical `libpython`.
+- **The full C compatibility layer**
+  ([c-library-compatibility.md](c-library-compatibility.md), stages 2–3) — the
+  standing rule that every system capability ships with its C spelling, draining
+  the absence table toward "portable C builds on danos"; `fork` is the one
+  permanent exception.
+
+## Related
+
+- [python-on-danos.md](python-on-danos.md) — the design note this executes.
+- [c-library-compatibility.md](c-library-compatibility.md) — P0's design.
+- [character-devices-and-tty.md](character-devices-and-tty.md) — P1's design.
+- [zig-self-hosting.md](zig-self-hosting.md) — shares P1; its fork makes P0's
+  triple prettier but gates nothing here.
+- [os-development/process-management.md](os-development/process-management.md) —
+  the spawn/exit surface P5 extends.
diff --git a/docs/python-on-danos.md b/docs/python-on-danos.md
new file mode 100644
index 0000000..fc464c1
--- /dev/null
+++ b/docs/python-on-danos.md
@@ -0,0 +1,261 @@
+# Python on danos: the CPython milestone
+
+A design note (not built yet) on bringing **CPython** to danos, compiled with the Zig
+toolchain (`zig cc`). Like [zig-self-hosting.md](zig-self-hosting.md), it is
+forward-looking: it sets a direction and the decisions that follow from it.
+
+## Why Python, and why now
+
+The Zig self-hosting road is gated on a compiler fork and a long std-library seam.
+Python is the **stop-gap that removes the wait**: a working CPython gives danos a way
+to write programs — services, tools, application prototypes — *without* the Zig
+compiler being self-hosted, and it brings the pure-Python package ecosystem along as
+a bonus. The intended division of labour:
+
+- **Zig** — the kernel, drivers, and anything on a data plane (interrupt paths,
+  DMA rings, block I/O). Unchanged.
+- **Python** — the control plane and the prototyping surface: services that are
+  event loops over IPC, policy logic that changes often, application experiments,
+  and eventually the shell.
+
+Python is also the scripting language for the terminal-and-shell arc: the first
+real danos application is planned as a terminal, a terminal wants a shell, a shell
+wants a scripting language — and [xonsh](https://xon.sh) (a shell written in
+Python) marks where that road can end.
+
+### Non-goals
+
+- **No drivers in Python.** Interrupt handling, ring management, and DMA stay in
+  Zig. Python may *supervise and configure* drivers; it does not sit in their hot
+  paths (interpreter overhead and garbage-collection pauses in an interrupt path
+  are disqualifying).
+- **No dynamic loading during bring-up, no `pip`.** The whole arc here ships
+  statically linked. Dynamic libraries are a real *later* milestone
+  ([dynamic-libraries.md](dynamic-libraries.md)) — an application-layer
+  facility that unlocks `ctypes` and loadable extension modules; the operating
+  system itself stays static and lean regardless (the size doctrine below).
+  `pip` stays out either way until a networking track exists.
+- **No fork.** `os.fork` will not exist. This costs almost nothing (see "The
+  spawn model fits").
+
+## The realization that shapes everything: the compiler is not the obstacle
+
+`zig cc` is a full Clang-based C cross-compiler, and CPython is portable C with
+official precedent for stranger targets than danos — the WASI port is upstream
+tier-2, and it runs **without fork, without dynamic loading, and without working
+threads**. Every "CPython can't possibly run there" objection has already been
+answered upstream by a target *more* constrained than danos.
+
+What CPython actually needs is a **C environment**: headers and a `libc.a`. danos
+has neither — and that is the whole project. In the language of the Zig roadmap's
+three doors, this is the **door-2-shaped work** (the deferred "musl door"), not the
+`std.os.danos` seam: CPython never touches Zig's std.
+
+### The same surface, a third time
+
+The Zig roadmap observed that door 1 (`std.os.danos`) and door 2 (a libc) implement
+the *same* ~30 danos-facing operations at different layers. CPython consumes that
+identical surface through C spellings. So nothing here is throwaway: the
+danos-native operations backing `runtime.os` are the same ones the libc bottoms out
+in, and the gaps this track must close (stdio byte streams, cwd, environment,
+entropy) are **exactly the Phase-1 gaps the Zig roadmap already lists**. The two
+tracks share a road until Python forks off at "build the libc."
+
+## Where danos stands: coverage vs. the gaps
+
+Judged against the minimal CPython configuration (static, WASI-like):
+
+| CPython need | danos today | Gap |
+|--------------|-------------|-----|
+| open/read/write/close/lseek, readdir | VFS + FAT via `runtime.fs` | none — wrap in C |
+| mkdir / unlink / rename / truncate | done (self-hosting Phase 2) | none |
+| stat with mtime | done (`wall_clock` + FAT mtime) | none |
+| mmap/munmap (object allocator) | native syscalls | none |
+| monotonic + wall clock | `clock` + `wall_clock` syscalls | none |
+| a place for `Lib/` | FAT boot image | none — better than WASI has it |
+| fork / exec | not needed (subprocess disabled at first) | — |
+| dynamic loading | not needed (static extension modules) | — |
+| getcwd / chdir | — | **missing** (shared with Zig Phase 1) |
+| environment variables | `Init` has no env | **missing** (can start empty) |
+| entropy | — | **missing** (hash seed; `PYTHONHASHSEED` pins it meanwhile) |
+| byte-stream stdin/stdout (fd 0/1/2) | `debug_write` out; structured `InputEvent` in | **missing** (shared with Zig Phase 1; the REPL needs it) |
+| signals | — | stubs suffice (WASI precedent); M17 signals-over-IPC maps on later |
+| threads | native `thread_spawn`/futex | build threadless first; a pthread subset later (xonsh needs it) |
+
+The clustering repeats the Zig roadmap's: **files, memory, and time are done; the
+work is the C packaging plus the small seam pieces** (tty bytes, cwd, env, entropy).
+
+## The libc decision: hand-rolled in Zig, computation lifted from musl
+
+Two viable shapes were considered:
+
+| Option | What it is | Verdict |
+|--------|-----------|---------|
+| **Mini-libc in Zig** | C-ABI-exporting Zig library over `runtime.os`/`runtime.fs`, shipped as headers + `libc.a`. | **Take this.** Reuses the danos-native surface directly; no Linux assumptions to fight. |
+| **Port musl** | Full musl with a danos syscall backend. | Defer, again. musl assumes Linux syscall semantics in places; heavier than the need. |
+
+The trick that makes the mini-libc tractable: musl's `string/`, `math/` (libm —
+CPython needs essentially all of it), and number-conversion layers are **pure
+computation with no syscalls**. Lift those wholesale (MIT-licensed, designed to
+compile standalone) and hand-write only:
+
+- the OS-facing bottom: fds, `mmap`, clocks, `exit`, `getcwd` — thin C-ABI wrappers
+  over `runtime.os`;
+- a `FILE*` stdio layer (buffered, over the fd layer);
+- `malloc` over danos `mmap` (a simple allocator is fine; CPython does its own
+  small-object arena management above it);
+- the headers (`stdio.h`, `stdlib.h`, `string.h`, `math.h`, `errno.h`, …).
+
+Estimate: **100–150 functions**, of which the hard 40% (libm, string, printf/strtod
+cores) are lifted, not written. Correctness hot spots are `strtod`/`dtoa` (Python's
+float repr round-trips through them) — another reason to lift musl's, not improvise.
+
+## C interop: static extension modules, not ctypes
+
+"Python can interface with C libraries" is true on danos with one important
+correction: **`ctypes` does not work at first** — it is built on `dlopen` + libffi,
+both of which arrive only with the [dynamic-libraries](dynamic-libraries.md)
+milestone (D2). Until then the interop story is the other, older one:
+
+- **Extension modules statically linked into the interpreter** via CPython's
+  `Modules/Setup` mechanism (the standard route for embedded/static builds).
+- **Zig speaks C ABI natively**, so danos extension modules are written in Zig
+  against `Python.h` — no C required. Two modules are planned from the start:
+  - **`danos`** — the system module: endpoints, send/receive, capability passing,
+    spawn, exit notification. This is what makes a Python *service* possible: an
+    event loop over IPC, speaking the same wire protocols as Zig services.
+  - **UI toolkit bindings** — the in-progress danos UI toolkit exposed to Python,
+    so application prototypes drive real windows.
+
+The package story follows: **pure-Python packages work** (unpack into
+`Lib/site-packages` on the FAT image); packages with C extensions must be
+cross-compiled and baked into the interpreter — a curated set chosen per image,
+not `pip install`. That is the honest shape of the stop-gap.
+
+## The roadmap
+
+### Phase 0 — Toolchain + libc bring-up
+
+`zig cc -target x86_64-freestanding-none` plus `-isystem` the danos headers and the
+mini-libc archive. No compiler fork required — this track deliberately avoids the
+Zig roadmap's Phase-0 gate (if the fork lands first, the triple becomes a clean
+`x86_64-danos`; nothing else changes). Exit criterion: a **hello-world C program**
+compiles on the host and runs on danos, printing via the libc's `write`.
+
+### Phase 1 — The shared seam pieces
+
+The same list as Zig self-hosting Phase 1, closed once for both tracks:
+
+- fd 0/1/2 as console **byte** streams (output exists as `debug_write`; input is a
+  new small thing — cooked line input first, raw mode when the REPL wants editing);
+- `getcwd`/`chdir`;
+- environment variables (an empty block is a valid start);
+- an entropy syscall or service (until then, builds pin `PYTHONHASHSEED`).
+
+### Phase 2 — Cross-compile CPython, minimal configuration
+
+Pin one CPython release (3.13 — strongest WASI-era cross-compile support). The
+mechanics are well-trodden upstream since 3.11:
+
+- a same-version **build-Python on the host** (`--with-build-python`);
+- a `config.site` cache answering what configure cannot probe cross
+  (`ac_cv_file__dev_ptmx=no` and friends);
+- a `config.sub` patch so `x86_64-unknown-danos` parses;
+- `--disable-shared`, static `Modules/Setup` with a minimal module set
+  (`posix`, `errno`, `_io`, `_codecs`, `time`, `math`, …);
+- `Lib/` shipped on the FAT image; `PYTHONHOME` pointed at it.
+
+Exit criterion: `python -c 'print(2**100)'` runs on danos under QEMU.
+
+### Phase 3 — Terminal + REPL: the first real application
+
+Depends on the display track's font rendering (already its stated next step) and
+Phase 1's tty. A terminal emulator drawing a `python` REPL is the milestone demo:
+interactive, self-evidently real, and it needs **zero** process-control machinery.
+
+### Phase 4 — The `danos` module and Python services
+
+Write the `danos` extension module and the UI-toolkit bindings; prototype one real
+service in Python (a policy-shaped one — e.g. hot-plug policy or a settings
+service) speaking the existing IPC protocols. This is the payoff phase for
+"prototyping a service or application."
+
+### Phase 5 — Process control, then the shell
+
+The shell — any shell, in any language — forces the surface danos has deferred so
+far: **exec-of-path, argv/envp passing, numeric exit status (`WEXITSTATUS`, not the
+categorical `ExitReason`), fd inheritance, and pipes.** That is a kernel/VFS
+milestone cluster of its own. Then, in order:
+
+1. `subprocess` enabled in CPython (maps onto danos spawn — see below);
+2. a **small Python shell** (a few hundred lines over `subprocess` + line input, no
+   job control) — the forcing function that reveals which process-control pieces
+   actually matter;
+3. **explicitly deferred:** a pthread subset over `thread_spawn`/futex
+   (create/join/mutex/condition/thread-locals), signals via M17 signals-over-IPC,
+   termios job control — and then **xonsh**, which wants all three.
+
+### The spawn model fits
+
+One genuinely good alignment: **CPython does not need fork.** `subprocess` maps
+cleanly onto a posix_spawn-style model — exactly what danos has — and the existing
+exit-notification-via-endpoint is a *better* fit for `Popen.wait` than Unix's
+`wait` semantics. `os.fork` simply won't exist, as on Windows, and almost nothing
+in practice cares.
+
+## Risks and gotchas
+
+- **Binary size — and the size doctrine that makes it acceptable.** danos's
+  leanness mandate applies to the **operating system**: the kernel and the system
+  services stay small (the kernel is measured in kilobytes, not megabytes), and
+  nothing in this track changes that — Python never enters the OS layer. An
+  **application** budget is different: a statically-linked CPython with its
+  module set will be tens of megabytes in ReleaseSafe (the measured ~2×
+  safety-check factor compounds it), and that is *allowed* — applications live
+  on the FAT image, not in the kernel's world. It still shapes the image, and it
+  means every Python service shares one interpreter binary + per-service
+  scripts, so the spawn model needs **argv** before "run this .py" works at all.
+- **FAT mtime granularity is 2 seconds.** CPython's `.pyc` cache validation is
+  mtime-based by default; a rapid edit-run cycle can see stale bytecode. Use
+  hash-based `.pyc` invalidation (PEP 552, `--invalidation-mode checked-hash` at
+  freeze time) or accept the quirk during bring-up.
+- **FAT name lookups are case-insensitive.** Long file names preserve case but
+  match insensitively — the same world Python inhabits on Windows/macOS, so
+  importlib copes, but two modules differing only by case cannot coexist on the
+  image.
+- **`strtod`/float repr correctness.** Python's float round-tripping is exacting;
+  lift musl's conversions rather than writing them, and run CPython's float tests
+  early.
+- **Threadless build is load-bearing, initially.** Like WASI, the first builds have
+  no working `threading`. The escape hatch is real (danos has native threads and
+  futexes; a pthread subset is Phase-5 work) but keep the configuration honestly
+  single-threaded until then.
+- **The test suite is the porting harness.** CPython ships its own conformance
+  suite; getting `test_builtin`, `test_int`, `test_float`, `test_io` running on
+  danos early converts "it seems to work" into a checklist. Budget image space for
+  the test `Lib/` tree during bring-up.
+- **Entropy before exposure.** `PYTHONHASHSEED=0` is fine for bring-up and wrong
+  forever; hash randomization exists because attacker-controlled dict keys are a
+  denial-of-service vector. Land the entropy source before any Python service
+  parses external input.
+
+## Related
+
+- [python-on-danos-milestones.md](python-on-danos-milestones.md) — the execution
+  plan (P0–P5) for this note.
+- [c-library-compatibility.md](c-library-compatibility.md) — the mini-libc
+  (libdanos-c) design behind Phase 0.
+- [character-devices-and-tty.md](character-devices-and-tty.md) — the stream-node /
+  console / no-pty design behind Phase 1.
+- [zig-self-hosting.md](zig-self-hosting.md) — the sibling track; shares Phase 1,
+  diverges at the libc.
+- [os-development/syscall.md](os-development/syscall.md) — the kernel ABI the
+  mini-libc bottoms out in.
+- [os-development/vdso.md](os-development/vdso.md) — the public ABI boundary the
+  `danos` extension module wraps.
+- [os-development/sysv.md](os-development/sysv.md) — the entry stack (argv/envp)
+  the spawn-argv work extends.
+- [device-driver-development/ipc.md](device-driver-development/ipc.md) — the IPC
+  surface Python services speak.
+- [file-system-development/danos-file-system-hierarchy-FSH.md](file-system-development/danos-file-system-hierarchy-FSH.md)
+  — where `Lib/` and `site-packages` land on the image.