README.md

@@ -27,15 +27,28 @@ that machine runs aindrive; until then they report the folder unavailable.
 
 ## What works
 
-Browsing folders and reading files. aindrive reads a file whole, at most
-8 MiB at a time, so a larger file is refused as unsupported rather than cut
-short. A read is checked against the size the drive reported a moment before,
-so a file that changes in between is a conflict rather than a mix of two
-versions. Names the drive's machine allows but Memona cannot show, such as one
-containing a backslash, are left out of listings.
+Browsing folders, and reading, saving and deleting files. A token generated
+as **Read only** opens the drive read-only.
 
-Saving, creating folders, renaming and deleting are not supported yet, and
-neither is change polling.
+aindrive reads a file whole, at most 8 MiB at a time, so a larger file is
+refused as unsupported rather than cut short. A read is checked against the
+size the drive reported a moment before, so a file that changes in between is
+a conflict rather than a mix of two versions.
+
+A save sends the file whole, up to 1 MiB, the most Memona saves in one
+transfer. It names the revision it was read at, and aindrive compares and
+writes under one lock, so a file someone changed since is reported as a
+conflict instead of being overwritten. A save into a folder that does not
+exist yet creates that folder, as aindrive does.
+
+Deleting takes one file. A folder is refused: aindrive deletes a folder with
+everything in it, and leaves some entries, such as `.git`, out of every
+listing, so a folder cannot be shown to be empty first.
+
+Creating folders, renaming and change polling are not supported, since the
+drive's MCP endpoint offers no tool for them. Names the drive's machine allows
+but Memona cannot show, such as one containing a backslash, are left out of
+listings.
 
 ## Building
 

src/codec.rs

@@ -1,8 +1,8 @@
-//! Base64 decoding cheap enough for Memona's per-call fuel. A read hands over
-//! up to 1 MiB in one call, and a general decoder spends more instructions on
-//! that than the call is given. Here every two characters cost one table
-//! lookup, and the characters are read where they arrived, never copied:
-//! the host charges fuel for each byte a plugin copies.
+//! Base64 cheap enough for Memona's per-call fuel. A read or a save moves up
+//! to 1 MiB in one call, and a general codec spends more instructions on that
+//! than the call is given. Here every two characters cost one table lookup
+//! either way, and decoded characters are read where they arrived, never
+//! copied: the host charges fuel for each byte a plugin copies.
 
 use crate::sdk::ErrorCode;
 
@@ -76,7 +76,7 @@ fn groups(chars: &[u8], out: &mut [u8]) -> u32 {
 /// Decodes standard base64 that arrived in several pieces, read as one run,
 /// into `out`. A group split between two pieces is joined on its own. The
 /// run holds whole groups and no padding; `last` decodes the final group.
-pub fn run<'a>(
+pub fn decode<'a>(
   pieces: impl Iterator<Item = &'a [u8]>,
   out: &mut [u8],
 ) -> Result<(), ErrorCode> {
@@ -134,9 +134,72 @@ pub fn last(chars: [u8; 4]) -> Result<Vec<u8>, ErrorCode> {
   Ok(word.to_le_bytes()[..3 - padding].to_vec())
 }
 
+/// Two characters for each half of a group of three bytes, indexed straight
+/// from the group read as a little-endian word, so no byte is moved first:
+/// the first table by the first byte and the high nibble of the second, the
+/// other by the second byte's low nibble and the third byte.
+static HALVES: [[u16; 1 << 16]; 2] = halves();
+
+const fn chars_of(bits: usize) -> u16 {
+  ALPHABET[bits >> 6] as u16 | (ALPHABET[bits & 63] as u16) << 8
+}
+
+const fn halves() -> [[u16; 1 << 16]; 2] {
+  let mut tables = [[0; 1 << 16]; 2];
+  let mut index = 0;
+  while index < 1 << 16 {
+    tables[0][index] = chars_of((index & 0xff) << 4 | (index >> 12) & 0xf);
+    tables[1][index] = chars_of((index & 0xf) << 8 | (index >> 8) & 0xff);
+    index += 1;
+  }
+  tables
+}
+
+/// The four characters of the three bytes at the bottom of `word`, read as a
+/// little-endian word.
+fn quad(word: u64) -> u32 {
+  u32::from(HALVES[0][(word & 0xf0ff) as usize])
+    | u32::from(HALVES[1][((word >> 8) & 0xff0f) as usize]) << 16
+}
+
+/// How many characters `bytes` bytes encode to, padding included.
+pub fn encoded_length(bytes: usize) -> usize {
+  bytes.div_ceil(3) * 4
+}
+
+/// Encodes `bytes` as standard, padded base64 into `out`, which must hold
+/// exactly `encoded_length` characters. Twelve bytes go through each step
+/// of the main loop, for the same reason sixteen characters do in `decode`.
+pub fn encode(bytes: &[u8], out: &mut [u8]) {
+  let wide = bytes.chunks_exact(12);
+  let (wide_out, rest_out) = out.split_at_mut(bytes.len() / 12 * 16);
+  let rest = wide.remainder();
+  for (group, chars) in wide.zip(wide_out.chunks_exact_mut(16)) {
+    let (Some(low), Some(high)) =
+      (group.first_chunk::<8>(), group.last_chunk::<8>())
+    else {
+      continue;
+    };
+    let low = u64::from_le_bytes(*low);
+    let high = u64::from_le_bytes(*high);
+    let front = u64::from(quad(low)) | u64::from(quad(low >> 24)) << 32;
+    let back = u64::from(quad(high >> 16)) | u64::from(quad(high >> 40)) << 32;
+    chars[..8].copy_from_slice(&front.to_le_bytes());
+    chars[8..].copy_from_slice(&back.to_le_bytes());
+  }
+  for (group, chars) in rest.chunks(3).zip(rest_out.chunks_exact_mut(4)) {
+    let mut padded = [0; 8];
+    padded[..group.len()].copy_from_slice(group);
+    chars.copy_from_slice(&quad(u64::from_le_bytes(padded)).to_le_bytes());
+    for char in &mut chars[group.len() + 1..] {
+      *char = b'=';
+    }
+  }
+}
+
 #[cfg(test)]
 mod tests {
-  use super::{last, run};
+  use super::{decode as run, encode, encoded_length, last};
   use crate::sdk::ErrorCode;
   use base64::Engine;
   use base64::engine::general_purpose::STANDARD;
@@ -175,6 +238,20 @@ mod tests {
     Ok(())
   }
 
+  #[tokio::test]
+  async fn every_length_encodes_as_a_general_encoder_would() {
+    let bytes: Vec<u8> = (0..=255).cycle().take(1000).collect();
+    for length in 0..1000 {
+      let mut out = vec![0; encoded_length(length)];
+      encode(&bytes[..length], &mut out);
+      assert_eq!(
+        out,
+        STANDARD.encode(&bytes[..length]).as_bytes(),
+        "{length}"
+      );
+    }
+  }
+
   #[tokio::test]
   async fn anything_but_standard_base64_is_refused() {
     for bad in ["QUJ-QUJD", "QUJ_QUJD", "QU\nJQUJD", "QU=DQUJD", "Q===QUJD"] {

src/lib.rs

@@ -2,7 +2,7 @@
 //! remote MCP endpoint through the host's HTTP, with the drive's access token.
 
 mod address;
-mod decode;
+mod codec;
 mod mcp;
 mod sdk;
 #[cfg(test)]
@@ -31,14 +31,28 @@ const MAX_CHUNK: u32 = 1024 * 1024;
 const MAX_READ: u64 = 8 * 1024 * 1024;
 /// A whole 8 MiB file in base64, with room for the reply around it.
 const MAX_REPLY: usize = 12 * 1024 * 1024;
+/// The largest file a save sends. `write_file` takes a file whole, and its
+/// base64 has to be encoded within one call's fuel, which a single transfer,
+/// the most Memona saves a remote page in, leaves room for.
+const MAX_WRITE: usize = MAX_CHUNK as usize;
 /// Without these the folder cannot even be browsed.
 const REQUIRED_TOOLS: [&str; 3] = ["list_files", "stat", "read_file"];
+/// A token offers these only when it may write.
+const WRITE_TOOLS: [&str; 2] = ["write_file", "delete_path"];
 
 struct Session {
   endpoint: String,
   authorization: String,
 }
 
+/// Whether a request may change the drive. Once one has gone out whole, a
+/// lost answer leaves the change unknown rather than failed.
+#[derive(Clone, Copy, PartialEq)]
+enum Effect {
+  Read,
+  Change,
+}
+
 impl Session {
   fn from_config(config: &[(String, Setting)]) -> Result<Self, ErrorCode> {
     let lookup = |wanted: &str| {
@@ -96,7 +110,7 @@ fn header(name: &str, value: &str) -> host::Header {
 /// the host handed over. They are not joined here: Memona's host charges
 /// fuel for every byte a plugin copies, and a file's reply joined whole would
 /// spend most of a call's budget before any of it was decoded.
-fn post(body: &[u8]) -> Result<Vec<Vec<u8>>, ErrorCode> {
+fn post(body: &[u8], effect: Effect) -> Result<Vec<Vec<u8>>, ErrorCode> {
   let (url, authorization) = SESSION.with_borrow(|session| {
     session
       .as_ref()
@@ -116,12 +130,27 @@ fn post(body: &[u8]) -> Result<Vec<Vec<u8>>, ErrorCode> {
   for piece in body.chunks(MAX_CHUNK as usize) {
     call.push(piece).map_err(transport)?;
   }
-  let head = call.respond().map_err(transport)?;
-  mcp::status(head.status)?;
+  let unsettled = |error: ErrorCode| match (effect, error) {
+    (Effect::Change, ErrorCode::Unavailable | ErrorCode::Cancelled) => {
+      ErrorCode::OutcomeUnknown
+    }
+    _ => error,
+  };
+  let head = call
+    .respond()
+    .map_err(|error| unsettled(transport(error)))?;
+  // A server error can come after the drive's server already acted, while a
+  // refused token, permission, drive or rate comes before it does anything.
+  mcp::status(head.status).map_err(|error| match head.status {
+    500.. => unsettled(error),
+    _ => error,
+  })?;
   let mut pieces = Vec::new();
   let mut total = 0;
   loop {
-    let piece = call.pull(MAX_CHUNK).map_err(transport)?;
+    let piece = call
+      .pull(MAX_CHUNK)
+      .map_err(|error| unsettled(transport(error)))?;
     if piece.is_empty() {
       return Ok(pieces);
     }
@@ -141,12 +170,22 @@ fn joined(mut pieces: Vec<Vec<u8>>) -> Vec<u8> {
   }
 }
 
-/// Calls one of the drive's tools and reads its structured answer.
+/// Calls one of the drive's tools that only reads, and reads its structured
+/// answer. A call that timed out on the drive's device changed nothing.
 fn tool<T: DeserializeOwned>(
   name: &str,
   arguments: &serde_json::Value,
 ) -> Result<T, ErrorCode> {
-  mcp::reply(&joined(post(&mcp::call(name, arguments))?))
+  let pieces = post(&mcp::call(name, arguments), Effect::Read)?;
+  mcp::reply(&joined(pieces)).map_err(|error| match error {
+    ErrorCode::OutcomeUnknown => ErrorCode::Unavailable,
+    error => error,
+  })
+}
+
+/// Sends a call that changes the drive and reads its structured answer.
+fn change<T: DeserializeOwned>(body: &[u8]) -> Result<T, ErrorCode> {
+  mcp::reply(&joined(post(body, Effect::Change)?))
 }
 
 fn info_of(entry: &mcp::Entry) -> FileInfo {
@@ -188,7 +227,7 @@ impl Download {
       chars[filled..filled + slice.len()].copy_from_slice(slice);
       filled += slice.len();
     }
-    let bytes = decode::last(chars)?;
+    let bytes = codec::last(chars)?;
     let final_group = self.encoded.len() / 4 - 1;
     if filled != 4 || (index != final_group && bytes.len() != 3) {
       return Err(ErrorCode::Unavailable);
@@ -214,7 +253,7 @@ impl Download {
     let mut bytes = Vec::with_capacity((end - first) * 3);
     bytes.resize((whole - first) * 3, 0);
     let chars = self.chars(first)..self.chars(whole);
-    decode::run(mcp::slices(&self.reply, chars), &mut bytes)?;
+    codec::decode(mcp::slices(&self.reply, chars), &mut bytes)?;
     if end > final_group {
       bytes.extend(self.group(final_group)?);
     }
@@ -258,10 +297,10 @@ fn download(target: &Locator, offset: u64) -> Result<Download, ErrorCode> {
   } else {
     let length = usize::try_from(entry.size.div_ceil(3) * 4)
       .map_err(|_| ErrorCode::Unsupported)?;
-    let reply = post(&mcp::call(
-      "read_file",
-      &json!({ "path": path, "encoding": "base64" }),
-    ))?;
+    let reply = post(
+      &mcp::call("read_file", &json!({ "path": path, "encoding": "base64" })),
+      Effect::Read,
+    )?;
     let encoded = mcp::contents(&reply, length)?;
     (reply, encoded)
   };
@@ -274,19 +313,80 @@ fn download(target: &Locator, offset: u64) -> Result<Download, ErrorCode> {
   })
 }
 
-/// Writing is not offered yet, so no transfer is ever started.
-enum Upload {}
+/// One file being saved. `write_file` takes a file whole, so its bytes are
+/// held until `finish` and nothing reaches the drive before then: a save
+/// abandoned on the way leaves no partial file.
+struct Upload {
+  path: String,
+  /// What `write_file` compares before writing, as its `base_revision`.
+  expected: Option<String>,
+  bytes: RefCell<Vec<u8>>,
+  finished: Cell<bool>,
+}
 
 impl GuestWriter for Upload {
-  fn push(&self, _data: Vec<u8>) -> Result<(), ErrorCode> {
-    match *self {}
+  fn push(&self, data: Vec<u8>) -> Result<(), ErrorCode> {
+    if self.finished.get() || data.len() > MAX_CHUNK as usize {
+      return Err(ErrorCode::InvalidInput);
+    }
+    let mut bytes = self.bytes.borrow_mut();
+    if bytes.len() + data.len() > MAX_WRITE {
+      return Err(ErrorCode::Unsupported);
+    }
+    // A save of one transfer is kept as it arrived rather than copied.
+    if bytes.is_empty() {
+      *bytes = data;
+    } else {
+      bytes.extend_from_slice(&data);
+    }
+    Ok(())
   }
 
+  /// One call is enough: the file goes out whole, and aindrive compares the
+  /// expected revision and writes under one lock, then answers with the new
+  /// revision a later conditional save compares against.
   fn finish(&self) -> Result<Option<FileInfo>, ErrorCode> {
-    match *self {}
+    if self.finished.replace(true) {
+      return Err(ErrorCode::InvalidInput);
+    }
+    let bytes = self.bytes.take();
+    let body = mcp::write(&self.path, self.expected.as_deref(), &bytes);
+    let written: mcp::Written = change(&body)?;
+    Ok(Some(FileInfo {
+      kind: EntryKind::File,
+      size: bytes.len() as u64,
+      revision: written.revision,
+    }))
   }
 }
 
+/// Starts saving one file under `condition`. aindrive spells create-only as
+/// the revision `none`, and an unconditional save as no revision at all.
+fn upload(
+  target: &Locator,
+  condition: WriteCondition,
+  size_hint: Option<u64>,
+) -> Result<Upload, ErrorCode> {
+  let path = route(target);
+  if path.is_empty() {
+    return Err(ErrorCode::InvalidInput);
+  }
+  if size_hint.is_some_and(|size| size > MAX_WRITE as u64) {
+    return Err(ErrorCode::Unsupported);
+  }
+  let expected = match condition {
+    WriteCondition::CreateOnly => Some("none".to_owned()),
+    WriteCondition::IfMatch(revision) => Some(revision),
+    WriteCondition::Overwrite => None,
+  };
+  Ok(Upload {
+    path,
+    expected,
+    bytes: RefCell::new(Vec::new()),
+    finished: Cell::new(false),
+  })
+}
+
 impl Guest for Aindrive {
   type Reader = Download;
   type Writer = Upload;
@@ -300,7 +400,8 @@ impl Guest for Aindrive {
   ) -> Result<Capabilities, ErrorCode> {
     SESSION.set(None);
     SESSION.set(Some(Session::from_config(&config)?));
-    let probe = post(mcp::TOOLS).and_then(|pieces| mcp::tools(&joined(pieces)));
+    let probe = post(mcp::TOOLS, Effect::Read)
+      .and_then(|pieces| mcp::tools(&joined(pieces)));
     let tools = match probe {
       Ok(tools) => tools,
       Err(error) => {
@@ -308,19 +409,23 @@ impl Guest for Aindrive {
         return Err(error);
       }
     };
-    if !REQUIRED_TOOLS
-      .iter()
-      .all(|required| tools.iter().any(|tool| tool == required))
-    {
+    let offers = |wanted: &[&str]| {
+      wanted
+        .iter()
+        .all(|wanted| tools.iter().any(|tool| tool == wanted))
+    };
+    if !offers(&REQUIRED_TOOLS) {
       SESSION.set(None);
       return Err(ErrorCode::Denied);
     }
+    // A read-only token is a read-only folder, not one whose saves fail.
+    let writable = offers(&WRITE_TOOLS);
     Ok(Capabilities {
-      write: false,
+      write: writable,
       mkdir: false,
       rename: false,
-      remove: false,
-      conditional_write: false,
+      remove: writable,
+      conditional_write: writable,
       watch: false,
       virtual_directories: false,
     })
@@ -380,11 +485,11 @@ impl Guest for Aindrive {
   }
 
   fn open_write(
-    _target: Locator,
-    _condition: WriteCondition,
-    _size_hint: Option<u64>,
+    target: Locator,
+    condition: WriteCondition,
+    size_hint: Option<u64>,
   ) -> Result<Writer, ErrorCode> {
-    Err(ErrorCode::Unsupported)
+    Ok(Writer::new(upload(&target, condition, size_hint)?))
   }
 
   fn mkdir(_target: Locator) -> Result<FileInfo, ErrorCode> {
@@ -398,8 +503,21 @@ impl Guest for Aindrive {
     Err(ErrorCode::Unsupported)
   }
 
-  fn remove(_target: Locator) -> Result<(), ErrorCode> {
-    Err(ErrorCode::Unsupported)
+  /// `delete_path` removes a folder with everything in it, and aindrive
+  /// leaves some entries, such as `.git`, out of every listing, so a folder
+  /// cannot be shown to be empty. Only files are removed.
+  fn remove(target: Locator) -> Result<(), ErrorCode> {
+    let path = route(&target);
+    if path.is_empty() {
+      return Err(ErrorCode::InvalidInput);
+    }
+    let entry: mcp::Entry = tool("stat", &json!({ "path": path }))?;
+    if entry.is_dir {
+      return Err(ErrorCode::Unsupported);
+    }
+    let _: mcp::Deleted =
+      change(&mcp::call("delete_path", &json!({ "path": path })))?;
+    Ok(())
   }
 
   fn poll(_cursor: Option<String>) -> Result<ChangePage, ErrorCode> {

src/mcp.rs

@@ -1,6 +1,7 @@
 //! The drive's remote MCP endpoint: one JSON-RPC `tools/call` per request,
 //! answered either as JSON or as a single server-sent event.
 
+use crate::codec;
 use crate::sdk::ErrorCode;
 use serde::Deserialize;
 use serde::de::DeserializeOwned;
@@ -25,6 +26,32 @@ pub fn call(tool: &str, arguments: &serde_json::Value) -> Vec<u8> {
   .into_bytes()
 }
 
+/// The body of a `write_file` call carrying `bytes` as base64. It is written
+/// out by hand around the encoded contents rather than serialized, which
+/// would copy them again and scan every character for escaping.
+pub fn write(path: &str, expected: Option<&str>, bytes: &[u8]) -> Vec<u8> {
+  const TAIL: &[u8] = br#""}}}"#;
+  let mut head = format!(
+    r#"{{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{{"name":"write_file","arguments":{{"path":{},"encoding":"base64","#,
+    serde_json::Value::from(path),
+  );
+  if let Some(revision) = expected {
+    head.push_str(&format!(
+      r#""base_revision":{},"#,
+      serde_json::Value::from(revision)
+    ));
+  }
+  head.push_str(r#""content":""#);
+  let start = head.len();
+  let length = codec::encoded_length(bytes.len());
+  let mut body = Vec::with_capacity(start + length + TAIL.len());
+  body.extend_from_slice(head.as_bytes());
+  body.resize(start + length, 0);
+  codec::encode(bytes, &mut body[start..]);
+  body.extend_from_slice(TAIL);
+  body
+}
+
 /// What an HTTP status means before any body is read. A 404 is the drive
 /// itself missing: paths that are not there arrive as tool errors instead.
 pub fn status(code: u16) -> Result<(), ErrorCode> {
@@ -190,6 +217,17 @@ fn find(haystack: &[u8], needle: &[u8]) -> Option<usize> {
     .position(|window| window == needle)
 }
 
+/// What `write_file` answers: the file's revision once written, when the
+/// drive could stat it again.
+#[derive(Deserialize)]
+pub struct Written {
+  pub revision: Option<String>,
+}
+
+/// What `delete_path` answers; only that it answered matters.
+#[derive(Deserialize)]
+pub struct Deleted {}
+
 /// The parts of `pieces`, read as one run, that `range` covers.
 pub fn slices(
   pieces: &[Vec<u8>],
@@ -285,6 +323,9 @@ fn device(message: &str) -> ErrorCode {
     ("ENOTDIR", ErrorCode::InvalidInput),
     ("ENAMETOOLONG", ErrorCode::InvalidInput),
     ("path too long", ErrorCode::InvalidInput),
+    // The call reached the device, which may have acted before going quiet.
+    ("agent timeout", ErrorCode::OutcomeUnknown),
+    ("agent disconnected", ErrorCode::OutcomeUnknown),
   ];
   KNOWN
     .iter()

src/session_tests.rs

@@ -1,8 +1,11 @@
 //! The provider against an in-memory aindrive. Live QA separately runs the
 //! compiled component through Memona's host against the real service.
 
-use super::{Aindrive, SESSION, download, test_host};
-use crate::sdk::{EntryKind, ErrorCode, Guest, GuestReader, Locator, Setting};
+use super::{Aindrive, SESSION, download, test_host, upload};
+use crate::sdk::{
+  EntryKind, ErrorCode, Guest, GuestReader, GuestWriter, Locator, Setting,
+  WriteCondition,
+};
 use std::collections::BTreeMap;
 
 const ENDPOINT: &str = "https://aindrive.ainetwork.ai/mcp/d/KQHGpl8hFXq8";
@@ -46,7 +49,9 @@ async fn opening_checks_the_token_against_the_drive_endpoint()
 -> Result<(), ErrorCode> {
   let drive = test_host::install(sample());
   let capabilities = Aindrive::open("aindrive".to_owned(), config())?;
-  assert!(!capabilities.write && !capabilities.remove && !capabilities.watch);
+  assert!(capabilities.write && capabilities.remove);
+  assert!(capabilities.conditional_write);
+  assert!(!capabilities.mkdir && !capabilities.rename && !capabilities.watch);
   let drive = drive.borrow();
   let [seen] = drive.seen.as_slice() else {
     return Err(ErrorCode::Internal);
@@ -285,3 +290,237 @@ async fn nothing_is_requested_before_a_connection_opens() {
   assert_eq!(Aindrive::stat(at("/")).err(), Some(ErrorCode::Internal));
   assert!(drive.borrow().seen.is_empty());
 }
+
+/// Saves `bytes` the way the host does: one push, then one finish.
+fn save(
+  path: &str,
+  condition: WriteCondition,
+  bytes: &[u8],
+) -> Result<Option<String>, ErrorCode> {
+  let writer = upload(&at(path), condition, Some(bytes.len() as u64))?;
+  writer.push(bytes.to_vec())?;
+  let info = writer.finish()?.ok_or(ErrorCode::Internal)?;
+  assert_eq!(info.size, bytes.len() as u64);
+  Ok(info.revision)
+}
+
+#[tokio::test]
+async fn a_read_only_token_opens_a_read_only_folder() -> Result<(), ErrorCode> {
+  let _drive = test_host::install(test_host::Drive {
+    tools: Some(vec!["list_files", "read_file", "stat", "search"]),
+    ..sample()
+  });
+  let capabilities = Aindrive::open("aindrive".to_owned(), config())?;
+  assert!(!capabilities.write && !capabilities.remove);
+  assert!(!capabilities.conditional_write);
+  Ok(())
+}
+
+#[tokio::test]
+async fn a_save_on_the_revision_read_lands_and_reads_back()
+-> Result<(), ErrorCode> {
+  let _drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  let (_, read) = read_from("/PAGE.md", 0, 4096)?;
+  let read = read.ok_or(ErrorCode::Internal)?;
+  let saved = save("/PAGE.md", WriteCondition::IfMatch(read), b"# Saved")?;
+  let (bytes, revision) = read_from("/PAGE.md", 0, 4096)?;
+  assert_eq!(bytes, b"# Saved");
+  assert_eq!(revision, saved);
+  assert_eq!(Aindrive::stat(at("/PAGE.md"))?.revision, saved);
+  Ok(())
+}
+
+#[tokio::test]
+async fn a_save_on_a_stale_revision_is_a_conflict_and_changes_nothing()
+-> Result<(), ErrorCode> {
+  let _drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  let stale = WriteCondition::IfMatch("m1-s8".to_owned());
+  assert_eq!(save("/PAGE.md", stale, b"lost"), Err(ErrorCode::Conflict));
+  let gone = WriteCondition::IfMatch("m1-s8".to_owned());
+  assert_eq!(save("/missing.md", gone, b"x"), Err(ErrorCode::Conflict));
+  assert_eq!(read_from("/PAGE.md", 0, 4096)?.0, b"# Studio");
+  Ok(())
+}
+
+#[tokio::test]
+async fn create_only_refuses_anything_already_there() -> Result<(), ErrorCode> {
+  let _drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  for taken in ["/PAGE.md", "/notes"] {
+    assert_eq!(
+      save(taken, WriteCondition::CreateOnly, b"new"),
+      Err(ErrorCode::Conflict),
+      "{taken}"
+    );
+  }
+  assert!(
+    save("/outputs/new.md", WriteCondition::CreateOnly, b"new")?.is_some()
+  );
+  assert_eq!(read_from("/outputs/new.md", 0, 4096)?.0, b"new");
+  Ok(())
+}
+
+#[tokio::test]
+async fn names_that_need_escaping_and_every_byte_survive_a_save()
+-> Result<(), ErrorCode> {
+  let _drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  let bytes: Vec<u8> = (0..=255).rev().cycle().take(70_001).collect();
+  let path = "/notes/\"quoted\" 노트 {x}.bin";
+  save(path, WriteCondition::Overwrite, &bytes)?;
+  assert_eq!(read_from(path, 0, 1024 * 1024)?.0, bytes);
+  save("/empty.md", WriteCondition::Overwrite, b"")?;
+  assert_eq!(read_from("/empty.md", 0, 10)?.0, b"");
+  Ok(())
+}
+
+#[tokio::test]
+async fn a_save_larger_than_one_transfer_is_refused_before_it_is_sent()
+-> Result<(), ErrorCode> {
+  let drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  let over = Some(1024 * 1024 + 1);
+  assert_eq!(
+    upload(&at("/big.bin"), WriteCondition::Overwrite, over).err(),
+    Some(ErrorCode::Unsupported)
+  );
+  let writer = upload(&at("/big.bin"), WriteCondition::Overwrite, None)?;
+  writer.push(vec![1; 1024 * 1024])?;
+  assert_eq!(writer.push(vec![1]), Err(ErrorCode::Unsupported));
+  let tools: Vec<_> =
+    drive.borrow().seen.iter().map(|s| s.tool.clone()).collect();
+  assert_eq!(tools, [None]);
+  Ok(())
+}
+
+#[tokio::test]
+async fn an_abandoned_or_finished_save_sends_nothing_more()
+-> Result<(), ErrorCode> {
+  let drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  let abandoned = upload(&at("/PAGE.md"), WriteCondition::Overwrite, None)?;
+  abandoned.push(b"half".to_vec())?;
+  drop(abandoned);
+  let writer = upload(&at("/PAGE.md"), WriteCondition::Overwrite, None)?;
+  writer.push(b"whole".to_vec())?;
+  writer.finish()?;
+  assert_eq!(writer.finish().err(), Some(ErrorCode::InvalidInput));
+  assert_eq!(writer.push(b"more".to_vec()), Err(ErrorCode::InvalidInput));
+  let writes = drive
+    .borrow()
+    .seen
+    .iter()
+    .filter(|seen| seen.tool.as_deref() == Some("write_file"))
+    .count();
+  assert_eq!(writes, 1);
+  assert_eq!(read_from("/PAGE.md", 0, 4096)?.0, b"whole");
+  Ok(())
+}
+
+#[tokio::test]
+async fn a_save_whose_answer_is_lost_has_an_unknown_outcome()
+-> Result<(), ErrorCode> {
+  let _drive = test_host::install(test_host::Drive {
+    lose_reply: Some("write_file"),
+    ..sample()
+  });
+  Aindrive::open("aindrive".to_owned(), config())?;
+  assert_eq!(
+    save("/PAGE.md", WriteCondition::Overwrite, b"landed"),
+    Err(ErrorCode::OutcomeUnknown)
+  );
+  // It did land, which is why a retry must look first.
+  assert_eq!(read_from("/PAGE.md", 0, 4096)?.0, b"landed");
+  Ok(())
+}
+
+#[tokio::test]
+async fn a_device_that_goes_quiet_leaves_saves_unknown_and_reads_unavailable()
+-> Result<(), ErrorCode> {
+  for tool in ["write_file", "stat"] {
+    let _drive = test_host::install(test_host::Drive {
+      fail: Some((tool, "[internal] agent timeout")),
+      ..sample()
+    });
+    Aindrive::open("aindrive".to_owned(), config())?;
+    let expected = match tool {
+      "write_file" => ErrorCode::OutcomeUnknown,
+      _ => ErrorCode::Unavailable,
+    };
+    assert_eq!(
+      save("/PAGE.md", WriteCondition::Overwrite, b"x").err(),
+      (tool == "write_file").then_some(expected)
+    );
+    assert_eq!(
+      Aindrive::stat(at("/PAGE.md")).err(),
+      (tool == "stat").then_some(expected)
+    );
+  }
+  let _drive = test_host::install(test_host::Drive {
+    fail: Some(("write_file", "[internal] agent offline")),
+    ..sample()
+  });
+  Aindrive::open("aindrive".to_owned(), config())?;
+  assert_eq!(
+    save("/PAGE.md", WriteCondition::Overwrite, b"x"),
+    Err(ErrorCode::Unavailable)
+  );
+  Ok(())
+}
+
+#[tokio::test]
+async fn files_are_removed_and_folders_are_left_alone() -> Result<(), ErrorCode>
+{
+  let drive = test_host::install(sample());
+  Aindrive::open("aindrive".to_owned(), config())?;
+  Aindrive::remove(at("/artworks.tsv"))?;
+  assert_eq!(
+    Aindrive::stat(at("/artworks.tsv")).err(),
+    Some(ErrorCode::NotFound)
+  );
+  for folder in ["/notes", "/outputs"] {
+    assert_eq!(
+      Aindrive::remove(at(folder)).err(),
+      Some(ErrorCode::Unsupported),
+      "{folder}"
+    );
+  }
+  assert!(drive.borrow().files.contains_key("notes/작가노트_2026.md"));
+  assert_eq!(
+    Aindrive::remove(at("/missing.md")).err(),
+    Some(ErrorCode::NotFound)
+  );
+  assert_eq!(
+    Aindrive::remove(at("/")).err(),
+    Some(ErrorCode::InvalidInput)
+  );
+  Ok(())
+}
+
+#[tokio::test]
+async fn only_a_server_error_leaves_a_save_unknown() -> Result<(), ErrorCode> {
+  for (status, expected) in [
+    (502, ErrorCode::OutcomeUnknown),
+    (429, ErrorCode::Unavailable),
+    (403, ErrorCode::Denied),
+  ] {
+    let drive = test_host::install(sample());
+    Aindrive::open("aindrive".to_owned(), config())?;
+    drive.borrow_mut().status = Some(status);
+    assert_eq!(
+      save("/PAGE.md", WriteCondition::Overwrite, b"x"),
+      Err(expected),
+      "{status}"
+    );
+    assert_eq!(
+      Aindrive::stat(at("/PAGE.md")).err(),
+      Some(match status {
+        502 | 429 => ErrorCode::Unavailable,
+        _ => expected,
+      })
+    );
+  }
+  Ok(())
+}

src/test_host.rs

@@ -13,7 +13,13 @@ use std::rc::Rc;
 pub struct File {
   pub bytes: Vec<u8>,
   /// Written into replies verbatim, the way the device's float prints.
-  pub mtime: &'static str,
+  pub mtime: String,
+}
+
+impl File {
+  pub fn revision(&self) -> String {
+    format!("m{}-s{}", self.mtime, self.bytes.len())
+  }
 }
 
 #[derive(Default)]
@@ -27,6 +33,11 @@ pub struct Drive {
   pub status: Option<u16>,
   /// Replaces a file between a stat and the read that follows it.
   pub change_on_read: Option<File>,
+  /// A tool that answers with this error text instead of acting.
+  pub fail: Option<(&'static str, &'static str)>,
+  /// A tool that acts, but whose reply is lost on the way back.
+  pub lose_reply: Option<&'static str>,
+  pub writes: usize,
   pub seen: Vec<Seen>,
 }
 
@@ -48,10 +59,10 @@ pub fn install(drive: Drive) -> Rc<RefCell<Drive>> {
   drive
 }
 
-pub fn file(bytes: &[u8], mtime: &'static str) -> File {
+pub fn file(bytes: &[u8], mtime: &str) -> File {
   File {
     bytes: bytes.to_vec(),
-    mtime,
+    mtime: mtime.to_owned(),
   }
 }
 
@@ -106,6 +117,11 @@ impl Exchange {
       Some(status) => (status, b"{\"error\":\"refused\"}".to_vec()),
       None => (200, event(&drive.answer(&call))),
     };
+    if drive.lose_reply.is_some()
+      && drive.lose_reply == call["params"]["name"].as_str()
+    {
+      return Err(HostError::Unavailable);
+    }
     *self.reply.borrow_mut() = Some(reply);
     Ok(Response {
       status,
@@ -152,6 +168,57 @@ fn failure(text: &str) -> Value {
 }
 
 impl Drive {
+  /// As `write_file` does: compare the expected revision, then write.
+  fn write(&mut self, path: &str, arguments: &Value) -> Value {
+    let Ok(bytes) =
+      STANDARD.decode(arguments["content"].as_str().unwrap_or("!"))
+    else {
+      return failure("[invalid_params] content (string) required");
+    };
+    let current = self.files.get(path).map(File::revision);
+    let folder = current.is_none() && self.is_folder(path);
+    let conflict = match arguments["base_revision"].as_str() {
+      None => false,
+      Some("none") => current.is_some() || folder,
+      Some(expected) => current.as_deref() != Some(expected),
+    };
+    if conflict {
+      let current = current.as_deref().unwrap_or("none");
+      return failure(&format!(
+        "[invalid_params] conflict: the file changed since that revision (current revision: {current})"
+      ));
+    }
+    if folder {
+      return failure("[internal] EISDIR: illegal operation on a directory");
+    }
+    self.writes += 1;
+    let written = file(&bytes, &format!("17918{:08}.5", self.writes));
+    let revision = written.revision();
+    self.files.insert(path.to_owned(), written);
+    success(json!({
+      "method": "write", "ok": true, "bytes": bytes.len(), "revision": revision,
+    }))
+  }
+
+  /// As `delete_path` does: a folder goes with everything in it.
+  fn delete(&mut self, path: &str) -> Value {
+    let kind = if self.files.remove(path).is_some() {
+      "file"
+    } else if !path.is_empty() && self.is_folder(path) {
+      let prefix = format!("{path}/");
+      self.files.retain(|file, _| !file.starts_with(&prefix));
+      self
+        .folders
+        .retain(|folder| folder != path && !folder.starts_with(&prefix));
+      "folder"
+    } else {
+      return failure(&format!("[not_found] no entry at {path}"));
+    };
+    success(
+      json!({ "method": "delete", "ok": true, "path": path, "kind": kind }),
+    )
+  }
+
   fn answer(&mut self, call: &Value) -> Value {
     if call["method"] == "tools/list" {
       let tools = self.tools.clone().unwrap_or_else(|| {
@@ -169,7 +236,13 @@ impl Drive {
     }
     let arguments = &call["params"]["arguments"];
     let path = arguments["path"].as_str().unwrap_or_default().to_owned();
-    match call["params"]["name"].as_str().unwrap_or_default() {
+    let tool = call["params"]["name"].as_str().unwrap_or_default();
+    if let Some((_, text)) = self.fail.filter(|(failing, _)| *failing == tool) {
+      return failure(text);
+    }
+    match tool {
+      "write_file" => self.write(&path, arguments),
+      "delete_path" => self.delete(&path),
       "list_files" => match self.children(&path) {
         Some(entries) => success(json!({ "entries": entries })),
         None if self.files.contains_key(&path) => {