Make the filament source explicit, and print from the AMS

`start_uploaded_file` hardcoded `use_ams: false` / `ams_mapping: ""`, which
silently overrode whatever the slicer had chosen. On a P1P with an AMS and a
project sliced for it, that is not a missing feature - it points the print at
the external spool. Confirmed on 2026-10-11: the print started, heated to
250C, reached 1%, then the printer paused itself with `print_error: 50364420`
and `tray_now: 255`. Nothing in the command's reply said so; `result` was
`success`, because the command had been accepted.

So `FilamentSource` is now a required argument with no default, because there
is no safe default - picking wrong draws the wrong filament or none. The
`ExternalSpool` variant sends the empty *string* the schema's trailing
`oneOf` demands, and `ams` takes slots per project filament index and pads to
every tray the printer has, per the padding rule in
`schemas/bambulab/v1/common/ams/ams_mapping.schema.json`. It refuses an empty
mapping (padding `[]` would mean `use_ams: true` with nothing mapped), one
longer than the printer has trays, and any negative that is not -1.

Confirmed live with the real mapping: `ams_mapping: [3,-1,-1,-1,-1]`,
`use_ams: true` took the printer to RUNNING, `tray_now` became 3 - the slot
the project was sliced for - and it ran past 1% to 13% with `print_error: 0`,
where the external-spool attempt had paused.

Finding the mapping needed reading the project back off the card, so FTPS
gains `download`/`download_file` and the probe a `fetch` step. The archive's
`Metadata/slice_info.config` lists each plate's filaments with
`tray_info_idx` and colour - here one PETG GFG99 at `#2850E0`, matching AMS
slot 3's `2850E0FF` - and `Metadata/plate_1.gcode` is present, which also
confirms the hardcoded `param`.

`upload_project_file` is left on the external spool and is the one remaining
place that can pick wrong, because it is vendor-generic and an AMS mapping is
a Bambu concept. It is unreachable - there is no start-print command on the
uplink - and the fix is for it to derive the mapping from the `.3mf` it
already holds, which needs a zip reader and so a dependency decision.

src/printer/bambu.rs

@@ -226,6 +226,82 @@ pub struct BambuPrinter {
     connection: Option<(super::bambu_mqtt::BambuMqttSession, BambuProtocolVersion)>,
 }
 
+/// Where a project print draws its filament from.
+///
+/// This exists because the alternative - hardcoding `use_ams: false` -
+/// silently overrode the slicer's own choice. Confirmed on a P1P
+/// (2026-10-11): a project sliced for AMS slot 3 started, heated to
+/// 250C, reached 1% and then the printer paused itself with
+/// `print_error: 50364420` and `tray_now: 255`, the external spool. The
+/// print was never going to work, and nothing in the command's reply
+/// said so - `result` was `success`, because the command was accepted.
+///
+/// So this is a required argument rather than an option with a default.
+/// There is no safe default: picking the wrong source draws the wrong
+/// filament or none at all.
+pub enum FilamentSource {
+    /// The external spool (`vt_tray`). `use_ams: false`, and
+    /// `ams_mapping` must be the empty *string* - the schema's trailing
+    /// `oneOf` ties those two together.
+    ExternalSpool,
+    /// The AMS, with a mapping from the project's filament indices to
+    /// AMS slots. Build it with `FilamentSource::ams`.
+    Ams(Vec<i32>),
+}
+
+impl FilamentSource {
+    /// `slots[i]` is the AMS slot the project's filament index `i` should
+    /// draw from, or `-1` for an index this project does not use.
+    ///
+    /// `tray_count` is every tray the printer has, AMS slots *and* external
+    /// spools - 5 for a P1P with one AMS. The mapping is padded to that
+    /// length with `-1`, because
+    /// `schemas/bambulab/v1/common/ams/ams_mapping.schema.json` says to
+    /// always pad it and an under-length array is one of the ways this goes
+    /// wrong quietly.
+    pub fn ams(slots: impl Into<Vec<i32>>, tray_count: usize) -> Result<Self, PrinterError> {
+        let mut slots: Vec<i32> = slots.into();
+
+        if slots.is_empty() {
+            return Err(PrinterError(
+                "an AMS mapping needs at least one filament index; use ExternalSpool instead".into(),
+            ));
+        }
+        if slots.len() > tray_count {
+            return Err(PrinterError(format!(
+                "an AMS mapping of {} entries cannot fit a printer with {tray_count} trays",
+                slots.len()
+            )));
+        }
+        if let Some(bad) = slots.iter().find(|slot| **slot < -1) {
+            return Err(PrinterError(format!(
+                "{bad} is not a valid AMS slot: use -1 for an unused filament index"
+            )));
+        }
+
+        slots.resize(tray_count, -1);
+        Ok(Self::Ams(slots))
+    }
+
+    /// Writes `use_ams` and `ams_mapping` into a `print.project_file` body.
+    ///
+    /// The two always move together: a real mapping requires
+    /// `use_ams: true`, and the no-AMS case requires `ams_mapping` to be the
+    /// empty string rather than an empty array.
+    fn apply(&self, body: &mut serde_json::Value) {
+        match self {
+            Self::ExternalSpool => {
+                body["use_ams"] = serde_json::Value::Bool(false);
+                body["ams_mapping"] = serde_json::Value::String(String::new());
+            }
+            Self::Ams(slots) => {
+                body["use_ams"] = serde_json::Value::Bool(true);
+                body["ams_mapping"] = serde_json::json!(slots);
+            }
+        }
+    }
+}
+
 impl BambuPrinter {
     pub fn new(
         id: &str,
@@ -541,6 +617,30 @@ impl BambuPrinter {
         self.list_dir(super::bambu_ftps::CACHE_DIR).await
     }
 
+    /// Reads one file back off the printer's SD card.
+    ///
+    /// Read-only, and the counterpart to `upload_file`. Needed because a
+    /// `.3mf` sitting on the card is the only record of which filament its
+    /// plates were sliced for, which is what `ams_mapping` has to agree
+    /// with.
+    pub async fn download_file(&self, path: &str) -> Result<Vec<u8>, PrinterError> {
+        let connector = self
+            .tls
+            .build_connector()
+            .map_err(|err| PrinterError(format!("{}: TLS setup failed: {err}", self.base.name)))?;
+
+        let host = self.base.host.clone();
+        let access_code = self.access_code.clone();
+        let path = path.to_string();
+
+        tokio::task::spawn_blocking(move || {
+            super::bambu_ftps::download(&host, &access_code, connector, &path)
+        })
+        .await
+        .map_err(|err| PrinterError(format!("{}: FTPS download thread failed: {err}", self.base.name)))?
+        .map_err(|err| PrinterError(format!("{}: FTPS download failed: {err}", self.base.name)))
+    }
+
     /// `list_cache` for any directory on the printer's SD card.
     ///
     /// Read-only, and the way to answer "what is the path to this file?"
@@ -726,7 +826,11 @@ impl BambuPrinter {
     /// check which the printer accepts before relying on that field, and
     /// fix continuum-schemas if it's wrong. The confirmed body above sets
     /// neither, so this is still open.
-    pub async fn start_uploaded_file(&self, file_name: &str) -> Result<(), PrinterError> {
+    pub async fn start_uploaded_file(
+        &self,
+        file_name: &str,
+        filament: &FilamentSource,
+    ) -> Result<(), PrinterError> {
         let (session, version) =
             self.connection.as_ref().ok_or_else(|| PrinterError(format!("{} is not connected", self.base.name)))?;
 
@@ -755,10 +859,9 @@ impl BambuPrinter {
             }
             body["subtask_name"] = serde_json::Value::String(String::new());
 
-            // See point 3 in this method's doc comment - these two are a
-            // pair, not independent switches.
-            body["use_ams"] = serde_json::Value::Bool(false);
-            body["ams_mapping"] = serde_json::Value::String(String::new());
+            // These two are a pair, not independent switches - see
+            // `FilamentSource::apply`.
+            filament.apply(&mut body);
         } else {
             // The root, for the same reason: it is where `upload_file` puts
             // things. Unverified - see this method's doc comment, which
@@ -917,7 +1020,21 @@ impl GenericPrinter for BambuPrinter {
             super::bambu_ftps::CACHE_DIR
         );
 
-        self.start_uploaded_file(&file_name).await
+        // The external spool, because this trait method has no way to know
+        // any better: `upload_project_file` is vendor-generic and an AMS
+        // mapping is a Bambu concept, so it cannot appear in the signature.
+        //
+        // That makes this the one remaining place that can silently pick the
+        // wrong filament source - see `FilamentSource` for what that does to
+        // a real print. It is not currently reachable: there is no
+        // start-print command on the uplink, so nothing calls this. The fix
+        // is for this method to *derive* the mapping from the `.3mf` it
+        // already holds in `data` - `Metadata/slice_info.config` lists each
+        // plate's filaments with their `tray_info_idx` and colour, which is
+        // exactly what has to be matched against the AMS report. That needs
+        // a zip reader, which is a dependency decision, which is why it is
+        // not done here yet.
+        self.start_uploaded_file(&file_name, &FilamentSource::ExternalSpool).await
     }
 
     /// Builds the real JSON payload straight from continuum-schemas'
@@ -943,3 +1060,70 @@ impl GenericPrinter for BambuPrinter {
         session.publish("print", &body).await
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The padding rule from
+    /// `schemas/bambulab/v1/common/ams/ams_mapping.schema.json`: always pad
+    /// to every tray the printer has. An under-length array is one of the
+    /// ways a mapping goes wrong quietly.
+    #[test]
+    fn an_ams_mapping_is_padded_to_the_tray_count() {
+        let FilamentSource::Ams(slots) = FilamentSource::ams(vec![3], 5).expect("valid") else {
+            panic!("expected an AMS source");
+        };
+        assert_eq!(slots, vec![3, -1, -1, -1, -1]);
+    }
+
+    #[test]
+    fn an_already_full_length_mapping_is_left_alone() {
+        let FilamentSource::Ams(slots) =
+            FilamentSource::ams(vec![3, -1, 0, -1, -1], 5).expect("valid")
+        else {
+            panic!("expected an AMS source");
+        };
+        assert_eq!(slots, vec![3, -1, 0, -1, -1]);
+    }
+
+    #[test]
+    fn an_empty_mapping_is_refused_rather_than_padded_into_nonsense() {
+        // Padding `[]` would produce `[-1,-1,-1,-1,-1]`: `use_ams: true` with
+        // every filament unmapped, which is not a thing to send a printer.
+        assert!(FilamentSource::ams(Vec::new(), 5).is_err());
+    }
+
+    #[test]
+    fn a_mapping_longer_than_the_printer_has_trays_is_refused() {
+        assert!(FilamentSource::ams(vec![0, 1, 2, 3, 0, 1], 5).is_err());
+    }
+
+    #[test]
+    fn minus_one_is_the_only_negative_slot_allowed() {
+        assert!(FilamentSource::ams(vec![-1], 5).is_ok());
+        assert!(FilamentSource::ams(vec![-2], 5).is_err());
+    }
+
+    /// The pair that cost a real print: `use_ams` and `ams_mapping` have to
+    /// move together, and the no-AMS case needs the empty *string* rather
+    /// than an empty array - the schema's trailing `oneOf` says so.
+    #[test]
+    fn the_external_spool_sends_an_empty_string_not_an_empty_array() {
+        let mut body = serde_json::json!({});
+        FilamentSource::ExternalSpool.apply(&mut body);
+        assert_eq!(body["use_ams"], serde_json::json!(false));
+        assert_eq!(body["ams_mapping"], serde_json::json!(""));
+    }
+
+    #[test]
+    fn an_ams_source_sends_an_array_and_turns_use_ams_on() {
+        let mut body = serde_json::json!({});
+        FilamentSource::ams(vec![3], 5).expect("valid").apply(&mut body);
+        assert_eq!(body["use_ams"], serde_json::json!(true));
+        assert_eq!(body["ams_mapping"], serde_json::json!([3, -1, -1, -1, -1]));
+        // Never the external-spool spelling while use_ams is on: that
+        // combination is what silently printed from the wrong source.
+        assert!(!body["ams_mapping"].is_string());
+    }
+}

src/printer/bambu_ftps.rs

@@ -116,6 +116,42 @@ pub fn upload(
     Ok(written)
 }
 
+/// Blocking: reads one file off the printer's SD card.
+///
+/// Read-only. The reason this exists: a `.3mf` already on the card is the
+/// only record of which filament each of its plates was sliced for, and
+/// `ams_mapping` cannot be built without that - see
+/// `schemas/bambulab/v1/common/ams/ams_mapping.schema.json`. Printing a
+/// project with the wrong mapping is not a cosmetic error; it draws the
+/// wrong filament, or none.
+pub fn download(
+    host: &str,
+    access_code: &str,
+    connector: native_tls::TlsConnector,
+    path: &str,
+) -> anyhow::Result<Vec<u8>> {
+    let mut ftp = NativeTlsFtpStream::connect_secure_implicit(
+        (host, FTPS_PORT),
+        NativeTlsConnector::from(connector),
+        host,
+    )?;
+
+    ftp.login(FTPS_USER, access_code)?;
+    // A `.3mf` is a zip archive; letting the server treat it as text is free
+    // corruption, exactly as on the way up.
+    ftp.transfer_type(FileType::Binary)?;
+
+    // `retr_as_buffer` rather than `retr_as_stream`: suppaftp 12's
+    // `TransferStream` finalizes itself on drop, and the files being read
+    // here are project archives of a few hundred KB, so there is nothing to
+    // gain from streaming them.
+    let bytes = ftp.retr_as_buffer(path)?.into_inner();
+
+    let _ = ftp.quit();
+
+    Ok(bytes)
+}
+
 /// Blocking: lists `cache/` on the printer.
 ///
 /// Read-only, and the counterpart to `upload`. The previous version of this

src/printer/mod.rs

@@ -35,7 +35,9 @@ mod klipper;
 mod marlin_serial;
 mod prusa;
 
-pub use bambu::{BambuPrinter, BambuProtocolPreference, BambuProtocolVersion, BambuTls};
+pub use bambu::{
+    BambuPrinter, BambuProtocolPreference, BambuProtocolVersion, BambuTls, FilamentSource,
+};
 pub use bambu_commands::{
     AmsState, AmsTray, AmsUnit, BambuReport, BambuRequest, CommandEnvelope, EmptyTray, GetVersionRequest,
     GetVersionResponse, LoadedTray, ModuleInfo, PrintReport, PushallRequest, XcamSettings,