diff --git a/src/printer/bambu.rs b/src/printer/bambu.rs
index e6fb4ad..7afc039 100644
--- a/src/printer/bambu.rs
+++ b/src/printer/bambu.rs
@@ -384,6 +384,7 @@ pub const CHAMBER_LIGHT: &str = "chamber_light";
 /// 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.
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub enum FilamentSource {
     /// The external spool (`vt_tray`). `use_ams: false`, and
     /// `ams_mapping` must be the empty *string* - the schema's trailing
@@ -1090,6 +1091,20 @@ impl BambuPrinter {
     /// - **`url` spelling.** `ftp:///<name>` - empty host, triple slash,
     ///   resolved against the card's root - is accepted. `file:///` was
     ///   never needed and remains untested.
+    /// - **An accepted start is not a successful print, and the failure can
+    ///   be physical.** Confirmed end to end on 2026-10-15 via
+    ///   `upload_project_file`: the mapping derived correctly
+    ///   (`ams_mapping` slot 3, and the printer echoed `tray_tar: 3`), the
+    ///   upload matched byte for byte, the printer went
+    ///   `IDLE -> PREPARE -> RUNNING`, heated to 255C - and then paused
+    ///   itself at stage 54 with `print_error: 0x07008003` and
+    ///   `hms: [{attr: 0x07002300, code: 0x00020004}]`, i.e.
+    ///   `HMS_0700_2300_0002_0004`, which is a broken-filament fault on the
+    ///   slot it was loading. Nothing in this code was wrong; the filament
+    ///   was. Which is the point: `command_outcome` can only report what
+    ///   the printer said about the *command*, so anything that wants to
+    ///   know whether a print is actually running has to watch
+    ///   `gcode_state`, `print_error` and `hms` afterwards.
     /// - **`use_ams: false` / `ams_mapping: ""`.** Accepted by the printer -
     ///   and *wrong* on an AMS-equipped machine. Confirmed on a second run
     ///   the same day, against a project the owner had sliced in Bambu
@@ -1187,6 +1202,24 @@ impl BambuPrinter {
             // things. Unverified - see this method's doc comment, which
             // records what the hardware said about the `cache/` spelling.
             body["param"] = serde_json::Value::String(format!("/{file_name}"));
+
+            // `filament` has nowhere to go here: `gcode_file` takes only
+            // `command` and `param` - no `use_ams`, no `ams_mapping` - so a
+            // bare gcode draws whatever its own tool changes select. That is
+            // correct, but it means this argument is silently discarded for
+            // half the inputs this method accepts, and "a filament choice
+            // quietly overridden" is precisely the bug `FilamentSource` was
+            // introduced to stop. So say so rather than letting a caller
+            // believe its mapping took effect.
+            if !matches!(filament, FilamentSource::ExternalSpool) {
+                tracing::warn!(
+                    printer = %self.base.name,
+                    file = file_name,
+                    ?filament,
+                    "ignoring the filament source: a bare .gcode has no use_ams/ams_mapping \
+                     field to put it in, so the gcode's own tool changes decide"
+                );
+            }
         }
 
         // A correlated `request`, not a fire-and-forget publish. This was