Report the OctoPrint API as what it is, and let the operator name it

Reporting a detected printer as `prusa_link` asserted a product nothing on
the wire mentions. What a probe observes is that something speaks the
OctoPrint REST API; PrusaLink is one implementation of it, OctoPrint itself
is another, and without a credential the two are indistinguishable - every
PrusaLink identity endpoint requires digest auth.

So discovery now reports `vendor = "octoprint"`, the protocol it actually
saw, and carries `ambiguous_with` so the rendered entry prints a comment
offering `prusa_link` directly above the `vendor` line. Changing it is a
one-word edit in the file the operator is already pasting into, which is the
right place for a question only they can answer. The exception is a body
that names itself: `/api/version`'s `text` field reads "PrusaLink 0.7.0" or
"OctoPrint 1.9.3", so on the rare unauthenticated 200 the answer is known
and no choice is offered.

For that to mean anything, `octoprint` had to become a vendor `fleet.rs`
accepts - otherwise discovery printed a block that would not load. It maps
to the same client as `prusa_link`, because they speak the same API, with an
`OctoPrintFlavour` recording which. The flavour is stored now, while the
client is still a stub, precisely because no probe can settle it: it is the
operator's answer, and without somewhere to keep it the real client would
have to ask them a second time. `PrusaLinkPrinter` is accordingly now
`OctoPrintPrinter` - one client for the API family - since a struct called
PrusaLink holding a flavour that says "Generic" is a contradiction someone
would later have to unpick.

**Found by the new test, and the reason it exists:** `fleet::load` did *not*
reject an entry whose credential was blank. `require_field` took an
`Option`, so `access_code = ""` arrived as `Some("")` and loaded happily -
which falsified the claim this feature was built on, that printing a block
with an empty credential is safe because pasting it in unfinished fails
loudly at startup. It does now. An empty credential is not a usable one; it
only moves the failure somewhere worse, where a Bambu printer is refused at
MQTT and a PrusaLink one 401s on every poll with nothing saying the field is
blank. This is the same trap as `??`-instead-of-`||` on an environment
variable that ships empty, which the backend's `auth/edge_secret.ts` and
`s3/index.ts` both document; it bites identically on a TOML key.

tests/discovery_to_fleet.rs is new and closes the loop: discovery and fleet
are coupled by nothing but a string, unit tests on either side cannot catch
a drift between them, and the symptom of a drift is an operator pasting in
exactly what they were told to and the gateway refusing to start. It renders
a candidate for every vendor, fills in the credential the way a human would,
loads it for real, and checks the handle matches the vendor asked for - not
merely that something loaded, since `octoprint` and `prusa_link` share a
client and a mapping that collapsed them would still load and still be
wrong.

Verification: the Moonraker probe now runs end to end over real HTTP against
a stand-in serving Moonraker's documented `/printer/info`, and correctly
reports a klipper entry named from the returned hostname. Port 80 cannot be
bound without root here, so the PrusaLink path is covered by a test that
takes a real socket answering a real digest challenge - the one link unit
tests on the classifier cannot reach, and the one PrusaLink is identified by.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/printer_polymorphism.rs

@@ -10,7 +10,7 @@
 
 use continuum_proxy::printer::{
     BambuPrinter, BambuProtocolPreference, BambuTls, GenericPrinter, KlipperPrinter, PrinterHandle,
-    PrusaLinkPrinter, PrusaSerialPrinter,
+    OctoPrintFlavour, OctoPrintPrinter, PrusaSerialPrinter,
 };
 
 #[tokio::main]
@@ -36,7 +36,13 @@ async fn main() {
             BambuTls::BundledCa,
             BambuProtocolPreference::Auto,
         )),
-        PrinterHandle::PrusaLink(PrusaLinkPrinter::new("p3", "MK4", "192.168.1.51", "prusa-api-key")),
+        PrinterHandle::OctoPrint(OctoPrintPrinter::new(
+            "p3",
+            "MK4",
+            "192.168.1.51",
+            "prusa-api-key",
+            OctoPrintFlavour::PrusaLink,
+        )),
         PrinterHandle::PrusaSerial(PrusaSerialPrinter::new("p4", "MK3S+", "/dev/ttyACM0", 115_200)),
         PrinterHandle::Klipper(KlipperPrinter::new("p5", "Voron 2.4", "192.168.1.52")),
         // No host configured - this one will hit the Err path.

printers.example.toml

@@ -17,6 +17,12 @@
 # - so every block it prints still needs that one field before this file will
 # load. Note that only `bambu` has a working client today; the others are
 # found and reported, but their drivers in src/printer/ are still stubs.
+#
+# One thing discovery cannot answer for you: `octoprint` vs `prusa_link`.
+# PrusaLink implements OctoPrint's API, so from the network the two are
+# identical - discovery suggests `octoprint` (the protocol it actually saw)
+# and leaves a comment offering the other. Change it if you know it is a
+# Prusa serving its own API.
 
 [[printer]]
 id = "p1p"

src/discovery/mod.rs

@@ -98,7 +98,15 @@ pub const SSDP_PORTS: [u16; 2] = [2021, 1990];
 pub enum Vendor {
     Bambu,
     Klipper,
+    /// Prusa's own implementation of the OctoPrint API, on an MK4/XL/MINI.
     PrusaLink,
+    /// OctoPrint itself, or anything else serving its API.
+    ///
+    /// This is the honest default for a printer found by probing, because
+    /// what a probe observes is "something here speaks the OctoPrint API" -
+    /// never which implementation. Reporting `prusa_link` would assert a
+    /// product nothing on the wire names.
+    Octoprint,
     /// Marlin-family firmware over a raw serial port. Named for Prusa
     /// because that is what `fleet.rs` calls it, but
     /// `printer/marlin_serial.rs` drives any Marlin board the same way -
@@ -117,6 +125,7 @@ impl Vendor {
             Vendor::Bambu => "bambu",
             Vendor::Klipper => "klipper",
             Vendor::PrusaLink => "prusa_link",
+            Vendor::Octoprint => "octoprint",
             Vendor::PrusaSerial => "prusa_serial",
         }
     }
@@ -131,7 +140,7 @@ impl Vendor {
             Vendor::Bambu => true,
             // `printer/klipper.rs` and `printer/prusa.rs` are stubs whose
             // methods return without doing anything.
-            Vendor::Klipper | Vendor::PrusaLink => false,
+            Vendor::Klipper | Vendor::PrusaLink | Vendor::Octoprint => false,
             // `marlin_serial.rs` is a real transport, but
             // `PrusaSerialPrinter`'s command methods on top of it are not.
             Vendor::PrusaSerial => false,
@@ -177,6 +186,14 @@ pub struct Candidate {
     /// the machine by. Not rendered into the TOML - nothing in `fleet.rs`
     /// takes a version, and a stale one in a config file would be a lie.
     pub detail: Option<String>,
+    /// Another vendor this could equally be, when the probe genuinely cannot
+    /// tell - which today means only OctoPrint versus PrusaLink.
+    ///
+    /// Carried rather than resolved by guessing, because the operator knows
+    /// what they own and the network does not. It renders as a comment above
+    /// `vendor`, so changing it is a one-word edit in the file they are
+    /// already pasting into.
+    pub ambiguous_with: Option<Vendor>,
     pub source: Source,
 }
 
@@ -190,6 +207,7 @@ impl Candidate {
             name: None,
             bind_state: None,
             detail: None,
+            ambiguous_with: None,
             source,
         }
     }
@@ -222,6 +240,13 @@ impl Candidate {
         out.push_str("[[printer]]\n");
         out.push_str(&format!("id = {}\n", toml_string(id)));
         out.push_str(&format!("name = {}\n", toml_string(&name)));
+        if let Some(other) = self.ambiguous_with {
+            out.push_str(&format!(
+                "# vendor: change to {:?} if that is what this is - nothing on the network\n\
+                 # distinguishes them, so this is a guess only you can settle.\n",
+                other.toml_value()
+            ));
+        }
         out.push_str(&format!("vendor = {}\n", toml_string(self.vendor.toml_value())));
 
         match self.vendor {
@@ -238,9 +263,10 @@ impl Candidate {
             // The only field `fleet.rs` wants, and Moonraker needs no
             // credential from us - it authorizes by client address.
             Vendor::Klipper => out.push_str(&format!("host = {}\n", toml_string(&self.host))),
-            Vendor::PrusaLink => {
+            // One client, two flavours - same required fields.
+            Vendor::PrusaLink | Vendor::Octoprint => {
                 out.push_str(&format!("host = {}\n", toml_string(&self.host)));
-                out.push_str("api_key = \"\"   # REQUIRED: PrusaLink web UI > Settings > API key\n");
+                out.push_str("api_key = \"\"   # REQUIRED: the printer's or OctoPrint's web UI > API key\n");
             }
             Vendor::PrusaSerial => {
                 out.push_str(&format!("port = {}\n", toml_string(&self.host)));
@@ -621,6 +647,7 @@ mod tests {
             (Vendor::Bambu, "192.168.0.21", vec!["host", "sn", "access_code"]),
             (Vendor::Klipper, "192.168.0.50", vec!["host"]),
             (Vendor::PrusaLink, "192.168.0.51", vec!["host", "api_key"]),
+            (Vendor::Octoprint, "192.168.0.60", vec!["host", "api_key"]),
             (Vendor::PrusaSerial, "/dev/ttyACM0", vec!["port", "baud_rate"]),
         ];
         for (vendor, host, expected) in cases {
@@ -645,7 +672,7 @@ mod tests {
 
     #[test]
     fn a_vendor_with_a_stub_client_says_so_in_its_entry() {
-        for vendor in [Vendor::Klipper, Vendor::PrusaLink, Vendor::PrusaSerial] {
+        for vendor in [Vendor::Klipper, Vendor::PrusaLink, Vendor::Octoprint, Vendor::PrusaSerial] {
             let entry = Candidate::new("192.168.0.50", vendor, Source::HttpApi).to_toml_entry("x");
             assert!(entry.contains("WARNING"), "{vendor:?} is a stub and must say so:\n{entry}");
             assert!(entry.contains("stub"), "{vendor:?}:\n{entry}");

src/discovery/net.rs

@@ -188,20 +188,26 @@ pub fn parse_moonraker_info(host: &str, body: &[u8]) -> anyhow::Result<Candidate
 /// vendor, and the one field that is still missing is the API key, which no
 /// probe could ever supply.
 ///
-/// **This cannot distinguish PrusaLink from OctoPrint**, and that is not a
-/// bug to fix here: PrusaLink deliberately implements OctoPrint's API, so the
-/// two are indistinguishable from outside without credentials. An OctoPrint
-/// instance reported as `prusa_link` is a wrong guess an operator can see and
-/// correct; silently skipping a real PrusaLink would be worse.
+/// **What this detects is the OctoPrint API, not a product.** PrusaLink
+/// deliberately implements that API, so without a credential the two are
+/// indistinguishable - and `vendor = "octoprint"` is what gets reported,
+/// because that is what was actually observed. Naming `prusa_link` would
+/// assert a product nothing on the wire mentions. The candidate carries
+/// `ambiguous_with` so the rendered entry offers the other choice as a
+/// one-word edit.
+///
+/// The exception is a body that names itself: `/api/version`'s `text` field
+/// reads `PrusaLink 0.7.0` or `OctoPrint 1.9.3`, so on the rare
+/// unauthenticated 200 the answer is known rather than guessed.
 pub async fn probe_prusalink(host: &str, timeout: Duration) -> anyhow::Result<Candidate> {
     let url = format!("http://{host}:{PRUSALINK_PORT}/api/version");
     let (status, body, challenge) = http_get(&url, timeout).await?;
-    classify_prusalink(host, status, &body, challenge)
+    classify_octoprint_api(host, status, &body, challenge)
 }
 
 /// The decision `probe_prusalink` makes once the response is in hand, split
 /// out so the 401 path can be tested without a printer.
-pub fn classify_prusalink(
+pub fn classify_octoprint_api(
     host: &str,
     status: u16,
     body: &[u8],
@@ -209,7 +215,7 @@ pub fn classify_prusalink(
 ) -> anyhow::Result<Candidate> {
     match status {
         // Some builds answer unauthenticated. Take the detail if so.
-        200 => parse_prusalink_version(host, body),
+        200 => parse_octoprint_version(host, body),
         401 => {
             // A bare 401 is *not* enough. Plenty of things on a LAN password
             // protect port 80 - routers, NASes, cameras - and accepting any
@@ -225,10 +231,15 @@ pub fn classify_prusalink(
                     "{host} wants {challenge:?} on /api/version, not Digest -                      some other password-protected web server, not PrusaLink"
                 );
             }
-            let mut candidate = Candidate::new(host, Vendor::PrusaLink, Source::HttpApi);
+            // Reported as the generic flavour: a digest challenge proves the
+            // OctoPrint API is here and proves nothing about which
+            // implementation serves it.
+            let mut candidate = Candidate::new(host, Vendor::Octoprint, Source::HttpApi);
+            candidate.ambiguous_with = Some(Vendor::PrusaLink);
             // The challenge is worth quoting back: `Digest realm="Printer API"`
             // is what an operator can check against what they expect there.
-            candidate.detail = Some(format!("PrusaLink or OctoPrint - needs an API key ({challenge})"));
+            candidate.detail =
+                Some(format!("an OctoPrint-compatible API needing a key - OctoPrint or PrusaLink ({challenge})"));
             Ok(candidate)
         }
         other => anyhow::bail!("{host} answered /api/version with HTTP {other}, which is not PrusaLink"),
@@ -236,7 +247,10 @@ pub fn classify_prusalink(
 }
 
 /// Reads an `/api/version` body, for the builds that answer without auth.
-pub fn parse_prusalink_version(host: &str, body: &[u8]) -> anyhow::Result<Candidate> {
+///
+/// This is the one path that can name the implementation rather than the
+/// protocol, because `text` says which it is.
+pub fn parse_octoprint_version(host: &str, body: &[u8]) -> anyhow::Result<Candidate> {
     let parsed: serde_json::Value = serde_json::from_slice(body)
         .map_err(|err| anyhow::anyhow!("{host} answered /api/version with something that is not JSON: {err}"))?;
     // `api` is the field OctoPrint's contract guarantees; without it this is
@@ -244,12 +258,25 @@ pub fn parse_prusalink_version(host: &str, body: &[u8]) -> anyhow::Result<Candid
     if parsed.get("api").is_none() {
         anyhow::bail!("{host} answered /api/version without an `api` field - not PrusaLink");
     }
-    let mut candidate = Candidate::new(host, Vendor::PrusaLink, Source::HttpApi);
-    let text = parsed.get("text").and_then(|v| v.as_str()).unwrap_or("PrusaLink");
+    let text = parsed.get("text").and_then(|v| v.as_str()).unwrap_or("");
+    let lowered = text.to_ascii_lowercase();
+    // `text` names the implementation when it is there, so this is the only
+    // place the distinction is known rather than left to the operator.
+    let (vendor, ambiguous_with) = if lowered.contains("prusalink") {
+        (Vendor::PrusaLink, None)
+    } else if lowered.contains("octoprint") {
+        (Vendor::Octoprint, None)
+    } else {
+        (Vendor::Octoprint, Some(Vendor::PrusaLink))
+    };
+
+    let mut candidate = Candidate::new(host, vendor, Source::HttpApi);
+    candidate.ambiguous_with = ambiguous_with;
     let firmware = parsed.get("firmware").and_then(|v| v.as_str());
+    let described = if text.is_empty() { "an OctoPrint-compatible API" } else { text };
     candidate.detail = Some(match firmware {
-        Some(firmware) => format!("{text}, firmware {firmware}"),
-        None => text.to_string(),
+        Some(firmware) => format!("{described}, firmware {firmware}"),
+        None => described.to_string(),
     });
     candidate.name = parsed.get("hostname").and_then(|v| v.as_str()).map(str::to_string);
     Ok(candidate)
@@ -409,20 +436,54 @@ mod tests {
         assert!(parse_moonraker_info("192.168.0.50", b"<html>hello</html>").is_err());
     }
 
+    /// `text` names the implementation, so this is the one case where the
+    /// product is known rather than inferred - and it must be taken, not
+    /// flattened into the generic answer.
     #[test]
-    fn a_prusalink_reply_names_the_firmware() {
-        let found = parse_prusalink_version("192.168.0.51", PRUSALINK_VERSION).expect("PrusaLink");
+    fn a_body_that_names_itself_prusalink_is_reported_as_prusalink() {
+        let found = parse_octoprint_version("192.168.0.51", PRUSALINK_VERSION).expect("PrusaLink");
         assert_eq!(found.vendor, Vendor::PrusaLink);
+        assert_eq!(found.ambiguous_with, None, "`text` settled it - nothing to offer the operator");
         assert_eq!(found.detail.as_deref(), Some("PrusaLink 0.7.0, firmware 3.10.1-4697"));
         // The API key is the field no probe can supply, so it must be
         // rendered empty and labelled rather than omitted.
         let parsed: toml::Value = toml::from_str(&found.to_toml_entry("mk4")).expect("valid TOML");
         assert_eq!(parsed["printer"][0]["api_key"].as_str(), Some(""));
+        assert_eq!(parsed["printer"][0]["vendor"].as_str(), Some("prusa_link"));
+    }
+
+    #[test]
+    fn a_body_that_names_itself_octoprint_is_reported_as_octoprint() {
+        let body = br#"{"api":"0.1","server":"1.9.3","text":"OctoPrint 1.9.3"}"#;
+        let found = parse_octoprint_version("192.168.0.60", body).expect("OctoPrint");
+        assert_eq!(found.vendor, Vendor::Octoprint);
+        assert_eq!(found.ambiguous_with, None);
+    }
+
+    /// The common case: the API answers but names no product, so the vendor
+    /// reported is the protocol that was observed and the alternative is
+    /// offered rather than guessed away.
+    #[test]
+    fn an_unnamed_octoprint_api_reports_the_protocol_and_offers_the_choice() {
+        let body = br#"{"api":"2.0.0","version":"2.1.2"}"#;
+        let found = parse_octoprint_version("192.168.0.51", body).expect("the API");
+        assert_eq!(found.vendor, Vendor::Octoprint);
+        assert_eq!(found.ambiguous_with, Some(Vendor::PrusaLink));
+
+        let entry = found.to_toml_entry("mk4");
+        assert_eq!(
+            toml::from_str::<toml::Value>(&entry).expect("valid TOML")["printer"][0]["vendor"].as_str(),
+            Some("octoprint")
+        );
+        // The other choice has to be a one-word edit in the file the
+        // operator is already pasting into.
+        assert!(entry.contains("prusa_link"), "the alternative must be offered:\n{entry}");
+        assert!(entry.contains("only you can settle"), "{entry}");
     }
 
     #[test]
     fn a_web_server_that_is_not_octoprint_shaped_is_refused() {
-        let err = parse_prusalink_version("192.168.0.51", br#"{"version":"1.0"}"#).expect_err("must refuse");
+        let err = parse_octoprint_version("192.168.0.51", br#"{"version":"1.0"}"#).expect_err("must refuse");
         assert!(err.to_string().contains("not PrusaLink"), "{err}");
     }
 
@@ -448,7 +509,7 @@ var g_Lan = 394;"#,
             b"",
         ];
         for body in real_bodies {
-            let err = classify_prusalink("192.168.0.1", 200, body, None).expect_err("not a printer");
+            let err = classify_octoprint_api("192.168.0.1", 200, body, None).expect_err("not a printer");
             assert!(err.to_string().contains("not JSON") || err.to_string().contains("not PrusaLink"), "{err}");
         }
     }
@@ -457,17 +518,58 @@ var g_Lan = 394;"#,
     fn a_password_protected_router_is_not_a_printer() {
         // The false positive that a bare "401 means PrusaLink" would create.
         // Routers, NASes and cameras all password protect port 80.
-        let basic = classify_prusalink("192.168.0.1", 401, b"", Some(r#"Basic realm="RouterOS""#.into()));
+        let basic = classify_octoprint_api("192.168.0.1", 401, b"", Some(r#"Basic realm="RouterOS""#.into()));
         let err = basic.expect_err("Basic auth is not PrusaLink");
         assert!(err.to_string().contains("not Digest"), "{err}");
 
-        let silent = classify_prusalink("192.168.0.1", 401, b"", None);
+        let silent = classify_octoprint_api("192.168.0.1", 401, b"", None);
         assert!(silent.is_err(), "a 401 with no challenge names nothing");
 
         // What a real PrusaLink answers, which must still be found.
-        let found = classify_prusalink("192.168.0.51", 401, b"", Some(r#"Digest realm="Printer API""#.into()))
-            .expect("a digest challenge on /api/version is PrusaLink");
-        assert_eq!(found.vendor, Vendor::PrusaLink);
+        let found = classify_octoprint_api("192.168.0.51", 401, b"", Some(r#"Digest realm="Printer API""#.into()))
+            .expect("a digest challenge on /api/version is the OctoPrint API");
+        // Generic, because a challenge proves the API is here and proves
+        // nothing about which implementation serves it.
+        assert_eq!(found.vendor, Vendor::Octoprint);
+        assert_eq!(found.ambiguous_with, Some(Vendor::PrusaLink));
+        assert!(found.detail.unwrap().contains("Printer API"));
+    }
+
+    /// The one link between a real socket and `classify_octoprint_api` that
+    /// unit tests on the classifier cannot reach: whether the challenge
+    /// header actually survives the trip. PrusaLink is identified *by* that
+    /// header, so if `http_get` dropped it every PrusaLink would be reported
+    /// as "no challenge - not PrusaLink". Port 80 cannot be bound without
+    /// root here, so this stands in for the real probe's plumbing.
+    #[tokio::test]
+    async fn a_digest_challenge_survives_the_round_trip_and_identifies_the_api() {
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").expect("a port");
+        let port = listener.local_addr().expect("an address").port();
+        std::thread::spawn(move || {
+            use std::io::{Read, Write};
+            let (mut socket, _) = listener.accept().expect("one connection");
+            let mut scratch = [0u8; 1024];
+            let _ = socket.read(&mut scratch);
+            // Exactly what PrusaLink answers an unauthenticated GET.
+            let _ = socket.write_all(
+                b"HTTP/1.1 401 Unauthorized\r\n\
+                  WWW-Authenticate: Digest realm=\"Printer API\", nonce=\"abc\", qop=\"auth\"\r\n\
+                  Content-Length: 0\r\n\r\n",
+            );
+        });
+
+        let (status, body, challenge) =
+            http_get(&format!("http://127.0.0.1:{port}/api/version"), Duration::from_secs(5))
+                .await
+                .expect("the stand-in answers");
+        assert_eq!(status, 401);
+        assert!(challenge.as_deref().unwrap_or_default().contains("Digest"), "{challenge:?}");
+
+        // And the classifier turns that into the generic flavour with the
+        // alternative offered - the whole path, end to end.
+        let found = classify_octoprint_api("127.0.0.1", status, &body, challenge).expect("the OctoPrint API");
+        assert_eq!(found.vendor, Vendor::Octoprint);
+        assert_eq!(found.ambiguous_with, Some(Vendor::PrusaLink));
         assert!(found.detail.unwrap().contains("Printer API"));
     }
 

src/fleet.rs

@@ -8,8 +8,8 @@ use std::path::Path;
 use serde::Deserialize;
 
 use crate::printer::{
-    BambuPrinter, BambuProtocolPreference, BambuTls, KlipperPrinter, PrinterHandle, PrusaLinkPrinter,
-    PrusaSerialPrinter,
+    BambuPrinter, BambuProtocolPreference, BambuTls, KlipperPrinter, OctoPrintFlavour, OctoPrintPrinter,
+    PrinterHandle, PrusaSerialPrinter,
 };
 
 #[derive(Debug, Deserialize)]
@@ -25,7 +25,12 @@ struct PrinterEntry {
     /// One of: "bambu" (auto-detects MQTT v5-vs-v3.1.1/BambuMQTT v2-vs-v1 -
     /// the default, since a firmware update can move the same physical
     /// printer between them), "bambu_v1"/"bambu_v2" (force a specific one),
-    /// "prusa_link", "prusa_serial", "klipper".
+    /// "prusa_link", "octoprint", "prusa_serial", "klipper".
+    ///
+    /// `prusa_link` and `octoprint` are the same REST client with a different
+    /// flavour - see `OctoPrintFlavour`. Pick `octoprint` if it is OctoPrint
+    /// on a Pi and `prusa_link` if it is an MK4/XL/MINI serving its own API;
+    /// discovery cannot tell, so it suggests `octoprint` and says so.
     vendor: String,
     host: Option<String>,
     access_code: Option<String>,
@@ -71,11 +76,15 @@ fn build_printer(entry: PrinterEntry) -> anyhow::Result<PrinterHandle> {
         "bambu" => build_bambu(entry, BambuProtocolPreference::Auto),
         "bambu_v1" => build_bambu(entry, BambuProtocolPreference::ForceV1),
         "bambu_v2" => build_bambu(entry, BambuProtocolPreference::ForceV2),
-        "prusa_link" => {
-            let host = require_field(&entry.id, "host", entry.host)?;
-            let api_key = require_field(&entry.id, "api_key", entry.api_key)?;
-            Ok(PrinterHandle::PrusaLink(PrusaLinkPrinter::new(&entry.id, &entry.name, &host, &api_key)))
-        }
+        // Two names for one client, because PrusaLink implements OctoPrint's
+        // API. The distinction is kept rather than collapsed: the two differ
+        // in which endpoints they offer beyond `/api/version`, and - the
+        // reason it has to come from config - nothing on the network can tell
+        // them apart, since every PrusaLink identity endpoint needs a
+        // credential. So this is the operator's answer, and `octoprint` is
+        // the honest default for a printer found by discovery.
+        "prusa_link" => build_octoprint(entry, OctoPrintFlavour::PrusaLink),
+        "octoprint" => build_octoprint(entry, OctoPrintFlavour::Generic),
         "prusa_serial" => {
             let port = require_field(&entry.id, "port", entry.port)?;
             Ok(PrinterHandle::PrusaSerial(PrusaSerialPrinter::new(&entry.id, &entry.name, &port, entry.baud_rate)))
@@ -88,6 +97,12 @@ fn build_printer(entry: PrinterEntry) -> anyhow::Result<PrinterHandle> {
     }
 }
 
+fn build_octoprint(entry: PrinterEntry, flavour: OctoPrintFlavour) -> anyhow::Result<PrinterHandle> {
+    let host = require_field(&entry.id, "host", entry.host)?;
+    let api_key = require_field(&entry.id, "api_key", entry.api_key)?;
+    Ok(PrinterHandle::OctoPrint(OctoPrintPrinter::new(&entry.id, &entry.name, &host, &api_key, flavour)))
+}
+
 fn build_bambu(entry: PrinterEntry, preference: BambuProtocolPreference) -> anyhow::Result<PrinterHandle> {
     let host = require_field(&entry.id, "host", entry.host)?;
     let access_code = require_field(&entry.id, "access_code", entry.access_code)?;
@@ -96,6 +111,31 @@ fn build_bambu(entry: PrinterEntry, preference: BambuProtocolPreference) -> anyh
     Ok(PrinterHandle::Bambu(BambuPrinter::new(&entry.id, &entry.name, &host, &access_code, &sn, tls, preference)))
 }
 
+/// A required field, where **empty counts as missing**.
+///
+/// The emptiness check is the load-bearing half. `Option` only catches an
+/// absent key, so `access_code = ""` arrived here as `Some("")` and loaded
+/// happily - and an empty credential is not a usable one, it just moves the
+/// failure somewhere worse. A Bambu printer configured that way reaches MQTT
+/// and is refused, a PrusaLink one gets a 401 on every poll, and nothing in
+/// either chain says "that field is blank in printers.toml".
+///
+/// It is also what makes a half-finished entry safe. `discovery` prints
+/// blocks with `access_code = ""`/`api_key = ""` for an operator to
+/// complete, and pasting one in and forgetting to finish it has to fail at
+/// startup naming the field - which is only true because of this check.
+/// `tests/discovery_to_fleet.rs` asserts exactly that, and is how the gap
+/// was found.
+///
+/// This is the same trap as `??`-instead-of-`||` on an environment variable
+/// that ships empty, which `continuum-backend`'s `auth/edge_secret.ts` and
+/// `s3/index.ts` both document. It bites identically on a TOML key.
 fn require_field(printer_id: &str, field: &str, value: Option<String>) -> anyhow::Result<String> {
-    value.ok_or_else(|| anyhow::anyhow!("{printer_id}: missing required field '{field}'"))
+    match value {
+        Some(value) if !value.trim().is_empty() => Ok(value),
+        Some(_) => Err(anyhow::anyhow!(
+            "{printer_id}: required field '{field}' is empty - fill it in              (discovery leaves it blank on purpose; an empty value is not a usable one)"
+        )),
+        None => Err(anyhow::anyhow!("{printer_id}: missing required field '{field}'")),
+    }
 }

src/printer/mod.rs

@@ -53,7 +53,7 @@ pub use bambu_commands::{
     XcamSettings,
 };
 pub use klipper::KlipperPrinter;
-pub use prusa::{PrusaLinkPrinter, PrusaSerialPrinter};
+pub use prusa::{OctoPrintFlavour, OctoPrintPrinter, PrusaSerialPrinter};
 
 /// A tiny error type: just a message. `String` would work too - this exists
 /// mainly so `?` has something concrete to convert into everywhere.
@@ -188,7 +188,7 @@ impl PrintControl {
 /// The closed set of printer kinds this fleet can talk to.
 pub enum PrinterHandle {
     Bambu(BambuPrinter),
-    PrusaLink(PrusaLinkPrinter),
+    OctoPrint(OctoPrintPrinter),
     PrusaSerial(PrusaSerialPrinter),
     Klipper(KlipperPrinter),
 }
@@ -205,7 +205,7 @@ impl PrinterHandle {
     pub async fn print_status(&self) -> Option<PrintReport> {
         match self {
             PrinterHandle::Bambu(p) => p.print_status().await,
-            PrinterHandle::PrusaLink(_) | PrinterHandle::PrusaSerial(_) | PrinterHandle::Klipper(_) => None,
+            PrinterHandle::OctoPrint(_) | PrinterHandle::PrusaSerial(_) | PrinterHandle::Klipper(_) => None,
         }
     }
 
@@ -234,7 +234,7 @@ impl PrinterHandle {
     pub async fn refresh_status(&self) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.pushall().await,
-            PrinterHandle::PrusaLink(_) | PrinterHandle::PrusaSerial(_) | PrinterHandle::Klipper(_) => Ok(()),
+            PrinterHandle::OctoPrint(_) | PrinterHandle::PrusaSerial(_) | PrinterHandle::Klipper(_) => Ok(()),
         }
     }
 }
@@ -247,7 +247,7 @@ impl GenericPrinter for PrinterHandle {
     async fn control_print(&mut self, action: PrintControl) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.control_print(action).await,
-            PrinterHandle::PrusaLink(p) => p.control_print(action).await,
+            PrinterHandle::OctoPrint(p) => p.control_print(action).await,
             PrinterHandle::PrusaSerial(p) => p.control_print(action).await,
             PrinterHandle::Klipper(p) => p.control_print(action).await,
         }
@@ -256,7 +256,7 @@ impl GenericPrinter for PrinterHandle {
     fn base(&self) -> &PrinterBase {
         match self {
             PrinterHandle::Bambu(p) => p.base(),
-            PrinterHandle::PrusaLink(p) => p.base(),
+            PrinterHandle::OctoPrint(p) => p.base(),
             PrinterHandle::PrusaSerial(p) => p.base(),
             PrinterHandle::Klipper(p) => p.base(),
         }
@@ -265,7 +265,7 @@ impl GenericPrinter for PrinterHandle {
     async fn connect(&mut self) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.connect().await,
-            PrinterHandle::PrusaLink(p) => p.connect().await,
+            PrinterHandle::OctoPrint(p) => p.connect().await,
             PrinterHandle::PrusaSerial(p) => p.connect().await,
             PrinterHandle::Klipper(p) => p.connect().await,
         }
@@ -274,7 +274,7 @@ impl GenericPrinter for PrinterHandle {
     async fn publish(&mut self, payload: String) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.publish(payload).await,
-            PrinterHandle::PrusaLink(p) => p.publish(payload).await,
+            PrinterHandle::OctoPrint(p) => p.publish(payload).await,
             PrinterHandle::PrusaSerial(p) => p.publish(payload).await,
             PrinterHandle::Klipper(p) => p.publish(payload).await,
         }
@@ -283,7 +283,7 @@ impl GenericPrinter for PrinterHandle {
     async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.set_fan_speed(percent).await,
-            PrinterHandle::PrusaLink(p) => p.set_fan_speed(percent).await,
+            PrinterHandle::OctoPrint(p) => p.set_fan_speed(percent).await,
             PrinterHandle::PrusaSerial(p) => p.set_fan_speed(percent).await,
             PrinterHandle::Klipper(p) => p.set_fan_speed(percent).await,
         }
@@ -292,7 +292,7 @@ impl GenericPrinter for PrinterHandle {
     async fn upload_project_file(&mut self, file_name: String, data: Vec<u8>) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.upload_project_file(file_name, data).await,
-            PrinterHandle::PrusaLink(p) => p.upload_project_file(file_name, data).await,
+            PrinterHandle::OctoPrint(p) => p.upload_project_file(file_name, data).await,
             PrinterHandle::PrusaSerial(p) => p.upload_project_file(file_name, data).await,
             PrinterHandle::Klipper(p) => p.upload_project_file(file_name, data).await,
         }
@@ -301,7 +301,7 @@ impl GenericPrinter for PrinterHandle {
     async fn send_gcode_line(&mut self, gcode_line: String) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.send_gcode_line(gcode_line).await,
-            PrinterHandle::PrusaLink(p) => p.send_gcode_line(gcode_line).await,
+            PrinterHandle::OctoPrint(p) => p.send_gcode_line(gcode_line).await,
             PrinterHandle::PrusaSerial(p) => p.send_gcode_line(gcode_line).await,
             PrinterHandle::Klipper(p) => p.send_gcode_line(gcode_line).await,
         }

src/printer/prusa.rs

@@ -1,28 +1,62 @@
 use super::{GenericPrinter, PrinterBase, PrinterError};
 
-/****************** PrusaLink ******************/
+/****************** OctoPrint API family ******************/
 
-pub struct PrusaLinkPrinter {
-    base: PrinterBase,
-    pub api_key: String,
+/// Which implementation of the OctoPrint REST API is on the other end.
+///
+/// PrusaLink deliberately implements OctoPrint's API, so one client serves
+/// both - but not identically. PrusaLink adds its own `/api/v1/*` (`info`,
+/// `status`, `job`) which is where the useful detail lives, while OctoPrint
+/// proper has the older `/api/printer` and `/api/job` and a plugin surface
+/// PrusaLink does not have.
+///
+/// This is recorded from config now, while the client is still a stub,
+/// because **no probe can settle it**: both answer `/api/version`, and every
+/// PrusaLink identity endpoint needs a credential, so from outside the two
+/// are indistinguishable (see `discovery::net::classify_octoprint_api`). It
+/// is the operator's answer to a question only they can answer, so it needs
+/// somewhere to live the moment they are asked - otherwise the real client
+/// would have to ask them a second time.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum OctoPrintFlavour {
+    /// Prusa's own firmware-hosted implementation, on an MK4/XL/MINI.
+    PrusaLink,
+    /// OctoPrint itself, usually on a Raspberry Pi in front of any printer.
+    Generic,
 }
 
-impl PrusaLinkPrinter {
-    pub fn new(id: &str, name: &str, host: &str, api_key: &str) -> Self {
-        Self {
-            base: PrinterBase { id: id.into(), name: name.into(), host: host.into() },
-            api_key: api_key.into(),
+impl OctoPrintFlavour {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            OctoPrintFlavour::PrusaLink => "PrusaLink",
+            OctoPrintFlavour::Generic => "OctoPrint",
         }
     }
 }
 
-impl GenericPrinter for PrusaLinkPrinter {
+pub struct OctoPrintPrinter {
+    base: PrinterBase,
+    pub api_key: String,
+    pub flavour: OctoPrintFlavour,
+}
+
+impl OctoPrintPrinter {
+    pub fn new(id: &str, name: &str, host: &str, api_key: &str, flavour: OctoPrintFlavour) -> Self {
+        Self {
+            base: PrinterBase { id: id.into(), name: name.into(), host: host.into() },
+            api_key: api_key.into(),
+            flavour,
+        }
+    }
+}
+
+impl GenericPrinter for OctoPrintPrinter {
     /// OVERRIDE: real version is `PUT /api/v1/job/{id}/pause`, `/resume`, or
     /// `DELETE /api/v1/job/{id}` to cancel - PrusaLink addresses the running
     /// job by id rather than the printer, so a real implementation has to
     /// fetch the current job first.
     async fn control_print(&mut self, action: super::PrintControl) -> Result<(), PrinterError> {
-        println!("[{}] PrusaLink job control: {}", self.base.name, action.as_str());
+        println!("[{}] {} job control: {}", self.base.name, self.flavour.as_str(), action.as_str());
         Ok(())
     }
 
@@ -35,7 +69,7 @@ impl GenericPrinter for PrusaLinkPrinter {
     /// plain REST API (real version: `GET /api/v1/status` with the API
     /// key, see `adapters::prusalink`), no persistent session to hold.
     async fn connect(&mut self) -> Result<(), PrinterError> {
-        println!("[{}] connecting over HTTP (PrusaLink)", self.base.name);
+        println!("[{}] connecting over HTTP ({})", self.base.name, self.flavour.as_str());
         Ok(())
     }
 

tests/discovery_to_fleet.rs

@@ -0,0 +1,136 @@
+//! Proves the blocks `discovery` prints are blocks `fleet` can load.
+//!
+//! The two are coupled by nothing but a string - `Vendor::toml_value` has to
+//! match what `fleet::build_printer` matches on, and the fields rendered have
+//! to be the fields it requires. Unit tests on either side cannot catch a
+//! drift between them, and the symptom of a drift is the worst kind: an
+//! operator pastes in exactly what they were told to and the gateway refuses
+//! to start.
+//!
+//! So this renders a candidate for every vendor, fills in the credential the
+//! way a human would, and loads the result for real.
+
+use std::io::Write;
+
+use continuum_proxy::discovery::{Candidate, Source, Vendor};
+use continuum_proxy::fleet;
+use continuum_proxy::printer::{GenericPrinter, PrinterHandle};
+
+/// Fills in the credential a human is told to supply, so the entry loads.
+fn with_credentials(entry: &str) -> String {
+    entry
+        .replace("access_code = \"\"", "access_code = \"12345678\"")
+        .replace("api_key = \"\"", "api_key = \"an-api-key\"")
+}
+
+fn load(toml: &str) -> anyhow::Result<Vec<PrinterHandle>> {
+    let mut file = tempfile();
+    file.write_all(toml.as_bytes()).expect("write the fleet file");
+    fleet::load(&file.path)
+}
+
+/// A self-deleting temporary file, rather than a dependency for four lines.
+struct TempToml {
+    path: std::path::PathBuf,
+    handle: Option<std::fs::File>,
+}
+
+impl TempToml {
+    fn write_all(&mut self, bytes: &[u8]) -> std::io::Result<()> {
+        self.handle.as_mut().expect("still open").write_all(bytes)?;
+        self.handle.as_mut().expect("still open").flush()
+    }
+}
+
+impl Drop for TempToml {
+    fn drop(&mut self) {
+        self.handle.take();
+        let _ = std::fs::remove_file(&self.path);
+    }
+}
+
+fn tempfile() -> TempToml {
+    // The pid and a counter, so two tests in one binary cannot collide.
+    use std::sync::atomic::{AtomicUsize, Ordering};
+    static NEXT: AtomicUsize = AtomicUsize::new(0);
+    let path =
+        std::env::temp_dir().join(format!("continuum-fleet-{}-{}.toml", std::process::id(), NEXT.fetch_add(1, Ordering::Relaxed)));
+    let handle = std::fs::File::create(&path).expect("create a temp fleet file");
+    TempToml { path, handle: Some(handle) }
+}
+
+fn candidate(vendor: Vendor, host: &str) -> Candidate {
+    let mut candidate = Candidate::new(host, vendor, Source::HttpApi);
+    // Bambu needs one, and for every other vendor it is ignored.
+    candidate.serial = Some("01S09B441900143".to_string());
+    candidate
+}
+
+#[test]
+fn every_rendered_entry_loads_as_the_printer_it_claims_to_be() {
+    let cases = [
+        (Vendor::Bambu, "192.168.0.20"),
+        (Vendor::Klipper, "192.168.0.50"),
+        (Vendor::PrusaLink, "192.168.0.51"),
+        (Vendor::Octoprint, "192.168.0.60"),
+        (Vendor::PrusaSerial, "/dev/ttyACM0"),
+    ];
+    for (vendor, host) in cases {
+        let entry = candidate(vendor, host).to_toml_entry("found");
+        let fleet = load(&with_credentials(&entry))
+            .unwrap_or_else(|err| panic!("{vendor:?} rendered an entry fleet::load rejects: {err}\n{entry}"));
+        assert_eq!(fleet.len(), 1, "{vendor:?}");
+        assert_eq!(fleet[0].base().id, "found", "{vendor:?}");
+
+        // The vendor string has to resolve to the matching handle, not just
+        // to *a* handle - `octoprint` and `prusa_link` share a client, so a
+        // mapping that collapsed them would still load and still be wrong.
+        let matched = matches!(
+            (vendor, &fleet[0]),
+            (Vendor::Bambu, PrinterHandle::Bambu(_))
+                | (Vendor::Klipper, PrinterHandle::Klipper(_))
+                | (Vendor::PrusaLink | Vendor::Octoprint, PrinterHandle::OctoPrint(_))
+                | (Vendor::PrusaSerial, PrinterHandle::PrusaSerial(_))
+        );
+        assert!(matched, "{vendor:?} loaded as the wrong handle");
+    }
+}
+
+/// The flavour is the operator's answer to a question no probe can settle, so
+/// it has to survive the trip from the file into the printer.
+#[test]
+fn the_octoprint_flavour_an_operator_chose_reaches_the_client() {
+    use continuum_proxy::printer::OctoPrintFlavour;
+
+    for (vendor, expected) in
+        [(Vendor::Octoprint, OctoPrintFlavour::Generic), (Vendor::PrusaLink, OctoPrintFlavour::PrusaLink)]
+    {
+        let entry = with_credentials(&candidate(vendor, "192.168.0.51").to_toml_entry("found"));
+        let fleet = load(&entry).expect("loads");
+        let PrinterHandle::OctoPrint(printer) = &fleet[0] else {
+            panic!("{vendor:?} should load as an OctoPrint-family client");
+        };
+        assert_eq!(printer.flavour, expected, "{vendor:?}");
+    }
+}
+
+/// An entry straight out of discovery, credential still blank, must be
+/// refused - and refused in a way that names the printer and the field.
+#[test]
+fn an_entry_with_the_credential_left_blank_is_refused_by_name() {
+    for vendor in [Vendor::Bambu, Vendor::PrusaLink, Vendor::Octoprint] {
+        let entry = candidate(vendor, "192.168.0.20").to_toml_entry("unfinished");
+        let Err(err) = load(&entry) else {
+            panic!("{vendor:?}: a blank credential must not load:\n{entry}");
+        };
+        let message = err.to_string();
+        // This is the whole reason discovery prints instead of writing the
+        // file: the failure is loud, and it says which printer and which
+        // field.
+        assert!(message.contains("unfinished"), "{vendor:?}: {message}");
+        assert!(
+            message.contains("access_code") || message.contains("api_key"),
+            "{vendor:?} must name the missing field: {message}"
+        );
+    }
+}