feat(neuron,cortex-core): source-aware loader (scheme:org/name)

Phase 1 of plan-source-aware-loader-preflight. Makes neuron's
loader treat `huggingface:org/name` and `helexa:org/name` as
first-class distinct sources with per-source endpoint + cache,
while staying backwards-compatible with bare `org/name` ids.
Zero behavior change for existing operator configs.

Motivation: helexa is adding an EU-hosted registry
(`registry.helexa.ai`) alongside HF. Both speak HF-compatible
wire format, but the bytes, jurisdiction, trust root, and cache
namespace are distinct. The loader needs to disambiguate which
registry serves a given model id, and to keep their caches from
colliding on disk when both happen to host the same `org/name`.

What lands:

- `cortex-core::source` — new module. `ModelSourceId { scheme,
  org, name }` with `FromStr` accepting both `scheme:org/name`
  and bare `org/name`. `Display` round-trips. `repo_path()`
  emits the `org/name` half for the hf-hub `Api::model(...)`
  call regardless of which scheme/endpoint we're hitting.
  Rejects malformed input with typed `ParseError` variants
  (empty scheme, missing slash, scheme with `/`, name with
  `:`, etc.).

- `neuron::config::CandleHarnessConfig` gains
  `default_source: Option<String>` and
  `sources: HashMap<String, SourceConfig>`. `SourceConfig`
  mirrors what `hf_hub::ApiBuilder` consumes: endpoint URL,
  optional `auth_env` (env var name read at startup so secrets
  stay out of TOML), and optional cache_dir. Defaults
  synthesise a `huggingface` entry pointing at
  `https://huggingface.co` with the legacy `hf_cache` field as
  its cache_dir — so existing configs that only set `hf_cache`
  keep working unchanged.

- `CandleHarness::new(bind_url, &CandleHarnessConfig)` replaces
  `CandleHarness::new(bind_url, hf_cache)`. Resolves every
  configured source's auth env var and cache dir up front so
  `hf_api_for(scheme)` is a pure HashMap lookup on the hot
  load path. Only the `huggingface` scheme gets the legacy
  `HF_HUB_CACHE`/`HF_HOME` env-var fallback chain; other
  schemes resolve to whatever the operator typed.

- `hf_api()` -> `hf_api_for(scheme)`. Builds an
  `hf_hub::Api` with the source's endpoint, cache_dir, and
  auth token. Errors with a useful message naming the
  configured schemes when an unknown scheme is requested.

- `CandleHarness::load_model` parses `spec.model_id` into a
  `ModelSourceId`, substitutes `default_source` for bare ids,
  and threads the parsed source through `preflight`,
  `resolve_files`, `resolve_dense_files`, `load_arch_gguf`,
  `load_arch_dense`, and `load_tp`. The hf-hub `Api::model()`
  call now uses `source_id.repo_path()` so registry calls hit
  the right URL shape regardless of scheme.

- `preflight()` signature gains a `&ModelSourceId` parameter
  (it's the canonical id for log lines and error display);
  `RepoFetchFailed.model_id` etc. now carry the
  scheme-qualified form so operator-visible errors echo
  exactly what was configured.

- `neuron.example.toml` documents the new
  `[harness.candle.sources.*]` table with commented-out
  examples for `huggingface` (explicit override) and `helexa`.

Tests:

- 13 new unit tests in `cortex-core::source` covering parse /
  display round-trip, default-scheme substitution semantics,
  and every `ParseError` variant.
- 6 new unit tests in `neuron::config` covering the
  `effective_sources` synth (legacy `hf_cache` carry-through,
  explicit override preservation, helexa-alongside-huggingface)
  and `effective_default_source` fallback.
- 2 new unit tests in `harness::candle::tests` covering
  multi-scheme `hf_api_for` routing, including the
  "unknown scheme" error path naming configured schemes.
- Preflight integration tests updated to construct
  `ModelSourceId` and assert against the scheme-qualified
  error form.

CI gate: cargo fmt --check, cargo clippy --workspace
--all-targets -- -D warnings, cargo test --workspace (all 24
test groups ok, zero failures).

Out of scope (Phase 3):
- Cortex catalogue `source` field — independent of Phase 1+2,
  ships when the registry comes online.
- `helexa` source endpoint itself — separate project; this
  PR adds the client-side rails only.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/cortex-core/src/lib.rs

@@ -7,4 +7,5 @@ pub mod metrics;
 pub mod node;
 pub mod openai;
 pub mod responses;
+pub mod source;
 pub mod translate;

crates/cortex-core/src/source.rs

@@ -0,0 +1,267 @@
+//! Scheme-qualified model identifiers.
+//!
+//! cortex/neuron historically resolves every model id through hf-hub
+//! against `https://huggingface.co`. Helexa is adding an EU-hosted
+//! registry (`registry.helexa.ai`) alongside HF — both speak the same
+//! HF-compatible wire format, but the bytes, jurisdiction, and trust
+//! root differ. Model ids therefore need a scheme:
+//!
+//!   - `huggingface:Qwen/Qwen3.6-27B`         — HF-hosted bytes
+//!   - `helexa:Qwen/Qwen3.6-27B-Uncensored`  — helexa registry bytes
+//!   - `helexa:SomeOperator/CustomFinetune`  — operator publishing
+//!     under the helexa namespace; same scheme handles all `org/name`
+//!     pairs hosted in that registry.
+//!
+//! Bare `org/name` parses with an empty scheme; the caller (typically
+//! a harness) substitutes its configured default scheme so existing
+//! configs keep working through the transition.
+
+use serde::{Deserialize, Serialize};
+use std::fmt;
+use std::str::FromStr;
+
+/// Parsed `scheme:org/name`. Bare `org/name` produces an empty scheme
+/// — call `with_default_scheme` (or check `is_scheme_unset`) to
+/// resolve before using.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub struct ModelSourceId {
+    pub scheme: String,
+    pub org: String,
+    pub name: String,
+}
+
+/// Errors from `ModelSourceId::from_str`. Carries the offending input
+/// so log lines / API errors can echo what the operator typed.
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum ParseError {
+    #[error("empty model id")]
+    Empty,
+    #[error("model id '{0}' is missing the '/' between org and name")]
+    MissingSlash(String),
+    #[error("model id '{0}' has an empty scheme before ':'")]
+    EmptyScheme(String),
+    #[error("model id '{0}' has an empty org")]
+    EmptyOrg(String),
+    #[error("model id '{0}' has an empty name")]
+    EmptyName(String),
+    #[error("model id '{0}' has a scheme containing '/' which is reserved for org/name")]
+    SchemeContainsSlash(String),
+    #[error("model id '{0}' has a name containing ':' which is reserved for the scheme prefix")]
+    NameContainsColon(String),
+}
+
+impl ModelSourceId {
+    /// Construct directly from already-validated parts. Used by tests
+    /// and call sites that have the fields separately; the public API
+    /// for parsing user input is `FromStr`.
+    pub fn new(scheme: impl Into<String>, org: impl Into<String>, name: impl Into<String>) -> Self {
+        Self {
+            scheme: scheme.into(),
+            org: org.into(),
+            name: name.into(),
+        }
+    }
+
+    /// True when this id parsed from a bare `org/name` (no scheme
+    /// prefix). The harness substitutes its configured default in
+    /// `with_default_scheme` before resolving against a registry.
+    pub fn is_scheme_unset(&self) -> bool {
+        self.scheme.is_empty()
+    }
+
+    /// Substitute `default` for an empty scheme. No-op when the scheme
+    /// is already set. Returns self by value so it composes neatly:
+    /// `id.parse::<ModelSourceId>()?.with_default_scheme("huggingface")`.
+    pub fn with_default_scheme(mut self, default: &str) -> Self {
+        if self.scheme.is_empty() {
+            self.scheme = default.to_string();
+        }
+        self
+    }
+
+    /// The `org/name` half — what an hf-hub `Api::model(...)` call
+    /// expects regardless of which scheme/endpoint we're hitting.
+    pub fn repo_path(&self) -> String {
+        format!("{}/{}", self.org, self.name)
+    }
+}
+
+impl fmt::Display for ModelSourceId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.scheme.is_empty() {
+            write!(f, "{}/{}", self.org, self.name)
+        } else {
+            write!(f, "{}:{}/{}", self.scheme, self.org, self.name)
+        }
+    }
+}
+
+impl FromStr for ModelSourceId {
+    type Err = ParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if s.is_empty() {
+            return Err(ParseError::Empty);
+        }
+        // Scheme split. Only the *first* colon counts — anything after
+        // belongs to org/name (and would be rejected separately because
+        // `:` isn't allowed there).
+        let (scheme, rest) = match s.split_once(':') {
+            Some((scheme, rest)) => {
+                if scheme.is_empty() {
+                    return Err(ParseError::EmptyScheme(s.to_string()));
+                }
+                if scheme.contains('/') {
+                    return Err(ParseError::SchemeContainsSlash(s.to_string()));
+                }
+                (scheme.to_string(), rest)
+            }
+            None => (String::new(), s),
+        };
+        let (org, name) = rest
+            .split_once('/')
+            .ok_or_else(|| ParseError::MissingSlash(s.to_string()))?;
+        if org.is_empty() {
+            return Err(ParseError::EmptyOrg(s.to_string()));
+        }
+        if name.is_empty() {
+            return Err(ParseError::EmptyName(s.to_string()));
+        }
+        if name.contains(':') {
+            return Err(ParseError::NameContainsColon(s.to_string()));
+        }
+        Ok(Self {
+            scheme,
+            org: org.to_string(),
+            name: name.to_string(),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_qualified() {
+        let id: ModelSourceId = "huggingface:Qwen/Qwen3.6-27B".parse().unwrap();
+        assert_eq!(id.scheme, "huggingface");
+        assert_eq!(id.org, "Qwen");
+        assert_eq!(id.name, "Qwen3.6-27B");
+        assert_eq!(id.repo_path(), "Qwen/Qwen3.6-27B");
+        assert!(!id.is_scheme_unset());
+    }
+
+    #[test]
+    fn parses_helexa_scheme() {
+        let id: ModelSourceId = "helexa:SomeOperator/Qwen3.6-27B-Uncensored"
+            .parse()
+            .unwrap();
+        assert_eq!(id.scheme, "helexa");
+        assert_eq!(id.org, "SomeOperator");
+        assert_eq!(id.name, "Qwen3.6-27B-Uncensored");
+    }
+
+    #[test]
+    fn parses_bare_id_with_empty_scheme() {
+        let id: ModelSourceId = "Qwen/Qwen3-30B-A3B-Instruct".parse().unwrap();
+        assert_eq!(id.scheme, "");
+        assert_eq!(id.org, "Qwen");
+        assert_eq!(id.name, "Qwen3-30B-A3B-Instruct");
+        assert!(id.is_scheme_unset());
+    }
+
+    #[test]
+    fn substitutes_default_scheme_only_when_unset() {
+        let id: ModelSourceId = "Qwen/Q3".parse().unwrap();
+        assert_eq!(id.with_default_scheme("huggingface").scheme, "huggingface");
+
+        let id: ModelSourceId = "helexa:Qwen/Q3".parse().unwrap();
+        assert_eq!(
+            id.with_default_scheme("huggingface").scheme,
+            "helexa",
+            "default substitution must not override an explicit scheme"
+        );
+    }
+
+    #[test]
+    fn display_roundtrips_qualified_id() {
+        let s = "helexa:Helexa/Qwen3.6-27B";
+        let id: ModelSourceId = s.parse().unwrap();
+        assert_eq!(id.to_string(), s);
+    }
+
+    #[test]
+    fn display_roundtrips_bare_id() {
+        let s = "Qwen/Q3";
+        let id: ModelSourceId = s.parse().unwrap();
+        assert_eq!(id.to_string(), s);
+    }
+
+    #[test]
+    fn rejects_empty() {
+        assert_eq!("".parse::<ModelSourceId>().unwrap_err(), ParseError::Empty);
+    }
+
+    #[test]
+    fn rejects_missing_slash() {
+        match "Qwen".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::MissingSlash(s) => assert_eq!(s, "Qwen"),
+            other => panic!("expected MissingSlash, got {other:?}"),
+        }
+        match "huggingface:Qwen".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::MissingSlash(s) => assert_eq!(s, "huggingface:Qwen"),
+            other => panic!("expected MissingSlash, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn rejects_empty_scheme() {
+        match ":Qwen/Q3".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::EmptyScheme(s) => assert_eq!(s, ":Qwen/Q3"),
+            other => panic!("expected EmptyScheme, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn rejects_scheme_with_slash() {
+        match "hugg/ingface:Q/N".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::SchemeContainsSlash(s) => assert_eq!(s, "hugg/ingface:Q/N"),
+            other => panic!("expected SchemeContainsSlash, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn rejects_empty_org_or_name() {
+        match "huggingface:/N".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::EmptyOrg(_) => {}
+            other => panic!("expected EmptyOrg, got {other:?}"),
+        }
+        match "huggingface:Q/".parse::<ModelSourceId>().unwrap_err() {
+            ParseError::EmptyName(_) => {}
+            other => panic!("expected EmptyName, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn rejects_name_with_colon() {
+        match "huggingface:Q/N:weird"
+            .parse::<ModelSourceId>()
+            .unwrap_err()
+        {
+            ParseError::NameContainsColon(s) => assert_eq!(s, "huggingface:Q/N:weird"),
+            other => panic!("expected NameContainsColon, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn serde_roundtrips_via_struct() {
+        // We serialize as a struct (scheme/org/name fields) so the
+        // shape is self-describing in API payloads. Callers that want
+        // the compact `scheme:org/name` string use `Display`/`FromStr`.
+        let id = ModelSourceId::new("helexa", "Helexa", "Qwen3.6-27B");
+        let json = serde_json::to_string(&id).unwrap();
+        let back: ModelSourceId = serde_json::from_str(&json).unwrap();
+        assert_eq!(back, id);
+    }
+}

crates/neuron/src/config.rs

@@ -6,8 +6,18 @@ use figment::{
     providers::{Env, Format, Toml},
 };
 use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
+/// Default scheme name applied to bare `org/name` model ids when no
+/// `[harness.candle.default_source]` is set. Keeps existing operator
+/// configs (which know nothing about schemes) working unchanged.
+pub const DEFAULT_SOURCE_SCHEME: &str = "huggingface";
+
+/// Endpoint URL for the default huggingface source, used when no
+/// `[harness.candle.sources.huggingface]` is configured.
+pub const DEFAULT_HF_ENDPOINT: &str = "https://huggingface.co";
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct NeuronConfig {
     #[serde(default = "default_port")]
@@ -37,8 +47,88 @@ pub struct HarnessSettings {
 pub struct CandleHarnessConfig {
     /// HuggingFace cache directory for model weights.
     /// When unset, defers to hf-hub's default (~/.cache/huggingface).
+    ///
+    /// Retained for back-compat — operators with existing
+    /// `hf_cache = "..."` configs continue to work. Treated as the
+    /// `huggingface` source's cache_dir when a sources table isn't
+    /// provided.
     #[serde(default)]
     pub hf_cache: Option<PathBuf>,
+
+    /// Default source scheme applied to bare `org/name` model ids
+    /// (those without an explicit `scheme:` prefix). When unset, falls
+    /// back to `DEFAULT_SOURCE_SCHEME` ("huggingface").
+    #[serde(default)]
+    pub default_source: Option<String>,
+
+    /// Per-scheme source endpoints. Each entry maps a scheme name
+    /// (`huggingface`, `helexa`, an operator's mirror tag, …) to its
+    /// endpoint URL, optional auth env var, and optional cache
+    /// directory.
+    ///
+    /// When absent or missing the `huggingface` key, the loader
+    /// synthesises a `huggingface` entry pointing at
+    /// `https://huggingface.co` with `hf_cache` (above) as its
+    /// cache_dir. This keeps single-source configs ergonomic.
+    #[serde(default)]
+    pub sources: HashMap<String, SourceConfig>,
+}
+
+/// Per-scheme source configuration. Mirrors the shape `hf_hub::ApiBuilder`
+/// needs: endpoint URL, optional auth token (read from an env var so
+/// secrets stay out of the config file), and optional cache directory
+/// disambiguated per source to prevent mirror-vs-canonical collisions.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct SourceConfig {
+    /// Base URL of the registry. Must speak the HF-compatible wire
+    /// format (siblings listing at
+    /// `/api/models/{org}/{name}[/revision/{rev}]`, blob fetch at
+    /// `/{org}/{name}/resolve/{rev}/{path}`).
+    pub endpoint: String,
+
+    /// Environment variable name to read for the bearer token used
+    /// against this source. `None` = anonymous. Reading from env
+    /// (vs. literal token in the config) keeps secrets out of TOML.
+    #[serde(default)]
+    pub auth_env: Option<String>,
+
+    /// Cache directory for this source. The hf-hub
+    /// `models--{org}--{name}/snapshots/...` tree lives directly
+    /// under this path, so distinct sources serving the same
+    /// `org/name` cannot collide on disk.
+    ///
+    /// `None` means "share the harness `hf_cache` directory" — only
+    /// safe when the operator has exactly one source configured.
+    #[serde(default)]
+    pub cache_dir: Option<PathBuf>,
+}
+
+impl CandleHarnessConfig {
+    /// Resolve the effective sources map for this config, synthesising
+    /// a `huggingface` entry from legacy fields (`hf_cache`) when the
+    /// operator hasn't supplied a sources table. Idempotent.
+    ///
+    /// Returns a fresh map rather than mutating self so the original
+    /// (operator-typed) config can still be serialized back to TOML
+    /// for diagnostics.
+    pub fn effective_sources(&self) -> HashMap<String, SourceConfig> {
+        let mut out = self.sources.clone();
+        out.entry(DEFAULT_SOURCE_SCHEME.to_string())
+            .or_insert_with(|| SourceConfig {
+                endpoint: DEFAULT_HF_ENDPOINT.to_string(),
+                auth_env: Some("HF_TOKEN".to_string()),
+                cache_dir: self.hf_cache.clone(),
+            });
+        out
+    }
+
+    /// Effective default scheme. Falls back to `DEFAULT_SOURCE_SCHEME`
+    /// when the operator hasn't pinned one.
+    pub fn effective_default_source(&self) -> &str {
+        self.default_source
+            .as_deref()
+            .unwrap_or(DEFAULT_SOURCE_SCHEME)
+    }
 }
 
 fn default_port() -> u16 {
@@ -65,3 +155,109 @@ impl Default for NeuronConfig {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn effective_sources_synthesises_huggingface_when_absent() {
+        let cfg = CandleHarnessConfig::default();
+        let sources = cfg.effective_sources();
+        assert!(sources.contains_key("huggingface"));
+        let hf = &sources["huggingface"];
+        assert_eq!(hf.endpoint, DEFAULT_HF_ENDPOINT);
+        assert_eq!(hf.auth_env.as_deref(), Some("HF_TOKEN"));
+        assert!(hf.cache_dir.is_none());
+    }
+
+    #[test]
+    fn effective_sources_carries_legacy_hf_cache_into_synth_entry() {
+        // Existing operator configs only set `hf_cache = "/archive3/..."`
+        // — the synth must pick that up so the loader keeps using the
+        // operator's storage.
+        let cfg = CandleHarnessConfig {
+            hf_cache: Some(PathBuf::from("/archive3/llm-cache")),
+            ..Default::default()
+        };
+        let sources = cfg.effective_sources();
+        assert_eq!(
+            sources["huggingface"].cache_dir.as_deref(),
+            Some(Path::new("/archive3/llm-cache"))
+        );
+    }
+
+    #[test]
+    fn effective_sources_preserves_explicit_huggingface_entry() {
+        // When an operator types out `[harness.candle.sources.huggingface]`
+        // explicitly, we must not clobber it with the synth defaults.
+        let mut sources = HashMap::new();
+        sources.insert(
+            "huggingface".to_string(),
+            SourceConfig {
+                endpoint: "https://huggingface.example.org".into(),
+                auth_env: Some("MY_TOKEN".into()),
+                cache_dir: Some(PathBuf::from("/operator-cache")),
+            },
+        );
+        let cfg = CandleHarnessConfig {
+            hf_cache: Some(PathBuf::from("/legacy-cache")),
+            sources,
+            ..Default::default()
+        };
+        let effective = cfg.effective_sources();
+        assert_eq!(
+            effective["huggingface"].endpoint,
+            "https://huggingface.example.org"
+        );
+        assert_eq!(
+            effective["huggingface"].auth_env.as_deref(),
+            Some("MY_TOKEN")
+        );
+        assert_eq!(
+            effective["huggingface"].cache_dir.as_deref(),
+            Some(Path::new("/operator-cache"))
+        );
+    }
+
+    #[test]
+    fn effective_sources_includes_helexa_alongside_synth_huggingface() {
+        let mut sources = HashMap::new();
+        sources.insert(
+            "helexa".to_string(),
+            SourceConfig {
+                endpoint: "https://registry.helexa.ai".into(),
+                auth_env: Some("HELEXA_TOKEN".into()),
+                cache_dir: Some(PathBuf::from("/archive3/llm-cache/helexa")),
+            },
+        );
+        let cfg = CandleHarnessConfig {
+            hf_cache: Some(PathBuf::from("/archive3/llm-cache/huggingface")),
+            sources,
+            ..Default::default()
+        };
+        let effective = cfg.effective_sources();
+        assert_eq!(effective.len(), 2);
+        assert_eq!(effective["helexa"].endpoint, "https://registry.helexa.ai");
+        // huggingface still gets synth-derived from legacy hf_cache.
+        assert_eq!(
+            effective["huggingface"].cache_dir.as_deref(),
+            Some(Path::new("/archive3/llm-cache/huggingface"))
+        );
+    }
+
+    #[test]
+    fn effective_default_source_falls_back() {
+        let cfg = CandleHarnessConfig::default();
+        assert_eq!(cfg.effective_default_source(), DEFAULT_SOURCE_SCHEME);
+    }
+
+    #[test]
+    fn effective_default_source_honours_explicit() {
+        let cfg = CandleHarnessConfig {
+            default_source: Some("helexa".into()),
+            ..Default::default()
+        };
+        assert_eq!(cfg.effective_default_source(), "helexa");
+    }
+}

crates/neuron/src/harness/candle.rs

@@ -44,7 +44,13 @@ use tracing::Instrument;
 /// In-process candle harness. Owns the loaded model registry.
 pub struct CandleHarness {
     models: Arc<RwLock<HashMap<String, LoadedHandle>>>,
-    hf_cache: Option<PathBuf>,
+    /// Post-resolution source map: scheme → endpoint/token/cache. Built
+    /// in `new()` from the operator's `CandleHarnessConfig`; auth tokens
+    /// are read from their configured env vars at startup so secrets
+    /// don't leak through the config file.
+    sources: HashMap<String, ResolvedSource>,
+    /// Scheme to substitute for bare `org/name` model ids.
+    default_source: String,
     bind_url: String,
     /// One worker thread per CUDA device index that owns its
     /// `CudaContext` for the daemon's lifetime. Populated lazily by
@@ -968,20 +974,87 @@ async fn chunked_prefill_tp(
     last_logits.ok_or_else(|| anyhow::anyhow!("chunked_prefill_tp: no chunks produced"))
 }
 
+/// Per-scheme source after env-var resolution. The auth token is the
+/// already-read env-var value (or None for anonymous access), and the
+/// cache dir is the post-`resolve_hf_cache` path for the huggingface
+/// scheme and the operator's literal value for everything else.
+#[derive(Debug, Clone)]
+struct ResolvedSource {
+    endpoint: String,
+    auth_token: Option<String>,
+    cache_dir: Option<PathBuf>,
+}
+
 impl CandleHarness {
-    pub fn new(bind_url: String, hf_cache: Option<PathBuf>) -> Self {
-        let hf_cache = resolve_hf_cache(hf_cache);
-        if let Some(p) = &hf_cache {
-            tracing::info!(path = %p.display(), "candle harness using HuggingFace cache");
+    /// Construct a new harness for `bind_url` using `config`. Resolves
+    /// every configured source's auth env var and cache dir up front so
+    /// the hot load path (`hf_api_for`) is a pure HashMap lookup.
+    pub fn new(bind_url: String, config: &crate::config::CandleHarnessConfig) -> Self {
+        let raw_sources = config.effective_sources();
+        let default_source = config.effective_default_source().to_string();
+        let mut sources = HashMap::with_capacity(raw_sources.len());
+        for (scheme, src) in raw_sources.into_iter() {
+            // Only the huggingface source gets the legacy
+            // HF_HUB_CACHE/HF_HOME env-var fallback chain — other
+            // schemes resolve to whatever the operator typed.
+            let cache_dir = if scheme == crate::config::DEFAULT_SOURCE_SCHEME {
+                resolve_hf_cache(src.cache_dir.clone())
+            } else {
+                src.cache_dir.clone()
+            };
+            let auth_token = src
+                .auth_env
+                .as_deref()
+                .and_then(|var| std::env::var(var).ok())
+                .filter(|v| !v.is_empty());
+            if let Some(p) = &cache_dir {
+                tracing::info!(
+                    scheme = %scheme,
+                    endpoint = %src.endpoint,
+                    cache = %p.display(),
+                    auth = auth_token.is_some(),
+                    "candle harness source resolved"
+                );
+            } else {
+                tracing::info!(
+                    scheme = %scheme,
+                    endpoint = %src.endpoint,
+                    auth = auth_token.is_some(),
+                    "candle harness source resolved (no cache dir; using hf-hub default)"
+                );
+            }
+            sources.insert(
+                scheme,
+                ResolvedSource {
+                    endpoint: src.endpoint,
+                    auth_token,
+                    cache_dir,
+                },
+            );
+        }
+        if !sources.contains_key(&default_source) {
+            tracing::warn!(
+                default_source,
+                "configured default_source has no matching [harness.candle.sources.*] entry; \
+                 bare model ids will fail to resolve until this is fixed"
+            );
         }
         Self {
             models: Arc::new(RwLock::new(HashMap::new())),
-            hf_cache,
+            sources,
+            default_source,
             bind_url,
             device_workers: Arc::new(RwLock::new(HashMap::new())),
         }
     }
 
+    /// Scheme to substitute for bare `org/name` model ids. Mirrors the
+    /// effective default from the operator's config, exposed for the
+    /// load path's `ModelSourceId::with_default_scheme`.
+    pub(crate) fn default_source_scheme(&self) -> &str {
+        &self.default_source
+    }
+
     /// Pick a candle `Device` for the requested indices. Without the
     /// `cuda` feature, or if CUDA initialisation fails, falls back to CPU.
     fn pick_device(devices: &[u32]) -> Result<Device> {
@@ -1033,14 +1106,33 @@ impl CandleHarness {
         Ok(handle)
     }
 
-    /// Build an hf-hub API client pre-configured with the harness's
-    /// `hf_cache` (when one is set).
-    fn hf_api(&self) -> Result<hf_hub::api::tokio::Api> {
-        let mut builder = hf_hub::api::tokio::ApiBuilder::new();
-        if let Some(cache) = &self.hf_cache {
+    /// Build an hf-hub API client for the given scheme. The scheme
+    /// must be present in the operator's configured `sources` table
+    /// (the synth `huggingface` entry counts). Each source carries its
+    /// own endpoint, optional bearer token, and cache directory, so
+    /// the same `org/name` served by two registries cannot collide on
+    /// disk.
+    pub(crate) fn hf_api_for(&self, scheme: &str) -> Result<hf_hub::api::tokio::Api> {
+        let src = self.sources.get(scheme).ok_or_else(|| {
+            let mut configured: Vec<&str> = self.sources.keys().map(String::as_str).collect();
+            configured.sort();
+            anyhow::anyhow!(
+                "no source configured for scheme '{scheme}'; \
+                 configured: {configured:?}. Add a \
+                 [harness.candle.sources.{scheme}] block to neuron.toml \
+                 with endpoint = '...'."
+            )
+        })?;
+        let mut builder = hf_hub::api::tokio::ApiBuilder::new().with_endpoint(src.endpoint.clone());
+        if let Some(cache) = &src.cache_dir {
             builder = builder.with_cache_dir(cache.clone());
         }
-        builder.build().context("build hf-hub API")
+        if let Some(token) = &src.auth_token {
+            builder = builder.with_token(Some(token.clone()));
+        }
+        builder
+            .build()
+            .with_context(|| format!("build hf-hub API for scheme '{scheme}'"))
     }
 
     /// Resolve a dense (bf16/fp16 safetensors) model to its local file
@@ -1053,18 +1145,21 @@ impl CandleHarness {
     async fn resolve_dense_files(
         &self,
         spec: &ModelSpec,
+        source_id: &cortex_core::source::ModelSourceId,
     ) -> Result<(PathBuf, PathBuf, Vec<PathBuf>)> {
-        let api = self.hf_api()?;
-        let repo = api.model(spec.model_id.clone());
+        let api = self.hf_api_for(&source_id.scheme)?;
+        let repo = api.model(source_id.repo_path());
+        let display_id = source_id.to_string();
+        let _ = spec; // reserved for future use (quant-aware filtering)
 
         let config_path = repo
             .get("config.json")
             .await
-            .with_context(|| format!("fetch config.json from {}", spec.model_id))?;
+            .with_context(|| format!("fetch config.json from {display_id}"))?;
         let tokenizer_path = repo
             .get("tokenizer.json")
             .await
-            .with_context(|| format!("fetch tokenizer.json from {}", spec.model_id))?;
+            .with_context(|| format!("fetch tokenizer.json from {display_id}"))?;
 
         // Prefer the sharded layout (most HF dense models > 5B ship it).
         let safetensors_paths = match repo.get("model.safetensors.index.json").await {
@@ -1111,9 +1206,10 @@ impl CandleHarness {
     async fn load_arch_gguf(
         &self,
         spec: &ModelSpec,
+        source_id: &cortex_core::source::ModelSourceId,
         device: &Device,
     ) -> Result<(PathBuf, ModelArch)> {
-        let (gguf_path, tokenizer_path) = self.resolve_files(spec).await?;
+        let (gguf_path, tokenizer_path) = self.resolve_files(spec, source_id).await?;
         let device_for_load = device.clone();
         let gguf_path_for_load = gguf_path.clone();
         let model_id_for_log = spec.model_id.clone();
@@ -1176,10 +1272,11 @@ impl CandleHarness {
     async fn load_arch_dense(
         &self,
         spec: &ModelSpec,
+        source_id: &cortex_core::source::ModelSourceId,
         device: &Device,
     ) -> Result<(PathBuf, ModelArch)> {
         let (config_path, tokenizer_path, safetensors_paths) =
-            self.resolve_dense_files(spec).await?;
+            self.resolve_dense_files(spec, source_id).await?;
         let device_for_load = device.clone();
         let model_id_for_log = spec.model_id.clone();
 
@@ -1290,14 +1387,20 @@ impl CandleHarness {
 
     /// Resolve a model spec to local GGUF and tokenizer file paths via
     /// hf-hub. Downloads on first use; subsequent calls are cached.
-    async fn resolve_files(&self, spec: &ModelSpec) -> Result<(PathBuf, PathBuf)> {
-        let api = self.hf_api()?;
-        let repo = api.model(spec.model_id.clone());
+    async fn resolve_files(
+        &self,
+        spec: &ModelSpec,
+        source_id: &cortex_core::source::ModelSourceId,
+    ) -> Result<(PathBuf, PathBuf)> {
+        let api = self.hf_api_for(&source_id.scheme)?;
+        let repo_path = source_id.repo_path();
+        let repo = api.model(repo_path.clone());
+        let display_id = source_id.to_string();
 
         let info = repo
             .info()
             .await
-            .with_context(|| format!("fetch HF repo info for {}", spec.model_id))?;
+            .with_context(|| format!("fetch HF repo info for {display_id}"))?;
 
         let quant = spec.quant.as_deref().unwrap_or("");
         let quant_lc = quant.to_lowercase();
@@ -1309,15 +1412,14 @@ impl CandleHarness {
             .find(|name| quant_lc.is_empty() || name.to_lowercase().contains(&quant_lc))
             .ok_or_else(|| {
                 anyhow::anyhow!(
-                    "no GGUF file matching quant {:?} in repo {}",
+                    "no GGUF file matching quant {:?} in repo {display_id}",
                     spec.quant,
-                    spec.model_id
                 )
             })?
             .to_string();
 
         tracing::info!(
-            model = %spec.model_id,
+            model = %display_id,
             file = %gguf_filename,
             "resolving GGUF (may be cached)"
         );
@@ -1331,27 +1433,28 @@ impl CandleHarness {
         // tokenizer.json lives in the base non-GGUF repo. Derive the
         // base repo id by stripping a `-GGUF` / `-gguf` suffix; if
         // there's no such suffix the same repo is used (works for
-        // non-GGUF model_ids).
-        let tokenizer_repo_id = spec
-            .model_id
+        // non-GGUF model_ids). Stripping happens on the repo_path
+        // (scheme already accounted for) so this composes cleanly with
+        // helexa-scheme GGUF repos too.
+        let tokenizer_repo_path = repo_path
             .strip_suffix("-GGUF")
-            .or_else(|| spec.model_id.strip_suffix("-gguf"))
-            .unwrap_or(spec.model_id.as_str())
+            .or_else(|| repo_path.strip_suffix("-gguf"))
+            .unwrap_or(&repo_path)
             .to_string();
-        let tokenizer_repo = if tokenizer_repo_id == spec.model_id {
+        let tokenizer_repo = if tokenizer_repo_path == repo_path {
             repo
         } else {
             tracing::debug!(
-                from = %spec.model_id,
-                to = %tokenizer_repo_id,
+                from = %repo_path,
+                to = %tokenizer_repo_path,
                 "tokenizer.json sourced from base repo (GGUF suffix stripped)"
             );
-            api.model(tokenizer_repo_id.clone())
+            api.model(tokenizer_repo_path.clone())
         };
         let tokenizer_path = tokenizer_repo
             .get("tokenizer.json")
             .await
-            .with_context(|| format!("fetch tokenizer.json from {tokenizer_repo_id}"))?;
+            .with_context(|| format!("fetch tokenizer.json from {tokenizer_repo_path}"))?;
         Ok((gguf_path, tokenizer_path))
     }
 
@@ -2002,6 +2105,16 @@ impl Harness for CandleHarness {
             }
         }
 
+        // Parse the model id, substituting the harness's default
+        // source for bare `org/name` entries so existing operator
+        // configs keep working unchanged. Stored on the request-local
+        // path so downstream resolve_* can ask the right registry.
+        let source_id = spec
+            .model_id
+            .parse::<cortex_core::source::ModelSourceId>()
+            .with_context(|| format!("parse model id '{}' as scheme:org/name", spec.model_id))?
+            .with_default_scheme(self.default_source_scheme());
+
         // Preflight: classify the source repo and apply the
         // tp/quant/source feasibility table before any device
         // allocation, NCCL handshake, or weight fetch. Failures bubble
@@ -2011,8 +2124,8 @@ impl Harness for CandleHarness {
         // dispatch — downstream `resolve_files` / `resolve_dense_files`
         // re-run their own substring match — but the structured error
         // surface is the main payoff.
-        let api = self.hf_api()?;
-        super::preflight::preflight(&api, spec)
+        let api = self.hf_api_for(&source_id.scheme)?;
+        super::preflight::preflight(&api, &source_id, spec)
             .await
             .map_err(anyhow::Error::new)?;
 
@@ -2020,7 +2133,7 @@ impl Harness for CandleHarness {
         if tp_size > 1 {
             #[cfg(feature = "cuda")]
             {
-                return self.load_tp(spec, tp_size).await;
+                return self.load_tp(spec, &source_id, tp_size).await;
             }
             #[cfg(not(feature = "cuda"))]
             {
@@ -2048,7 +2161,7 @@ impl Harness for CandleHarness {
         let (tokenizer_path, arch_local, arch_handle) = if let Some(w) = &worker {
             // CUDA path: resolve, then load in the worker.
             if spec.quant.is_some() {
-                let (gguf_path, tokenizer_path) = self.resolve_files(spec).await?;
+                let (gguf_path, tokenizer_path) = self.resolve_files(spec, &source_id).await?;
                 let handle = w
                     .load_gguf(gguf_path, spec.model_id.clone())
                     .await
@@ -2056,7 +2169,7 @@ impl Harness for CandleHarness {
                 (tokenizer_path, None, Some(handle))
             } else {
                 let (config_path, tokenizer_path, safetensors_paths) =
-                    self.resolve_dense_files(spec).await?;
+                    self.resolve_dense_files(spec, &source_id).await?;
                 let handle = w
                     .load_dense(config_path, safetensors_paths, spec.model_id.clone())
                     .await
@@ -2066,9 +2179,9 @@ impl Harness for CandleHarness {
         } else {
             // CPU path: legacy spawn_blocking + Arc<Mutex<ModelArch>>.
             let (tokenizer_path, arch) = if spec.quant.is_some() {
-                self.load_arch_gguf(spec, &device).await?
+                self.load_arch_gguf(spec, &source_id, &device).await?
             } else {
-                self.load_arch_dense(spec, &device).await?
+                self.load_arch_dense(spec, &source_id, &device).await?
             };
             (tokenizer_path, Some(Arc::new(Mutex::new(arch))), None)
         };
@@ -2226,7 +2339,12 @@ impl CandleHarness {
     /// `spec.devices` carries the per-rank CUDA device indices (one
     /// entry per rank, in rank order); defaults to `0..tp_size`.
     #[cfg(feature = "cuda")]
-    async fn load_tp(&self, spec: &ModelSpec, tp_size: u32) -> Result<()> {
+    async fn load_tp(
+        &self,
+        spec: &ModelSpec,
+        source_id: &cortex_core::source::ModelSourceId,
+        tp_size: u32,
+    ) -> Result<()> {
         use std::sync::Arc as StdArc;
         use tokio::sync::Mutex as TMutex;
 
@@ -2251,7 +2369,7 @@ impl CandleHarness {
 
         // 1. Resolve config + tokenizer + safetensors via hf-hub.
         let (config_path, tokenizer_path, safetensors_paths) =
-            self.resolve_dense_files(spec).await?;
+            self.resolve_dense_files(spec, source_id).await?;
         let config_json = std::fs::read_to_string(&config_path).context("read config.json")?;
         // Reject unsupported architectures *before* spawning the worker
         // pool and fanning out NCCL — otherwise we'd burn the pool
@@ -4023,4 +4141,75 @@ mod tests {
             "DriverError(CUDA_ERROR_ILLEGAL_ADDRESS, \"an illegal memory access was encountered\")"
         ));
     }
+
+    /// Phase 1 of plan-source-aware-loader: harness must resolve each
+    /// configured scheme to its own endpoint+cache, and reject schemes
+    /// the operator hasn't configured with a useful error.
+    #[test]
+    fn hf_api_for_routes_per_scheme() {
+        use crate::config::{CandleHarnessConfig, SourceConfig};
+        use std::collections::HashMap;
+
+        let mut sources = HashMap::new();
+        sources.insert(
+            "huggingface".to_string(),
+            SourceConfig {
+                endpoint: "https://huggingface.example.org".into(),
+                auth_env: None,
+                cache_dir: Some(std::path::PathBuf::from("/tmp/hf-cache")),
+            },
+        );
+        sources.insert(
+            "helexa".to_string(),
+            SourceConfig {
+                endpoint: "https://registry.helexa.example.ai".into(),
+                auth_env: None,
+                cache_dir: Some(std::path::PathBuf::from("/tmp/helexa-cache")),
+            },
+        );
+        let cfg = CandleHarnessConfig {
+            sources,
+            default_source: Some("huggingface".into()),
+            ..Default::default()
+        };
+        let harness = CandleHarness::new("http://localhost:13131".into(), &cfg);
+
+        // Both configured schemes build cleanly.
+        harness
+            .hf_api_for("huggingface")
+            .expect("huggingface scheme should build");
+        harness
+            .hf_api_for("helexa")
+            .expect("helexa scheme should build");
+
+        // Unknown scheme errors with a message that names the configured
+        // set so the operator can act on it.
+        let err = harness
+            .hf_api_for("does-not-exist")
+            .expect_err("unknown scheme should error");
+        let msg = format!("{err:#}");
+        assert!(
+            msg.contains("does-not-exist") && msg.contains("huggingface") && msg.contains("helexa"),
+            "error must list configured schemes: {msg}"
+        );
+
+        assert_eq!(harness.default_source_scheme(), "huggingface");
+    }
+
+    /// Operator with only `hf_cache` set (no `sources` table) still
+    /// gets a working `huggingface` source pointed at HF.
+    #[test]
+    fn hf_api_for_synthesises_huggingface_from_legacy_hf_cache() {
+        use crate::config::CandleHarnessConfig;
+
+        let cfg = CandleHarnessConfig {
+            hf_cache: Some(std::path::PathBuf::from("/archive3/llm-cache")),
+            ..Default::default()
+        };
+        let harness = CandleHarness::new("http://localhost:13131".into(), &cfg);
+        harness
+            .hf_api_for("huggingface")
+            .expect("synth huggingface source should build");
+        assert_eq!(harness.default_source_scheme(), "huggingface");
+    }
 }

crates/neuron/src/harness/mod.rs

@@ -115,7 +115,7 @@ impl HarnessRegistry {
                 "candle" => {
                     let harness = Arc::new(candle::CandleHarness::new(
                         bind_url.to_string(),
-                        settings.candle.hf_cache.clone(),
+                        &settings.candle,
                     ));
                     registry.candle = Some(Arc::clone(&harness));
                     registry.harnesses.insert("candle".into(), harness);

crates/neuron/src/harness/preflight.rs

@@ -22,6 +22,7 @@
 //! cleanly when Phase 1 lands.
 
 use cortex_core::harness::ModelSpec;
+use cortex_core::source::ModelSourceId;
 use hf_hub::api::tokio::Api;
 use serde::Serialize;
 
@@ -115,13 +116,22 @@ pub enum PreflightError {
 /// One network round-trip (`repo.info()`); no blob fetches. Returns
 /// `Ok(PlacementPlan)` when the requested combination is feasible, or
 /// a structured `PreflightError` describing what's wrong.
-pub async fn preflight(api: &Api, spec: &ModelSpec) -> Result<PlacementPlan, PreflightError> {
-    let repo = api.model(spec.model_id.clone());
+///
+/// `api` must already be configured for the scheme `source_id` belongs
+/// to — caller (typically `CandleHarness::load_model`) builds it via
+/// `hf_api_for(&source_id.scheme)`. Only the `org/name` portion of the
+/// id is sent to the registry.
+pub async fn preflight(
+    api: &Api,
+    source_id: &ModelSourceId,
+    spec: &ModelSpec,
+) -> Result<PlacementPlan, PreflightError> {
+    let repo = api.model(source_id.repo_path());
     let info = repo
         .info()
         .await
         .map_err(|e| PreflightError::RepoFetchFailed {
-            model_id: spec.model_id.clone(),
+            model_id: source_id.to_string(),
             cause: format!("{e}"),
         })?;
 
@@ -132,13 +142,13 @@ pub async fn preflight(api: &Api, spec: &ModelSpec) -> Result<PlacementPlan, Pre
     match (&format, tp_size, spec.quant.as_deref()) {
         // No weights at all — nothing to do.
         (SourceFormat::Empty, _, _) => Err(PreflightError::EmptyRepo {
-            model_id: spec.model_id.clone(),
+            model_id: source_id.to_string(),
         }),
 
         // GGUF-only + TP: not supported. Today's HauhauCS failure.
         (SourceFormat::Gguf { quants }, tp, _) if tp > 1 => {
             Err(PreflightError::TpRequiresSafetensors {
-                model_id: spec.model_id.clone(),
+                model_id: source_id.to_string(),
                 tp_size: tp,
                 gguf_quants: quants.clone(),
                 suggestion: format!(
@@ -154,13 +164,13 @@ pub async fn preflight(api: &Api, spec: &ModelSpec) -> Result<PlacementPlan, Pre
             let picked = pick_gguf_file(&filenames, requested.unwrap_or(""));
             match picked {
                 Some(fname) => Ok(PlacementPlan {
-                    model_id: spec.model_id.clone(),
+                    model_id: source_id.to_string(),
                     format: format.clone(),
                     tp_size,
                     picked_quant_file: Some(fname),
                 }),
                 None => Err(PreflightError::QuantNotFound {
-                    model_id: spec.model_id.clone(),
+                    model_id: source_id.to_string(),
                     requested: requested.unwrap_or("").to_string(),
                     available: quants.clone(),
                     nearest: nearest_quant(requested.unwrap_or(""), quants),
@@ -174,7 +184,7 @@ pub async fn preflight(api: &Api, spec: &ModelSpec) -> Result<PlacementPlan, Pre
         // on disk, since it needs the parsed JSON.
         (SourceFormat::DenseSafetensors { .. } | SourceFormat::Mixed { .. }, _, _) => {
             Ok(PlacementPlan {
-                model_id: spec.model_id.clone(),
+                model_id: source_id.to_string(),
                 format: format.clone(),
                 tp_size,
                 picked_quant_file: None,
@@ -431,14 +441,20 @@ mod tests {
         format: &SourceFormat,
         filenames: &[&str],
     ) -> Result<PlacementPlan, PreflightError> {
+        // Tests parse spec.model_id with the default scheme so the
+        // assertions can keep comparing against bare "org/name".
+        let source_id: ModelSourceId = spec
+            .model_id
+            .parse::<ModelSourceId>()
+            .expect("test spec.model_id must parse");
         let tp_size = spec.tensor_parallel.unwrap_or(1);
         match (format, tp_size, spec.quant.as_deref()) {
             (SourceFormat::Empty, _, _) => Err(PreflightError::EmptyRepo {
-                model_id: spec.model_id.clone(),
+                model_id: source_id.to_string(),
             }),
             (SourceFormat::Gguf { quants }, tp, _) if tp > 1 => {
                 Err(PreflightError::TpRequiresSafetensors {
-                    model_id: spec.model_id.clone(),
+                    model_id: source_id.to_string(),
                     tp_size: tp,
                     gguf_quants: quants.clone(),
                     suggestion: format!(
@@ -451,13 +467,13 @@ mod tests {
                 let picked = pick_gguf_file(filenames, requested.unwrap_or(""));
                 match picked {
                     Some(fname) => Ok(PlacementPlan {
-                        model_id: spec.model_id.clone(),
+                        model_id: source_id.to_string(),
                         format: format.clone(),
                         tp_size,
                         picked_quant_file: Some(fname),
                     }),
                     None => Err(PreflightError::QuantNotFound {
-                        model_id: spec.model_id.clone(),
+                        model_id: source_id.to_string(),
                         requested: requested.unwrap_or("").to_string(),
                         available: quants.clone(),
                         nearest: nearest_quant(requested.unwrap_or(""), quants),
@@ -466,7 +482,7 @@ mod tests {
             }
             (SourceFormat::DenseSafetensors { .. } | SourceFormat::Mixed { .. }, _, _) => {
                 Ok(PlacementPlan {
-                    model_id: spec.model_id.clone(),
+                    model_id: source_id.to_string(),
                     format: format.clone(),
                     tp_size,
                     picked_quant_file: None,

crates/neuron/tests/preflight.rs

@@ -12,6 +12,7 @@ use axum::http::StatusCode;
 use axum::response::{IntoResponse, Json};
 use axum::routing::get;
 use cortex_core::harness::ModelSpec;
+use cortex_core::source::ModelSourceId;
 use neuron::harness::preflight::{PreflightError, SourceFormat, preflight};
 use serde_json::{Value, json};
 use std::sync::Arc;
@@ -89,6 +90,15 @@ fn spec(model_id: &str, tp: Option<u32>, quant: Option<&str>) -> ModelSpec {
     }
 }
 
+/// Build a `ModelSourceId` from a bare `org/name` test input,
+/// substituting the default scheme so the mock route key matches.
+fn sid(model_id: &str) -> ModelSourceId {
+    model_id
+        .parse::<ModelSourceId>()
+        .expect("test model_id parses")
+        .with_default_scheme("huggingface")
+}
+
 #[tokio::test]
 async fn preflight_gguf_tp_rejected_over_http() {
     let cache = tempfile::tempdir().expect("tempdir");
@@ -107,7 +117,7 @@ async fn preflight_gguf_tp_rejected_over_http() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("HauhauCS/Qwen3.6", Some(2), Some("q6k"));
-    let err = preflight(&api, &s).await.unwrap_err();
+    let err = preflight(&api, &sid(&s.model_id), &s).await.unwrap_err();
     match err {
         PreflightError::TpRequiresSafetensors {
             model_id,
@@ -115,7 +125,9 @@ async fn preflight_gguf_tp_rejected_over_http() {
             gguf_quants,
             ..
         } => {
-            assert_eq!(model_id, "HauhauCS/Qwen3.6");
+            // Scheme prefix surfaces in error display now that
+            // preflight is source-aware.
+            assert_eq!(model_id, "huggingface:HauhauCS/Qwen3.6");
             assert_eq!(tp_size, 2);
             assert_eq!(gguf_quants.len(), 3);
         }
@@ -140,7 +152,7 @@ async fn preflight_gguf_quant_suggestion_over_http() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("HauhauCS/Qwen3.6", Some(1), Some("q6k"));
-    let err = preflight(&api, &s).await.unwrap_err();
+    let err = preflight(&api, &sid(&s.model_id), &s).await.unwrap_err();
     match err {
         PreflightError::QuantNotFound {
             requested,
@@ -176,7 +188,9 @@ async fn preflight_dense_safetensors_tp_ok() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("Qwen/Q3-30B", Some(2), Some("q5k"));
-    let plan = preflight(&api, &s).await.expect("dense+tp should succeed");
+    let plan = preflight(&api, &sid(&s.model_id), &s)
+        .await
+        .expect("dense+tp should succeed");
     assert_eq!(plan.tp_size, 2);
     assert!(plan.picked_quant_file.is_none());
     assert!(matches!(
@@ -197,7 +211,7 @@ async fn preflight_gguf_single_gpu_good_quant() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("HauhauCS/Qwen3.6", Some(1), Some("q6_k_p"));
-    let plan = preflight(&api, &s)
+    let plan = preflight(&api, &sid(&s.model_id), &s)
         .await
         .expect("good quant should succeed");
     assert_eq!(plan.tp_size, 1);
@@ -219,7 +233,7 @@ async fn preflight_repo_fetch_failed_on_404() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("DoesNot/Exist", Some(1), None);
-    let err = preflight(&api, &s).await.unwrap_err();
+    let err = preflight(&api, &sid(&s.model_id), &s).await.unwrap_err();
     assert!(
         matches!(err, PreflightError::RepoFetchFailed { .. }),
         "expected RepoFetchFailed, got {err:?}"
@@ -238,7 +252,7 @@ async fn preflight_empty_repo_rejected() {
 
     let api = build_api(&endpoint, cache.path());
     let s = spec("Empty/Repo", Some(1), None);
-    let err = preflight(&api, &s).await.unwrap_err();
+    let err = preflight(&api, &sid(&s.model_id), &s).await.unwrap_err();
     assert!(
         matches!(err, PreflightError::EmptyRepo { .. }),
         "expected EmptyRepo, got {err:?}"
@@ -264,6 +278,8 @@ async fn preflight_mixed_repo_prefers_safetensors() {
     // TP=2 + quant should succeed via the dense path even though a
     // GGUF is present — the dense path handles ISQ.
     let s = spec("Mixed/Repo", Some(2), Some("q5k"));
-    let plan = preflight(&api, &s).await.expect("mixed should succeed");
+    let plan = preflight(&api, &sid(&s.model_id), &s)
+        .await
+        .expect("mixed should succeed");
     assert!(matches!(plan.format, SourceFormat::Mixed { .. }));
 }

neuron.example.toml

@@ -22,7 +22,9 @@ name = "candle"
 # HuggingFace cache directory for model weights.
 #
 # Resolution order (first hit wins):
-#   1. `hf_cache` here in this file.
+#   1. `hf_cache` here in this file (applies to the synth `huggingface`
+#      source only — see [harness.candle.sources.*] below for explicit
+#      per-source paths).
 #   2. `HF_HUB_CACHE` env var — same convention as the Python
 #      `huggingface_hub` library, so an existing cache directory shared
 #      with other tooling can be reused without per-tool config.
@@ -36,6 +38,32 @@ name = "candle"
 #     Environment=HF_HUB_CACHE=/archive/hf-cache
 # hf_cache = "/var/lib/neuron/hf-cache"
 
+# Default scheme applied to bare `org/name` model ids (those without a
+# `scheme:` prefix). Defaults to "huggingface" when unset. Set to
+# "helexa" to make `default_models = [{ model_id = "Helexa/Foo" }]`
+# resolve via the helexa registry without prefixing every entry.
+# default_source = "huggingface"
+
+# Per-scheme source endpoints. Each scheme maps to an HF-compatible
+# registry. The `huggingface` source is auto-synthesised pointing at
+# `https://huggingface.co` when omitted; declare it explicitly here to
+# override the endpoint, auth env, or cache dir.
+#
+# [harness.candle.sources.huggingface]
+# endpoint = "https://huggingface.co"
+# auth_env = "HF_TOKEN"          # optional bearer token via env var
+# cache_dir = "/archive3/llm-cache/huggingface"
+#
+# Add helexa (or any operator-run mirror speaking the HF-compatible
+# wire format) by adding another sources entry. Caches are
+# disambiguated per scheme so a mirror serving the same `org/name` as
+# HF cannot collide on disk.
+#
+# [harness.candle.sources.helexa]
+# endpoint = "https://registry.helexa.ai"
+# auth_env = "HELEXA_TOKEN"
+# cache_dir = "/archive3/llm-cache/helexa"
+
 # -- Default models ----------------------------------------------------------
 # Models listed here are loaded automatically when the neuron service
 # activates. Loading is sequential — a slow or failing entry doesn't