diff --git a/hive-ag3nt/src/events.rs b/hive-ag3nt/src/events.rs
index e16e3b7..f7e9c89 100644
--- a/hive-ag3nt/src/events.rs
+++ b/hive-ag3nt/src/events.rs
@@ -310,22 +310,27 @@ pub enum TurnState {
     Compacting,
 }
 
-/// Default claude model when nothing's been set at runtime. Overridable
-/// via the `HIVE_DEFAULT_MODEL` env var (set from `hyperhive.model` in
-/// the container's `agent.nix`). The operator can also switch at runtime
-/// via `/model <name>` in the web terminal; the chosen model is persisted
-/// to the state dir so it survives restarts.
+/// Compiled-in fallback model used when neither `HIVE_DEFAULT_MODEL` nor a
+/// persisted runtime override is present.
 pub const DEFAULT_MODEL: &str = "haiku";
 
-/// Return the initial default model name: `HIVE_DEFAULT_MODEL` env var if
-/// set to a non-empty string, otherwise `DEFAULT_MODEL`.
+/// Return the model declared in `HIVE_DEFAULT_MODEL` (set from
+/// `hyperhive.model` in `agent.nix`), or `None` if the env var is absent /
+/// empty.  When `Some`, this takes precedence over any persisted runtime
+/// override so that nix config changes always take effect on rebuild.
 #[must_use]
-pub fn default_model() -> &'static str {
+pub fn configured_model() -> Option<&'static str> {
     // Leak once at startup — acceptable for a single config value.
     std::env::var("HIVE_DEFAULT_MODEL")
         .ok()
         .filter(|s| !s.trim().is_empty())
-        .map_or(DEFAULT_MODEL, |s| Box::leak(s.into_boxed_str()))
+        .map(|s| &*Box::leak(s.into_boxed_str()))
+}
+
+/// Return the model to use when no config and no persisted override exist.
+#[must_use]
+pub fn default_model() -> &'static str {
+    configured_model().unwrap_or(DEFAULT_MODEL)
 }
 
 /// Context-window size in tokens for a given model name.
@@ -441,7 +446,13 @@ impl Bus {
             }
         };
         let (tx, _) = broadcast::channel(CHANNEL_CAPACITY);
-        let initial_model = load_model().unwrap_or_else(|| default_model().to_owned());
+        // Priority: HIVE_DEFAULT_MODEL (from hyperhive.model in agent.nix) >
+        // persisted runtime override > compiled-in DEFAULT_MODEL.
+        // The nix config always wins on rebuild; the persisted file is kept
+        // for within-session tracking only (see persist_model / set_model).
+        let initial_model = configured_model()
+            .map(str::to_owned)
+            .unwrap_or_else(|| load_model().unwrap_or_else(|| DEFAULT_MODEL.to_owned()));
         // Restore rate_limited from the sentinel file — if the harness
         // crashed while parked, we should still show the right status on
         // cold load until the next turn clears it.
diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index 9127ac8..64a8b02 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -2,6 +2,11 @@
   pkgs,
   lib,
   config,
+  # Flake inputs routed through _module.args by the agent flake.nix.
+  # Default to {} so the module evaluates cleanly even when the agent
+  # flake doesn't set up the routing pattern (e.g. during standalone
+  # nixos-rebuild without a flake wrapper).
+  flakeInputs ? { },
   ...
 }:
 {
@@ -20,13 +25,12 @@
     default = "haiku";
     example = "sonnet";
     description = ''
-      Default claude model for this agent. Sets the `HIVE_DEFAULT_MODEL`
-      environment variable consumed by the harness at boot; if no
-      persisted model choice exists in the agent's state dir the harness
-      falls back to this value. The operator can still switch the model at
-      runtime via the per-agent web UI — that choice is persisted to the
-      state dir and takes precedence over this default until the agent is
-      purged.
+      Claude model for this agent. Sets the `HIVE_DEFAULT_MODEL`
+      environment variable; the harness applies it at boot and it takes
+      priority over any persisted runtime override. The operator can still
+      switch the model at runtime via the per-agent web UI — that choice
+      is tracked in the state dir for the current session but is reset by
+      any rebuild that changes this option.
 
       Valid values are the short model names that `claude --model` accepts:
       `"haiku"`, `"sonnet"`, `"opus"` (or any future identifier). Context
@@ -320,6 +324,42 @@
   };
 
   config = {
+    assertions = [
+      # Guard the inputs-routed-as-output pattern: the agent flake.nix is
+      # expected to set `_module.args.flakeInputs = builtins.removeAttrs inputs ["self"]`.
+      # If `self` leaks into flakeInputs the agent gets a spurious attrset
+      # entry that can shadow real inputs and is almost certainly a bug.
+      {
+        assertion = !(builtins.hasAttr "self" flakeInputs);
+        message = ''
+          hyperhive: `flakeInputs` must not contain "self".
+          In your agent flake.nix, use:
+            _module.args.flakeInputs = builtins.removeAttrs inputs [ "self" ];
+        '';
+      }
+      # hyperhive.model must be a non-empty string — an empty value causes
+      # the harness to pass an invalid model flag to claude.
+      {
+        assertion = config.hyperhive.model != "";
+        message = "hyperhive.model must not be empty (set it to e.g. \"haiku\" or \"sonnet\")";
+      }
+      # hyperhive.forge.url must look like an HTTP URL when non-default.
+      {
+        assertion =
+          config.hyperhive.forge.url == ""
+          || lib.hasPrefix "http://" config.hyperhive.forge.url
+          || lib.hasPrefix "https://" config.hyperhive.forge.url;
+        message = "hyperhive.forge.url must be an http:// or https:// URL (got: \"${config.hyperhive.forge.url}\")";
+      }
+      # hyperhive.icon must reference an SVG file when set.
+      {
+        assertion =
+          config.hyperhive.icon == null
+          || lib.hasSuffix ".svg" (toString config.hyperhive.icon);
+        message = "hyperhive.icon must point to an .svg file";
+      }
+    ];
+
     environment.etc."hyperhive/extra-mcp.json".text = builtins.toJSON config.hyperhive.extraMcpServers;
 
     # Operator-set per-agent icon (hyperhive.icon). When configured, the