diff --git a/nix/templates/harness-base.nix b/nix/templates/harness-base.nix
index e58829d8..d80fc572 100644
--- a/nix/templates/harness-base.nix
+++ b/nix/templates/harness-base.nix
@@ -134,6 +134,29 @@ in
     '';
   };
 
+  options.hyperhive.effortLevel = lib.mkOption {
+    type = lib.types.enum [
+      "medium"
+      "high"
+      "xhigh"
+    ];
+    default = "medium";
+    example = "high";
+    description = ''
+      Baseline claude effort level for this agent. Rendered into the
+      `HYPERHIVE_EFFORT_LEVEL` environment variable; the harness resolves
+      effort as operator-override-file → this env → built-in `"medium"`,
+      and passes the result to `claude --effort` at turn launch.
+
+      `"medium"` (the default) keeps token spend low and works well with
+      the harness across families. `"high"` matches the current platform
+      default; `"xhigh"` is recommended for coding / high-autonomy work on
+      capable models (Opus 4.8+) at higher token cost. The operator can
+      override at runtime per-agent via the web UI (applied on the next
+      session); any rebuild that changes this option resets that override.
+    '';
+  };
+
   options.hyperhive.allowedBashPatterns = lib.mkOption {
     type = lib.types.listOf lib.types.str;
     default = [ ];
@@ -912,6 +935,10 @@ in
       # surfaces it to the frontend; an empty value falls back to the
       # built-in default list.
       HIVE_AVAILABLE_MODELS = lib.concatStringsSep "," config.hyperhive.availableModels;
+      # HYPERHIVE_EFFORT_LEVEL is the per-agent baseline effort (see
+      # hyperhive.effortLevel). The harness resolves operator-override-file
+      # → this env → "medium" and passes it to claude --effort at turn launch.
+      HYPERHIVE_EFFORT_LEVEL = config.hyperhive.effortLevel;
       HIVE_ASSETS_DIR = "${pkgs.hyperhive-assets}/share/hyperhive";
       SHELL = "${pkgs.bashInteractive}/bin/bash";
     }