diff --git a/cells/repo/ci.nix b/cells/repo/ci.nix
index b0798f3..e3868a2 100644
--- a/cells/repo/ci.nix
+++ b/cells/repo/ci.nix
@@ -3,8 +3,19 @@
 in
   cilib.mkCI {
     pipelines."default" = {
-      stages = ["build" "deploy"];
+      stages = ["test" "build" "deploy"];
       jobs = {
+        "test:lib" = {
+          stage = "test";
+          script = [
+            "nix run .#tests -- --junit=junit.xml"
+          ];
+          allow_failure = true;
+          artifacts = {
+            when = "always";
+            reports.junit = "junit.xml";
+          };
+        };
         "docs" = {
           stage = "build";
           script = [
diff --git a/cells/repo/docs.nix b/cells/repo/docs.nix
index eda2205..4b52a41 100644
--- a/cells/repo/docs.nix
+++ b/cells/repo/docs.nix
@@ -36,6 +36,7 @@ in
           {"API Reference" = "api.md";}
           {"Related Libraries" = "libraries.md";}
           {"Direnv Integration" = "direnv.md";}
+          {"Debugging" = "debugging.md";}
         ];
         markdown_extensions = [
           {
diff --git a/cells/repo/flake.lock b/cells/repo/flake.lock
index b99a60f..fb920c3 100644
--- a/cells/repo/flake.lock
+++ b/cells/repo/flake.lock
@@ -3,11 +3,11 @@
     "devshell": {
       "locked": {
         "dir": "lib",
-        "lastModified": 1758204313,
-        "narHash": "sha256-ainbY0Oajb1HMdvy+A8QxF/P5qwcbEzJGEY5pzKdDdc=",
+        "lastModified": 1767218348,
+        "narHash": "sha256-8MJqwH9sRMuHH+RsB7iqWyWD30TgmpiYKEvegAULggs=",
         "owner": "rensa-nix",
         "repo": "devshell",
-        "rev": "7d0c4bc78d9f017a739b0c7eb2f4e563118353e6",
+        "rev": "7a9b7e5d9f162a1fa3edfdc0169cdc29d3a67f8e",
         "type": "gitlab"
       },
       "original": {
@@ -20,11 +20,11 @@
     "devtools-lib": {
       "locked": {
         "dir": "lib",
-        "lastModified": 1766237846,
-        "narHash": "sha256-7n3WFabnf49SFHXD/e4zfLit3BniZZOecUXfBm3d1d0=",
+        "lastModified": 1767214272,
+        "narHash": "sha256-gvW7flZ60xdv3Z3Ksec5jcRjW2sqRHsGoJdwsNWQVPk=",
         "owner": "rensa-nix",
         "repo": "devtools",
-        "rev": "5c8712f48c5bcb6b79d3da1afd1268f84f5e35e3",
+        "rev": "f40e59c32c48cdbf4cbc621c2f0f11e7bb80dbd3",
         "type": "gitlab"
       },
       "original": {
@@ -55,11 +55,11 @@
     "nixmkdocs": {
       "locked": {
         "dir": "lib",
-        "lastModified": 1766062227,
-        "narHash": "sha256-jhr5CUi9eDeMIAJn7ayXP8Wr+Y2loV5EhdDIKDkRIdw=",
+        "lastModified": 1766404754,
+        "narHash": "sha256-EjBe6x6BT8ckPirMWhSf1GfaFxORYxR/Uu71FvSAm60=",
         "owner": "TECHNOFAB",
         "repo": "nixmkdocs",
-        "rev": "cb0bb5dc3382e8ba5d81324a2f1fd94ccd5a5df4",
+        "rev": "cfa9606eeeb9288e2799896d7d42b3d3860f9ccb",
         "type": "gitlab"
       },
       "original": {
@@ -69,12 +69,31 @@
         "type": "gitlab"
       }
     },
+    "nixtest-lib": {
+      "locked": {
+        "dir": "lib",
+        "lastModified": 1765728058,
+        "narHash": "sha256-V3FXECl1oTxEtGteNz3o3GJs/X8asSn1TxRpZ2F+htU=",
+        "owner": "TECHNOFAB",
+        "repo": "nixtest",
+        "rev": "2477ad31ae3aa4134e1bb5eeddbebe0cb64ccb57",
+        "type": "gitlab"
+      },
+      "original": {
+        "dir": "lib",
+        "owner": "TECHNOFAB",
+        "ref": "v1.2.1",
+        "repo": "nixtest",
+        "type": "gitlab"
+      }
+    },
     "root": {
       "inputs": {
         "devshell": "devshell",
         "devtools-lib": "devtools-lib",
         "nix-gitlab-ci-lib": "nix-gitlab-ci-lib",
         "nixmkdocs": "nixmkdocs",
+        "nixtest-lib": "nixtest-lib",
         "soonix-lib": "soonix-lib",
         "treefmt-nix": "treefmt-nix"
       }
@@ -99,11 +118,11 @@
     "treefmt-nix": {
       "flake": false,
       "locked": {
-        "lastModified": 1766000401,
-        "narHash": "sha256-+cqN4PJz9y0JQXfAK5J1drd0U05D5fcAGhzhfVrDlsI=",
+        "lastModified": 1767122417,
+        "narHash": "sha256-yOt/FTB7oSEKQH9EZMFMeuldK1HGpQs2eAzdS9hNS/o=",
         "owner": "numtide",
         "repo": "treefmt-nix",
-        "rev": "42d96e75aa56a3f70cab7e7dc4a32868db28e8fd",
+        "rev": "dec15f37015ac2e774c84d0952d57fcdf169b54d",
         "type": "github"
       },
       "original": {
diff --git a/cells/repo/flake.nix b/cells/repo/flake.nix
index 46a3c1d..8180fcc 100644
--- a/cells/repo/flake.nix
+++ b/cells/repo/flake.nix
@@ -5,6 +5,7 @@
     soonix-lib.url = "gitlab:TECHNOFAB/soonix?dir=lib";
     nix-gitlab-ci-lib.url = "gitlab:TECHNOFAB/nix-gitlab-ci/3.1.2?dir=lib";
     devtools-lib.url = "gitlab:rensa-nix/devtools?dir=lib";
+    nixtest-lib.url = "gitlab:TECHNOFAB/nixtest/v1.2.1?dir=lib";
     treefmt-nix = {
       url = "github:numtide/treefmt-nix";
       flake = false;
@@ -18,6 +19,8 @@
       doclib = i.nixmkdocs.lib {inherit (i.parent) pkgs;};
       soonix = i.soonix-lib.lib {inherit (i.parent) pkgs;};
       cilib = i.nix-gitlab-ci-lib.lib {inherit (i.parent) pkgs;};
+      ntlib = i.nixtest-lib.lib {inherit (i.parent) pkgs;};
+      rensa = import "${i.parent.self}/lib" {inherit (i.parent.pkgs) lib;};
       treefmt = import i.treefmt-nix;
     };
 }
diff --git a/cells/repo/tests.nix b/cells/repo/tests.nix
new file mode 100644
index 0000000..fa75087
--- /dev/null
+++ b/cells/repo/tests.nix
@@ -0,0 +1,10 @@
+{inputs, ...}: let
+  inherit (inputs) pkgs ntlib rensa;
+in {
+  tests = ntlib.mkNixtest {
+    modules = ntlib.autodiscover {dir = "${inputs.self}/tests";};
+    args = {
+      inherit pkgs ntlib rensa;
+    };
+  };
+}
diff --git a/docs/api.md b/docs/api.md
index 2500ca3..a4620f8 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -8,7 +8,7 @@ The main entry point for creating a Rensa flake.
 
 - `inputs`: The flake inputs.
 - `cellsFrom`: Path to the directory containing your cells.
-- `cellBlocks`: A list of blocks to load for each cell.
+- `cellBlocks`: A list of blocks to load for each cell. Defaults to `[rensa.blocks.autodiscover]`.
 - `transformInputs` (optional): A function to transform inputs before they are passed to cells.
 
 ### Returns
@@ -23,7 +23,7 @@ The underlying builder function used by `buildWith`. It returns the raw Rensa ou
 
 - `inputs`: The flake inputs.
 - `cellsFrom`: Path to the directory containing your cells.
-- `cellBlocks`: A list of blocks to load for each cell.
+- `cellBlocks`: A list of blocks to load for each cell. Defaults to `[rensa.blocks.autodiscover]`.
 - `transformInputs` (optional): A function to transform inputs.
 
 ### Returns
@@ -56,6 +56,45 @@ by passing through actions etc. from a cell.
 
 Currently not really used.
 
+### `autodiscover`
+
+```nix
+autodiscover
+# or
+(autodiscover "cellName")
+```
+
+Automatically discovers and loads all cell blocks by scanning the cells directory for `.nix` files.
+When `autodiscover` is present in `cellBlocks`, Rensa will:
+
+1. Walk through all cells in `cellsFrom` (or specific cell if provided).
+1. Find all `.nix` files (excluding `flake.nix`).
+1. Find all directories containing `default.nix`.
+1. Generate `simple` block definitions for each discovered block.
+
+**Usage patterns:**
+
+```nix
+# full autodiscovery
+cellBlocks = with rensa.blocks; [
+  autodiscover
+];
+# or
+cellBlocks = with rensa.blocks; [
+  (autodiscover "backend")  # only discover blocks from backend cell
+  (autodiscover "frontend") # only discover blocks from frontend cell
+];
+# mixed
+cellBlocks = with rensa.blocks; [
+  (simple "myBlock")  # explicit block (takes precedence)
+  autodiscover        # discover all others
+];
+```
+
+!!! note
+
+    When a block is both explicitly defined and discovered, the explicit definition takes precedence.
+
 ## `rensa.select`
 
 Helper to select specific outputs from the generated flake.
diff --git a/docs/debugging.md b/docs/debugging.md
new file mode 100644
index 0000000..2f0343b
--- /dev/null
+++ b/docs/debugging.md
@@ -0,0 +1,38 @@
+# Debugging
+
+## Error Context
+
+To help with debugging, Rensa uses Nix's `builtins.addErrorContext` to add context
+about where an error is happening.
+
+Use the following command to get just these logs from Rensa and please include that (or the whole stacktrace) in any issues or bug reports :)
+
+```sh
+nix ... --show-trace 2> >(grep "… \[ren\]")
+```
+
+## Trace Verbose
+
+There are also some `traceVerbose` calls in Rensa, so to debug non-failing executions
+just run Nix with the `--trace-verbose` flag.
+
+The output could look like this (example with this repo's flake):
+
+```sh
+trace: [ren] loading cell block ci, type ci, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-darwin
+trace: [ren] loading cell block devShells, type devShells, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-darwin
+trace: [ren] loading cell block docs, type docs, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-darwin
+trace: [ren] loading cell block soonix, type soonix, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-darwin
+trace: [ren] loading cell block ci, type ci, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-linux
+trace: [ren] loading cell block devShells, type devShells, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-linux
+trace: [ren] loading cell block docs, type docs, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-linux
+trace: [ren] loading cell block soonix, type soonix, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system aarch64-linux
+trace: [ren] loading cell block ci, type ci, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-darwin
+trace: [ren] loading cell block devShells, type devShells, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-darwin
+trace: [ren] loading cell block docs, type docs, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-darwin
+trace: [ren] loading cell block soonix, type soonix, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-darwin
+trace: [ren] loading cell block ci, type ci, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-linux
+trace: [ren] loading cell block devShells, type devShells, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-linux
+trace: [ren] loading cell block docs, type docs, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-linux
+trace: [ren] loading cell block soonix, type soonix, from cell /nix/store/7acz6q9360fkg0x187yx43d1vwpv850l-cells/repo, for system x86_64-linux
+```
diff --git a/flake.lock b/flake.lock
index 4baf8fb..8792074 100644
--- a/flake.lock
+++ b/flake.lock
@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1766125104,
-        "narHash": "sha256-l/YGrEpLromL4viUo5GmFH3K5M1j0Mb9O+LiaeCPWEM=",
+        "lastModified": 1767151656,
+        "narHash": "sha256-ujL2AoYBnJBN262HD95yer7QYUmYp5kFZGYbyCCKxq8=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "7d853e518814cca2a657b72eeba67ae20ebf7059",
+        "rev": "f665af0cdb70ed27e1bd8f9fdfecaf451260fc55",
         "type": "github"
       },
       "original": {
@@ -18,11 +18,11 @@
     },
     "nixpkgs-lib": {
       "locked": {
-        "lastModified": 1765674936,
-        "narHash": "sha256-k00uTP4JNfmejrCLJOwdObYC9jHRrr/5M/a/8L2EIdo=",
+        "lastModified": 1766884708,
+        "narHash": "sha256-x8nyRwtD0HMeYtX60xuIuZJbwwoI7/UKAdCiATnQNz0=",
         "owner": "nix-community",
         "repo": "nixpkgs.lib",
-        "rev": "2075416fcb47225d9b68ac469a5c4801a9c4dd85",
+        "rev": "15177f81ad356040b4460a676838154cbf7f6213",
         "type": "github"
       },
       "original": {
diff --git a/flake.nix b/flake.nix
index a47fa5a..52b2326 100644
--- a/flake.nix
+++ b/flake.nix
@@ -24,12 +24,14 @@
         (simple "docs")
         (simple "ci")
         (simple "soonix")
+        (simple "tests")
       ];
     } {
       packages = rensa.select inputs.self [
         ["repo" "docs"]
         ["repo" "ci" "packages"]
         ["repo" "soonix" "packages"]
+        ["repo" "tests"]
       ];
     };
 }
diff --git a/lib/blocks/default.nix b/lib/blocks/default.nix
index 7f83c8d..ebaea9a 100644
--- a/lib/blocks/default.nix
+++ b/lib/blocks/default.nix
@@ -6,6 +6,17 @@
   dynamic = name: {
     inherit name;
     type = name;
+    cli = true;
+    actions = args: {};
     # TODO: dynamic actions
   };
+  autodiscover = {
+    name = "__autodiscover";
+    type = "__autodiscover";
+    _functor = self: cell:
+      self
+      // {
+        inherit cell;
+      };
+  };
 }
diff --git a/lib/core/autodiscover.nix b/lib/core/autodiscover.nix
new file mode 100644
index 0000000..9bec7c2
--- /dev/null
+++ b/lib/core/autodiscover.nix
@@ -0,0 +1,98 @@
+{
+  l,
+  cellsFrom,
+  cellBlocks,
+}: let
+  # find autodiscover blocks
+  autodiscoverBlocks = l.filter (block: block.name or "" == "__autodiscover") cellBlocks;
+  explicitBlocks = l.filter (block: block.name or "" != "__autodiscover") cellBlocks;
+  hasAutodiscover = (l.length autodiscoverBlocks) > 0;
+
+  # discover all block names from a single cell's directory
+  discoverBlocksFromCell = cellPath: let
+    cellContents = l.readDir cellPath;
+    # filter for .nix files (but not flake.nix) and dirs with default.nix
+    blockNames = l.unique (
+      l.filter (name: name != null) (
+        l.mapAttrsToList (
+          name: type:
+            if type == "regular" && l.hasSuffix ".nix" name && name != "flake.nix"
+            then l.removeSuffix ".nix" name
+            else if type == "directory" && l.pathExists (cellPath + "/${name}/default.nix")
+            then name
+            else null
+        )
+        cellContents
+      )
+    );
+  in
+    builtins.addErrorContext "[ren] while discovering blocks from cell ${cellPath}"
+    blockNames;
+
+  # discover all blocks from all cells or specific cells
+  discoverAllBlocks = let
+    # check if any autodiscover blocks have cell-specific targets (eg. `autodiscover "a"`)
+    cellSpecificDiscoveries = l.filter (block: block ? cell) autodiscoverBlocks;
+    globalDiscoveries = l.filter (block: !(block ? cell)) autodiscoverBlocks;
+
+    cellSpecificBlocks = l.flatten (
+      l.map (
+        block: let
+          cellPath = cellsFrom + "/${block.cell}";
+        in
+          if l.pathExists cellPath
+          then discoverBlocksFromCell cellPath
+          else []
+      )
+      cellSpecificDiscoveries
+    );
+
+    # global autodiscover
+    globalBlocks = builtins.addErrorContext "[ren] while discovering global blocks" (
+      if (l.length globalDiscoveries) > 0
+      then let
+        cells = l.readDir cellsFrom;
+        allBlockNames = l.unique (
+          l.flatten (
+            l.mapAttrsToList (
+              cellName: cellType:
+                if cellType == "directory"
+                then discoverBlocksFromCell (cellsFrom + "/${cellName}")
+                else []
+            )
+            cells
+          )
+        );
+      in
+        allBlockNames
+      else []
+    );
+
+    allBlockNames = l.unique (cellSpecificBlocks ++ globalBlocks);
+  in
+    builtins.addErrorContext "[ren] while discovering all blocks"
+    l.map (name: {
+      inherit name;
+      type = name;
+    })
+    allBlockNames;
+
+  discoveredBlocks =
+    if hasAutodiscover
+    then discoverAllBlocks
+    else [];
+
+  # merge the explicit and autodiscovered blocks (explicit taking precedence)
+  allBlocks = explicitBlocks ++ discoveredBlocks;
+  uniqueBlocks =
+    l.foldl' (
+      acc: block:
+      # first occurence of name wins
+        if l.any (b: b.name == block.name) acc
+        then acc
+        else acc ++ [block]
+    ) []
+    allBlocks;
+in
+  builtins.addErrorContext "[ren] while autodiscovering cell blocks"
+  uniqueBlocks
diff --git a/lib/core/builder.nix b/lib/core/builder.nix
index 86c2a65..7a8307b 100644
--- a/lib/core/builder.nix
+++ b/lib/core/builder.nix
@@ -2,6 +2,8 @@
   l,
   utils,
   loader,
+  autodiscover,
+  blocks,
 }: let
   inherit (utils) accumulate;
   inherit (loader) createCellLoader;
@@ -9,7 +11,7 @@
   build = {
     inputs,
     cellsFrom,
-    cellBlocks,
+    cellBlocks ? [blocks.autodiscover],
     transformInputs ? system: i: i,
     ...
   } @ args: let
@@ -28,26 +30,31 @@
         ]
       );
 
+    cellBlocks' = autodiscover {inherit l cellsFrom cellBlocks;};
+
     cells = res.output;
 
     loadOutputFor = system: let
       loadCell = createCellLoader {
-        inherit inputs system cells cellsFrom cellBlocks transformInputs;
+        inherit inputs system cells cellsFrom transformInputs;
+        cellBlocks = cellBlocks';
       };
 
       cells' = l.mapAttrsToList (cell: type: cell) (l.readDir cellsFrom);
       res = accumulate (l.map loadCell cells');
-    in [
-      {${system} = res.output;}
-      {${system} = res.actions;}
-      {
-        name = system;
-        value = res.init;
-      }
-    ];
+    in
+      builtins.addErrorContext "[ren] while loading output for ${system}" [
+        {${system} = res.output;}
+        {${system} = res.actions;}
+        {
+          name = system;
+          value = res.init;
+        }
+      ];
 
     res = accumulate (l.map loadOutputFor systems);
   in
+    builtins.addErrorContext "[ren] while building rensa output"
     res.output
     // {
       __ren = {
@@ -89,10 +96,12 @@
     recursiveUpdate = lhs: rhs:
       recursiveUpdateUntil g lhs rhs;
   in
-    build args
-    // {
-      __functor = l.flip recursiveUpdate;
-    };
+    builtins.addErrorContext "[ren] while building rensa output (buildWith)" (
+      build args
+      // {
+        __functor = l.flip recursiveUpdate;
+      }
+    );
 in {
   inherit build buildWith;
 }
diff --git a/lib/core/call-flake.nix b/lib/core/call-flake.nix
index 6bd325a..5961981 100644
--- a/lib/core/call-flake.nix
+++ b/lib/core/call-flake.nix
@@ -112,4 +112,5 @@ ref: inputOverrides: let
     )
     lockFile.nodes;
 in
+  builtins.addErrorContext "[ren] while loading flake lockfile ${ref}"
   allNodes.${lockFile.root}
diff --git a/lib/core/default.nix b/lib/core/default.nix
index 25a3f44..3276941 100644
--- a/lib/core/default.nix
+++ b/lib/core/default.nix
@@ -1,11 +1,13 @@
 {
   l,
   utils,
+  blocks,
 }: let
   paths = import ./paths.nix;
   callFlake = import ./call-flake.nix;
+  autodiscover = import ./autodiscover.nix;
   loader = import ./loader.nix {inherit l utils paths callFlake;};
-  builder = import ./builder.nix {inherit l utils loader;};
+  builder = import ./builder.nix {inherit l utils loader autodiscover blocks;};
 in {
   inherit (builder) build buildWith;
 }
diff --git a/lib/core/loader.nix b/lib/core/loader.nix
index eec8e70..1c6a0fc 100644
--- a/lib/core/loader.nix
+++ b/lib/core/loader.nix
@@ -33,14 +33,18 @@
             }).outputs
           else {};
       in
-        importSignatureFor system updatedCell cells additionalInputs;
+        builtins.addErrorContext "[ren] while getting input signature for ${blockP}" (
+          importSignatureFor system updatedCell cells additionalInputs
+        );
 
       import' = importPath: let
         block = import importPath;
       in
         if l.typeOf block == "set"
         then block
-        else block signature;
+        else
+          builtins.addErrorContext "[ren] while importing block at ${importPath}"
+          (block signature);
 
       importPaths =
         if isFile
@@ -101,18 +105,20 @@
     in
       optionalLoad (isFile || isDir) (
         assert l.assertMsg isAttrs "cell block does not return an attrset: ${importPaths.displayPath}";
-          l.traceVerbose "[ren] loading cell block ${cellBlock.name}, type ${cellBlock.type}, from cell ${cellP}"
-          [
-            {${cellBlock.name} = imported;}
-            {${cellBlock.name} = l.mapAttrs (_: set: set.actions) extracted;}
-            ({
-                cellBlock = cellBlock.name;
-                blockType = cellBlock.type;
-                targets = l.mapAttrsToList (_: set: set.init) extracted;
-              }
-              // (l.optionalAttrs (l.pathExists blockP.readmeDir) {readme = blockP.readmeDir;})
-              // (l.optionalAttrs (l.pathExists blockP.readme) {inherit (blockP) readme;}))
-          ]
+          l.traceVerbose "[ren] loading cell block ${cellBlock.name}, type ${cellBlock.type}, from cell ${cellP}, for system ${system}" (
+            builtins.addErrorContext "[ren] loading cell block ${cellBlock.name}, type ${cellBlock.type}, from cell ${cellP}, for system ${system}"
+            [
+              {${cellBlock.name} = imported;}
+              {${cellBlock.name} = l.mapAttrs (_: set: set.actions) extracted;}
+              ({
+                  cellBlock = cellBlock.name;
+                  blockType = cellBlock.type;
+                  targets = l.mapAttrsToList (_: set: set.init) extracted;
+                }
+                // (l.optionalAttrs (l.pathExists blockP.readmeDir) {readme = blockP.readmeDir;})
+                // (l.optionalAttrs (l.pathExists blockP.readme) {inherit (blockP) readme;}))
+            ]
+          )
       );
   in
     loadCellBlock;
@@ -133,7 +139,7 @@
       # fixes `ìnfinite recursion` errors when accessing cell attributes from sibling blocks
       # example: cells/test/a.nix returns `cell.b`, so the same thing as cells/test/b.nix.
       # this would previously fail with infinite recursion, this makes it work:
-      cell = l.listToAttrs (l.concatMap (
+      cell = builtins.addErrorContext "[ren] while accessing cell '${cellName}' siblings" (l.listToAttrs (l.concatMap (
           block: let
             blockP = paths.cellBlockPath cellP block;
             exists = l.pathExists blockP.file || l.pathExists blockP.dir;
@@ -148,18 +154,19 @@
             ]
             else []
         )
-        cellBlocks');
+        cellBlocks'));
       loadCellBlock = createCellBlockLoader {inherit inputs system cells cell transformInputs;};
       res = accumulate (l.map (loadCellBlock cellName cellP) cellBlocks');
-    in [
-      {${cellName} = res.output;}
-      {${cellName} = res.actions;}
-      ({
-          cell = cellName;
-          cellBlocks = res.init;
-        }
-        // (l.optionalAttrs (l.pathExists cellP.readme) {inherit (cellP) readme;}))
-    ];
+    in
+      builtins.addErrorContext "[ren] while loading cell ${cellName}" [
+        {${cellName} = res.output;}
+        {${cellName} = res.actions;}
+        ({
+            cell = cellName;
+            cellBlocks = res.init;
+          }
+          // (l.optionalAttrs (l.pathExists cellP.readme) {inherit (cellP) readme;}))
+      ];
   in
     loadCell;
 in {
diff --git a/lib/default.nix b/lib/default.nix
index d76620f..498d8d1 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -1,9 +1,9 @@
 {lib}: let
   l = builtins // lib;
-  utils = import ./utils {inherit l;};
-  core = import ./core {inherit l utils;};
-  compat = import ./compat {inherit l;};
   blocks = import ./blocks;
+  utils = import ./utils {inherit l;};
+  compat = import ./compat {inherit l;};
+  core = import ./core {inherit l utils blocks;};
 in {
   inherit (compat) filter select get;
   inherit (core) build buildWith;
diff --git a/lib/flake.lock b/lib/flake.lock
index f7915ba..293dc4e 100644
--- a/lib/flake.lock
+++ b/lib/flake.lock
@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs-lib": {
       "locked": {
-        "lastModified": 1754184128,
-        "narHash": "sha256-AjhoyBL4eSyXf01Bmc6DiuaMrJRNdWopmdnMY0Pa/M0=",
+        "lastModified": 1766884708,
+        "narHash": "sha256-x8nyRwtD0HMeYtX60xuIuZJbwwoI7/UKAdCiATnQNz0=",
         "owner": "nix-community",
         "repo": "nixpkgs.lib",
-        "rev": "02e72200e6d56494f4a7c0da8118760736e41b60",
+        "rev": "15177f81ad356040b4460a676838154cbf7f6213",
         "type": "github"
       },
       "original": {
diff --git a/tests/autodiscovery_test.nix b/tests/autodiscovery_test.nix
new file mode 100644
index 0000000..51c9205
--- /dev/null
+++ b/tests/autodiscovery_test.nix
@@ -0,0 +1,63 @@
+{
+  pkgs,
+  ntlib,
+  ...
+}: {
+  suites."Autodiscovery" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "find nix files";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          mkdir -p "testcell"
+          echo '{ foo = "bar"; }' > "testcell/packages.nix"
+          echo '{ baz = "qux"; }' > "testcell/devShells.nix"
+          echo '{ ignored = true; }' > "testcell/flake.nix"
+
+          result=$(nix eval --impure --expr '
+            let
+              lib = import ${pkgs.path}/lib;
+              autodiscover = import ${../lib/core/autodiscover.nix} {
+                l = builtins // lib;
+                cellsFrom = ./.;
+                cellBlocks = [ { name = "__autodiscover"; } ];
+              };
+            in
+              builtins.length (builtins.filter (b: b.name == "packages" || b.name == "devShells") autodiscover)
+          ')
+
+          assert_eq "$result" "2" "should discover 2 blocks"
+        '';
+      }
+      {
+        name = "finds directories";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          mkdir -p "testcell/packages"
+          echo '{ hello = "world"; }' > "testcell/packages/default.nix"
+
+          result=$(nix eval --impure --expr '
+            let
+              lib = import ${pkgs.path}/lib;
+              autodiscover = import ${../lib/core/autodiscover.nix} {
+                l = builtins // lib;
+                cellsFrom = ./.;
+                cellBlocks = [ { name = "__autodiscover"; } ];
+              };
+            in
+              builtins.any (b: b.name == "packages") autodiscover
+          ')
+
+          assert "$result == true" "Should discover directory with default.nix"
+        '';
+      }
+    ];
+  };
+}
diff --git a/tests/builder_test.nix b/tests/builder_test.nix
new file mode 100644
index 0000000..a520b43
--- /dev/null
+++ b/tests/builder_test.nix
@@ -0,0 +1,90 @@
+{
+  pkgs,
+  ntlib,
+  rensa,
+  ...
+}: {
+  suites."Builder" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "has system outputs";
+        expected = true;
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          builtins.hasAttr "x86_64-linux" testFlake;
+      }
+      {
+        name = "has ren metadata";
+        expected = true;
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          builtins.hasAttr "__ren" testFlake;
+      }
+      {
+        name = "merges outputs";
+        expected = {
+          packages = {
+            test-package = "merged";
+          };
+        };
+        actual = let
+          testFlake =
+            rensa.buildWith {
+              inputs = {};
+              cellsFrom = ../cells;
+              cellBlocks = with rensa.blocks; [
+                (simple "test")
+              ];
+              systems = ["x86_64-linux"];
+            } {
+              packages = {
+                test-package = "merged";
+              };
+            };
+        in {
+          packages = testFlake.packages;
+        };
+      }
+      {
+        name = "eval test";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          result=$(nix eval --impure --expr '
+            let
+              rensa = import ${../lib} {lib = import ${pkgs.path}/lib;};
+              testFlake = rensa.build {
+                inputs = { test = "value"; };
+                cellsFrom = ${../cells};
+                cellBlocks = [];
+                systems = [ "x86_64-linux" ];
+              };
+            in
+              testFlake.__ren.__schema
+          ')
+
+          assert "$result == \"v0\"" "should contain version"
+        '';
+      }
+    ];
+  };
+}
diff --git a/tests/filter_test.nix b/tests/filter_test.nix
new file mode 100644
index 0000000..f45f1a3
--- /dev/null
+++ b/tests/filter_test.nix
@@ -0,0 +1,51 @@
+{rensa, ...}: {
+  suites."Filter" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "filter by cell";
+        expected = {
+          x86_64-linux = {};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo" "other"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "docs-value";};
+                ci = {value = "ci-value";};
+              };
+              other = {
+                packages = {value = "other-packages";};
+              };
+            };
+          };
+        in
+          rensa.filter (_: cell: cell == "repo") testFlake [["*" "*"]];
+      }
+      {
+        name = "filter by block";
+        expected = {
+          x86_64-linux = {};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "docs-value";};
+                ci = {value = "ci-value";};
+                packages = {value = "packages-value";};
+              };
+            };
+          };
+        in
+          rensa.filter (block: _: block == "docs") testFlake [["repo" "*"]];
+      }
+    ];
+  };
+}
diff --git a/tests/loader_test.nix b/tests/loader_test.nix
new file mode 100644
index 0000000..9732293
--- /dev/null
+++ b/tests/loader_test.nix
@@ -0,0 +1,57 @@
+{
+  pkgs,
+  ntlib,
+  rensa,
+  ...
+}: {
+  suites."Loader" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "cell sibling access";
+        expected = {
+          hello = "world";
+        };
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          testFlake.x86_64-linux.test.test;
+      }
+      {
+        name = "load file";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          mkdir -p "cells/testcell"
+          echo '{ hello = "world"; }' > "cells/testcell/packages.nix"
+
+          cat > "flake.nix" << 'EOF'
+          {
+            outputs = inputs: let
+              rensa = import ${../lib} { lib = import "${pkgs.path}/lib"; };
+            in rensa.build {
+              inputs = {};
+              cellsFrom = ./cells;
+              cellBlocks = with rensa.blocks; [ (simple "packages") ];
+              systems = [ "x86_64-linux" ];
+            };
+          }
+          EOF
+
+          result=$(nix eval --impure .#x86_64-linux.testcell.packages.hello)
+
+          assert "$result == \"world\"" "should equal to world"
+        '';
+      }
+    ];
+  };
+}
diff --git a/tests/select_test.nix b/tests/select_test.nix
new file mode 100644
index 0000000..823e101
--- /dev/null
+++ b/tests/select_test.nix
@@ -0,0 +1,49 @@
+{rensa, ...}: {
+  suites."Select" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "single block";
+        expected = {
+          x86_64-linux = {value = "test-value";};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "test-value";};
+                ci = {packages = {value = "ci-packages";};};
+              };
+            };
+          };
+        in
+          rensa.select testFlake [["repo" "docs"]];
+      }
+      {
+        name = "multiple blocks";
+        expected = {
+          x86_64-linux = {value = "ci-packages";};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                ci = {packages = {value = "ci-packages";};};
+                other = {value = "ignored";};
+              };
+            };
+          };
+        in
+          rensa.select testFlake [
+            ["repo" "ci" "packages"]
+          ];
+      }
+    ];
+  };
+}