diff --git a/.gitignore b/.gitignore
index 09edf7a7..14c56aab 100644
--- a/.gitignore
+++ b/.gitignore
@@ -56,6 +56,16 @@ microwaveprop-*.tar
 # Git worktrees
 /.worktrees/
 
+# Nix dev shell runtime state (local to each machine)
+.nix-postgres/
+.nix-mix/
+.nix-hex/
+.nix-services-started
+.direnv/
+
+# Generated by pre-commit-hooks.nix (configuration lives in nix/shell.nix)
+.pre-commit-config.yaml
+
 # OS files
 .DS_Store
 Thumbs.db
diff --git a/.tool-versions b/.tool-versions
index 088edd21..5e93d4ea 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1,2 +1,2 @@
-erlang 29.0.2
+erlang 29.0.3
 elixir 1.20.2-otp-29
diff --git a/Dockerfile b/Dockerfile
index c9c55e28..412a0553 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,8 +11,8 @@
 #   - codeberg.org/gmcintire/prop-base - prebuilt runtime base (see Dockerfile.base)
 #   - Ex: docker.io/hexpm/elixir:1.19.5-erlang-28.4.1-debian-trixie-20260316-slim
 #
-ARG ELIXIR_VERSION=1.20.1
-ARG OTP_VERSION=29.0.2
+ARG ELIXIR_VERSION=1.20.2
+ARG OTP_VERSION=29.0.3
 ARG DEBIAN_VERSION=trixie-20260518-slim
 
 ARG BUILDER_IMAGE="docker.io/hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION}-debian-${DEBIAN_VERSION}"
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 00000000..7b0eb01f
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,124 @@
+{
+  "nodes": {
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1767039857,
+        "narHash": "sha256-vNpUSpF5Nuw8xvDLj2KCwwksIbjua2LZCqhV1LNRDns=",
+        "owner": "NixOS",
+        "repo": "flake-compat",
+        "rev": "5edf11c44bc78a0d334f6334cdaf7d60d732daab",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1769996383,
+        "narHash": "sha256-AnYjnFWgS49RlqX7LrC4uA+sCCDBj0Ry/WOJ5XWAsa0=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "57928607ea566b5db3ad13af0e57e921e6b12381",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "pre-commit-hooks",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1709087332,
+        "narHash": "sha256-HG2cCnktfHsKV0s4XW83gU3F57gaTljL9KNSuG6bnQs=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1784007870,
+        "narHash": "sha256-djcLt/JJphyNt4eDY9XTly+/WbCK5lqWq9lSgCmJkkQ=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "18b9261cb3294b6d2a06d03f96872827b8fe2698",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "pre-commit-hooks": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "gitignore": "gitignore",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1769939035,
+        "narHash": "sha256-Fok2AmefgVA0+eprw2NDwqKkPGEI5wvR+twiZagBvrg=",
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "rev": "a8ca480175326551d6c4121498316261cbb5b260",
+        "type": "github"
+      },
+      "original": {
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nixpkgs": "nixpkgs",
+        "pre-commit-hooks": "pre-commit-hooks",
+        "systems": "systems"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 00000000..586536cb
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,69 @@
+{
+  description = "Prop — Microwave propagation prediction for NTMS";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+    flake-parts = {
+      url = "github:hercules-ci/flake-parts";
+      inputs.nixpkgs-lib.follows = "nixpkgs";
+    };
+
+    systems.url = "github:nix-systems/default";
+
+    pre-commit-hooks = {
+      url = "github:cachix/pre-commit-hooks.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    inputs@{
+      self,
+      nixpkgs,
+      flake-parts,
+      systems,
+      pre-commit-hooks,
+    }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = builtins.filter (s: s != "x86_64-darwin") (import systems);
+
+      perSystem =
+        {
+          config,
+          self',
+          inputs',
+          pkgs,
+          system,
+          ...
+        }:
+        {
+          _module.args.pkgs = import nixpkgs {
+            inherit system;
+            config = {
+              allowUnfree = true;
+            };
+            overlays = [
+              # Pin Erlang 29.0.3 and Elixir 1.20.2 (matches .tool-versions)
+              (final: prev: {
+                erlang = prev.beam.interpreters.erlang_29;
+                elixir = prev.beam.packages.erlang_29.elixir_1_20;
+                beamPackages = prev.beam.packages.erlang_29.extend (
+                  self: super: {
+                    elixir = super.elixir_1_20;
+                  }
+                );
+              })
+            ];
+          };
+
+          # Development shell with PostgreSQL, Rust, Elixir LSP
+          devShells.default = pkgs.callPackage ./nix/shell.nix {
+            pre-commit-hooks = inputs.pre-commit-hooks.lib.${system};
+          };
+
+          # Nix formatter for flake files
+          formatter = pkgs.nixfmt;
+        };
+    };
+}
diff --git a/nix.md b/nix.md
new file mode 100644
index 00000000..6cbf93ab
--- /dev/null
+++ b/nix.md
@@ -0,0 +1,155 @@
+# Nix + direnv Setup
+
+This project uses [Nix flakes](https://nixos.wiki/wiki/Flakes) to provide a reproducible dev environment that activates automatically when you `cd` into the project directory. No more "works on my machine" — every dev gets the same Elixir, Erlang, Rust, and PostgreSQL.
+
+## How It Works
+
+1. **Nix flakes** (`flake.nix`) declare every tool and library the project needs, pinned via `flake.lock`.
+2. **direnv** watches for `.envrc` and loads/unloads the environment as you enter/leave the directory.
+3. **nix-direnv** bridges the two, so `use flake` in `.envrc` triggers `nix develop`.
+
+The result: `cd` into the repo and you're ready to run `mix phx.server`. `cd` out and your shell is clean. No global installs, no version conflicts, no manual setup.
+
+## File Layout
+
+| File | Purpose |
+|---|---|
+| `flake.nix` | Top-level flake: pins nixpkgs, Erlang/Elixir versions, defines dev shell |
+| `flake.lock` | Locked hashes for every dependency — reproducible across machines |
+| `nix/shell.nix` | Dev shell definition: packages, env vars, PostgreSQL service scripts, pre-commit hooks |
+| `shell.nix` | Flake-compat shim so `nix-shell` works without `--extra-experimental-features flakes` |
+| `.envrc` | Contains `use flake` (gitignored — create it yourself on each machine) |
+
+## One-Time Setup
+
+### 1. Install Nix (multi-user)
+
+```bash
+sh <(curl -L https://nixos.org/nix/install)
+```
+
+Follow the prompts. The multi-user install (`--daemon`) is recommended on macOS. When it finishes, open a new terminal so the Nix daemon is running.
+
+Enable flakes. Add this to `~/.config/nix/nix.conf` (create the file if needed):
+```
+experimental-features = nix-command flakes
+```
+
+Verify:
+```bash
+nix --version
+nix flake --help
+```
+
+### 2. Install direnv
+
+```bash
+brew install direnv
+```
+
+Add the direnv hook to your shell's rc file. Running `brew info direnv` will print the exact line — pick the one for your shell:
+
+**fish** (`~/.config/fish/config.fish`):
+```fish
+direnv hook fish | source
+```
+
+**zsh** (`~/.zshrc`):
+```zsh
+eval "$(direnv hook zsh)"
+```
+
+### 3. Install nix-direnv
+
+```bash
+brew install nix-direnv
+```
+
+Then configure direnv to load it. Create (or append to) `~/.config/direnv/direnvrc`:
+```bash
+mkdir -p ~/.config/direnv
+cat > ~/.config/direnv/direnvrc << 'EOF'
+source "$(brew --prefix nix-direnv)/share/nix-direnv/direnvrc"
+EOF
+```
+
+If you installed nix-direnv via `nix profile` instead of Homebrew, the source line is:
+```
+source $HOME/.nix-profile/share/nix-direnv/direnvrc
+```
+
+### 4. Create `.envrc`
+
+```bash
+echo 'use flake' > .envrc
+```
+
+This is gitignored — each dev creates their own. You can add personal overrides (e.g., extra env vars) here.
+
+### 5. Restart your shell
+
+Open a new terminal tab or `exec $SHELL` so the direnv hook takes effect.
+
+### 6. First time entering the project directory
+
+```bash
+cd ~/dev/prop
+direnv allow   # one-time approval — thereafter it loads automatically
+```
+
+The first build may take several minutes while Nix downloads and compiles packages (Erlang, Elixir, Rust, PostgreSQL, etc.). Subsequent entries are instant — direnv caches the shell.
+
+## Daily Use
+
+Once set up, just `cd` into the repo. The shell activates automatically:
+
+- **PostgreSQL** starts on port 5432 with `prop_dev` and `prop_test` databases created
+- **`start-services`** / **`stop-services`** commands are available to manually control PostgreSQL
+- **Mix/Hex** are sandboxed to `.nix-mix` / `.nix-hex` (doesn't pollute `~/.mix`)
+- **Pre-commit hooks** (mix format, credo, nixfmt, cargo fmt, cargo clippy) install automatically
+- **Services stop** automatically when you `cd` out of the directory
+
+## Troubleshooting
+
+**`direnv: error .envrc is blocked`**
+Run `direnv allow` in the project directory.
+
+**`nix develop` fails with "experimental Nix feature 'nix-command' is disabled"**
+Add `experimental-features = nix-command flakes` to `~/.config/nix/nix.conf`.
+
+**Empty shell, no Elixir available**
+Run `nix develop --command elixir --version` to see build output. Check for syntax errors in `flake.nix` or `nix/shell.nix`:
+```bash
+nix flake check
+```
+
+**Flake can't see new or modified files**
+Nix flakes only see files tracked by git. If you added a new nix file, stage it first:
+```bash
+git add -N <file>
+```
+
+**Nix build fails with a hash mismatch in `shell.nix` (flake-compat)**
+The `flake-compat` tarball hash in `shell.nix` is stale. Update it to the actual hash shown in the error message.
+
+**direnv not triggering on `cd`**
+Check your shell's rc file has the hook line from step 2. Run `direnv status` in the project directory.
+
+**I added a new package to `nix/shell.nix` — how do I reload?**
+direnv watches `flake.nix` and `flake.lock` for changes and rebuilds automatically. If it doesn't pick up the change, run `direnv reload`.
+
+## Updating Dependencies
+
+The flake pins everything via `flake.lock`. To update all inputs to their latest versions:
+
+```bash
+nix flake update
+```
+
+To update just nixpkgs (e.g., to get a newer Erlang/Elixir):
+
+```bash
+nix flake lock --update-input nixpkgs
+```
+
+Commit the updated `flake.lock` so everyone gets the same versions.
diff --git a/nix/shell.nix b/nix/shell.nix
new file mode 100644
index 00000000..e3e8a9bf
--- /dev/null
+++ b/nix/shell.nix
@@ -0,0 +1,280 @@
+{
+  lib,
+  stdenv,
+  mkShell,
+  writeShellScriptBin,
+  # Elixir/Erlang
+  elixir,
+  # Database
+  postgresql_16,
+  # Rust toolchain
+  rustup,
+  rust-analyzer,
+  # Development tools
+  git-lfs,
+  inotify-tools,
+  # LSPs and formatters
+  elixir-ls,
+  nixfmt,
+  shellcheck,
+  # Pre-commit hooks
+  pre-commit-hooks,
+}:
+
+let
+  # PostgreSQL data directory (local to project)
+  pgDataDir = ".nix-postgres";
+  pgPort = "5432";
+  pgHost = "localhost";
+
+  # Service management flag
+  servicesFlag = ".nix-services-started";
+
+  # Script to start PostgreSQL
+  startServices = writeShellScriptBin "start-services" ''
+        set -e
+
+        echo "Starting development services..."
+
+        # Check if already started
+        if [ -f "${servicesFlag}" ]; then
+          echo "Services already running (remove ${servicesFlag} to force restart)"
+          exit 0
+        fi
+
+        # Initialize PostgreSQL if needed
+        if [ ! -d "${pgDataDir}" ]; then
+          echo "Initializing PostgreSQL database..."
+          ${postgresql_16}/bin/initdb -D ${pgDataDir} -U $USER --encoding=UTF8 --locale=en_US.UTF-8
+
+          # Configure PostgreSQL for development (fast but unsafe for production)
+          cat >> ${pgDataDir}/postgresql.conf << EOF
+    # Development configuration
+    max_connections = 100
+    shared_buffers = 128MB
+    fsync = off
+    synchronous_commit = off
+    full_page_writes = off
+    EOF
+        fi
+
+        # Check if PostgreSQL is already running on this port
+        if ${postgresql_16}/bin/pg_isready -h ${pgHost} -p ${pgPort} > /dev/null 2>&1; then
+          echo "PostgreSQL already running on port ${pgPort}"
+        else
+          # Start PostgreSQL
+          echo "Starting PostgreSQL on port ${pgPort}..."
+          ${postgresql_16}/bin/pg_ctl -D ${pgDataDir} -l ${pgDataDir}/logfile -o "-p ${pgPort}" start
+
+          # Wait for PostgreSQL to be ready
+          for i in {1..30}; do
+            if ${postgresql_16}/bin/pg_isready -h ${pgHost} -p ${pgPort} > /dev/null 2>&1; then
+              break
+            fi
+            if [ $i -eq 30 ]; then
+              echo "PostgreSQL failed to start within 30 seconds"
+              echo "Check ${pgDataDir}/logfile for details"
+              exit 1
+            fi
+            sleep 1
+          done
+          echo "PostgreSQL started successfully"
+        fi
+
+        # Create postgres role if it doesn't exist (needed by some tools)
+        ${postgresql_16}/bin/psql -h ${pgHost} -p ${pgPort} -U $USER -d postgres -tc \
+          "SELECT 1 FROM pg_roles WHERE rolname='postgres'" | grep -q 1 || \
+          ${postgresql_16}/bin/psql -h ${pgHost} -p ${pgPort} -U $USER -d postgres -c \
+            "CREATE ROLE postgres WITH SUPERUSER LOGIN;"
+
+        # Create development and test databases
+        for db in prop_dev prop_test; do
+          ${postgresql_16}/bin/psql -h ${pgHost} -p ${pgPort} -U $USER -d postgres -tc \
+            "SELECT 1 FROM pg_database WHERE datname='$db'" | grep -q 1 || \
+            ${postgresql_16}/bin/createdb -h ${pgHost} -p ${pgPort} -U $USER $db
+        done
+
+        # Mark services as started
+        touch ${servicesFlag}
+
+        echo ""
+        echo "Development environment ready!"
+        echo ""
+        echo "PostgreSQL: ${pgHost}:${pgPort} (databases: prop_dev, prop_test)"
+        echo ""
+  '';
+
+  # Script to stop PostgreSQL
+  stopServices = writeShellScriptBin "stop-services" ''
+    set -e
+
+    echo "Stopping development services..."
+
+    # Stop PostgreSQL
+    if [ -d "${pgDataDir}" ]; then
+      ${postgresql_16}/bin/pg_ctl -D ${pgDataDir} stop -m fast 2>/dev/null || true
+      echo "PostgreSQL stopped"
+    fi
+
+    # Remove services flag
+    rm -f ${servicesFlag}
+
+    echo "Services stopped successfully"
+  '';
+
+  # Pre-commit hooks configuration
+  pre-commit-check = pre-commit-hooks.run {
+    src = ../.;
+    hooks = {
+      # Elixir formatting
+      mix-format = {
+        enable = true;
+        name = "mix format";
+        entry = "mix format --check-formatted";
+        files = "\\.(ex|exs)$";
+        pass_filenames = false;
+      };
+
+      # Credo linting
+      credo = {
+        enable = true;
+        name = "credo";
+        entry = "mix credo --strict";
+        files = "\\.(ex|exs)$";
+        pass_filenames = false;
+      };
+
+      # Rust formatting and linting
+      cargo-fmt = {
+        enable = true;
+        name = "cargo fmt";
+        entry = "cargo fmt --all --check";
+        files = "\\.rs$";
+        pass_filenames = false;
+      };
+
+      cargo-clippy = {
+        enable = true;
+        name = "cargo clippy";
+        entry = "cargo clippy --all-targets -- -D warnings";
+        files = "\\.rs$";
+        pass_filenames = false;
+      };
+
+      # Nix formatting
+      nixfmt = {
+        enable = true;
+        entry = "${nixfmt}/bin/nixfmt";
+      };
+
+      # Shellcheck for shell scripts
+      shellcheck.enable = true;
+    };
+  };
+
+in
+mkShell {
+  name = "prop-dev";
+
+  # Development tools
+  buildInputs = [
+    # Elixir/Erlang
+    elixir
+
+    # Database
+    postgresql_16
+
+    # Rust toolchain
+    rustup
+    rust-analyzer
+
+    # Development tools
+    git-lfs
+
+    # LSPs and formatters
+    elixir-ls
+    nixfmt
+    shellcheck
+
+    # Service management scripts
+    startServices
+    stopServices
+  ]
+  ++ lib.optionals stdenv.isLinux [
+    # Linux-only tools
+    inotify-tools # For Mix file watching
+  ];
+
+  # Environment variables
+  shellHook = ''
+    # Set Mix and Hex home to local directories (avoid polluting $HOME)
+    export MIX_HOME="$PWD/.nix-mix"
+    export HEX_HOME="$PWD/.nix-hex"
+    mkdir -p "$MIX_HOME" "$HEX_HOME"
+
+    # PostgreSQL connection
+    export PGHOST="${pgHost}"
+    export PGPORT="${pgPort}"
+    export PGUSER="$USER"
+    export DATABASE_URL="ecto://$USER@${pgHost}:${pgPort}/prop_dev"
+
+    # Development secrets (DO NOT use in production)
+    export SECRET_KEY_BASE="dev_secret_key_base_at_least_64_bytes_long_for_phoenix_to_accept_it_safely"
+    export RELEASE_COOKIE="dev_release_cookie_for_distributed_erlang"
+
+    # Development environment
+    export MIX_ENV=dev
+    export PHX_HOST=localhost
+    export PORT=4000
+
+    # Ensure Erlang can find NIF libraries
+    export ERL_AFLAGS="-kernel shell_history enabled"
+
+    # Rust: set up stable toolchain if not already installed
+    if ! rustup show active-toolchain 2>/dev/null | grep -q stable; then
+      rustup default stable 2>/dev/null || true
+    fi
+
+    # Install Hex and Rebar3 if not already installed
+    mix local.hex --force --if-missing > /dev/null 2>&1
+    mix local.rebar --force --if-missing > /dev/null 2>&1
+
+    # Auto-start services on first shell entry
+    if [ ! -f "${servicesFlag}" ]; then
+      ${startServices}/bin/start-services
+
+      # Run migrations if databases exist and Mix deps are available
+      if [ -d "deps" ] && mix ecto.migrate 2>/dev/null; then
+        echo "Database migrations applied"
+      fi
+    fi
+
+    # Install pre-commit hooks
+    ${pre-commit-check.shellHook}
+
+    # Welcome message
+    echo ""
+    echo "╔═══════════════════════════════════════════════════════════╗"
+    echo "║                                                           ║"
+    echo "║  Prop — Microwave Propagation Prediction                  ║"
+    echo "║                                                           ║"
+    echo "╚═══════════════════════════════════════════════════════════╝"
+    echo ""
+    echo "Available commands:"
+    echo "  mix phx.server       - Start Phoenix development server"
+    echo "  mix test             - Run test suite"
+    echo "  mix precommit        - Run pre-commit checks (format, credo, test)"
+    echo "  start-services       - Start PostgreSQL"
+    echo "  stop-services        - Stop PostgreSQL"
+    echo "  mix ecto.reset       - Reset database (drop, create, migrate, seed)"
+    echo "  cargo build          - Build Rust crates (cd rust/prop_grid_rs)"
+    echo "  cargo test           - Run Rust tests"
+    echo ""
+    echo "Services:"
+    echo "  PostgreSQL: ${pgHost}:${pgPort}"
+    echo ""
+    echo "Documentation:"
+    echo "  See CLAUDE.md and nix.md for more information"
+    echo ""
+  '';
+}
diff --git a/shell.nix b/shell.nix
new file mode 100644
index 00000000..821da66d
--- /dev/null
+++ b/shell.nix
@@ -0,0 +1,7 @@
+# Compatibility shim for developers using `nix-shell` instead of `nix develop`
+# This file uses flake-compat to provide the development shell from flake.nix
+
+(import (fetchTarball {
+  url = "https://github.com/edolstra/flake-compat/archive/35bb57c0c8d8b62bbfd284272c928ceb64ddbde9.tar.gz";
+  sha256 = "sha256-4gtG9iQuiKITOjNQQeQIpoIB6b16fm+504Ch3sNKLd8=";
+}) { src = ./.; }).shellNix.default