diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 2705b2b..471cb04 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -1,7 +1,26 @@
+<!--
+PR title must follow Conventional Commits (enforced by CI, and it becomes
+the changelog entry via release-please):
+
+  type(scope): short description
+
+- type: feat | fix | docs | style | refactor | perf | test | build | ci | chore | revert
+- scope: optional, e.g. the app or feature area you touched
+- description: lowercase, imperative mood ("add PNG export", not "Added PNG export")
+
+Examples:
+  feat(stats): add PNG export for charts
+  fix: correct datepicker padding in production build
+-->
+
 ## Summary
 
-<!-- What does this change and why? -->
+<!-- What does this PR do and why? A few sentences is enough. -->
+
+## How to test
+
+<!-- Steps to verify the change. For UI changes, add a screenshot. -->
 
-## Related issues
+---
 
-<!-- Closes #123, Refs #456 -->
+<!-- Does this close any open issues? Use "Closes #123" if so. -->
diff --git a/.gitignore b/.gitignore
index 7b70820..cd8f692 100644
--- a/.gitignore
+++ b/.gitignore
@@ -420,3 +420,7 @@ CLAUDE.md
 AGENTS.md
 DESIGN.md
 .claude/
+
+# Lefthook local overrides (personal, not shared)
+.lefthook-local.yml
+lefthook-local.yml
diff --git a/.lefthook.yml b/.lefthook.yml
new file mode 100644
index 0000000..9de3957
--- /dev/null
+++ b/.lefthook.yml
@@ -0,0 +1,43 @@
+# Lefthook Git Hooks Configuration
+min_version: 1.9.0
+assert_lefthook_installed: true
+colors: true
+output:
+  - summary
+  - execution
+  - execution_out
+  - execution_info
+
+pre-commit:
+  parallel: true
+  commands:
+    dotnet-fmt:
+      glob: "*.cs"
+      run: dotnet csharpier check .
+      fail_text: "C# formatting issues found! Run 'just format-dotnet' to fix them"
+
+    ts-lint:
+      root: "clients/typescript/"
+      glob: "*.{ts,tsx,js,jsx,json}"
+      run: deno task lint
+      fail_text: "Linting failed! Run 'cd clients/typescript && deno task lint' to see errors"
+
+    ts-fmt:
+      root: "clients/typescript/"
+      glob: "*.{ts,tsx,js,jsx,json}"
+      run: deno task fmt
+      fail_text: "Formatting issues found! Run 'just format-ts' to fix them"
+
+pre-push:
+  parallel: true
+  commands:
+    dotnet-build:
+      glob: "*.{cs,csproj,sln}"
+      run: dotnet build JsonApiToolkit.sln --configuration Release
+      fail_text: "Build failed! Run 'dotnet build JsonApiToolkit.sln' to see errors"
+
+    ts-check:
+      root: "clients/typescript/"
+      glob: "*.{ts,tsx,js,jsx}"
+      run: deno task check
+      fail_text: "Type checking failed! Run 'cd clients/typescript && deno task check' to see errors"
diff --git a/docs/contributing.md b/docs/contributing.md
index 89c22f3..58ad8a5 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -2,22 +2,42 @@
 
 ## Prerequisites
 
-- .NET 10 SDK 
+- .NET 10 SDK, [Deno](https://deno.com) 2.x, [just](https://github.com/casey/just), [lefthook](https://lefthook.dev)
+  (all pinned in `mise.toml`; run `mise install` if you use [mise](https://mise.jdx.dev))
 - `uv` for the docs site (`brew install uv` or `mise use uv`)
 
 ## Setup
 
 ```bash
-dotnet tool restore   # csharpier + dotnet-api-docs
-dotnet restore
+just setup   # dotnet tool restore, dotnet restore, lefthook install
 ```
 
+This installs git hooks (lefthook) that format-check on commit and type-check
+on push; see `.lefthook.yml`. CI (`ci-cd.yml`, `typescript-ci.yml`,
+`contract-tests.yml`) is still the source of truth and runs the full suites.
+
 ## Daily Commands
 
+Run `just` (no arguments) to list all recipes, or `just --list`.
+
+```bash
+just format          # format .NET + TypeScript
+just check           # format-check + type-check everything, no fixes
+just test            # .NET tests + TypeScript unit tests, in parallel
+just test-contract   # build & run samples/ContractApi, run the Deno contract suite against it
+just test-all        # check + test + test-contract (what CI runs)
+```
+
+Narrower recipes (`format-dotnet`, `format-ts`, `test-dotnet`, `test-ts`,
+`lint`, `clean`) are also available; see the `justfile`.
+
+Equivalent plain commands, if you don't have `just`:
+
 ```bash
 dotnet build --configuration Release
 dotnet test  --configuration Release
-dotnet csharpier format .  
+dotnet csharpier format .
+cd clients/typescript && deno task test
 ```
 
 ## Docs
diff --git a/justfile b/justfile
new file mode 100644
index 0000000..6d1b10e
--- /dev/null
+++ b/justfile
@@ -0,0 +1,134 @@
+set dotenv-load := false
+
+sample := "samples/ContractApi"
+ts := "clients/typescript"
+
+# List available recipes
+default:
+    @just --list
+
+# ---------------------------------------------------------------------------
+# Setup
+# ---------------------------------------------------------------------------
+
+# Full bootstrap: dotnet tools/packages, git hooks
+[group('setup')]
+setup: tools restore hooks
+    @echo "Setup complete. Run 'just test' to verify everything works."
+
+# Restore local dotnet tools (csharpier)
+[group('setup')]
+tools:
+    dotnet tool restore
+
+# Restore NuGet packages
+[group('setup')]
+restore:
+    dotnet restore --locked-mode
+
+# Install lefthook git hooks
+[group('setup')]
+hooks:
+    lefthook install
+
+# ---------------------------------------------------------------------------
+# Quality
+# ---------------------------------------------------------------------------
+
+# Format all code (.NET + TypeScript) in parallel
+[group('quality')]
+format:
+    #!/usr/bin/env bash
+    set -uo pipefail
+    pids=()
+    dotnet csharpier format . & pids+=($!)
+    (cd {{ts}} && deno task fmt:fix) & pids+=($!)
+    code=0
+    for pid in "${pids[@]}"; do wait "$pid" || code=1; done
+    exit $code
+
+# Format .NET only (CSharpier)
+[group('quality')]
+format-dotnet:
+    dotnet csharpier format .
+
+# Format the TypeScript client only (deno fmt)
+[group('quality')]
+format-ts:
+    cd {{ts}} && deno task fmt:fix
+
+# Lint the TypeScript client (deno lint)
+[group('quality')]
+lint:
+    cd {{ts}} && deno task lint
+
+# Type-check and format-check everything, no fixes (.NET + TypeScript) in parallel
+[group('quality')]
+check:
+    #!/usr/bin/env bash
+    set -uo pipefail
+    pids=()
+    dotnet csharpier check . & pids+=($!)
+    (cd {{ts}} && deno task check && deno task lint && deno task fmt) & pids+=($!)
+    code=0
+    for pid in "${pids[@]}"; do wait "$pid" || code=1; done
+    exit $code
+
+# Run .NET + TypeScript unit tests in parallel (excludes the contract suite; see test-contract)
+[group('quality')]
+test:
+    #!/usr/bin/env bash
+    set -uo pipefail
+    pids=()
+    dotnet test --configuration Release & pids+=($!)
+    (cd {{ts}} && deno task test) & pids+=($!)
+    code=0
+    for pid in "${pids[@]}"; do wait "$pid" || code=1; done
+    exit $code
+
+# .NET tests only (JsonApiToolkit.Tests)
+[group('quality')]
+test-dotnet:
+    dotnet test --configuration Release
+
+# TypeScript client unit tests only
+[group('quality')]
+test-ts:
+    cd {{ts}} && deno task test
+
+# Build & run ContractApi (default + all-strict-opt-ins), run the Deno contract suite, then stop the servers
+[group('quality')]
+test-contract:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    dotnet build {{sample}} --configuration Release
+    ASPNETCORE_URLS=http://localhost:5198 \
+        dotnet run --project {{sample}} --configuration Release --no-build &
+    default_pid=$!
+    JSONAPI_STRICT=true ASPNETCORE_URLS=http://localhost:5199 \
+        dotnet run --project {{sample}} --configuration Release --no-build &
+    strict_pid=$!
+    trap 'kill $default_pid $strict_pid 2>/dev/null || true' EXIT
+
+    for url in http://localhost:5198/articles http://localhost:5199/articles; do
+        timeout 60 bash -c "until curl -sf -o /dev/null $url; do sleep 1; done"
+    done
+
+    cd {{ts}} && deno task test:contract
+
+# Everything CI runs: format check, unit tests, and the contract suite
+[group('quality')]
+test-all: check test test-contract
+
+# ---------------------------------------------------------------------------
+# Clean
+# ---------------------------------------------------------------------------
+
+# Remove build artifacts (.NET bin/obj)
+[group('clean')]
+clean:
+    #!/usr/bin/env bash
+    set -uo pipefail
+    dotnet clean JsonApiToolkit.sln > /dev/null 2>&1 || true
+    find . -type d \( -name bin -o -name obj \) -prune -exec rm -rf {} +
+    echo "Cleaned build artifacts."
diff --git a/mise.toml b/mise.toml
index 70aaafc..39620f4 100644
--- a/mise.toml
+++ b/mise.toml
@@ -1,2 +1,5 @@
 [tools]
 dotnet = "10.0.201"
+deno = "2.7.13"
+just = "latest"
+lefthook = "latest"