diff --git a/.github/workflows/Check.yml b/.github/workflows/Check.yml
index eb6ca72..2243cd7 100644
--- a/.github/workflows/Check.yml
+++ b/.github/workflows/Check.yml
@@ -33,18 +33,15 @@ jobs:
       - uses: julia-actions/cache@v2
       - name: Install dependencies
         run: |
-          julia --project --code-coverage=${{ env.coverage_flag }} -e 'using Pkg; Pkg.instantiate(); Pkg.precompile()'
+          julia --color=yes --project --code-coverage=${{ env.coverage_flag }} -e 'using Pkg; Pkg.instantiate(); Pkg.precompile()'
       - name: Run Runic
         run: |
-          julia --project --code-coverage=${{ env.coverage_flag }} -m Runic -i $(git ls-files -- '*.jl')
+          julia --color=yes --project --code-coverage=${{ env.coverage_flag }} -m Runic --check --diff $(git ls-files -- '*.jl')
         if: ${{ matrix.version == 'nightly' }}
       - name: Run Runic
         run: |
-          julia --project --code-coverage=${{ env.coverage_flag }} -e 'using Runic; exit(Runic.main(ARGS))' -- -i $(git ls-files -- '*.jl')
+          julia --color=yes --project --code-coverage=${{ env.coverage_flag }} -e 'using Runic; exit(Runic.main(ARGS))' -- --check --diff $(git ls-files -- '*.jl')
         if: ${{ matrix.version != 'nightly' }}
-      - name: Check formatting diff
-        run: |
-          git diff --color=always --exit-code
       - uses: julia-actions/julia-processcoverage@v1
       - uses: codecov/codecov-action@v4
         with:
diff --git a/src/Runic.jl b/src/Runic.jl
index a233c2f..c5f3612 100644
--- a/src/Runic.jl
+++ b/src/Runic.jl
@@ -125,6 +125,16 @@ function Context(
     )
     fmt_io = IOBuffer()
     fmt_tree = nothing
+    # Set up buffers
+    src_pos = position(src_io)
+    @assert src_pos == 0
+    fmt_pos = position(fmt_io)
+    @assert fmt_pos == 0
+    nb = write(fmt_io, read(src_io, span(src_tree)))
+    @assert nb == span(src_tree)
+    # Reset IO positions to the beginning
+    seek(src_io, src_pos)
+    seek(fmt_io, fmt_pos)
     # Debug mode enforces verbose and assert
     verbose = debug ? true : verbose
     assert = debug ? true : assert
@@ -440,16 +450,10 @@ end
 # Entrypoint
 function format_tree!(ctx::Context)
     root = ctx.src_tree
-    # Write the root node to the output IO so that the formatter can read it if needed
-    src_pos = position(ctx.src_io)
-    @assert src_pos == 0
+    # Verify buffers
+    @assert position(ctx.src_io) == 0
     fmt_pos = position(ctx.fmt_io)
     @assert fmt_pos == 0
-    nb = write(ctx.fmt_io, read(ctx.src_io, span(root)))
-    @assert nb == span(root)
-    # Reset IOs so that the offsets are correct
-    seek(ctx.src_io, src_pos)
-    seek(ctx.fmt_io, fmt_pos)
     # Set the root to the current node
     root′ = root
     itr = 0
diff --git a/src/main.jl b/src/main.jl
index 8e9bc3d..29bc2fd 100644
--- a/src/main.jl
+++ b/src/main.jl
@@ -173,6 +173,15 @@ function main(argv)
         return panic("option `-i` is incompatible with stdin as input")
     end
 
+    # --diff currently requires git
+    git = nothing
+    if diff
+        git = Sys.which("git")
+        if git === nothing
+            return panic("option `-d, --diff` requires `git` to be installed")
+        end
+    end
+
     # Loop over the input files
     for inputfile in inputfiles
         # Read the input
@@ -266,7 +275,7 @@ function main(argv)
             end
         elseif changed || !inplace
             try
-                write(output, take!(ctx.fmt_io))
+                write(output, seekstart(ctx.fmt_io))
             catch err
                 print_progress && errln()
                 panic("could not write to output: ", err)
@@ -276,7 +285,15 @@ function main(argv)
             print_progress && okln()
         end
         if diff
-            error("todo")
+            mktempdir() do dir
+                A = joinpath(dir, "A.jl")
+                B = joinpath(dir, "B.jl")
+                write(A, ctx.src_str)
+                write(B, seekstart(ctx.fmt_io))
+                # ignorestatus because --no-index implies --exit-code
+                cmd = `$(git) --no-pager diff --no-index --color=always A.jl B.jl`
+                run(setenv(ignorestatus(cmd); dir = dir))
+            end
         end
 
     end # inputfile loop