diff --git a/.github/workflows/coveralls.yml b/.github/workflows/coveralls.yml
index a3d37cd3..25d98c28 100644
--- a/.github/workflows/coveralls.yml
+++ b/.github/workflows/coveralls.yml
@@ -5,7 +5,20 @@ on:
 name: Code coverage with llvm-cov
 
 jobs:
+  Prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      pr_changed_source: ${{ steps.classify_pr.outputs.pr_changed_source }}
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Determine what files the PR changes.
+        id: classify_pr
+        run: contrib/classify-pr.sh ${{ github.event.pull_request.head.sha }} ${{ github.event.pull_request.base.sha }}
+
   Coveralls:
+    needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Code coverage - stable toolchain
     runs-on: ubuntu-latest
     strategy:
diff --git a/.github/workflows/fuzz.yml b/.github/workflows/fuzz.yml
index 3a0e4d1a..232d8812 100644
--- a/.github/workflows/fuzz.yml
+++ b/.github/workflows/fuzz.yml
@@ -9,8 +9,20 @@ on:
   pull_request:
 
 jobs:
+  Prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      pr_changed_source: ${{ steps.classify_pr.outputs.pr_changed_source }}
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Determine what files the PR changes.
+        id: classify_pr
+        run: contrib/classify-pr.sh ${{ github.event.pull_request.head.sha }} ${{ github.event.pull_request.base.sha }}
+
   fuzz:
-    if: ${{ !github.event.act }}
+    needs: Prepare
+    if: ${{ !github.event.act }} && needs.Prepare.outputs.pr_changed_source == 'true'
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -64,7 +76,8 @@ jobs:
           path: executed_${{ matrix.fuzz_target }}
 
   verify-execution:
-    if: ${{ !github.event.act }}
+    needs: Prepare
+    if: ${{ !github.event.act }} && needs.Prepare.outputs.pr_changed_source == 'true'
     needs: fuzz
     runs-on: ubuntu-latest
     steps:
diff --git a/.github/workflows/rust.yml b/.github/workflows/rust.yml
index 169e2e5f..6081d431 100644
--- a/.github/workflows/rust.yml
+++ b/.github/workflows/rust.yml
@@ -14,9 +14,13 @@ jobs:
       crates: ${{ steps.get_matrix.outputs.crates }}
       deps: ${{ steps.get_matrix.outputs.deps }}
       nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
+      pr_changed_source: ${{ steps.classify_pr.outputs.pr_changed_source }}
     steps:
       - name: Checkout Crate
         uses: actions/checkout@v4
+      - name: Determine what files the PR changes.
+        id: classify_pr
+        run: contrib/classify-pr.sh ${{ github.event.pull_request.head.sha }} ${{ github.event.pull_request.base.sha }}
       - name: Read nightly version
         id: read_toolchain
         run: echo "nightly_version=$(cat .github/nightly-version)" >> $GITHUB_OUTPUT
@@ -26,6 +30,7 @@ jobs:
 
   Stable:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Test - stable toolchain
     runs-on: ubuntu-latest
     strategy:
@@ -47,6 +52,7 @@ jobs:
 
   Beta:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Test - beta toolchain
     runs-on: ubuntu-latest
     strategy:
@@ -67,6 +73,7 @@ jobs:
 
   Nightly:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Test - nightly toolchain
     runs-on: ubuntu-latest
     strategy:
@@ -91,6 +98,7 @@ jobs:
 
   MSRV:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Test - 1.56.1 toolchain
     runs-on: ubuntu-latest
     strategy:
@@ -112,6 +120,8 @@ jobs:
         run: ./contrib/run_task.sh ${{ matrix.crate }} ${{ matrix.task }}
 
   Arch32bit:
+    needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Test 32-bit version
     runs-on: ubuntu-latest
     steps:
@@ -129,8 +139,9 @@ jobs:
         run: cargo test --target i686-unknown-linux-gnu
 
   Cross:
+    needs: Prepare
+    if: ${{ !github.event.act }} && needs.Prepare.outputs.pr_changed_source == 'true'
     name: Cross test
-    if: ${{ !github.event.act }}
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Crate
@@ -146,6 +157,7 @@ jobs:
 
   Embedded:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     runs-on: ubuntu-latest
     env:
       RUSTFLAGS: "-C link-arg=-Tlink.x"
@@ -171,6 +183,7 @@ jobs:
 
   ASAN:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Address sanitizer     # hashes crate only.
     runs-on: ubuntu-latest
     strategy:
@@ -193,6 +206,7 @@ jobs:
 
   WASM:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: WebAssembly Build # hashes crate only.
     runs-on: ubuntu-latest
     strategy:
@@ -211,6 +225,7 @@ jobs:
 
   Schemars:
     needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     name: Schemars
     runs-on: ubuntu-latest
     strategy:
@@ -228,6 +243,8 @@ jobs:
         run: ./contrib/run_task.sh ${{ matrix.crate }} ${{ matrix.task }}
 
   Kani:
+    needs: Prepare
+    if: needs.Prepare.outputs.pr_changed_source == 'true'
     runs-on: ubuntu-20.04
     steps:
       - name: 'Checkout your code.'
diff --git a/contrib/classify-pr.sh b/contrib/classify-pr.sh
new file mode 100755
index 00000000..e72fba5d
--- /dev/null
+++ b/contrib/classify-pr.sh
@@ -0,0 +1,29 @@
+#!/bin/sh
+
+if [ "$#" -ne 2 ]; then
+  echo "Usage: $0 <tip of master> <tip of PR>"
+  exit 1
+fi
+
+pr_tip=$1
+master_tip=$2
+
+# When called on `pull_request`, GA fails to pull down master by default.
+# When called on `push`, GA fails to pull down the PR by default, only its merge commit.
+# The simplest way to deal with this is to just pull both explicitly.
+git fetch origin "$master_tip":master_tip
+git fetch origin "$pr_tip":pr_tip
+
+pr_base=$(git merge-base master_tip pr_tip)
+
+echo "Using  master $master_tip"
+echo "Using  PR tip $pr_tip"
+echo "Using PR base $pr_base"
+
+# If something modifies any non-markdown file, it's considered a source code change.
+if git diff --name-only "$pr_base" "$pr_tip" | grep -qv "^.md$"; then
+    echo "pr_changed_source=true" >> "$GITHUB_OUTPUT"
+else
+    echo "pr_changed_source=false" >> "$GITHUB_OUTPUT"
+fi
+