diff --git a/.github/workflows/rainix-build-pointers.yaml b/.github/workflows/rainix-build-pointers.yaml
index 4d6ba13..2b49b16 100644
--- a/.github/workflows/rainix-build-pointers.yaml
+++ b/.github/workflows/rainix-build-pointers.yaml
@@ -30,6 +30,17 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache only (NOT out/): this is a
+      # clean-build determinism check (assert committed pointers match a fresh
+      # BuildPointers run), so out/ is regenerated fresh each run while cache/
+      # still speeds recompilation.
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: cache
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       # Use rainix's sol-shell directly (slim — no rust/node/chromium).
       # Mixed-language consumers can ship their heavy default devShell without
       # paying for it on this purely Solidity-side check.
diff --git a/.github/workflows/rainix-copy-artifacts.yaml b/.github/workflows/rainix-copy-artifacts.yaml
index bbfd073..92d54ed 100644
--- a/.github/workflows/rainix-copy-artifacts.yaml
+++ b/.github/workflows/rainix-copy-artifacts.yaml
@@ -29,6 +29,16 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache only (NOT out/): out/ is
+      # regenerated fresh each run so the committed-artifact assert stays a true
+      # clean-build check, while cache/ still speeds recompilation.
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: cache
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       - name: Install soldeer dependencies
         if: hashFiles('soldeer.lock') != ''
         run: nix develop github:rainlanguage/rainix#sol-shell -c forge soldeer install
diff --git a/.github/workflows/rainix-manual-sol-artifacts.yaml b/.github/workflows/rainix-manual-sol-artifacts.yaml
index b8d4689..74f894b 100644
--- a/.github/workflows/rainix-manual-sol-artifacts.yaml
+++ b/.github/workflows/rainix-manual-sol-artifacts.yaml
@@ -37,6 +37,17 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache + artifacts so unchanged
+      # contracts aren't recompiled (forge build is the dominant CI cost).
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: |
+            cache
+            out
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       - name: Install soldeer dependencies
         if: hashFiles('soldeer.lock') != ''
         run: nix develop github:rainlanguage/rainix#sol-shell -c forge soldeer install
diff --git a/.github/workflows/rainix-sol-static.yaml b/.github/workflows/rainix-sol-static.yaml
index 78ce9ad..593ca3a 100644
--- a/.github/workflows/rainix-sol-static.yaml
+++ b/.github/workflows/rainix-sol-static.yaml
@@ -30,6 +30,17 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache + artifacts so unchanged
+      # contracts aren't recompiled (forge build is the dominant CI cost).
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: |
+            cache
+            out
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       - name: Install soldeer dependencies
         if: hashFiles('soldeer.lock') != ''
         run: nix develop github:rainlanguage/rainix#sol-shell -c forge soldeer install
diff --git a/.github/workflows/rainix-sol-test.yaml b/.github/workflows/rainix-sol-test.yaml
index dd0c1df..305e925 100644
--- a/.github/workflows/rainix-sol-test.yaml
+++ b/.github/workflows/rainix-sol-test.yaml
@@ -59,6 +59,17 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache + artifacts so unchanged
+      # contracts aren't recompiled (forge build is the dominant CI cost).
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: |
+            cache
+            out
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       - name: Install soldeer dependencies
         if: hashFiles('soldeer.lock') != ''
         run: nix develop github:rainlanguage/rainix#sol-shell -c forge soldeer install
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 38f9188..d018b1e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -54,6 +54,17 @@ jobs:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
           gc-max-store-size-linux: 8G
+      # Cache Foundry's incremental compilation cache + artifacts so unchanged
+      # contracts aren't recompiled (forge build is the dominant CI cost).
+      - name: Cache Foundry build
+        uses: actions/cache@v4
+        with:
+          path: |
+            cache
+            out
+          key: foundry-${{ runner.os }}-${{ hashFiles('src/**/*.sol', 'test/**/*.sol', 'script/**/*.sol', 'foundry.toml', 'soldeer.lock', 'remappings.txt') }}
+          restore-keys: |
+            foundry-${{ runner.os }}-
       - run: nix develop ../.. --command forge soldeer install
       - name: Run ${{ matrix.task }}
         env: