diff --git a/README.md b/README.md
index 18add49..1a534a0 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,17 @@ The `FENV_DOCKER_TAG` argument is automatically provided and ensures your extend
 
 The `/cache` directory is backed by a Docker volume that persists between container runs. This makes it an ideal location for build and test caches (as shown in the example above with Go's module cache and golangci-lint cache).
 
+### Image Namespacing
+
+Extended images are automatically namespaced by project to prevent collisions when multiple projects use fenv. The image name format is `fenv-ext-{project_name}:{tag}`.
+
+The project name is determined in the following order:
+1. Explicit `FENV_PROJECT_NAME` environment variable
+2. Git remote repository name (e.g., `myproject` from `git@github.com:user/myproject.git`)
+3. Current directory name as a fallback
+
+This ensures that different projects can have different extended images without interfering with each other, even if they happen to be at the same git commit hash.
+
 ## Local Development
 
 Use the `fenv.sh` script to manage your development environment:
@@ -182,6 +193,7 @@ jobs:
 **User-configurable:**
 - `FENV_FDB_VER`: FoundationDB version (default: `7.1.61`)
 - `FENV_CACHE_VOLUME`: Custom cache volume name (optional)
+- `FENV_PROJECT_NAME`: Project name for namespacing extended images (optional, auto-derived from git remote or directory name)
 
 **Auto-computed (available after running fenv):**
 - `FENV_DOCKER_TAG`: Docker tag for the base fenv image
@@ -198,6 +210,7 @@ jobs:
 |------|-------------|----------|---------|
 | `fdb_ver` | FoundationDB version | No | `7.1.61` |
 | `ext_dockerfile` | Path to custom Dockerfile for extending fenv image | No | - |
+| `project_name` | Project name for namespacing extended images | No | Repository name |
 
 ### GitHub Action Outputs
 
diff --git a/action.yaml b/action.yaml
index 82bf3b0..17efb57 100644
--- a/action.yaml
+++ b/action.yaml
@@ -13,6 +13,11 @@ inputs:
     required: false
     default: ''
 
+  project_name:
+    description: 'Project name for namespacing extended images (defaults to repo name)'
+    required: false
+    default: ''
+
 outputs:
   fenv_docker_tag:
     description: 'Docker image tag for the build container'
@@ -90,6 +95,18 @@ runs:
       env:
         FENV_FDB_VER: ${{ inputs.fdb_ver }}
 
+    - name: Derive project name
+      if: steps.check_ext_dockerfile.outputs.should_build == 'true'
+      id: project_name
+      shell: bash
+      run: |
+        if [[ -n "${{ inputs.project_name }}" ]]; then
+          echo "name=${{ inputs.project_name }}" >> $GITHUB_OUTPUT
+        else
+          # Extract repo name from GITHUB_REPOSITORY (org/repo -> repo)
+          echo "name=$(basename ${{ github.repository }})" >> $GITHUB_OUTPUT
+        fi
+
     # Create inline bake file for building both images
     - name: Create bake configuration
       if: steps.check_ext_dockerfile.outputs.should_build == 'true'
@@ -114,10 +131,10 @@ runs:
           }
           context    = "${{ github.workspace }}"
           dockerfile = "${{ inputs.ext_dockerfile }}"
-          tags       = ["fenv-ext:${{ steps.docker_ext.outputs.tag }}"]
+          tags       = ["fenv-ext-${{ steps.project_name.outputs.name }}:${{ steps.docker_ext.outputs.tag }}"]
           platforms  = ["linux/amd64"]
-          cache-from = ["type=gha,scope=fenv-ext-${{ inputs.fdb_ver }}"]
-          cache-to   = ["type=gha,mode=max,scope=fenv-ext-${{ inputs.fdb_ver }}"]
+          cache-from = ["type=gha,scope=fenv-ext-${{ steps.project_name.outputs.name }}-${{ inputs.fdb_ver }}"]
+          cache-to   = ["type=gha,mode=max,scope=fenv-ext-${{ steps.project_name.outputs.name }}-${{ inputs.fdb_ver }}"]
           args = {
             FENV_DOCKER_TAG = "${{ steps.docker.outputs.tag }}"
           }
@@ -158,8 +175,10 @@ runs:
         # Export extended image variables
         if [[ "${{ steps.check_ext_dockerfile.outputs.should_build }}" == "true" ]]; then
           echo "FENV_EXT_DOCKER_TAG=${{ steps.docker_ext.outputs.tag }}" >> $GITHUB_ENV
+          echo "FENV_PROJECT_NAME=${{ steps.project_name.outputs.name }}" >> $GITHUB_ENV
           echo "FENV_EXT_IMAGE_BUILT=true" >> $GITHUB_ENV
         else
           echo "FENV_EXT_DOCKER_TAG=" >> $GITHUB_ENV
+          echo "FENV_PROJECT_NAME=" >> $GITHUB_ENV
           echo "FENV_EXT_IMAGE_BUILT=false" >> $GITHUB_ENV
         fi
diff --git a/fenv.sh b/fenv.sh
index d4e31d1..e1a2652 100755
--- a/fenv.sh
+++ b/fenv.sh
@@ -111,6 +111,35 @@ while [[ $# -gt 0 ]]; do
 done
 
 
+# derive_project_name determines a unique name for the project.
+# It uses FENV_PROJECT_NAME if set, otherwise derives from git remote
+# URL or directory name as a fallback.
+
+function derive_project_name {
+  # Use explicit project name if provided
+  if [[ -n "${FENV_PROJECT_NAME:-}" ]]; then
+    echo "${FENV_PROJECT_NAME}"
+    return
+  fi
+
+  # Try to derive from git remote URL
+  if git -C "${CALLING_DIR}" remote get-url origin &>/dev/null; then
+    local remote_url
+    remote_url="$(git -C "${CALLING_DIR}" remote get-url origin)"
+    # Extract repo name from URL (handles both https and ssh formats)
+    # e.g., https://github.com/user/repo.git -> repo
+    # e.g., git@github.com:user/repo.git -> repo
+    local repo_name
+    repo_name="$(basename "${remote_url}" .git)"
+    echo "${repo_name}"
+    return
+  fi
+
+  # Fallback to directory name
+  basename "${CALLING_DIR}"
+}
+
+
 # Build variables required by docker commands.
 
 FENV_FDB_VER="${FENV_FDB_VER:-7.1.61}"
@@ -131,9 +160,14 @@ if [[ -f "${CALLING_DIR}/fenv/docker_tag.sh" ]]; then
   export FENV_EXT_DOCKER_TAG
 fi
 
+# Derive project name for namespacing extended images
+FENV_PROJECT_NAME="$(derive_project_name)"
+echo "FENV_PROJECT_NAME=${FENV_PROJECT_NAME}"
+export FENV_PROJECT_NAME
+
 # Select which image to use for compose based on whether --docker was provided.
 if [[ -n "${EXT_DOCKERFILE:-}" ]]; then
-  FENV_IMAGE="fenv-ext:${FENV_EXT_DOCKER_TAG}"
+  FENV_IMAGE="fenv-ext-${FENV_PROJECT_NAME}:${FENV_EXT_DOCKER_TAG}"
 else
   FENV_IMAGE="fenv:${FENV_DOCKER_TAG}"
 fi
@@ -163,7 +197,7 @@ if [[ -n "${DO_BUILD:-}" ]]; then
     (set -x; docker build \
       --platform linux/amd64 \
       --build-arg "FENV_DOCKER_TAG=${FENV_DOCKER_TAG}" \
-      --tag "fenv-ext:${FENV_EXT_DOCKER_TAG}" \
+      --tag "fenv-ext-${FENV_PROJECT_NAME}:${FENV_EXT_DOCKER_TAG}" \
       --file "${EXT_DOCKERFILE}" \
       "${CALLING_DIR}")
   fi