diff --git a/.cursorrules b/.cursorrules
index 0a0308c..66b0e72 100644
--- a/.cursorrules
+++ b/.cursorrules
@@ -28,6 +28,11 @@ Critical Rules:
    the relevant documentation (README, DEVELOPMENT.md, inline docs) in
    the same commit or PR. Do not leave documentation out of sync with
    code.
+9. Ensure the DevRail Makefile is active. GNU Make prefers GNUmakefile
+   over makefile over Makefile. If the project has a GNUmakefile or
+   makefile, the DevRail Makefile will be silently ignored. Merge
+   project-specific targets into the DevRail Makefile, or add
+   `include Makefile` to the existing GNUmakefile.
 
 Quick Reference:
 
diff --git a/.opencode/agents.yaml b/.opencode/agents.yaml
index 7211f17..8d6142d 100644
--- a/.opencode/agents.yaml
+++ b/.opencode/agents.yaml
@@ -33,6 +33,11 @@ agents:
          the relevant documentation (README, DEVELOPMENT.md, inline docs) in
          the same commit or PR. Do not leave documentation out of sync with
          code.
+      9. Ensure the DevRail Makefile is active. GNU Make prefers GNUmakefile
+         over makefile over Makefile. If the project has a GNUmakefile or
+         makefile, the DevRail Makefile will be silently ignored. Merge
+         project-specific targets into the DevRail Makefile, or add
+         `include Makefile` to the existing GNUmakefile.
 
       Quick Reference:
 
diff --git a/AGENTS.md b/AGENTS.md
index 6d238eb..6c6afb1 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -13,6 +13,7 @@ See DEVELOPMENT.md for the complete reference.
 6. **Use the shared logging library.** No raw `echo` for status messages. Use `log_info`, `log_warn`, `log_error`, `log_debug`, and `die` from `lib/log.sh`.
 7. **Never suppress failing checks.** When a lint, format, security, or test check fails, fix the underlying issue. Never comment out code, add suppression annotations, disable rules, or mark CI jobs as allowed-to-fail to bypass a failing check.
 8. **Update documentation when changing behavior.** When a change affects public interfaces, configuration, CLI usage, or setup steps, update the relevant documentation (README, DEVELOPMENT.md, inline docs) in the same commit or PR. Do not leave documentation out of sync with code.
+9. **Ensure the DevRail `Makefile` is active.** GNU Make prefers `GNUmakefile` over `makefile` over `Makefile`. If the project has a `GNUmakefile` or `makefile`, the DevRail `Makefile` will be silently ignored. Merge project-specific targets into the DevRail `Makefile`, or add `include Makefile` to the existing `GNUmakefile`.
 
 ## Quick Reference
 
diff --git a/CLAUDE.md b/CLAUDE.md
index 5fc40d7..1159ffd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -13,6 +13,7 @@ See DEVELOPMENT.md for the complete reference.
 6. **Use the shared logging library.** No raw `echo` for status messages. Use `log_info`, `log_warn`, `log_error`, `log_debug`, and `die` from `lib/log.sh`.
 7. **Never suppress failing checks.** When a lint, format, security, or test check fails, fix the underlying issue. Never comment out code, add suppression annotations, disable rules, or mark CI jobs as allowed-to-fail to bypass a failing check.
 8. **Update documentation when changing behavior.** When a change affects public interfaces, configuration, CLI usage, or setup steps, update the relevant documentation (README, DEVELOPMENT.md, inline docs) in the same commit or PR. Do not leave documentation out of sync with code.
+9. **Ensure the DevRail `Makefile` is active.** GNU Make prefers `GNUmakefile` over `makefile` over `Makefile`. If the project has a `GNUmakefile` or `makefile`, the DevRail `Makefile` will be silently ignored. Merge project-specific targets into the DevRail `Makefile`, or add `include Makefile` to the existing `GNUmakefile`.
 
 ## Quick Reference