diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 25a1d436..f0d8a9c8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -57,6 +57,14 @@ jobs:
     - name: Run tests
       run: uv run pytest tests/unit tests/test_console.py -n auto --dist worksteal
 
+    - name: Lint - no raw str(relative_to) patterns
+      run: |
+        # Fail if any code uses str(x.relative_to(y)) instead of portable_relpath()
+        if grep -rn --include="*.py" -P 'str\([^)]*\.relative_to\(' src/apm_cli/ | grep -v portable_relpath | grep -v '\.pyc'; then
+          echo "::error::Found raw str(path.relative_to()) calls. Use portable_relpath() from apm_cli.utils.paths instead."
+          exit 1
+        fi
+
     - name: Install UPX
       run: |
         sudo apt-get update
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3bf9a253..03a4bbcd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Systematic Windows path compatibility hardening: added `portable_relpath()` utility, migrated ~23 `relative_to()` call sites to use `.resolve()` on both sides with `.as_posix()` output, and added CI lint guard to block raw `str(path.relative_to())` regressions (#419, #422)
 - Cross-platform YAML encoding: all `apm.yml` read/write operations now use explicit UTF-8 encoding via centralized `yaml_io` helpers, preventing silent mojibake on Windows cp1252 terminals; non-ASCII characters (e.g. accented author names) are preserved as real UTF-8 instead of `\xNN` escape sequences (#387, #433) -- based on #388 by @alopezsanchez
 - SSL certificate verification failures in PyInstaller binary on systems without python.org Python installed; bundled `certifi` CA bundle is now auto-configured via runtime hook (#429)
 - Virtual package types (files, collections, subdirectories) now respect `ARTIFACTORY_ONLY=1`, matching the primary zip-archive proxy-only behavior (#418)
diff --git a/src/apm_cli/commands/uninstall/engine.py b/src/apm_cli/commands/uninstall/engine.py
index 134f4a79..ed20129b 100644
--- a/src/apm_cli/commands/uninstall/engine.py
+++ b/src/apm_cli/commands/uninstall/engine.py
@@ -6,6 +6,7 @@
 from ...constants import APM_MODULES_DIR, APM_YML_FILENAME
 from ...core.command_logger import CommandLogger
 from ...utils.path_security import PathTraversalError, safe_rmtree
+from ...utils.paths import portable_relpath
 
 from ...deps.lockfile import LockFile
 from ...models.apm_package import APMPackage, DependencyReference
@@ -132,7 +133,7 @@ def _remove_packages_from_disk(packages_to_remove, apm_modules_dir, logger):
             try:
                 safe_rmtree(package_path, apm_modules_dir)
                 logger.progress(f"Removed {package} from apm_modules/")
-                logger.verbose_detail(f"    Path: {package_path.relative_to(apm_modules_dir)}")
+                logger.verbose_detail(f"    Path: {portable_relpath(package_path, apm_modules_dir)}")
                 removed += 1
                 deleted_pkg_paths.append(package_path)
             except Exception as e:
@@ -212,7 +213,7 @@ def _cleanup_transitive_orphans(lockfile, packages_to_remove, apm_modules_dir, a
             try:
                 safe_rmtree(orphan_path, apm_modules_dir)
                 logger.progress(f"Removed transitive dependency {orphan_key} from apm_modules/")
-                logger.verbose_detail(f"    Path: {orphan_path.relative_to(apm_modules_dir)}")
+                logger.verbose_detail(f"    Path: {portable_relpath(orphan_path, apm_modules_dir)}")
                 removed += 1
                 deleted_orphan_paths.append(orphan_path)
             except Exception as e:
diff --git a/src/apm_cli/compilation/agents_compiler.py b/src/apm_cli/compilation/agents_compiler.py
index 00ae1828..a1619648 100644
--- a/src/apm_cli/compilation/agents_compiler.py
+++ b/src/apm_cli/compilation/agents_compiler.py
@@ -19,6 +19,7 @@
     find_chatmode_by_name
 )
 from .link_resolver import resolve_markdown_links, validate_link_targets
+from ..utils.paths import portable_relpath
 
 
 @dataclass
@@ -438,10 +439,7 @@ def _compile_claude_md(self, config: CompilationConfig, primitives: PrimitiveCol
                 f"CLAUDE.md Preview: Would generate {len(claude_result.placements)} files"
             ]
             for claude_path in claude_result.content_map.keys():
-                try:
-                    rel_path = claude_path.relative_to(self.base_dir)
-                except ValueError:
-                    rel_path = claude_path
+                rel_path = portable_relpath(claude_path, self.base_dir)
                 preview_lines.append(f"  {rel_path}")
             
             return CompilationResult(
@@ -534,10 +532,7 @@ def _compile_claude_md(self, config: CompilationConfig, primitives: PrimitiveCol
             f"Generated {files_written} CLAUDE.md files:"
         ]
         for placement in claude_result.placements:
-            try:
-                rel_path = placement.claude_path.relative_to(self.base_dir)
-            except ValueError:
-                rel_path = placement.claude_path
+            rel_path = portable_relpath(placement.claude_path, self.base_dir)
             summary_lines.append(f"- {rel_path} ({len(placement.instructions)} instructions)")
         
         return CompilationResult(
@@ -628,12 +623,7 @@ def validate_primitives(self, primitives: PrimitiveCollection) -> List[str]:
         for primitive in primitives.all_primitives():
             primitive_errors = primitive.validate()
             if primitive_errors:
-                try:
-                    # Try to get relative path, but fall back to absolute if it fails
-                    file_path = str(primitive.file_path.relative_to(self.base_dir))
-                except ValueError:
-                    # File is outside base_dir, use absolute path
-                    file_path = str(primitive.file_path)
+                file_path = portable_relpath(primitive.file_path, self.base_dir)
                 
                 for error in primitive_errors:
                     # Treat validation errors as warnings instead of hard errors
@@ -645,10 +635,7 @@ def validate_primitives(self, primitives: PrimitiveCollection) -> List[str]:
                 primitive_dir = primitive.file_path.parent
                 link_errors = validate_link_targets(primitive.content, primitive_dir)
                 if link_errors:
-                    try:
-                        file_path = str(primitive.file_path.relative_to(self.base_dir))
-                    except ValueError:
-                        file_path = str(primitive.file_path)
+                    file_path = portable_relpath(primitive.file_path, self.base_dir)
                     
                     for link_error in link_errors:
                         self.warnings.append(f"{file_path}: {link_error}")
@@ -784,11 +771,7 @@ def _display_placement_preview(self, distributed_result) -> None:
         self._log("progress", "")
         
         for placement in distributed_result.placements:
-            try:
-                rel_path = placement.agents_path.relative_to(self.base_dir.resolve())
-            except ValueError:
-                # Fallback for path resolution issues
-                rel_path = placement.agents_path
+            rel_path = portable_relpath(placement.agents_path, self.base_dir)
             self._log("verbose_detail", f"{rel_path}")
             self._log("verbose_detail", f"   Instructions: {len(placement.instructions)}")
             self._log("verbose_detail", f"   Patterns: {', '.join(sorted(placement.coverage_patterns))}")
@@ -808,18 +791,12 @@ def _display_trace_info(self, distributed_result, primitives: PrimitiveCollectio
         self._log("progress", "")
         
         for placement in distributed_result.placements:
-            try:
-                rel_path = placement.agents_path.relative_to(self.base_dir.resolve())
-            except ValueError:
-                rel_path = placement.agents_path
+            rel_path = portable_relpath(placement.agents_path, self.base_dir)
             self._log("verbose_detail", f"{rel_path}")
             
             for instruction in placement.instructions:
                 source = getattr(instruction, 'source', 'local')
-                try:
-                    inst_path = instruction.file_path.relative_to(self.base_dir.resolve())
-                except ValueError:
-                    inst_path = instruction.file_path
+                inst_path = portable_relpath(instruction.file_path, self.base_dir)
                 
                 self._log("verbose_detail", f"   * {instruction.apply_to or 'no pattern'} <- {source} {inst_path}")
             self._log("verbose_detail", "")
@@ -836,10 +813,7 @@ def _generate_placement_summary(self, distributed_result) -> str:
         lines = ["Distributed AGENTS.md Placement Summary:", ""]
         
         for placement in distributed_result.placements:
-            try:
-                rel_path = placement.agents_path.resolve().relative_to(self.base_dir.resolve()).as_posix()
-            except ValueError:
-                rel_path = str(placement.agents_path)
+            rel_path = portable_relpath(placement.agents_path, self.base_dir)
             lines.append(f"{rel_path}")
             lines.append(f"   Instructions: {len(placement.instructions)}")
             lines.append(f"   Patterns: {', '.join(sorted(placement.coverage_patterns))}")
@@ -866,10 +840,7 @@ def _generate_distributed_summary(self, distributed_result, config: CompilationC
         ]
         
         for placement in distributed_result.placements:
-            try:
-                rel_path = placement.agents_path.resolve().relative_to(self.base_dir.resolve()).as_posix()
-            except ValueError:
-                rel_path = str(placement.agents_path)
+            rel_path = portable_relpath(placement.agents_path, self.base_dir)
             lines.append(f"- {rel_path} ({len(placement.instructions)} instructions)")
         
         lines.extend([
diff --git a/src/apm_cli/compilation/claude_formatter.py b/src/apm_cli/compilation/claude_formatter.py
index 4c2f7079..f06e5ef3 100644
--- a/src/apm_cli/compilation/claude_formatter.py
+++ b/src/apm_cli/compilation/claude_formatter.py
@@ -18,6 +18,7 @@
 
 from ..primitives.models import Instruction, PrimitiveCollection, Chatmode
 from ..version import get_version
+from ..utils.paths import portable_relpath
 from .constants import BUILD_ID_PLACEHOLDER
 from .constitution import read_constitution
 
@@ -298,10 +299,7 @@ def _generate_claude_content(
                             source = placement.source_attribution.get(
                                 str(instruction.file_path), 'local'
                             )
-                            try:
-                                rel_path = instruction.file_path.relative_to(self.base_dir)
-                            except ValueError:
-                                rel_path = instruction.file_path
+                            rel_path = portable_relpath(instruction.file_path, self.base_dir)
                             
                             sections.append(f"<!-- Source: {source} {rel_path} -->")
                         
diff --git a/src/apm_cli/compilation/context_optimizer.py b/src/apm_cli/compilation/context_optimizer.py
index 2e827634..4fc4181d 100644
--- a/src/apm_cli/compilation/context_optimizer.py
+++ b/src/apm_cli/compilation/context_optimizer.py
@@ -21,6 +21,7 @@
     CompilationResults, ProjectAnalysis, OptimizationDecision, OptimizationStats,
     PlacementStrategy, PlacementSummary
 )
+from ..utils.paths import portable_relpath
 
 # CRITICAL: Shadow Click commands to prevent namespace collision
 # When this module is imported during 'apm compile', Click's active context
@@ -349,7 +350,7 @@ def get_compilation_results(
             instruction_patterns_detected=len(self._optimization_decisions),
             max_depth=max((a.depth for a in self._directory_cache.values()), default=0),
             constitution_detected=constitution_detected,
-            constitution_path=str(constitution_path.relative_to(self.base_dir)) if constitution_detected else None
+            constitution_path=portable_relpath(constitution_path, self.base_dir) if constitution_detected else None
         )
         
         # Create placement summaries
@@ -421,7 +422,7 @@ def _analyze_project_structure(self) -> None:
             
             # Calculate depth for analysis
             try:
-                relative_path = current_path.relative_to(self.base_dir)
+                relative_path = current_path.resolve().relative_to(self.base_dir.resolve())
                 depth = len(relative_path.parts)
             except ValueError:
                 depth = 0
@@ -512,7 +513,7 @@ def _should_exclude_path(self, path: Path) -> bool:
         except (OSError, FileNotFoundError):
             resolved = path.absolute()
         try:
-            rel_path = resolved.relative_to(self.base_dir)
+            rel_path = resolved.relative_to(self.base_dir.resolve())
         except ValueError:
             # Path is not relative to base_dir, don't exclude
             return False
@@ -655,8 +656,8 @@ def _solve_placement_optimization(
             if intended_dir:
                 # Place in the intended directory (e.g., docs/ for docs/**/*.md)
                 placement = intended_dir
-                reasoning = f"No matching files found, placed in intended directory '{intended_dir.relative_to(self.base_dir)}'"
-                self._warnings.append(f"Pattern '{pattern}' matches no files - placing in intended directory '{intended_dir.relative_to(self.base_dir)}'")
+                reasoning = f"No matching files found, placed in intended directory '{portable_relpath(intended_dir, self.base_dir)}'"
+                self._warnings.append(f"Pattern '{pattern}' matches no files - placing in intended directory '{portable_relpath(intended_dir, self.base_dir)}'")
             else:
                 # Fallback to root for global patterns
                 placement = self.base_dir
@@ -796,7 +797,7 @@ def _file_matches_pattern(self, file_path: Path, pattern: str) -> bool:
                 try:
                     # Resolve both paths to handle symlinks and path inconsistencies
                     resolved_file = file_path.resolve()
-                    rel_path = resolved_file.relative_to(self.base_dir)
+                    rel_path = resolved_file.relative_to(self.base_dir.resolve())
                     
                     # Use cached glob results instead of repeated glob calls
                     matches = self._cached_glob(expanded_pattern)
@@ -810,10 +811,8 @@ def _file_matches_pattern(self, file_path: Path, pattern: str) -> bool:
             else:
                 # For non-recursive patterns, use fnmatch as before
                 try:
-                    # Resolve both paths to handle symlinks and path inconsistencies
-                    resolved_file = file_path.resolve()
-                    rel_path = resolved_file.relative_to(self.base_dir)
-                    if fnmatch.fnmatch(str(rel_path), expanded_pattern):
+                    rel_str = portable_relpath(file_path, self.base_dir)
+                    if fnmatch.fnmatch(rel_str, expanded_pattern):
                         return True
                 except ValueError:
                     pass
@@ -1112,7 +1111,7 @@ def _find_minimal_coverage_placement(self, matching_directories: Set[Path]) -> O
             return None
             
         # Convert to relative paths for easier analysis
-        relative_dirs = [d.relative_to(self.base_dir) for d in matching_directories]
+        relative_dirs = [d.resolve().relative_to(self.base_dir.resolve()) for d in matching_directories]
         
         # Find the lowest common ancestor that covers all directories
         if len(relative_dirs) == 1:
@@ -1166,7 +1165,7 @@ def _is_hierarchically_covered(self, target_dir: Path, placement_dir: Path) -> b
         """
         try:
             # Check if target is the same as placement or is a subdirectory of placement
-            target_dir.relative_to(placement_dir)
+            target_dir.resolve().relative_to(placement_dir.resolve())
             return True
         except ValueError:
             # target_dir is not under placement_dir
@@ -1286,8 +1285,8 @@ def _is_child_directory(self, child: Path, parent: Path) -> bool:
             bool: True if child is subdirectory of parent.
         """
         try:
-            child.relative_to(parent)
-            return child != parent
+            child.resolve().relative_to(parent.resolve())
+            return child.resolve() != parent.resolve()
         except ValueError:
             return False
     
diff --git a/src/apm_cli/compilation/distributed_compiler.py b/src/apm_cli/compilation/distributed_compiler.py
index 686e92e6..48c3214b 100644
--- a/src/apm_cli/compilation/distributed_compiler.py
+++ b/src/apm_cli/compilation/distributed_compiler.py
@@ -20,6 +20,7 @@
 from .link_resolver import UnifiedLinkResolver
 from ..output.formatters import CompilationFormatter
 from ..output.models import CompilationResults
+from ..utils.paths import portable_relpath
 
 # CRITICAL: Shadow Click commands to prevent namespace collision
 set = builtins.set
@@ -241,7 +242,7 @@ def analyze_directory_structure(self, instructions: List[Instruction]) -> Direct
                 directories[abs_dir].add(pattern)
                 
                 # Calculate depth and parent relationships
-                depth = len(abs_dir.relative_to(self.base_dir).parts)
+                depth = len(abs_dir.resolve().relative_to(self.base_dir.resolve()).parts)
                 depth_map[abs_dir] = depth
                 
                 if depth > 0:
@@ -541,10 +542,7 @@ def _generate_agents_content(
                     # Add source attribution for individual instructions
                     if placement.source_attribution:
                         source = placement.source_attribution.get(str(instruction.file_path), 'local')
-                        try:
-                            rel_path = instruction.file_path.relative_to(self.base_dir)
-                        except ValueError:
-                            rel_path = instruction.file_path
+                        rel_path = portable_relpath(instruction.file_path, self.base_dir)
                         
                         sections.append(f"<!-- Source: {source} {rel_path} -->")
                     
@@ -612,7 +610,7 @@ def _find_orphaned_agents_files(self, generated_paths: List[Path]) -> List[Path]
         for agents_file in self.base_dir.rglob("AGENTS.md"):
             # Skip files that are outside our project or in special directories
             try:
-                relative_path = agents_file.relative_to(self.base_dir)
+                relative_path = agents_file.resolve().relative_to(self.base_dir.resolve())
                 
                 # Skip files in certain directories that shouldn't be cleaned
                 skip_dirs = {".git", ".apm", "node_modules", "__pycache__", ".pytest_cache", "apm_modules"}
@@ -645,13 +643,13 @@ def _generate_orphan_warnings(self, orphaned_files: List[Path]) -> List[str]:
         
         # Professional warning format with readable list for multiple files
         if len(orphaned_files) == 1:
-            rel_path = orphaned_files[0].relative_to(self.base_dir)
+            rel_path = portable_relpath(orphaned_files[0], self.base_dir)
             warning_messages.append(f"Orphaned AGENTS.md found: {rel_path} - run 'apm compile --clean' to remove")
         else:
             # For multiple files, create a single multi-line warning message
             file_list = []
             for file_path in orphaned_files[:5]:  # Show first 5
-                rel_path = file_path.relative_to(self.base_dir)
+                rel_path = portable_relpath(file_path, self.base_dir)
                 file_list.append(f"  * {rel_path}")
             if len(orphaned_files) > 5:
                 file_list.append(f"  * ...and {len(orphaned_files) - 5} more")
@@ -681,14 +679,14 @@ def _cleanup_orphaned_files(self, orphaned_files: List[Path], dry_run: bool = Fa
             # In dry-run mode, just report what would be cleaned
             cleanup_messages.append(f"Would clean up {len(orphaned_files)} orphaned AGENTS.md files")
             for file_path in orphaned_files:
-                rel_path = file_path.relative_to(self.base_dir)
+                rel_path = portable_relpath(file_path, self.base_dir)
                 cleanup_messages.append(f"  * {rel_path}")
         else:
             # Actually perform the cleanup
             cleanup_messages.append(f"Cleaning up {len(orphaned_files)} orphaned AGENTS.md files")
             for file_path in orphaned_files:
                 try:
-                    rel_path = file_path.relative_to(self.base_dir)
+                    rel_path = portable_relpath(file_path, self.base_dir)
                     file_path.unlink()
                     cleanup_messages.append(f"  + Removed {rel_path}")
                 except Exception as e:
diff --git a/src/apm_cli/compilation/template_builder.py b/src/apm_cli/compilation/template_builder.py
index 8aa20267..e9b945d2 100644
--- a/src/apm_cli/compilation/template_builder.py
+++ b/src/apm_cli/compilation/template_builder.py
@@ -5,6 +5,7 @@
 from pathlib import Path
 from typing import List, Dict, Optional, Tuple
 from ..primitives.models import Instruction, Chatmode
+from ..utils.paths import portable_relpath
 
 
 @dataclass
@@ -45,12 +46,12 @@ def build_conditional_sections(instructions: List[Instruction]) -> str:
                 try:
                     # Try to get relative path for cleaner display
                     if instruction.file_path.is_absolute():
-                        relative_path = instruction.file_path.relative_to(Path.cwd())
+                        relative_path = portable_relpath(instruction.file_path, Path.cwd())
                     else:
-                        relative_path = instruction.file_path
+                        relative_path = str(instruction.file_path)
                 except (ValueError, OSError):
                     # Fall back to absolute or given path if relative fails
-                    relative_path = instruction.file_path
+                    relative_path = instruction.file_path.as_posix()
                 
                 sections.append(f"<!-- Source: {relative_path} -->")
                 sections.append(content)
diff --git a/src/apm_cli/integration/agent_integrator.py b/src/apm_cli/integration/agent_integrator.py
index a98eafc1..fa6c86da 100644
--- a/src/apm_cli/integration/agent_integrator.py
+++ b/src/apm_cli/integration/agent_integrator.py
@@ -9,6 +9,7 @@
 from typing import List, Dict
 
 from apm_cli.integration.base_integrator import BaseIntegrator, IntegrationResult
+from apm_cli.utils.paths import portable_relpath
 
 
 class AgentIntegrator(BaseIntegrator):
@@ -161,7 +162,7 @@ def integrate_package_agents(self, package_info, project_root: Path,
         for source_file in agent_files:
             target_filename = self.get_target_filename(source_file, package_info.package.name)
             target_path = agents_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
             
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
@@ -176,7 +177,7 @@ def integrate_package_agents(self, package_info, project_root: Path,
             if claude_agents_dir:
                 claude_filename = self.get_target_filename_claude(source_file, package_info.package.name)
                 claude_target = claude_agents_dir / claude_filename
-                claude_rel = str(claude_target.relative_to(project_root))
+                claude_rel = portable_relpath(claude_target, project_root)
                 if not self.check_collision(claude_target, claude_rel, managed_files, force, diagnostics=diagnostics):
                     self.copy_agent(source_file, claude_target)
                     target_paths.append(claude_target)
@@ -185,7 +186,7 @@ def integrate_package_agents(self, package_info, project_root: Path,
             if cursor_agents_dir:
                 cursor_filename = self.get_target_filename_cursor(source_file, package_info.package.name)
                 cursor_target = cursor_agents_dir / cursor_filename
-                cursor_rel = str(cursor_target.relative_to(project_root))
+                cursor_rel = portable_relpath(cursor_target, project_root)
                 if not self.check_collision(cursor_target, cursor_rel, managed_files, force, diagnostics=diagnostics):
                     self.copy_agent(source_file, cursor_target)
                     target_paths.append(cursor_target)
@@ -263,7 +264,7 @@ def integrate_package_agents_claude(self, package_info, project_root: Path,
         for source_file in agent_files:
             target_filename = self.get_target_filename_claude(source_file, package_info.package.name)
             target_path = agents_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
             
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
@@ -382,7 +383,7 @@ def integrate_package_agents_cursor(self, package_info, project_root: Path,
         for source_file in agent_files:
             target_filename = self.get_target_filename_cursor(source_file, package_info.package.name)
             target_path = agents_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
             
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
@@ -452,7 +453,7 @@ def integrate_package_agents_opencode(self, package_info, project_root: Path,
                 source_file, package_info.package.name
             )
             target_path = agents_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
 
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
diff --git a/src/apm_cli/integration/command_integrator.py b/src/apm_cli/integration/command_integrator.py
index 6cac15c6..883f7ef5 100644
--- a/src/apm_cli/integration/command_integrator.py
+++ b/src/apm_cli/integration/command_integrator.py
@@ -9,6 +9,7 @@
 import frontmatter
 
 from apm_cli.integration.base_integrator import BaseIntegrator, IntegrationResult
+from apm_cli.utils.paths import portable_relpath
 
 # Re-export for backward compat (tests import CommandIntegrationResult)
 CommandIntegrationResult = IntegrationResult
@@ -136,7 +137,7 @@ def integrate_package_commands(self, package_info, project_root: Path,
                 base_name = prompt_file.stem
             
             target_path = commands_dir / f"{base_name}.md"
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
             
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
@@ -224,7 +225,7 @@ def integrate_package_commands_opencode(self, package_info, project_root: Path,
                 base_name = prompt_file.stem
 
             target_path = commands_dir / f"{base_name}.md"
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
 
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
diff --git a/src/apm_cli/integration/hook_integrator.py b/src/apm_cli/integration/hook_integrator.py
index 0ace6ade..2d122c27 100644
--- a/src/apm_cli/integration/hook_integrator.py
+++ b/src/apm_cli/integration/hook_integrator.py
@@ -50,6 +50,7 @@
 from dataclasses import dataclass, field
 
 from apm_cli.integration.base_integrator import BaseIntegrator
+from apm_cli.utils.paths import portable_relpath
 
 
 @dataclass
@@ -313,7 +314,7 @@ def integrate_package_hooks(self, package_info, project_root: Path,
             stem = hook_file.stem
             target_filename = f"{package_name}-{stem}.json"
             target_path = hooks_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
 
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 continue
diff --git a/src/apm_cli/integration/instruction_integrator.py b/src/apm_cli/integration/instruction_integrator.py
index fb1166f4..2d6f36c1 100644
--- a/src/apm_cli/integration/instruction_integrator.py
+++ b/src/apm_cli/integration/instruction_integrator.py
@@ -12,6 +12,7 @@
 from typing import Dict, List, Optional, Set
 
 from apm_cli.integration.base_integrator import BaseIntegrator, IntegrationResult
+from apm_cli.utils.paths import portable_relpath
 
 
 class InstructionIntegrator(BaseIntegrator):
@@ -78,7 +79,7 @@ def integrate_package_instructions(
 
         for source_file in instruction_files:
             target_path = instructions_dir / source_file.name
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
 
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
@@ -227,7 +228,7 @@ def integrate_package_instructions_cursor(
             mdc_name = f"{stem}.mdc"
 
             target_path = rules_dir / mdc_name
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
 
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
diff --git a/src/apm_cli/integration/prompt_integrator.py b/src/apm_cli/integration/prompt_integrator.py
index 0d33ff58..444831df 100644
--- a/src/apm_cli/integration/prompt_integrator.py
+++ b/src/apm_cli/integration/prompt_integrator.py
@@ -4,6 +4,7 @@
 from typing import List, Dict
 
 from apm_cli.integration.base_integrator import BaseIntegrator, IntegrationResult
+from apm_cli.utils.paths import portable_relpath
 
 
 class PromptIntegrator(BaseIntegrator):
@@ -111,7 +112,7 @@ def integrate_package_prompts(self, package_info, project_root: Path,
         for source_file in prompt_files:
             target_filename = self.get_target_filename(source_file, package_info.package.name)
             target_path = prompts_dir / target_filename
-            rel_path = str(target_path.relative_to(project_root))
+            rel_path = portable_relpath(target_path, project_root)
             
             if self.check_collision(target_path, rel_path, managed_files, force, diagnostics=diagnostics):
                 files_skipped += 1
diff --git a/src/apm_cli/security/gate.py b/src/apm_cli/security/gate.py
index ee71663d..e8ac9f96 100644
--- a/src/apm_cli/security/gate.py
+++ b/src/apm_cli/security/gate.py
@@ -13,6 +13,7 @@
 from typing import Dict, List, Literal, Optional
 
 from .content_scanner import ContentScanner, ScanFinding
+from ..utils.paths import portable_relpath
 
 
 # ---------------------------------------------------------------------------
@@ -102,7 +103,7 @@ def scan_files(
                 except OSError:
                     continue
                 if file_findings:
-                    rel = str(fpath.relative_to(root))
+                    rel = portable_relpath(fpath, root)
                     findings_by_file[rel] = file_findings
 
         return SecurityGate._build_verdict(
diff --git a/src/apm_cli/utils/__init__.py b/src/apm_cli/utils/__init__.py
index dd6df7e8..d397b8d1 100644
--- a/src/apm_cli/utils/__init__.py
+++ b/src/apm_cli/utils/__init__.py
@@ -21,6 +21,8 @@
     CATEGORY_ERROR,
 )
 
+from .paths import portable_relpath
+
 __all__ = [
     '_rich_success',
     '_rich_error',
@@ -37,4 +39,5 @@
     'CATEGORY_OVERWRITE',
     'CATEGORY_WARNING',
     'CATEGORY_ERROR',
+    'portable_relpath',
 ]
\ No newline at end of file
diff --git a/src/apm_cli/utils/paths.py b/src/apm_cli/utils/paths.py
new file mode 100644
index 00000000..f084bf6c
--- /dev/null
+++ b/src/apm_cli/utils/paths.py
@@ -0,0 +1,27 @@
+"""Cross-platform path utilities for APM CLI.
+
+Centralises the resolve-then-relativise-then-posixify pattern so every
+call site gets Windows-safe, forward-slash relative paths by default.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+
+def portable_relpath(path: Path, base: Path) -> str:
+    """Return a forward-slash relative path, resolving both sides first.
+
+    Handles Windows 8.3 short names (e.g. ``RUNNER~1`` vs ``runneradmin``)
+    and ensures consistent POSIX output on every platform.
+
+    When *path* is not under *base* (or resolution fails), falls back to
+    a resolved absolute POSIX path.
+    """
+    try:
+        return path.resolve().relative_to(base.resolve()).as_posix()
+    except (ValueError, OSError, RuntimeError):
+        try:
+            return path.resolve().as_posix()
+        except (OSError, RuntimeError):
+            return path.as_posix()
diff --git a/tests/unit/test_portable_relpath.py b/tests/unit/test_portable_relpath.py
new file mode 100644
index 00000000..946a17f5
--- /dev/null
+++ b/tests/unit/test_portable_relpath.py
@@ -0,0 +1,96 @@
+"""Unit tests for apm_cli.utils.paths.portable_relpath().
+
+Covers:
+- Basic relative path computation with forward slashes
+- .resolve() on both sides (handles Windows 8.3 short-name mismatches)
+- Fallback to POSIX path when path is not under base
+- Edge cases: same directory, deeply nested, trailing slashes
+"""
+
+import pytest
+
+from apm_cli.utils.paths import portable_relpath
+
+
+class TestPortableRelpath:
+    """Tests for portable_relpath()."""
+
+    def test_simple_relative(self, tmp_path):
+        """Basic child path returns forward-slash relative string."""
+        child = tmp_path / "sub" / "file.md"
+        child.parent.mkdir(parents=True, exist_ok=True)
+        child.touch()
+        result = portable_relpath(child, tmp_path)
+        assert result == "sub/file.md"
+
+    def test_deeply_nested(self, tmp_path):
+        """Deeply nested paths use forward slashes throughout."""
+        child = tmp_path / "a" / "b" / "c" / "d.txt"
+        child.parent.mkdir(parents=True, exist_ok=True)
+        child.touch()
+        result = portable_relpath(child, tmp_path)
+        assert result == "a/b/c/d.txt"
+
+    def test_same_directory(self, tmp_path):
+        """Path equal to base returns '.'."""
+        result = portable_relpath(tmp_path, tmp_path)
+        assert result == "."
+
+    def test_file_in_base(self, tmp_path):
+        """File directly in base returns just the filename."""
+        child = tmp_path / "README.md"
+        child.touch()
+        result = portable_relpath(child, tmp_path)
+        assert result == "README.md"
+
+    def test_fallback_when_not_under_base(self, tmp_path):
+        """Returns POSIX-style resolved path when path is not under base."""
+        other = tmp_path / "other"
+        other.mkdir()
+        base = tmp_path / "base"
+        base.mkdir()
+        result = portable_relpath(other, base)
+        # Should fall back to resolved absolute POSIX path
+        assert result == other.resolve().as_posix()
+        assert "\\" not in result
+
+    def test_result_never_contains_backslash(self, tmp_path):
+        """Returned string never contains backslashes (Windows safety)."""
+        child = tmp_path / "src" / "apm_cli" / "utils" / "paths.py"
+        child.parent.mkdir(parents=True, exist_ok=True)
+        child.touch()
+        result = portable_relpath(child, tmp_path)
+        assert "\\" not in result
+        assert result == "src/apm_cli/utils/paths.py"
+
+    def test_resolve_handles_symlinks(self, tmp_path):
+        """Symlinked paths resolve to the same result."""
+        real_dir = tmp_path / "real"
+        real_dir.mkdir()
+        child = real_dir / "file.txt"
+        child.touch()
+
+        link_dir = tmp_path / "link"
+        try:
+            link_dir.symlink_to(real_dir)
+        except OSError:
+            pytest.skip("symlinks not supported on this platform")
+
+        linked_child = link_dir / "file.txt"
+        # Both should resolve to the same thing
+        result_real = portable_relpath(child, tmp_path)
+        result_link = portable_relpath(linked_child, tmp_path)
+        assert result_real == result_link
+
+    def test_nonexistent_path_still_works(self, tmp_path):
+        """Works even if the path doesn't exist on disk (resolve still works)."""
+        fake = tmp_path / "does" / "not" / "exist.md"
+        result = portable_relpath(fake, tmp_path)
+        assert result == "does/not/exist.md"
+
+    def test_return_type_is_str(self, tmp_path):
+        """Always returns a str, never a Path."""
+        child = tmp_path / "file.txt"
+        child.touch()
+        result = portable_relpath(child, tmp_path)
+        assert isinstance(result, str)