diff --git a/src/fromager/bootstrap_requirement_resolver.py b/src/fromager/bootstrap_requirement_resolver.py
index 4f0752e7..1c54c9ce 100644
--- a/src/fromager/bootstrap_requirement_resolver.py
+++ b/src/fromager/bootstrap_requirement_resolver.py
@@ -51,9 +51,11 @@ def __init__(
         self.prev_graph = prev_graph
         # Session-level resolution cache to avoid re-resolving same requirements
         # Key: (requirement_string, pre_built) to distinguish source vs prebuilt
-        # Value: list of (url, version) tuples sorted by version (highest first)
+        # Value: tuple of (url, version) tuples sorted by version (highest first)
+        # Values are stored as immutable tuples to prevent accidental corruption
+        # when callers modify the returned reference.
         self._resolved_requirements: dict[
-            tuple[str, bool], list[tuple[str, Version]]
+            tuple[str, bool], tuple[tuple[str, Version], ...]
         ] = {}
 
     def resolve(
@@ -106,7 +108,7 @@ def resolve(
         cached_result = self.get_cached_resolution(req, pre_built)
         if cached_result is not None:
             logger.debug(f"resolved {req} from cache")
-            return cached_result if return_all_versions else [cached_result[0]]
+            return list(cached_result) if return_all_versions else [cached_result[0]]
 
         # Resolve using strategies
         results = self._resolve(req, req_type, parent_req, pre_built)
@@ -182,15 +184,17 @@ def get_cached_resolution(
         self,
         req: Requirement,
         pre_built: bool,
-    ) -> list[tuple[str, Version]] | None:
+    ) -> tuple[tuple[str, Version], ...] | None:
         """Get a cached resolution result if it exists.
 
+        Returns an immutable tuple to prevent accidental cache corruption.
+
         Args:
             req: Package requirement to look up in cache
             pre_built: Whether looking for prebuilt or source resolution
 
         Returns:
-            List of (url, version) tuples if cached, None otherwise
+            Tuple of (url, version) tuples if cached, None otherwise
         """
         cache_key = (str(req), pre_built)
         return self._resolved_requirements.get(cache_key)
@@ -203,6 +207,9 @@ def cache_resolution(
     ) -> None:
         """Cache a resolution result.
 
+        The result is stored as an immutable tuple to prevent accidental
+        corruption when callers modify the original list.
+
         Used by Bootstrapper to cache git URL resolutions that are
         handled externally (outside this resolver).
 
@@ -212,7 +219,7 @@ def cache_resolution(
             result: List of (url, version) tuples
         """
         cache_key = (str(req), pre_built)
-        self._resolved_requirements[cache_key] = result
+        self._resolved_requirements[cache_key] = tuple(result)
 
     def _resolve_from_graph(
         self,
diff --git a/src/fromager/bootstrapper.py b/src/fromager/bootstrapper.py
index 2f1096fb..a2eb9a3e 100644
--- a/src/fromager/bootstrapper.py
+++ b/src/fromager/bootstrapper.py
@@ -224,7 +224,9 @@ def resolve_versions(
             cached_result = self._resolver.get_cached_resolution(req, pre_built=False)
             if cached_result is not None:
                 logger.debug(f"resolved {req} from cache")
-                return cached_result if return_all_versions else [cached_result[0]]
+                return (
+                    list(cached_result) if return_all_versions else [cached_result[0]]
+                )
 
             logger.info("resolving source via URL, ignoring any plugins")
             source_url, resolved_version = self._resolve_version_from_git_url(req=req)
diff --git a/tests/test_bootstrap_requirement_resolver.py b/tests/test_bootstrap_requirement_resolver.py
index 1f7b04d2..710dca71 100644
--- a/tests/test_bootstrap_requirement_resolver.py
+++ b/tests/test_bootstrap_requirement_resolver.py
@@ -562,6 +562,86 @@ def test_resolve_auto_routes_to_source(
         assert version == Version("2.0")
 
 
+def test_cache_resolution_stores_immutable_tuple(tmp_context: WorkContext) -> None:
+    """cache_resolution() stores an immutable tuple, not the original list."""
+    resolver = BootstrapRequirementResolver(tmp_context)
+    req = Requirement("mypkg>=1.0")
+    original = [("https://example.com/mypkg-1.0.tar.gz", Version("1.0"))]
+
+    resolver.cache_resolution(req, pre_built=False, result=original)
+    cached = resolver.get_cached_resolution(req, pre_built=False)
+
+    # Cached value should be a tuple
+    assert isinstance(cached, tuple)
+
+    # Mutating the original list must not affect the cache
+    original.append(("https://example.com/mypkg-2.0.tar.gz", Version("2.0")))
+    cached_after = resolver.get_cached_resolution(req, pre_built=False)
+    assert cached_after is not None
+    assert len(cached_after) == 1
+
+
+def test_get_cached_resolution_returns_immutable(tmp_context: WorkContext) -> None:
+    """get_cached_resolution() returns a tuple that cannot be mutated."""
+    resolver = BootstrapRequirementResolver(tmp_context)
+    req = Requirement("mypkg>=1.0")
+
+    resolver.cache_resolution(
+        req,
+        pre_built=False,
+        result=[("https://example.com/mypkg-1.0.tar.gz", Version("1.0"))],
+    )
+    cached = resolver.get_cached_resolution(req, pre_built=False)
+    assert cached is not None
+
+    with pytest.raises(AttributeError):
+        cached.append(("https://example.com/bad.tar.gz", Version("2.0")))  # type: ignore[attr-defined, union-attr]
+
+    with pytest.raises(TypeError):
+        cached[0] = ("https://example.com/bad.tar.gz", Version("2.0"))  # type: ignore[index]
+
+
+@patch("fromager.resolver.find_all_matching_from_provider")
+def test_resolve_cache_returns_independent_lists(
+    mock_resolve: MagicMock,
+    tmp_context: WorkContext,
+) -> None:
+    """resolve() returns independent list copies from the cache, not shared references."""
+    req = Requirement("mypkg>=1.0")
+    mock_resolve.return_value = [
+        ("https://example.com/mypkg-2.0.tar.gz", Version("2.0")),
+        ("https://example.com/mypkg-1.5.tar.gz", Version("1.5")),
+    ]
+
+    resolver = BootstrapRequirementResolver(tmp_context)
+
+    # First call populates cache
+    results1 = resolver.resolve(
+        req=req,
+        req_type=RequirementType.INSTALL,
+        parent_req=None,
+        pre_built=False,
+        return_all_versions=True,
+    )
+
+    # Mutate the returned list
+    results1.append(("https://example.com/injected.tar.gz", Version("9.9")))
+
+    # Second call should return clean cached data, unaffected by mutation
+    results2 = resolver.resolve(
+        req=req,
+        req_type=RequirementType.INSTALL,
+        parent_req=None,
+        pre_built=False,
+        return_all_versions=True,
+    )
+
+    assert len(results2) == 2
+    assert results1 is not results2
+    # Only called once — second call used cache
+    mock_resolve.assert_called_once()
+
+
 @patch("fromager.resolver.find_all_matching_from_provider")
 def test_resolve_prebuilt_after_source_uses_separate_cache(
     mock_resolve: MagicMock,