diff --git a/e2e/test_bootstrap_prerelease.sh b/e2e/test_bootstrap_prerelease.sh
index 46682ed5..eb180d11 100755
--- a/e2e/test_bootstrap_prerelease.sh
+++ b/e2e/test_bootstrap_prerelease.sh
@@ -27,8 +27,11 @@ fi
 
 $pass
 
-CONSTRAINTS_FILE="$OUTDIR/flit_core_constraints.txt"
-echo "flit_core==2.0rc3" > $CONSTRAINTS_FILE
+CONSTRAINTS_FILE_1="$OUTDIR/flit_core_constraints.txt"
+echo "flit_core==2.0rc3" > $CONSTRAINTS_FILE_1
+
+CONSTRAINTS_FILE_2="$OUTDIR/misc_constraints.txt"
+echo "setuptools>=75.8.0" > $CONSTRAINTS_FILE_2
 
 DEBUG_RESOLVER=true fromager \
   --verbose \
@@ -36,7 +39,8 @@ DEBUG_RESOLVER=true fromager \
   --sdists-repo="$OUTDIR/sdists-repo" \
   --wheels-repo="$OUTDIR/wheels-repo" \
   --work-dir="$OUTDIR/work-dir" \
-  --constraints-file="$CONSTRAINTS_FILE" \
+  --constraints-file="$CONSTRAINTS_FILE_1" \
+  --constraints-file="$CONSTRAINTS_FILE_2" \
   bootstrap $DIST || true
 
 
@@ -46,4 +50,4 @@ if ! grep -q "flit_core: new toplevel dependency flit_core<2.0.1 resolves to 2.0
   pass=false
 fi
 
-$pass
\ No newline at end of file
+$pass
diff --git a/src/fromager/__main__.py b/src/fromager/__main__.py
index 8666e987..2468e417 100644
--- a/src/fromager/__main__.py
+++ b/src/fromager/__main__.py
@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import logging
+import os
 import pathlib
 import sys
 
@@ -107,8 +108,20 @@
 @click.option(
     "-c",
     "--constraints-file",
+    # for click.Path, click splits env vars on os.pathsep (':')
+    type=clickext.ClickPath(exists=True, file_okay=True, dir_okay=False),
+    multiple=True,
+    default=[],
+    help="location of constraints files (multiple use)",
+)
+@click.option(
+    "--constraints-url",
+    # for str, click splits env vars on space
     type=str,
-    help="location of the constraints file",
+    multiple=True,
+    default=[],
+    callback=clickext.verify_url_callback,
+    help="remote location of constraints file as URL (multiple use)",
 )
 @click.option(
     "--cleanup/--no-cleanup",
@@ -142,7 +155,8 @@ def main(
     patches_dir: pathlib.Path,
     settings_file: pathlib.Path,
     settings_dir: pathlib.Path,
-    constraints_file: str,
+    constraints_file: tuple[pathlib.Path],
+    constraints_url: tuple[str],
     cleanup: bool,
     variant: str,
     jobs: int | None,
@@ -195,13 +209,19 @@ def main(
             logger.info(f"variant: {variant}")
             logger.info(f"patches dir: {patches_dir}")
             logger.info(f"maximum concurrent jobs: {jobs}")
-            logger.info(f"constraints file: {constraints_file}")
+            logger.info(
+                f"constraints files: {os.pathsep.join(str(p) for p in constraints_file)}"
+            )
+            logger.info(f"constraints URLs: {' '.join(constraints_url)}")
             logger.info(f"network isolation: {network_isolation}")
             overrides.log_overrides()
 
     if network_isolation and not SUPPORTS_NETWORK_ISOLATION:
         ctx.fail(f"network isolation is not available: {NETWORK_ISOLATION_ERROR}")
 
+    constraints: list[str] = list(constraints_url)
+    constraints.extend(os.fspath(p) for p in constraints_file)
+
     wkctx = context.WorkContext(
         active_settings=packagesettings.Settings.from_files(
             settings_file=settings_file,
@@ -210,7 +230,7 @@ def main(
             variant=variant,
             max_jobs=jobs,
         ),
-        constraints_file=constraints_file,
+        constraints_files=constraints,
         patches_dir=patches_dir,
         sdists_repo=sdists_repo,
         wheels_repo=wheels_repo,
diff --git a/src/fromager/clickext.py b/src/fromager/clickext.py
index 76938ccd..a00a9ed7 100644
--- a/src/fromager/clickext.py
+++ b/src/fromager/clickext.py
@@ -7,6 +7,19 @@
 from . import requirements_file
 
 
+def verify_url_callback(
+    ctx: click.Context, param: click.Parameter, value: tuple[str]
+) -> tuple[str]:
+    for item in value:
+        if not item.startswith(("https://", "http://", "file://")):
+            raise click.BadParameter(
+                f"value must be a http, https, or file URL, got {item}",
+                ctx=ctx,
+                param=param,
+            )
+    return value
+
+
 class ClickPath(click.Path):
     """ClickPath that returns pathlib.Path"""
 
diff --git a/src/fromager/constraints.py b/src/fromager/constraints.py
index 81e616e2..91523c3b 100644
--- a/src/fromager/constraints.py
+++ b/src/fromager/constraints.py
@@ -20,6 +20,9 @@ def __init__(self) -> None:
     def __iter__(self) -> typing.Iterable[NormalizedName]:
         yield from self._data
 
+    def __bool__(self) -> bool:
+        return bool(self._data)
+
     def add_constraint(self, unparsed: str) -> None:
         """Add new constraint, must not conflict with any existing constraints"""
         req = Requirement(unparsed)
@@ -34,12 +37,18 @@ def add_constraint(self, unparsed: str) -> None:
             self._data[canon_name] = req
 
     def load_constraints_file(self, constraints_file: str | pathlib.Path) -> None:
-        """Load constraints from a constraints file"""
+        """Load constraints from a constraints file or URI"""
         logger.info("loading constraints from %s", constraints_file)
         content = requirements_file.parse_requirements_file(constraints_file)
         for line in content:
             self.add_constraint(line)
 
+    def dump_constraints_file(self, constraints_file: pathlib.Path) -> None:
+        """Dump all constraints into a file"""
+        with constraints_file.open("w", encoding="utf-8") as f:
+            for req in self._data.values():
+                f.write(f"{req}\n")
+
     def get_constraint(self, name: str) -> Requirement | None:
         return self._data.get(canonicalize_name(name))
 
diff --git a/src/fromager/context.py b/src/fromager/context.py
index ddc6f166..b7a37db8 100644
--- a/src/fromager/context.py
+++ b/src/fromager/context.py
@@ -8,7 +8,7 @@
 from packaging.utils import NormalizedName, canonicalize_name
 from packaging.version import Version
 
-from . import constraints, dependency_graph, packagesettings, request_session
+from . import constraints, dependency_graph, packagesettings
 
 logger = logging.getLogger(__name__)
 
@@ -21,7 +21,7 @@ class WorkContext:
     def __init__(
         self,
         active_settings: packagesettings.Settings | None,
-        constraints_file: str | None,
+        constraints_files: list[str],
         patches_dir: pathlib.Path,
         sdists_repo: pathlib.Path,
         wheels_repo: pathlib.Path,
@@ -41,13 +41,6 @@ def __init__(
                 max_jobs=max_jobs,
             )
         self.settings = active_settings
-        self.input_constraints_uri: str | None
-        self.constraints = constraints.Constraints()
-        if constraints_file is not None:
-            self.input_constraints_uri = constraints_file
-            self.constraints.load_constraints_file(constraints_file)
-        else:
-            self.input_constraints_uri = None
         self.sdists_repo = pathlib.Path(sdists_repo).absolute()
         self.sdists_downloads = self.sdists_repo / "downloads"
         self.sdists_builds = self.sdists_repo / "builds"
@@ -64,10 +57,16 @@ def __init__(
         self.network_isolation = network_isolation
         self.settings_dir = settings_dir
 
-        self._constraints_filename = self.work_dir / "constraints.txt"
-
         self.dependency_graph = dependency_graph.DependencyGraph()
 
+        self.constraints = constraints.Constraints()
+        self._constraints_filename: pathlib.Path | None = None
+        if constraints_files:
+            # local or remote (HTTPS) files
+            for cf in constraints_files:
+                self.constraints.load_constraints_file(cf)
+            self._constraints_filename = self.work_dir / "combined-constraints.txt"
+
         # storing metrics
         self.time_store: dict[str, dict[str, float]] = collections.defaultdict(
             dict[str, float]
@@ -84,19 +83,10 @@ def pip_wheel_server_args(self) -> list[str]:
 
     @property
     def pip_constraint_args(self) -> list[str]:
-        if not self.input_constraints_uri:
+        if self._constraints_filename is None:
             return []
-
-        if self.input_constraints_uri.startswith(("https://", "http://", "file://")):
-            path_to_constraints_file = self.work_dir / "input-constraints.txt"
-            if not path_to_constraints_file.exists():
-                response = request_session.session.get(self.input_constraints_uri)
-                path_to_constraints_file.write_text(response.text)
-        else:
-            path_to_constraints_file = pathlib.Path(self.input_constraints_uri)
-
-        path_to_constraints_file = path_to_constraints_file.absolute()
-        return ["--constraint", os.fspath(path_to_constraints_file)]
+        self.constraints.dump_constraints_file(self._constraints_filename)
+        return ["--constraint", os.fspath(self._constraints_filename)]
 
     def write_to_graph_to_file(self):
         with open(self.work_dir / "graph.json", "w") as f:
diff --git a/tests/conftest.py b/tests/conftest.py
index 933612f0..8809b044 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -20,7 +20,7 @@ def tmp_context(tmp_path: pathlib.Path) -> context.WorkContext:
     variant = "cpu"
     ctx = context.WorkContext(
         active_settings=None,
-        constraints_file=None,
+        constraints_files=[],
         patches_dir=patches_dir,
         sdists_repo=tmp_path / "sdists-repo",
         wheels_repo=tmp_path / "wheels-repo",
@@ -46,7 +46,7 @@ def testdata_context(
             variant=variant,
             max_jobs=None,
         ),
-        constraints_file=None,
+        constraints_files=[],
         patches_dir=overrides / "patches",
         sdists_repo=tmp_path / "sdists-repo",
         wheels_repo=tmp_path / "wheels-repo",
diff --git a/tests/test_constraints.py b/tests/test_constraints.py
index 6e0c96e0..f601a192 100644
--- a/tests/test_constraints.py
+++ b/tests/test_constraints.py
@@ -64,6 +64,15 @@ def test_load_non_existant_constraints_file(tmp_path: pathlib.Path):
         c.load_constraints_file(non_existant_file)
 
 
+def test_magic_methods():
+    c = constraints.Constraints()
+    assert not c
+    assert list(c) == []
+    c.add_constraint("flit_core==2.0rc3")
+    assert c
+    assert list(c) == ["flit-core"]
+
+
 def test_load_constraints_file(tmp_path: pathlib.Path):
     constraint_file = tmp_path / "constraint.txt"
     constraint_file.write_text("egg\ntorch==3.1.0 # comment\n")
@@ -71,3 +80,13 @@ def test_load_constraints_file(tmp_path: pathlib.Path):
     c.load_constraints_file(constraint_file)
     assert list(c) == ["egg", "torch"]  # type: ignore
     assert c.get_constraint("torch") == Requirement("torch==3.1.0")
+
+
+def test_dump_constraints_file(tmp_path: pathlib.Path):
+    c = constraints.Constraints()
+    c.add_constraint("foo<=1.1")
+    c.add_constraint("bar>=2.0")
+
+    constraint_file = tmp_path / "constraint.txt"
+    c.dump_constraints_file(constraint_file)
+    assert constraint_file.read_text().split("\n") == ["foo<=1.1", "bar>=2.0", ""]
diff --git a/tests/test_context.py b/tests/test_context.py
index 6b9f1cdb..3874209c 100644
--- a/tests/test_context.py
+++ b/tests/test_context.py
@@ -8,18 +8,21 @@ def test_pip_constraints_args(tmp_path):
     constraints_file.write_text("\n")  # the file has to exist
     ctx = context.WorkContext(
         active_settings=None,
-        constraints_file=str(constraints_file),
+        constraints_files=[constraints_file],
         patches_dir=tmp_path / "overrides/patches",
         sdists_repo=tmp_path / "sdists-repo",
         wheels_repo=tmp_path / "wheels-repo",
         work_dir=tmp_path / "work-dir",
     )
     ctx.setup()
-    assert ["--constraint", os.fspath(constraints_file)] == ctx.pip_constraint_args
+    assert ctx.pip_constraint_args == [
+        "--constraint",
+        os.fspath(ctx.work_dir / "combined-constraints.txt"),
+    ]
 
     ctx = context.WorkContext(
         active_settings=None,
-        constraints_file=None,
+        constraints_files=[],
         patches_dir=tmp_path / "overrides/patches",
         sdists_repo=tmp_path / "sdists-repo",
         wheels_repo=tmp_path / "wheels-repo",