diff --git a/private/orfs_design.bzl b/private/orfs_design.bzl
index 3a0f3c2b..cb607fcd 100644
--- a/private/orfs_design.bzl
+++ b/private/orfs_design.bzl
@@ -44,7 +44,7 @@ def _convert_sources(sources, pkg):
             result[var] = converted
     return result
 
-def orfs_design(name = None, platform = None, design = None, designs = None, mock_openroad = None, mock_yosys = None):  # buildifier: disable=unused-variable
+def orfs_design(name = None, platform = None, design = None, designs = None, mock_openroad = None, mock_yosys = None, user_arguments = []):  # buildifier: disable=unused-variable
     """Create orfs_flow() targets for a design based on its parsed config.mk.
 
     Call this from a design's BUILD.bazel:
@@ -66,6 +66,12 @@ def orfs_design(name = None, platform = None, design = None, designs = None, moc
         mock_yosys: Label for mock-yosys binary. When set, lint flow uses
             this instead of real Yosys for synthesis.
             Example: "//mock/yosys/src/bin:yosys".
+        user_arguments: List of variable names from config.mk that are
+            project-specific (read by user-supplied .tcl/.mk, not by
+            ORFS itself, e.g. FP_PDN_RAIL_OFFSET in a custom pdn.cfg).
+            Routed through orfs_flow(user_arguments=...) to bypass the
+            variables.yaml validator instead of being checked as known
+            ORFS arguments.
     """
     if designs == None:
         fail("orfs_design() requires designs argument: pass DESIGNS from @orfs_designs//:designs.bzl")
@@ -158,6 +164,13 @@ def orfs_design(name = None, platform = None, design = None, designs = None, moc
     # Real flow — uses Docker image with real OpenROAD/Yosys
     arguments = dict(config["arguments"])
 
+    # Move caller-flagged design-specific knobs out of arguments and into
+    # user_arguments so they bypass the variables.yaml validator.
+    user_args = {}
+    for var in user_arguments:
+        if var in arguments:
+            user_args[var] = arguments.pop(var)
+
     # Default SYNTH_NUM_PARTITIONS to a static value so that the action graph
     # is identical across machines and remote cache hits are possible.  Users
     # who prefer local parallelism over caching can pass NUM_CPUS explicitly.
@@ -174,6 +187,7 @@ def orfs_design(name = None, platform = None, design = None, designs = None, moc
         verilog_files = verilog_files,
         pdk = "//flow:" + platform,
         arguments = arguments,
+        user_arguments = user_args,
         sources = sources,
         macros = macros if macros else [],
         stage_data = {"synth": extra_data} if extra_data else {},
diff --git a/private/stages.bzl b/private/stages.bzl
index 8c1391a8..28e8afa7 100644
--- a/private/stages.bzl
+++ b/private/stages.bzl
@@ -6,9 +6,31 @@ load("//private:utils.bzl", "flatten", "set", "union")
 # A stage argument is used in one or more stages. This is metainformation
 # about the ORFS code that there is no known nice way for ORFS to
 # provide.
-BAZEL_VARIABLE_TO_STAGES = {}
+#
+# Variables auto-injected by bazel-orfs (not present in ORFS variables.yaml)
+# are registered here so that check_variables() does not flag them as
+# unknown when downstream callers see them in arguments.
+BAZEL_VARIABLE_TO_STAGES = {
+    # Set in orfs_design.bzl when SYNTH_HIERARCHICAL=1.
+    "SYNTH_NUM_PARTITIONS": ["synth"],
+}
 
-BAZEL_STAGE_TO_VARIABLES = {}
+BAZEL_STAGE_TO_VARIABLES = {
+    stage: [v for v, stages in BAZEL_VARIABLE_TO_STAGES.items() if stage in stages]
+    for stage in [
+        "synth",
+        "floorplan",
+        "place",
+        "cts",
+        "grt",
+        "route",
+        "final",
+        "generate_abstract",
+        "generate_metadata",
+        "test",
+        "update_rules",
+    ]
+}
 
 ALL_STAGES = [
     "synth",
@@ -135,7 +157,8 @@ ORFS_STAGE_TO_VARIABLES = {
 }
 
 ALL_STAGE_TO_VARIABLES = {
-    stage: ORFS_STAGE_TO_VARIABLES.get(stage, [])
+    stage: ORFS_STAGE_TO_VARIABLES.get(stage, []) +
+           BAZEL_STAGE_TO_VARIABLES.get(stage, [])
     for stage in ALL_STAGES
 }
 
diff --git a/synth_partition.sh b/synth_partition.sh
index bf3deb51..f37ca48f 100755
--- a/synth_partition.sh
+++ b/synth_partition.sh
@@ -17,13 +17,18 @@ NUM_PARTITIONS=${SYNTH_NUM_PARTITIONS:?}
 KEPT_JSON="$RESULTS_DIR/kept_modules.json"
 OUTPUT="$RESULTS_DIR/partition_${PARTITION_ID}.v"
 
-# Parse module list from JSON using sed (avoids python wrapper issues)
-# JSON format: {"modules": ["mod1", "mod2", ...]}
-ALL_MODULES=$(sed 's/.*\[//;s/\].*//;s/"//g;s/,/\n/g;s/ //g' "$KEPT_JSON")
+# Parse module list from JSON. Module names can contain '[' and ']'
+# (slang elaborates parameterized instances to names like
+# 'foo$bar.gen_tiles[0].i_tile.gen_banks[3]'), so a greedy sed regex is
+# unsafe. Module names cannot contain '"', so extracting all quoted
+# strings and skipping the first ("modules" key) is correct.
+ALL_MODULES=$(grep -oE '"[^"]+"' "$KEPT_JSON" | tail -n +2 | sed 's/"//g')
 
-# When SYNTH_SKIP_KEEP is set, the keep-hierarchy discovery was skipped.
+# When SYNTH_SKIP_KEEP=1, the keep-hierarchy discovery was skipped.
 # Partitions read from canonical RTLIL and run full synthesis (coarse+fine).
-if [ -n "${SYNTH_SKIP_KEEP:-}" ]; then
+# ORFS variables.yaml defaults SYNTH_SKIP_KEEP to "0", so test for "1"
+# explicitly rather than -n (which is true for any non-empty string).
+if [ "${SYNTH_SKIP_KEEP:-0}" = "1" ]; then
   CHECKPOINT="$RESULTS_DIR/1_1_yosys_canonicalize.rtlil"
   # Validate that every SYNTH_KEEP_MODULES entry exists in the canonical RTLIL
   RTLIL_MODULES_FILE=$(mktemp)