diff --git a/cmd/sin-code/internal/spec/check.go b/cmd/sin-code/internal/spec/check.go
index 61e2d412..143cda64 100644
--- a/cmd/sin-code/internal/spec/check.go
+++ b/cmd/sin-code/internal/spec/check.go
@@ -17,6 +17,33 @@ import (
 // DefaultCheckTimeout is the per-criterion timeout if not configured.
 const DefaultCheckTimeout = 60 * time.Second
 
+// Policy controls how spec-drift failures affect exit codes (issue #157).
+//   - PolicyOff:   never block, exit 0 even with must-failures
+//   - PolicyWarn:  print warnings, exit 0 (advisory mode)
+//   - PolicyError: block on must-failures, exit 1 (CI gate mode)
+type Policy string
+
+const (
+	PolicyOff   Policy = "off"
+	PolicyWarn  Policy = "warn"
+	PolicyError Policy = "error"
+)
+
+// ParsePolicy normalises a raw policy string. Unknown values
+// default to PolicyError (fail-closed; the verify gate is sacred).
+func ParsePolicy(s string) Policy {
+	switch strings.ToLower(strings.TrimSpace(s)) {
+	case "off":
+		return PolicyOff
+	case "warn", "warning":
+		return PolicyWarn
+	case "error", "strict", "":
+		return PolicyError
+	default:
+		return PolicyError
+	}
+}
+
 // CheckResult is the outcome of running a single criterion's verify
 // command. The ID matches the Criterion.ID from the parsed spec.
 type CheckResult struct {
@@ -54,6 +81,18 @@ func (r *CheckReport) HasFailures() bool {
 	return false
 }
 
+// ShouldBlock returns true if the report should cause the calling
+// CLI to exit non-zero, given the active policy. PolicyOff and
+// PolicyWarn never block; PolicyError blocks on must-failures.
+func (r *CheckReport) ShouldBlock(p Policy) bool {
+	switch p {
+	case PolicyOff, PolicyWarn:
+		return false
+	default:
+		return r.HasFailures()
+	}
+}
+
 // Check runs every criterion's verify: command in s. The per-command
 // timeout is enforced via context.WithTimeout. Output is truncated to
 // 4KB per criterion to keep the report small.
diff --git a/cmd/sin-code/internal/spec/policy_test.go b/cmd/sin-code/internal/spec/policy_test.go
new file mode 100644
index 00000000..33b764a8
--- /dev/null
+++ b/cmd/sin-code/internal/spec/policy_test.go
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: MIT
+// Purpose: tests for issue #157 — spec.drift policy key.
+package spec
+
+import "testing"
+
+func TestParsePolicy(t *testing.T) {
+	cases := map[string]Policy{
+		"off":     PolicyOff,
+		"OFF":     PolicyOff,
+		"  Off ":  PolicyOff,
+		"warn":    PolicyWarn,
+		"warning": PolicyWarn,
+		"error":   PolicyError,
+		"strict":  PolicyError,
+		"":        PolicyError, // empty defaults to error (fail-closed)
+		"bogus":   PolicyError, // unknown defaults to error
+	}
+	for in, want := range cases {
+		if got := ParsePolicy(in); got != want {
+			t.Errorf("ParsePolicy(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestCheckReport_ShouldBlock(t *testing.T) {
+	// Empty report: no block.
+	empty := &CheckReport{}
+	if empty.ShouldBlock(PolicyError) {
+		t.Error("empty report should not block even in error mode")
+	}
+	if empty.ShouldBlock(PolicyOff) {
+		t.Error("empty report should not block in off mode")
+	}
+
+	// Report with a must-failure.
+	failing := &CheckReport{
+		Results: []CheckResult{{ID: "R1", Passed: false, Priority: Must}},
+	}
+	if !failing.ShouldBlock(PolicyError) {
+		t.Error("must-failure should block in error mode")
+	}
+	if failing.ShouldBlock(PolicyWarn) {
+		t.Error("must-failure should NOT block in warn mode")
+	}
+	if failing.ShouldBlock(PolicyOff) {
+		t.Error("must-failure should NOT block in off mode")
+	}
+
+	// Report with only should-priority failures: never blocks.
+	shouldFail := &CheckReport{
+		Results: []CheckResult{{ID: "R1", Passed: false, Priority: Should}},
+	}
+	if shouldFail.ShouldBlock(PolicyError) {
+		t.Error("should-failure should never block")
+	}
+
+	// Skipped failures: never block.
+	skipped := &CheckReport{
+		Results: []CheckResult{{ID: "R1", Passed: false, Skipped: true, Priority: Must}},
+	}
+	if skipped.ShouldBlock(PolicyError) {
+		t.Error("skipped failures should never block")
+	}
+}
diff --git a/cmd/sin-code/spec_cmd.go b/cmd/sin-code/spec_cmd.go
index 7a471fb1..79ab6eb7 100644
--- a/cmd/sin-code/spec_cmd.go
+++ b/cmd/sin-code/spec_cmd.go
@@ -145,6 +145,7 @@ func newSpecCheckCmd() *cobra.Command {
 		timeout time.Duration
 		drift   bool
 		root    string
+		policy  string
 	)
 	c := &cobra.Command{
 		Use:   "check [file.spec.md]",
@@ -163,10 +164,22 @@ source tree under --root (default: current dir).
   sin-code spec check --all --json                      # machine-readable report
   sin-code spec check --all --drift                     # + signature drift
   sin-code spec check --all --drift --root ./cmd/...   # scope the walk
-  sin-code spec check --timeout 30s ...                # override per-criterion timeout`,
+  sin-code spec check --timeout 30s ...                # override per-criterion timeout
+  sin-code spec check --policy off|warn|error          # drift strictness (issue #157)`,
+  // The --policy default reads from SIN_SPEC_DRIFT env var, then falls
+  // back to "error" (CI gate mode; the verify gate is sacred).
 		Args: cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			ctx := context.Background()
+			// Resolve policy: --policy flag > SIN_SPEC_DRIFT env > "error".
+			polRaw := policy
+			if polRaw == "" {
+				polRaw = os.Getenv("SIN_SPEC_DRIFT")
+			}
+			pol := spec.ParsePolicy(polRaw)
+			if !asJSON {
+				fmt.Fprintf(cmd.OutOrStdout(), "policy: %s\n", pol)
+			}
 			paths, err := collectSpecPaths(args, all)
 			if err != nil {
 				return err
@@ -229,8 +242,8 @@ source tree under --root (default: current dir).
 					return err
 				}
 			}
-			if anyFailure {
-				return fmt.Errorf("spec check: at least one must-priority criterion or signature drifted")
+			if anyFailure && pol == spec.PolicyError {
+				return fmt.Errorf("spec check: at least one must-priority criterion or signature drifted (policy=%s)", pol)
 			}
 			return nil
 		},
@@ -240,6 +253,7 @@ source tree under --root (default: current dir).
 	c.Flags().DurationVar(&timeout, "timeout", spec.DefaultCheckTimeout, "per-criterion timeout")
 	c.Flags().BoolVar(&drift, "drift", false, "also run the Spec<->Code signature drift check")
 	c.Flags().StringVar(&root, "root", ".", "root directory for the signature drift walk")
+	c.Flags().StringVar(&policy, "policy", "", "drift strictness: off|warn|error (overrides SIN_SPEC_DRIFT env; default error)")
 	return c
 }
 
diff --git a/scripts/spec-drift-check.sh b/scripts/spec-drift-check.sh
index 671a843c..a2941626 100755
--- a/scripts/spec-drift-check.sh
+++ b/scripts/spec-drift-check.sh
@@ -19,6 +19,13 @@ set -euo pipefail
 # see the right tree.
 cd "$(git rev-parse --show-toplevel)"
 
+# Policy: SIN_SPEC_DRIFT env var overrides the default (error).
+#   off   — never block (developers opt-in)
+#   warn  — print warnings, never block
+#   error — block on must-failures (CI gate mode; default)
+: "${SIN_SPEC_DRIFT:=error}"
+export SIN_SPEC_DRIFT
+
 # Locate the sin-code binary. Prefer the user's PATH; fall back to the
 # locally-built binary at ./sin-code (dev workflow).
 SIN_BIN="${SIN_BIN:-sin-code}"