diff --git a/src/gate/schema-change.test.ts b/src/gate/schema-change.test.ts
new file mode 100644
index 0000000..0b31abe
--- /dev/null
+++ b/src/gate/schema-change.test.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { gateSchemaChange } from "./schema-change.ts";
+
+describe("gateSchemaChange", () => {
+  it("blocks a schema change by default", () => {
+    expect(gateSchemaChange({ changed: true })).toEqual({
+      conclusion: "failure",
+      message: expect.stringContaining("changes the database schema"),
+    });
+  });
+
+  it("softens to a non-blocking warning under a warn policy", () => {
+    expect(
+      gateSchemaChange({ changed: true }, { "schema-drift": "warn" })?.conclusion,
+    ).toBe("neutral");
+  });
+
+  it("drops the gate under an off policy", () => {
+    expect(
+      gateSchemaChange({ changed: true }, { "schema-drift": "off" }),
+    ).toBeNull();
+  });
+
+  it("passes when the run reports no schema change", () => {
+    expect(gateSchemaChange({ changed: false })).toBeNull();
+  });
+
+  it("passes when the API returned no schema-change signal", () => {
+    expect(gateSchemaChange(null)).toBeNull();
+    expect(gateSchemaChange(undefined)).toBeNull();
+  });
+});
diff --git a/src/gate/schema-change.ts b/src/gate/schema-change.ts
new file mode 100644
index 0000000..8d6de8f
--- /dev/null
+++ b/src/gate/schema-change.ts
@@ -0,0 +1,43 @@
+// The schema-change gate (Site#3289). The API already computes the real schema
+// diff between the PR and the baseline (`runMetadata.schemaChange`); this gate
+// only decides the check outcome from it. It blocks by default so a person
+// validates the migration before merge — the value is a human eyeball on every
+// schema change, independent of any cost delta. A repo softens it to `warn` or
+// `off` via the schema-drift policy (#3500). Detection is not this gate's job.
+
+import type { RepoPolicyConfig } from "@query-doctor/core";
+import { resolveVerdict } from "./policy.ts";
+
+/** The schema diff the API returns on a run (`runMetadata.schemaChange`). */
+export interface SchemaChangeSignal {
+  changed: boolean;
+}
+
+export interface SchemaGateResult {
+  /** `failure` blocks the check; `neutral` is a surfaced, non-blocking warning. */
+  conclusion: "failure" | "neutral";
+  message: string;
+}
+
+const MESSAGE =
+  "This PR changes the database schema — validate the migration before merge. " +
+  "To stop schema changes from blocking this repo, set the schema-drift check " +
+  "to warn or off in CI settings.";
+
+/**
+ * Decide the schema-change gate from the API's schema diff and the repo policy.
+ * Returns the check outcome when the PR changes the schema and the policy
+ * surfaces it, or `null` when there is no change or the policy is `off`.
+ */
+export function gateSchemaChange(
+  schemaChange: SchemaChangeSignal | null | undefined,
+  config: RepoPolicyConfig = {},
+): SchemaGateResult | null {
+  if (!schemaChange?.changed) return null;
+  const { conclusion, surfaced } = resolveVerdict(
+    { condition: "schema-drift", verdictClass: "finding" },
+    config,
+  );
+  if (!surfaced || conclusion === "success") return null;
+  return { conclusion, message: MESSAGE };
+}
diff --git a/src/main.ts b/src/main.ts
index 521d649..1a94264 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -18,6 +18,7 @@ import { formatCost, queryPreview } from "./reporters/github/github.ts";
 import { fetchPrChangedFiles } from "./gate/changed-files.ts";
 import { evaluateTestPresence } from "./gate/test-presence.ts";
 import { gateRegression } from "./gate/regression.ts";
+import { gateSchemaChange } from "./gate/schema-change.ts";
 import { resolveVerdict } from "./gate/policy.ts";
 import { DEFAULT_CONFIG } from "./config.ts";
 import { ApiClient } from "./remote/api-client.ts";
@@ -276,6 +277,22 @@ async function runInCI(
       }
     }
 
+    // Schema-change gate (Site#3289). The API already computed the schema diff
+    // (`runMetadata.schemaChange`) and the comment renders it; this only decides
+    // the check. Block by default so a person validates the migration; a repo
+    // softens it to `warn`/`off` via the schema-drift policy.
+    const schemaGate = gateSchemaChange(
+      reportContext.runMetadata?.schemaChange,
+      "conditionPolicies" in config ? config.conditionPolicies : undefined,
+    );
+    if (schemaGate) {
+      if (schemaGate.conclusion === "failure") {
+        core.setFailed(schemaGate.message);
+      } else {
+        core.warning(schemaGate.message);
+      }
+    }
+
     // Block PR if regressions exceed thresholds, or if a brand-new query ships
     // with a high-confidence index recommendation (#3281). New queries have no
     // baseline so they can never regress; the new-query gate catches the missing