diff --git a/src/graph/PaymentGraphChecker.ts b/src/graph/PaymentGraphChecker.ts
index 8bcac41..578e51f 100644
--- a/src/graph/PaymentGraphChecker.ts
+++ b/src/graph/PaymentGraphChecker.ts
@@ -59,6 +59,49 @@ export interface PaymentGraphCheckerOptions {
   horizonUrl: string;
 }
 
+/** A single weighted edge in a payment graph. */
+export interface PaymentGraphEdge {
+  /** Source account of the hop. */
+  from: string;
+  /** Destination account of the hop. */
+  to: string;
+  /**
+   * Amount carried by this hop. Zero is allowed and represents a
+   * pass-through hop; a negative value is never valid.
+   */
+  weight: number | bigint;
+}
+
+/** A payment graph expressed as a set of weighted edges. */
+export interface PaymentGraph {
+  edges: PaymentGraphEdge[];
+}
+
+/** A single problem found by {@link PaymentGraphChecker.checkGraph}. */
+export interface NegativeEdgeWeightIssue {
+  code: "NEGATIVE_EDGE_WEIGHT";
+  /** Source account of the offending edge. */
+  from: string;
+  /** Destination account of the offending edge. */
+  to: string;
+  /** The offending weight, as supplied. */
+  weight: number | bigint;
+  /** Human-readable description naming the offending edge. */
+  message: string;
+}
+
+/** Result of validating the structure of a payment graph. */
+export interface GraphValidationResult {
+  /** `true` only when no issues were found. */
+  valid: boolean;
+  /** Every issue found, in edge order. Empty when `valid` is `true`. */
+  issues: NegativeEdgeWeightIssue[];
+}
+
+function isNegativeWeight(weight: number | bigint): boolean {
+  return typeof weight === "bigint" ? weight < 0n : weight < 0;
+}
+
 export class PaymentGraphChecker {
   private cache = new Map<string, CacheEntry>();
   private cacheTTL: number;
@@ -69,6 +112,42 @@ export class PaymentGraphChecker {
     this.horizonUrl = options.horizonUrl;
   }
 
+  /**
+   * Validate the structure of a weighted payment graph.
+   *
+   * A negative edge weight is always a defect: traversing the graph greedily
+   * treats a negative hop as if it adds funds, which allows a route to be
+   * repeated for unbounded extraction. Zero-weight edges are legitimate and
+   * represent pass-through hops, so only strictly negative weights fail.
+   *
+   * @param graph - The graph to validate, either as `{ edges }` or as a bare
+   *                array of edges.
+   * @returns A result listing every offending edge; `valid` is `true` when
+   *          no edge has a negative weight.
+   */
+  checkGraph(graph: PaymentGraph | PaymentGraphEdge[]): GraphValidationResult {
+    const edges = Array.isArray(graph) ? graph : (graph?.edges ?? []);
+    const issues: NegativeEdgeWeightIssue[] = [];
+
+    for (const edge of edges) {
+      if (!edge || !isNegativeWeight(edge.weight)) {
+        continue;
+      }
+
+      issues.push({
+        code: "NEGATIVE_EDGE_WEIGHT",
+        from: edge.from,
+        to: edge.to,
+        weight: edge.weight,
+        message:
+          `Negative edge weight ${String(edge.weight)} on edge ` +
+          `${edge.from} -> ${edge.to}`,
+      });
+    }
+
+    return { valid: issues.length === 0, issues };
+  }
+
   /**
    * Check if valid payment paths exist for all recipients in an invoice.
    */
diff --git a/test/paymentGraphChecker.test.ts b/test/paymentGraphChecker.test.ts
index dd8d64c..2e1a212 100644
--- a/test/paymentGraphChecker.test.ts
+++ b/test/paymentGraphChecker.test.ts
@@ -162,4 +162,89 @@ describe("PaymentGraphChecker", () => {
       expect(fetchMock).toHaveBeenCalled();
     });
   });
+
+  describe("checkGraph() — negative edge weights", () => {
+    const edge = (from: string, to: string, weight: number | bigint) => ({
+      from,
+      to,
+      weight,
+    });
+
+    it("accepts a graph whose edges all have positive weights", () => {
+      const result = checker.checkGraph({
+        edges: [edge(sourceAccount, recipientA, 100), edge(recipientA, recipientB, 50)],
+      });
+
+      expect(result.valid).toBe(true);
+      expect(result.issues).toEqual([]);
+    });
+
+    it("allows zero-weight edges, which represent pass-through hops", () => {
+      const result = checker.checkGraph({
+        edges: [edge(sourceAccount, recipientA, 0), edge(recipientA, recipientB, 0n)],
+      });
+
+      expect(result.valid).toBe(true);
+      expect(result.issues).toEqual([]);
+    });
+
+    it("accepts an empty graph", () => {
+      expect(checker.checkGraph({ edges: [] }).valid).toBe(true);
+      expect(checker.checkGraph([]).valid).toBe(true);
+    });
+
+    it("fails a graph containing a negative-weight edge", () => {
+      const result = checker.checkGraph({
+        edges: [edge(sourceAccount, recipientA, 100), edge(recipientA, recipientB, -5)],
+      });
+
+      expect(result.valid).toBe(false);
+      expect(result.issues).toHaveLength(1);
+      expect(result.issues[0]?.code).toBe("NEGATIVE_EDGE_WEIGHT");
+    });
+
+    it("names the offending edge, source to target, in the message", () => {
+      const result = checker.checkGraph({
+        edges: [edge(recipientA, recipientB, -5)],
+      });
+
+      const message = result.issues[0]?.message ?? "";
+      expect(message).toContain(recipientA);
+      expect(message).toContain(recipientB);
+      expect(message).toContain("-5");
+      expect(result.issues[0]?.from).toBe(recipientA);
+      expect(result.issues[0]?.to).toBe(recipientB);
+    });
+
+    it("detects negative bigint weights", () => {
+      const result = checker.checkGraph({
+        edges: [edge(sourceAccount, recipientA, -1n)],
+      });
+
+      expect(result.valid).toBe(false);
+      expect(result.issues[0]?.weight).toBe(-1n);
+    });
+
+    it("reports every offending edge, in edge order", () => {
+      const result = checker.checkGraph({
+        edges: [
+          edge(sourceAccount, recipientA, -1),
+          edge(recipientA, recipientB, 10),
+          edge(recipientB, sourceAccount, -2n),
+        ],
+      });
+
+      expect(result.valid).toBe(false);
+      expect(result.issues).toHaveLength(2);
+      expect(result.issues[0]?.to).toBe(recipientA);
+      expect(result.issues[1]?.to).toBe(sourceAccount);
+    });
+
+    it("accepts a bare array of edges as well as { edges }", () => {
+      const edges = [edge(sourceAccount, recipientA, -3)];
+
+      expect(checker.checkGraph(edges)).toEqual(checker.checkGraph({ edges }));
+      expect(checker.checkGraph(edges).valid).toBe(false);
+    });
+  });
 });