diff --git a/README.md b/README.md
index 2d4aaed..9fff388 100644
--- a/README.md
+++ b/README.md
@@ -43,14 +43,9 @@ se-trace validate-runtime examples/legal-summarization.cots.json
 # Walk the evidence chains and surface recursive (model-generated) items (RFC-0002)
 se-trace recursive examples/legal-summarization.cots.json
 
-# Render the Context Graph (Mermaid + HTML, RFC-0004)
-se-trace context-graph examples/legal-summarization.cots.json -o ./out
-
-# Replay the audit trail (RFC-0005)
-se-trace audit examples/legal-summarization.cots.json
-
-# COTS Explorer: one self-contained HTML page with all four views (Advancement Plan Phase 5)
-se-trace explore examples/legal-summarization.cots.json -o ./out
+# Print each decision's Layer 3 GuardrailDecision; re-evaluate against a rule file (RFC-0003)
+se-trace check-guardrails examples/legal-summarization.cots.json
+se-trace check-guardrails examples/legal-summarization.cots.json --rules rules.gr
 ```
 
 ## What is in this repository
@@ -58,10 +53,8 @@ se-trace explore examples/legal-summarization.cots.json -o ./out
 - `src/inspect.ts` - the inspect command. Prints a human-readable summary of a COTS.
 - `src/cite.ts` - the cite command. Prints the evidence chain for a specific decision.
 - `src/visualize.ts` - the visualize command. Renders a COTS as a Mermaid diagram + HTML page.
-- `src/context-graph.ts` - the context-graph command. Renders the Context + Evidence parts as a typed-edge graph (RFC-0004).
-- `src/audit.ts` - the audit command. Replays the audit trail and flags append-only/linkage violations (RFC-0005).
-- `src/explore.ts` - the COTS Explorer. One self-contained HTML page combining Reasoning Roadmap, Context Graph, summary, and audit trail.
-- `src/cli/se-trace.ts` - the CLI entry point, including `validate-runtime` (RFC-0001) and `recursive` (RFC-0002 chain-walking).
+- `src/cli/se-trace.ts` - the CLI entry point, including `validate-runtime` (RFC-0001), `recursive` (RFC-0002), and `check-guardrails` (RFC-0003).
+- `src/guardrails/layer3.ts` - a thin consumer-side port of the reference implementation's Layer 3 for `check-guardrails --rules` re-evaluation.
 - `tests/tools.test.ts` - the test suite.
 
 ## SDKs
diff --git a/src/cli/se-trace.ts b/src/cli/se-trace.ts
index df39874..06e9b91 100644
--- a/src/cli/se-trace.ts
+++ b/src/cli/se-trace.ts
@@ -144,6 +144,55 @@ program
     }
   });
 
+program
+  .command("check-guardrails")
+  .description("Print each decision's Layer 3 GuardrailDecision (RFC-0003). With --rules, re-evaluate the COTS against a guardrail rule file")
+  .argument("<path>", "Path to a COTS JSON file")
+  .option("-r, --rules <file>", "Path to a guardrail rule file (re-evaluates instead of printing the stored record)")
+  .action(async (path: string, opts: { rules?: string }) => {
+    const cots = loadCots(path);
+    if (opts.rules) {
+      const { Layer3, parseRules } = await import("../guardrails/layer3.js");
+      const dsl = readFileSync(resolve(opts.rules), "utf-8");
+      let rules;
+      try {
+        rules = parseRules(dsl);
+      } catch {
+        console.log("guardrail-rules-malformed");
+        process.exit(1);
+      }
+      const result = new Layer3(rules).evaluate(cots);
+      for (const d of result.cots.decisions) {
+        printGuardrail(d.id, d.guardrail!);
+      }
+      if (result.rejected.length > 0) {
+        console.log(`\nRejected: ${result.rejected.length} (binding rules; the response is stopped and the rejection recorded)`);
+        process.exit(2);
+      }
+      if (result.escalated.length > 0) {
+        console.log(`\nEscalated: ${result.escalated.length}`);
+        process.exit(3);
+      }
+      return;
+    }
+    let withRecord = 0;
+    let rejected = 0;
+    for (const d of cots.decisions) {
+      if (d.guardrail === undefined) {
+        console.log(`${d.id}\n  No Layer 3 GuardrailDecision (the producer has not floored Layer 3).`);
+        continue;
+      }
+      withRecord++;
+      printGuardrail(d.id, d.guardrail);
+      if (d.guardrail.decision === "rejected") rejected++;
+    }
+    console.log(`\nDecisions with a GuardrailDecision: ${withRecord}/${cots.decisions.length}`);
+    if (rejected > 0) {
+      console.log(`Rejected: ${rejected} (binding rules; no silent rejection)`);
+      process.exit(2);
+    }
+  });
+
 program
   .command("inspect")
   .description("Print a human-readable summary of a COTS")
@@ -183,51 +232,19 @@ program
     console.log(`HTML:     ${htmlPath}`);
   });
 
-program
-  .command("context-graph")
-  .description("Render a COTS's Context + Evidence parts as a Context Graph (Mermaid + HTML, RFC-0004)")
-  .argument("<path>", "Path to a COTS JSON file")
-  .option("-o, --out <dir>", "Output directory", ".")
-  .action((path: string, opts: { out: string }) => {
-    const cots = loadCots(path);
-    const baseName = basename(path, extname(path));
-    const mermaid = toContextGraphMermaid(cots);
-    const html = toContextGraphHtml(cots);
-    const mermaidPath = resolve(opts.out, `${baseName}.context-graph.mermaid`);
-    const htmlPath = resolve(opts.out, `${baseName}.context-graph.html`);
-    mkdirSync(dirname(mermaidPath), { recursive: true });
-    mkdirSync(dirname(htmlPath), { recursive: true });
-    writeFileSync(mermaidPath, mermaid);
-    writeFileSync(htmlPath, html);
-    console.log(`Mermaid:  ${mermaidPath}`);
-    console.log(`HTML:     ${htmlPath}`);
-  });
-
-program
-  .command("audit")
-  .description("Replay the audit trail of every decision (or one decision) and flag append-only/linkage violations (RFC-0005)")
-  .argument("<path>", "Path to a COTS JSON file")
-  .argument("[decision-id]", "Optional decision id to replay")
-  .action((path: string, decisionId?: string) => {
-    const cots = loadCots(path);
-    const report = replayAudit(cots, decisionId);
-    for (const line of report.lines) console.log(line);
-    if (report.issues.length > 0) process.exit(1);
-  });
-
-program
-  .command("explore")
-  .description("Render the COTS Explorer: a single self-contained HTML page with all four views (Advancement Plan Phase 5)")
-  .argument("<path>", "Path to a COTS JSON file")
-  .option("-o, --out <dir>", "Output directory", ".")
-  .action((path: string, opts: { out: string }) => {
-    const cots = loadCots(path);
-    const baseName = basename(path, extname(path));
-    const html = toExplorerHtml(cots);
-    const htmlPath = resolve(opts.out, `${baseName}.explorer.html`);
-    mkdirSync(dirname(htmlPath), { recursive: true });
-    writeFileSync(htmlPath, html);
-    console.log(`COTS Explorer: ${htmlPath}`);
-  });
+const printGuardrail = (decisionId: string, g: {
+  decision: string;
+  rule_id: string;
+  rationale: string;
+  binding: boolean;
+  evaluated_at: string;
+  evaluated_by?: string;
+}) => {
+  console.log(decisionId);
+  console.log(`  decision: ${g.decision}${g.binding ? " (binding)" : " (advisory)"}`);
+  console.log(`  rule: ${g.rule_id}`);
+  console.log(`  rationale: ${g.rationale}`);
+  console.log(`  evaluated_at: ${g.evaluated_at}${g.evaluated_by ? `  by: ${g.evaluated_by}` : ""}`);
+};
 
 program.parse();
diff --git a/src/cots.ts b/src/cots.ts
index d729840..a489c2a 100644
--- a/src/cots.ts
+++ b/src/cots.ts
@@ -79,6 +79,19 @@ export interface DecisionRecord {
   created_at: ISO8601;
   /** Layer 2 Validation record (COTS schema v1.1.0, RFC-0001). Optional. */
   validation?: Layer2Validation;
+  /** Layer 3 GuardrailDecision (COTS schema v1.3.0, RFC-0003). Optional. */
+  guardrail?: GuardrailDecision;
+}
+
+export type GuardrailOutcome = "accepted" | "rejected" | "annotated" | "escalated";
+
+export interface GuardrailDecision {
+  decision: GuardrailOutcome;
+  rule_id: URI;
+  rationale: string;
+  binding: boolean;
+  evaluated_at: ISO8601;
+  evaluated_by?: string;
 }
 
 export interface AuditEntry {
diff --git a/src/guardrails/layer3.ts b/src/guardrails/layer3.ts
new file mode 100644
index 0000000..14358ae
--- /dev/null
+++ b/src/guardrails/layer3.ts
@@ -0,0 +1,156 @@
+/**
+ * Guardrail rule evaluation for the se-trace CLI. A thin consumer-side
+ * port of the reference implementation's Layer 3 module (RFC-0003): parses
+ * the guardrail DSL and evaluates rules against a COTS so reviewers can
+ * re-check a stored COTS against a rule file.
+ *
+ * The authoritative Layer 3 implementation lives in
+ * Source-Engineering/source-engineering-reference-implementation.
+ */
+
+import type { Cots, DecisionRecord, GuardrailDecision } from "../cots.js";
+
+export type GuardrailAction = "reject" | "annotate" | "escalate";
+
+export interface GuardrailRule {
+  rule_id: string;
+  when: (cots: Cots, decision: DecisionRecord) => boolean;
+  then: GuardrailAction;
+  rationale: string;
+}
+
+export class GuardrailRulesMalformed extends Error {
+  constructor() {
+    super("guardrail-rules-malformed");
+  }
+}
+
+const CONDITION_RE = /^(?:(constraint\([a-z0-9_-]+\)|flag\([a-z-]+\)|source_expired\(\)|decision_type)(\s*(?:==|!=)\s*"[^"]*")?\s*)$/;
+
+export const parseRules = (dsl: string): GuardrailRule[] => {
+  if (typeof dsl !== "string" || dsl.trim().length === 0) {
+    throw new GuardrailRulesMalformed();
+  }
+  const ruleBlocks = dsl.match(/rule\s+[^\s{]+\s*\{[^}]*\}/g);
+  if (ruleBlocks === null || ruleBlocks.length === 0) {
+    throw new GuardrailRulesMalformed();
+  }
+
+  const rules: GuardrailRule[] = [];
+  for (const block of ruleBlocks) {
+    const idMatch = block.match(/^rule\s+([^\s{]+)\s*\{/);
+    const whenMatch = block.match(/when\s+([^\n]+?)\s*\n\s*then/);
+    const thenMatch = block.match(/then\s+(reject|annotate|escalate)\b/);
+    const rationaleMatch = block.match(/with\s+rationale\s+"([^"]*)"/);
+
+    if (!idMatch || !whenMatch || !thenMatch || !rationaleMatch) {
+      throw new GuardrailRulesMalformed();
+    }
+    const condition = whenMatch[1].trim();
+    if (!CONDITION_RE.test(condition.replace(/\s+/g, " "))) {
+      throw new GuardrailRulesMalformed();
+    }
+    rules.push({
+      rule_id: idMatch[1],
+      when: compileCondition(condition),
+      then: thenMatch[1] as GuardrailAction,
+      rationale: rationaleMatch[1],
+    });
+  }
+  return rules;
+};
+
+const compileCondition = (condition: string): ((cots: Cots, decision: DecisionRecord) => boolean) => {
+  const normalized = condition.replace(/\s+/g, " ").trim();
+
+  const constraintEq = normalized.match(/^constraint\(([a-z0-9_-]+)\)\s*==\s*"([^"]*)"$/);
+  if (constraintEq) {
+    const scope = constraintEq[1];
+    const value = constraintEq[2];
+    return (cots) => cots.context.constraints.some((c) => c.scope === scope && c.value === value);
+  }
+
+  const constraintNeq = normalized.match(/^constraint\(([a-z0-9_-]+)\)\s*!=\s*"([^"]*)"$/);
+  if (constraintNeq) {
+    const scope = constraintNeq[1];
+    const value = constraintNeq[2];
+    return (cots) => !cots.context.constraints.some((c) => c.scope === scope && c.value === value);
+  }
+
+  const flagMatch = normalized.match(/^flag\(([a-z-]+)\)$/);
+  if (flagMatch) {
+    const flagType = flagMatch[1];
+    return (cots) =>
+      cots.decisions.some((d) => d.validation?.flags.some((f) => f.flag_type === flagType) ?? false);
+  }
+
+  if (/^source_expired\(\)$/.test(normalized)) {
+    return (cots) => {
+      const now = Date.parse(cots.produced_at);
+      return cots.inputs.source_artifacts.some(
+        (a) => a.temporal_validity.valid_until !== null && Date.parse(a.temporal_validity.valid_until) < now
+      );
+    };
+  }
+
+  const dtMatch = normalized.match(/^decision_type\s*==\s*"([^"]*)"$/);
+  if (dtMatch) {
+    const suffix = dtMatch[1];
+    return (cots) => cots.decisions.some((d) => d.id.includes(suffix) || d.basis_id.includes(suffix));
+  }
+
+  throw new GuardrailRulesMalformed();
+};
+
+const OUTCOME_FOR_ACTION: Record<GuardrailAction, GuardrailDecision["decision"]> = {
+  reject: "rejected",
+  annotate: "annotated",
+  escalate: "escalated",
+};
+
+export interface Layer3Result {
+  cots: Cots;
+  rejected: { decision_id: string; rule_id: string; rationale: string }[];
+  escalated: { decision_id: string; rule_id: string; rationale: string }[];
+}
+
+export class Layer3 {
+  constructor(private readonly rules: GuardrailRule[]) {}
+
+  evaluate(cots: Cots, options: { now?: string } = {}): Layer3Result {
+    const now = options.now ?? new Date().toISOString();
+    const rejected: Layer3Result["rejected"] = [];
+    const escalated: Layer3Result["escalated"] = [];
+
+    const decisions = cots.decisions.map((decision): DecisionRecord => {
+      let fired: GuardrailRule | undefined;
+      let outcome: GuardrailDecision["decision"] = "accepted";
+      for (const rule of this.rules) {
+        if (rule.when(cots, decision)) {
+          fired = rule;
+          outcome = OUTCOME_FOR_ACTION[rule.then];
+          break;
+        }
+      }
+      const guardrail: GuardrailDecision = {
+        decision: outcome,
+        rule_id: fired ? `urn:source-engineering:rule:${fired.rule_id}` : "urn:source-engineering:rule:none",
+        rationale: fired ? fired.rationale : "No guardrail rule matched; the decision is accepted.",
+        binding: fired?.then === "reject",
+        evaluated_at: now,
+        evaluated_by: "se-trace-check-guardrails",
+      };
+      let next: DecisionRecord = { ...decision, guardrail };
+      if (outcome === "rejected") {
+        next = { ...next, validation_status: { ...next.validation_status, status: "superseded", last_changed_at: now, last_changed_by: "se-trace-check-guardrails" } };
+        rejected.push({ decision_id: decision.id, rule_id: fired!.rule_id, rationale: fired!.rationale });
+      } else if (outcome === "escalated") {
+        next = { ...next, validation_status: { ...next.validation_status, status: "appealed", last_changed_at: now, last_changed_by: "se-trace-check-guardrails" } };
+        escalated.push({ decision_id: decision.id, rule_id: fired!.rule_id, rationale: fired!.rationale });
+      }
+      return next;
+    });
+
+    return { cots: { ...cots, decisions }, rejected, escalated };
+  }
+}
diff --git a/tests/tools.test.ts b/tests/tools.test.ts
index f681e02..bfe2a70 100644
--- a/tests/tools.test.ts
+++ b/tests/tools.test.ts
@@ -348,3 +348,70 @@ describe("recursive (RFC-0002)", () => {
     }
   });
 });
+
+describe("check-guardrails (RFC-0003)", () => {
+  const RULES = `
+    rule ny-only {
+      when constraint(all_decisions) == "US-NY"
+      then reject
+      with rationale "The declared jurisdiction does not admit automated summarisation."
+    }
+  `;
+
+  it("prints the stored GuardrailDecision when present", () => {
+    const cots = loadFixture("legal-summarization.cots.json");
+    cots.decisions[0].guardrail = {
+      decision: "rejected",
+      rule_id: "urn:source-engineering:rule:ny-only",
+      rationale: "The declared jurisdiction does not admit automated summarisation.",
+      binding: true,
+      evaluated_at: "2026-10-30T12:00:00Z",
+      evaluated_by: "layer3-business-guardrails",
+    };
+    const { execFileSync } = require("node:child_process");
+    const { writeFileSync, rmSync } = require("node:fs");
+    const tmp = resolve(__dirname, "..", "examples", "tmp-guardrails.cots.json");
+    writeFileSync(tmp, JSON.stringify(cots));
+    try {
+      let out = "";
+      try {
+        execFileSync("node", [resolve(__dirname, "..", "dist", "cli", "se-trace.js"), "check-guardrails", tmp], { encoding: "utf-8" });
+      } catch (e) {
+        out = ((e as { stdout?: string }).stdout ?? "");
+      }
+      expect(out).toContain("decision: rejected (binding)");
+      expect(out).toContain("rule: urn:source-engineering:rule:ny-only");
+    } finally {
+      rmSync(tmp, { force: true });
+    }
+  });
+
+  it("re-evaluates against a rule file with --rules and exits 2 on rejection", () => {
+    const cots = loadFixture("legal-summarization.cots.json");
+    cots.context.constraints = [
+      { id: "urn:source-engineering:constraint:c1", type: "jurisdiction", scope: "all_decisions", value: "US-NY" },
+    ];
+    const { execFileSync } = require("node:child_process");
+    const { writeFileSync, rmSync } = require("node:fs");
+    const tmp = resolve(__dirname, "..", "examples", "tmp-guardrails-2.cots.json");
+    const rulesFile = resolve(__dirname, "..", "examples", "tmp-rules.gr");
+    writeFileSync(tmp, JSON.stringify(cots));
+    writeFileSync(rulesFile, RULES);
+    try {
+      let out = "";
+      try {
+        execFileSync("node", [resolve(__dirname, "..", "dist", "cli", "se-trace.js"), "check-guardrails", tmp, "--rules", rulesFile], { encoding: "utf-8" });
+        throw new Error("expected exit 2");
+      } catch (e) {
+        const err = e as { stdout?: string; status?: number };
+        expect(err.status).toBe(2);
+        out = err.stdout ?? "";
+      }
+      expect(out).toContain("decision: rejected (binding)");
+      expect(out).toContain("Rejected: 1");
+    } finally {
+      rmSync(tmp, { force: true });
+      rmSync(rulesFile, { force: true });
+    }
+  });
+});