diff --git a/tools/alignment/detect-clause-drift.test.ts b/tools/alignment/detect-clause-drift.test.ts
new file mode 100644
index 0000000000..9ac985ac94
--- /dev/null
+++ b/tools/alignment/detect-clause-drift.test.ts
@@ -0,0 +1,62 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { findClauseReferences } from "./detect-clause-drift.ts";
+
+describe("findClauseReferences", () => {
+  // Use a unique OS temp directory rather than a fixed relative path so a
+  // changed CWD can never make the afterEach rmSync delete an unexpected dir.
+  let testDir: string;
+
+  beforeEach(() => {
+    testDir = mkdtempSync(join(tmpdir(), "clause-ref-"));
+    writeFileSync(join(testDir, "file1.md"), "This file references HC-1 and SD-2.");
+    writeFileSync(join(testDir, "file2.ts"), "This file references DIR-3.");
+    writeFileSync(join(testDir, "file3.txt"), "This file has no references.");
+  });
+
+  afterEach(() => {
+    rmSync(testDir, { recursive: true, force: true });
+  });
+
+  it("finds all references to valid alignment clauses in the directory", () => {
+    const references = findClauseReferences(testDir);
+    expect(references.size).toBe(3);
+    expect(references.get("HC-1")).toEqual([join(testDir, "file1.md")]);
+    expect(references.get("SD-2")).toEqual([join(testDir, "file1.md")]);
+    expect(references.get("DIR-3")).toEqual([join(testDir, "file2.ts")]);
+  });
+
+  it("ignores out-of-range clause IDs (word boundaries + bounded ranges)", () => {
+    writeFileSync(
+      join(testDir, "bad.md"),
+      "HC-0 and SD-99 and DIR-8 and XHC-1 are not valid clause refs.",
+    );
+    const references = findClauseReferences(testDir);
+    expect(references.has("HC-0")).toBe(false);
+    expect(references.has("SD-99")).toBe(false);
+    expect(references.has("DIR-8")).toBe(false);
+  });
+
+  it("matches multiple clauses on one line without skipping (no shared lastIndex)", () => {
+    const dir = mkdtempSync(join(tmpdir(), "clause-multi-"));
+    try {
+      writeFileSync(join(dir, "multi.md"), "HC-1 HC-2 HC-3 on one line");
+      const references = findClauseReferences(dir);
+      expect(references.get("HC-1")).toEqual([join(dir, "multi.md")]);
+      expect(references.get("HC-2")).toEqual([join(dir, "multi.md")]);
+      expect(references.get("HC-3")).toEqual([join(dir, "multi.md")]);
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  it("skips ignored directories (e.g. node_modules)", () => {
+    const nested = join(testDir, "node_modules");
+    mkdirSync(nested, { recursive: true });
+    writeFileSync(join(nested, "dep.md"), "Should be ignored: HC-7.");
+    const references = findClauseReferences(testDir);
+    expect(references.has("HC-7")).toBe(false);
+  });
+});
diff --git a/tools/alignment/detect-clause-drift.ts b/tools/alignment/detect-clause-drift.ts
new file mode 100644
index 0000000000..66385e433c
--- /dev/null
+++ b/tools/alignment/detect-clause-drift.ts
@@ -0,0 +1,168 @@
+#!/usr/bin/env bun
+// detect-clause-drift.ts — alignment-clause cross-reference (blast-radius) scan.
+//
+// B-0058.4 (decomposed from B-0058): pre-renegotiation impact survey.
+// Scans the repository working tree for references to alignment clauses
+// (HC-1..HC-7, SD-1..SD-9, DIR-1..DIR-5) from docs/ALIGNMENT.md and
+// reports which files reference each clause. Answers "who depends on
+// this clause, and what breaks if it moves?" BEFORE an ALIGNMENT.md
+// renegotiation is accepted.
+//
+// Distinct from audit_clause_drift.ts (B-0058 slice 2), which diffs
+// docs/ALIGNMENT.md between two git refs to detect WHAT changed. This
+// tool detects WHO references the clauses (the blast radius). The two
+// compose: audit_clause_drift.ts names the changed clauses; this tool
+// surveys their cross-references across the working tree.
+//
+// Usage:
+//   bun tools/alignment/detect-clause-drift.ts            # all clauses
+//   bun tools/alignment/detect-clause-drift.ts HC-1       # one clause
+//   bun tools/alignment/detect-clause-drift.ts --json
+//
+// Exit codes:
+//   0  Clean run (references emitted, or none found)
+//   2  Script error / bad args
+
+import { readFileSync, readdirSync } from "node:fs";
+import { join } from "node:path";
+import { spawnSync } from "node:child_process";
+
+type ExitCode = 0 | 2;
+
+// Canonical valid clause set + matcher, aligned with
+// tools/alignment/audit_clause_coverage.ts extractClauses(). Word
+// boundaries + bounded numeric ranges prevent false positives on
+// out-of-range IDs (HC-0, SD-99, etc.). A fresh RegExp per line keeps
+// the matcher stateless (no shared /g lastIndex skipping matches).
+const CLAUSE_PATTERN = "\\b(HC-[1-7]|SD-[1-9]|DIR-[1-5])\\b";
+
+// Heavy / non-source trees that would make a full walk take minutes and
+// drown the signal. references/ (and references/upstreams/) is the big
+// one — gigabytes of mirrored OTHER-repo source per the repo convention
+// (.claude/rules/references-upstreams-not-our-code-search-excludes.md).
+const IGNORE_DIRS: readonly string[] = [
+  "node_modules", ".git", ".vscode", ".idea", "dist", "build",
+  "bin", "obj", "target", "references",
+];
+const IGNORE_EXTS: readonly string[] = [
+  ".png", ".jpg", ".jpeg", ".gif", ".svg", ".ico", ".pdf",
+  ".zip", ".gz", ".tar", ".DS_Store",
+];
+
+export interface ClauseMatch {
+  readonly file: string;
+  readonly line: number;
+  readonly clause: string;
+  readonly text: string;
+}
+
+/** Resolve the git repository root so the scan covers the whole repo
+ *  regardless of the caller's CWD. Falls back to the given dir. */
+function repoRoot(fallback: string): string {
+  const res = spawnSync("git", ["rev-parse", "--show-toplevel"], {
+    encoding: "utf-8",
+  });
+  if (res.status === 0 && typeof res.stdout === "string") {
+    const root = res.stdout.trim();
+    if (root.length > 0) return root;
+  }
+  return fallback;
+}
+
+function searchInFile(filePath: string): ClauseMatch[] {
+  if (IGNORE_EXTS.some((ext) => filePath.endsWith(ext))) return [];
+
+  const matches: ClauseMatch[] = [];
+  let content: string;
+  try {
+    content = readFileSync(filePath, "utf-8");
+  } catch {
+    // Unreadable / binary file — skip silently.
+    return matches;
+  }
+
+  const lines = content.split("\n");
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+    if (line === undefined) continue;
+    // Fresh RegExp per line: matchAll over a non-shared instance avoids
+    // the cross-line lastIndex skipping that a module-level /g regex has.
+    for (const m of line.matchAll(new RegExp(CLAUSE_PATTERN, "g"))) {
+      const clause = m[0];
+      matches.push({ file: filePath, line: i + 1, clause, text: line.trim() });
+    }
+  }
+  return matches;
+}
+
+function searchInDirectory(dirPath: string): ClauseMatch[] {
+  let allMatches: ClauseMatch[] = [];
+  const entries = readdirSync(dirPath, { withFileTypes: true });
+  for (const entry of entries) {
+    if (IGNORE_DIRS.includes(entry.name)) continue;
+    const fullPath = join(dirPath, entry.name);
+    if (entry.isDirectory()) {
+      allMatches = allMatches.concat(searchInDirectory(fullPath));
+    } else if (entry.isFile()) {
+      allMatches = allMatches.concat(searchInFile(fullPath));
+    }
+  }
+  return allMatches;
+}
+
+/** Build a clause → referencing-files map for the given root. */
+export function findClauseReferences(dirPath: string): Map<string, string[]> {
+  const references = new Map<string, string[]>();
+  for (const match of searchInDirectory(dirPath)) {
+    const files = references.get(match.clause) ?? [];
+    if (!files.includes(match.file)) files.push(match.file);
+    references.set(match.clause, files);
+  }
+  return references;
+}
+
+export function main(argv: readonly string[]): ExitCode {
+  const json = argv.includes("--json");
+  const targetClause = argv.find((a) => !a.startsWith("--"));
+
+  const root = repoRoot(process.cwd());
+  const allMatches = searchInDirectory(root);
+
+  const filtered = targetClause
+    ? allMatches.filter(
+        (m) => m.clause.toUpperCase() === targetClause.toUpperCase(),
+      )
+    : allMatches;
+
+  if (json) {
+    const grouped: Record<string, ClauseMatch[]> = {};
+    for (const m of filtered) (grouped[m.clause] ??= []).push(m);
+    process.stdout.write(
+      JSON.stringify({ root, references: grouped }, null, 2) + "\n",
+    );
+    return 0;
+  }
+
+  process.stdout.write(`Alignment-clause references under ${root}\n`);
+  if (filtered.length === 0) {
+    process.stdout.write("No alignment clause references found.\n");
+    return 0;
+  }
+
+  const groupedByClause: Record<string, ClauseMatch[]> = {};
+  for (const m of filtered) (groupedByClause[m.clause] ??= []).push(m);
+
+  for (const clause of Object.keys(groupedByClause).sort()) {
+    const group = groupedByClause[clause];
+    if (group === undefined) continue;
+    process.stdout.write(`\n--- ${group.length} references to ${clause} ---\n`);
+    for (const match of group) {
+      process.stdout.write(`${match.file}:${match.line} - ${match.text}\n`);
+    }
+  }
+  return 0;
+}
+
+if (import.meta.main) {
+  process.exit(main(process.argv.slice(2)));
+}