diff --git a/package.json b/package.json
index 77a7de2d..6ee6e7ea 100644
--- a/package.json
+++ b/package.json
@@ -73,6 +73,7 @@
     "sdk:test": "jest --testPathIgnorePatterns /node_modules/ tests/e2e/ --passWithNoTests",
     "check:egress": "node scripts/check-egress.cjs",
     "check:doc-version": "node scripts/check-doc-version.cjs",
+    "check:workspace": "node scripts/check-workspace.cjs",
     "test:prod-bundle-key": "node scripts/check-prod-supabase-bundle.mjs",
     "verify": "tsc --noEmit && jest --testPathIgnorePatterns /node_modules/ tests/e2e/ --passWithNoTests",
     "test:wrapper-readiness": "node tests/e2e/wrapper-reach-preflight.mjs && jest tests/e2e/wrapper-readiness.test.ts",
diff --git a/scripts/check-workspace.cjs b/scripts/check-workspace.cjs
new file mode 100644
index 00000000..6effbf21
--- /dev/null
+++ b/scripts/check-workspace.cjs
@@ -0,0 +1,48 @@
+#!/usr/bin/env node
+'use strict';
+/**
+ * Refuse repo-specific work in the WRONG checkout — a durable guard for the launcher-misrouting defect
+ * (DECISION 3, #679 sprint). The session launcher has bound a TrustShell task to the old `ihh-ica` project
+ * while the working dir was the `trustshell-tl` worktree, so `start_session` resolved the wrong repo and
+ * work risked landing (or silently failing) against the wrong history. The session-binding itself is
+ * app-level config this script cannot change; this is the in-repo assertion so any loop or session can gate
+ * its own work and fail LOUDLY instead of silently misrouting:
+ *
+ *   node scripts/check-workspace.cjs                 # expects origin = DealAppSeo/trustshell
+ *   node scripts/check-workspace.cjs owner/repo      # expects that repo
+ *   TRUSTSHELL_EXPECT_REPO=owner/repo node scripts/check-workspace.cjs
+ *
+ * Exit 0 when origin matches, 1 (with a loud message) otherwise. No deps; safe to call at task startup.
+ */
+const { execSync } = require('node:child_process');
+
+/** PURE. Does a git remote URL (https or ssh, optional `.git` / trailing slash) point at `owner/repo`? */
+function workspaceMatches(remoteUrl, expected) {
+  const m = String(remoteUrl)
+    .trim()
+    .match(/github\.com[/:]([^/\s]+\/[^/\s]+?)(?:\.git)?\/?$/i);
+  return m ? m[1].toLowerCase() === String(expected).toLowerCase() : false;
+}
+
+/** Resolve the current origin + branch and assert it is `expected`. Returns the process exit code. */
+function main(argv, env, run) {
+  const expected = argv[2] || env.TRUSTSHELL_EXPECT_REPO || 'DealAppSeo/trustshell';
+  const sh = run || ((cmd) => execSync(cmd, { encoding: 'utf8' }).trim());
+  let remote = '';
+  let branch = '';
+  try { remote = sh('git remote get-url origin'); } catch { /* no remote */ }
+  try { branch = sh('git rev-parse --abbrev-ref HEAD'); } catch { /* detached / no repo */ }
+  if (!workspaceMatches(remote, expected)) {
+    process.stderr.write(
+      `WORKSPACE MISROUTED: origin is "${remote || '(none)'}", expected ${expected}. ` +
+        `Refusing repo-specific work in the wrong checkout — cd to the ${expected} worktree first.\n`,
+    );
+    return 1;
+  }
+  process.stderr.write(`workspace OK: ${expected}${branch ? ` @ ${branch}` : ''}\n`);
+  return 0;
+}
+
+module.exports = { workspaceMatches, main };
+
+if (require.main === module) process.exit(main(process.argv, process.env));
diff --git a/tests/check-workspace.test.ts b/tests/check-workspace.test.ts
new file mode 100644
index 00000000..f33eaae3
--- /dev/null
+++ b/tests/check-workspace.test.ts
@@ -0,0 +1,38 @@
+/**
+ * The workspace guard refuses repo-specific work in the wrong checkout (DECISION 3, #679 sprint): the launcher
+ * bound a TrustShell task to the old ihh-ica project. `workspaceMatches` is pure; `main` is run with an injected
+ * shell so the test asserts both exit paths without depending on the checkout it runs in.
+ */
+const { workspaceMatches, main } = require('../scripts/check-workspace.cjs') as {
+  workspaceMatches: (remoteUrl: string, expected: string) => boolean;
+  main: (argv: string[], env: NodeJS.ProcessEnv, run?: (cmd: string) => string) => number;
+};
+
+describe('check-workspace: refuse the wrong checkout (DECISION 3, #679)', () => {
+  it('matches https and ssh forms of the expected repo, with or without .git / case / trailing slash', () => {
+    for (const u of [
+      'https://github.com/DealAppSeo/trustshell.git',
+      'https://github.com/DealAppSeo/trustshell',
+      'git@github.com:DealAppSeo/trustshell.git',
+      'https://github.com/dealappseo/TrustShell/',
+    ]) {
+      expect(workspaceMatches(u, 'DealAppSeo/trustshell')).toBe(true);
+    }
+  });
+
+  it('rejects the ihh-ica misroute, a different repo, and empty/garbage remotes', () => {
+    expect(workspaceMatches('https://github.com/DealAppSeo/ihh-ica.git', 'DealAppSeo/trustshell')).toBe(false);
+    expect(workspaceMatches('git@github.com:DealAppSeo/repid-engine.git', 'DealAppSeo/trustshell')).toBe(false);
+    expect(workspaceMatches('', 'DealAppSeo/trustshell')).toBe(false);
+    expect(workspaceMatches('not-a-url', 'DealAppSeo/trustshell')).toBe(false);
+  });
+
+  it('main() exits 0 on a matching origin and 1 (loudly) on a misroute — injected shell, no real git', () => {
+    const asTrustshell = (cmd: string) => (cmd.includes('remote get-url') ? 'https://github.com/DealAppSeo/trustshell.git' : 'main');
+    const asIhhIca = (cmd: string) => (cmd.includes('remote get-url') ? 'https://github.com/DealAppSeo/ihh-ica.git' : 'ica-build');
+    expect(main(['node', 'check-workspace.cjs'], {} as NodeJS.ProcessEnv, asTrustshell)).toBe(0);
+    expect(main(['node', 'check-workspace.cjs'], {} as NodeJS.ProcessEnv, asIhhIca)).toBe(1);
+    // explicit expected-repo arg wins
+    expect(main(['node', 'check-workspace.cjs', 'DealAppSeo/repid-engine'], {} as NodeJS.ProcessEnv, (cmd) => (cmd.includes('remote get-url') ? 'git@github.com:DealAppSeo/repid-engine.git' : 'main'))).toBe(0);
+  });
+});