diff --git a/src/utils/git-spawn.ts b/src/utils/git-spawn.ts
index 6b699e7d..4426d1a4 100644
--- a/src/utils/git-spawn.ts
+++ b/src/utils/git-spawn.ts
@@ -12,11 +12,15 @@
  * git child does this, not only after a trust decision. An empty
  * `diff.external` still tries to exec, so a read-only `diff` also passes
  * `--no-ext-diff` and `--no-textconv` and keeps git's own diff. Repo
- * `filter.*` and `diff.*` drivers are blanked the same way, including ones
- * pulled in by `include.path`, `includeIf`, or `extensions.worktreeConfig`.
+ * `filter.*`, `merge.*.driver`, and `diff.*` drivers are blanked for every
+ * `runGit` child (read-only and mutating), including ones pulled in by
+ * `include.path`, `includeIf`, or `extensions.worktreeConfig`.
  * Only `local` and `worktree` scopes are overridden. System and global config
  * stay in effect, and `command` scope is ignored. Mutating commands
- * (worktree add, apply) keep ssh and credential helpers.
+ * (worktree add, apply, add) get the same discovered executable-config
+ * overrides so repo `filter.*` and `merge.*.driver` programs do not run;
+ * they still skip the unconditional read-only blanks for ssh, pager, and
+ * credential helpers (unless those keys are set locally).
  *
  * `GIT_*` on the child comes from the environment captured before a project
  * `.env`. A project file cannot point git at another repo or config.
@@ -72,7 +76,7 @@ function withoutExternalDiff(args: readonly string[]): string[] {
 
 /** Repo config keys whose values are programs. */
 const EXEC_CONFIG_KEY =
-  /^(?:filter\..+\.(?:clean|smudge|process)|diff\..+\.(?:textconv|command)|core\.(?:fsmonitor|hooksPath|sshCommand|pager)|credential\.helper)$/;
+  /^(?:filter\..+\.(?:clean|smudge|process)|merge\..+\.driver|diff\..+\.(?:textconv|command)|core\.(?:fsmonitor|hooksPath|sshCommand|pager)|credential\.helper)$/;
 
 /**
  * Git matches `--get-regexp` against the canonical lowercase key, so
@@ -85,7 +89,7 @@ const EXEC_CONFIG_KEY =
  * own git children (those already force `core.hooksPath` to `/dev/null`).
  */
 const EXEC_CONFIG_REGEXP =
-  '^(filter\\..*\\.(clean|smudge|process)|diff\\..*\\.(textconv|command)|diff\\.external|core\\.(fsmonitor|hookspath|sshcommand|pager)|credential\\.helper)$';
+  '^(filter\\..*\\.(clean|smudge|process)|merge\\..*\\.driver|diff\\..*\\.(textconv|command)|diff\\.external|core\\.(fsmonitor|hookspath|sshcommand|pager)|credential\\.helper)$';
 
 const FILTER_DRIVER = /^filter\.(.+)\.(?:clean|smudge|process)$/i;
 
@@ -455,6 +459,8 @@ export interface RunGitOptions {
   /**
    * Default true. Pass false for mutating git (worktree add, apply, add).
    * `core.fsmonitor` and `core.hooksPath` are overridden either way.
+   * Discovered local/worktree executable-config overrides (filters, merge
+   * drivers, …) are injected for both read-only and mutating calls.
    */
   readOnly?: boolean;
 }
@@ -467,10 +473,10 @@ export async function runGit(
   const env: Record<string, string> = { ...(opts.env ?? {}) };
   if (readOnly) env.GIT_PAGER = 'cat';
   let gitArgs = hardenedGitArgs(args, { readOnly });
-  if (readOnly) {
-    const discovered = await executableConfigOverrides(opts.cwd, opts.signal);
-    gitArgs = insertBeforeSubcommand(gitArgs, discovered);
-  }
+  // Mutating git (worktree add, add -A -N, apply --3way) must also blank
+  // repo filter/merge drivers; those call sites pass readOnly: false.
+  const discovered = await executableConfigOverrides(opts.cwd, opts.signal);
+  gitArgs = insertBeforeSubcommand(gitArgs, discovered);
   return runProcess('git', {
     args: gitArgs,
     cwd: opts.cwd,
diff --git a/test/git-spawn.spec.mjs b/test/git-spawn.spec.mjs
index e6af7c39..840cbd55 100644
--- a/test/git-spawn.spec.mjs
+++ b/test/git-spawn.spec.mjs
@@ -107,6 +107,7 @@ describe('git spawn', { concurrency: 1 }, () => {
         'local\tfilter.x.clean /tmp/clean.sh',
         'worktree\tfilter.x.smudge /tmp/smudge.sh',
         'local\tfilter.x.process /tmp/process.sh',
+        'local\tmerge.x.driver /tmp/merge.sh',
         'local\tdiff.x.textconv /tmp/textconv.sh',
         'worktree\tdiff.x.command /tmp/command.sh',
         'local\tcore.fsmonitor /tmp/fs.sh',
@@ -120,6 +121,7 @@ describe('git spawn', { concurrency: 1 }, () => {
     assert.ok(args.includes('filter.x.smudge='));
     assert.ok(args.includes('filter.x.process='));
     assert.ok(args.includes('filter.x.required=false'));
+    assert.ok(args.includes('merge.x.driver='));
     assert.ok(args.includes('diff.x.textconv='));
     assert.ok(args.includes('diff.x.command='));
     assert.ok(args.includes('core.fsmonitor='));
diff --git a/test/worktree-isolation-o79.spec.mjs b/test/worktree-isolation-o79.spec.mjs
new file mode 100644
index 00000000..1bd7f6a2
--- /dev/null
+++ b/test/worktree-isolation-o79.spec.mjs
@@ -0,0 +1,181 @@
+/**
+ * O79: mutating runGit (worktree add, add -A -N, apply --3way) must not run
+ * untrusted repo filter/merge drivers.
+ */
+import test, { describe } from 'node:test';
+import assert from 'node:assert/strict';
+import { execFileSync } from 'node:child_process';
+import fs from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+
+import {
+  createWorktree,
+  collectWorktreePatch,
+  applyWorktreePatch,
+  cleanupWorktree,
+} from '../dist/core/subagent/worktree-isolation.js';
+import { runGit } from '../dist/utils/git-spawn.js';
+
+function git(cwd, args, opts = {}) {
+  return execFileSync('git', args, { cwd, stdio: 'ignore', ...opts });
+}
+
+async function markerWritten(marker) {
+  try {
+    const text = await fs.readFile(marker, 'utf8');
+    return text.includes('ran');
+  } catch {
+    return false;
+  }
+}
+
+describe('O79 worktree isolation git exec', { concurrency: 1 }, () => {
+  test('createWorktree does not run untrusted filter.smudge', async (t) => {
+    const root = await fs.mkdtemp(path.join(os.tmpdir(), 'moss-o79-smudge-'));
+    t.after(() => fs.rm(root, { recursive: true, force: true }));
+    const repo = path.join(root, 'repo');
+    await fs.mkdir(repo);
+    const marker = path.join(root, 'SMUDGE_RAN');
+    git(repo, ['init', '-q']);
+    git(repo, ['config', 'user.email', 'o79@example.test']);
+    git(repo, ['config', 'user.name', 'o79']);
+    await fs.writeFile(path.join(repo, '.gitattributes'), '*.txt filter=pwn\n');
+    await fs.writeFile(path.join(repo, 'a.txt'), 'hello\n');
+    git(repo, ['add', '.']);
+    git(repo, ['commit', '-q', '-m', 'init']);
+    const cmd = `sh -c 'echo ran >> "${marker.replace(/\\/g, '/')}"; cat'`;
+    git(repo, ['config', 'filter.pwn.smudge', cmd]);
+    git(repo, ['config', 'filter.pwn.clean', 'cat']);
+
+    const probe = path.join(root, 'probe');
+    git(repo, ['worktree', 'add', '-q', '--detach', probe, 'HEAD']);
+    assert.equal(await markerWritten(marker), true, 'fixture: plain worktree add runs smudge');
+    await fs.rm(marker, { force: true });
+    git(repo, ['worktree', 'remove', '--force', probe]);
+
+    const lease = await createWorktree({ parentWorkspace: repo, runId: 'o79-smudge' });
+    t.after(() => cleanupWorktree({ parentWorkspace: repo, worktreePath: lease.worktreePath }));
+    assert.equal(await markerWritten(marker), false, 'createWorktree ran untrusted smudge');
+  });
+
+  test('collectWorktreePatch add -A -N path does not run filter.clean', async (t) => {
+    const root = await fs.mkdtemp(path.join(os.tmpdir(), 'moss-o79-clean-'));
+    t.after(() => fs.rm(root, { recursive: true, force: true }));
+    const repo = path.join(root, 'repo');
+    await fs.mkdir(repo);
+    const marker = path.join(root, 'CLEAN_RAN');
+    const script = path.join(root, 'clean.sh');
+    await fs.writeFile(script, `#!/bin/sh\necho ran >> ${JSON.stringify(marker)}\ncat\n`);
+    await fs.chmod(script, 0o755);
+
+    git(repo, ['init', '-q']);
+    git(repo, ['config', 'user.email', 'o79@example.test']);
+    git(repo, ['config', 'user.name', 'o79']);
+    await fs.writeFile(path.join(repo, '.gitattributes'), '*.txt filter=pwn\n');
+    await fs.writeFile(path.join(repo, 'a.txt'), 'hello\n');
+    git(repo, ['add', '.']);
+    git(repo, ['commit', '-q', '-m', 'init']);
+    git(repo, ['config', 'filter.pwn.clean', script]);
+    git(repo, ['config', 'filter.pwn.smudge', 'cat']);
+    git(repo, ['config', 'filter.pwn.required', 'true']);
+
+    await fs.writeFile(path.join(repo, 'a.txt'), 'hello\nchanged\n');
+    const future = new Date(Date.now() + 10_000);
+    await fs.utimes(path.join(repo, 'a.txt'), future, future);
+    await fs.rm(marker, { force: true });
+    git(repo, ['add', '-A']);
+    assert.equal(await markerWritten(marker), true, 'fixture: raw git add -A runs clean');
+    git(repo, ['reset', '-q']);
+    await fs.rm(marker, { force: true });
+
+    const lease = await createWorktree({ parentWorkspace: repo, runId: 'o79-clean' });
+    t.after(() => cleanupWorktree({ parentWorkspace: repo, worktreePath: lease.worktreePath }));
+    await fs.writeFile(path.join(lease.worktreePath, 'a.txt'), 'hello\nlease\n');
+    await fs.utimes(path.join(lease.worktreePath, 'a.txt'), future, future);
+    await fs.rm(marker, { force: true });
+
+    await collectWorktreePatch({ parentWorkspace: repo, lease });
+    assert.equal(
+      await markerWritten(marker),
+      false,
+      'collectWorktreePatch (add -A -N) ran untrusted clean'
+    );
+
+    // Same hardening entry point as add -A -N: mutating runGit must blank
+    // filter.clean. Plain `git add -A` runs clean; prove overrides apply.
+    await fs.writeFile(path.join(lease.worktreePath, 'a.txt'), 'hello\nlease2\n');
+    await fs.utimes(path.join(lease.worktreePath, 'a.txt'), future, future);
+    await fs.rm(marker, { force: true });
+    await runGit(['add', '-A'], { cwd: lease.worktreePath, readOnly: false, timeout: 60_000 });
+    assert.equal(await markerWritten(marker), false, 'mutating runGit add -A ran untrusted clean');
+  });
+
+  test('applyWorktreePatch --3way does not run untrusted merge driver', async (t) => {
+    const root = await fs.mkdtemp(path.join(os.tmpdir(), 'moss-o79-merge-'));
+    t.after(() => fs.rm(root, { recursive: true, force: true }));
+    const repo = path.join(root, 'repo');
+    await fs.mkdir(repo);
+    const marker = path.join(root, 'MERGE_RAN');
+    const script = path.join(root, 'merge.sh');
+    await fs.writeFile(
+      script,
+      `#!/bin/sh\necho ran >> ${JSON.stringify(marker)}\necho driver > "$1"\n`
+    );
+    await fs.chmod(script, 0o755);
+
+    git(repo, ['init', '-q']);
+    git(repo, ['config', 'user.email', 'o79@example.test']);
+    git(repo, ['config', 'user.name', 'o79']);
+    await fs.writeFile(path.join(repo, '.gitattributes'), '*.js merge=pwn\n');
+    await fs.writeFile(path.join(repo, 'same.js'), 'value = 0\n');
+    git(repo, ['add', '.']);
+    git(repo, ['commit', '-q', '-m', 'init']);
+    git(repo, ['config', 'merge.pwn.driver', `${script} %A`]);
+    git(repo, ['config', 'merge.pwn.name', 'pwn']);
+
+    const leaseA = await createWorktree({ parentWorkspace: repo, runId: 'o79-merge-a' });
+    const leaseB = await createWorktree({ parentWorkspace: repo, runId: 'o79-merge-b' });
+    t.after(async () => {
+      await cleanupWorktree({ parentWorkspace: repo, worktreePath: leaseA.worktreePath });
+      await cleanupWorktree({ parentWorkspace: repo, worktreePath: leaseB.worktreePath });
+    });
+
+    await fs.writeFile(path.join(leaseA.worktreePath, 'same.js'), 'value = 100\n');
+    await fs.writeFile(path.join(leaseB.worktreePath, 'same.js'), 'value = 200\n');
+    const patchA = await collectWorktreePatch({ parentWorkspace: repo, lease: leaseA });
+    const patchB = await collectWorktreePatch({ parentWorkspace: repo, lease: leaseB });
+
+    const first = await applyWorktreePatch({
+      parentWorkspace: repo,
+      patchPath: patchA.patchPath,
+    });
+    assert.equal(first.status, 'merged', 'first lease should merge cleanly');
+
+    // Fixture: raw git apply --3way invokes the merge driver on the clash.
+    const rawPatch = path.join(root, 'raw-b.patch');
+    await fs.copyFile(patchB.patchPath, rawPatch);
+    await fs.rm(marker, { force: true });
+    try {
+      execFileSync('git', ['apply', '--3way', rawPatch], { cwd: repo, stdio: 'ignore' });
+    } catch {
+      /* conflict or driver rewrite is fine */
+    }
+    assert.equal(await markerWritten(marker), true, 'fixture: raw apply --3way runs merge driver');
+    git(repo, ['reset', '--hard', 'HEAD']);
+    // Re-apply first lease so the second apply has a real clash again.
+    const reapply = await applyWorktreePatch({
+      parentWorkspace: repo,
+      patchPath: patchA.patchPath,
+    });
+    assert.equal(reapply.status, 'merged');
+
+    await fs.rm(marker, { force: true });
+    await applyWorktreePatch({ parentWorkspace: repo, patchPath: patchB.patchPath });
+    assert.equal(
+      await markerWritten(marker),
+      false,
+      'applyWorktreePatch --3way ran untrusted merge driver'
+    );
+  });
+});