diff --git a/packages/coding-agent/src/utils/git.ts b/packages/coding-agent/src/utils/git.ts
index b75856369..16b74f5e8 100644
--- a/packages/coding-agent/src/utils/git.ts
+++ b/packages/coding-agent/src/utils/git.ts
@@ -1430,14 +1430,18 @@ export async function detachGitDir(worktreeRoot: string, sourceCommonDir: string
 		if (isEnoent(err)) return "no-git";
 		throw err;
 	}
-	const parentCommon = path.resolve(sourceCommonDir);
-	const isoCommon = path.resolve(
-		(
-			await runText(worktreeRoot, ["rev-parse", "--path-format=absolute", "--git-common-dir"], {
-				readOnly: true,
-			})
-		).trim(),
-	);
+	// Canonicalize both sides before comparing: `rev-parse` resolves symlinks
+	// (macOS `/tmp` → `/private/tmp`) while callers derive `sourceCommonDir`
+	// lexically from the session cwd. A lexical mismatch here would silently
+	// classify a shared linked-worktree copy as "independent" and skip the
+	// detach entirely — leaving the parent-mutation leak in place.
+	const parentCommon = await fs.promises.realpath(sourceCommonDir).catch(() => path.resolve(sourceCommonDir));
+	const isoCommonRaw = (
+		await runText(worktreeRoot, ["rev-parse", "--path-format=absolute", "--git-common-dir"], {
+			readOnly: true,
+		})
+	).trim();
+	const isoCommon = await fs.promises.realpath(isoCommonRaw).catch(() => path.resolve(isoCommonRaw));
 	// A full-copy `.git` already resolves to its own object DB — leave it alone.
 	if (isoCommon !== parentCommon) return "independent";
 
diff --git a/packages/coding-agent/test/task/worktree.test.ts b/packages/coding-agent/test/task/worktree.test.ts
index 0d0dcf8e0..b75fd35ba 100644
--- a/packages/coding-agent/test/task/worktree.test.ts
+++ b/packages/coding-agent/test/task/worktree.test.ts
@@ -753,6 +753,31 @@ describe("detachGitDir", () => {
 		expect((await runGit(iso, ["rev-list", "HEAD"])).split("\n")).toHaveLength(1);
 	});
 
+	it("detaches when sourceCommonDir is reached through a symlinked path", async () => {
+		const { wt, commonDir, baseSha } = await makeLinkedWorktree();
+		// Alias the main checkout through a symlink and hand detachGitDir the
+		// lexical (un-canonicalized) common dir — the shape ensureIsolation
+		// produces when the session cwd traverses a symlink (macOS /tmp,
+		// symlinked project dirs). The shared-common-dir gate must still match,
+		// or the detach silently no-ops and the parent leak survives.
+		const aliasBase = await fs.mkdtemp(path.join(os.tmpdir(), "omp-detach-alias-"));
+		tempDirs.push(aliasBase);
+		const aliasMain = path.join(aliasBase, "main-link");
+		await fs.symlink(path.dirname(commonDir), aliasMain);
+		const aliasCommonDir = path.join(aliasMain, ".git");
+
+		const iso = await copyTree(wt);
+		expect(await git.detachGitDir(iso, aliasCommonDir)).toBe("detached");
+
+		// Isolation is fully functional: task branch + commit stay private.
+		await runGit(iso, ["checkout", "-q", "-b", "feature/a", baseSha]);
+		await fs.writeFile(path.join(iso, "a.txt"), "task a\n");
+		await runGit(iso, ["add", "a.txt"]);
+		await runGit(iso, ["commit", "-q", "-m", "task a"]);
+		expect(await runGit(wt, ["rev-parse", "--abbrev-ref", "HEAD"])).toBe("feature/parent");
+		expect(await runGit(wt, ["branch", "--format=%(refname:short)"])).not.toContain("feature/a");
+	});
+
 	it("keeps ensureIsolation from mutating a linked-worktree parent (rcopy backend)", async () => {
 		const { wt, baseSha } = await makeLinkedWorktree();
 		vi.spyOn(natives, "isoResolve").mockReturnValue({