diff --git a/packages/core/src/__tests__/git-repository.test.ts b/packages/core/src/__tests__/git-repository.test.ts
index bb0cff60ca..973109b6b4 100644
--- a/packages/core/src/__tests__/git-repository.test.ts
+++ b/packages/core/src/__tests__/git-repository.test.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { execFile } from "node:child_process";
-import { existsSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { existsSync, mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { promisify } from "node:util";
@@ -107,4 +107,17 @@ describe("ensureGitRepositoryForProjectPath", () => {
       ensureGitRepositoryForProjectPath(projectPath, { runner }),
     ).rejects.toBeInstanceOf(GitRepositoryInitializationError);
   });
+
+  it("skips git init for a workspace-mode project root (.fusion/workspace.json present)", async () => {
+    const projectPath = tempDir("fusion-git-workspace-");
+    // Simulate workspace init: .fusion/workspace.json exists, root is non-git
+    mkdirSync(join(projectPath, ".fusion"), { recursive: true });
+    writeFileSync(join(projectPath, ".fusion", "workspace.json"), JSON.stringify({ repos: ["repo-a"] }));
+
+    const outcome = await ensureGitRepositoryForProjectPath(projectPath);
+
+    expect(outcome).toBe("existing");
+    // No .git should be created at the workspace root
+    expect(existsSync(join(projectPath, ".git"))).toBe(false);
+  });
 });
diff --git a/packages/core/src/git-repository.ts b/packages/core/src/git-repository.ts
index adaebeb2ed..2199bec314 100644
--- a/packages/core/src/git-repository.ts
+++ b/packages/core/src/git-repository.ts
@@ -41,6 +41,20 @@ export async function ensureGitRepositoryForProjectPath(
   const runner = options.runner ?? runGitCommand;
   const timeout = options.timeoutMs ?? DEFAULT_GIT_TIMEOUT_MS;
 
+  /*
+  FNXC:Workspace 2026-06-24-10:00:
+  A workspace-mode project root is intentionally NOT a git repository — it is a parent
+  directory containing multiple git sub-repos (detected at init time and recorded in
+  .fusion/workspace.json). Running `git init` here would create a stray empty repo at the
+  workspace root, poisoning every downstream git command: the executor sets the session cwd
+  to this root (browse-only), and `git rev-parse --abbrev-ref HEAD` fails on the unborn HEAD
+  with "ambiguous argument 'HEAD'". Detect workspace mode via the config file and skip the
+  git-init so the root stays non-git, matching the workspace execution contract (KTD1).
+  */
+  if (await loadWorkspaceConfig(projectPath)) {
+    return "existing";
+  }
+
   if (await isInsideGitWorkTree(projectPath, runner, timeout)) {
     return "existing";
   }