fix: make git-checkout 'branch' optional to allow creating branches from HEAD (#109)

Agent-Logs-Url: https://github.com/huberp/agentloop/sessions/7eb12d9e-a4bd-4d06-af62-5f99a864ef42

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: huberp <4027454+huberp@users.noreply.github.com>

Author: Copilot

src/__tests__/git-tools.test.ts

@@ -533,6 +533,22 @@ describe("git-checkout — create and switch (newBranch)", () => {
     const afterStatus = await git.status();
     expect(afterStatus.current).toBe("feat/new");
   });
+
+  it("creates a new branch from HEAD when 'branch' is omitted", async () => {
+    const git = simpleGit(repo);
+
+    const raw = await gitCheckout().execute({
+      newBranch: "feat/from-head",
+      cwd: repo,
+    });
+    const result = JSON.parse(raw);
+
+    expect(result.success).toBe(true);
+    expect(result.branch).toBe("feat/from-head");
+
+    const afterStatus = await git.status();
+    expect(afterStatus.current).toBe("feat/from-head");
+  });
 });
 
 describe("git-checkout — outside a Git repository", () => {

src/tools/git-checkout.ts

@@ -5,13 +5,22 @@ import type { ToolDefinition } from "./registry";
 const schema = z.object({
   branch: z
     .string()
-    .describe("Branch name, tag, or commit hash to check out"),
+    .optional()
+    .describe(
+      "Branch name, tag, or commit hash to check out. " +
+        "Required when switching to an existing branch. " +
+        "When 'newBranch' is also provided this is used as the start-point; " +
+        "omit it to base the new branch on the current HEAD."
+    ),
   newBranch: z
     .string()
     .optional()
     .describe(
-      "When provided, creates a new branch with this name at 'branch' and checks it out. " +
-        "Equivalent to `git checkout -b <newBranch> <branch>`."
+      "When provided, creates a new branch with this name and checks it out. " +
+        "If 'branch' is also provided, the new branch is based on that ref " +
+        "(equivalent to `git checkout -b <newBranch> <branch>`). " +
+        "If 'branch' is omitted, the new branch starts from the current HEAD " +
+        "(equivalent to `git checkout -b <newBranch>`)."
     ),
   cwd: z.string().optional().describe("Repository path (defaults to process.cwd())"),
 });
@@ -27,8 +36,11 @@ export const toolDefinition: ToolDefinition = {
   name: "git-checkout",
   description:
     "Switches the working tree to the given branch, tag, or commit. " +
-    "When 'newBranch' is supplied, creates that branch at the given ref and checks it out " +
-    "(equivalent to `git checkout -b <newBranch> <branch>`). " +
+    "When 'newBranch' is supplied, creates that branch and checks it out. " +
+    "If 'branch' is also provided it is used as the start-point " +
+    "(equivalent to `git checkout -b <newBranch> <branch>`); " +
+    "otherwise the new branch is based on the current HEAD " +
+    "(equivalent to `git checkout -b <newBranch>`). " +
     "Permission: cautious (modifies working-tree state).",
   schema,
   permissions: "cautious",
@@ -37,7 +49,7 @@ export const toolDefinition: ToolDefinition = {
     newBranch,
     cwd,
   }: {
-    branch: string;
+    branch?: string;
     newBranch?: string;
     cwd?: string;
   }): Promise<string> => {
@@ -46,11 +58,23 @@ export const toolDefinition: ToolDefinition = {
       const git = simpleGit(repoPath);
 
       if (newBranch) {
-        // Create and switch to a new branch based on the given ref
-        await git.checkoutBranch(newBranch, branch);
+        if (branch) {
+          // Create and switch to a new branch based on the given ref
+          await git.checkoutBranch(newBranch, branch);
+        } else {
+          // Create and switch to a new branch from the current HEAD
+          await git.checkout(["-b", newBranch]);
+        }
         return JSON.stringify({ success: true, branch: newBranch } as GitCheckoutResult);
       }
 
+      if (!branch) {
+        return JSON.stringify({
+          success: false,
+          error: "'branch' is required when 'newBranch' is not provided",
+        } as GitCheckoutResult);
+      }
+
       // Switch to an existing branch, tag, or commit
       await git.checkout(branch);
       return JSON.stringify({ success: true, branch } as GitCheckoutResult);