fix: clear error when new branch name collides with existing branch namespace

src/commands/worktree/switch.rs

@@ -1031,6 +1031,21 @@ fn execute_switch(
                         Repository::SLOW_OPERATION_DELAY_MS,
                         progress_msg,
                     ) {
+                        // A new branch whose name is a path prefix of (or sits
+                        // under) an existing branch can't be created: git stores
+                        // refs as file paths, so `release` and `release/2026.4`
+                        // can't coexist. Surface that as a clear, actionable
+                        // error instead of git's raw "cannot lock ref" text.
+                        if *create_branch
+                            && let Some(conflicting) =
+                                detect_branch_namespace_conflict(repo, &branch)
+                        {
+                            return Err(GitError::BranchNamespaceConflict {
+                                branch: branch.clone(),
+                                conflicting,
+                            }
+                            .into());
+                        }
                         return Err(worktree_creation_error(
                             &e,
                             branch.clone(),
@@ -1209,6 +1224,28 @@ fn execute_switch(
     }
 }
 
+/// Detect a git ref directory/file (D/F) conflict for a branch about to be
+/// created, returning an existing branch it collides with.
+///
+/// Git stores refs as file paths under `refs/heads/`, so a branch name can't
+/// be both a file and a directory: creating `release` fails when
+/// `release/2026.4` exists, and creating `release/foo` fails when `release`
+/// exists. This inspects the cached local-branch inventory (no extra
+/// subprocess) for either shape and returns the first colliding branch.
+fn detect_branch_namespace_conflict(repo: &Repository, branch: &str) -> Option<String> {
+    let prefix = format!("{branch}/");
+    repo.local_branches()
+        .ok()?
+        .iter()
+        .map(|b| b.name.as_str())
+        .find(|name| {
+            // `branch` is a directory prefix of an existing branch, or an
+            // existing branch is a directory prefix of `branch`.
+            name.starts_with(&prefix) || branch.starts_with(&format!("{name}/"))
+        })
+        .map(String::from)
+}
+
 /// Build a `GitError::WorktreeCreationFailed` from a failed `git worktree add`,
 /// extracting the underlying command output for the error message.
 fn worktree_creation_error(

src/git/error.rs

@@ -463,6 +463,16 @@ pub enum GitError {
         /// The git command that failed, shown separately from git output
         command: Option<FailedCommand>,
     },
+    /// A new branch can't be created because its name collides with the
+    /// directory namespace of an existing branch. Git stores refs as file
+    /// paths, so `release` and `release/2026.4` can't both exist — one would
+    /// have to be a file and a directory at the same path.
+    BranchNamespaceConflict {
+        /// The branch the user tried to create.
+        branch: String,
+        /// An existing branch whose name collides with `branch`.
+        conflicting: String,
+    },
     WorktreeRemovalFailed {
         branch: String,
         path: PathBuf,
@@ -668,6 +678,13 @@ impl GitError {
                 None => cformat!("Failed to create worktree for <bold>{branch}</>"),
             },
 
+            GitError::BranchNamespaceConflict {
+                branch,
+                conflicting,
+            } => cformat!(
+                "Cannot create branch <bold>{branch}</> — it collides with existing branch <bold>{conflicting}</>"
+            ),
+
             GitError::WorktreeRemovalFailed { branch, path, .. } => {
                 let path_display = format_path_for_display(path);
                 cformat!(
@@ -1008,6 +1025,21 @@ impl GitError {
                 Ok(())
             }
 
+            GitError::BranchNamespaceConflict {
+                branch,
+                conflicting,
+            } => {
+                let title = self.title();
+                write!(
+                    f,
+                    "{}\n{}",
+                    error_message(&title),
+                    hint_message(cformat!(
+                        "Git stores branches as file paths, so <underline>{branch}</> and <underline>{conflicting}</> can't both exist. Pick a different name, or rename the existing branch."
+                    ))
+                )
+            }
+
             GitError::WorktreeRemovalFailed {
                 error,
                 remaining_entries,

tests/integration_tests/switch.rs

@@ -130,6 +130,23 @@ fn test_switch_create_existing_branch_error(mut repo: TestRepo) {
     );
 }
 
+/// Creating a branch whose name is a path prefix of an existing branch is a git
+/// ref directory/file conflict: `release` can't be created while `release/2026.4`
+/// exists. Regression for #3527 — surface a clear namespace-collision error
+/// instead of git's raw "cannot lock ref" text.
+#[rstest]
+fn test_switch_create_branch_namespace_conflict(repo: TestRepo) {
+    // An existing branch living under the `release/` namespace.
+    repo.run_git(&["branch", "release/2026.4"]);
+
+    // `release` can't be both a branch and a directory of branches.
+    snapshot_switch(
+        "switch_create_namespace_conflict",
+        &repo,
+        &["--create", "release"],
+    );
+}
+
 /// When --execute is passed and the branch already exists, the error hint should
 /// include --execute and trailing args in the suggested command.
 #[rstest]

tests/snapshots/integration__integration_tests__switch__switch_create_namespace_conflict.snap

@@ -0,0 +1,53 @@
+---
+source: tests/integration_tests/switch.rs
+info:
+  program: wt
+  args:
+    - switch
+    - "--create"
+    - release
+  env:
+    APPDATA: "[TEST_CONFIG_HOME]"
+    CLAUDE_CONFIG_DIR: "[TEST_CLAUDE_CONFIG]"
+    CLICOLOR_FORCE: "1"
+    COLUMNS: "500"
+    GIT_AUTHOR_DATE: "2025-01-01T00:00:00Z"
+    GIT_COMMITTER_DATE: "2025-01-01T00:00:00Z"
+    GIT_CONFIG_GLOBAL: "[TEST_GIT_CONFIG]"
+    GIT_CONFIG_SYSTEM: /dev/null
+    GIT_TERMINAL_PROMPT: "0"
+    HOME: "[TEST_HOME]"
+    LANG: C
+    LC_ALL: C
+    LLVM_PROFILE_FILE: "[LLVM_PROFILE_FILE]"
+    MOCK_CONFIG_DIR: "[MOCK_CONFIG_DIR]"
+    OPENCODE_CONFIG_DIR: "[TEST_OPENCODE_CONFIG]"
+    PATH: "[PATH]"
+    TERM: alacritty
+    USERPROFILE: "[TEST_HOME]"
+    WORKTRUNK_APPROVALS_PATH: "[TEST_APPROVALS]"
+    WORKTRUNK_CONFIG_PATH: "[TEST_CONFIG]"
+    WORKTRUNK_SYSTEM_CONFIG_PATH: "[TEST_SYSTEM_CONFIG]"
+    WORKTRUNK_TEST_BASH_INSTALLED: "0"
+    WORKTRUNK_TEST_CLAUDE_INSTALLED: "0"
+    WORKTRUNK_TEST_CODEX_INSTALLED: "0"
+    WORKTRUNK_TEST_DELAYED_STREAM_MS: "-1"
+    WORKTRUNK_TEST_EPOCH: "1735776000"
+    WORKTRUNK_TEST_FISH_INSTALLED: "0"
+    WORKTRUNK_TEST_GEMINI_INSTALLED: "0"
+    WORKTRUNK_TEST_NUSHELL_ENV: "0"
+    WORKTRUNK_TEST_OPENCODE_INSTALLED: "0"
+    WORKTRUNK_TEST_PARENT_SHELL: ""
+    WORKTRUNK_TEST_POWERSHELL_ENV: "0"
+    WORKTRUNK_TEST_POWERSHELL_INSTALLED: "0"
+    WORKTRUNK_TEST_SKIP_URL_HEALTH_CHECK: "1"
+    WORKTRUNK_TEST_ZSH_INSTALLED: "0"
+    XDG_CONFIG_HOME: "[TEST_CONFIG_HOME]"
+---
+success: false
+exit_code: 1
+----- stdout -----
+
+----- stderr -----
+✗ Cannot create branch release — it collides with existing branch release/2026.4
+↳ Git stores branches as file paths, so release and release/2026.4 can't both exist. Pick a different name, or rename the existing branch.