diff --git a/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/.openspec.yaml b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/.openspec.yaml
new file mode 100644
index 0000000..1b4051e
--- /dev/null
+++ b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-04-27
diff --git a/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/proposal.md b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/proposal.md
new file mode 100644
index 0000000..fe27b1d
--- /dev/null
+++ b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/proposal.md
@@ -0,0 +1,7 @@
+# Proposal: reuse the active agent worktree on branch start
+
+`gx branch start` currently creates a fresh timestamped branch even when it is invoked from inside an existing `agent/*` worktree. That copies the active sandbox into a nested sandbox and splits follow-up work away from the lane the user selected.
+
+- reuse the current `agent/*` worktree by default when `branch start` runs inside it
+- keep an explicit `--new` / `--no-reuse` escape hatch for intentional child lanes
+- preserve downstream parser compatibility for reused branch-start output
diff --git a/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/specs/reuse-existing-agent-worktree-on-repeate/spec.md b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/specs/reuse-existing-agent-worktree-on-repeate/spec.md
new file mode 100644
index 0000000..6a72f1f
--- /dev/null
+++ b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/specs/reuse-existing-agent-worktree-on-repeate/spec.md
@@ -0,0 +1,19 @@
+## ADDED Requirements
+
+### Requirement: Branch start reuses current agent worktree
+When `gx branch start` runs from a worktree whose current branch starts with `agent/`, the command SHALL reuse that existing branch and worktree by default instead of creating another timestamped branch/worktree from it.
+
+#### Scenario: Follow-up agent starts inside an existing sandbox
+- **GIVEN** the current working tree is an agent worktree on branch `agent/codex/example`
+- **WHEN** `gx branch start "continue work" "codex"` is run from that worktree
+- **THEN** the command reports `Reusing existing branch: agent/codex/example`
+- **AND** the reported worktree path is the current worktree
+- **AND** no nested agent worktree is created.
+
+### Requirement: Branch start keeps an explicit new-lane escape hatch
+When a caller intentionally needs a child or parallel lane from inside an existing agent worktree, the command SHALL provide an explicit option to bypass reuse and create a new branch/worktree.
+
+#### Scenario: Caller opts out of reuse
+- **GIVEN** the current working tree is an agent worktree
+- **WHEN** `gx branch start --new "parallel work" "codex"` is run
+- **THEN** the command may create a new isolated branch/worktree using the existing startup behavior.
diff --git a/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/tasks.md b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/tasks.md
new file mode 100644
index 0000000..add4047
--- /dev/null
+++ b/openspec/changes/agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17/tasks.md
@@ -0,0 +1,32 @@
+## 1. Spec
+
+- [x] 1.1 Capture why branch start inside an active agent sandbox should attach instead of cloning the sandbox.
+- [x] 1.2 Define the reuse behavior and explicit new-lane escape hatch.
+
+## 2. Implementation
+
+- [x] 2.1 Update branch-start runtime and template scripts to reuse the current `agent/*` worktree by default.
+- [x] 2.2 Keep parser surfaces compatible with reused branch-start output.
+- [x] 2.3 Add focused regression coverage for the existing-worktree reuse path.
+
+## 3. Verification
+
+- [x] 3.1 Run targeted branch-start regression tests.
+- [x] 3.2 Run template parity and script syntax checks.
+- [x] 3.3 Run `openspec validate agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17 --type change --strict`.
+- [x] 3.4 Run `openspec validate --specs`.
+
+Verification evidence:
+- `node --test test/branch.test.js` (pass, 30/30)
+- `node --test test/metadata.test.js` (pass, 24/24)
+- `npm test` (pass, 288 passed, 1 skipped)
+- `bash -n scripts/agent-branch-start.sh`, `bash -n templates/scripts/agent-branch-start.sh`, `bash -n scripts/agent-branch-merge.sh`, `bash -n templates/scripts/agent-branch-merge.sh` (pass)
+- `git diff --check` (pass)
+- `openspec validate agent-codex-reuse-existing-agent-worktree-on-repeate-2026-04-27-18-17 --type change --strict` (pass)
+- `openspec validate --specs` (pass; no spec items found)
+
+## 4. Cleanup
+
+- [ ] 4.1 Commit, push, open/update PR, merge, and clean up the worktree.
+- [ ] 4.2 Record PR URL + final `MERGED` state in the completion handoff.
+- [ ] 4.3 Confirm the sandbox worktree is gone.
diff --git a/scripts/agent-branch-merge.sh b/scripts/agent-branch-merge.sh
index ac47f6d..f3192b5 100755
--- a/scripts/agent-branch-merge.sh
+++ b/scripts/agent-branch-merge.sh
@@ -288,7 +288,7 @@ if [[ -z "$TARGET_BRANCH" ]]; then
   fi
 
   printf '%s\n' "$start_output"
-  TARGET_BRANCH="$(printf '%s\n' "$start_output" | sed -n 's/^\[agent-branch-start\] Created branch: //p' | head -n 1)"
+  TARGET_BRANCH="$(printf '%s\n' "$start_output" | sed -n -E 's/^\[agent-branch-start\] (Created branch|Reusing existing branch): //p' | head -n 1)"
   target_worktree="$(printf '%s\n' "$start_output" | sed -n 's/^\[agent-branch-start\] Worktree: //p' | head -n 1)"
   if [[ -z "$TARGET_BRANCH" || -z "$target_worktree" ]]; then
     echo "[agent-branch-merge] Unable to parse target branch/worktree from agent-branch-start output." >&2
diff --git a/scripts/agent-branch-start.sh b/scripts/agent-branch-start.sh
index bfeab2f..da4f484 100755
--- a/scripts/agent-branch-start.sh
+++ b/scripts/agent-branch-start.sh
@@ -15,6 +15,7 @@ OPENSPEC_CHANGE_SLUG_OVERRIDE="${GUARDEX_OPENSPEC_CHANGE_SLUG:-}"
 OPENSPEC_CAPABILITY_SLUG_OVERRIDE="${GUARDEX_OPENSPEC_CAPABILITY_SLUG:-}"
 OPENSPEC_MASTERPLAN_LABEL_RAW="${GUARDEX_OPENSPEC_MASTERPLAN_LABEL-masterplan}"
 OPENSPEC_TIER_RAW="${GUARDEX_OPENSPEC_TIER:-T3}"
+REUSE_EXISTING_RAW="${GUARDEX_BRANCH_START_REUSE_EXISTING:-true}"
 PRINT_NAME_ONLY=0
 POSITIONAL_ARGS=()
 
@@ -58,6 +59,14 @@ while [[ $# -gt 0 ]]; do
       OPENSPEC_TIER_RAW="${2:-$OPENSPEC_TIER_RAW}"
       shift 2
       ;;
+    --reuse-existing|--reuse)
+      REUSE_EXISTING_RAW="true"
+      shift
+      ;;
+    --new|--no-reuse|--no-reuse-existing)
+      REUSE_EXISTING_RAW="false"
+      shift
+      ;;
     --in-place|--allow-in-place)
       echo "[agent-branch-start] In-place branch mode is disabled." >&2
       echo "[agent-branch-start] This command always creates an isolated worktree to keep your active checkout unchanged." >&2
@@ -78,7 +87,7 @@ while [[ $# -gt 0 ]]; do
       ;;
     -*)
       echo "[agent-branch-start] Unknown option: $1" >&2
-      echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--print-name-only]" >&2
+      echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--reuse-existing|--new] [--print-name-only]" >&2
       exit 1
       ;;
     *)
@@ -90,7 +99,7 @@ done
 
 if [[ "${#POSITIONAL_ARGS[@]}" -gt 3 ]]; then
   echo "[agent-branch-start] Too many positional arguments." >&2
-  echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>]" >&2
+  echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--reuse-existing|--new]" >&2
   exit 1
 fi
 
@@ -254,6 +263,7 @@ normalize_bool() {
 }
 
 OPENSPEC_AUTO_INIT="$(normalize_bool "$OPENSPEC_AUTO_INIT_RAW" "1")"
+REUSE_EXISTING_WORKTREE="$(normalize_bool "$REUSE_EXISTING_RAW" "1")"
 
 normalize_tier() {
   local raw="${1:-}"
@@ -370,6 +380,22 @@ resolve_worktree_leaf() {
   printf '%s' "${branch_name//\//__}"
 }
 
+print_reused_agent_worktree() {
+  local branch_name="$1"
+  local worktree_path="$2"
+
+  echo "[agent-branch-start] Reusing existing branch: ${branch_name}"
+  echo "[agent-branch-start] Worktree: ${worktree_path}"
+  echo "[agent-branch-start] OpenSpec tier: ${OPENSPEC_TIER}"
+  echo "[agent-branch-start] OpenSpec change: existing worktree"
+  echo "[agent-branch-start] OpenSpec plan: existing worktree"
+  echo "[agent-branch-start] Next steps:"
+  echo "  cd \"${worktree_path}\""
+  echo "  gx locks claim --branch \"${branch_name}\" <file...>"
+  echo "  # continue work in this existing sandbox"
+  echo "  gx branch finish --branch \"${branch_name}\" --via-pr --wait-for-merge"
+}
+
 has_local_changes() {
   local root="$1"
   if ! git -C "$root" diff --quiet; then
@@ -550,6 +576,12 @@ if [[ "$BASE_BRANCH_EXPLICIT" -eq 1 && -z "$BASE_BRANCH" ]]; then
   exit 1
 fi
 
+current_branch="$(git -C "$repo_root" rev-parse --abbrev-ref HEAD 2>/dev/null || true)"
+if [[ "$REUSE_EXISTING_WORKTREE" -eq 1 && "$current_branch" == agent/* ]]; then
+  print_reused_agent_worktree "$current_branch" "$repo_root"
+  exit 0
+fi
+
 task_slug="$(sanitize_slug "$TASK_NAME" "task")"
 agent_slug="$(normalize_role "$AGENT_NAME")"
 if [[ "$WORKTREE_ROOT_EXPLICIT" -eq 0 ]]; then
diff --git a/src/cli/main.js b/src/cli/main.js
index 7952faf..2bb68a1 100755
--- a/src/cli/main.js
+++ b/src/cli/main.js
@@ -408,8 +408,9 @@ function runSetupBootstrapInternal(options) {
 }
 
 function extractAgentBranchStartMetadata(output) {
-  const branchMatch = String(output || '').match(/^\[agent-branch-start\] Created branch: (.+)$/m);
-  const worktreeMatch = String(output || '').match(/^\[agent-branch-start\] Worktree: (.+)$/m);
+  const outputText = String(output || '');
+  const branchMatch = outputText.match(/^\[agent-branch-start\] (?:Created branch|Reusing existing branch): (.+)$/m);
+  const worktreeMatch = outputText.match(/^\[agent-branch-start\] Worktree: (.+)$/m);
   return {
     branch: branchMatch ? branchMatch[1].trim() : '',
     worktreePath: worktreeMatch ? worktreeMatch[1].trim() : '',
@@ -3466,7 +3467,7 @@ function pivot(rawArgs) {
   }
   const stdoutText = String(result.stdout || '');
   const wtMatch = stdoutText.match(/^\[agent-branch-start\] Worktree:\s+(.+)$/m);
-  const branchMatch = stdoutText.match(/^\[agent-branch-start\] Created branch:\s+(.+)$/m);
+  const branchMatch = stdoutText.match(/^\[agent-branch-start\] (?:Created branch|Reusing existing branch):\s+(.+)$/m);
   if (wtMatch) {
     const wtPath = wtMatch[1].trim();
     process.stdout.write('\n');
diff --git a/src/sandbox/index.js b/src/sandbox/index.js
index 3fe15fd..0e665ab 100644
--- a/src/sandbox/index.js
+++ b/src/sandbox/index.js
@@ -65,8 +65,9 @@ function assertProtectedMainWriteAllowed(options, commandName) {
 }
 
 function extractAgentBranchStartMetadata(output) {
-  const branchMatch = String(output || '').match(/^\[agent-branch-start\] Created branch: (.+)$/m);
-  const worktreeMatch = String(output || '').match(/^\[agent-branch-start\] Worktree: (.+)$/m);
+  const outputText = String(output || '');
+  const branchMatch = outputText.match(/^\[agent-branch-start\] (?:Created branch|Reusing existing branch): (.+)$/m);
+  const worktreeMatch = outputText.match(/^\[agent-branch-start\] Worktree: (.+)$/m);
   return {
     branch: branchMatch ? branchMatch[1].trim() : '',
     worktreePath: worktreeMatch ? worktreeMatch[1].trim() : '',
diff --git a/templates/scripts/agent-branch-merge.sh b/templates/scripts/agent-branch-merge.sh
index ac47f6d..f3192b5 100755
--- a/templates/scripts/agent-branch-merge.sh
+++ b/templates/scripts/agent-branch-merge.sh
@@ -288,7 +288,7 @@ if [[ -z "$TARGET_BRANCH" ]]; then
   fi
 
   printf '%s\n' "$start_output"
-  TARGET_BRANCH="$(printf '%s\n' "$start_output" | sed -n 's/^\[agent-branch-start\] Created branch: //p' | head -n 1)"
+  TARGET_BRANCH="$(printf '%s\n' "$start_output" | sed -n -E 's/^\[agent-branch-start\] (Created branch|Reusing existing branch): //p' | head -n 1)"
   target_worktree="$(printf '%s\n' "$start_output" | sed -n 's/^\[agent-branch-start\] Worktree: //p' | head -n 1)"
   if [[ -z "$TARGET_BRANCH" || -z "$target_worktree" ]]; then
     echo "[agent-branch-merge] Unable to parse target branch/worktree from agent-branch-start output." >&2
diff --git a/templates/scripts/agent-branch-start.sh b/templates/scripts/agent-branch-start.sh
index bfeab2f..da4f484 100755
--- a/templates/scripts/agent-branch-start.sh
+++ b/templates/scripts/agent-branch-start.sh
@@ -15,6 +15,7 @@ OPENSPEC_CHANGE_SLUG_OVERRIDE="${GUARDEX_OPENSPEC_CHANGE_SLUG:-}"
 OPENSPEC_CAPABILITY_SLUG_OVERRIDE="${GUARDEX_OPENSPEC_CAPABILITY_SLUG:-}"
 OPENSPEC_MASTERPLAN_LABEL_RAW="${GUARDEX_OPENSPEC_MASTERPLAN_LABEL-masterplan}"
 OPENSPEC_TIER_RAW="${GUARDEX_OPENSPEC_TIER:-T3}"
+REUSE_EXISTING_RAW="${GUARDEX_BRANCH_START_REUSE_EXISTING:-true}"
 PRINT_NAME_ONLY=0
 POSITIONAL_ARGS=()
 
@@ -58,6 +59,14 @@ while [[ $# -gt 0 ]]; do
       OPENSPEC_TIER_RAW="${2:-$OPENSPEC_TIER_RAW}"
       shift 2
       ;;
+    --reuse-existing|--reuse)
+      REUSE_EXISTING_RAW="true"
+      shift
+      ;;
+    --new|--no-reuse|--no-reuse-existing)
+      REUSE_EXISTING_RAW="false"
+      shift
+      ;;
     --in-place|--allow-in-place)
       echo "[agent-branch-start] In-place branch mode is disabled." >&2
       echo "[agent-branch-start] This command always creates an isolated worktree to keep your active checkout unchanged." >&2
@@ -78,7 +87,7 @@ while [[ $# -gt 0 ]]; do
       ;;
     -*)
       echo "[agent-branch-start] Unknown option: $1" >&2
-      echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--print-name-only]" >&2
+      echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--reuse-existing|--new] [--print-name-only]" >&2
       exit 1
       ;;
     *)
@@ -90,7 +99,7 @@ done
 
 if [[ "${#POSITIONAL_ARGS[@]}" -gt 3 ]]; then
   echo "[agent-branch-start] Too many positional arguments." >&2
-  echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>]" >&2
+  echo "Usage: $0 [task] [agent] [base] [--worktree-root <path>] [--reuse-existing|--new]" >&2
   exit 1
 fi
 
@@ -254,6 +263,7 @@ normalize_bool() {
 }
 
 OPENSPEC_AUTO_INIT="$(normalize_bool "$OPENSPEC_AUTO_INIT_RAW" "1")"
+REUSE_EXISTING_WORKTREE="$(normalize_bool "$REUSE_EXISTING_RAW" "1")"
 
 normalize_tier() {
   local raw="${1:-}"
@@ -370,6 +380,22 @@ resolve_worktree_leaf() {
   printf '%s' "${branch_name//\//__}"
 }
 
+print_reused_agent_worktree() {
+  local branch_name="$1"
+  local worktree_path="$2"
+
+  echo "[agent-branch-start] Reusing existing branch: ${branch_name}"
+  echo "[agent-branch-start] Worktree: ${worktree_path}"
+  echo "[agent-branch-start] OpenSpec tier: ${OPENSPEC_TIER}"
+  echo "[agent-branch-start] OpenSpec change: existing worktree"
+  echo "[agent-branch-start] OpenSpec plan: existing worktree"
+  echo "[agent-branch-start] Next steps:"
+  echo "  cd \"${worktree_path}\""
+  echo "  gx locks claim --branch \"${branch_name}\" <file...>"
+  echo "  # continue work in this existing sandbox"
+  echo "  gx branch finish --branch \"${branch_name}\" --via-pr --wait-for-merge"
+}
+
 has_local_changes() {
   local root="$1"
   if ! git -C "$root" diff --quiet; then
@@ -550,6 +576,12 @@ if [[ "$BASE_BRANCH_EXPLICIT" -eq 1 && -z "$BASE_BRANCH" ]]; then
   exit 1
 fi
 
+current_branch="$(git -C "$repo_root" rev-parse --abbrev-ref HEAD 2>/dev/null || true)"
+if [[ "$REUSE_EXISTING_WORKTREE" -eq 1 && "$current_branch" == agent/* ]]; then
+  print_reused_agent_worktree "$current_branch" "$repo_root"
+  exit 0
+fi
+
 task_slug="$(sanitize_slug "$TASK_NAME" "task")"
 agent_slug="$(normalize_role "$AGENT_NAME")"
 if [[ "$WORKTREE_ROOT_EXPLICIT" -eq 0 ]]; then
diff --git a/test/branch.test.js b/test/branch.test.js
index beb87b5..05194ab 100644
--- a/test/branch.test.js
+++ b/test/branch.test.js
@@ -117,6 +117,38 @@ test('agent-branch-start prefers current protected branch over stale configured
 });
 
 
+test('agent-branch-start reuses the current agent worktree instead of cloning it', () => {
+  const { repoDir } = createBootstrappedRepo({ committed: true });
+
+  let result = runBranchStart(['--tier', 'T1', 'rust repair snapshot selection', 'bot'], repoDir, {
+    GUARDEX_OPENSPEC_AUTO_INIT: 'true',
+  });
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+  const firstBranch = extractCreatedBranch(result.stdout);
+  const firstWorktree = extractCreatedWorktree(result.stdout);
+
+  result = runBranchStart(['--tier', 'T1', 'continue rust worktree', 'bot'], firstWorktree, {
+    GUARDEX_OPENSPEC_AUTO_INIT: 'true',
+  });
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+  assert.match(result.stdout, new RegExp(`Reusing existing branch: ${escapeRegexLiteral(firstBranch)}`));
+  assert.equal(extractCreatedWorktree(result.stdout), firstWorktree);
+  assert.equal(
+    fs.existsSync(path.join(firstWorktree, '.omx', 'agent-worktrees')),
+    false,
+    'branch start inside an agent worktree must not create nested worktrees',
+  );
+
+  const worktreeList = runCmd('git', ['worktree', 'list', '--porcelain'], repoDir);
+  assert.equal(worktreeList.status, 0, worktreeList.stderr || worktreeList.stdout);
+  assert.equal(
+    (worktreeList.stdout.match(/^branch refs\/heads\/agent\//gm) || []).length,
+    1,
+    'only the original agent branch should remain registered',
+  );
+});
+
+
 test('agent-branch-start moves protected-branch local changes into the new agent worktree', () => {
   const repoDir = initRepoOnBranch('main');
   seedCommit(repoDir);