diff --git a/.changeset/silly-finches-travel.md b/.changeset/silly-finches-travel.md
new file mode 100644
index 000000000..635103460
--- /dev/null
+++ b/.changeset/silly-finches-travel.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3563
+---
+**Ultraplan runtime gate now detects Claude Code correctly** - runtime gating no longer relies on `CLAUDE_CODE_VERSION`; it uses Claude Code marker env vars and preserves the minimum supported version floor. Fixes #3561.
diff --git a/get-shit-done/workflows/ultraplan-phase.md b/get-shit-done/workflows/ultraplan-phase.md
index a6a448c50..d8c12db79 100644
--- a/get-shit-done/workflows/ultraplan-phase.md
+++ b/get-shit-done/workflows/ultraplan-phase.md
@@ -29,7 +29,16 @@ Use /gsd:plan-phase for stable local planning.
 Check that the session is running inside Claude Code:
 
 ```bash
-echo "$CLAUDE_CODE_VERSION"
+if [ "$CLAUDECODE" = "1" ] || [ -n "$CLAUDE_CODE_ENTRYPOINT" ]; then
+  CC_VERSION="$(claude --version 2>/dev/null | grep -Eo '[0-9]+\.[0-9]+\.[0-9]+' | head -n1)"
+  if [ -n "$CC_VERSION" ] && [ "$(printf '%s\n' "2.1.91" "$CC_VERSION" | sort -V | head -n1)" = "2.1.91" ]; then
+    echo "claude-code:${CC_VERSION}"
+  else
+    echo ""
+  fi
+else
+  echo ""
+fi
 ```
 
 If the output is empty or unset, display the following error and exit:
diff --git a/tests/ultraplan-phase.test.cjs b/tests/ultraplan-phase.test.cjs
index bcb799742..2ea6ec310 100644
--- a/tests/ultraplan-phase.test.cjs
+++ b/tests/ultraplan-phase.test.cjs
@@ -86,8 +86,15 @@ describe('ultraplan-phase workflow beta marker', () => {
 describe('ultraplan-phase workflow runtime gate', () => {
   const content = fs.readFileSync(WF_PATH, 'utf-8');
 
-  test('checks CLAUDE_CODE_VERSION to detect Claude Code runtime', () => {
-    assert.ok(content.includes('CLAUDE_CODE_VERSION'), 'workflow must gate on CLAUDE_CODE_VERSION env var');
+  test('checks Claude Code runtime markers instead of version env var', () => {
+    assert.ok(
+      content.includes('CLAUDECODE') || content.includes('CLAUDE_CODE_ENTRYPOINT'),
+      'workflow must gate on Claude Code runtime marker env vars'
+    );
+    assert.ok(
+      !content.includes('CLAUDE_CODE_VERSION'),
+      'workflow must not gate on CLAUDE_CODE_VERSION'
+    );
   });
 
   test('error message references /gsd-plan-phase as local alternative', () => {