diff --git a/.changeset/503-antigravity-agent-local-detection.md b/.changeset/503-antigravity-agent-local-detection.md
new file mode 100644
index 000000000..3d723b039
--- /dev/null
+++ b/.changeset/503-antigravity-agent-local-detection.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 512
+---
+**`/gsd:update` now detects local Antigravity (`.agent`) installs** — local Antigravity installs live in `./.agent/`, but the update detection cascade in `update.md` only knew the global `.gemini/antigravity{,-ide,-cli}` layout, so a local install was misclassified as `claude` and refreshed Claude artifacts instead. `.agent` → antigravity is now mapped across all four runtime-dir surfaces (path classifier, `RUNTIME_DIRS`, local-scope discovery loop, and the post-update cache-clear loop). (#503)
diff --git a/get-shit-done/workflows/update.md b/get-shit-done/workflows/update.md
index 3246f4f6f..22ab36fdb 100644
--- a/get-shit-done/workflows/update.md
+++ b/get-shit-done/workflows/update.md
@@ -17,6 +17,7 @@ First, derive `PREFERRED_CONFIG_DIR` and `PREFERRED_RUNTIME` from the invoking p
 - Path contains `/.gemini/antigravity-ide/` -> `antigravity`
 - Path contains `/.gemini/antigravity-cli/` -> `antigravity`
 - Path contains `/.gemini/antigravity/` -> `antigravity`
+- Path contains `/.agent/` -> `antigravity` (local Antigravity install dir; see bin/install.js getDirName('antigravity'))
 - Path contains `/.gemini/` -> `gemini`
 - Path contains `/.config/kilo/` or `/.kilo/`, or `PREFERRED_CONFIG_DIR` contains `kilo.json` / `kilo.jsonc` -> `kilo`
 - Path contains `/.config/opencode/` or `/.opencode/`, or `PREFERRED_CONFIG_DIR` contains `opencode.json` / `opencode.jsonc` -> `opencode`
@@ -39,7 +40,7 @@ expand_home() {
 # Using an array instead of a space-separated string ensures correct
 # iteration in both bash and zsh (zsh does not word-split unquoted
 # variables by default). Fixes #1173.
-RUNTIME_DIRS=( "claude:.claude" "opencode:.config/opencode" "opencode:.opencode" "antigravity:.gemini/antigravity-ide" "antigravity:.gemini/antigravity-cli" "antigravity:.gemini/antigravity" "gemini:.gemini" "kilo:.config/kilo" "kilo:.kilo" "codex:.codex" )
+RUNTIME_DIRS=( "claude:.claude" "opencode:.config/opencode" "opencode:.opencode" "antigravity:.gemini/antigravity-ide" "antigravity:.gemini/antigravity-cli" "antigravity:.gemini/antigravity" "antigravity:.agent" "gemini:.gemini" "kilo:.config/kilo" "kilo:.kilo" "codex:.codex" )
 ENV_RUNTIME_DIRS=()
 
 # PREFERRED_CONFIG_DIR / PREFERRED_RUNTIME should be set from execution_context
@@ -100,7 +101,7 @@ if [ -n "$PREFERRED_CONFIG_DIR" ] && { [ -f "$PREFERRED_CONFIG_DIR/get-shit-done
     printf '%s' "$p"
   }
   normalized_preferred="$(normalize_path "$PREFERRED_CONFIG_DIR")"
-  for dir in .claude .config/opencode .opencode .gemini/antigravity-ide .gemini/antigravity-cli .gemini/antigravity .gemini .config/kilo .kilo .codex; do
+  for dir in .claude .config/opencode .opencode .gemini/antigravity-ide .gemini/antigravity-cli .gemini/antigravity .agent .gemini .config/kilo .kilo .codex; do
     resolved_local="$(cd "./$dir" 2>/dev/null && pwd)"
     normalized_local="$(normalize_path "$resolved_local")"
     if [ -n "$normalized_local" ] && [ "$normalized_local" = "$normalized_preferred" ]; then
@@ -606,7 +607,7 @@ for dir in "${CACHE_DIRS[@]}"; do
   fi
 done
 
-for dir in .claude .config/opencode .opencode .gemini/antigravity-ide .gemini/antigravity-cli .gemini/antigravity .gemini .config/kilo .kilo .codex; do
+for dir in .claude .config/opencode .opencode .gemini/antigravity-ide .gemini/antigravity-cli .gemini/antigravity .agent .gemini .config/kilo .kilo .codex; do
   rm -f "./$dir/cache/gsd-update-check.json"
   rm -f "$HOME/$dir/cache/gsd-update-check.json"
 done
diff --git a/tests/bug-503-update-agent-antigravity-detection.test.cjs b/tests/bug-503-update-agent-antigravity-detection.test.cjs
new file mode 100644
index 000000000..d503c1d30
--- /dev/null
+++ b/tests/bug-503-update-agent-antigravity-detection.test.cjs
@@ -0,0 +1,75 @@
+// allow-test-rule: source-text-is-the-product
+// update.md is a workflow file whose text IS the contract the runtime loads
+// and executes (the embedded bash detection cascade). Asserting on its text
+// tests the deployed behavior. Per CONTRIBUTING.md exception matrix.
+
+/**
+ * Bug #503: /gsd:update misclassifies local Antigravity (.agent) installs as claude
+ *
+ * The installer places a LOCAL Antigravity install in ./.agent/
+ * (bin/install.js: getDirName('antigravity') === '.agent'). But the
+ * /gsd:update detection cascade in get-shit-done/workflows/update.md only
+ * knew the GLOBAL Antigravity layout (.gemini/antigravity{,-ide,-cli}), so a
+ * local .agent install fell through to the `Otherwise -> claude` default and
+ * the update refreshed Claude artifacts instead of the Antigravity install.
+ *
+ * The cascade has three coupled detection surfaces; .agent must be mapped to
+ * antigravity in all three:
+ *   1. the execution_context path classifier,
+ *   2. the RUNTIME_DIRS candidate array,
+ *   3. the LOCAL-scope discovery `for dir in ...` loop.
+ */
+
+const { describe, test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const path = require('path');
+
+const UPDATE_MD = fs.readFileSync(
+  path.join(__dirname, '..', 'get-shit-done', 'workflows', 'update.md'),
+  'utf-8'
+);
+
+describe('/gsd:update detects local Antigravity (.agent) installs (#503)', () => {
+  test('execution_context classifier maps a /.agent/ path to antigravity', () => {
+    // A rule line of the form: Path contains `/.agent/` -> `antigravity`
+    const hasAgentClassifierRule =
+      /\/\.agent\/[^\n]*->[^\n]*antigravity/.test(UPDATE_MD);
+    assert.ok(
+      hasAgentClassifierRule,
+      'update.md classifier must map a `/.agent/` path to the `antigravity` runtime'
+    );
+  });
+
+  test('RUNTIME_DIRS candidate array includes antigravity:.agent', () => {
+    const runtimeDirsLine = UPDATE_MD
+      .split('\n')
+      .find((l) => l.includes('RUNTIME_DIRS=('));
+    assert.ok(runtimeDirsLine, 'RUNTIME_DIRS array must exist in update.md');
+    assert.ok(
+      runtimeDirsLine.includes('antigravity:.agent'),
+      `RUNTIME_DIRS must contain "antigravity:.agent", got: ${runtimeDirsLine}`
+    );
+  });
+
+  test('every runtime-dir `for dir in` loop includes .agent', () => {
+    // Both the LOCAL-scope discovery loop AND the post-update cache-clear loop
+    // enumerate the runtime config dirs as a literal `.claude ... .codex` list.
+    // The same root cause (.agent missing from the runtime-dir list) breaks
+    // detection in the first and leaves a stale update indicator in the second,
+    // so ALL such loops must include .agent.
+    const runtimeDirLoops = UPDATE_MD
+      .split('\n')
+      .filter((l) => /for dir in .*\.claude.*\.codex/.test(l));
+    assert.ok(
+      runtimeDirLoops.length >= 2,
+      `expected at least 2 runtime-dir loops in update.md, found ${runtimeDirLoops.length}`
+    );
+    for (const loop of runtimeDirLoops) {
+      assert.ok(
+        /(^|\s)\.agent(\s|$)/.test(loop),
+        `every runtime-dir loop must include .agent, got: ${loop.trim()}`
+      );
+    }
+  });
+});