diff --git a/.changeset/quote-decimal-frontmatter-scalars.md b/.changeset/quote-decimal-frontmatter-scalars.md
new file mode 100644
index 000000000..ef50f1c88
--- /dev/null
+++ b/.changeset/quote-decimal-frontmatter-scalars.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 4165
+---
+**Decimal-shaped frontmatter scalars (e.g. a `22.10` phase id) are now quoted on write**, so a spec-compliant YAML reader preserves them as the exact string instead of reloading `22.10` as the float `22.1` — which collided with `22.1`, a different phase. Exponent, hex, octal and binary forms are quoted likewise. All-digit values (integer counts and zero-padded ids like `02`) stay unquoted as a deliberate scoped trade-off; `gsd_state_version` is now written `"1.0"`, matching the quoted form in the STATE.md template. (#4053)
diff --git a/src/frontmatter.cts b/src/frontmatter.cts
index d13afd600..a301d1f63 100644
--- a/src/frontmatter.cts
+++ b/src/frontmatter.cts
@@ -913,6 +913,23 @@ function agentScalarNeedsDoubleQuoting(s: string): boolean {
   return false;
 }
 
+/**
+ * #4053 — Quote a numeric-looking scalar that is not all-digit (`22.10`,
+ * `1.0`, `1e3`, `0x1F`) so a spec YAML reader keeps it a string: bare `22.10`
+ * reloads as the float 22.1 and collides with `22.1`, a different phase.
+ *
+ * All-digit strings stay bare as a deliberate, scoped trade-off — not a
+ * safety guarantee. A leading-zero value (`02`, `017`) also mis-parses under
+ * a spec reader (to 2, 17). It is left unquoted because zero-padded ids
+ * (`plan: 01`, `phase: 02`) are the pervasive GSD convention, quoting them
+ * all is the blanket quoting #4053 asked to avoid, and the loss is padding
+ * rather than identity: `02` and `2` normalize to the same phase; `22.1` and
+ * `22.10` do not.
+ */
+function generalScalarNeedsNumericQuoting(s: string): boolean {
+  return YAML_NUMERIC_RE.test(s) && !/^\d+$/.test(s);
+}
+
 function reconstructFrontmatter(obj: Frontmatter): string {
   const lines: string[] = [];
   // #3257: read the full-line-comment channel (set by parseGuardedYamlRegion when comments
@@ -978,12 +995,12 @@ function reconstructFrontmatter(obj: Frontmatter): string {
         } else {
           // eslint-disable-next-line @typescript-eslint/no-base-to-string
           const sv = String(subval);
-          lines.push(`  ${subkey}: ${sv.includes(':') || sv.includes('#') || scalarNeedsDoubleQuoting(sv) ? `"${escapeDoubleQuotedScalar(sv)}"` : sv}`);
+          lines.push(`  ${subkey}: ${sv.includes(':') || sv.includes('#') || scalarNeedsDoubleQuoting(sv) || generalScalarNeedsNumericQuoting(sv) ? `"${escapeDoubleQuotedScalar(sv)}"` : sv}`);
         }
       }
     } else {
       const sv = String(value);
-      if (sv.includes(':') || sv.includes('#') || sv.startsWith('[') || sv.startsWith('{') || scalarNeedsDoubleQuoting(sv)) {
+      if (sv.includes(':') || sv.includes('#') || sv.startsWith('[') || sv.startsWith('{') || scalarNeedsDoubleQuoting(sv) || generalScalarNeedsNumericQuoting(sv)) {
         lines.push(`${key}: "${escapeDoubleQuotedScalar(sv)}"`);
       } else {
         lines.push(`${key}: ${sv}`);
diff --git a/tests/frontmatter.test.cjs b/tests/frontmatter.test.cjs
index 4a1652e1e..b408cf79a 100644
--- a/tests/frontmatter.test.cjs
+++ b/tests/frontmatter.test.cjs
@@ -249,6 +249,63 @@ describe('reconstructFrontmatter', () => {
     assert.ok(hashResult.includes('"value # note"'), 'should quote value with hash');
   });
 
+  describe('#4053 — decimal-shaped numeric scalars survive a spec YAML reader', () => {
+    const yaml = require('js-yaml');
+    const lineFor = (obj, key) =>
+      reconstructFrontmatter(obj).split('\n').find((l) => l.startsWith(`${key}:`));
+
+    test('a decimal phase id is quoted so js-yaml keeps it a string', () => {
+      assert.strictEqual(lineFor({ current_phase: '22.1' }, 'current_phase'), 'current_phase: "22.1"');
+      assert.strictEqual(lineFor({ current_phase: '22.10' }, 'current_phase'), 'current_phase: "22.10"');
+      assert.strictEqual(lineFor({ current_phase: '22.0' }, 'current_phase'), 'current_phase: "22.0"');
+    });
+
+    test('"22.1" and "22.10" no longer collide under js-yaml', () => {
+      const load = (v) => yaml.load(reconstructFrontmatter({ current_phase: v })).current_phase;
+      const a = load('22.1');
+      const b = load('22.10');
+      assert.strictEqual(a, '22.1');
+      assert.strictEqual(b, '22.10'); // was the float 22.1 before the fix
+      assert.notStrictEqual(a, b);
+      assert.strictEqual(typeof b, 'string'); // was 'number' before the fix
+    });
+
+    test('a nested decimal value is also quoted', () => {
+      assert.strictEqual(
+        reconstructFrontmatter({ progress: { ratio: '1.10' } }),
+        'progress:\n  ratio: "1.10"',
+      );
+    });
+
+    test('plain integer phase ids and counts stay bare — no idempotency churn', () => {
+      assert.strictEqual(lineFor({ current_phase: '3' }, 'current_phase'), 'current_phase: 3');
+      assert.strictEqual(lineFor({ current_phase: '10' }, 'current_phase'), 'current_phase: 10');
+      assert.strictEqual(
+        reconstructFrontmatter({ progress: { completed_phases: '2', percent: '40' } }),
+        'progress:\n  completed_phases: 2\n  percent: 40',
+      );
+    });
+
+    test('exponent, hex, octal, binary and sexagesimal forms are quoted and survive js-yaml', () => {
+      for (const v of ['1e3', '0x1F', '0o17', '0b101', '12:30']) {
+        assert.strictEqual(lineFor({ k: v }, 'k'), `k: "${v}"`, v);
+        assert.strictEqual(yaml.load(reconstructFrontmatter({ k: v })).k, v, v);
+      }
+    });
+
+    test('a leading-zero all-digit id stays bare — the documented, scoped trade-off', () => {
+      assert.strictEqual(lineFor({ current_phase: '02' }, 'current_phase'), 'current_phase: 02');
+      assert.strictEqual(lineFor({ plan: '01' }, 'plan'), 'plan: 01');
+    });
+
+    test('free-text with no YAML-special characters stays unquoted — no blanket quoting', () => {
+      assert.strictEqual(
+        lineFor({ current_phase_name: 'Test Phase' }, 'current_phase_name'),
+        'current_phase_name: Test Phase',
+      );
+    });
+  });
+
   test('serializes nested objects with proper indentation', () => {
     const result = reconstructFrontmatter({ tech: { added: 'prisma', patterns: 'repo' } });
     assert.ok(result.includes('tech:'), 'should have parent key');
@@ -312,7 +369,7 @@ describe('reconstructFrontmatter', () => {
       `comment should survive reconstruct; got:\n${reconstructed}`,
     );
     // data identity preserved alongside the comment.
-    assert.ok(reconstructed.includes('gsd_state_version: 1.0'));
+    assert.ok(reconstructed.includes('gsd_state_version: "1.0"'));
     assert.ok(reconstructed.includes('current_phase: 3'));
     assert.ok(reconstructed.includes('status: executing'));
     // the reconstructed output re-parses to the same data (idempotent round-trip).
diff --git a/tests/state-transition.test.cjs b/tests/state-transition.test.cjs
index c2ac1dcc7..8eedd6053 100644
--- a/tests/state-transition.test.cjs
+++ b/tests/state-transition.test.cjs
@@ -1879,7 +1879,7 @@ describe('ADR-1769 Phase 4: milestoneSwitch transition — milestone reset', ()
 
   test('gsd_state_version is preserved across the reset', () => {
     const result = transitionCore(milestoneBody(), { kind: 'milestoneSwitch', version: 'v2.0', name: 'New Milestone' }, deps);
-    assert.ok(/gsd_state_version:\s*1\.0/.test(result.content), 'gsd_state_version must be preserved');
+    assert.ok(/gsd_state_version:\s*"?1\.0"?/.test(result.content), 'gsd_state_version must be preserved');
   });
 
   test('Current Position section is reset to "Not started (defining requirements)"', () => {
diff --git a/tests/state.test.cjs b/tests/state.test.cjs
index ee531fec1..5e38d1f7e 100644
--- a/tests/state.test.cjs
+++ b/tests/state.test.cjs
@@ -842,7 +842,7 @@ describe('STATE.md frontmatter sync', () => {
 
     const content = fs.readFileSync(path.join(tmpDir, '.planning', 'STATE.md'), 'utf-8');
     assert.ok(content.startsWith('---\n'), 'should start with frontmatter delimiter');
-    assert.ok(content.includes('gsd_state_version: 1.0'), 'should have version field');
+    assert.ok(content.includes('gsd_state_version: "1.0"'), 'should have version field');
     assert.ok(content.includes('current_phase: 02'), 'frontmatter should have current phase');
     assert.ok(content.includes('**Current Phase:** 02'), 'body field should be preserved');
     assert.ok(content.includes('**Status:** Executing Plan 1'), 'updated field in body');
@@ -7126,7 +7126,7 @@ describe('ADR-3408 §8.5 Matrix (#3471): stale-but-present, and the report resid
       }), tmp);
 
       const expected = [
-        '---', 'gsd_state_version: 1.0', 'status: unknown', 'last_updated: "2023-11-14T22:13:20.000Z"',
+        '---', 'gsd_state_version: "1.0"', 'status: unknown', 'last_updated: "2023-11-14T22:13:20.000Z"',
         'stopped_at: Phase 5, curated stop', 'paused_at: Phase 5, curated pause',
         'current_phase: 5', 'current_phase_name: Curated Name', 'current_plan: 05-02-plan',
         'last_activity_desc: curated activity desc',