diff --git a/src/resources/extensions/gsd/doctor.ts b/src/resources/extensions/gsd/doctor.ts
index 5e74e0a42..c7daa6b47 100644
--- a/src/resources/extensions/gsd/doctor.ts
+++ b/src/resources/extensions/gsd/doctor.ts
@@ -960,7 +960,7 @@ export async function runGSDDoctor(basePath: string, options?: { fix?: boolean;
           fixable: true,
         });
         dryRunCanFix("all_tasks_done_roadmap_not_checked", `mark ${slice.id} done in roadmap`);
-        if (shouldFix("all_tasks_done_roadmap_not_checked") && (hasSliceSummary || issues.some(issue => issue.code === "all_tasks_done_missing_slice_summary" && issue.unitId === unitId))) {
+        if (shouldFix("all_tasks_done_roadmap_not_checked") && (hasSliceSummary || existsSync(join(slicePath, `${slice.id}-SUMMARY.md`)))) {
           await markSliceDoneInRoadmap(basePath, milestoneId, slice.id, fixesApplied);
         }
       }
diff --git a/src/resources/extensions/gsd/tests/doctor-completion-deferral.test.ts b/src/resources/extensions/gsd/tests/doctor-completion-deferral.test.ts
index 26beebbdb..86c723d8c 100644
--- a/src/resources/extensions/gsd/tests/doctor-completion-deferral.test.ts
+++ b/src/resources/extensions/gsd/tests/doctor-completion-deferral.test.ts
@@ -80,7 +80,7 @@ test("COMPLETION_TRANSITION_CODES only contains slice summary code", () => {
   );
 });
 
-test("fixLevel:task — fixes roadmap checkbox and UAT stub immediately, defers only summary (#1808)", async () => {
+test("fixLevel:task — fixes UAT stub immediately, defers summary and roadmap checkbox (#1808, #1910)", async () => {
   const tmp = makeTmp("partial-deferral");
   try {
     buildScaffold(tmp);
@@ -101,15 +101,16 @@ test("fixLevel:task — fixes roadmap checkbox and UAT stub immediately, defers
     const sliceUatPath = join(tmp, ".gsd", "milestones", "M001", "slices", "S01", "S01-UAT.md");
     assert.ok(existsSync(sliceUatPath), "should have created UAT stub immediately");
 
-    // Roadmap checkbox SHOULD be marked done (mechanical bookkeeping, no longer deferred)
+    // Roadmap checkbox must NOT be checked without summary on disk (#1910).
+    // Checking it without the summary causes deriveState() to skip complete-slice.
     const roadmapContent = readFileSync(join(tmp, ".gsd", "milestones", "M001", "M001-ROADMAP.md"), "utf8");
-    assert.ok(roadmapContent.includes("- [x] **S01"), "roadmap should show S01 as checked");
+    assert.ok(roadmapContent.includes("- [ ] **S01"), "roadmap must NOT be checked without summary on disk (#1910)");
   } finally {
     rmSync(tmp, { recursive: true, force: true });
   }
 });
 
-test("fixLevel:task — session crash after last task leaves roadmap and UAT consistent (#1808)", async () => {
+test("fixLevel:task — session crash after last task leaves UAT consistent, roadmap deferred with summary (#1808, #1910)", async () => {
   const tmp = makeTmp("crash-consistency");
   try {
     buildScaffold(tmp);
@@ -121,13 +122,7 @@ test("fixLevel:task — session crash after last task leaves roadmap and UAT con
     // A new session starts and runs doctor again at task level.
     const report2 = await runGSDDoctor(tmp, { fix: true, fixLevel: "task" });
 
-    // The only remaining issue should be the deferred summary.
-    // Roadmap and UAT should already be fixed from the first run.
     const remainingCodes = report2.issues.map(i => i.code);
-    assert.ok(
-      !remainingCodes.includes("all_tasks_done_roadmap_not_checked"),
-      "roadmap should already be fixed from first doctor run"
-    );
     assert.ok(
       !remainingCodes.includes("all_tasks_done_missing_slice_uat"),
       "UAT should already be fixed from first doctor run"
@@ -137,6 +132,16 @@ test("fixLevel:task — session crash after last task leaves roadmap and UAT con
       remainingCodes.includes("all_tasks_done_missing_slice_summary"),
       "summary should still be detected as missing (deferred)"
     );
+    // Roadmap should still be unchecked because summary doesn't exist (#1910)
+    assert.ok(
+      remainingCodes.includes("all_tasks_done_roadmap_not_checked"),
+      "roadmap should still be unchecked — summary does not exist on disk (#1910)"
+    );
+    // Must NOT produce the cascade error from checking roadmap without summary
+    assert.ok(
+      !remainingCodes.includes("slice_checked_missing_summary"),
+      "must not produce slice_checked_missing_summary (#1910)"
+    );
   } finally {
     rmSync(tmp, { recursive: true, force: true });
   }
diff --git a/src/resources/extensions/gsd/tests/doctor-fixlevel.test.ts b/src/resources/extensions/gsd/tests/doctor-fixlevel.test.ts
index 8308dab6b..5ee3be354 100644
--- a/src/resources/extensions/gsd/tests/doctor-fixlevel.test.ts
+++ b/src/resources/extensions/gsd/tests/doctor-fixlevel.test.ts
@@ -63,7 +63,7 @@ Done.
 `);
 }
 
-test("fixLevel:task — defers only summary stub, fixes roadmap and UAT immediately (#1808)", async () => {
+test("fixLevel:task — defers summary stub and roadmap checkbox, fixes UAT immediately (#1808, #1910)", async () => {
   const tmp = makeTmp("task-level");
   try {
     buildScaffold(tmp);
@@ -79,13 +79,14 @@ test("fixLevel:task — defers only summary stub, fixes roadmap and UAT immediat
     const sliceSummaryPath = join(tmp, ".gsd", "milestones", "M001", "slices", "S01", "S01-SUMMARY.md");
     assert.ok(!existsSync(sliceSummaryPath), "should NOT have created summary stub");
 
-    // Roadmap SHOULD be marked done (mechanical bookkeeping, no longer deferred)
+    // Roadmap must NOT be checked without summary on disk (#1910)
     const roadmapContent = readFileSync(join(tmp, ".gsd", "milestones", "M001", "M001-ROADMAP.md"), "utf8");
-    assert.ok(roadmapContent.includes("- [x] **S01"), "roadmap should show S01 as checked");
+    assert.ok(roadmapContent.includes("- [ ] **S01"), "roadmap must NOT be checked without summary (#1910)");
 
-    // Fixes applied should NOT include summary but SHOULD include roadmap
+    // Fixes applied should NOT include summary or roadmap
     for (const f of report.fixesApplied) {
       assert.ok(!f.includes("SUMMARY"), `should not have fixed summary: ${f}`);
+      assert.ok(!f.includes("ROADMAP") && !f.includes("roadmap"), `should not have fixed roadmap: ${f}`);
     }
   } finally {
     rmSync(tmp, { recursive: true, force: true });
diff --git a/src/resources/extensions/gsd/tests/doctor-roadmap-summary-atomicity.test.ts b/src/resources/extensions/gsd/tests/doctor-roadmap-summary-atomicity.test.ts
new file mode 100644
index 000000000..63cbee5cd
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/doctor-roadmap-summary-atomicity.test.ts
@@ -0,0 +1,167 @@
+/**
+ * Regression test for #1910: Doctor marks roadmap checkbox at fixLevel="task"
+ * without summary on disk, causing deriveState() to skip complete-slice and
+ * hard-stop at validating-milestone.
+ *
+ * The roadmap checkbox must only be marked when the slice summary actually
+ * exists on disk (either pre-existing or created in the current doctor run).
+ * At fixLevel="task", the summary is deferred (COMPLETION_TRANSITION_CODES),
+ * so the roadmap checkbox must also be deferred.
+ */
+
+import { mkdirSync, writeFileSync, rmSync, readFileSync, existsSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import test from "node:test";
+import assert from "node:assert/strict";
+import { runGSDDoctor } from "../doctor.ts";
+
+function makeTmp(name: string): string {
+  const dir = join(tmpdir(), `doctor-roadmap-summary-${name}-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+  mkdirSync(dir, { recursive: true });
+  return dir;
+}
+
+/**
+ * Build a minimal .gsd structure: milestone with one slice, one task
+ * marked done with a summary — but no slice summary and roadmap unchecked.
+ * This is the state after the last task completes.
+ */
+function buildScaffold(base: string) {
+  const gsd = join(base, ".gsd");
+  const m = join(gsd, "milestones", "M001");
+  const s = join(m, "slices", "S01", "tasks");
+  mkdirSync(s, { recursive: true });
+
+  writeFileSync(join(m, "M001-ROADMAP.md"), `# M001: Test
+
+## Slices
+
+- [ ] **S01: Test Slice** \`risk:low\` \`depends:[]\`
+  > Demo text
+`);
+
+  writeFileSync(join(m, "slices", "S01", "S01-PLAN.md"), `# S01: Test Slice
+
+**Goal:** test
+
+## Tasks
+
+- [x] **T01: Do stuff** \`est:5m\`
+`);
+
+  writeFileSync(join(s, "T01-SUMMARY.md"), `---
+id: T01
+parent: S01
+milestone: M001
+duration: 5m
+verification_result: passed
+completed_at: 2026-01-01
+---
+
+# T01: Do stuff
+
+Done.
+`);
+}
+
+test("fixLevel:task — must NOT mark roadmap checkbox when summary does not exist on disk (#1910)", async () => {
+  const tmp = makeTmp("no-roadmap-without-summary");
+  try {
+    buildScaffold(tmp);
+
+    const report = await runGSDDoctor(tmp, { fix: true, fixLevel: "task" });
+
+    // Doctor should detect both issues
+    const codes = report.issues.map(i => i.code);
+    assert.ok(codes.includes("all_tasks_done_missing_slice_summary"), "should detect missing summary");
+    assert.ok(codes.includes("all_tasks_done_roadmap_not_checked"), "should detect unchecked roadmap");
+
+    // Summary should NOT exist (deferred at task level)
+    const sliceSummaryPath = join(tmp, ".gsd", "milestones", "M001", "slices", "S01", "S01-SUMMARY.md");
+    assert.ok(!existsSync(sliceSummaryPath), "summary should NOT be created (deferred)");
+
+    // CRITICAL: Roadmap checkbox must NOT be checked without summary on disk.
+    // If it is checked, deriveState() sees the milestone as complete and skips
+    // the summarizing phase, causing a hard-stop at validating-milestone.
+    const roadmapContent = readFileSync(join(tmp, ".gsd", "milestones", "M001", "M001-ROADMAP.md"), "utf8");
+    assert.ok(
+      roadmapContent.includes("- [ ] **S01"),
+      "roadmap must NOT mark S01 as checked when summary does not exist on disk"
+    );
+  } finally {
+    rmSync(tmp, { recursive: true, force: true });
+  }
+});
+
+test("fixLevel:task — consecutive runs must not produce slice_checked_missing_summary (#1910)", async () => {
+  const tmp = makeTmp("no-cascade-error");
+  try {
+    buildScaffold(tmp);
+
+    // First doctor run at task level
+    await runGSDDoctor(tmp, { fix: true, fixLevel: "task" });
+
+    // Second doctor run — if the first run incorrectly checked the roadmap,
+    // this run would detect slice_checked_missing_summary (the cascade error
+    // described in the issue's forensic evidence).
+    const report2 = await runGSDDoctor(tmp, { fix: true, fixLevel: "task" });
+    const codes2 = report2.issues.map(i => i.code);
+
+    assert.ok(
+      !codes2.includes("slice_checked_missing_summary"),
+      "must not produce slice_checked_missing_summary — roadmap should not have been checked without summary"
+    );
+  } finally {
+    rmSync(tmp, { recursive: true, force: true });
+  }
+});
+
+test("fixLevel:all — roadmap checkbox IS marked because summary is created in same run (#1910)", async () => {
+  const tmp = makeTmp("all-level-creates-both");
+  try {
+    buildScaffold(tmp);
+
+    const report = await runGSDDoctor(tmp, { fix: true });
+
+    // At fixLevel:all, summary stub is created first, then roadmap is checked.
+    // Both should be fixed.
+    const sliceSummaryPath = join(tmp, ".gsd", "milestones", "M001", "slices", "S01", "S01-SUMMARY.md");
+    assert.ok(existsSync(sliceSummaryPath), "summary should be created at fixLevel:all");
+
+    const roadmapContent = readFileSync(join(tmp, ".gsd", "milestones", "M001", "M001-ROADMAP.md"), "utf8");
+    assert.ok(roadmapContent.includes("- [x] **S01"), "roadmap should show S01 as checked at fixLevel:all");
+  } finally {
+    rmSync(tmp, { recursive: true, force: true });
+  }
+});
+
+test("fixLevel:task — roadmap IS marked when summary already exists on disk (#1910)", async () => {
+  const tmp = makeTmp("summary-preexists");
+  try {
+    buildScaffold(tmp);
+
+    // Pre-create the slice summary (as if complete-slice already ran)
+    const sliceSummaryPath = join(tmp, ".gsd", "milestones", "M001", "slices", "S01", "S01-SUMMARY.md");
+    writeFileSync(sliceSummaryPath, `---
+id: S01
+milestone: M001
+---
+
+# S01: Test Slice
+
+Summary content.
+`);
+
+    const report = await runGSDDoctor(tmp, { fix: true, fixLevel: "task" });
+
+    // Summary exists, so roadmap SHOULD be checked even at task level
+    const roadmapContent = readFileSync(join(tmp, ".gsd", "milestones", "M001", "M001-ROADMAP.md"), "utf8");
+    assert.ok(
+      roadmapContent.includes("- [x] **S01"),
+      "roadmap should be checked when summary already exists on disk"
+    );
+  } finally {
+    rmSync(tmp, { recursive: true, force: true });
+  }
+});