diff --git a/.github/scripts/post-coverage.cjs b/.github/scripts/post-coverage.cjs
new file mode 100644
index 0000000..e41f2fd
--- /dev/null
+++ b/.github/scripts/post-coverage.cjs
@@ -0,0 +1,35 @@
+/**
+ * Posts JMAP spec coverage report as a PR comment.
+ *
+ * @param {object} params
+ * @param {import('@actions/github/lib/utils').GitHub} params.github
+ * @param {import('@actions/github').context} params.context
+ */
+module.exports = async ({ github, context }) => {
+  const report = process.env.COVERAGE_REPORT;
+
+  if (!report) {
+    console.log('No coverage report found in COVERAGE_REPORT env var');
+    return;
+  }
+
+  const body = `## JMAP Spec Coverage
+
+${report}
+`;
+
+  const prNumber = context.payload.pull_request?.number;
+  if (!prNumber) {
+    console.log('Not a pull request context, skipping comment');
+    return;
+  }
+
+  await github.rest.issues.createComment({
+    owner: context.repo.owner,
+    repo: context.repo.repo,
+    issue_number: prNumber,
+    body
+  });
+
+  console.log('Coverage comment posted successfully');
+};
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 60e4a3b..8bd7fa8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -129,3 +129,47 @@ jobs:
       - name: Stop test server
         if: always()
         run: pnpm test:server:stop
+
+  spec-coverage:
+    name: Spec Coverage
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    permissions:
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 10.17.0
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "24"
+          cache: "pnpm"
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run spec tests
+        run: pnpm test:spec
+
+      - name: Generate coverage report
+        id: coverage
+        run: |
+          {
+            echo 'report<<COVERAGE_EOF'
+            pnpm coverage:spec --markdown
+            echo 'COVERAGE_EOF'
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Post coverage comment
+        uses: actions/github-script@v7
+        env:
+          COVERAGE_REPORT: ${{ steps.coverage.outputs.report }}
+        with:
+          script: |
+            const script = require('./.github/scripts/post-coverage.cjs')
+            await script({ github, context })
diff --git a/scripts/spec-coverage.ts b/scripts/spec-coverage.ts
index b728fe0..c2620a3 100644
--- a/scripts/spec-coverage.ts
+++ b/scripts/spec-coverage.ts
@@ -21,6 +21,7 @@ import {
 interface CliArgs {
   json: boolean
   byType: boolean
+  markdown: boolean
   help: boolean
 }
 
@@ -29,6 +30,7 @@ function parseArgs(): CliArgs {
   return {
     json: args.includes('--json'),
     byType: args.includes('--by-type'),
+    markdown: args.includes('--markdown'),
     help: args.includes('--help') || args.includes('-h')
   }
 }
@@ -45,12 +47,14 @@ Usage:
 Options:
   --json      Output as JSON
   --by-type   Group coverage by object type
+  --markdown  Output as GitHub-flavored markdown table
   --help, -h  Show this help message
 
 Examples:
   pnpm coverage:spec           # Show coverage table
   pnpm coverage:spec --by-type # Show coverage grouped by type
   pnpm coverage:spec --json    # Output as JSON for CI/scripts
+  pnpm coverage:spec --markdown # Output markdown for PR comments
 `)
 }
 
@@ -145,6 +149,31 @@ function printJson() {
   console.log(JSON.stringify(result, null, 2))
 }
 
+function printMarkdown() {
+  const byType = getCompletenessByType()
+  const completeness = getCompleteness()
+
+  // Header with overall progress
+  console.log(`**Overall: ${completeness.implemented}/${completeness.total} methods (${completeness.percentage.toFixed(1)}%)**`)
+  console.log('')
+
+  // Table for each object type
+  for (const [type, data] of Object.entries(byType).sort((a, b) => a[0].localeCompare(b[0]))) {
+    console.log(`### ${type} (${data.implemented}/${data.total})`)
+    console.log('')
+    console.log('| Method | Status |')
+    console.log('|--------|--------|')
+
+    for (const method of data.methods) {
+      const name = method.name.split('/')[1]
+      const status = method.implemented ? '✅' : '⬚'
+      console.log(`| ${name} | ${status} |`)
+    }
+
+    console.log('')
+  }
+}
+
 function main() {
   const args = parseArgs()
 
@@ -158,6 +187,11 @@ function main() {
     return
   }
 
+  if (args.markdown) {
+    printMarkdown()
+    return
+  }
+
   if (args.byType) {
     printByType()
     return