diff --git a/tool/build-flutter-packages.mjs b/tool/build-flutter-packages.mjs
new file mode 100644
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+++ b/tool/build-flutter-packages.mjs
@@ -0,0 +1,298 @@
+#!/usr/bin/env node
+/**
+ * Builds every Flutter package that has a `web/package.json` file, using the
+ * lower bound of the Flutter SDK constraint in each package's `pubspec.yaml`
+ * (`environment.flutter`).
+ *
+ * Verification: pinned versions are checked against `fvm list` before
+ * building. If a version is missing, the tool exits with an actionable error.
+ *
+ * After a successful build, writes `build/web/package.json` by merging:
+ *   - name, description, version  →  derived from pubspec.yaml
+ *   - all other fields            →  carried over from web/package.json
+ *
+ * A package opts in to this tool by having both `pubspec.yaml` and
+ * `web/package.json` inside its directory.
+ *
+ * Usage: node tool/build-flutter-packages.mjs
+ */
+
+import { spawnSync } from 'node:child_process';
+import { existsSync, readFileSync, renameSync, writeFileSync } from 'node:fs';
+import { homedir } from 'node:os';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = join(dirname(fileURLToPath(import.meta.url)), '..');
+
+// ─── pubspec.yaml parser ────────────────────────────────────────────────────
+//
+// Extracts only the fields this tool needs:
+//   name, description, version  (top-level scalars / block scalars)
+//   environment.flutter         (nested scalar)
+
+function parsePubspec(pubspecPath) {
+  const lines = readFileSync(pubspecPath, 'utf8').split('\n');
+  const result = {};
+  let i = 0;
+
+  while (i < lines.length) {
+    const line = lines[i];
+    const trimmed = line.trimStart();
+    const indent = line.length - trimmed.length;
+
+    // Only process non-empty, non-comment, top-level lines.
+    if (indent !== 0 || !trimmed || trimmed.startsWith('#')) {
+      i++;
+      continue;
+    }
+
+    const colonIdx = trimmed.indexOf(':');
+    if (colonIdx === -1) { i++; continue; }
+
+    const key = trimmed.slice(0, colonIdx).trim();
+    const afterColon = trimmed.slice(colonIdx + 1).trim();
+
+    if (['name', 'description', 'version'].includes(key)) {
+      // Block scalar (">-", ">", "|-", "|"): collect indented continuation lines.
+      if (afterColon === '>-' || afterColon === '>' || afterColon === '|-' || afterColon === '|') {
+        const folded = afterColon.startsWith('>');
+        const blockLines = [];
+        i++;
+        while (i < lines.length) {
+          const bl = lines[i];
+          // A non-empty line with no leading whitespace means we're back at top-level.
+          if (bl.trimStart() === bl && bl.trim() !== '') break;
+          if (bl.trim()) blockLines.push(bl.trim());
+          i++;
+        }
+        result[key] = folded ? blockLines.join(' ') : blockLines.join('\n');
+        continue;
+      }
+      // Simple / quoted scalar.
+      result[key] = afterColon.replace(/^["']|["']$/g, '');
+    } else if (key === 'environment') {
+      // Parse the nested environment mapping.
+      i++;
+      while (i < lines.length) {
+        const el = lines[i];
+        const eTrimmed = el.trimStart();
+        const eIndent = el.length - eTrimmed.length;
+        // A non-empty, non-comment line at indent 0 means we left the block.
+        if (eIndent === 0 && eTrimmed && !eTrimmed.startsWith('#')) break;
+        if (eTrimmed.startsWith('flutter:')) {
+          result.flutterConstraint = eTrimmed
+            .slice('flutter:'.length)
+            .trim()
+            .replace(/^["']|["']$/g, '');
+        }
+        i++;
+      }
+      continue;
+    }
+
+    i++;
+  }
+
+  return result;
+}
+
+// ─── FVM helpers ────────────────────────────────────────────────────────────
+
+/** Returns the set of Flutter versions installed locally according to FVM. */
+function getInstalledFvmVersions() {
+  const result = spawnSync('fvm', ['list'], { encoding: 'utf8' });
+  if (result.status !== 0 || result.error) {
+    console.error('✗  `fvm list` failed — is FVM installed and on PATH?');
+    process.exit(1);
+  }
+  // Strip ANSI escape codes before parsing.
+  const plain = result.stdout.replace(/\x1b\[[0-9;]*m/g, '');
+  const versions = new Set();
+  for (const line of plain.split('\n')) {
+    // Table data rows are delimited by │; the version is the first cell.
+    if (!line.includes('│')) continue;
+    const firstCell = line.split('│')[1]?.trim() ?? '';
+    if (/^\d+\.\d+\.\d+$/.test(firstCell)) versions.add(firstCell);
+  }
+  return versions;
+}
+
+/**
+ * Extracts the lower bound version from a Flutter SDK constraint string.
+ *
+ * Examples:
+ *   "3.41.1"           → "3.41.1"  (exact pin is its own lower bound)
+ *   ">=3.6.0 <4.0.0"  → "3.6.0"
+ *   ">=3.6.0"          → "3.6.0"
+ *
+ * Returns null when no lower bound can be determined (absent, malformed, or
+ * uses an exclusive operator — see extractLowerBound callers for the error).
+ */
+function extractLowerBound(constraint) {
+  if (!constraint) return null;
+  // Exact pin — no operators.
+  if (/^\d+\.\d+\.\d+$/.test(constraint)) return constraint;
+  // Inclusive lower bound only: ">=x.y.z ..." → x.y.z.
+  // Exclusive lower bounds (">x.y.z") are intentionally not matched because
+  // x.y.z itself would not satisfy the constraint.
+  const m = constraint.match(/>=\s*(\d+\.\d+\.\d+)/);
+  return m ? m[1] : null;
+}
+
+/**
+ * Resolves the path to the `flutter` binary for a given FVM-installed version.
+ * Returns null if the binary cannot be found on disk.
+ */
+function resolveFvmFlutterBin(version) {
+  const fvmHome = process.env.FVM_HOME ?? join(homedir(), 'fvm');
+  const candidate = join(fvmHome, 'versions', version, 'bin', 'flutter');
+  return existsSync(candidate) ? candidate : null;
+}
+
+// ─── Package discovery ──────────────────────────────────────────────────────
+
+/**
+ * Returns all workspace packages that have a `web/` directory, sourced from
+ * `melos list --dir-exists=web --json` so discovery matches the workspace globs
+ * in melos.yaml exactly.
+ */
+function discoverPackages() {
+  const result = spawnSync('melos', ['list', '--dir-exists=web', '--json'], { encoding: 'utf8', cwd: repoRoot });
+  if (result.error || result.status !== 0) {
+    console.error('✗  `melos list --dir-exists=web --json` failed — is Melos installed and on PATH?');
+    if (result.error) console.error(`   ${result.error.message}`);
+    process.exit(result.status ?? 1);
+  }
+
+  let packages;
+  try {
+    packages = JSON.parse(result.stdout);
+  } catch {
+    console.error('✗  Failed to parse `melos list --dir-exists=web --json` output.');
+    process.exit(1);
+  }
+
+  return packages.map(({ location }) => ({
+    dir: location,
+    pubspecPath: join(location, 'pubspec.yaml'),
+    webPkgPath: join(location, 'web', 'package.json'),
+  }));
+}
+
+// ─── Conversion helpers ─────────────────────────────────────────────────────
+
+//"tap_burst_web_component" → "tap-burst-web-component"
+function toNpmName(n) {
+  return n.replaceAll('_', '-');
+}
+
+// ─── Main ───────────────────────────────────────────────────────────────────
+
+const installedFvmVersions = getInstalledFvmVersions();
+if (installedFvmVersions.size === 0) {
+  console.error('✗  No Flutter versions installed in FVM.');
+  process.exit(1);
+}
+
+const packages = discoverPackages();
+
+if (packages.length === 0) {
+  console.error(
+    '✗  No packages found.\n' +
+    '   Add a web/package.json to a Flutter package directory to opt in.',
+  );
+  process.exit(1);
+}
+
+console.log(`Found ${packages.length} package(s) to build.\n`);
+
+for (const pkg of packages) {
+  const { name, description, version, flutterConstraint } = parsePubspec(pkg.pubspecPath);
+
+  if (!name || !version) {
+    console.error(`✗  ${pkg.dir}: pubspec.yaml is missing required fields (name, version).`);
+    process.exit(1);
+  }
+
+  console.log(`▶  Building ${name}…`);
+
+  // Resolve the Flutter executable to use for this package.
+  let flutterCmd;
+
+  const targetVersion = extractLowerBound(flutterConstraint);
+
+  if (!targetVersion) {
+    console.error(`✗  ${name}: could not determine a Flutter version from constraint: "${flutterConstraint ?? '(none)'}".`);
+    if (flutterConstraint && /^>\s*\d/.test(flutterConstraint) && !/^>=/.test(flutterConstraint)) {
+      console.error('   Exclusive lower bounds (">x.y.z") are not supported. Use ">=" instead.');
+    }
+    process.exit(1);
+  }
+
+  if (!installedFvmVersions.has(targetVersion)) {
+    console.error(`✗  Flutter ${targetVersion} is not installed in FVM.`);
+    console.error(`   Run: fvm install ${targetVersion}`);
+    process.exit(1);
+  }
+
+  const bin = resolveFvmFlutterBin(targetVersion);
+  if (!bin) {
+    console.error(
+      `✗  Flutter ${targetVersion} is listed by FVM but the binary was not found on disk.`,
+    );
+    process.exit(1);
+  }
+
+  flutterCmd = bin;
+
+  // Print the actual Flutter version before building.
+  const versionResult = spawnSync(flutterCmd, ['--version'], { encoding: 'utf8' });
+  if (versionResult.error || versionResult.status !== 0) {
+    console.error(`✗  Failed to run \`flutter --version\` for ${name} (${flutterCmd}).`);
+    if (versionResult.error) console.error(`   ${versionResult.error.message}`);
+    process.exit(versionResult.status ?? 1);
+  }
+  process.stdout.write(`\x1b[32m${versionResult.stdout}\x1b[0m`);
+
+  // Run: <flutter> build web
+  const buildResult = spawnSync(flutterCmd, ['build', 'web', '--profile'], {
+    cwd: pkg.dir,
+    stdio: 'inherit',
+  });
+  if (buildResult.status !== 0) {
+    console.error(`✗  Flutter build failed for ${name} (exit code ${buildResult.status ?? '?'}).`);
+    process.exit(buildResult.status ?? 1);
+  }
+
+  // Write build/web/package.json
+  const webPkg = JSON.parse(readFileSync(pkg.webPkgPath, 'utf8'));
+  const outputPkg = {
+    ...webPkg,
+    name: toNpmName(name),
+    description: description ?? '',
+    version,
+  };
+
+  const outDir = join(pkg.dir, 'build', 'web');
+  const outPath = join(outDir, 'package.json');
+  writeFileSync(outPath, JSON.stringify(outputPkg, null, 2) + '\n', 'utf8');
+
+  // Pack the build output into a tarball.
+  const packResult = spawnSync('npm', ['pack'], { cwd: outDir, stdio: 'inherit' });
+  if (packResult.status !== 0) {
+    console.error(`✗  npm pack failed for ${name} (exit code ${packResult.status ?? '?'}).`);
+    process.exit(packResult.status ?? 1);
+  }
+
+  // Rename <name>-<version>.tgz → <name>.tgz (drop the version suffix).
+  const npmName = toNpmName(name);
+  renameSync(
+    join(outDir, `${npmName}-${version}.tgz`),
+    join(outDir, `${npmName}.tgz`),
+  );
+
+  console.log(`✓  ${name} → build/web/${npmName}.tgz\n`);
+}
+
+console.log('All packages built successfully.');