diff --git a/apps/api/src/api-deploy-selection.test.ts b/apps/api/src/api-deploy-selection.test.ts
new file mode 100644
index 00000000..5ce4f49f
--- /dev/null
+++ b/apps/api/src/api-deploy-selection.test.ts
@@ -0,0 +1,164 @@
+import { expect, test } from "bun:test"
+import { spawnSync } from "node:child_process"
+import { existsSync, mkdirSync, mkdtempSync, readFileSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs"
+import { tmpdir } from "node:os"
+import { dirname, join, relative, resolve } from "node:path"
+import ts from "typescript"
+import fc from "fast-check"
+import { apiUnaffectedByPaths, canSkipApiBuild } from "../../../scripts/api-deploy-selection"
+
+const root = resolve(import.meta.dir, "../../..")
+
+test("only nonempty website-only content changes leave the API unchanged", () => {
+  for (const path of ["apps/web/src/docs/guide.md", "apps/web/src/styles.css", "apps/web/scripts/build.ts", "apps/web/media/demo.png", "apps/web/public/favicon.ico"]) {
+    expect(apiUnaffectedByPaths([path])).toBe(true)
+    for (const input of ["api/index.ts", "apps/api/src/config.ts", "src/index.ts", "package.json", "bun.lock", "tsconfig.json", "vercel.json", "scripts/api-deploy-selection.ts"]) {
+      expect(apiUnaffectedByPaths([path, input])).toBe(false)
+      expect(apiUnaffectedByPaths([input, path])).toBe(false)
+    }
+  }
+  for (const paths of [[], ["apps/web/package.json"], ["apps/web/bun.lock"], ["apps/web/src/package.json"], ["apps/web/src/.npmrc"], ["apps/web/src/../package.json"], ["apps/web/src//file.ts"], ["apps/web-old/src/a.ts"]]) {
+    expect(apiUnaffectedByPaths(paths)).toBe(false)
+  }
+})
+
+test("selection requires exact current HEAD, available ancestor, and a nonempty safe diff", () => {
+  const directory = mkdtempSync(join(tmpdir(), "slopcamera-api-selection-"))
+  const git = (...args: string[]) => {
+    const result = spawnSync("git", args, { cwd: directory, encoding: "utf8" })
+    if (result.status !== 0) throw new Error(result.stderr)
+    return result.stdout.trim()
+  }
+  try {
+    git("init", "-q")
+    git("config", "user.email", "test@example.invalid")
+    git("config", "user.name", "Test")
+    writeFileSync(join(directory, "shared.txt"), "base")
+    git("add", ".")
+    git("commit", "-qm", "base")
+    const base = git("rev-parse", "HEAD")
+    mkdirSync(join(directory, "apps/web/src"), { recursive: true })
+    writeFileSync(join(directory, "apps/web/src/page.html"), "site")
+    git("add", ".")
+    git("commit", "-qm", "site")
+    const site = git("rev-parse", "HEAD")
+    expect(canSkipApiBuild(directory, base, site)).toBe(true)
+    expect(canSkipApiBuild(directory, site, site)).toBe(false)
+    expect(canSkipApiBuild(directory, undefined, site)).toBe(false)
+    expect(canSkipApiBuild(directory, "f".repeat(40), site)).toBe(false)
+    expect(canSkipApiBuild(directory, base, base)).toBe(false)
+    expect(canSkipApiBuild(directory, "HEAD~1", site)).toBe(false)
+    expect(canSkipApiBuild(directory, base, undefined)).toBe(false)
+    writeFileSync(join(directory, "shared.txt"), "changed")
+    git("add", ".")
+    git("commit", "-qm", "shared")
+    expect(canSkipApiBuild(directory, base, git("rev-parse", "HEAD"))).toBe(false)
+    git("checkout", "-q", "--detach", base)
+    expect(canSkipApiBuild(directory, site, base)).toBe(false)
+  } finally {
+    rmSync(directory, { recursive: true, force: true })
+  }
+})
+
+function assertLocalImportClosureOutsideWeb(directory: string, entries: readonly string[]): Set<string> {
+  const config = ts.readConfigFile(join(directory, "tsconfig.json"), ts.sys.readFile)
+  if (config.error) throw new Error(ts.flattenDiagnosticMessageText(config.error.messageText, "\n"))
+  const parsed = ts.parseJsonConfigFileContent(config.config, ts.sys, directory)
+  const pending = [...entries]
+  const visited = new Set<string>()
+  const localPath = (file: string): string | undefined => {
+    const physical = realpathSync(file)
+    const path = relative(directory, physical)
+    if (path.startsWith("apps/web/")) throw new Error(`Website dependency: ${path}`)
+    // Third-party packages are fixed by the unchanged root manifests/lockfile.
+    // Workspace symlinks resolve into the source tree and must be followed,
+    // even when TypeScript calls their bare import an external library.
+    if (path.startsWith("../") || path.startsWith("node_modules/")) return undefined
+    return physical
+  }
+  while (pending.length > 0) {
+    const file = localPath(pending.pop()!)
+    if (!file || visited.has(file)) continue
+    visited.add(file)
+    const source = readFileSync(file, "utf8")
+    const ast = ts.createSourceFile(file, source, ts.ScriptTarget.Latest, true)
+    const imports: string[] = []
+    const walk = (node: ts.Node): void => {
+      if ((ts.isImportDeclaration(node) || ts.isExportDeclaration(node)) && node.moduleSpecifier && ts.isStringLiteral(node.moduleSpecifier)) imports.push(node.moduleSpecifier.text)
+      if (ts.isImportTypeNode(node) && ts.isLiteralTypeNode(node.argument) && ts.isStringLiteral(node.argument.literal)) imports.push(node.argument.literal.text)
+      if (ts.isCallExpression(node) && (node.expression.kind === ts.SyntaxKind.ImportKeyword || (ts.isIdentifier(node.expression) && node.expression.text === "require"))) {
+        const argument = node.arguments[0]
+        if (!argument || !ts.isStringLiteral(argument)) throw new Error(`Unresolved dynamic dependency in ${relative(directory, file)}`)
+        imports.push(argument.text)
+      }
+      if (ts.isNewExpression(node) && ts.isIdentifier(node.expression) && node.expression.text === "URL" && node.arguments?.length === 2) {
+        const argument = node.arguments[0]
+        if (argument && ts.isStringLiteral(argument) && argument.text.startsWith(".")) localPath(resolve(dirname(file), argument.text))
+      }
+      ts.forEachChild(node, walk)
+    }
+    walk(ast)
+    for (const specifier of imports) {
+      const resolved = new Set<string>()
+      const typed = ts.resolveModuleName(specifier, file, parsed.options, ts.sys).resolvedModule
+      if (typed && existsSync(typed.resolvedFileName)) resolved.add(typed.resolvedFileName)
+      // Resolve runtime exports as well: a package's .d.ts may not mention
+      // the modules loaded by its actual JavaScript implementation.
+      try {
+        const runtime = Bun.resolveSync(specifier, dirname(file))
+        if (existsSync(runtime)) resolved.add(runtime)
+      } catch {
+        // Type-only declarations need no runtime export; unresolved local
+        // paths still fail below instead of being treated as external.
+      }
+      const asset = resolve(dirname(file), specifier)
+      if (specifier.startsWith(".") && existsSync(asset)) resolved.add(asset)
+      if (resolved.size === 0 && (specifier.startsWith(".") || Object.hasOwn(parsed.options.paths ?? {}, specifier))) {
+        throw new Error(`Unresolved local dependency ${specifier} in ${relative(directory, file)}`)
+      }
+      for (const dependency of resolved) {
+        const local = localPath(dependency)
+        if (local && /\.[cm]?[jt]sx?$/u.test(local)) pending.push(local)
+      }
+    }
+  }
+  return visited
+}
+
+test("API local module and static asset imports stay outside the skipped website tree", () => {
+  const files = spawnSync("git", ["ls-files", "api"], { cwd: root, encoding: "utf8" })
+  expect(files.status).toBe(0)
+  const entries = files.stdout.trim().split("\n").filter((path) => path.endsWith(".ts")).map((path) => join(root, path))
+  expect(entries.length).toBeGreaterThan(0)
+  expect(assertLocalImportClosureOutsideWeb(root, entries).size).toBeGreaterThan(10)
+})
+
+test("follows a workspace package's real runtime export beyond its type declarations", () => {
+  const directory = realpathSync(mkdtempSync(join(tmpdir(), "slopcamera-api-closure-")))
+  try {
+    for (const path of ["api", "packages/local", "node_modules", "apps/web/src"]) mkdirSync(join(directory, path), { recursive: true })
+    writeFileSync(join(directory, "tsconfig.json"), JSON.stringify({ compilerOptions: { moduleResolution: "Bundler", module: "Preserve" } }))
+    writeFileSync(join(directory, "api/entry.ts"), 'import "local-package";')
+    writeFileSync(join(directory, "packages/local/package.json"), JSON.stringify({ name: "local-package", type: "module", exports: { types: "./index.d.ts", import: "./index.js" } }))
+    writeFileSync(join(directory, "packages/local/index.d.ts"), 'export {};')
+    writeFileSync(join(directory, "packages/local/index.js"), 'import "../../apps/web/src/runtime.js";')
+    writeFileSync(join(directory, "apps/web/src/runtime.js"), 'export const source = "web";')
+    symlinkSync(join(directory, "packages/local"), join(directory, "node_modules/local-package"), "dir")
+    expect(() => assertLocalImportClosureOutsideWeb(directory, [join(directory, "api/entry.ts")])).toThrow("Website dependency")
+  } finally {
+    rmSync(directory, { recursive: true, force: true })
+  }
+})
+
+
+
+test("adding a shared input never turns a selected build into a skip", () => {
+  fc.assert(fc.property(
+    fc.array(fc.constantFrom("apps/web/src/page.html", "apps/web/scripts/build.ts", "apps/web/media/demo.png")),
+    fc.string(),
+    (webPaths, suffix) => {
+      expect(apiUnaffectedByPaths([...webPaths, `src/${suffix}`])).toBe(false)
+      expect(apiUnaffectedByPaths([`src/${suffix}`, ...webPaths])).toBe(false)
+    },
+  ))
+})
diff --git a/scripts/api-deploy-selection.ts b/scripts/api-deploy-selection.ts
new file mode 100644
index 00000000..a63d7b40
--- /dev/null
+++ b/scripts/api-deploy-selection.ts
@@ -0,0 +1,38 @@
+import { spawnSync } from "node:child_process"
+
+/**
+ * Only site content/implementation directories, never workspace manifests.
+ * apps/api/src/api-deploy-selection.test.ts guards local module/static-asset
+ * imports. Future API filesystem reads from these directories must remove
+ * those paths from skip eligibility; import analysis cannot prove such reads.
+ */
+export function apiUnaffectedByPaths(paths: readonly string[]): boolean {
+  return paths.length > 0 && paths.every((path) =>
+    /^(?:apps\/web\/(?:src|public|media|scripts)\/)/u.test(path)
+    && !path.split("/").some((part) => part === ".." || part === "." || part === "")
+    && !/(?:^|\/)(?:package\.json|bun\.lockb?|package-lock\.json|pnpm-lock\.yaml|yarn\.lock|bunfig\.toml|\.[^/]+)$/u.test(path))
+}
+
+export function canSkipApiBuild(root: string, previous: string | undefined, current: string | undefined): boolean {
+  if (!previous || !current || !/^[a-f0-9]{40}$/u.test(previous) || !/^[a-f0-9]{40}$/u.test(current)) return false
+  const git = (...args: string[]) => spawnSync("git", args, { cwd: root, encoding: "utf8" })
+  if (git("rev-parse", "--verify", "HEAD").stdout?.trim() !== current) return false
+  if (git("cat-file", "-e", `${previous}^{commit}`).status !== 0) return false
+  if (git("merge-base", "--is-ancestor", previous, current).status !== 0) return false
+  const diff = git("diff", "--no-renames", "--name-only", "-z", previous, current, "--")
+  if (diff.status !== 0 || diff.error) return false
+  return apiUnaffectedByPaths(diff.stdout.split("\0").filter(Boolean))
+}
+
+if (import.meta.main) {
+  // Vercel runs this before install: unavailable history or uncertain input
+  // always builds. Empty diffs also build, preserving explicit redeploys.
+  let skip = false
+  try {
+    skip = canSkipApiBuild(process.cwd(), process.env.VERCEL_GIT_PREVIOUS_SHA, process.env.VERCEL_GIT_COMMIT_SHA)
+  } catch {
+    // An optimization must never prevent a deployment on an unknown input.
+  }
+  console.log(skip ? "API unchanged: website-only changes; skipping API build." : "Building API: shared, API, or unverified inputs.")
+  process.exitCode = skip ? 0 : 1
+}
diff --git a/vercel.json b/vercel.json
index 30c1fa94..920a7cfe 100644
--- a/vercel.json
+++ b/vercel.json
@@ -1,5 +1,6 @@
 {
   "$schema": "https://openapi.vercel.sh/vercel.json",
+  "ignoreCommand": "bun scripts/api-deploy-selection.ts",
   "buildCommand": "echo 'slopcamera-api: functions only'",
   "functions": {
     "api/**/*.ts": {