diff --git a/doc/DEVELOPING.md b/doc/DEVELOPING.md
index a60bf9403d2..3dedf6a52b9 100644
--- a/doc/DEVELOPING.md
+++ b/doc/DEVELOPING.md
@@ -4086,3 +4086,18 @@ one-time config-file seed preserves existing choices, then later starts use the
 database without loading home `.env` files or repairing worktree config. See
 [Cloud runtime configuration](cloud-runtime-configuration.md) for field ownership,
 secret handling, initialization order and migration interfaces.
+
+### Candidate compatibility with trusted test-project typechecks
+
+Trusted PR CI checks the server in low-memory layers, then runs:
+
+```sh
+node scripts/typecheck-test-projects.mjs --low-memory --reuse-declarations
+```
+
+Run that command immediately after a successful server typecheck on the same
+commit. It checks maintenance support, lifecycle, Runner E2E, and Speko projects
+against fresh server declarations and rejects accidental server source imports.
+For a standalone check, omit `--reuse-declarations` so the driver prepares and
+builds those declarations first. The ordinary project configs remain available
+to editors and Vitest; CI uses their separate `tsconfig.layers.json` files.
diff --git a/scripts/run-tsc.mjs b/scripts/run-tsc.mjs
new file mode 100644
index 00000000000..143c5f341e7
--- /dev/null
+++ b/scripts/run-tsc.mjs
@@ -0,0 +1,47 @@
+#!/usr/bin/env node
+// Runs the calling package's tsc, honoring the repository's low-memory mode.
+//
+// TypeScript 7 is a native binary, so Node heap flags do not limit it. Each
+// checker keeps its own copy of the types, which makes the checker count the
+// most useful control a package has over its peak memory.
+// scripts/typecheck-low-memory.mjs sets PAPERCLIP_TYPECHECK_LOW_MEMORY=1 so the
+// largest programs check on a single thread.
+//
+// A build that only needs JavaScript sets PAPERCLIP_TSC_EMIT_ONLY=1. That build
+// skips type checks and declarations, which use most of the memory of an emit.
+// The Docker image uses this mode.
+//
+// Usage: node ../scripts/run-tsc.mjs [tsc arguments]
+
+import { spawnSync } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+export const LOW_MEMORY_VARIABLE = "PAPERCLIP_TYPECHECK_LOW_MEMORY";
+export const EMIT_ONLY_VARIABLE = "PAPERCLIP_TSC_EMIT_ONLY";
+
+export function lowMemoryTscArgs(env = process.env) {
+  return env[LOW_MEMORY_VARIABLE] === "1" ? ["--singleThreaded"] : [];
+}
+
+export function emitOnlyTscArgs(env = process.env) {
+  return env[EMIT_ONLY_VARIABLE] === "1"
+    ? ["--noCheck", "--declaration", "false", "--declarationMap", "false"]
+    : [];
+}
+
+export function runTsc(args, { cwd = process.cwd(), env = process.env } = {}) {
+  const tsc = path.join(cwd, "node_modules", "typescript", "bin", "tsc");
+  // tsc only accepts -b as its first argument, so the caller's arguments go first.
+  const result = spawnSync(process.execPath, [tsc, ...args, ...lowMemoryTscArgs(env), ...emitOnlyTscArgs(env)], {
+    cwd,
+    env,
+    stdio: "inherit",
+  });
+  if (result.error) throw result.error;
+  return { status: result.status ?? 1, signal: result.signal };
+}
+
+if (process.argv[1] && path.resolve(process.argv[1]) === fileURLToPath(import.meta.url)) {
+  process.exitCode = runTsc(process.argv.slice(2)).status;
+}
diff --git a/scripts/typecheck-low-memory.mjs b/scripts/typecheck-low-memory.mjs
new file mode 100644
index 00000000000..22d44f7b07f
--- /dev/null
+++ b/scripts/typecheck-low-memory.mjs
@@ -0,0 +1,41 @@
+#!/usr/bin/env node
+// Runs the workspace typecheck with less memory, but more slowly. It checks one
+// package at a time, and scripts/run-tsc.mjs uses one thread for the largest
+// programs. See "Typecheck memory" in doc/DEVELOPING.md.
+//
+// Usage: node scripts/typecheck-low-memory.mjs
+
+import { spawnSync } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { LOW_MEMORY_VARIABLE } from "./run-tsc.mjs";
+
+export const STEPS = [
+  ["run", "preflight:workspace-links"],
+  ["-r", "--workspace-concurrency=1", "typecheck"],
+];
+
+export function runPnpm(args, options) {
+  const result = spawnSync("pnpm", args, {
+    ...options,
+    stdio: "inherit",
+    // A bare `pnpm` on Windows is pnpm.cmd, which needs the shell to resolve.
+    shell: process.platform === "win32",
+  });
+  if (result.error) throw result.error;
+  return result.status ?? 1;
+}
+
+export function typecheckLowMemory({ cwd, env = process.env, run = runPnpm }) {
+  for (const args of STEPS) {
+    const status = run(args, { cwd, env: { ...env, [LOW_MEMORY_VARIABLE]: "1" } });
+    if (status !== 0) return status;
+  }
+  return 0;
+}
+
+if (process.argv[1] && path.resolve(process.argv[1]) === fileURLToPath(import.meta.url)) {
+  process.exitCode = typecheckLowMemory({
+    cwd: path.resolve(path.dirname(fileURLToPath(import.meta.url)), ".."),
+  });
+}
diff --git a/scripts/typecheck-test-projects.mjs b/scripts/typecheck-test-projects.mjs
new file mode 100644
index 00000000000..8fd8cef014f
--- /dev/null
+++ b/scripts/typecheck-test-projects.mjs
@@ -0,0 +1,136 @@
+#!/usr/bin/env node
+// Typechecks the test projects outside `pnpm -r`.
+//
+// Each project imports server source through relative paths, so its tsconfig.json
+// is one program over most of the server. Its tsconfig.layers.json reads that
+// source as the declarations from server/scripts/typecheck-layers.mjs, which this
+// script builds first. It then fails if a project still loads server source,
+// because that is what makes these programs large. See "Typecheck memory" in
+// doc/DEVELOPING.md.
+//
+// --reuse-declarations skips the build. Use it only straight after a server
+// typecheck on the same commit, because stale declarations pass silently.
+//
+// Usage: node scripts/typecheck-test-projects.mjs [--low-memory] [--reuse-declarations] [project ...]
+
+import { spawnSync } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { LOW_MEMORY_VARIABLE, runTsc } from "./run-tsc.mjs";
+import { runPnpm } from "./typecheck-low-memory.mjs";
+import { isTestFile, typecheckLayers } from "../server/scripts/typecheck-layers.mjs";
+
+export const PROJECTS = [
+  "server/src/__tests__/helpers/maintenance",
+  "tests/lifecycle-baseline",
+  "tests/runner-e2e",
+  "tests/speko-native",
+];
+
+// The same steps as the server's typecheck script.
+export const PREPARE_STEPS = [
+  ["--filter", "@paperclipai/paperclip-runner", "run", "build:typescript"],
+  ["--filter", "@paperclipai/plugin-sdk", "ensure-build-deps"],
+];
+
+/**
+ * Finds the server source in a program's file list, as paths relative to the
+ * server. Test support files have no layer, and declaration and JSON files add
+ * little, so only other source files count.
+ */
+export function findServerSource(files, serverRoot) {
+  return files
+    .map((file) => path.relative(serverRoot, file).split(path.sep).join("/"))
+    .filter(
+      (file) =>
+        file.startsWith("src/") && /\.[cm]?[jt]sx?$/.test(file) && !/\.d\.[cm]?ts$/.test(file) && !isTestFile(file),
+    );
+}
+
+function listProgramFiles(config, { cwd, env }) {
+  const tsc = path.join(cwd, "node_modules", "typescript", "bin", "tsc");
+  const result = spawnSync(process.execPath, [tsc, "-p", config, "--listFilesOnly"], {
+    cwd,
+    env,
+    encoding: "utf8",
+    maxBuffer: 1 << 26,
+  });
+  if (result.error) throw result.error;
+  return { status: result.status ?? 1, files: result.stdout.split("\n").filter(Boolean), output: result.stdout };
+}
+
+export function typecheckTestProjects({
+  root,
+  projects = PROJECTS,
+  reuseDeclarations = false,
+  env = process.env,
+  pnpm = runPnpm,
+  run = runTsc,
+  listFiles = listProgramFiles,
+  log = console.log,
+  logError = console.error,
+}) {
+  const serverRoot = path.join(root, "server");
+  if (!reuseDeclarations) {
+    for (const args of PREPARE_STEPS) {
+      const status = pnpm(args, { cwd: root, env });
+      if (status !== 0) return status;
+    }
+
+    const layers = typecheckLayers({
+      serverRoot,
+      declarationsOnly: true,
+      run: (args, options) => run(args, { ...options, env }),
+      log,
+    });
+    // Report the server's own errors first. They would repeat in each project.
+    if (layers !== 0) return layers;
+  }
+
+  let exitCode = 0;
+  for (const project of projects) {
+    log(`typecheck:test-projects: ${project}`);
+    const config = path.posix.join(project, "tsconfig.layers.json");
+    const { status, signal } = run(["-p", config], { cwd: root, env });
+    if (signal) return status || 1;
+    if (status !== 0) exitCode = status;
+
+    const listing = listFiles(config, { cwd: root, env });
+    if (listing.status !== 0) {
+      logError(`typecheck:test-projects: could not list the files of ${project}\n${listing.output ?? ""}`);
+      exitCode ||= listing.status;
+      continue;
+    }
+    const source = findServerSource(listing.files, serverRoot);
+    if (source.length > 0) {
+      logError(
+        `typecheck:test-projects: ${project} loads ${source.length} server source files. ` +
+          `It must read them as layer declarations. First files: ${source.slice(0, 5).join(", ")}`,
+      );
+      exitCode ||= 1;
+    }
+  }
+  return exitCode;
+}
+
+if (process.argv[1] && path.resolve(process.argv[1]) === fileURLToPath(import.meta.url)) {
+  const args = process.argv.slice(2);
+  const flags = args.filter((arg) => arg.startsWith("--"));
+  const projects = args.filter((arg) => !arg.startsWith("--"));
+  const unknownFlags = flags.filter((flag) => !["--low-memory", "--reuse-declarations"].includes(flag));
+  const unknown = projects.filter((project) => !PROJECTS.includes(project));
+  if (unknownFlags.length > 0) {
+    console.error(`Unknown option: ${unknownFlags.join(", ")}`);
+    process.exitCode = 2;
+  } else if (unknown.length > 0) {
+    console.error(`Unknown project: ${unknown.join(", ")}. Known projects: ${PROJECTS.join(", ")}`);
+    process.exitCode = 2;
+  } else {
+    process.exitCode = typecheckTestProjects({
+      root: path.resolve(path.dirname(fileURLToPath(import.meta.url)), ".."),
+      projects: projects.length > 0 ? projects : PROJECTS,
+      reuseDeclarations: flags.includes("--reuse-declarations"),
+      env: args.includes("--low-memory") ? { ...process.env, [LOW_MEMORY_VARIABLE]: "1" } : process.env,
+    });
+  }
+}
diff --git a/server/package.json b/server/package.json
index 0265e98d04e..6556e9164a2 100644
--- a/server/package.json
+++ b/server/package.json
@@ -43,7 +43,7 @@
     "build:runtime": "node scripts/bundle-runtime.mjs",
     "start": "node dist/index.js",
     "prepare:runner-vendor": "node scripts/prepare-runner-vendor.mjs",
-    "typecheck": "pnpm run prepare:runner-vendor && pnpm --filter @paperclipai/plugin-sdk ensure-build-deps && tsc --noEmit",
+    "typecheck": "pnpm --filter @paperclipai/paperclip-runner run build:typescript && pnpm --filter @paperclipai/plugin-sdk ensure-build-deps && node scripts/typecheck-layers.mjs",
     "postinstall": "node scripts/install-copilot-assets.mjs"
   },
   "dependencies": {
diff --git a/server/scripts/typecheck-layers.mjs b/server/scripts/typecheck-layers.mjs
new file mode 100644
index 00000000000..95b0f22e9ee
--- /dev/null
+++ b/server/scripts/typecheck-layers.mjs
@@ -0,0 +1,280 @@
+// Typechecks the server in layers so that no single tsc process holds the whole
+// program in memory.
+//
+// tsc keeps every type it creates until it exits, so its peak grows with the
+// source it checks. The server is about 1,700 source files and 500 test files,
+// and one program over all of them needs more than 8 GiB at any setting. Most
+// of those files are not in an import cycle, so they can be checked separately
+// and read back as declarations:
+//
+//   below  files the largest import cycle depends on, plus unrelated files
+//   cycle  the largest import cycle, reading `below` as declarations
+//   above  files that depend on the cycle, reading both as declarations
+//   tests  test files, reading all three as declarations
+//
+// The layers come from the import graph on every run, so nothing has to move
+// and no list can go stale. The import scan does not have to be perfect: if it
+// misses an edge, a file imports one outside its layer's references and tsc
+// fails with TS6307 instead of skipping the file.
+//
+// server/tsconfig.json stays the single program that editors and `pnpm build`
+// use.
+//
+// Projects outside the server read the source layers as declarations through a
+// `tsconfig.layers.json` that references tsconfig.declarations.json in the output
+// directory. --declarations-only builds those layers and skips the tests layer.
+//
+// Usage: node scripts/typecheck-layers.mjs [--declarations-only]
+
+import { mkdirSync, readdirSync, readFileSync, rmSync, writeFileSync } from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { runTsc } from "../../scripts/run-tsc.mjs";
+
+const IMPORT_SPECIFIER = /(?:from\s+|import\s*\(\s*|import\s+)["'](\.[^"']+)["']/g;
+const OUTPUT_DIRECTORY = path.join("node_modules", ".cache", "paperclip-typecheck");
+
+export const isTestFile = (file) => /\.test\.tsx?$/.test(file) || file.split("/").includes("__tests__");
+// tsconfig.json excludes this tree. A test reaches its helpers through imports.
+const isExcludedFromProgram = (file) => file.startsWith("src/__tests__/");
+
+export function listSourceFiles(serverRoot) {
+  const files = [];
+  const walk = (directory) => {
+    for (const entry of readdirSync(path.join(serverRoot, directory), { withFileTypes: true })) {
+      const relative = `${directory}/${entry.name}`;
+      if (entry.isDirectory()) walk(relative);
+      else if (/\.tsx?$/.test(entry.name)) files.push(relative);
+    }
+  };
+  walk("src");
+  return files.sort();
+}
+
+/** Resolves the relative imports of each file. `files` maps a path to its text. */
+export function readImports(files) {
+  const imports = new Map();
+  for (const [file, text] of files) {
+    const modules = new Set();
+    const json = new Set();
+    for (const [, specifier] of text.matchAll(IMPORT_SPECIFIER)) {
+      const target = path.posix.join(path.posix.dirname(file), specifier);
+      if (target.endsWith(".json")) {
+        json.add(target);
+        continue;
+      }
+      const candidates = [
+        target,
+        target.replace(/\.js$/, ".ts"),
+        target.replace(/\.js$/, ".tsx"),
+        `${target}.ts`,
+        `${target}/index.ts`,
+      ];
+      const resolved = candidates.find((candidate) => files.has(candidate));
+      if (resolved) modules.add(resolved);
+    }
+    imports.set(file, { modules, json });
+  }
+  return imports;
+}
+
+/** Tarjan's algorithm, iterative because import chains can be very deep. */
+function largestCycle(graph) {
+  const index = new Map();
+  const low = new Map();
+  const onStack = new Set();
+  const stack = [];
+  let next = 0;
+  let largest = [];
+
+  for (const root of graph.keys()) {
+    if (index.has(root)) continue;
+    const work = [{ node: root, edges: [...graph.get(root)], position: 0 }];
+    index.set(root, next);
+    low.set(root, next);
+    next += 1;
+    stack.push(root);
+    onStack.add(root);
+    while (work.length > 0) {
+      const frame = work[work.length - 1];
+      if (frame.position < frame.edges.length) {
+        const edge = frame.edges[frame.position];
+        frame.position += 1;
+        if (!index.has(edge)) {
+          index.set(edge, next);
+          low.set(edge, next);
+          next += 1;
+          stack.push(edge);
+          onStack.add(edge);
+          work.push({ node: edge, edges: [...graph.get(edge)], position: 0 });
+        } else if (onStack.has(edge)) {
+          low.set(frame.node, Math.min(low.get(frame.node), index.get(edge)));
+        }
+        continue;
+      }
+      work.pop();
+      const parent = work[work.length - 1];
+      if (parent) low.set(parent.node, Math.min(low.get(parent.node), low.get(frame.node)));
+      if (low.get(frame.node) === index.get(frame.node)) {
+        const component = [];
+        let member;
+        do {
+          member = stack.pop();
+          onStack.delete(member);
+          component.push(member);
+        } while (member !== frame.node);
+        if (component.length > largest.length) largest = component;
+      }
+    }
+  }
+  return largest.length > 1 ? new Set(largest) : new Set();
+}
+
+function reachableFrom(start, graph) {
+  const seen = new Set(start);
+  const pending = [...start];
+  while (pending.length > 0) {
+    for (const edge of graph.get(pending.pop()) ?? []) {
+      if (!seen.has(edge)) {
+        seen.add(edge);
+        pending.push(edge);
+      }
+    }
+  }
+  return seen;
+}
+
+/**
+ * Sorts every file into the layers, in check order. `files` maps a path to its
+ * text. Declaration files are ambient, so every layer lists them.
+ */
+export function planLayers(files) {
+  const declarations = [...files.keys()].filter((file) => file.endsWith(".d.ts"));
+  const candidates = new Map([...files].filter(([file]) => !file.endsWith(".d.ts")));
+  const imports = readImports(candidates);
+
+  const graph = new Map();
+  const reversed = new Map();
+  for (const file of candidates.keys()) {
+    if (!isTestFile(file)) {
+      graph.set(file, new Set());
+      reversed.set(file, new Set());
+    }
+  }
+  for (const file of graph.keys()) {
+    for (const target of imports.get(file).modules) {
+      if (graph.has(target)) {
+        graph.get(file).add(target);
+        reversed.get(target).add(file);
+      }
+    }
+  }
+
+  const cycle = largestCycle(graph);
+  const dependents = reachableFrom(cycle, reversed);
+  const above = [...dependents].filter((file) => !cycle.has(file));
+  const below = [...graph.keys()].filter((file) => !dependents.has(file));
+  const tests = [...candidates.keys()].filter((file) => isTestFile(file) && !isExcludedFromProgram(file));
+
+  const layer = (name, members, references) => {
+    const sorted = [...members].sort();
+    const json = new Set(sorted.flatMap((file) => [...imports.get(file).json]));
+    return { name, sourceFiles: sorted, files: [...sorted, ...[...json].sort(), ...declarations], references };
+  };
+  return [
+    layer("below", below, []),
+    layer("cycle", cycle, ["below"]),
+    layer("above", above, ["below", "cycle"]),
+    layer("tests", tests, ["below", "cycle", "above"]),
+  ].filter((candidate) => candidate.sourceFiles.length > 0);
+}
+
+/** Builds the tsconfig for one layer. Paths are relative to the output directory. */
+export function layerConfig(layer, { emptyLayers = new Set() } = {}) {
+  const fromOutput = (file) => path.posix.join("../../..", file);
+  const references = layer.references
+    .filter((name) => !emptyLayers.has(name))
+    .map((name) => ({ path: `./tsconfig.${name}.json` }));
+  const output =
+    layer.name === "tests"
+      ? { composite: false, declaration: false, declarationMap: false, sourceMap: false, noEmit: true }
+      : {
+          composite: true,
+          declaration: true,
+          declarationMap: false,
+          sourceMap: false,
+          emitDeclarationOnly: true,
+          outDir: "./dts",
+          tsBuildInfoFile: `./${layer.name}.tsbuildinfo`,
+        };
+  return {
+    extends: fromOutput("tsconfig.json"),
+    compilerOptions: { ...output, rootDir: fromOutput("src") },
+    files: layer.files.map(fromOutput),
+    include: [],
+    references,
+  };
+}
+
+/** Builds the config other projects reference to read the source layers as declarations. */
+export function declarationsConfig(layers) {
+  return {
+    compilerOptions: { composite: true },
+    files: [],
+    references: layers
+      .filter((layer) => layer.name !== "tests")
+      .map((layer) => ({ path: `./tsconfig.${layer.name}.json` })),
+  };
+}
+
+export function typecheckLayers({
+  serverRoot,
+  declarationsOnly = false,
+  run = runTsc,
+  log = console.log,
+}) {
+  const sourceFiles = listSourceFiles(serverRoot);
+  const layers = planLayers(
+    new Map(sourceFiles.map((file) => [file, readFileSync(path.join(serverRoot, file), "utf8")])),
+  );
+  const planned = new Set(layers.map((layer) => layer.name));
+  const emptyLayers = new Set(["below", "cycle", "above", "tests"].filter((name) => !planned.has(name)));
+
+  const outputRoot = path.join(serverRoot, OUTPUT_DIRECTORY);
+  rmSync(outputRoot, { recursive: true, force: true });
+  mkdirSync(outputRoot, { recursive: true });
+  // Without this, the path under node_modules would make the declarations CommonJS.
+  writeFileSync(path.join(outputRoot, "package.json"), `${JSON.stringify({ type: "module" })}\n`);
+  for (const layer of layers) {
+    writeFileSync(
+      path.join(outputRoot, `tsconfig.${layer.name}.json`),
+      `${JSON.stringify(layerConfig(layer, { emptyLayers }), null, 2)}\n`,
+    );
+  }
+
+  writeFileSync(
+    path.join(outputRoot, "tsconfig.declarations.json"),
+    `${JSON.stringify(declarationsConfig(layers), null, 2)}\n`,
+  );
+
+  let exitCode = 0;
+  for (const layer of layers) {
+    if (declarationsOnly && layer.name === "tests") continue;
+    log(`typecheck:layers: ${layer.name} (${layer.sourceFiles.length} files)`);
+    const config = path.join(OUTPUT_DIRECTORY, `tsconfig.${layer.name}.json`);
+    const { status, signal } = run(["-p", config], { cwd: serverRoot });
+    // A killed process leaves no declarations, so later layers would only add noise.
+    if (signal) return status || 1;
+    // Later layers still run after type errors. Each file is checked in one
+    // layer, so this reports everything a single tsc run would.
+    if (status !== 0) exitCode = status;
+  }
+  return exitCode;
+}
+
+if (process.argv[1] && path.resolve(process.argv[1]) === fileURLToPath(import.meta.url)) {
+  process.exitCode = typecheckLayers({
+    serverRoot: path.resolve(path.dirname(fileURLToPath(import.meta.url)), ".."),
+    declarationsOnly: process.argv.includes("--declarations-only"),
+  });
+}
diff --git a/server/scripts/typecheck-layers.test.mjs b/server/scripts/typecheck-layers.test.mjs
new file mode 100644
index 00000000000..53ee817ad32
--- /dev/null
+++ b/server/scripts/typecheck-layers.test.mjs
@@ -0,0 +1,749 @@
+import { spawnSync } from "node:child_process";
+import { existsSync, mkdirSync, mkdtempSync, readdirSync, readFileSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+import { afterEach, describe, expect, it } from "vitest";
+
+import { emitOnlyTscArgs, lowMemoryTscArgs, runTsc } from "../../scripts/run-tsc.mjs";
+import { STEPS, typecheckLowMemory } from "../../scripts/typecheck-low-memory.mjs";
+import { findServerSource, PREPARE_STEPS, PROJECTS, typecheckTestProjects } from "../../scripts/typecheck-test-projects.mjs";
+import { declarationsConfig, layerConfig, listSourceFiles, planLayers, readImports, typecheckLayers } from "./typecheck-layers.mjs";
+
+const serverRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const sourcesOf = (layers, name) => layers.find((layer) => layer.name === name)?.sourceFiles;
+
+describe("readImports", () => {
+  it("resolves every import form that TypeScript follows", () => {
+    const files = new Map([
+      [
+        "src/a.ts",
+        [
+          `import { b } from "./b.js";`,
+          `export * from "./c.js";`,
+          `import "./d.js";`,
+          `const e = await import("./e.js");`,
+          `type F = typeof import("./f.js");`,
+          `import g from "./g/index.js";`,
+          `import h from "./h.js";`,
+          `import data from "./data.json" with { type: "json" };`,
+          `import { z } from "zod";`,
+        ].join("\n"),
+      ],
+      ["src/b.ts", ""],
+      ["src/c.ts", ""],
+      ["src/d.ts", ""],
+      ["src/e.ts", ""],
+      ["src/f.ts", ""],
+      ["src/g/index.ts", ""],
+      ["src/h.tsx", ""],
+    ]);
+
+    const { modules, json } = readImports(files).get("src/a.ts");
+
+    expect([...modules].sort()).toEqual([
+      "src/b.ts",
+      "src/c.ts",
+      "src/d.ts",
+      "src/e.ts",
+      "src/f.ts",
+      "src/g/index.ts",
+      "src/h.tsx",
+    ]);
+    expect([...json]).toEqual(["src/data.json"]);
+  });
+});
+
+describe("planLayers", () => {
+  it("separates the largest cycle from what it depends on and what depends on it", () => {
+    const layers = planLayers(
+      new Map([
+        ["src/leaf.ts", ""],
+        ["src/unrelated.ts", ""],
+        ["src/one.ts", `import "./two.js"; import "./leaf.js";`],
+        ["src/two.ts", `import "./one.js";`],
+        ["src/route.ts", `import "./one.js";`],
+        ["src/route.test.ts", `import "./route.js";`],
+        ["src/types/ambient.d.ts", ""],
+      ]),
+    );
+
+    expect(sourcesOf(layers, "below")).toEqual(["src/leaf.ts", "src/unrelated.ts"]);
+    expect(sourcesOf(layers, "cycle")).toEqual(["src/one.ts", "src/two.ts"]);
+    expect(sourcesOf(layers, "above")).toEqual(["src/route.ts"]);
+    expect(sourcesOf(layers, "tests")).toEqual(["src/route.test.ts"]);
+    for (const layer of layers) expect(layer.files).toContain("src/types/ambient.d.ts");
+  });
+
+  it("keeps test support files in the test layer and leaves src/__tests__ to imports", () => {
+    const layers = planLayers(
+      new Map([
+        ["src/a.ts", ""],
+        ["src/services/__tests__/fixtures.ts", ""],
+        ["src/__tests__/helpers/db.ts", ""],
+        ["src/__tests__/big.test.ts", ""],
+      ]),
+    );
+
+    expect(sourcesOf(layers, "below")).toEqual(["src/a.ts"]);
+    expect(sourcesOf(layers, "tests")).toEqual(["src/services/__tests__/fixtures.ts"]);
+  });
+
+  it("lists imported JSON in the layer that imports it", () => {
+    const layers = planLayers(
+      new Map([
+        ["src/a.ts", `import data from "./data.json";`],
+        ["src/one.ts", `import "./two.js"; import data from "./cycle.json";`],
+        ["src/two.ts", `import "./one.js";`],
+      ]),
+    );
+
+    expect(layers.find((layer) => layer.name === "below").files).toContain("src/data.json");
+    expect(layers.find((layer) => layer.name === "cycle").files).toContain("src/cycle.json");
+  });
+
+  it("omits layers with no files, as when the source has no cycle", () => {
+    const layers = planLayers(new Map([["src/a.ts", ""], ["src/b.ts", `import "./a.js";`]]));
+
+    expect(layers.map((layer) => layer.name)).toEqual(["below"]);
+    expect(sourcesOf(layers, "below")).toEqual(["src/a.ts", "src/b.ts"]);
+  });
+
+  it("covers the real server: one layer per file, and no import reaches a later layer", () => {
+    const sourceFiles = listSourceFiles(serverRoot);
+    const files = new Map(
+      sourceFiles.map((file) => [file, readFileSync(path.join(serverRoot, file), "utf8")]),
+    );
+    const layers = planLayers(files);
+
+    const planned = layers.flatMap((layer) => layer.sourceFiles);
+    expect(new Set(planned).size).toBe(planned.length);
+    expect(planned.sort()).toEqual(
+      sourceFiles.filter((file) => !file.endsWith(".d.ts") && !file.startsWith("src/__tests__/")).sort(),
+    );
+
+    const position = new Map(
+      layers.flatMap((layer, index) => layer.sourceFiles.map((file) => [file, index])),
+    );
+    const imports = readImports(new Map([...files].filter(([file]) => !file.endsWith(".d.ts"))));
+    for (const layer of layers.filter((candidate) => candidate.name !== "tests")) {
+      for (const file of layer.sourceFiles) {
+        for (const target of imports.get(file).modules) {
+          if (position.has(target)) expect(position.get(target)).toBeLessThanOrEqual(position.get(file));
+        }
+      }
+    }
+  });
+});
+
+describe("layerConfig", () => {
+  const layer = { name: "cycle", files: ["src/a.ts"], references: ["below"] };
+
+  it("emits declarations for source layers and references earlier ones", () => {
+    const config = layerConfig(layer);
+
+    expect(config.compilerOptions).toMatchObject({ composite: true, emitDeclarationOnly: true, outDir: "./dts" });
+    expect(config.extends).toBe("../../../tsconfig.json");
+    expect(config.files).toEqual(["../../../src/a.ts"]);
+    expect(config.references).toEqual([{ path: "./tsconfig.below.json" }]);
+  });
+
+  it("emits nothing for tests and drops references to empty layers", () => {
+    const config = layerConfig(
+      { name: "tests", files: [], references: ["below", "cycle"] },
+      { emptyLayers: new Set(["cycle"]) },
+    );
+
+    expect(config.compilerOptions).toMatchObject({ composite: false, noEmit: true });
+    expect(config.references).toEqual([{ path: "./tsconfig.below.json" }]);
+  });
+});
+
+describe("declarationsConfig", () => {
+  it("references the source layers that exist, and not tests", () => {
+    const config = declarationsConfig([{ name: "below" }, { name: "above" }, { name: "tests" }]);
+
+    expect(config.compilerOptions.composite).toBe(true);
+    expect(config.files).toEqual([]);
+    expect(config.references).toEqual([{ path: "./tsconfig.below.json" }, { path: "./tsconfig.above.json" }]);
+  });
+});
+
+describe("typecheckLayers", () => {
+  const roots = [];
+
+  afterEach(() => {
+    for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+  });
+
+  function serverFixture() {
+    const root = mkdtempSync(path.join(tmpdir(), "typecheck-layers-"));
+    roots.push(root);
+    mkdirSync(path.join(root, "src"));
+    writeFileSync(path.join(root, "src", "leaf.ts"), "");
+    writeFileSync(path.join(root, "src", "one.ts"), `import "./two.js"; import "./leaf.js";`);
+    writeFileSync(path.join(root, "src", "two.ts"), `import "./one.js";`);
+    writeFileSync(path.join(root, "src", "route.ts"), `import "./one.js";`);
+    writeFileSync(path.join(root, "src", "route.test.ts"), `import "./route.js";`);
+    return root;
+  }
+
+  it("runs the layers in order and marks the output directory as ESM", () => {
+    const root = serverFixture();
+    const configs = [];
+
+    const status = typecheckLayers({
+      serverRoot: root,
+      log: () => {},
+      run: (args) => {
+        configs.push(path.basename(args[1]));
+        return { status: 0, signal: null };
+      },
+    });
+
+    expect(status).toBe(0);
+    expect(configs).toEqual([
+      "tsconfig.below.json",
+      "tsconfig.cycle.json",
+      "tsconfig.above.json",
+      "tsconfig.tests.json",
+    ]);
+    const output = path.join(root, "node_modules", ".cache", "paperclip-typecheck");
+    expect(JSON.parse(readFileSync(path.join(output, "package.json"), "utf8"))).toEqual({ type: "module" });
+  });
+
+  it("skips the tests layer when only declarations are needed", () => {
+    const root = serverFixture();
+    const configs = [];
+
+    const status = typecheckLayers({
+      serverRoot: root,
+      declarationsOnly: true,
+      log: () => {},
+      run: (args) => {
+        configs.push(path.basename(args[1]));
+        return { status: 0, signal: null };
+      },
+    });
+
+    expect(status).toBe(0);
+    expect(configs).toEqual(["tsconfig.below.json", "tsconfig.cycle.json", "tsconfig.above.json"]);
+    const output = path.join(root, "node_modules", ".cache", "paperclip-typecheck");
+    expect(JSON.parse(readFileSync(path.join(output, "tsconfig.declarations.json"), "utf8")).references).toEqual([
+      { path: "./tsconfig.below.json" },
+      { path: "./tsconfig.cycle.json" },
+      { path: "./tsconfig.above.json" },
+    ]);
+  });
+
+  it("reports type errors from every layer", () => {
+    const root = serverFixture();
+    const configs = [];
+
+    const status = typecheckLayers({
+      serverRoot: root,
+      log: () => {},
+      run: (args) => {
+        configs.push(args[1]);
+        return { status: configs.length === 1 ? 2 : 0, signal: null };
+      },
+    });
+
+    expect(status).toBe(2);
+    expect(configs).toHaveLength(4);
+  });
+
+  it("stops when a process is killed, because later layers have no declarations", () => {
+    const root = serverFixture();
+    const configs = [];
+
+    const status = typecheckLayers({
+      serverRoot: root,
+      log: () => {},
+      run: (args) => {
+        configs.push(args[1]);
+        return { status: 1, signal: "SIGKILL" };
+      },
+    });
+
+    expect(status).toBe(1);
+    expect(configs).toHaveLength(1);
+  });
+});
+
+describe("typecheckLayers with the real compiler", () => {
+  const tsc = path.join(serverRoot, "node_modules", "typescript", "bin", "tsc");
+  const roots = [];
+
+  afterEach(() => {
+    for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+  });
+
+  function compilerFixture() {
+    const root = mkdtempSync(path.join(tmpdir(), "typecheck-layers-real-"));
+    roots.push(root);
+    mkdirSync(path.join(root, "src"));
+    writeFileSync(path.join(root, "package.json"), `${JSON.stringify({ type: "module" })}\n`);
+    writeFileSync(
+      path.join(root, "tsconfig.json"),
+      `${JSON.stringify({
+        compilerOptions: {
+          target: "ES2023",
+          module: "NodeNext",
+          moduleResolution: "NodeNext",
+          strict: true,
+          types: [],
+          outDir: "dist",
+          rootDir: "src",
+          resolveJsonModule: true,
+          skipLibCheck: true,
+        },
+        include: ["src"],
+      })}\n`,
+    );
+    const sources = {
+      "leaf.ts": `export const leaf = 1;\n`,
+      "one.ts": `import { two } from "./two.js";\nimport { leaf } from "./leaf.js";\nexport const one = () => leaf + two();\n`,
+      "two.ts": `import { one } from "./one.js";\nexport const two = (): number => one.length;\n`,
+      "route.ts": `import { one } from "./one.js";\nexport const route = () => one();\n`,
+      "route.test.ts": `import { route } from "./route.js";\nexport const checked: number = route();\n`,
+    };
+    for (const [name, text] of Object.entries(sources)) {
+      writeFileSync(path.join(root, "src", name), text);
+    }
+    return root;
+  }
+
+  function check(root) {
+    const output = [];
+    const status = typecheckLayers({
+      serverRoot: root,
+      log: () => {},
+      run: (args, { cwd }) => {
+        const result = spawnSync(process.execPath, [tsc, ...args], { cwd, encoding: "utf8" });
+        output.push(result.stdout, result.stderr);
+        return { status: result.status ?? 1, signal: result.signal };
+      },
+    });
+    return { status, output: output.join("") };
+  }
+
+  it("emits declarations between layers and checks every layer", () => {
+    const root = compilerFixture();
+
+    const { status, output } = check(root);
+
+    expect(output).toBe("");
+    expect(status).toBe(0);
+    const declarations = path.join(root, "node_modules", ".cache", "paperclip-typecheck", "dts");
+    for (const name of ["leaf.d.ts", "one.d.ts", "two.d.ts", "route.d.ts"]) {
+      expect(existsSync(path.join(declarations, name))).toBe(true);
+    }
+    expect(existsSync(path.join(declarations, "route.test.d.ts"))).toBe(false);
+  }, 60_000);
+
+  it("lets a project outside the server read the source layers as declarations", () => {
+    const root = compilerFixture();
+    const outside = path.join(root, "outside");
+    mkdirSync(outside);
+    writeFileSync(path.join(outside, "use.ts"), `import { route } from "../src/route.js";\nexport const used: number = route();\n`);
+    const config = path.join(outside, "tsconfig.layers.json");
+    writeFileSync(
+      config,
+      `${JSON.stringify({
+        extends: "../tsconfig.json",
+        compilerOptions: { noEmit: true, rootDir: "..", outDir: null },
+        include: ["*.ts"],
+        references: [{ path: "../node_modules/.cache/paperclip-typecheck/tsconfig.declarations.json" }],
+      })}\n`,
+    );
+    const buildDeclarations = () =>
+      typecheckLayers({
+        serverRoot: root,
+        declarationsOnly: true,
+        log: () => {},
+        run: (args, { cwd }) => {
+          const result = spawnSync(process.execPath, [tsc, ...args], { cwd, encoding: "utf8" });
+          return { status: result.status ?? 1, signal: result.signal };
+        },
+      });
+    const tscOutside = (...args) => spawnSync(process.execPath, [tsc, "-p", config, ...args], { cwd: root, encoding: "utf8" });
+
+    expect(buildDeclarations()).toBe(0);
+    const passing = tscOutside();
+    expect(passing.stdout).toBe("");
+    expect(passing.status).toBe(0);
+    const loaded = tscOutside("--listFilesOnly").stdout;
+    expect(loaded).toContain("paperclip-typecheck/dts/route.d.ts");
+    expect(loaded).not.toContain("src/route.ts");
+    expect(findServerSource(loaded.split("\n").filter(Boolean), root)).toEqual([]);
+
+    const layered = readFileSync(config, "utf8");
+    const { references: _references, ...withoutReferences } = JSON.parse(layered);
+    writeFileSync(config, `${JSON.stringify(withoutReferences)}\n`);
+    const allSource = tscOutside("--listFilesOnly").stdout.split("\n").filter(Boolean);
+    expect(findServerSource(allSource, root)).toContain("src/route.ts");
+    writeFileSync(config, layered);
+
+    writeFileSync(path.join(root, "src", "route.ts"), `import { one } from "./one.js";\nexport const route = () => String(one());\n`);
+    expect(buildDeclarations()).toBe(0);
+    const failing = tscOutside();
+    expect(failing.stdout).toContain("outside/use.ts");
+    expect(failing.stdout).toContain("TS2322");
+    expect(failing.status).not.toBe(0);
+  }, 60_000);
+
+  it("reports a changed export in the layers that depend on it", () => {
+    const root = compilerFixture();
+    writeFileSync(path.join(root, "src", "leaf.ts"), `export const renamedLeaf = 1;\n`);
+
+    const { status, output } = check(root);
+
+    expect(status).not.toBe(0);
+    expect(output).toMatch(/src\/one\.ts\(\d+,\d+\): error TS\d+/);
+    expect(output).toContain("leaf");
+  }, 60_000);
+
+  it("reports a type change through declarations in the test layer", () => {
+    const root = compilerFixture();
+    writeFileSync(
+      path.join(root, "src", "route.ts"),
+      `import { one } from "./one.js";\nexport const route = (): string => String(one());\n`,
+    );
+
+    const { status, output } = check(root);
+
+    expect(status).not.toBe(0);
+    expect(output).toMatch(/src\/route\.test\.ts\(\d+,\d+\): error TS2322/);
+  }, 60_000);
+
+  // Emitted declarations keep imports of .d.mts and JSON files, which tsc does not
+  // copy to the output directory. If those imports stopped resolving, skipLibCheck
+  // would hide it and the types would become `any`, so these errors would disappear.
+  it("keeps types from .d.mts and JSON imports in later layers", () => {
+    const root = compilerFixture();
+    mkdirSync(path.join(root, "src", "scripts"));
+    writeFileSync(path.join(root, "src", "scripts", "helper.mjs"), `export {};\n`);
+    writeFileSync(path.join(root, "src", "scripts", "helper.d.mts"), `export interface Manifest { version: number }\n`);
+    writeFileSync(path.join(root, "src", "catalog.json"), `${JSON.stringify({ tools: [{ name: "a" }] })}\n`);
+    writeFileSync(
+      path.join(root, "src", "leaf.ts"),
+      [
+        `import catalog from "./catalog.json" with { type: "json" };`,
+        `import type { Manifest } from "./scripts/helper.mjs";`,
+        `export type { Manifest };`,
+        `export type Tool = (typeof catalog.tools)[number];`,
+        `export const leaf = 1;`,
+        ``,
+      ].join("\n"),
+    );
+    const misuse = [
+      `import type { Manifest, Tool } from "./leaf.js";`,
+      `export const manifest: Manifest = { wrong: 1 };`,
+      `export const tool: Tool = { wrong: 1 };`,
+      ``,
+    ].join("\n");
+    writeFileSync(path.join(root, "src", "cycle-misuse.ts"), `import { one } from "./one.js";\nexport const used = one;\n${misuse}`);
+    writeFileSync(
+      path.join(root, "src", "two.ts"),
+      `import { one } from "./one.js";\nimport { used } from "./cycle-misuse.js";\nexport const two = (): number => one.length + used.length;\n`,
+    );
+    writeFileSync(path.join(root, "src", "misuse.test.ts"), misuse);
+
+    const { status, output } = check(root);
+
+    expect(status).not.toBe(0);
+    for (const file of ["cycle-misuse", "misuse\\.test"]) {
+      expect(output).toMatch(new RegExp(`src/${file}\\.ts\\(\\d+,\\d+\\): error TS2353.*'Manifest'`));
+      expect(output).toMatch(new RegExp(`src/${file}\\.ts\\(\\d+,\\d+\\): error TS2353.*'\\{ name: string; \\}'`));
+    }
+  }, 60_000);
+});
+
+describe("typecheckLowMemory", () => {
+  it("runs every step in low-memory mode and stops at a failure", () => {
+    const calls = [];
+    const run = (args, { env }) => {
+      calls.push([args, env.PAPERCLIP_TYPECHECK_LOW_MEMORY]);
+      return calls.length === 1 ? 0 : 3;
+    };
+
+    expect(typecheckLowMemory({ cwd: serverRoot, env: {}, run })).toBe(3);
+    expect(calls).toEqual(STEPS.map((args) => [args, "1"]));
+  });
+
+  it("does not run the typecheck after a failed preflight", () => {
+    const calls = [];
+
+    const status = typecheckLowMemory({
+      cwd: serverRoot,
+      env: {},
+      run: (args) => {
+        calls.push(args);
+        return 1;
+      },
+    });
+
+    expect(status).toBe(1);
+    expect(calls).toHaveLength(1);
+  });
+});
+
+describe("typecheckTestProjects", () => {
+  const roots = [];
+
+  afterEach(() => {
+    for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+  });
+
+  function repositoryFixture() {
+    const root = mkdtempSync(path.join(tmpdir(), "typecheck-test-projects-"));
+    roots.push(root);
+    mkdirSync(path.join(root, "server", "src"), { recursive: true });
+    writeFileSync(path.join(root, "server", "src", "leaf.ts"), "");
+    writeFileSync(path.join(root, "server", "src", "one.ts"), `import "./two.js"; import "./leaf.js";`);
+    writeFileSync(path.join(root, "server", "src", "two.ts"), `import "./one.js";`);
+    writeFileSync(path.join(root, "server", "src", "route.ts"), `import "./one.js";`);
+    writeFileSync(path.join(root, "server", "src", "route.test.ts"), `import "./route.js";`);
+    return root;
+  }
+
+  function runWith(
+    root,
+    {
+      pnpmStatus = () => 0,
+      tscResult = () => ({ status: 0, signal: null }),
+      listFiles = () => ({ status: 0, files: [] }),
+      ...options
+    } = {},
+  ) {
+    const calls = [];
+    const errors = [];
+    const status = typecheckTestProjects({
+      root,
+      listFiles,
+      log: () => {},
+      logError: (message) => errors.push(message),
+      pnpm: (args, { env }) => {
+        calls.push({ tool: "pnpm", args, env });
+        return pnpmStatus(args);
+      },
+      run: (args, { env }) => {
+        const target = path.basename(path.dirname(args[1])) === "paperclip-typecheck" ? path.basename(args[1]) : args[1];
+        calls.push({ tool: "tsc", target, env });
+        return tscResult(target);
+      },
+      ...options,
+    });
+    return { status, calls, errors };
+  }
+
+  it("prepares, builds declarations without the tests layer, and then checks the projects", () => {
+    const env = { MARKER: "1" };
+
+    const { status, calls } = runWith(repositoryFixture(), { projects: ["tests/a", "tests/b"], env });
+
+    expect(status).toBe(0);
+    expect(calls.filter((call) => call.tool === "pnpm").map((call) => call.args)).toEqual(PREPARE_STEPS);
+    expect(calls.filter((call) => call.tool === "tsc").map((call) => call.target)).toEqual([
+      "tsconfig.below.json",
+      "tsconfig.cycle.json",
+      "tsconfig.above.json",
+      "tests/a/tsconfig.layers.json",
+      "tests/b/tsconfig.layers.json",
+    ]);
+    expect(calls.every((call) => call.env === env)).toBe(true);
+  });
+
+  it("stops when the preparation or the server layers fail", () => {
+    const prepare = runWith(repositoryFixture(), { pnpmStatus: () => 3 });
+    expect(prepare.status).toBe(3);
+    expect(prepare.calls).toHaveLength(1);
+
+    const layers = runWith(repositoryFixture(), {
+      projects: ["tests/a"],
+      tscResult: (target) => ({ status: target === "tsconfig.cycle.json" ? 2 : 0, signal: null }),
+    });
+    expect(layers.status).toBe(2);
+    expect(layers.calls.some((call) => call.target === "tests/a/tsconfig.layers.json")).toBe(false);
+  });
+
+  it("checks every project after a type error, and stops when a process is killed", () => {
+    const errors = runWith(repositoryFixture(), {
+      projects: ["tests/a", "tests/b"],
+      tscResult: (target) => ({ status: target === "tests/a/tsconfig.layers.json" ? 2 : 0, signal: null }),
+    });
+    expect(errors.status).toBe(2);
+    expect(errors.calls.at(-1).target).toBe("tests/b/tsconfig.layers.json");
+
+    const killed = runWith(repositoryFixture(), {
+      projects: ["tests/a", "tests/b"],
+      tscResult: (target) =>
+        target === "tests/a/tsconfig.layers.json" ? { status: 1, signal: "SIGKILL" } : { status: 0, signal: null },
+    });
+    expect(killed.status).toBe(1);
+    expect(killed.calls.at(-1).target).toBe("tests/a/tsconfig.layers.json");
+  });
+
+  it("reuses declarations without preparing or building layers", () => {
+    const { status, calls } = runWith(repositoryFixture(), { projects: ["tests/a"], reuseDeclarations: true });
+
+    expect(status).toBe(0);
+    expect(calls.map((call) => call.target ?? call.tool)).toEqual(["tests/a/tsconfig.layers.json"]);
+  });
+
+  it("fails when a project loads server source, even if its types pass", () => {
+    const root = repositoryFixture();
+    const server = path.join(root, "server");
+
+    const { status, errors } = runWith(root, {
+      projects: ["tests/a", "tests/b"],
+      reuseDeclarations: true,
+      listFiles: (config) => ({
+        status: 0,
+        files: config.startsWith("tests/a/") ? [path.join(server, "src", "route.ts")] : [],
+      }),
+    });
+
+    expect(status).toBe(1);
+    expect(errors).toHaveLength(1);
+    expect(errors[0]).toContain("tests/a loads 1 server source files");
+    expect(errors[0]).toContain("src/route.ts");
+  });
+
+  it("fails when a project's files cannot be listed, and keeps a type error's status", () => {
+    const listing = runWith(repositoryFixture(), {
+      projects: ["tests/a"],
+      reuseDeclarations: true,
+      listFiles: () => ({ status: 3, files: [], output: "TS6053" }),
+    });
+    expect(listing.status).toBe(3);
+    expect(listing.errors[0]).toContain("TS6053");
+
+    const both = runWith(repositoryFixture(), {
+      projects: ["tests/a"],
+      reuseDeclarations: true,
+      tscResult: () => ({ status: 2, signal: null }),
+      listFiles: () => ({ status: 3, files: [], output: "" }),
+    });
+    expect(both.status).toBe(2);
+  });
+
+  it("counts only source files that a layer should provide", () => {
+    const server = path.join(path.sep, "repo", "server");
+    const file = (relative) => path.join(server, ...relative.split("/"));
+
+    expect(
+      findServerSource(
+        [
+          file("src/route.ts"),
+          file("src/view.tsx"),
+          file("src/legacy.js"),
+          file("src/types/express.d.ts"),
+          file("src/data.json"),
+          file("src/route.test.ts"),
+          file("src/__tests__/helpers/home.ts"),
+          file("node_modules/.cache/paperclip-typecheck/dts/route.d.ts"),
+          file("test-baselines/lifecycle.test.ts"),
+          path.join(path.sep, "repo", "tests", "a.ts"),
+        ],
+        server,
+      ),
+    ).toEqual(["src/route.ts", "src/view.tsx", "src/legacy.js"]);
+  });
+
+  it("covers every project under tests/, or names why it does not", () => {
+    // This project imports one small server file, so it stays one small program.
+    const singlePrograms = ["tests/runner-acceptance"];
+    const testsRoot = path.join(serverRoot, "..", "tests");
+    const projects = readdirSync(testsRoot, { withFileTypes: true })
+      .filter((entry) => entry.isDirectory() && existsSync(path.join(testsRoot, entry.name, "tsconfig.json")))
+      .map((entry) => `tests/${entry.name}`);
+
+    expect(projects.sort()).toEqual(
+      [...PROJECTS.filter((project) => project.startsWith("tests/")), ...singlePrograms].sort(),
+    );
+  });
+
+  it("prepares the same way as the server typecheck", () => {
+    const { scripts } = JSON.parse(readFileSync(path.join(serverRoot, "package.json"), "utf8"));
+    for (const step of PREPARE_STEPS) {
+      expect(scripts.typecheck).toContain(`pnpm ${step.join(" ")}`);
+    }
+  });
+
+  it.each(PROJECTS)("keeps the generated references out of the tsconfig.json of %s", (project) => {
+    const directory = path.join(serverRoot, "..", project);
+    const readConfig = (name) => JSON.parse(readFileSync(path.join(directory, name), "utf8"));
+
+    // Vitest reads tsconfig.json and fails when a referenced config does not exist yet.
+    expect(readConfig("tsconfig.json").references).toBeUndefined();
+    const layered = readConfig("tsconfig.layers.json");
+    expect(layered.extends).toBe("./tsconfig.json");
+    expect(layered.references.map((reference) => path.resolve(directory, reference.path))).toEqual([
+      path.join(serverRoot, "node_modules", ".cache", "paperclip-typecheck", "tsconfig.declarations.json"),
+    ]);
+  });
+});
+
+describe("runTsc", () => {
+  it("emits JavaScript without checks or declarations in emit-only mode", () => {
+    const root = mkdtempSync(path.join(tmpdir(), "run-tsc-emit-only-"));
+    try {
+      mkdirSync(path.join(root, "src"));
+      mkdirSync(path.join(root, "node_modules"));
+      symlinkSync(
+        realpathSync(path.join(serverRoot, "node_modules", "typescript")),
+        path.join(root, "node_modules", "typescript"),
+      );
+      writeFileSync(
+        path.join(root, "tsconfig.json"),
+        `${JSON.stringify({
+          compilerOptions: { declaration: true, declarationMap: true, types: [], outDir: "dist", rootDir: "src" },
+          include: ["src"],
+        })}\n`,
+      );
+      writeFileSync(path.join(root, "src", "a.ts"), `export const value: number = "not a number";\n`);
+
+      const { status } = runTsc([], { cwd: root, env: { ...process.env, PAPERCLIP_TSC_EMIT_ONLY: "1" } });
+
+      expect(status).toBe(0);
+      expect(existsSync(path.join(root, "dist", "a.js"))).toBe(true);
+      expect(existsSync(path.join(root, "dist", "a.d.ts"))).toBe(false);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  }, 60_000);
+
+  it("accepts build mode with low-memory mode on", () => {
+    const { status } = runTsc(["-b", "--dry"], {
+      cwd: serverRoot,
+      env: { ...process.env, PAPERCLIP_TYPECHECK_LOW_MEMORY: "1" },
+    });
+
+    expect(status).toBe(0);
+  });
+});
+
+describe("emitOnlyTscArgs", () => {
+  it("removes checks and declarations only in emit-only mode", () => {
+    expect(emitOnlyTscArgs({ PAPERCLIP_TSC_EMIT_ONLY: "1" })).toEqual([
+      "--noCheck",
+      "--declaration",
+      "false",
+      "--declarationMap",
+      "false",
+    ]);
+    expect(emitOnlyTscArgs({})).toEqual([]);
+  });
+
+});
+
+describe("lowMemoryTscArgs", () => {
+  it("uses one thread only when low-memory mode is on", () => {
+    expect(lowMemoryTscArgs({ PAPERCLIP_TYPECHECK_LOW_MEMORY: "1" })).toEqual(["--singleThreaded"]);
+    expect(lowMemoryTscArgs({ PAPERCLIP_TYPECHECK_LOW_MEMORY: "0" })).toEqual([]);
+    expect(lowMemoryTscArgs({})).toEqual([]);
+  });
+});
diff --git a/server/src/__tests__/helpers/maintenance/tsconfig.json b/server/src/__tests__/helpers/maintenance/tsconfig.json
new file mode 100644
index 00000000000..864a6b58de6
--- /dev/null
+++ b/server/src/__tests__/helpers/maintenance/tsconfig.json
@@ -0,0 +1,8 @@
+{
+  "extends": "../../../../../tsconfig.base.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "rootDir": "../../../../.."
+  },
+  "include": ["*.ts", "../../maintenance-*.test.ts", "../../decision-target-work.test.ts", "../../../types/express.d.ts"]
+}
diff --git a/server/src/__tests__/helpers/maintenance/tsconfig.layers.json b/server/src/__tests__/helpers/maintenance/tsconfig.layers.json
new file mode 100644
index 00000000000..589c934fec9
--- /dev/null
+++ b/server/src/__tests__/helpers/maintenance/tsconfig.layers.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": false,
+    "declarationMap": false
+  },
+  "references": [{ "path": "../../../../node_modules/.cache/paperclip-typecheck/tsconfig.declarations.json" }]
+}
diff --git a/tests/lifecycle-baseline/tsconfig.json b/tests/lifecycle-baseline/tsconfig.json
index 0fa94c55b6f..e6a31f8654b 100644
--- a/tests/lifecycle-baseline/tsconfig.json
+++ b/tests/lifecycle-baseline/tsconfig.json
@@ -7,5 +7,5 @@
     "types": ["node"],
     "typeRoots": ["../../cli/node_modules/@types", "../../server/node_modules/@types"]
   },
-  "include": ["./*.ts", "../../server/test-baselines/*.ts"]
+  "include": ["./*.ts", "../../server/test-baselines/*.ts", "../../server/src/types/express.d.ts"]
 }
diff --git a/tests/lifecycle-baseline/tsconfig.layers.json b/tests/lifecycle-baseline/tsconfig.layers.json
new file mode 100644
index 00000000000..f1c68632282
--- /dev/null
+++ b/tests/lifecycle-baseline/tsconfig.layers.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": false,
+    "declarationMap": false
+  },
+  "references": [{ "path": "../../server/node_modules/.cache/paperclip-typecheck/tsconfig.declarations.json" }]
+}
diff --git a/tests/runner-e2e/api-skill-update-prerequisite.test.ts b/tests/runner-e2e/api-skill-update-prerequisite.test.ts
index 3ebcd1addac..f810dc14c47 100644
--- a/tests/runner-e2e/api-skill-update-prerequisite.test.ts
+++ b/tests/runner-e2e/api-skill-update-prerequisite.test.ts
@@ -18,7 +18,7 @@ it("uses the exact authenticated update endpoint and returns real successful pre
 it.each(["skill_source_ref_unavailable", "skill_source_github_rate_limited", "skill_source_github_read_failed"])("retains only the supported closed %s/status while withholding private body fields", async code => {
   const { api } = fixture(); vi.stubGlobal("fetch", vi.fn(async () => Response.json({ code, error: "Bearer private-error-canary", details: { code, status: 429, token: "secret-value", repository: "private-origin" } }, { status: 422 })));
   const error = await api.getSkillSourceUpdateStatus(company, skill).catch(error => error);
-  expect(error).toBeInstanceOf(SkillSourceUpdateReadError); expect(error).toMatchObject({ status: 422, reason: code, providerStatus: 429 });
+  expect(error).toBeInstanceOf(SkillSourceUpdateReadError); if (!(error instanceof SkillSourceUpdateReadError)) throw error; expect(error).toMatchObject({ status: 422, reason: code, providerStatus: 429 });
   expect(JSON.stringify(error)).not.toMatch(/private-error|secret-value|private-origin|Bearer/); expect(error.message).not.toMatch(/private-error|secret-value|private-origin|Bearer/);
 });
 it.each(["unknown code", "mismatched code", "oversized", "invalid JSON", "missing code", "invalid provider status"])("does not infer a reason or leak response bytes for %s", async fault => {
@@ -32,7 +32,7 @@ it.each(["unknown code", "mismatched code", "oversized", "invalid JSON", "missin
   if (fault === "invalid provider status") body = JSON.stringify({ code, details: { status: "private-error-canary" } });
   vi.stubGlobal("fetch", vi.fn(async () => new Response(body, { status: 422 })));
   const error = await api.getSkillSourceUpdateStatus(company, skill).catch(error => error);
-  expect(error).toBeInstanceOf(SkillSourceUpdateReadError);
+  expect(error).toBeInstanceOf(SkillSourceUpdateReadError); if (!(error instanceof SkillSourceUpdateReadError)) throw error;
   expect(error.reason).toBe(fault === "invalid provider status" ? code : "unclassified"); expect(error.providerStatus).toBeNull();
   expect(JSON.stringify(error)).not.toContain("private-error-canary"); expect(error.message).not.toContain("private-error-canary");
 });
diff --git a/tests/runner-e2e/chat-catalog-domains/external-personal-secret.ts b/tests/runner-e2e/chat-catalog-domains/external-personal-secret.ts
index aeaed2f3d5d..7b642cdbea0 100644
--- a/tests/runner-e2e/chat-catalog-domains/external-personal-secret.ts
+++ b/tests/runner-e2e/chat-catalog-domains/external-personal-secret.ts
@@ -48,7 +48,7 @@ interface PrivateFixture {
   endpoint: Awaited<ReturnType<typeof secretTestEndpoint>>;
 }
 const fixtures = new WeakMap<ChatCatalogContext, PrivateFixture>();
-interface PersonalSetupAgent { id: string; companyId: string; lifecycleState: string; status: string; lifecycleError?: string | null; lifecycleHolds: string[]; adapterType: string; adapterConfig: Record<string, unknown> }
+interface PersonalSetupAgent { id: string; companyId: string; lifecycleState: string; status: string; lifecycleError?: string | null; lifecycleHolds: string[]; adapterType: string; adapterConfig: Record<string, unknown> & { env?: Record<string, unknown> } }
 const missingBindingLifecycleError = "Execution configuration check failed. Paperclip could not complete this step. It will retry automatically. If this continues, contact your instance administrator.";
 export function personalSecretUnboundConfigHash(config: Record<string, unknown>): string {
   const env = { ...(config.env as Record<string, unknown> | undefined) }; delete env.PUBLISHING_API_KEY;
diff --git a/tests/runner-e2e/chat-catalog-domains/identity-questions-and-approvals.ts b/tests/runner-e2e/chat-catalog-domains/identity-questions-and-approvals.ts
index 17c2b9673b0..9fcfbdb009b 100644
--- a/tests/runner-e2e/chat-catalog-domains/identity-questions-and-approvals.ts
+++ b/tests/runner-e2e/chat-catalog-domains/identity-questions-and-approvals.ts
@@ -2,7 +2,7 @@ import { chatCatalogNativePhaseDeadline, chatCatalogNativePhaseIsTimely, chatCat
 import { expect } from "@playwright/test";
 import { createIssueThreadInteractionSchema } from "../../../packages/shared/src/validators/issue.js";
 import { pollUntil } from "../api.js";
-import { checks, project, entity, hasLink, putDocument } from "./internal-support.js";
+import { checks, project, entity, hasLink, putDocument, type Entity } from "./internal-support.js";
 import { identityDefinition, identityObservation, observeIdentity, openStandingChat, readAgentResource, waitForReply, remainingIdentityObservationBudget, type Interaction } from "./identity-support.js";
 import type { ChatCatalogContext } from "../chat-catalog-contract.js";
 import { createAuthenticatedPerson, enrollPrerequisitePerson } from "./identity-auth.js";
diff --git a/tests/runner-e2e/chat-catalog-domains/internal-goal-hierarchy-v2.test.ts b/tests/runner-e2e/chat-catalog-domains/internal-goal-hierarchy-v2.test.ts
index b53bcee58fb..89d5b42ef17 100644
--- a/tests/runner-e2e/chat-catalog-domains/internal-goal-hierarchy-v2.test.ts
+++ b/tests/runner-e2e/chat-catalog-domains/internal-goal-hierarchy-v2.test.ts
@@ -49,7 +49,7 @@ it.each(["wrong parent", "missing child", "foreign goal", "no saved state", "mis
   expect(grade_goals(o).passed).toBe(false);
 });
 it("does not allow a retry to repair an invalid first parent or first foreign owner", () => {
-  for (const field of ["parentId", "ownerAgentId"]) {
+  for (const field of ["parentId", "ownerAgentId"] as const) {
     const o = valid(), first = o.state.firstCreation as InternalObservation;
     first.goals[1]![field] = "foreign";
     expect(grade_goals(o).passed).toBe(false);
diff --git a/tests/runner-e2e/chat-catalog-domains/internal-goals.ts b/tests/runner-e2e/chat-catalog-domains/internal-goals.ts
index 01d75a6844f..8e9341b2adb 100644
--- a/tests/runner-e2e/chat-catalog-domains/internal-goals.ts
+++ b/tests/runner-e2e/chat-catalog-domains/internal-goals.ts
@@ -1,5 +1,5 @@
 import type { ChatCatalogCase } from "../chat-catalog-contract.js";
-import { project, task, goal, putDocument, entity, observation, checks, hasLink, sameTask, type Entity, type InternalObservation } from "./internal-support.js";
+import { project, task, goal, putDocument, entity, observation, checks, hasLink, sameTask, internalResultLink, type Entity, type InternalObservation } from "./internal-support.js";
 import { makeDefinition, retryDefinition, fixtureWorker } from "./internal-definition.js";
 
 export function grade_goals(value: unknown) {
diff --git a/tests/runner-e2e/source-sdk-audit-capture.test.ts b/tests/runner-e2e/source-sdk-audit-capture.test.ts
index a80d8d17ecb..3f3cc13faeb 100644
--- a/tests/runner-e2e/source-sdk-audit-capture.test.ts
+++ b/tests/runner-e2e/source-sdk-audit-capture.test.ts
@@ -21,7 +21,7 @@ async function fixture(){
   await writeFile(path.join(dir,"index.js"),body);const outputs=createHash("sha256").update(JSON.stringify(["index.js",body.length])).update(body).digest("hex");
   await writeFile(path.join(dir,".paperclip-build-complete"),JSON.stringify({sources:"a".repeat(64),outputs}));
  }
- const context={root:handoff,private:true,runId:"123",attempt:"1",executionIds:[id],cellReportProducer:await inspectCellReportProducer()};
+ const context={root:handoff,private:true as const,runId:"123",attempt:"1",executionIds:[id],cellReportProducer:await inspectCellReportProducer()};
  const source=path.join(temp,"artifacts-private/cases",scenario,"snapshots/source-sdk-privacy-observations.json"),destination=path.join(cell,"snapshots/source-sdk-privacy-observations.json");
  const reject=()=>retainPreparedSourceSdk({repositoryRoot:repo,temporaryRoot:temp,executionIds:[id],sourceRevision:"a".repeat(40),installed:false});
  const copy=()=>copySharedEvidence(capture,cell,temp,[secret],{executionId:id,context});
diff --git a/tests/runner-e2e/tsconfig.layers.json b/tests/runner-e2e/tsconfig.layers.json
new file mode 100644
index 00000000000..f1c68632282
--- /dev/null
+++ b/tests/runner-e2e/tsconfig.layers.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": false,
+    "declarationMap": false
+  },
+  "references": [{ "path": "../../server/node_modules/.cache/paperclip-typecheck/tsconfig.declarations.json" }]
+}
diff --git a/tests/speko-native/tsconfig.layers.json b/tests/speko-native/tsconfig.layers.json
new file mode 100644
index 00000000000..f1c68632282
--- /dev/null
+++ b/tests/speko-native/tsconfig.layers.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": false,
+    "declarationMap": false
+  },
+  "references": [{ "path": "../../server/node_modules/.cache/paperclip-typecheck/tsconfig.declarations.json" }]
+}