diff --git a/docs/TEST_INTEGRITY.md b/docs/TEST_INTEGRITY.md
index d4eb51b..44c659d 100644
--- a/docs/TEST_INTEGRITY.md
+++ b/docs/TEST_INTEGRITY.md
@@ -2635,3 +2635,83 @@ test_integrity_note:
     did_relax_any_assertion: false
     if_true_explain_spec_basis: ""
 ```
+
+# Candidate engine and artifact sealing Phase A (2026-10-01)
+
+```yaml
+test_integrity_note:
+  change_type: new_tests
+  scenarios:
+    - id: SCN-candidate-engine-pure-fixture
+      given: Hermetic frozen fixture descriptors, bounded command data, explicit environment and a fixture artifact directory
+      when: The isolated Phase-A engine observes a child execution
+      then: Only bounded parent-observed terminal data and a nonpromotable SourceAttestation are returned
+    - id: SCN-artifact-manifest-seal
+      given: Fixed regular artifact bytes in a hermetic temporary directory
+      when: The parent seals the directory
+      then: Canonical sorted paths, byte lengths and SHA-256 values are write-once and verify unchanged
+  observable_outcomes:
+    - Raw exit, error, signal, timeout and overflow observations; zero exit plus overflow is not success
+    - No receipt identity, writer capability, registered-state handle or consumer eligibility
+    - Capped canonical manifests and refusals for links, traversal, special files, overwrite and post-seal drift
+  determinism_controls:
+    - Fixed bytes, injected executors and isolated temporary directories
+    - No retries, sleeps, network, registered control-plane state or provider process
+  anti_cheat_rationale:
+    prevents:
+      - Hard-coded PASS or fixture receipt promotion
+      - Ambient environment or registered-state capability leakage
+      - Snapshot/golden rubber-stamping and unbounded output masking
+      - Link, traversal, overwrite or post-seal artifact substitution
+  relaxation:
+    did_relax_any_assertion: false
+    if_true_explain_spec_basis: ""
+```
+
+# Candidate engine and artifact sealing Phase A corrective boundaries (2026-10-01)
+
+```yaml
+test_integrity_note:
+  change_type: new_tests
+  scenarios:
+    - id: SCN-candidate-engine-boundary-limits
+      given: "A fixture run with candidate execution limits"
+      when: "A required bound is missing, zero, oversized, or supplied to the injected native-shaped executor"
+      then: "Execution refuses before start or receives finite timeout and output caps without ambient defaults"
+    - id: SCN-candidate-engine-utf8-overflow
+      given: "A parent observes multi-byte UTF-8 output larger than its byte cap"
+      when: "The child exits zero with output overflow"
+      then: "Only a valid byte-bounded prefix is retained and the terminal observation is non-success"
+    - id: SCN-candidate-engine-immutable-input-and-environment
+      given: "A candidate input byte snapshot and explicit deterministic environment"
+      when: "The digest or byte length is absent or mismatched, an unallowlisted environment is requested, or caller data mutates"
+      then: "The run refuses before execution or preserves its independent immutable snapshot"
+    - id: SCN-artifact-manifest-bytewise-and-caps
+      given: "Fixed artifacts with bytewise-distinct UTF-8 paths and configured evidence limits"
+      when: "The parent seals files at or beyond count, file, aggregate, path, and manifest limits"
+      then: "Canonical UTF-8 byte ordering is stable and every over-limit artifact refuses"
+    - id: SCN-artifact-manifest-root-and-manifest-boundaries
+      given: "A candidate artifact root or final manifest path"
+      when: "A link, directory/special node, pre-existing seal, or oversized manifest is encountered"
+      then: "No path is followed or parsed as evidence and write-once sealing refuses"
+    - id: SCN-artifact-manifest-complete-drift
+      given: "A previously sealed manifest"
+      when: "An artifact is added or the manifest is replaced"
+      then: "Verification rejects drift or an invalid seal"
+  observable_outcomes:
+    - "UTF-8 retained bytes never exceed maxOutputBytes; overflow including exit zero is non-success"
+    - "Missing or mismatched input byte snapshots, unallowlisted environment names, and invalid bounds do not invoke the executor"
+    - "Manifest lstat/open/read caps, path ordering, link/special refusal, and seal-drift errors are externally observable"
+  determinism_controls:
+    - "Injected fixture executor, fixed byte buffers, temporary directories, and no ambient environment inheritance"
+    - "No network, registered control-plane state, provider process, or live receipt adapter"
+  anti_cheat_rationale:
+    prevents:
+      - "Character-count truncation retaining more bytes than the declared cap"
+      - "Ambient or syntactically valid but unapproved environment capability leakage"
+      - "Caller-supplied digest labels impersonating verified immutable input"
+      - "Link-following, unbounded manifest parsing, locale-dependent ordering, or post-seal substitution"
+  relaxation:
+    did_relax_any_assertion: false
+    if_true_explain_spec_basis: ""
+```
diff --git a/src/checks/artifact-manifest.mjs b/src/checks/artifact-manifest.mjs
new file mode 100644
index 0000000..5410c8a
--- /dev/null
+++ b/src/checks/artifact-manifest.mjs
@@ -0,0 +1,218 @@
+import { createHash } from 'node:crypto';
+import {
+  closeSync, constants, fstatSync, lstatSync, openSync, readdirSync, readSync, realpathSync, writeFileSync,
+} from 'node:fs';
+import { isAbsolute, join, relative, resolve } from 'node:path';
+import { TorchError } from '../kernel/errors.mjs';
+
+export const ARTIFACT_MANIFEST_FILE = '.torch-artifact-manifest-v1.json';
+export const ARTIFACT_MANIFEST_LIMITS = Object.freeze({
+  maxFiles: 64,
+  maxFileBytes: 1_048_576,
+  maxTotalBytes: 8_388_608,
+  maxPathBytes: 4_096,
+  maxManifestBytes: 65_536,
+});
+
+function fail(message, code, details) {
+  throw new TorchError(message, { code, details });
+}
+
+function canonicalJson(value) {
+  return JSON.stringify(value);
+}
+
+function digest(value) {
+  return createHash('sha256').update(canonicalJson(value)).digest('hex');
+}
+
+function limits(input = {}) {
+  if (!input || typeof input !== 'object' || Array.isArray(input)) fail('Artifact limits must be an object', 'CANDIDATE_ARTIFACT_LIMIT_INVALID');
+  const allowed = new Set(Object.keys(ARTIFACT_MANIFEST_LIMITS));
+  for (const key of Object.keys(input)) {
+    if (!allowed.has(key)) fail('Artifact limit is unsupported', 'CANDIDATE_ARTIFACT_LIMIT_INVALID', { key });
+  }
+  const result = {};
+  for (const [key, maximum] of Object.entries(ARTIFACT_MANIFEST_LIMITS)) {
+    const value = input[key] ?? maximum;
+    if (!Number.isInteger(value) || value < 1 || value > maximum) fail('Artifact limit is invalid', 'CANDIDATE_ARTIFACT_LIMIT_INVALID', { key, value, maximum });
+    result[key] = value;
+  }
+  return result;
+}
+
+function bytewisePathOrder(left, right) {
+  return Buffer.compare(Buffer.from(left, 'utf8'), Buffer.from(right, 'utf8'));
+}
+
+export function assertArtifactRelativePath(path, maxPathBytes = ARTIFACT_MANIFEST_LIMITS.maxPathBytes) {
+  if (typeof path !== 'string' || !path || isAbsolute(path) || path.includes('\\')
+    || path.includes('\0') || Buffer.byteLength(path, 'utf8') > maxPathBytes
+    || path.split('/').some((part) => !part || part === '.' || part === '..')) {
+    fail('Artifact path is not canonical relative evidence', 'CANDIDATE_ARTIFACT_PATH_INVALID', { path });
+  }
+  return path;
+}
+
+export function artifactNodeKind(stat, path) {
+  if (stat.isSymbolicLink()) fail('Artifact cannot be a link', 'CANDIDATE_ARTIFACT_LINK_REJECTED', { path });
+  if (stat.isDirectory()) return 'directory';
+  if (stat.isFile()) return 'file';
+  fail('Artifact must be a regular file or directory', 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED', { path });
+}
+
+function lstatOrNull(path) {
+  try { return lstatSync(path); }
+  catch (error) {
+    if (error?.code === 'ENOENT') return null;
+    throw error;
+  }
+}
+
+function readRegularFileCapped(path, limit, label) {
+  const beforePath = lstatOrNull(path);
+  if (!beforePath) fail(`${label} is missing`, 'CANDIDATE_ARTIFACT_MANIFEST_MISSING', { path });
+  artifactNodeKind(beforePath, path);
+  if (!beforePath.isFile()) fail(`${label} must be a regular file`, 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED', { path });
+  if (beforePath.size > limit) fail(`${label} exceeds its byte limit`, 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED', { path, limit });
+
+  let descriptor;
+  try {
+    descriptor = openSync(path, constants.O_RDONLY | constants.O_NOFOLLOW);
+    const before = fstatSync(descriptor);
+    if (!before.isFile()) fail(`${label} must be a regular file`, 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED', { path });
+    if (before.size > limit) fail(`${label} exceeds its byte limit`, 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED', { path, limit });
+    const buffer = Buffer.alloc(before.size + 1);
+    let offset = 0;
+    while (offset < buffer.length) {
+      const read = readSync(descriptor, buffer, offset, buffer.length - offset, offset);
+      if (read === 0) break;
+      offset += read;
+    }
+    const after = fstatSync(descriptor);
+    if (after.size !== before.size || offset !== before.size) {
+      fail(`${label} changed while being read`, 'CANDIDATE_ARTIFACT_SEAL_DRIFT', { path });
+    }
+    return buffer.subarray(0, before.size);
+  } catch (error) {
+    if (error instanceof TorchError) throw error;
+    fail(`${label} cannot be read as a regular file`, 'CANDIDATE_ARTIFACT_LINK_REJECTED', { path });
+  } finally {
+    if (descriptor !== undefined) closeSync(descriptor);
+  }
+}
+
+function inventory(root, configuredLimits) {
+  const files = [];
+  let totalBytes = 0;
+  function visit(current, parent = '') {
+    for (const name of readdirSync(current).sort(bytewisePathOrder)) {
+      if (!parent && name === ARTIFACT_MANIFEST_FILE) continue;
+      const path = parent ? `${parent}/${name}` : name;
+      assertArtifactRelativePath(path, configuredLimits.maxPathBytes);
+      const absolute = join(current, name);
+      const stat = lstatSync(absolute);
+      const kind = artifactNodeKind(stat, path);
+      if (kind === 'directory') visit(absolute, path);
+      else {
+        if (stat.size > configuredLimits.maxFileBytes) fail('Artifact exceeds per-file limit', 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED', { path });
+        totalBytes += stat.size;
+        if (files.length + 1 > configuredLimits.maxFiles || totalBytes > configuredLimits.maxTotalBytes) {
+          fail('Artifact manifest exceeds aggregate limits', 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED');
+        }
+        const bytes = readRegularFileCapped(absolute, configuredLimits.maxFileBytes, 'Artifact');
+        if (bytes.length !== stat.size || bytes.length > configuredLimits.maxFileBytes) {
+          fail('Artifact changed while being read', 'CANDIDATE_ARTIFACT_SEAL_DRIFT', { path });
+        }
+        files.push({ path, bytes: bytes.length, sha256: createHash('sha256').update(bytes).digest('hex') });
+      }
+    }
+  }
+  visit(root);
+  return { files: files.sort((left, right) => bytewisePathOrder(left.path, right.path)), totalBytes };
+}
+
+function validateManifest(manifest, configuredLimits) {
+  if (!manifest || manifest.schema !== 'torch.dev/artifact-manifest/v1alpha1' || !Array.isArray(manifest.files)
+    || !Number.isInteger(manifest.totalBytes) || typeof manifest.sealDigest !== 'string') {
+    fail('Artifact manifest is malformed', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+  }
+  const manifestKeys = new Set(['schema', 'files', 'totalBytes', 'sealDigest']);
+  for (const key of Object.keys(manifest)) {
+    if (!manifestKeys.has(key)) fail('Artifact manifest contains unsupported fields', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID', { key });
+  }
+  const seen = new Set();
+  let totalBytes = 0;
+  for (const record of manifest.files) {
+    if (!record || typeof record !== 'object' || Array.isArray(record)
+      || Object.keys(record).some((key) => !new Set(['path', 'bytes', 'sha256']).has(key))) {
+      fail('Artifact manifest record is invalid', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+    }
+    assertArtifactRelativePath(record.path, configuredLimits.maxPathBytes);
+    if (seen.has(record.path)) fail('Artifact manifest contains duplicate paths', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+    seen.add(record.path);
+    if (!Number.isInteger(record.bytes) || record.bytes < 0 || record.bytes > configuredLimits.maxFileBytes
+      || !/^[0-9a-f]{64}$/.test(record.sha256 ?? '')) fail('Artifact manifest record is invalid', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+    totalBytes += record.bytes;
+  }
+  if (manifest.files.length > configuredLimits.maxFiles || totalBytes !== manifest.totalBytes || totalBytes > configuredLimits.maxTotalBytes) {
+    fail('Artifact manifest limits do not match', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+  }
+  const sorted = [...manifest.files].sort((left, right) => bytewisePathOrder(left.path, right.path));
+  if (canonicalJson(sorted) !== canonicalJson(manifest.files)) fail('Artifact manifest paths are not canonical', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID');
+  const payload = { schema: manifest.schema, files: manifest.files, totalBytes: manifest.totalBytes };
+  if (digest(payload) !== manifest.sealDigest) fail('Artifact manifest seal digest is invalid', 'CANDIDATE_ARTIFACT_SEAL_INVALID');
+  return payload;
+}
+
+function ownedRoot(directory) {
+  if (typeof directory !== 'string' || !directory) fail('Artifact directory is missing', 'CANDIDATE_ARTIFACT_DIRECTORY_INVALID');
+  const initial = lstatOrNull(directory);
+  if (!initial) fail('Artifact directory is missing', 'CANDIDATE_ARTIFACT_DIRECTORY_INVALID');
+  if (initial.isSymbolicLink()) fail('Artifact directory cannot be a link', 'CANDIDATE_ARTIFACT_LINK_REJECTED', { directory });
+  if (!initial.isDirectory()) fail('Artifact directory is not a directory', 'CANDIDATE_ARTIFACT_DIRECTORY_INVALID');
+  const root = realpathSync(directory);
+  if (!lstatSync(root).isDirectory()) fail('Artifact directory is not a directory', 'CANDIDATE_ARTIFACT_DIRECTORY_INVALID');
+  return root;
+}
+
+export function sealArtifactManifest({ directory, limits: requestedLimits } = {}) {
+  const root = ownedRoot(directory);
+  const configuredLimits = limits(requestedLimits);
+  const path = join(root, ARTIFACT_MANIFEST_FILE);
+  if (lstatOrNull(path)) fail('Artifact manifest is already sealed', 'CANDIDATE_ARTIFACT_ALREADY_SEALED');
+  const records = inventory(root, configuredLimits);
+  const payload = { schema: 'torch.dev/artifact-manifest/v1alpha1', ...records };
+  const manifest = { ...payload, sealDigest: digest(payload) };
+  const serialized = Buffer.from(`${canonicalJson(manifest)}\n`, 'utf8');
+  if (serialized.length > configuredLimits.maxManifestBytes) {
+    fail('Artifact manifest exceeds its byte limit', 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED');
+  }
+  writeFileSync(path, serialized, { mode: 0o600, flag: 'wx' });
+  return Object.freeze({ ...manifest, manifestPath: path });
+}
+
+export function verifyArtifactManifest({ directory, limits: requestedLimits } = {}) {
+  const root = ownedRoot(directory);
+  const configuredLimits = limits(requestedLimits);
+  const path = join(root, ARTIFACT_MANIFEST_FILE);
+  const manifestStat = lstatOrNull(path);
+  if (!manifestStat) fail('Artifact manifest is missing', 'CANDIDATE_ARTIFACT_MANIFEST_MISSING');
+  artifactNodeKind(manifestStat, ARTIFACT_MANIFEST_FILE);
+  if (!manifestStat.isFile()) fail('Artifact manifest must be a regular file', 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED');
+  if (manifestStat.size > configuredLimits.maxManifestBytes) {
+    fail('Artifact manifest exceeds its byte limit', 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED');
+  }
+  let manifest;
+  try { manifest = JSON.parse(readRegularFileCapped(path, configuredLimits.maxManifestBytes, 'Artifact manifest').toString('utf8')); }
+  catch { fail('Artifact manifest cannot be parsed', 'CANDIDATE_ARTIFACT_MANIFEST_INVALID'); }
+  const payload = validateManifest(manifest, configuredLimits);
+  const current = inventory(root, configuredLimits);
+  if (canonicalJson(current) !== canonicalJson({ files: payload.files, totalBytes: payload.totalBytes })) {
+    fail('Artifact contents changed after sealing', 'CANDIDATE_ARTIFACT_SEAL_DRIFT');
+  }
+  if (resolve(path) !== join(root, ARTIFACT_MANIFEST_FILE) || relative(root, path).startsWith('..')) {
+    fail('Artifact manifest escaped its directory', 'CANDIDATE_ARTIFACT_DIRECTORY_INVALID');
+  }
+  return Object.freeze({ ...manifest, manifestPath: path, verified: true });
+}
diff --git a/src/checks/candidate-engine.mjs b/src/checks/candidate-engine.mjs
new file mode 100644
index 0000000..df52268
--- /dev/null
+++ b/src/checks/candidate-engine.mjs
@@ -0,0 +1,224 @@
+import { spawnSync } from 'node:child_process';
+import { createHash } from 'node:crypto';
+import { TorchError } from '../kernel/errors.mjs';
+
+export const CANDIDATE_ENGINE_LIMITS = Object.freeze({
+  maxInputBytes: 1_048_576,
+  maxDefinitionBytes: 65_536,
+  maxArgs: 64,
+  maxArgBytes: 4_096,
+  maxEnvironmentEntries: 3,
+  maxEnvironmentNameBytes: 128,
+  maxEnvironmentValueBytes: 4_096,
+  maxOutputBytes: 65_536,
+  maxArtifactPathBytes: 4_096,
+  timeoutMs: 60_000,
+});
+
+/** Deterministic variables only; no ambient process environment is inherited. */
+export const CANDIDATE_ENGINE_ENV_ALLOWLIST = Object.freeze({
+  LANG: Object.freeze(['C.UTF-8']),
+  LC_ALL: Object.freeze(['C']),
+  TZ: Object.freeze(['UTC']),
+});
+
+function fail(message, code, details) {
+  throw new TorchError(message, { code, details });
+}
+
+function byteLength(value) {
+  return Buffer.byteLength(value, 'utf8');
+}
+
+function sha256(bytes) {
+  return createHash('sha256').update(bytes).digest('hex');
+}
+
+function exactObject(value, allowed, label) {
+  if (!value || typeof value !== 'object' || Array.isArray(value)) {
+    fail(`${label} must be an object`, 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  for (const key of Object.keys(value)) {
+    if (!allowed.has(key)) fail(`${label} contains unsupported capability data`, 'CANDIDATE_ENGINE_CAPABILITY_FORBIDDEN', { key });
+  }
+}
+
+function boundedString(value, limit, label) {
+  if (typeof value !== 'string' || !value || byteLength(value) > limit) {
+    fail(`${label} is missing or exceeds its limit`, 'CANDIDATE_ENGINE_INPUT_INVALID', { label, limit });
+  }
+  return value;
+}
+
+function normalizeLimits(limits) {
+  exactObject(limits, new Set(Object.keys(CANDIDATE_ENGINE_LIMITS)), 'limits');
+  const normalized = {};
+  for (const [key, maximum] of Object.entries(CANDIDATE_ENGINE_LIMITS)) {
+    if (!Object.hasOwn(limits, key)) {
+      fail(`Candidate-engine limit is required: ${key}`, 'CANDIDATE_ENGINE_LIMIT_INVALID', { key });
+    }
+    const value = limits[key];
+    if (!Number.isInteger(value) || value < 1 || value > maximum) {
+      fail(`Invalid candidate-engine limit: ${key}`, 'CANDIDATE_ENGINE_LIMIT_INVALID', { key, value, maximum });
+    }
+    normalized[key] = value;
+  }
+  return Object.freeze(normalized);
+}
+
+function freeze(value) {
+  if (value && typeof value === 'object' && !Object.isFrozen(value)) {
+    for (const item of Object.values(value)) freeze(item);
+    Object.freeze(value);
+  }
+  return value;
+}
+
+function normalizeInput(input, limits) {
+  exactObject(input, new Set(['directory', 'digest', 'bytes', 'byteLength']), 'input');
+  const directory = boundedString(input.directory, limits.maxArtifactPathBytes, 'input.directory');
+  if (!Buffer.isBuffer(input.bytes) && !(input.bytes instanceof Uint8Array)) {
+    fail('Candidate input bytes are required', 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  const snapshot = Buffer.from(input.bytes);
+  if (snapshot.length > limits.maxInputBytes || !Number.isInteger(input.byteLength)
+    || input.byteLength !== snapshot.length || typeof input.digest !== 'string'
+    || !/^[0-9a-f]{64}$/i.test(input.digest)) {
+    fail('Candidate input must be a bounded immutable byte snapshot', 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  const observedDigest = sha256(snapshot);
+  if (input.digest.toLowerCase() !== observedDigest) {
+    fail('Candidate input digest does not bind the supplied bytes', 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  return freeze({ directory, digest: observedDigest, bytes: snapshot.length });
+}
+
+function normalizeDefinition(definition, limits) {
+  exactObject(definition, new Set(['command', 'args']), 'definition');
+  const command = boundedString(definition.command, limits.maxArgBytes, 'definition.command');
+  const args = normalizeArgs(definition.args ?? [], limits, 'definition.args');
+  const normalized = { command, args };
+  if (byteLength(JSON.stringify(normalized)) > limits.maxDefinitionBytes) {
+    fail('Candidate definition exceeds its limit', 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  return freeze(normalized);
+}
+
+function normalizeArgs(args, limits, label = 'args') {
+  if (!Array.isArray(args) || args.length > limits.maxArgs) {
+    fail(`${label} exceeds its count limit`, 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  return freeze(args.map((value) => boundedString(value, limits.maxArgBytes, label)));
+}
+
+function normalizeEnvironment(environment, limits) {
+  exactObject(environment, new Set(Object.keys(environment ?? {})), 'environment');
+  const entries = Object.entries(environment);
+  if (entries.length > limits.maxEnvironmentEntries) {
+    fail('Candidate environment exceeds its entry limit', 'CANDIDATE_ENGINE_INPUT_INVALID');
+  }
+  const normalized = {};
+  for (const [key, value] of entries.sort(([left], [right]) => Buffer.compare(Buffer.from(left, 'utf8'), Buffer.from(right, 'utf8')))) {
+    const allowedValues = CANDIDATE_ENGINE_ENV_ALLOWLIST[key];
+    if (!allowedValues || byteLength(key) > limits.maxEnvironmentNameBytes) {
+      fail('Candidate environment name is not allowlisted', 'CANDIDATE_ENGINE_ENVIRONMENT_INVALID', { key });
+    }
+    const bounded = boundedString(value, limits.maxEnvironmentValueBytes, `environment.${key}`);
+    if (!allowedValues.includes(bounded)) {
+      fail('Candidate environment value is not allowlisted', 'CANDIDATE_ENGINE_ENVIRONMENT_INVALID', { key });
+    }
+    normalized[key] = bounded;
+  }
+  return freeze(normalized);
+}
+
+function safeUtf8Prefix(bytes, limit) {
+  const capped = Math.min(bytes.length, limit);
+  const decoder = new TextDecoder('utf-8', { fatal: true });
+  for (let length = capped; length >= 0; length -= 1) {
+    try {
+      const text = decoder.decode(bytes.subarray(0, length));
+      if (byteLength(text) <= limit) return text;
+    } catch {
+      // Shorten until the prefix ends on a complete UTF-8 sequence.
+    }
+  }
+  return '';
+}
+
+function boundedOutput(value, limit) {
+  const output = Buffer.isBuffer(value) ? Buffer.from(value)
+    : Buffer.from(typeof value === 'string' ? value : value?.toString?.() ?? '', 'utf8');
+  const bytes = output.length;
+  const text = safeUtf8Prefix(output, bytes > limit ? limit : bytes);
+  return freeze({ bytes, retainedBytes: byteLength(text), text, overflow: bytes > limit });
+}
+
+function normalizeError(error) {
+  if (!error) return null;
+  return freeze({
+    code: typeof error.code === 'string' ? boundedOutput(error.code, 128).text : null,
+    message: boundedOutput(typeof error.message === 'string' ? error.message : String(error), 512).text,
+  });
+}
+
+function terminalObservation(result, limits) {
+  const stdout = boundedOutput(result?.stdout, limits.maxOutputBytes);
+  const stderr = boundedOutput(result?.stderr, limits.maxOutputBytes);
+  const error = normalizeError(result?.error);
+  const signal = typeof result?.signal === 'string' ? result.signal : null;
+  const exitStatus = Number.isInteger(result?.status) ? result.status : null;
+  const timedOut = result?.timedOut === true || error?.code === 'ETIMEDOUT';
+  const overflow = stdout.overflow || stderr.overflow || error?.code === 'ENOBUFS';
+  let kind = 'exit';
+  if (timedOut) kind = 'timeout';
+  else if (overflow) kind = 'overflow';
+  else if (signal) kind = 'signal';
+  else if (error) kind = 'error';
+  return freeze({
+    kind, exitStatus, signal, error, timedOut, overflow,
+    success: kind === 'exit' && exitStatus === 0,
+    stdout, stderr,
+  });
+}
+
+/**
+ * Fixture-only Phase-A seam. It deliberately exposes a raw parent observation,
+ * never a receipt, capability, ControlPlane handle, or registered project root.
+ */
+export function runCandidateEngine(options = {}) {
+  exactObject(options, new Set(['input', 'definition', 'args', 'environment', 'artifactDirectory', 'executor', 'limits']), 'candidate engine options');
+  const limits = normalizeLimits(options.limits);
+  const input = normalizeInput(options.input, limits);
+  const definition = normalizeDefinition(options.definition, limits);
+  const args = normalizeArgs(options.args ?? [], limits);
+  const environment = normalizeEnvironment(options.environment ?? {}, limits);
+  const artifactDirectory = boundedString(options.artifactDirectory, limits.maxArtifactPathBytes, 'artifactDirectory');
+  const executor = options.executor ?? ((command, commandArgs, execution) => spawnSync(command, commandArgs, execution));
+  if (typeof executor !== 'function') fail('Candidate executor must be a function', 'CANDIDATE_ENGINE_INPUT_INVALID');
+
+  const commandArgs = freeze([...definition.args, ...args]);
+  const execution = freeze({
+    cwd: input.directory,
+    env: environment,
+    encoding: 'utf8',
+    stdio: ['ignore', 'pipe', 'pipe'],
+    maxBuffer: limits.maxOutputBytes,
+    timeout: limits.timeoutMs,
+    killSignal: 'SIGTERM',
+    artifactDirectory,
+  });
+  let raw;
+  try {
+    raw = executor(definition.command, commandArgs, execution);
+  } catch (error) {
+    raw = { status: null, error };
+  }
+  const terminal = terminalObservation(raw, limits);
+  return freeze({
+    schema: 'torch.dev/candidate-engine-observation/v1alpha1',
+    fixtureOnly: true,
+    sourceAttestation: freeze({ schema: 'torch.dev/source-attestation/v1alpha1', promotion: 'nonpromotable' }),
+    input, definition, args, environment, artifactDirectory, limits, terminal,
+  });
+}
diff --git a/test/fleet/candidate-artifact-manifest.test.mjs b/test/fleet/candidate-artifact-manifest.test.mjs
new file mode 100644
index 0000000..e90fd4a
--- /dev/null
+++ b/test/fleet/candidate-artifact-manifest.test.mjs
@@ -0,0 +1,193 @@
+import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
+import {
+  mkdirSync, mkdtempSync, readFileSync, renameSync, symlinkSync, unlinkSync, writeFileSync,
+} from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import test from 'node:test';
+import {
+  ARTIFACT_MANIFEST_FILE, artifactNodeKind, assertArtifactRelativePath, sealArtifactManifest, verifyArtifactManifest,
+} from '../../src/checks/artifact-manifest.mjs';
+
+function fixture() {
+  return mkdtempSync(join(tmpdir(), 'torch-candidate-manifest-'));
+}
+
+function write(root, path, content) {
+  const target = join(root, path);
+  mkdirSync(join(target, '..'), { recursive: true });
+  writeFileSync(target, content);
+}
+
+function manifestBytesFor(path, content) {
+  const bytes = Buffer.from(content, 'utf8');
+  const payload = {
+    schema: 'torch.dev/artifact-manifest/v1alpha1',
+    files: [{ path, bytes: bytes.length, sha256: createHash('sha256').update(bytes).digest('hex') }],
+    totalBytes: bytes.length,
+  };
+  const manifest = {
+    ...payload,
+    sealDigest: createHash('sha256').update(JSON.stringify(payload)).digest('hex'),
+  };
+  return Buffer.byteLength(`${JSON.stringify(manifest)}\n`, 'utf8');
+}
+
+test('SCN-artifact-manifest-seal: regular fixture artifacts seal as canonical sorted hash and length evidence regardless of creation order', () => {
+  const first = fixture();
+  const second = fixture();
+  write(first, 'z/proof.txt', 'zeta');
+  write(first, 'a/proof.txt', 'alpha');
+  write(second, 'a/proof.txt', 'alpha');
+  write(second, 'z/proof.txt', 'zeta');
+  const left = sealArtifactManifest({ directory: first });
+  const right = sealArtifactManifest({ directory: second });
+  assert.deepEqual(left.files, right.files);
+  assert.equal(left.sealDigest, right.sealDigest);
+  assert.deepEqual(left.files.map((file) => file.path), ['a/proof.txt', 'z/proof.txt']);
+  assert.equal(left.totalBytes, 9);
+  assert.equal(verifyArtifactManifest({ directory: first }).verified, true);
+  assert.throws(() => sealArtifactManifest({ directory: first }), { code: 'CANDIDATE_ARTIFACT_ALREADY_SEALED' });
+});
+
+test('SCN-artifact-manifest-negative-paths: traversal, links, special files, caps and duplicate manifest paths refuse', () => {
+  assert.throws(() => assertArtifactRelativePath('../escape'), { code: 'CANDIDATE_ARTIFACT_PATH_INVALID' });
+  assert.throws(() => assertArtifactRelativePath('/absolute'), { code: 'CANDIDATE_ARTIFACT_PATH_INVALID' });
+
+  const linked = fixture();
+  write(linked, 'safe.txt', 'safe');
+  symlinkSync('safe.txt', join(linked, 'linked.txt'));
+  assert.throws(() => sealArtifactManifest({ directory: linked }), { code: 'CANDIDATE_ARTIFACT_LINK_REJECTED' });
+
+  const limited = fixture();
+  write(limited, 'large.txt', '12345');
+  assert.throws(() => sealArtifactManifest({ directory: limited, limits: { maxFileBytes: 4 } }),
+    { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+
+  assert.throws(() => artifactNodeKind({
+    isSymbolicLink: () => false, isDirectory: () => false, isFile: () => false,
+  }, 'pipe'), { code: 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED' });
+
+  const malformed = fixture();
+  write(malformed, 'proof.txt', 'proof');
+  sealArtifactManifest({ directory: malformed });
+  const path = join(malformed, ARTIFACT_MANIFEST_FILE);
+  const record = JSON.parse(readFileSync(path, 'utf8'));
+  record.files.push({ ...record.files[0] });
+  writeFileSync(path, JSON.stringify(record));
+  assert.throws(() => verifyArtifactManifest({ directory: malformed }), { code: 'CANDIDATE_ARTIFACT_MANIFEST_INVALID' });
+});
+
+test('SCN-artifact-manifest-drift: post-seal mutation or overwrite is detected and cannot become retained evidence', () => {
+  const root = fixture();
+  write(root, 'proof.txt', 'before');
+  sealArtifactManifest({ directory: root });
+  write(root, 'proof.txt', 'after');
+  assert.throws(() => verifyArtifactManifest({ directory: root }), { code: 'CANDIDATE_ARTIFACT_SEAL_DRIFT' });
+});
+
+test('SCN-artifact-manifest-bytewise-and-caps: UTF-8 bytewise path ordering and every declared cap are deterministic', () => {
+  const first = fixture();
+  const second = fixture();
+  write(first, 'ä.txt', 'umlaut');
+  write(first, 'z.txt', 'zed');
+  write(second, 'z.txt', 'zed');
+  write(second, 'ä.txt', 'umlaut');
+  const left = sealArtifactManifest({ directory: first });
+  const right = sealArtifactManifest({ directory: second });
+  assert.deepEqual(left.files.map((file) => file.path), ['z.txt', 'ä.txt']);
+  assert.deepEqual(left.files, right.files);
+  assert.equal(left.sealDigest, right.sealDigest);
+
+  const count = fixture();
+  write(count, 'one.txt', '1');
+  write(count, 'two.txt', '2');
+  assert.throws(() => sealArtifactManifest({ directory: count, limits: { maxFiles: 1 } }), { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+  const countPass = fixture();
+  write(countPass, 'one.txt', '1');
+  assert.equal(sealArtifactManifest({ directory: countPass, limits: { maxFiles: 1 } }).files.length, 1);
+
+  const perFile = fixture();
+  write(perFile, 'proof.txt', '1234');
+  assert.equal(sealArtifactManifest({ directory: perFile, limits: { maxFileBytes: 4 } }).files[0].bytes, 4);
+  const perFileFail = fixture();
+  write(perFileFail, 'proof.txt', '12345');
+  assert.throws(() => sealArtifactManifest({ directory: perFileFail, limits: { maxFileBytes: 4 } }), { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+
+  const aggregate = fixture();
+  write(aggregate, 'one.txt', '12');
+  write(aggregate, 'two.txt', '34');
+  assert.throws(() => sealArtifactManifest({ directory: aggregate, limits: { maxTotalBytes: 3 } }), { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+  const aggregatePass = fixture();
+  write(aggregatePass, 'one.txt', '12');
+  write(aggregatePass, 'two.txt', '34');
+  assert.equal(sealArtifactManifest({ directory: aggregatePass, limits: { maxTotalBytes: 4 } }).totalBytes, 4);
+
+  const paths = fixture();
+  write(paths, 'abcd.txt', 'x');
+  assert.throws(() => sealArtifactManifest({ directory: paths, limits: { maxPathBytes: 7 } }), { code: 'CANDIDATE_ARTIFACT_PATH_INVALID' });
+  const pathsPass = fixture();
+  write(pathsPass, 'abc.txt', 'x');
+  assert.equal(sealArtifactManifest({ directory: pathsPass, limits: { maxPathBytes: 7 } }).files.length, 1);
+
+  const manifestCap = fixture();
+  write(manifestCap, 'proof.txt', 'proof');
+  assert.throws(() => sealArtifactManifest({ directory: manifestCap, limits: { maxManifestBytes: 16 } }), { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+  const exactManifestBytes = manifestBytesFor('proof.txt', 'proof');
+  const manifestAtCap = fixture();
+  write(manifestAtCap, 'proof.txt', 'proof');
+  assert.equal(sealArtifactManifest({ directory: manifestAtCap, limits: { maxManifestBytes: exactManifestBytes } }).files.length, 1);
+  const manifestOverCap = fixture();
+  write(manifestOverCap, 'proof.txt', 'proof');
+  assert.throws(() => sealArtifactManifest({ directory: manifestOverCap, limits: { maxManifestBytes: exactManifestBytes - 1 } }),
+    { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+  assert.throws(() => sealArtifactManifest({ directory: fixture(), limits: { unknownCap: 1 } }), { code: 'CANDIDATE_ARTIFACT_LIMIT_INVALID' });
+});
+
+test('SCN-artifact-manifest-root-and-manifest-boundaries: root or manifest links, special nodes and oversized manifests never become evidence', () => {
+  const actual = fixture();
+  write(actual, 'proof.txt', 'proof');
+  const linkedRoot = `${actual}-link`;
+  symlinkSync(actual, linkedRoot);
+  assert.throws(() => sealArtifactManifest({ directory: linkedRoot }), { code: 'CANDIDATE_ARTIFACT_LINK_REJECTED' });
+
+  const sealed = fixture();
+  write(sealed, 'proof.txt', 'proof');
+  sealArtifactManifest({ directory: sealed });
+  const manifest = join(sealed, ARTIFACT_MANIFEST_FILE);
+  const replacement = `${manifest}.replacement`;
+  renameSync(manifest, replacement);
+  symlinkSync(replacement, manifest);
+  assert.throws(() => verifyArtifactManifest({ directory: sealed }), { code: 'CANDIDATE_ARTIFACT_LINK_REJECTED' });
+  assert.throws(() => sealArtifactManifest({ directory: sealed }), { code: 'CANDIDATE_ARTIFACT_ALREADY_SEALED' });
+
+  const oversized = fixture();
+  write(oversized, 'proof.txt', 'proof');
+  sealArtifactManifest({ directory: oversized });
+  writeFileSync(join(oversized, ARTIFACT_MANIFEST_FILE), 'x'.repeat(80));
+  assert.throws(() => verifyArtifactManifest({ directory: oversized, limits: { maxManifestBytes: 64 } }),
+    { code: 'CANDIDATE_ARTIFACT_LIMIT_EXCEEDED' });
+
+  const special = fixture();
+  write(special, 'proof.txt', 'proof');
+  sealArtifactManifest({ directory: special });
+  const specialManifest = join(special, ARTIFACT_MANIFEST_FILE);
+  unlinkSync(specialManifest);
+  mkdirSync(specialManifest);
+  assert.throws(() => verifyArtifactManifest({ directory: special }), { code: 'CANDIDATE_ARTIFACT_SPECIAL_REJECTED' });
+});
+
+test('SCN-artifact-manifest-complete-drift: adding artifacts or replacing the manifest cannot preserve a sealed record', () => {
+  const root = fixture();
+  write(root, 'proof.txt', 'before');
+  sealArtifactManifest({ directory: root });
+  write(root, 'added.txt', 'after');
+  assert.throws(() => verifyArtifactManifest({ directory: root }), { code: 'CANDIDATE_ARTIFACT_SEAL_DRIFT' });
+
+  const replaced = fixture();
+  write(replaced, 'proof.txt', 'before');
+  sealArtifactManifest({ directory: replaced });
+  writeFileSync(join(replaced, ARTIFACT_MANIFEST_FILE), JSON.stringify({ schema: 'torch.dev/artifact-manifest/v1alpha1', files: [], totalBytes: 0, sealDigest: '0'.repeat(64) }));
+  assert.throws(() => verifyArtifactManifest({ directory: replaced }), { code: 'CANDIDATE_ARTIFACT_SEAL_INVALID' });
+});
diff --git a/test/fleet/candidate-engine.test.mjs b/test/fleet/candidate-engine.test.mjs
new file mode 100644
index 0000000..83b625e
--- /dev/null
+++ b/test/fleet/candidate-engine.test.mjs
@@ -0,0 +1,186 @@
+import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
+import { mkdtempSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import test from 'node:test';
+import { CANDIDATE_ENGINE_LIMITS, runCandidateEngine } from '../../src/checks/candidate-engine.mjs';
+
+function engineLimits(overrides = {}) {
+  return { ...CANDIDATE_ENGINE_LIMITS, ...overrides };
+}
+
+function fixtureInput() {
+  const bytes = Buffer.from('fixture input bytes', 'utf8');
+  return {
+    directory: mkdtempSync(join(tmpdir(), 'torch-candidate-engine-')),
+    digest: createHash('sha256').update(bytes).digest('hex'),
+    bytes,
+    byteLength: bytes.length,
+  };
+}
+
+function fixtureOptions(overrides = {}) {
+  return {
+    input: fixtureInput(),
+    definition: { command: 'fixture-command', args: ['--fixed'] },
+    args: ['--bounded'],
+    environment: { TZ: 'UTC' },
+    artifactDirectory: mkdtempSync(join(tmpdir(), 'torch-candidate-artifacts-')),
+    executor: () => ({ status: 0, stdout: 'fixture output', stderr: '' }),
+    limits: engineLimits(),
+    ...overrides,
+  };
+}
+
+test('SCN-candidate-engine-pure-fixture: immutable bounded fixture execution returns only parent-observed nonpromotable evidence', () => {
+  let invocation;
+  const options = fixtureOptions({
+    executor: (command, args, execution) => {
+      invocation = { command, args, execution };
+      return { status: 0, stdout: 'fixture output', stderr: '' };
+    },
+  });
+  const observed = runCandidateEngine(options);
+  assert.equal(observed.fixtureOnly, true);
+  assert.deepEqual(observed.sourceAttestation, {
+    schema: 'torch.dev/source-attestation/v1alpha1', promotion: 'nonpromotable',
+  });
+  assert.equal(Object.hasOwn(observed.sourceAttestation, 'receiptId'), false);
+  assert.equal(observed.terminal.success, true);
+  assert.equal(observed.terminal.kind, 'exit');
+  assert.deepEqual(invocation.args, ['--fixed', '--bounded']);
+  assert.equal(invocation.execution.cwd, options.input.directory);
+  assert.deepEqual(invocation.execution.env, { TZ: 'UTC' });
+  assert.equal(Object.isFrozen(invocation.execution), true);
+  assert.equal(invocation.execution.timeout, CANDIDATE_ENGINE_LIMITS.timeoutMs);
+  assert.equal(invocation.execution.maxBuffer, CANDIDATE_ENGINE_LIMITS.maxOutputBytes);
+  assert.equal(Object.hasOwn(invocation.execution, 'token'), false);
+  assert.equal(Object.hasOwn(invocation.execution, 'receiptWriter'), false);
+  assert.equal(Object.hasOwn(invocation.execution, 'controlPlane'), false);
+});
+
+test('SCN-candidate-engine-limits: invalid bounded inputs, arguments, environment and artifact descriptors refuse before execution', () => {
+  let executions = 0;
+  const executor = () => { executions += 1; return { status: 0 }; };
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, input: { directory: '/tmp', digest: 'bad', bytes: Buffer.from('x'), byteLength: 1 } })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, args: ['x'.repeat(8)], limits: engineLimits({ maxArgBytes: 4 }) })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, environment: { bad: '1' } })),
+    { code: 'CANDIDATE_ENGINE_ENVIRONMENT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, artifactDirectory: 'x'.repeat(5), limits: engineLimits({ maxArtifactPathBytes: 4 }) })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, limits: engineLimits({ maxInputBytes: 4 }) })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, environment: { TZ: 'UTC', LANG: 'C.UTF-8' }, limits: engineLimits({ maxEnvironmentEntries: 1 }) })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.equal(executions, 0, 'refusal must occur before a candidate executor starts');
+});
+
+test('SCN-candidate-engine-terminal-observation: overflow, including exit zero plus ENOBUFS, is raw parent evidence and never success', () => {
+  const overflow = runCandidateEngine(fixtureOptions({
+    limits: engineLimits({ maxOutputBytes: 4 }),
+    executor: () => ({ status: 0, error: Object.assign(new Error('overflow'), { code: 'ENOBUFS' }), stdout: 'too many bytes' }),
+  }));
+  assert.equal(overflow.terminal.exitStatus, 0);
+  assert.equal(overflow.terminal.kind, 'overflow');
+  assert.equal(overflow.terminal.success, false);
+  assert.equal(overflow.terminal.overflow, true);
+
+  const timeout = runCandidateEngine(fixtureOptions({
+    executor: () => ({ status: null, timedOut: true, error: Object.assign(new Error('timeout'), { code: 'ETIMEDOUT' }) }),
+  }));
+  assert.equal(timeout.terminal.kind, 'timeout');
+  assert.equal(timeout.terminal.success, false);
+
+  const signal = runCandidateEngine(fixtureOptions({ executor: () => ({ status: null, signal: 'SIGTERM' }) }));
+  assert.equal(signal.terminal.kind, 'signal');
+  assert.equal(signal.terminal.success, false);
+});
+
+test('SCN-candidate-engine-capability-boundary: registered-state and writer-shaped inputs are rejected before execution', () => {
+  let executions = 0;
+  const options = fixtureOptions({ executor: () => { executions += 1; return { status: 0 }; } });
+  options.controlPlane = {};
+  assert.throws(() => runCandidateEngine(options), { code: 'CANDIDATE_ENGINE_CAPABILITY_FORBIDDEN' });
+  assert.equal(executions, 0);
+});
+
+test('SCN-candidate-engine-boundary-limits: every execution bound is explicit, finite and supplied to the native-shaped executor options', () => {
+  let executions = 0;
+  const executor = () => { executions += 1; return { status: 0 }; };
+  const missingTimeout = engineLimits();
+  delete missingTimeout.timeoutMs;
+  const missingOutput = engineLimits();
+  delete missingOutput.maxOutputBytes;
+  const missingInput = engineLimits();
+  delete missingInput.maxInputBytes;
+  for (const limits of [missingTimeout, missingOutput, missingInput, engineLimits({ timeoutMs: 0 }), engineLimits({ timeoutMs: 60_001 })]) {
+    assert.throws(() => runCandidateEngine(fixtureOptions({ executor, limits })), { code: 'CANDIDATE_ENGINE_LIMIT_INVALID' });
+  }
+  assert.equal(executions, 0, 'missing or invalid bounds refuse before execution');
+
+  let execution;
+  runCandidateEngine(fixtureOptions({ executor: (_command, _args, options) => { execution = options; return { status: 0 }; } }));
+  assert.deepEqual(Object.keys(execution).sort(), ['artifactDirectory', 'cwd', 'encoding', 'env', 'killSignal', 'maxBuffer', 'stdio', 'timeout']);
+  assert.equal(execution.timeout, CANDIDATE_ENGINE_LIMITS.timeoutMs);
+  assert.equal(execution.maxBuffer, CANDIDATE_ENGINE_LIMITS.maxOutputBytes);
+});
+
+test('SCN-candidate-engine-utf8-overflow: retained output is a valid UTF-8 prefix inside the byte cap and cannot become success', () => {
+  const fourBytes = runCandidateEngine(fixtureOptions({
+    limits: engineLimits({ maxOutputBytes: 4 }),
+    executor: () => ({ status: 0, stdout: '😀😀', stderr: '' }),
+  }));
+  assert.equal(fourBytes.terminal.stdout.text, '😀');
+  assert.equal(Buffer.byteLength(fourBytes.terminal.stdout.text, 'utf8'), 4);
+  assert.equal(fourBytes.terminal.overflow, true);
+  assert.equal(fourBytes.terminal.success, false);
+
+  const threeBytes = runCandidateEngine(fixtureOptions({
+    limits: engineLimits({ maxOutputBytes: 3 }),
+    executor: () => ({ status: 0, stdout: '😀😀', stderr: '' }),
+  }));
+  assert.equal(threeBytes.terminal.stdout.text, '');
+  assert.equal(Buffer.byteLength(threeBytes.terminal.stdout.text, 'utf8') <= 3, true);
+  assert.equal(threeBytes.terminal.stdout.text.includes('\uFFFD'), false);
+  assert.equal(threeBytes.terminal.success, false);
+});
+
+test('SCN-candidate-engine-immutable-input-and-environment: only matched byte snapshots and allowlisted deterministic environment reach the executor', () => {
+  let executions = 0;
+  const executor = () => { executions += 1; return { status: 0 }; };
+  const mismatched = fixtureInput();
+  mismatched.digest = '0'.repeat(64);
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, input: mismatched })), { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, input: { directory: '/tmp', digest: '0'.repeat(64), byteLength: 0 } })),
+    { code: 'CANDIDATE_ENGINE_INPUT_INVALID' });
+  assert.throws(() => runCandidateEngine(fixtureOptions({ executor, environment: { UNREVIEWED_SECRET: '1' } })),
+    { code: 'CANDIDATE_ENGINE_ENVIRONMENT_INVALID' });
+  assert.equal(executions, 0, 'invalid snapshot or environment refuses before execution');
+
+  const input = fixtureInput();
+  const definition = { command: 'fixture-command', args: ['--fixed'] };
+  const args = ['--bounded'];
+  const environment = { TZ: 'UTC' };
+  let invocation;
+  const observed = runCandidateEngine(fixtureOptions({
+    input, definition, args, environment,
+    executor: (_command, commandArgs, execution) => {
+      input.bytes.fill(0);
+      definition.args[0] = '--mutated';
+      args[0] = '--mutated';
+      environment.TZ = 'America/New_York';
+      invocation = { commandArgs, execution };
+      return { status: 0 };
+    },
+  }));
+  assert.deepEqual(invocation.commandArgs, ['--fixed', '--bounded']);
+  assert.deepEqual(invocation.execution.env, { TZ: 'UTC' });
+  assert.deepEqual(observed.input, {
+    directory: input.directory,
+    digest: createHash('sha256').update(Buffer.from('fixture input bytes')).digest('hex'),
+    bytes: Buffer.byteLength('fixture input bytes'),
+  });
+});