diff --git a/test/fixtures/overlay-nudges.ts b/test/fixtures/overlay-nudges.ts
new file mode 100644
index 00000000..9d9cecc4
--- /dev/null
+++ b/test/fixtures/overlay-nudges.ts
@@ -0,0 +1,179 @@
+/**
+ * Overlay-efficacy fixture registry.
+ *
+ * Each fixture defines a reproducible A/B test for one behavioral nudge
+ * embedded in a model-overlays/*.md file. The harness at
+ * test/skill-e2e-overlay-harness.test.ts iterates this registry and runs
+ * `fixture.trials` A/B trials per fixture, asserting `fixture.pass(arms)`.
+ *
+ * Adding a new overlay eval = one entry in this list. The harness handles
+ * arm wiring, concurrency, artifact storage, rate-limit retries, and the
+ * cross-harness diagnostic.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+import {
+  firstTurnParallelism,
+  type AgentSdkResult,
+} from '../helpers/agent-sdk-runner';
+
+const REPO_ROOT = path.resolve(__dirname, '..', '..');
+
+// ---------------------------------------------------------------------------
+// Types
+// ---------------------------------------------------------------------------
+
+export interface OverlayFixture {
+  /** Unique, lowercase/digits/dash only. Used in artifact paths. */
+  id: string;
+  /** Path to the overlay file, relative to repo root. */
+  overlayPath: string;
+  /** API model ID, not the overlay family name. */
+  model: string;
+  /** Integer >= 3. Trials per arm. */
+  trials: number;
+  /** Max concurrent queries for this fixture's arms. Default 3. */
+  concurrency?: number;
+  /** Populate the workspace dir before each trial. */
+  setupWorkspace: (dir: string) => void;
+  /** The prompt the model receives. Non-empty. */
+  userPrompt: string;
+  /** Compute the per-trial metric from the typed SDK result. */
+  metric: (r: AgentSdkResult) => number;
+  /** Acceptance predicate across all arms' per-trial metrics. */
+  pass: (arms: { overlay: number[]; off: number[] }) => boolean;
+}
+
+// ---------------------------------------------------------------------------
+// Validation
+// ---------------------------------------------------------------------------
+
+export function validateFixtures(fixtures: OverlayFixture[]): void {
+  const ids = new Set<string>();
+  for (const f of fixtures) {
+    if (!f.id || !/^[a-z0-9-]+$/.test(f.id)) {
+      throw new Error(
+        `fixture id must be non-empty, lowercase/digits/dash only: ${JSON.stringify(f.id)}`,
+      );
+    }
+    if (ids.has(f.id)) {
+      throw new Error(`duplicate fixture id: ${f.id}`);
+    }
+    ids.add(f.id);
+
+    if (!Number.isInteger(f.trials) || f.trials < 3) {
+      throw new Error(`${f.id}: trials must be an integer >= 3 (got ${f.trials})`);
+    }
+    if (
+      f.concurrency !== undefined &&
+      (!Number.isInteger(f.concurrency) || f.concurrency < 1)
+    ) {
+      throw new Error(
+        `${f.id}: concurrency must be an integer >= 1 (got ${f.concurrency})`,
+      );
+    }
+
+    if (!f.model) throw new Error(`${f.id}: model must be non-empty`);
+    if (!f.userPrompt) throw new Error(`${f.id}: userPrompt must be non-empty`);
+
+    if (path.isAbsolute(f.overlayPath) || f.overlayPath.includes('..')) {
+      throw new Error(
+        `${f.id}: overlayPath must be relative and must not contain '..' (got ${f.overlayPath})`,
+      );
+    }
+    const fullPath = path.resolve(REPO_ROOT, f.overlayPath);
+    if (!fs.existsSync(fullPath)) {
+      throw new Error(`${f.id}: overlay file not found at ${f.overlayPath}`);
+    }
+
+    for (const fn of ['setupWorkspace', 'metric', 'pass'] as const) {
+      if (typeof f[fn] !== 'function') {
+        throw new Error(`${f.id}: ${fn} must be a function`);
+      }
+    }
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Metric + predicate helpers
+// ---------------------------------------------------------------------------
+
+function mean(xs: number[]): number {
+  if (xs.length === 0) return 0;
+  return xs.reduce((a, b) => a + b, 0) / xs.length;
+}
+
+/**
+ * Standard fanout predicate: overlay mean beats off mean by at least 0.5
+ * parallel tool_use blocks in first turn, AND at least 3 of the overlay
+ * trials emit >= 2 parallel tool_use blocks.
+ *
+ * The combined rule catches both "overlay nudges every trial slightly"
+ * (mean) and "overlay sometimes triggers real fanout" (floor). A single
+ * 0.5 lift with every trial still emitting 1 call would be suspicious;
+ * this predicate rejects it.
+ */
+export function fanoutPass(arms: { overlay: number[]; off: number[] }): boolean {
+  const lift = mean(arms.overlay) - mean(arms.off);
+  const floorHits = arms.overlay.filter((n) => n >= 2).length;
+  return lift >= 0.5 && floorHits >= 3;
+}
+
+// ---------------------------------------------------------------------------
+// Fixtures
+// ---------------------------------------------------------------------------
+
+export const OVERLAY_FIXTURES: OverlayFixture[] = [
+  {
+    id: 'opus-4-7-fanout-toy',
+    overlayPath: 'model-overlays/opus-4-7.md',
+    model: 'claude-opus-4-7',
+    trials: 10,
+    concurrency: 3,
+    setupWorkspace: (dir) => {
+      fs.writeFileSync(path.join(dir, 'alpha.txt'), 'Alpha file: used in module A.\n');
+      fs.writeFileSync(path.join(dir, 'beta.txt'), 'Beta file: used in module B.\n');
+      fs.writeFileSync(path.join(dir, 'gamma.txt'), 'Gamma file: used in module C.\n');
+    },
+    userPrompt:
+      'Read alpha.txt, beta.txt, and gamma.txt and summarize each in one line.',
+    metric: (r) => firstTurnParallelism(r.assistantTurns[0]),
+    pass: fanoutPass,
+  },
+  {
+    id: 'opus-4-7-fanout-realistic',
+    overlayPath: 'model-overlays/opus-4-7.md',
+    model: 'claude-opus-4-7',
+    trials: 10,
+    concurrency: 3,
+    setupWorkspace: (dir) => {
+      fs.writeFileSync(
+        path.join(dir, 'app.ts'),
+        "import { config } from './config';\nimport { util } from './src/util';\n\nexport function main() { return config.name + ':' + util(); }\n",
+      );
+      fs.writeFileSync(
+        path.join(dir, 'config.ts'),
+        "export const config = { name: 'demo', version: 1 };\n",
+      );
+      fs.writeFileSync(
+        path.join(dir, 'README.md'),
+        '# demo project\n\nA small demo. Entry: `app.ts`. Config: `config.ts`.\n',
+      );
+      fs.mkdirSync(path.join(dir, 'src'), { recursive: true });
+      fs.writeFileSync(
+        path.join(dir, 'src', 'util.ts'),
+        "export function util() { return 'util-result'; }\n",
+      );
+    },
+    userPrompt:
+      'Audit this project: read app.ts, config.ts, and README.md, and glob for ' +
+      'every .ts file under src/. Summarize what you find in 3 bullet points.',
+    metric: (r) => firstTurnParallelism(r.assistantTurns[0]),
+    pass: fanoutPass,
+  },
+];
+
+// Validate at module load so a broken fixture fails fast at test startup,
+// not mid-run after burning API dollars.
+validateFixtures(OVERLAY_FIXTURES);
diff --git a/test/helpers/agent-sdk-runner.ts b/test/helpers/agent-sdk-runner.ts
new file mode 100644
index 00000000..1504384e
--- /dev/null
+++ b/test/helpers/agent-sdk-runner.ts
@@ -0,0 +1,465 @@
+/**
+ * Claude Agent SDK wrapper for the overlay-efficacy harness.
+ *
+ * This sits alongside session-runner.ts (which drives `claude -p` as a
+ * subprocess) but runs the model via the published @anthropic-ai/claude-agent-sdk
+ * instead. The SDK exposes the same harness primitives Claude Code itself uses,
+ * so overlay-driven behavior change is measured against a closer approximation
+ * of real Claude Code than the `claude -p` subprocess path provides.
+ *
+ * Explicit design rules (from plan review):
+ *   - Use SDK-exported SDKMessage types. No `| unknown` union collapse.
+ *   - Permission surface is explicit: bypassPermissions + settingSources:[] +
+ *     disallowedTools inverse. Without these, the SDK inherits user settings,
+ *     project .claude/, and local hooks, and arms are no longer comparable.
+ *   - Binary pinning via pathToClaudeCodeExecutable. Resolve with `which claude`
+ *     at setup time; the SDK would otherwise use its bundled binary.
+ *   - 3-shape rate-limit detection: thrown error, result-message error subtype,
+ *     mid-stream SDKRateLimitEvent. All three recover on retry.
+ *   - On retry, caller resets workspace via a setupWorkspace callback so
+ *     partial Bash side-effects don't contaminate the next attempt.
+ *   - Process-level semaphore caps concurrent queries across all callers in
+ *     the same bun-test process. Composes with bun's own --concurrent flag.
+ */
+
+import {
+  query,
+  type SDKMessage,
+  type SDKAssistantMessage,
+  type SDKResultMessage,
+  type SDKSystemMessage,
+  type PermissionMode,
+  type SettingSource,
+  type Options,
+} from '@anthropic-ai/claude-agent-sdk';
+import * as fs from 'fs';
+import * as path from 'path';
+import { execSync } from 'child_process';
+import type { SkillTestResult } from './session-runner';
+
+// ---------------------------------------------------------------------------
+// Types
+// ---------------------------------------------------------------------------
+
+export interface AgentSdkResult {
+  /** Full raw event stream for forensic recovery. */
+  events: SDKMessage[];
+  /** Assistant-typed subset, in order. */
+  assistantTurns: SDKAssistantMessage[];
+  /** Flat tool-call list, in order of emission. */
+  toolCalls: Array<{ tool: string; input: unknown; output: string }>;
+  /** Concatenated assistant text, newline-joined. */
+  output: string;
+  /** 'success' | 'error_during_execution' | 'error_max_turns' | ... */
+  exitReason: string;
+  turnsUsed: number;
+  durationMs: number;
+  firstResponseMs: number;
+  maxInterTurnMs: number;
+  costUsd: number;
+  model: string;
+  sdkVersion: string;
+  /** claude_code_version from the SDK's system/init event (authoritative). */
+  sdkClaudeCodeVersion: string;
+  /** Path to the claude binary we pinned. */
+  resolvedBinaryPath: string;
+  /** browse-error pattern scan for SkillTestResult parity. Always empty here. */
+  browseErrors: string[];
+}
+
+/** Signature matching `query()` from the SDK. DI hook for unit tests. */
+export type QueryProvider = typeof query;
+
+/** Subset of SDK Options['systemPrompt'] we support. */
+export type SystemPromptOption =
+  | string
+  | { type: 'preset'; preset: 'claude_code'; append?: string; excludeDynamicSections?: boolean };
+
+export interface RunAgentSdkOptions {
+  /**
+   * System prompt surface.
+   *   - bare string "" -> omit entirely (SDK default: no system prompt)
+   *   - bare string "...text..." -> REPLACE default with given text (use sparingly)
+   *   - { type:'preset', preset:'claude_code' } -> use Claude Code default
+   *   - { type:'preset', preset:'claude_code', append: "..." } -> default + append
+   *
+   * For overlay-efficacy measurement, the preset+append pattern is the right
+   * one: it measures "does adding overlay text to the REAL Claude Code system
+   * prompt change behavior" rather than "does the overlay alone (stripped of
+   * base scaffolding) change behavior".
+   */
+  systemPrompt: SystemPromptOption;
+  userPrompt: string;
+  workingDirectory: string;
+  model?: string;
+  maxTurns?: number;
+  allowedTools?: string[];
+  disallowedTools?: string[];
+  permissionMode?: PermissionMode;
+  settingSources?: SettingSource[];
+  env?: Record<string, string>;
+  pathToClaudeCodeExecutable?: string;
+  testName?: string;
+  runId?: string;
+  fixtureId?: string;
+  queryProvider?: QueryProvider;
+  /** Max 429 retries per call. Default 3. */
+  maxRetries?: number;
+  /**
+   * Caller provides this when retry should reset the workspace. The harness
+   * invokes it with a fresh dir after a rate-limit failure. When omitted,
+   * retries reuse the original workingDirectory (fine for read-only tests).
+   */
+  onRetry?: (freshDir: string) => void;
+}
+
+export class RateLimitExhaustedError extends Error {
+  readonly attempts: number;
+  constructor(attempts: number, cause?: unknown) {
+    super(`rate limit exhausted after ${attempts} attempts`);
+    this.name = 'RateLimitExhaustedError';
+    this.attempts = attempts;
+    if (cause !== undefined) (this as { cause?: unknown }).cause = cause;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Process-level semaphore for API concurrency
+// ---------------------------------------------------------------------------
+
+/**
+ * Bounded token bucket. Shared across all runAgentSdkTest calls in this
+ * process so that bun's --concurrent flag does not compound with in-test
+ * concurrency to blow past Anthropic's rate limits.
+ *
+ * Default capacity 3. Override via GSTACK_SDK_MAX_CONCURRENCY env var.
+ */
+class Semaphore {
+  private available: number;
+  private readonly queue: Array<() => void> = [];
+  constructor(capacity: number) {
+    this.available = capacity;
+  }
+  async acquire(): Promise<void> {
+    if (this.available > 0) {
+      this.available--;
+      return;
+    }
+    await new Promise<void>((resolve) => this.queue.push(resolve));
+  }
+  release(): void {
+    const next = this.queue.shift();
+    if (next) {
+      next();
+    } else {
+      this.available++;
+    }
+  }
+  /** For tests. Returns tokens currently in-flight. */
+  inFlight(): number {
+    // Not introspectable from outside without tracking; approximate.
+    return this.queue.length;
+  }
+}
+
+const DEFAULT_SDK_CONCURRENCY = Number(process.env.GSTACK_SDK_MAX_CONCURRENCY ?? 3);
+let _apiSemaphore: Semaphore | null = null;
+function getApiSemaphore(): Semaphore {
+  if (!_apiSemaphore) _apiSemaphore = new Semaphore(DEFAULT_SDK_CONCURRENCY);
+  return _apiSemaphore;
+}
+
+/** Test-only. Resets the process-level semaphore. */
+export function __resetSemaphoreForTests(capacity: number): void {
+  _apiSemaphore = new Semaphore(capacity);
+}
+
+// ---------------------------------------------------------------------------
+// Rate-limit detection
+// ---------------------------------------------------------------------------
+
+/** True if `err` looks like a rate-limit thrown from the SDK. */
+export function isRateLimitThrown(err: unknown): boolean {
+  if (!err || typeof err !== 'object') return false;
+  const msg = (err as { message?: string }).message ?? '';
+  const name = (err as { name?: string }).name ?? '';
+  const status = (err as { status?: number }).status;
+  return (
+    status === 429 ||
+    /rate.?limit|429|too many requests/i.test(msg) ||
+    /RateLimit/i.test(name)
+  );
+}
+
+/** True if a SDKResultMessage is a rate-limit-shaped error. */
+export function isRateLimitResult(msg: SDKMessage): boolean {
+  if (msg.type !== 'result') return false;
+  const r = msg as SDKResultMessage;
+  if (r.subtype === 'success') return false;
+  // subtype === 'error_during_execution' | 'error_max_turns' | 'error_max_budget_usd' | ...
+  if (r.subtype !== 'error_during_execution') return false;
+  const errs = (r as { errors?: string[] }).errors ?? [];
+  return errs.some((e) => /rate.?limit|429|too many requests/i.test(e));
+}
+
+/** True if mid-stream SDKRateLimitEvent indicates a blocking rate-limit. */
+export function isRateLimitEvent(msg: SDKMessage): boolean {
+  if (msg.type !== 'rate_limit_event') return false;
+  const info = (msg as { rate_limit_info?: { status?: string } }).rate_limit_info;
+  return info?.status === 'rejected';
+}
+
+// ---------------------------------------------------------------------------
+// Version resolution (cached)
+// ---------------------------------------------------------------------------
+
+let _sdkVersionCache: string | null = null;
+function resolveSdkVersion(): string {
+  if (_sdkVersionCache) return _sdkVersionCache;
+  try {
+    const pkgPath = require.resolve('@anthropic-ai/claude-agent-sdk/package.json');
+    const pkg = JSON.parse(fs.readFileSync(pkgPath, 'utf-8')) as { version?: string };
+    _sdkVersionCache = pkg.version ?? 'unknown';
+  } catch {
+    _sdkVersionCache = 'unknown';
+  }
+  return _sdkVersionCache;
+}
+
+export function resolveClaudeBinary(): string | null {
+  try {
+    return execSync('which claude', { encoding: 'utf-8' }).trim() || null;
+  } catch {
+    return null;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Main runner
+// ---------------------------------------------------------------------------
+
+/**
+ * Execute a single SDK query with retries. Returns a typed result.
+ *
+ * The retry loop treats 429 as recoverable and any other error as fatal.
+ * Exponential backoff: 1s, 2s, 4s. After maxRetries failures, throws
+ * RateLimitExhaustedError so the caller can decide what to do with the run.
+ */
+export async function runAgentSdkTest(
+  opts: RunAgentSdkOptions,
+): Promise<AgentSdkResult> {
+  const sem = getApiSemaphore();
+  const maxRetries = opts.maxRetries ?? 3;
+  const queryImpl: QueryProvider = opts.queryProvider ?? query;
+  const model = opts.model ?? 'claude-opus-4-7';
+
+  let attempt = 0;
+  let lastErr: unknown = null;
+
+  while (attempt <= maxRetries) {
+    await sem.acquire();
+    const startMs = Date.now();
+    try {
+      const sdkOpts: Options = {
+        model,
+        cwd: opts.workingDirectory,
+        maxTurns: opts.maxTurns ?? 5,
+        tools: opts.allowedTools ?? ['Read', 'Glob', 'Grep', 'Bash'],
+        disallowedTools: opts.disallowedTools,
+        allowedTools: opts.allowedTools ?? ['Read', 'Glob', 'Grep', 'Bash'],
+        permissionMode: opts.permissionMode ?? 'bypassPermissions',
+        allowDangerouslySkipPermissions:
+          (opts.permissionMode ?? 'bypassPermissions') === 'bypassPermissions',
+        settingSources: opts.settingSources ?? [],
+        env: opts.env,
+        pathToClaudeCodeExecutable: opts.pathToClaudeCodeExecutable,
+      };
+      // Empty bare string means "omit entirely" (SDK runs with no override).
+      // Any object or non-empty string is passed through.
+      if (typeof opts.systemPrompt === 'object' || opts.systemPrompt !== '') {
+        sdkOpts.systemPrompt = opts.systemPrompt;
+      }
+
+      const events: SDKMessage[] = [];
+      const assistantTurns: SDKAssistantMessage[] = [];
+      const toolCalls: Array<{ tool: string; input: unknown; output: string }> = [];
+      const assistantTextParts: string[] = [];
+      let firstResponseMs = 0;
+      let lastEventMs = startMs;
+      let maxInterTurnMs = 0;
+      let systemInitVersion = 'unknown';
+      let rateLimited: unknown = null;
+      let terminalResult: SDKResultMessage | null = null;
+
+      const q = queryImpl({
+        prompt: opts.userPrompt,
+        options: sdkOpts,
+      });
+
+      for await (const ev of q) {
+        const now = Date.now();
+        if (firstResponseMs === 0) firstResponseMs = now - startMs;
+        const interTurn = now - lastEventMs;
+        if (interTurn > maxInterTurnMs) maxInterTurnMs = interTurn;
+        lastEventMs = now;
+
+        events.push(ev);
+
+        if (ev.type === 'system' && (ev as SDKSystemMessage).subtype === 'init') {
+          systemInitVersion =
+            (ev as SDKSystemMessage).claude_code_version ?? 'unknown';
+        } else if (ev.type === 'assistant') {
+          const am = ev as SDKAssistantMessage;
+          assistantTurns.push(am);
+          const content = am.message?.content;
+          if (Array.isArray(content)) {
+            for (const block of content as Array<
+              | { type: 'text'; text?: string }
+              | { type: 'tool_use'; name?: string; input?: unknown }
+              | { type: string }
+            >) {
+              if (block.type === 'text') {
+                const t = (block as { text?: string }).text;
+                if (t) assistantTextParts.push(t);
+              } else if (block.type === 'tool_use') {
+                const tb = block as { name?: string; input?: unknown };
+                toolCalls.push({
+                  tool: tb.name ?? 'unknown',
+                  input: tb.input ?? {},
+                  output: '',
+                });
+              }
+            }
+          }
+        } else if (isRateLimitEvent(ev)) {
+          rateLimited = new Error(
+            `mid-stream rate limit: ${JSON.stringify(
+              (ev as { rate_limit_info?: unknown }).rate_limit_info,
+            )}`,
+          );
+        } else if (ev.type === 'result') {
+          terminalResult = ev as SDKResultMessage;
+          if (isRateLimitResult(ev)) {
+            rateLimited = new Error(
+              `result-message rate limit: ${((ev as { errors?: string[] }).errors ?? []).join('; ')}`,
+            );
+          }
+        }
+      }
+
+      if (rateLimited) {
+        throw rateLimited;
+      }
+      if (!terminalResult) {
+        throw new Error('query stream ended without a result event');
+      }
+
+      const durationMs = Date.now() - startMs;
+      const costUsd =
+        (terminalResult as { total_cost_usd?: number }).total_cost_usd ?? 0;
+      const turnsUsed =
+        (terminalResult as { num_turns?: number }).num_turns ??
+        assistantTurns.length;
+      const exitReason =
+        (terminalResult as { subtype?: string }).subtype ?? 'unknown';
+
+      return {
+        events,
+        assistantTurns,
+        toolCalls,
+        output: assistantTextParts.join('\n'),
+        exitReason,
+        turnsUsed,
+        durationMs,
+        firstResponseMs,
+        maxInterTurnMs,
+        costUsd,
+        model,
+        sdkVersion: resolveSdkVersion(),
+        sdkClaudeCodeVersion: systemInitVersion,
+        resolvedBinaryPath: opts.pathToClaudeCodeExecutable ?? 'sdk-default',
+        browseErrors: [],
+      };
+    } catch (err) {
+      lastErr = err;
+      const isRetryable = isRateLimitThrown(err);
+      if (!isRetryable || attempt >= maxRetries) {
+        if (isRetryable) {
+          throw new RateLimitExhaustedError(attempt + 1, err);
+        }
+        throw err;
+      }
+      attempt++;
+      // backoff: 1s, 2s, 4s
+      await new Promise((r) => setTimeout(r, 1000 * Math.pow(2, attempt - 1)));
+      // Let caller reset workspace since prior attempt may have partially
+      // mutated files via Bash.
+      if (opts.onRetry) {
+        opts.onRetry(opts.workingDirectory);
+      }
+    } finally {
+      sem.release();
+    }
+  }
+
+  throw new RateLimitExhaustedError(attempt + 1, lastErr);
+}
+
+// ---------------------------------------------------------------------------
+// Legacy shape mapper
+// ---------------------------------------------------------------------------
+
+/**
+ * Adapt AgentSdkResult to the legacy SkillTestResult shape so helpers that
+ * expect the old `claude -p` output (extractToolSummary, etc) work unchanged.
+ */
+export function toSkillTestResult(r: AgentSdkResult): SkillTestResult {
+  // Cost estimate: use SDK's authoritative cost; back-compute chars.
+  // session-runner.ts:30 requires inputChars/outputChars/estimatedTokens.
+  // These are rough; real consumers of CostEstimate use cost + turns.
+  const outputChars = r.output.length;
+  const inputChars = 0; // unknown from SDK path; not used for pass/fail
+  const estimatedTokens = Math.round((inputChars + outputChars) / 4);
+
+  // Build a flat transcript list mimicking the NDJSON shape:
+  // parseNDJSON emits [{ type: 'assistant', message: {...} }, ...].
+  // Use the SDK's assistantTurns directly since their shape matches.
+  const transcript: unknown[] = r.events.slice();
+
+  return {
+    toolCalls: r.toolCalls,
+    browseErrors: r.browseErrors,
+    exitReason: r.exitReason,
+    duration: r.durationMs,
+    output: r.output,
+    costEstimate: {
+      inputChars,
+      outputChars,
+      estimatedTokens,
+      estimatedCost: r.costUsd,
+      turnsUsed: r.turnsUsed,
+    },
+    transcript,
+    model: r.model,
+    firstResponseMs: r.firstResponseMs,
+    maxInterTurnMs: r.maxInterTurnMs,
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Metric helpers (re-exported for fixtures)
+// ---------------------------------------------------------------------------
+
+/**
+ * Count `tool_use` blocks in the first assistant turn of an SDK result.
+ * Returns 0 if there is no first turn or no content array.
+ *
+ * This is the core "fanout" metric. A turn with N tool_use blocks = N
+ * parallel tool invocations.
+ */
+export function firstTurnParallelism(firstTurn: SDKAssistantMessage | undefined): number {
+  if (!firstTurn) return 0;
+  const content = firstTurn.message?.content;
+  if (!Array.isArray(content)) return 0;
+  return (content as Array<{ type: string }>).filter((b) => b.type === 'tool_use').length;
+}