diff --git a/src/core/ai/gateway.ts b/src/core/ai/gateway.ts
index 10bdb642e..53d25b34f 100644
--- a/src/core/ai/gateway.ts
+++ b/src/core/ai/gateway.ts
@@ -642,8 +642,42 @@ function warnRecipesMissingBatchTokens(): void {
   }
 }
 
-/** Reset (for tests). */
-export function resetGateway(): void {
+/**
+ * Test-only reset baseline (#3554). The bunfig preload
+ * (`test/helpers/legacy-embedding-preload.ts`) pins the gateway to the legacy
+ * OpenAI/1536 config at process start, but `resetGateway()` used to wipe that
+ * pin to `_config = null`. The next test file's engine connect then
+ * reconfigured from the SHIPPED default (zembed-1 @ 1280) and every 1536-d
+ * fixture in that file exploded with `expected 1280 dimensions, not 1536` —
+ * a cross-file mine whose placement depended on shard bin-packing.
+ *
+ * When a baseline factory is registered, `resetGateway()` means "back to the
+ * test baseline" instead of "unconfigured": it clears everything as before,
+ * then re-applies the factory's config via `configureGateway()`. A factory
+ * (not a frozen config) so each re-application captures fresh
+ * `process.env`, matching the preload's original `applyLegacy()` semantics.
+ *
+ * Production is untouched: nothing in `src/` calls `resetGateway()` or this
+ * setter, so in production the baseline is never registered and
+ * `resetGateway()` still fully unconfigures. Same `__*ForTests` seam
+ * convention as `__setEmbedTransportForTests` above.
+ */
+let _resetBaseline: (() => AIGatewayConfig) | null = null;
+
+/**
+ * Register (or clear, with `null`) the config factory that `resetGateway()`
+ * re-applies. Called once by the bunfig test preload.
+ *
+ * @internal exported for tests; not part of the public gateway API.
+ */
+export function __setGatewayResetBaselineForTests(
+  factory: (() => AIGatewayConfig) | null,
+): void {
+  _resetBaseline = factory;
+}
+
+/** Clear every piece of module state. Shared by both reset flavors. */
+function clearGatewayState(): void {
   _config = null;
   _modelCache.clear();
   _shrinkState.clear();
@@ -655,6 +689,33 @@ export function resetGateway(): void {
   _extendedModels.clear();
 }
 
+/**
+ * Reset (for tests). Clears all module state (config, model cache, shrink
+ * state, transports, warned recipes, extended models), then — if a test
+ * baseline is registered — re-applies it so the gateway returns to the
+ * process-wide test default instead of an unconfigured limbo (#3554).
+ */
+export function resetGateway(): void {
+  clearGatewayState();
+  // configureGateway re-clears _modelCache/_shrinkState/_extendedModels and
+  // registers the baseline's models; transports are NOT touched by it, so a
+  // stale test transport can never leak back in through this path.
+  if (_resetBaseline) configureGateway(_resetBaseline());
+}
+
+/**
+ * Reset AND stay unconfigured, ignoring any registered baseline. For the
+ * handful of tests that assert genuine no-gateway behavior
+ * (`no_gateway_config` diagnosis, `isAvailable() === false`, graceful
+ * degradation paths). The preload's per-test beforeEach restores the
+ * baseline before the next test, so this cannot leak across tests.
+ *
+ * @internal exported for tests; not part of the public gateway API.
+ */
+export function __unconfigureGatewayForTests(): void {
+  clearGatewayState();
+}
+
 /**
  * Test-only seam. Replaces the function the gateway calls to embed a
  * sub-batch. Pass `null` to restore the real `embedMany` from the AI SDK.
diff --git a/test/ai/gateway-reset-baseline.test.ts b/test/ai/gateway-reset-baseline.test.ts
new file mode 100644
index 000000000..db521f659
--- /dev/null
+++ b/test/ai/gateway-reset-baseline.test.ts
@@ -0,0 +1,68 @@
+/**
+ * #3554 — resetGateway() must restore the test baseline, not unconfigure.
+ *
+ * The bunfig preload (test/helpers/legacy-embedding-preload.ts) pins the
+ * gateway to openai:text-embedding-3-large @ 1536 at process start and
+ * registers that config as the reset baseline. Before the fix,
+ * resetGateway() wiped the pin to _config = null; the next file's beforeAll
+ * engine-connect then reconfigured from the SHIPPED default (zembed-1 @
+ * 1280) and every 1536-d fixture in that file failed with
+ * `expected 1280 dimensions, not 1536`. Which file pairs collided depended
+ * on shard bin-packing, so adding ANY test file reshuffled the mines.
+ *
+ * These assertions pin the contract so it cannot silently rot again.
+ */
+import { describe, test, expect, afterEach } from 'bun:test';
+import {
+  configureGateway,
+  resetGateway,
+  __unconfigureGatewayForTests,
+  __setChatTransportForTests,
+  getEmbeddingModel,
+  getEmbeddingDimensions,
+  isAvailable,
+} from '../../src/core/ai/gateway.ts';
+
+afterEach(() => resetGateway());
+
+describe('resetGateway baseline restore (#3554)', () => {
+  test('immediately after resetGateway(), the preload baseline is live', () => {
+    resetGateway();
+    expect(getEmbeddingModel()).toBe('openai:text-embedding-3-large');
+    expect(getEmbeddingDimensions()).toBe(1536);
+  });
+
+  test('resetGateway() overwrites a file-local config back to the baseline', () => {
+    configureGateway({
+      embedding_model: 'zeroentropyai:zembed-1',
+      embedding_dimensions: 1280,
+      env: {},
+    });
+    expect(getEmbeddingDimensions()).toBe(1280);
+    resetGateway();
+    expect(getEmbeddingModel()).toBe('openai:text-embedding-3-large');
+    expect(getEmbeddingDimensions()).toBe(1536);
+  });
+
+  test('resetGateway() still clears test transports (no stale transport leaks back)', () => {
+    __setChatTransportForTests(async () => {
+      throw new Error('should have been cleared');
+    });
+    resetGateway();
+    // Baseline config sets no chat key in a keyless env, but the transport
+    // seam itself must be gone: isAvailable('chat') short-circuits to true
+    // whenever a chat transport is installed, so with a hard-unconfigured
+    // gateway it can only be true if the transport survived the reset.
+    __unconfigureGatewayForTests();
+    expect(isAvailable('chat')).toBe(false);
+  });
+
+  test('__unconfigureGatewayForTests() gives a genuinely unconfigured gateway', () => {
+    __unconfigureGatewayForTests();
+    expect(() => getEmbeddingDimensions()).toThrow(/not configured/);
+    expect(isAvailable('embedding')).toBe(false);
+    // And a plain reset brings the baseline back.
+    resetGateway();
+    expect(getEmbeddingDimensions()).toBe(1536);
+  });
+});
diff --git a/test/ai/gateway.test.ts b/test/ai/gateway.test.ts
index 730cc8b71..7445afb92 100644
--- a/test/ai/gateway.test.ts
+++ b/test/ai/gateway.test.ts
@@ -2,6 +2,7 @@ import { describe, test, expect, beforeEach, afterAll } from 'bun:test';
 import {
   configureGateway,
   resetGateway,
+  __unconfigureGatewayForTests,
   isAvailable,
   embed,
   getEmbeddingModel,
@@ -55,6 +56,9 @@ describe('gateway.isAvailable (silent-drop regression surface)', () => {
   beforeEach(() => resetGateway());
 
   test('returns false when gateway not configured', () => {
+    // resetGateway() restores the preload's test baseline (#3554); go
+    // genuinely unconfigured for this one assertion.
+    __unconfigureGatewayForTests();
     expect(isAvailable('embedding')).toBe(false);
   });
 
diff --git a/test/doctor-ze-checks.test.ts b/test/doctor-ze-checks.test.ts
index aebf5d33c..0bbbfa7e6 100644
--- a/test/doctor-ze-checks.test.ts
+++ b/test/doctor-ze-checks.test.ts
@@ -143,9 +143,10 @@ describe('checkEmbeddingWidthConsistency', () => {
   });
 
   test('gateway unconfigured: skips with ok', async () => {
-    // Reset gateway so requireConfig() throws.
-    const { resetGateway } = await import('../src/core/ai/gateway.ts');
-    resetGateway();
+    // Hard-unconfigure so requireConfig() throws — resetGateway() would
+    // restore the preload's test baseline (#3554).
+    const { __unconfigureGatewayForTests } = await import('../src/core/ai/gateway.ts');
+    __unconfigureGatewayForTests();
     const check = await checkEmbeddingWidthConsistency(engine);
     expect(check.status).toBe('ok');
     expect(check.message).toContain('gateway not configured');
diff --git a/test/e2e/skill-brain-first.test.ts b/test/e2e/skill-brain-first.test.ts
index 59cabeb9c..f733ff884 100644
--- a/test/e2e/skill-brain-first.test.ts
+++ b/test/e2e/skill-brain-first.test.ts
@@ -81,6 +81,11 @@ function copyFixturesIntoTempWorkspace(): Workspace {
 
 let workspace: Workspace;
 
+// Restore (not delete) after each test: the audit-dir preload sets
+// GBRAIN_AUDIT_DIR once at process start, and deleting it leaks the
+// operator's real ~/.gbrain/audit/ to every later file in the shard.
+const priorAuditDir = process.env.GBRAIN_AUDIT_DIR;
+
 beforeEach(() => {
   workspace = copyFixturesIntoTempWorkspace();
   // Redirect audit dir to the tempdir so the snapshot file doesn't pollute
@@ -89,7 +94,8 @@ beforeEach(() => {
 });
 
 afterEach(() => {
-  delete process.env.GBRAIN_AUDIT_DIR;
+  if (priorAuditDir === undefined) delete process.env.GBRAIN_AUDIT_DIR;
+  else process.env.GBRAIN_AUDIT_DIR = priorAuditDir;
   workspace.cleanup();
 });
 
diff --git a/test/embed-preflight.test.ts b/test/embed-preflight.test.ts
index 5a0a0ca84..4f9fecd87 100644
--- a/test/embed-preflight.test.ts
+++ b/test/embed-preflight.test.ts
@@ -6,7 +6,11 @@
  * process.env.
  */
 import { describe, test, expect, beforeEach, afterAll } from 'bun:test';
-import { configureGateway, resetGateway } from '../src/core/ai/gateway.ts';
+import {
+  configureGateway,
+  resetGateway,
+  __unconfigureGatewayForTests,
+} from '../src/core/ai/gateway.ts';
 import {
   validateEmbeddingCreds,
   formatEmbeddingCredsError,
@@ -22,18 +26,12 @@ import type { AIGatewayConfig } from '../src/core/ai/types.ts';
 // isAvailable('embedding') check. That's what made facts-backstop-gating
 // fail intermittently (bin-pack-dependent) on CI shard 10.
 //
-// Don't end on a bare resetGateway() either: the NEXT file's beforeAll
-// (often engine.initSchema, which sizes vector columns from ambient gateway
-// state) runs before the legacy-embedding-preload's per-test restore, so a
-// null gateway here would seed 1280-d schemas under 1536-d fixtures.
-// Restore the preload's legacy pin instead.
+// #3554: resetGateway() now restores the preload's legacy pin itself (the
+// preload registers it via __setGatewayResetBaselineForTests), so a bare
+// reset is safe here — the NEXT file's beforeAll sees the 1536-d baseline,
+// not a null gateway that would seed 1280-d schemas under 1536-d fixtures.
 afterAll(() => {
   resetGateway();
-  configureGateway({
-    embedding_model: 'openai:text-embedding-3-large',
-    embedding_dimensions: 1536,
-    env: { ...process.env },
-  });
 });
 
 function baseConfig(overrides: Partial<AIGatewayConfig> = {}): AIGatewayConfig {
@@ -138,7 +136,9 @@ describe('validateEmbeddingCreds', () => {
   });
 
   test('throws no_gateway_config when gateway was not configured', () => {
-    // resetGateway() in beforeEach already cleared _config.
+    // resetGateway() restores the preload's test baseline (#3554), so this
+    // test needs the hard variant to get a genuinely unconfigured gateway.
+    __unconfigureGatewayForTests();
     let caught: unknown;
     try { validateEmbeddingCreds(); } catch (e) { caught = e; }
     expect(caught).toBeInstanceOf(EmbeddingCredentialError);
diff --git a/test/foreground-chat-gateway-init.serial.test.ts b/test/foreground-chat-gateway-init.serial.test.ts
index 5bdc7879d..81b747cf4 100644
--- a/test/foreground-chat-gateway-init.serial.test.ts
+++ b/test/foreground-chat-gateway-init.serial.test.ts
@@ -9,7 +9,11 @@ import { afterEach, beforeEach, describe, expect, spyOn, test } from 'bun:test';
 import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
-import { isAvailable, resetGateway } from '../src/core/ai/gateway.ts';
+import {
+  __unconfigureGatewayForTests,
+  isAvailable,
+  resetGateway,
+} from '../src/core/ai/gateway.ts';
 import { runExtractConversationFacts } from '../src/commands/extract-conversation-facts.ts';
 import { runEnrich } from '../src/commands/enrich.ts';
 
@@ -27,7 +31,10 @@ beforeEach(() => {
   }));
   process.env.GBRAIN_HOME = home;
   process.env.OPENAI_API_KEY = 'test-key';
-  resetGateway();
+  // Hard-unconfigure: this suite exists to exercise the COLD-gateway path
+  // (#2590), and resetGateway() now restores the preload's test baseline
+  // (#3554), which would make configureGatewayIfUninitialized a no-op.
+  __unconfigureGatewayForTests();
 });
 
 afterEach(() => {
diff --git a/test/helpers/legacy-embedding-preload.ts b/test/helpers/legacy-embedding-preload.ts
index 257c21d35..ca76b5303 100644
--- a/test/helpers/legacy-embedding-preload.ts
+++ b/test/helpers/legacy-embedding-preload.ts
@@ -18,7 +18,11 @@
  * `configureGateway()` explicitly in their own beforeAll, which
  * overwrites this preload.
  */
-import { configureGateway, getEmbeddingDimensions } from '../../src/core/ai/gateway.ts';
+import {
+  configureGateway,
+  getEmbeddingDimensions,
+  __setGatewayResetBaselineForTests,
+} from '../../src/core/ai/gateway.ts';
 import { beforeEach } from 'bun:test';
 
 const LEGACY_CONFIG = {
@@ -26,12 +30,16 @@ const LEGACY_CONFIG = {
   embedding_dimensions: 1536,
 } as const;
 
-function applyLegacy() {
-  configureGateway({
+function legacyGatewayConfig() {
+  return {
     embedding_model: LEGACY_CONFIG.embedding_model,
     embedding_dimensions: LEGACY_CONFIG.embedding_dimensions,
     env: { ...process.env },
-  });
+  };
+}
+
+function applyLegacy() {
+  configureGateway(legacyGatewayConfig());
 }
 
 if (process.env.GBRAIN_DEBUG_PRELOAD === '1') {
@@ -41,6 +49,16 @@ if (process.env.GBRAIN_DEBUG_PRELOAD === '1') {
 // Initial application — covers tests that don't reset the gateway.
 applyLegacy();
 
+// #3554: make resetGateway() mean "back to this baseline" instead of
+// "unconfigured". Without this, a file whose teardown calls resetGateway()
+// leaves _config = null; the NEXT file's beforeAll engine-connect then
+// reconfigures from the shipped default (zembed-1 @ 1280) BEFORE the
+// beforeEach below can fire, and the 1280-sized schema rejects the file's
+// 1536-d fixtures. Which file pairs collide depends on shard bin-packing,
+// so adding any test file reshuffles the mines. A factory (not a frozen
+// config) so each re-application captures fresh process.env.
+__setGatewayResetBaselineForTests(legacyGatewayConfig);
+
 // Per-test re-application — handles tests that call `resetGateway()`
 // in their setup/teardown. Bun's preload allows registering global
 // hooks; this fires before every test in every file in the shard.
diff --git a/test/llm-intent-escalation.test.ts b/test/llm-intent-escalation.test.ts
index 35bb636ca..33ad461fc 100644
--- a/test/llm-intent-escalation.test.ts
+++ b/test/llm-intent-escalation.test.ts
@@ -15,6 +15,7 @@ import {
 } from '../src/core/search/llm-intent.ts';
 import {
   __setChatTransportForTests,
+  __unconfigureGatewayForTests,
   configureGateway,
   resetGateway,
 } from '../src/core/ai/gateway.ts';
@@ -122,7 +123,9 @@ describe('classifyModalityWithLLM — fail-open', () => {
   });
 
   test('Gateway not configured → returns fallback', async () => {
-    resetGateway();
+    // Hard-unconfigure: resetGateway() would restore the preload's test
+    // baseline (#3554), whose {...process.env} could make chat available.
+    __unconfigureGatewayForTests();
     // No configureGateway called → isAvailable('chat') returns false.
     expect(await classifyModalityWithLLM('q', 'text')).toBe('text');
   });
diff --git a/test/minions-shell.test.ts b/test/minions-shell.test.ts
index ff1f311f9..42f801cc1 100644
--- a/test/minions-shell.test.ts
+++ b/test/minions-shell.test.ts
@@ -282,12 +282,19 @@ describe('shell-audit: computeAuditFilename', () => {
 
 describe('shell-audit: write', () => {
   let tmpDir: string;
+  // #3554-sibling: the audit-dir preload sets GBRAIN_AUDIT_DIR once at
+  // process start; deleting it here (instead of restoring) let every file
+  // AFTER this one in the shard write audit fixtures to the operator's
+  // real ~/.gbrain/audit/ — and failed audit-dir-preload.test.ts whenever
+  // bin-packing placed it later in the shard. Restore the prior value.
+  const priorAuditDir = process.env.GBRAIN_AUDIT_DIR;
   beforeEach(() => {
     tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'shell-audit-test-'));
     process.env.GBRAIN_AUDIT_DIR = tmpDir;
   });
   afterAll(() => {
-    delete process.env.GBRAIN_AUDIT_DIR;
+    if (priorAuditDir === undefined) delete process.env.GBRAIN_AUDIT_DIR;
+    else process.env.GBRAIN_AUDIT_DIR = priorAuditDir;
   });
 
   test('GBRAIN_AUDIT_DIR env override resolves to the custom dir', () => {
diff --git a/test/v0_37_fix_wave.serial.test.ts b/test/v0_37_fix_wave.serial.test.ts
index 500309570..801254856 100644
--- a/test/v0_37_fix_wave.serial.test.ts
+++ b/test/v0_37_fix_wave.serial.test.ts
@@ -55,24 +55,20 @@ describe('v0.37 Lane A — defaults sweep', () => {
   test('A.5: embedding-column registry builtin defaults to ZE/1280 on empty config + gateway', async () => {
     // The registry's resolution chain is cfg > gateway > DEFAULT. With
     // no cfg AND no gateway, it should fall through to the canonical
-    // default (ZE/1280). Reset gateway first to exercise that path.
-    const { resetGateway } = await import('../src/core/ai/gateway.ts');
+    // default (ZE/1280). Hard-unconfigure first to exercise that path —
+    // resetGateway() would restore the preload's 1536 baseline (#3554).
+    const { __unconfigureGatewayForTests, resetGateway } = await import('../src/core/ai/gateway.ts');
     const { getEmbeddingColumnRegistry } = await import('../src/core/search/embedding-column.ts');
-    resetGateway();
+    __unconfigureGatewayForTests();
     try {
       const reg = getEmbeddingColumnRegistry({ engine: 'pglite' } as any);
       expect(reg['embedding']).toBeDefined();
       expect(reg['embedding'].provider).toBe('zeroentropyai:zembed-1');
       expect(reg['embedding'].dimensions).toBe(1280);
     } finally {
-      // Re-apply legacy preload defaults so the rest of the file's tests
-      // (and subsequent files in this shard) see a configured gateway.
-      const { configureGateway } = await import('../src/core/ai/gateway.ts');
-      configureGateway({
-        embedding_model: 'openai:text-embedding-3-large',
-        embedding_dimensions: 1536,
-        env: { ...process.env },
-      });
+      // Restore the preload's legacy baseline so the rest of the file's
+      // tests (and subsequent files in this shard) see a configured gateway.
+      resetGateway();
     }
   });