diff --git a/changelog.d/features/5696-layer-a-capability-filter.md b/changelog.d/features/5696-layer-a-capability-filter.md
new file mode 100644
index 0000000000..37d04132e3
--- /dev/null
+++ b/changelog.d/features/5696-layer-a-capability-filter.md
@@ -0,0 +1 @@
+- **feat(core):** add Layer A capability filter at router (#5696)
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 3e7f6cab2f..01c1cd027c 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -142,6 +142,8 @@ import {
   getExplicitModelOutputCap,
   resolveInputTokenCapForGate,
 } from "@/lib/modelCapabilities.ts";
+import { checkRequestCapabilityFit, deriveRequestCapabilityRequirements, buildCapabilityMismatchMessage } from "@/shared/constants/capabilities/capabilityFilter.ts";
+import { isFeatureFlagEnabled } from "@/shared/utils/featureFlags.ts";
 import { toPositiveInteger } from "../services/reasoningTokenBuffer.ts";
 import { normalizeThinkingForModel } from "@/shared/constants/modelSpecs.ts";
 import {
@@ -2638,7 +2640,16 @@ export async function handleChatCore({
     }
   }
   // === /Quota Share enforcement PRE-hook ===
-
+  if (isFeatureFlagEnabled("CAPABILITY_FILTER_ENABLED")) {
+    const fit = checkRequestCapabilityFit(getResolvedModelCapabilities({ provider, model: effectiveModel }),
+      deriveRequestCapabilityRequirements(body as Record<string, unknown>), provider);
+    if (!fit.compatible) {
+      const msg = buildCapabilityMismatchMessage(fit.terminalReason!, provider, effectiveModel);
+      log?.warn?.("CAPABILITY", msg);
+      trackPendingRequest(model, provider, connectionId, false);
+      return createErrorResult(400, msg, null, fit.terminalReason, "invalid_request_error");
+    }
+  }
   // Get executor for this provider (with optional upstream proxy routing)
   const executor = await resolveExecutorWithProxy(provider);
   const getExecutionCredentials = () =>
diff --git a/src/i18n/messages/en.json b/src/i18n/messages/en.json
index f55671d00c..3fafbd560a 100644
--- a/src/i18n/messages/en.json
+++ b/src/i18n/messages/en.json
@@ -12426,5 +12426,10 @@
     "degraded.source.circuitBreaker": "Circuit Breaker",
     "degraded.source.modelLockouts": "Model Lockouts",
     "degraded.source.count": "Connection Count"
-  }
+  },
+  "featureFlagCapabilityFilterEnabledDescription": "Reject requests before dispatch when the target model lacks required capabilities (vision, tools, structured output, context window). Protects direct single-provider requests that bypass the combo-layer compatibility filter.",
+  "capabilityFilter.visionMismatch": "Provider does not support vision for this image request",
+  "capabilityFilter.toolsMismatch": "Provider does not support tool calling",
+  "capabilityFilter.structuredOutputMismatch": "Provider does not support structured output",
+  "capabilityFilter.contextWindowMismatch": "Request exceeds provider context window"
 }
diff --git a/src/i18n/messages/pt-BR.json b/src/i18n/messages/pt-BR.json
index 7ba3d31af2..a415b0d2a6 100644
--- a/src/i18n/messages/pt-BR.json
+++ b/src/i18n/messages/pt-BR.json
@@ -12426,5 +12426,10 @@
     "degraded.source.circuitBreaker": "Disjuntor",
     "degraded.source.modelLockouts": "Bloqueios de Modelo",
     "degraded.source.count": "Contagem de Conexões"
-  }
+  },
+  "featureFlagCapabilityFilterEnabledDescription": "Rejeitar requisições antes do despacho quando o modelo alvo nao possui as capacidades necessarias (visao, ferramentas, saída estruturada, janela de contexto). Protege requisições diretas que ignoram o filtro de compatibilidade do combo.",
+  "capabilityFilter.visionMismatch": "O provedor nao suporta visao para esta requisicao de imagem",
+  "capabilityFilter.toolsMismatch": "O provedor nao suporta chamada de ferramentas",
+  "capabilityFilter.structuredOutputMismatch": "O provedor nao suporta saida estruturada",
+  "capabilityFilter.contextWindowMismatch": "A requisicao excede a janela de contexto do provedor"
 }
diff --git a/src/shared/constants/capabilities/capabilityFilter.ts b/src/shared/constants/capabilities/capabilityFilter.ts
new file mode 100644
index 0000000000..b0c0de8091
--- /dev/null
+++ b/src/shared/constants/capabilities/capabilityFilter.ts
@@ -0,0 +1,211 @@
+/**
+ * Layer A capability filter — shared, provider-agnostic module.
+ *
+ * Validates that a provider+model can satisfy the request's capability
+ * requirements (tools, vision, structured output, context window) BEFORE
+ * dispatch to the executor. Returns an early 400 if not, rather than
+ * letting the request fail downstream or produce garbage (e.g. a text-only
+ * model receiving image_url content and answering "image not provided").
+ *
+ * The logic mirrors what `filterTargetsByRequestCompatibility` in
+ * comboStructure.ts already does for combo-routed requests, but this
+ * module lives at the router layer (Layer A) so it also protects direct
+ * single-provider requests (`model:"openai/gpt-4o-mini"` without a combo).
+ *
+ * #5696
+ */
+
+import { getResolvedModelCapabilities } from "@/lib/modelCapabilities";
+import { evaluateContextLimit } from "@omniroute/open-sse/services/combo/contextOverrideGate";
+import { hasEstimableContent } from "@omniroute/open-sse/services/combo/knownContextOverflow";
+import { isRecord } from "@omniroute/open-sse/services/combo/comboData";
+import { providerSupportsEmulatedToolCalling } from "@omniroute/open-sse/services/combo/comboStructure";
+import { estimateTokens } from "@omniroute/open-sse/services/contextManager";
+
+// ── Types ─────────────────────────────────────────────────────────────────
+
+export type CapabilityFailure = "tools" | "vision" | "structured_output" | "context_window";
+
+export interface RequestCapabilityRequirements {
+  requiresTools: boolean;
+  requiresVision: boolean;
+  requiresStructuredOutput: boolean;
+  requiredContextTokens: number;
+  toolCount: number;
+}
+
+export interface CapabilityFilterResult {
+  compatible: boolean;
+  failures: CapabilityFailure[];
+  terminalReason?: string;
+}
+
+// ── Pure helpers (mirror the unexported helpers in comboStructure.ts) ──────
+
+function requestRequiresTools(body: Record<string, unknown>): boolean {
+  if (Array.isArray(body.tools) && body.tools.length > 0) return true;
+  if (Array.isArray(body.functions) && body.functions.length > 0) return true;
+  return false;
+}
+
+function requestRequiresStructuredOutput(body: Record<string, unknown>): boolean {
+  const responseFormat = isRecord(body.response_format) ? body.response_format : null;
+  const type = typeof responseFormat?.type === "string" ? responseFormat.type : null;
+  return type === "json_object" || type === "json_schema";
+}
+
+function estimateRequestInputTokens(body: Record<string, unknown>): number {
+  const estimatePayload: Record<string, unknown> = {};
+  for (const key of ["messages", "input", "tools", "functions", "response_format"]) {
+    if (hasEstimableContent(body[key])) estimatePayload[key] = body[key];
+  }
+  return Object.keys(estimatePayload).length > 0 ? estimateTokens(estimatePayload) : 0;
+}
+
+function getPositiveTokenCount(value: unknown): number {
+  const count = Number(value);
+  return Number.isFinite(count) && count > 0 ? Math.ceil(count) : 0;
+}
+
+function isMediaTypeImage(value: Record<string, unknown>): boolean {
+  const source = isRecord(value.source) ? value.source : null;
+  const mediaType = typeof source?.media_type === "string" ? source.media_type.toLowerCase() : "";
+  return mediaType.startsWith("image/");
+}
+
+function valueContainsImagePart(value: unknown, depth = 0): boolean {
+  if (depth > 8 || value === null || value === undefined) return false;
+  if (typeof value === "string") return value.startsWith("data:image/");
+  if (Array.isArray(value)) return value.some((entry) => valueContainsImagePart(entry, depth + 1));
+  if (!isRecord(value)) return false;
+
+  if (valueContainsImageType(value)) return true;
+  if (isMediaTypeImage(value)) return true;
+
+  return Object.values(value).some((entry) => valueContainsImagePart(entry, depth + 1));
+}
+
+function isContextOverflow(
+  capabilities: { maxInputTokens: number | null; contextWindow: number | null },
+  requirements: { requiredContextTokens: number }
+): boolean {
+  return evaluateContextLimit(
+    { maxInputTokens: capabilities.maxInputTokens, contextWindow: capabilities.contextWindow },
+    { estimatedInputTokens: requirements.requiredContextTokens, requiredContextTokens: requirements.requiredContextTokens }
+  ) === false;
+}
+
+function valueContainsImageType(value: Record<string, unknown>): boolean {
+  const type = typeof value.type === "string" ? value.type.toLowerCase() : null;
+  if (type === "image" || type === "image_url" || type === "input_image") return true;
+  if ("image_url" in value || "input_image" in value) return true;
+  return false;
+}
+
+// ── Public API ─────────────────────────────────────────────────────────────
+
+/**
+ * Derive capability requirements from a request body.
+ * Mirrors `deriveRequestCompatibilityRequirements` in comboStructure.ts.
+ */
+export function deriveRequestCapabilityRequirements(
+  body: Record<string, unknown>
+): RequestCapabilityRequirements {
+  const estimatedInputTokens = estimateRequestInputTokens(body);
+  const requestedOutputTokens = Math.max(
+    getPositiveTokenCount(body.max_tokens),
+    getPositiveTokenCount(body.max_completion_tokens)
+  );
+  return {
+    requiresTools: requestRequiresTools(body),
+    requiresVision: valueContainsImagePart(body.messages) || valueContainsImagePart(body.input),
+    requiresStructuredOutput: requestRequiresStructuredOutput(body),
+    requiredContextTokens: estimatedInputTokens + requestedOutputTokens,
+    toolCount: Array.isArray(body.tools) ? body.tools.length : 0,
+  };
+}
+
+/**
+ * Build a human-readable error message for a capability mismatch.
+ * Mirrors the i18n keys: capabilityFilter.visionMismatch / toolsMismatch / etc.
+ */
+export function buildCapabilityMismatchMessage(
+  terminalReason: string,
+  provider: string | null,
+  model: string | null
+): string {
+  const msgs: Record<string, string> = {
+    vision: `Provider '${provider}' does not support vision for this image request`,
+    tools: `Provider '${provider}' does not support tool calling`,
+    structured_output: `Provider '${provider}' does not support structured output`,
+    context_window: `Request exceeds the context window for ${provider}/${model}`,
+  };
+  return msgs[terminalReason] || `Provider '${provider}' does not support the required capabilities`;
+}
+
+/**
+ * Check whether a model's capabilities satisfy the request requirements.
+ *
+ * @param capabilities - Resolved model capabilities (from getResolvedModelCapabilities)
+ * @param requirements - Request capability requirements
+ * @param provider - Provider id or alias (needed for emulated-tool-calling bypass)
+ * @returns CapabilityFilterResult with compatibility verdict and failure details
+ */
+function collectCapabilityFailures(
+  capabilities: Record<string, unknown>,
+  requirements: RequestCapabilityRequirements,
+  provider?: string | null
+): CapabilityFailure[] {
+  const failures: CapabilityFailure[] = [];
+  const caps = capabilities as {
+    supportsTools: boolean | null;
+    toolCalling: boolean;
+    supportsVision: boolean | null;
+    structuredOutput: boolean | null;
+    contextWindow: number | null;
+    maxInputTokens: number | null;
+    maxOutputTokens: number | null;
+  };
+
+  if (requirements.requiresTools && (caps.supportsTools === false || !caps.toolCalling)
+    && !providerSupportsEmulatedToolCalling(provider)) {
+    failures.push("tools");
+  }
+  if (requirements.requiresVision && caps.supportsVision !== true) {
+    failures.push("vision");
+  }
+  if (requirements.requiresStructuredOutput && caps.structuredOutput === false) {
+    failures.push("structured_output");
+  }
+  if (requirements.requiredContextTokens > 0 && isContextOverflow(caps, requirements)) {
+    failures.push("context_window");
+  }
+  return failures;
+}
+
+function primaryFailure(failures: CapabilityFailure[]): CapabilityFailure {
+  if (failures.includes("vision")) return "vision";
+  if (failures.includes("tools")) return "tools";
+  if (failures.includes("structured_output")) return "structured_output";
+  return "context_window";
+}
+
+export function checkRequestCapabilityFit(
+  capabilities: {
+    supportsTools: boolean | null;
+    toolCalling: boolean;
+    supportsVision: boolean | null;
+    structuredOutput: boolean | null;
+    contextWindow: number | null;
+    maxInputTokens: number | null;
+    maxOutputTokens: number | null;
+  },
+  requirements: RequestCapabilityRequirements,
+  provider?: string | null
+): CapabilityFilterResult {
+  const failures = collectCapabilityFailures(capabilities as Record<string, unknown>, requirements, provider);
+  if (failures.length === 0) {
+    return { compatible: true, failures: [] };
+  }
+  return { compatible: false, failures, terminalReason: primaryFailure(failures) };
+}
\ No newline at end of file
diff --git a/src/shared/constants/featureFlagDefinitions.ts b/src/shared/constants/featureFlagDefinitions.ts
index e5cf433a61..4e0a8a4477 100644
--- a/src/shared/constants/featureFlagDefinitions.ts
+++ b/src/shared/constants/featureFlagDefinitions.ts
@@ -245,6 +245,18 @@ export const FEATURE_FLAG_DEFINITIONS: FeatureFlagDefinition[] = [
     requiresRestart: true,
     warningLevel: "info",
   },
+  {
+    key: "CAPABILITY_FILTER_ENABLED",
+    label: "Capability Filter",
+    description:
+      "Reject requests before dispatch when the target model lacks required capabilities (vision, tools, structured output, context window). Protects direct single-provider requests that bypass the combo-layer compatibility filter.",
+    descriptionI18nKey: "featureFlagCapabilityFilterEnabledDescription",
+    category: "policies",
+    defaultValue: "false",
+    type: "boolean",
+    requiresRestart: false,
+    warningLevel: "caution",
+  },
   {
     key: "RADAR_ENABLED",
     label: "Radar",
diff --git a/stryker.conf.json b/stryker.conf.json
index 71e0b21352..7a0cc3dfcc 100644
--- a/stryker.conf.json
+++ b/stryker.conf.json
@@ -39,7 +39,9 @@
   "incremental": true,
   "incrementalFile": "reports/mutation/stryker-incremental.json",
   "testRunner": "tap",
-  "plugins": ["@stryker-mutator/tap-runner"],
+  "plugins": [
+    "@stryker-mutator/tap-runner"
+  ],
   "tap": {
     "testFiles": [
       "tests/unit/7993-noauth-proxy-routing.test.ts",
@@ -333,7 +335,8 @@
       "tests/unit/upstream-timeout-model-override.test.ts",
       "tests/unit/usage-service-hardening.test.ts",
       "tests/unit/validate-response-quality.test.ts",
-      "tests/unit/xai-agent-tools-passthrough.test.ts"
+      "tests/unit/xai-agent-tools-passthrough.test.ts",
+      "tests/unit/capability-filter.test.ts"
     ],
     "nodeArgs": [
       "--import",
@@ -442,7 +445,11 @@
     ".worktrees",
     ".stryker-tmp"
   ],
-  "reporters": ["progress", "html", "json"],
+  "reporters": [
+    "progress",
+    "html",
+    "json"
+  ],
   "htmlReporter": {
     "fileName": "reports/mutation/mutation.html"
   },
diff --git a/tests/unit/capability-filter.test.ts b/tests/unit/capability-filter.test.ts
new file mode 100644
index 0000000000..5bcab12d76
--- /dev/null
+++ b/tests/unit/capability-filter.test.ts
@@ -0,0 +1,269 @@
+/**
+ * #5696 — Layer A capability filter unit tests.
+ *
+ * Tests the pure `checkRequestCapabilityFit` function and the
+ * `deriveRequestCapabilityRequirements` helper. The chatCore integration
+ * gate is tested via the feature flag assertion below.
+ *
+ * Note: `getResolvedModelCapabilities` requires a database connection, so
+ * the full integration path (capabilities → filter → error response) is
+ * tested by verifying the filter function's behavior with mock capabilities.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  checkRequestCapabilityFit,
+  deriveRequestCapabilityRequirements,
+  type RequestCapabilityRequirements,
+  type CapabilityFilterResult,
+} from "../../src/shared/constants/capabilities/capabilityFilter.ts";
+
+// ── Helpers ───────────────────────────────────────────────────────────────
+
+/** Minimal capabilities shape for filter testing. */
+function caps(overrides: Partial<{
+  supportsTools: boolean | null;
+  toolCalling: boolean;
+  supportsVision: boolean | null;
+  structuredOutput: boolean | null;
+  contextWindow: number | null;
+  maxInputTokens: number | null;
+  maxOutputTokens: number | null;
+}> = {}) {
+  return {
+    supportsTools: overrides.supportsTools ?? null,
+    toolCalling: overrides.toolCalling ?? true,
+    supportsVision: overrides.supportsVision ?? null,
+    structuredOutput: overrides.structuredOutput ?? null,
+    contextWindow: overrides.contextWindow ?? null,
+    maxInputTokens: overrides.maxInputTokens ?? null,
+    maxOutputTokens: overrides.maxOutputTokens ?? null,
+  };
+}
+
+function req(overrides: Partial<RequestCapabilityRequirements> = {}): RequestCapabilityRequirements {
+  return {
+    requiresTools: false,
+    requiresVision: false,
+    requiresStructuredOutput: false,
+    requiredContextTokens: 0,
+    toolCount: 0,
+    ...overrides,
+  };
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────
+
+test("checkRequestCapabilityFit: compatible when no requirements", () => {
+  const result = checkRequestCapabilityFit(caps(), req());
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: vision failure when model lacks vision", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsVision: false }),
+    req({ requiresVision: true })
+  );
+  assert.equal(result.compatible, false);
+  assert.deepEqual(result.failures, ["vision"]);
+  assert.equal(result.terminalReason, "vision");
+});
+
+test("checkRequestCapabilityFit: vision failure when model vision is unknown (null)", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsVision: null }),
+    req({ requiresVision: true })
+  );
+  assert.equal(result.compatible, false);
+  assert.deepEqual(result.failures, ["vision"]);
+  assert.equal(result.terminalReason, "vision");
+});
+
+test("checkRequestCapabilityFit: vision OK when model supports vision", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsVision: true }),
+    req({ requiresVision: true })
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: tools failure when model has no tool support", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsTools: false, toolCalling: false }),
+    req({ requiresTools: true }),
+    "openai"
+  );
+  assert.equal(result.compatible, false);
+  assert.deepEqual(result.failures, ["tools"]);
+  assert.equal(result.terminalReason, "tools");
+});
+
+test("checkRequestCapabilityFit: tools OK when model supports tools", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsTools: true, toolCalling: true }),
+    req({ requiresTools: true }),
+    "openai"
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: tools bypassed for emulated-tool provider", () => {
+  // chatgpt-web has toolCalling: "emulated" in the provider registry,
+  // so the filter must not reject it even when capabilities report false.
+  const result = checkRequestCapabilityFit(
+    caps({ supportsTools: false, toolCalling: false }),
+    req({ requiresTools: true }),
+    "chatgpt-web"
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: structured output failure when model does not support", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ structuredOutput: false }),
+    req({ requiresStructuredOutput: true })
+  );
+  assert.equal(result.compatible, false);
+  assert.deepEqual(result.failures, ["structured_output"]);
+  assert.equal(result.terminalReason, "structured_output");
+});
+
+test("checkRequestCapabilityFit: structured output OK when model supports", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ structuredOutput: true }),
+    req({ requiresStructuredOutput: true })
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: context window failure when tokens exceed window", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ contextWindow: 1000, maxInputTokens: 1000 }),
+    req({ requiredContextTokens: 2000 })
+  );
+  assert.equal(result.compatible, false);
+  assert.deepEqual(result.failures, ["context_window"]);
+  assert.equal(result.terminalReason, "context_window");
+});
+
+test("checkRequestCapabilityFit: context window OK when tokens fit", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ contextWindow: 10000, maxInputTokens: 10000 }),
+    req({ requiredContextTokens: 2000 })
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("checkRequestCapabilityFit: multiple failures reported", () => {
+  const result = checkRequestCapabilityFit(
+    caps({ supportsVision: false, supportsTools: false, toolCalling: false }),
+    req({ requiresVision: true, requiresTools: true }),
+    "openai"
+  );
+  assert.equal(result.compatible, false);
+  // vision is checked first, so it's the terminalReason
+  assert.ok(result.failures.length >= 1);
+  assert.ok(result.failures.includes("vision"));
+});
+
+test("checkRequestCapabilityFit: context window returns null (unknown) when no window data", () => {
+  // When contextWindow and maxInputTokens are both null, evaluateContextLimit
+  // returns null, which means compatible (no data to judge).
+  const result = checkRequestCapabilityFit(
+    caps({ contextWindow: null, maxInputTokens: null }),
+    req({ requiredContextTokens: 2000 })
+  );
+  assert.equal(result.compatible, true);
+  assert.deepEqual(result.failures, []);
+});
+
+test("deriveRequestCapabilityRequirements: no requirements from empty body", () => {
+  const requirements = deriveRequestCapabilityRequirements({});
+  assert.equal(requirements.requiresTools, false);
+  assert.equal(requirements.requiresVision, false);
+  assert.equal(requirements.requiresStructuredOutput, false);
+  assert.equal(requirements.requiredContextTokens, 0);
+  assert.equal(requirements.toolCount, 0);
+});
+
+test("deriveRequestCapabilityRequirements: detects tools from body", () => {
+  const requirements = deriveRequestCapabilityRequirements({
+    tools: [{ type: "function", function: { name: "test" } }],
+  });
+  assert.equal(requirements.requiresTools, true);
+  assert.equal(requirements.toolCount, 1);
+});
+
+test("deriveRequestCapabilityRequirements: detects vision from image_url", () => {
+  const requirements = deriveRequestCapabilityRequirements({
+    messages: [
+      { role: "user", content: [{ type: "image_url", image_url: { url: "https://example.com/img.jpg" } }] },
+    ],
+  });
+  assert.equal(requirements.requiresVision, true);
+});
+
+test("deriveRequestCapabilityRequirements: detects structured output from response_format", () => {
+  const requirements = deriveRequestCapabilityRequirements({
+    response_format: { type: "json_object" },
+  });
+  assert.equal(requirements.requiresStructuredOutput, true);
+});
+
+test("deriveRequestCapabilityRequirements: detects json_schema structured output", () => {
+  const requirements = deriveRequestCapabilityRequirements({
+    response_format: { type: "json_schema", json_schema: { name: "test", schema: {} } },
+  });
+  assert.equal(requirements.requiresStructuredOutput, true);
+});
+
+test("feature flag CAPABILITY_FILTER_ENABLED defaults to false", () => {
+  // This test verifies the feature flag definition ensures the gate is
+  // opt-in. The default value must be "false" per the plan.
+  import("../../src/shared/constants/featureFlagDefinitions.ts").then(
+    ({ FEATURE_FLAG_DEFINITIONS }) => {
+      const flag = FEATURE_FLAG_DEFINITIONS.find(
+        (d) => d.key === "CAPABILITY_FILTER_ENABLED"
+      );
+      assert.ok(flag, "CAPABILITY_FILTER_ENABLED flag must be defined");
+      assert.equal(flag.defaultValue, "false");
+      assert.equal(flag.type, "boolean");
+      assert.equal(flag.category, "policies");
+    }
+  );
+});
+
+test("error responses use buildErrorBody and do not leak stack traces", () => {
+  // Verify that capability mismatch errors route through buildErrorBody
+  // (createErrorResult) and never contain stack traces.
+  import("../../open-sse/utils/error.ts").then(({ createErrorResult }) => {
+    const result = createErrorResult(
+      400,
+      "Provider 'test' does not support vision for this image request",
+      null,
+      "vision",
+      "invalid_request_error"
+    );
+    assert.equal(result.status, 400);
+    assert.equal(result.error, "Provider 'test' does not support vision for this image request");
+    assert.equal(result.errorType, "invalid_request_error");
+    assert.equal(result.errorCode, "vision");
+
+    // Parse the response body and assert no stack leak
+    result.response.text().then((text) => {
+      const body = JSON.parse(text);
+      assert.ok(body.error.message, "error message must exist");
+      assert.equal(body.error.message.includes("at /"), false, "must not leak stack traces");
+      assert.equal(body.error.code, "vision");
+      assert.equal(body.error.type, "invalid_request_error");
+    });
+  });
+});
\ No newline at end of file