diff --git a/config/quality/eslint-suppressions.json b/config/quality/eslint-suppressions.json
index 54749b5b8a07..f20888c04c60 100644
--- a/config/quality/eslint-suppressions.json
+++ b/config/quality/eslint-suppressions.json
@@ -1818,11 +1818,6 @@
       "count": 11
     }
   },
-  "src/shared/validation/schemas/provider.ts": {
-    "@typescript-eslint/no-unused-vars": {
-      "count": 10
-    }
-  },
   "src/shared/validation/schemas/proxy.ts": {
     "@typescript-eslint/no-unused-vars": {
       "count": 11
diff --git a/src/shared/validation/schemas/provider.ts b/src/shared/validation/schemas/provider.ts
index 0fd4f9a1bbc9..246bdd2a07d6 100644
--- a/src/shared/validation/schemas/provider.ts
+++ b/src/shared/validation/schemas/provider.ts
@@ -1,22 +1,9 @@
 import { z } from "zod";
-import {
-  ACCOUNT_FALLBACK_STRATEGY_VALUES,
-  ROUTING_STRATEGY_VALUES,
-} from "@/shared/constants/routingStrategies";
-import { SUPPORTED_BATCH_ENDPOINTS } from "@/shared/constants/batchEndpoints";
-import { MAX_REQUEST_BODY_LIMIT_MB, MIN_REQUEST_BODY_LIMIT_MB } from "@/shared/constants/bodySize";
-import { COMBO_CONFIG_MODES } from "@/shared/constants/comboConfigMode";
 import {
   MODEL_SUPPORTED_ENDPOINT_VALUES,
   normalizeModelSupportedEndpoints,
 } from "@/shared/constants/modelSupportedEndpoints";
 import { providerAllowsOptionalApiKey } from "@/shared/constants/providers";
-import { HIDEABLE_SIDEBAR_ITEM_IDS } from "@/shared/constants/sidebarVisibility";
-import {
-  isForbiddenUpstreamHeaderName,
-  isForbiddenCustomHeaderName,
-} from "@/shared/constants/upstreamHeaders";
-import { MAX_TIMER_TIMEOUT_MS } from "@/shared/utils/runtimeTimeouts";
 import { validateProviderSpecificData } from "@/shared/validation/providerSpecificData";
 import {
   isReservedProviderPrefix,
@@ -89,6 +76,16 @@ const providerNodeIconUrlSchema = z
 // Same fix shape as #6562 (priority cap raised to 100_000).
 export const MAX_PROVIDER_CREDENTIAL_LENGTH = 100_000;
 
+// #15930: credentials arriving with this prefix are encrypted envelopes produced by a
+// credential store. `encrypt()` deliberately skips re-encrypting values that already
+// carry it, so an envelope would be stored verbatim, fail to decrypt at runtime, and
+// surface only as a misleading "Missing API key" on the connection test. Plaintext
+// keys from every provider (sk-…, tvly-…, JWTs, cookie headers, JSON storage state)
+// start with something else, so rejecting at validation time is near-zero-false-positive.
+// Checked inline rather than via lib/db/encryption's looksEncrypted() because this
+// schema is imported by client-side dashboard code and must stay free of node:crypto.
+const ENCRYPTED_ENVELOPE_PREFIX = "enc:v1:";
+
 export const createProviderSchema = z
   .object({
     provider: z.string().min(1).max(100),
@@ -123,6 +120,14 @@ export const createProviderSchema = z
         path: ["apiKey"],
       });
     }
+    if (apiKey.startsWith(ENCRYPTED_ENVELOPE_PREFIX)) {
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        message:
+          "apiKey looks like an encrypted envelope from a credential store, not a plaintext key — store the plaintext API key",
+        path: ["apiKey"],
+      });
+    }
 
     const cx =
       data.providerSpecificData && typeof data.providerSpecificData === "object"
@@ -641,6 +646,15 @@ export const updateProviderConnectionSchema = z
         path: [],
       });
     }
+    const apiKey = typeof value.apiKey === "string" ? value.apiKey.trim() : "";
+    if (apiKey.startsWith(ENCRYPTED_ENVELOPE_PREFIX)) {
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        message:
+          "apiKey looks like an encrypted envelope from a credential store, not a plaintext key — store the plaintext API key",
+        path: ["apiKey"],
+      });
+    }
   });
 
 export const providersBatchTestSchema = z
diff --git a/tests/unit/15930-encrypted-envelope-credential-rejected.test.ts b/tests/unit/15930-encrypted-envelope-credential-rejected.test.ts
new file mode 100644
index 000000000000..430c72afc7c7
--- /dev/null
+++ b/tests/unit/15930-encrypted-envelope-credential-rejected.test.ts
@@ -0,0 +1,65 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { createProviderSchema, updateProviderConnectionSchema } =
+  await import("../../src/shared/validation/schemas/provider.ts");
+
+// Foreign ciphertext envelope: right prefix, wrong (3-segment) shape — exactly what
+// another tool's credential store produces and what #15930 let slip into the DB.
+const ENVELOPE = "enc:v1:" + "a1b2c3d4e5f60718293a4b5c6d7e8f90".repeat(2);
+
+type SafeParseResult =
+  | { success: true }
+  | {
+      success: false;
+      error: { issues: Array<{ path: Array<string | number | symbol>; message: string }> };
+    };
+
+function apiKeyIssues(
+  schema: { safeParse: (d: unknown) => SafeParseResult },
+  data: unknown
+): string[] {
+  const result = schema.safeParse(data);
+  if (result.success) return [];
+  return result.error.issues.filter((i) => i.path.includes("apiKey")).map((i) => i.message);
+}
+
+test("create rejects an apiKey that is an encrypted envelope", () => {
+  const issues = apiKeyIssues(createProviderSchema, {
+    provider: "serper-search",
+    apiKey: ENVELOPE,
+    name: "andy+github@savage.hk",
+  });
+  assert.ok(
+    issues.some((m) => /encrypted envelope/i.test(m)),
+    `expected an encrypted-envelope issue on apiKey, got: ${JSON.stringify(issues)}`
+  );
+});
+
+test("create still accepts plaintext keys, including values that merely contain the prefix mid-string", () => {
+  for (const apiKey of ["767321c822493d35e9e4683d902fd0a9e5da51fd", "sk-enc:v1:not-an-envelope"]) {
+    const result = createProviderSchema.safeParse({
+      provider: "serper-search",
+      apiKey,
+      name: "andy+github@savage.hk",
+    });
+    assert.equal(result.success, true, `plaintext key rejected: ${apiKey}`);
+  }
+});
+
+test("update rejects an encrypted-envelope apiKey", () => {
+  const issues = apiKeyIssues(updateProviderConnectionSchema, { apiKey: ENVELOPE });
+  assert.ok(
+    issues.some((m) => /encrypted envelope/i.test(m)),
+    `expected an encrypted-envelope issue on apiKey, got: ${JSON.stringify(issues)}`
+  );
+});
+
+test("update still accepts plaintext keys and unrelated patches", () => {
+  assert.equal(
+    updateProviderConnectionSchema.safeParse({ apiKey: "sk-live-abc123" }).success,
+    true
+  );
+  assert.equal(updateProviderConnectionSchema.safeParse({ isActive: true }).success, true);
+  assert.equal(updateProviderConnectionSchema.safeParse({}).success, false); // "No valid fields to update" preserved
+});