diff --git a/clients/cli/__tests__/metadata.test.ts b/clients/cli/__tests__/metadata.test.ts
index ef8e2b37..55074dd4 100644
--- a/clients/cli/__tests__/metadata.test.ts
+++ b/clients/cli/__tests__/metadata.test.ts
@@ -911,6 +911,34 @@ describe("Metadata Tests", () => {
       }
     });
 
+    it("names the key but never the value when rejecting", async () => {
+      // The pair can carry a credential, and this message reaches stderr and
+      // CI logs.
+      const server = createTestServerHttp({
+        serverInfo: createTestServerInfo(),
+        tools: [createEchoTool()],
+      });
+      try {
+        await server.start();
+        const result = await runCli([
+          server.url,
+          "--cli",
+          "--method",
+          "tools/list",
+          "--metadata",
+          'credentials={"accessToken":"sk-live-nope","n":1e400}',
+          "--transport",
+          "http",
+        ]);
+        const output = `${result.stderr}${result.stdout}`;
+        expect(result.exitCode).not.toBe(0);
+        expect(output).toContain("credentials");
+        expect(output).not.toContain("sk-live");
+      } finally {
+        await server.stop();
+      }
+    });
+
     it("should handle metadata parsing validation", async () => {
       const server = createTestServerHttp({
         serverInfo: createTestServerInfo(),
diff --git a/clients/cli/src/cli.ts b/clients/cli/src/cli.ts
index c486a725..412fc5dc 100644
--- a/clients/cli/src/cli.ts
+++ b/clients/cli/src/cli.ts
@@ -504,8 +504,11 @@ function parseKeyValuePair(
   // and better than falling back to the literal string, which would also not
   // be what was asked for.
   if (!isSerializableJson(parsedValue)) {
+    // Names the key, never the value: the pair can carry a credential
+    // (`credentials={"accessToken":"…","n":1e400}`) and this message lands in
+    // stderr and CI logs.
     throw new Error(
-      `Invalid parameter value: ${value}. Numbers must be finite (a value like 1e400 overflows and cannot be sent).`,
+      `Invalid value for "${key}": numbers must be finite (a literal like 1e400 overflows to Infinity and cannot be sent).`,
     );
   }
 
diff --git a/clients/web/src/test/core/mcp/serverList.test.ts b/clients/web/src/test/core/mcp/serverList.test.ts
index 2ad70e4c..acca524f 100644
--- a/clients/web/src/test/core/mcp/serverList.test.ts
+++ b/clients/web/src/test/core/mcp/serverList.test.ts
@@ -1477,6 +1477,44 @@ describe("normalizeStoredMetadata (#1910)", () => {
     expect(warn).toHaveBeenCalledTimes(1);
   });
 
+  it("drops a value that parses but cannot be sent, keeping its siblings", () => {
+    // A hand-edited catalog reaches this untouched by any editor, so it is a
+    // real source of `1e400` — which parses to Infinity and serializes to null.
+    const out = normalizeStoredMetadata(
+      JSON.parse('{"good":1,"bad":1e400,"alsoGood":{"n":2}}'),
+    );
+    expect(out).toEqual({ good: 1, alsoGood: { n: 2 } });
+    expect(warn).toHaveBeenCalledTimes(1);
+  });
+
+  it("drops a non-serializable value nested inside an object", () => {
+    const out = normalizeStoredMetadata(JSON.parse('{"a":{"deep":[1e400]}}'));
+    expect(out).toEqual({});
+  });
+
+  it("applies the same check to a legacy pair value", () => {
+    const out = normalizeStoredMetadata([
+      { key: "ok", value: 1 },
+      { key: "bad", value: JSON.parse("1e400") },
+    ]);
+    expect(out).toEqual({ ok: 1 });
+  });
+
+  it("never prints the offending value in a warning", () => {
+    // `_meta` can hold credentials, and these warnings go to ordinary console
+    // output — so a diagnostic may name the key and the type, never the value.
+    normalizeStoredMetadata([
+      { value: { accessToken: "sk-live-nope" } },
+      { key: "leaky", value: JSON.parse('{"token":1e400}') },
+    ]);
+    normalizeStoredMetadata("sk-live-also-nope");
+    const printed = JSON.stringify(warn.mock.calls);
+    expect(printed).not.toContain("sk-live");
+    expect(printed).not.toContain("accessToken");
+    // The key is still named, so the warning stays actionable.
+    expect(printed).toContain("leaky");
+  });
+
   it("migrates a legacy file to the object shape on the next write", () => {
     // Read side accepts the pair array; write side emits only the object, so
     // one round-trip is the migration.
diff --git a/core/mcp/remote/node/server.ts b/core/mcp/remote/node/server.ts
index 0b3bffe5..8ceb79ae 100644
--- a/core/mcp/remote/node/server.ts
+++ b/core/mcp/remote/node/server.ts
@@ -1213,9 +1213,18 @@ export function createRemoteApp(
   // credentials are intentionally lost on first read (hard cutover per
   // #1358 decision 4). Users re-enter via the form or hand-edit into the
   // flat shape.
-  // A plain JSON object — the container `_meta` requires. Values are
-  // deliberately unchecked: `_meta` takes arbitrary JSON (#1910), and anything
-  // that survived `JSON.parse` already is.
+  // The *container* `_meta` requires — a plain JSON object.
+  //
+  // Value-level validity is deliberately not checked here. Values are
+  // arbitrary JSON (#1910), and the one class that survives `JSON.parse` yet
+  // cannot be sent — a non-finite number, from a literal like `1e400` in a
+  // hand-edited catalog — is filtered **per key** by
+  // `normalizeStoredMetadata`, which every read routes through. Rejecting the
+  // whole field here instead would cost a user every other key they had
+  // configured, for one bad value.
+  //
+  // Note the wire cannot carry the bad case at all: a client's own
+  // `JSON.stringify` turns `Infinity` into `null` before the request is sent.
   const isJsonObject = (v: unknown): v is RequestMetadata =>
     v !== null && typeof v === "object" && !Array.isArray(v);
   const isStringRecord = (v: unknown): v is Record<string, string> => {
diff --git a/core/mcp/serverList.ts b/core/mcp/serverList.ts
index 405bbca2..b21d736b 100644
--- a/core/mcp/serverList.ts
+++ b/core/mcp/serverList.ts
@@ -30,6 +30,7 @@ import {
 } from "../auth/secret-fields.js";
 import { toRecord } from "../json/jsonUtils.js";
 import type { JsonValue } from "../json/jsonUtils.js";
+import { isSerializableJson } from "../json/jsonUtils.js";
 
 // The full set of valid `type` discriminator values, used to reject anything
 // else read off disk so unknown strings can't propagate to narrowing sites.
@@ -226,9 +227,12 @@ export function normalizeStoredMetadata(
         entry === null ||
         typeof (entry as { key?: unknown }).key !== "string"
       ) {
+        // Structural message only — never the entry. A malformed legacy row
+        // still holds whatever the user put in it, which for `_meta` can be a
+        // credential, and this goes to the console of every client that reads
+        // the catalog.
         console.warn(
-          "Ignoring malformed legacy metadata entry (expected `{ key, value }`):",
-          entry,
+          "Ignoring malformed legacy metadata entry (expected `{ key, value }`).",
         );
         continue;
       }
@@ -236,13 +240,14 @@ export function normalizeStoredMetadata(
       if (key.trim() === "") continue;
       // `??`, not `||`: `null` is a legal `_meta` value and must survive the
       // migration as `null`. Only a genuinely absent `value` becomes `""`.
-      //
+      const resolved = value === undefined ? "" : value;
+      if (!keepSerializable(key, resolved)) continue;
       // `defineProperty` rather than `out[key] = …` because a legacy key of
       // `__proto__` would otherwise hit the prototype setter — silently
       // dropping the entry (or, for an object value, mutating the prototype)
       // instead of storing an own property under that name.
       Object.defineProperty(out, key, {
-        value: value === undefined ? "" : value,
+        value: resolved,
         writable: true,
         enumerable: true,
         configurable: true,
@@ -252,16 +257,51 @@ export function normalizeStoredMetadata(
   }
 
   if (typeof metadata === "object") {
-    return { ...(metadata as RequestMetadata) };
+    // A hand-edited catalog reaches this untouched by any editor, so it is a
+    // real source of values that parse but cannot be sent — `1e400` becomes
+    // `Infinity`, which the next serialization writes as `null`. Same
+    // invariant the web editor and the CLI enforce, applied per key so one bad
+    // value does not cost the rest.
+    const out: RequestMetadata = {};
+    for (const [key, value] of Object.entries(metadata as RequestMetadata)) {
+      if (keepSerializable(key, value)) out[key] = value;
+    }
+    return out;
   }
 
   console.warn(
-    "Ignoring malformed `metadata` (expected a JSON object):",
-    metadata,
+    `Ignoring malformed \`metadata\` (expected a JSON object, got ${describeJsonType(metadata)}).`,
   );
   return {};
 }
 
+/**
+ * Whether a metadata value can be sent as written, warning by key when it
+ * cannot.
+ *
+ * Names the key and the offending type, never the value: `_meta` carries
+ * whatever the user configured, credentials included, and these warnings reach
+ * ordinary console output.
+ */
+function keepSerializable(key: string, value: unknown): boolean {
+  if (isSerializableJson(value)) return true;
+  console.warn(
+    `Ignoring metadata key "${key}" — its value is not JSON that can be sent ` +
+      `(got ${describeJsonType(value)}; note \`1e400\` parses to Infinity).`,
+  );
+  return false;
+}
+
+/** A one-word type name for a diagnostic that must not disclose the value. */
+function describeJsonType(value: unknown): string {
+  if (value === null) return "null";
+  if (Array.isArray(value)) return "array";
+  if (typeof value === "number" && !Number.isFinite(value)) {
+    return Number.isNaN(value) ? "NaN" : "a non-finite number";
+  }
+  return typeof value;
+}
+
 /**
  * Validate a server's `oauth.authorizationParams` as it comes off disk, or
  * `undefined` when there is nothing usable. (#2018)