diff --git a/clients/cli/__tests__/metadata.test.ts b/clients/cli/__tests__/metadata.test.ts
index a4cb05fc..ef8e2b37 100644
--- a/clients/cli/__tests__/metadata.test.ts
+++ b/clients/cli/__tests__/metadata.test.ts
@@ -880,6 +880,37 @@ describe("Metadata Tests", () => {
       }
     });
 
+    it.each([
+      ["an overflowing literal", "n=1e400"],
+      ["a negative overflow", "n=-1e400"],
+      ["one nested in an object", 'o={"a":1e400}'],
+    ])("rejects %s rather than sending null", async (_label, pair) => {
+      // `JSON.parse` accepts these and yields ±Infinity, which
+      // `JSON.stringify` writes as `null` — so accepting the flag would
+      // transmit a value the user did not ask for.
+      const server = createTestServerHttp({
+        serverInfo: createTestServerInfo(),
+        tools: [createEchoTool()],
+      });
+      try {
+        await server.start();
+        const result = await runCli([
+          server.url,
+          "--cli",
+          "--method",
+          "tools/list",
+          "--metadata",
+          pair,
+          "--transport",
+          "http",
+        ]);
+        expect(result.exitCode).not.toBe(0);
+        expect(`${result.stderr}${result.stdout}`).toMatch(/finite/i);
+      } 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 6f9c8550..c486a725 100644
--- a/clients/cli/src/cli.ts
+++ b/clients/cli/src/cli.ts
@@ -25,6 +25,7 @@ import {
 } from "@inspector/core/mcp/node/index.js";
 import type { JsonValue } from "@inspector/core/mcp/index.js";
 import type { StrictJsonValue } from "@inspector/core/json/jsonUtils.js";
+import { isSerializableJson } from "@inspector/core/json/jsonUtils.js";
 import {
   canonicalUrlHost,
   isAllInterfacesHost,
@@ -492,9 +493,22 @@ function parseKeyValuePair(
   try {
     parsedValue = JSON.parse(val) as StrictJsonValue;
   } catch {
+    // Not JSON at all — a bare word or an unquoted string. Sent as a string,
+    // which is what the user plainly meant.
     parsedValue = val;
   }
 
+  // Valid JSON syntax is not the same as sendable JSON: `1e400` parses to
+  // `Infinity`, which `JSON.stringify` writes as `null`. Rejecting is better
+  // than accepting the flag and silently transmitting a different value —
+  // and better than falling back to the literal string, which would also not
+  // be what was asked for.
+  if (!isSerializableJson(parsedValue)) {
+    throw new Error(
+      `Invalid parameter value: ${value}. Numbers must be finite (a value like 1e400 overflows and cannot be sent).`,
+    );
+  }
+
   return { ...previous, [key as string]: parsedValue };
 }
 
diff --git a/clients/web/src/test/core/json/isSerializableJson.test.ts b/clients/web/src/test/core/json/isSerializableJson.test.ts
new file mode 100644
index 00000000..ea0d91f4
--- /dev/null
+++ b/clients/web/src/test/core/json/isSerializableJson.test.ts
@@ -0,0 +1,47 @@
+import { describe, it, expect } from "vitest";
+import { isSerializableJson } from "@inspector/core/json/jsonUtils.js";
+
+describe("isSerializableJson", () => {
+  it.each([
+    ["a string", "a"],
+    ["a finite number", 1.5],
+    ["zero", 0],
+    ["a negative number", -42],
+    ["a boolean", true],
+    ["null", null],
+    ["a nested object", { a: { b: [1, 2, { c: "d" }] } }],
+    ["an empty array", []],
+  ])("accepts %s", (_label, value) => {
+    expect(isSerializableJson(value)).toBe(true);
+  });
+
+  it.each([
+    ["NaN", NaN],
+    ["Infinity", Infinity],
+    ["-Infinity", -Infinity],
+  ])("rejects %s, which JSON.stringify rewrites to null", (_label, value) => {
+    expect(isSerializableJson(value)).toBe(false);
+    // The reason it matters, asserted rather than asserted-about.
+    expect(JSON.stringify({ v: value })).toBe('{"v":null}');
+  });
+
+  it.each([
+    ["undefined", undefined],
+    ["a function", () => {}],
+    ["a symbol", Symbol("s")],
+    ["a bigint", 10n],
+  ])("rejects %s, which is not JSON at all", (_label, value) => {
+    expect(isSerializableJson(value)).toBe(false);
+  });
+
+  it("descends into arrays and objects", () => {
+    expect(isSerializableJson({ a: [1, { b: Infinity }] })).toBe(false);
+    expect(isSerializableJson([[[NaN]]])).toBe(false);
+    expect(isSerializableJson({ a: [1, { b: 2 }] })).toBe(true);
+  });
+
+  it("rejects what `JSON.parse` produces for an overflowing literal", () => {
+    // The reachable path: valid JSON text that parses to an unsendable value.
+    expect(isSerializableJson(JSON.parse('{"n":1e400}'))).toBe(false);
+  });
+});
diff --git a/clients/web/src/utils/jsonObjectDraft.test.ts b/clients/web/src/utils/jsonObjectDraft.test.ts
index 83298caf..51e835a4 100644
--- a/clients/web/src/utils/jsonObjectDraft.test.ts
+++ b/clients/web/src/utils/jsonObjectDraft.test.ts
@@ -24,6 +24,25 @@ describe("parseJsonObjectDraft", () => {
     });
   });
 
+  it.each([
+    ["an overflowing literal", '{"n":1e400}'],
+    ["a negative overflow", '{"n":-1e400}'],
+    ["one nested in an array", '{"a":[1,1e400]}'],
+  ])("rejects %s, which would be sent as null", (_label, text) => {
+    // `JSON.parse` accepts these; `JSON.stringify` then writes `null`, so the
+    // editor would show one value and the wire would carry another.
+    const result = parseJsonObjectDraft(text);
+    expect(result.ok).toBe(false);
+    expect(result.ok === false && result.error).toMatch(/finite/);
+  });
+
+  it("still accepts large-but-finite numbers", () => {
+    expect(parseJsonObjectDraft('{"n":1e308}')).toEqual({
+      ok: true,
+      value: { n: 1e308 },
+    });
+  });
+
   it("rejects text that is not JSON", () => {
     expect(parseJsonObjectDraft('{"a":')).toEqual({
       ok: false,
diff --git a/clients/web/src/utils/jsonObjectDraft.ts b/clients/web/src/utils/jsonObjectDraft.ts
index 16bc545e..731d6b8f 100644
--- a/clients/web/src/utils/jsonObjectDraft.ts
+++ b/clients/web/src/utils/jsonObjectDraft.ts
@@ -1,4 +1,5 @@
 import type { StrictJsonObject } from "@inspector/core/json/jsonUtils.js";
+import { isSerializableJson } from "@inspector/core/json/jsonUtils.js";
 
 /**
  * What a JSON-object editor's draft text currently means: the object to emit,
@@ -49,5 +50,15 @@ export function parseJsonObjectDraft(text: string): JsonObjectDraft {
       error: "Must be a JSON object (`{ … }`) — changes are not applied",
     };
   }
+  // `JSON.parse` accepts numeric literals it cannot represent: `1e400` parses
+  // to `Infinity`, which `JSON.stringify` then writes as `null`. Accepting it
+  // here would show the user one value and send another.
+  if (!isSerializableJson(parsed)) {
+    return {
+      ok: false,
+      error:
+        "Numbers must be finite — a value like `1e400` overflows and would be sent as null",
+    };
+  }
   return { ok: true, value: parsed };
 }
diff --git a/core/json/jsonUtils.ts b/core/json/jsonUtils.ts
index a84de8fc..ba1533e9 100644
--- a/core/json/jsonUtils.ts
+++ b/core/json/jsonUtils.ts
@@ -37,6 +37,40 @@ export type StrictJsonValue =
 
 export type StrictJsonObject = { [key: string]: StrictJsonValue };
 
+/**
+ * Whether a value is JSON that survives `JSON.stringify` unchanged.
+ *
+ * {@link StrictJsonValue} is the *type*-level half of this and cannot express
+ * the rest: TypeScript has one `number`, but JSON has no encoding for `NaN` or
+ * `±Infinity`, so `JSON.stringify` rewrites them to `null`.
+ *
+ * That is reachable from ordinary input, not just from a careless caller —
+ * `JSON.parse("1e400")` returns `Infinity` on overflow, so text a user typed
+ * parses successfully, satisfies the type, and then reaches the wire as `null`
+ * while the editor still shows what they wrote. Checking at the boundary is the
+ * only place the distinction exists.
+ */
+export function isSerializableJson(value: unknown): value is StrictJsonValue {
+  if (value === null) return true;
+  switch (typeof value) {
+    case "string":
+    case "boolean":
+      return true;
+    case "number":
+      // Rejects NaN and ±Infinity; every other number round-trips.
+      return Number.isFinite(value);
+    case "object":
+      return Array.isArray(value)
+        ? value.every(isSerializableJson)
+        : Object.values(value as Record<string, unknown>).every(
+            isSerializableJson,
+          );
+    default:
+      // `undefined`, functions, symbols and bigints are not JSON at all.
+      return false;
+  }
+}
+
 /**
  * Widen a typed object to a generic string-keyed record so its keys can be
  * iterated or read/written generically. Many of the project's config/SDK types