diff --git a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
index b7160a79..e43deaa8 100644
--- a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
+++ b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
@@ -197,6 +197,26 @@ describe("EditReplayModal", () => {
     ).toBeInTheDocument();
   });
 
+  // `JSON.parse` rounds a whole number past 2^53−1 to the nearest double, so
+  // the draft shows digits the wire will not carry.
+  it("disables Send for a whole number that loses digits when parsed", async () => {
+    renderWithMantine(<EditReplayModal {...baseProps} method="tools/call" />);
+    await setAceText('{"name":"echo","arguments":{"id":9007199254740993}}');
+    expect(screen.getByRole("button", { name: "Send" })).toBeDisabled();
+    expect(
+      screen.getByText(/Whole numbers must be within/),
+    ).toBeInTheDocument();
+  });
+
+  it("disables Send for a non-string prompt argument", async () => {
+    renderWithMantine(<EditReplayModal {...baseProps} method="prompts/get" />);
+    await setAceText('{"name":"greet","arguments":{"count":2}}');
+    expect(screen.getByRole("button", { name: "Send" })).toBeDisabled();
+    expect(
+      screen.getByText(/`count` would be sent as JSON text/),
+    ).toBeInTheDocument();
+  });
+
   it("closes without sending when cancelled", async () => {
     const user = userEvent.setup();
     const onSend = vi.fn();
diff --git a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx
index eb28d3e1..c70d258f 100644
--- a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx
+++ b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx
@@ -1,7 +1,11 @@
 import { useState } from "react";
 import { Alert, Button, Group, Modal, Stack, Text } from "@mantine/core";
 import { JsonEditor } from "../../elements/JsonEditor/JsonEditor";
-import { isJsonObject } from "../../../utils/jsonObjectDraft";
+import {
+  hasRoundedInteger,
+  isJsonObject,
+  ROUNDED_INTEGER_ERROR,
+} from "../../../utils/jsonObjectDraft";
 import {
   replayableParams,
   reshapedReplayParam,
@@ -95,6 +99,9 @@ function parseParamsDraft(
         "Numbers must be finite — a value like `1e400` overflows and would be sent as null",
     };
   }
+  if (hasRoundedInteger(parsed)) {
+    return { ok: false, error: ROUNDED_INTEGER_ERROR };
+  }
   // Seeding from the projection is only half of it: a key the dispatcher does
   // not read can also be *added* to the draft, and Send would then discard an
   // edit the user was looking at. Checked with the same function the seed is
diff --git a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
index 08eab255..485beb5a 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx
@@ -1913,6 +1913,22 @@ describe("SchemaForm raw JSON (#2151)", () => {
     expect(screen.getByText(/Numbers must be finite/)).toBeInTheDocument();
   });
 
+  // The quieter half of the same defect the `1e400` case covers: a whole number
+  // past 2^53−1 is rounded by `JSON.parse`, so the editor would show digits the
+  // wire will not carry.
+  it("blocks submission for a whole number that loses digits when parsed", async () => {
+    const user = userEvent.setup();
+    const onValidityChange = vi.fn();
+    renderWithMantine(<RawHarness onValidityChange={onValidityChange} />);
+    await enableRawJson(user);
+
+    await setAceTextByLabel(/Arguments JSON/, '{"id":9007199254740993}');
+    expect(onValidityChange).toHaveBeenLastCalledWith(true);
+    expect(
+      screen.getByText(/Whole numbers must be within/),
+    ).toBeInTheDocument();
+  });
+
   // Nothing in `values` names a branch, so the picker's index is held by the
   // form. Editing the discriminator in the raw document is a change to which
   // branch is in effect, and it carries no other signal — without the re-derive
diff --git a/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx b/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
index a24649a8..e533c5fc 100644
--- a/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
+++ b/clients/web/src/components/groups/SchemaForm/SchemaForm.tsx
@@ -24,7 +24,11 @@ import {
 import { ClearButton } from "../../elements/ClearButton/ClearButton";
 import { EnlargeButton } from "../../elements/EnlargeButton/EnlargeButton";
 import { JsonEditor } from "../../elements/JsonEditor/JsonEditor";
-import { isJsonObject } from "../../../utils/jsonObjectDraft";
+import {
+  hasRoundedInteger,
+  isJsonObject,
+  ROUNDED_INTEGER_ERROR,
+} from "../../../utils/jsonObjectDraft";
 import { isSerializableJson } from "@inspector/core/json/jsonUtils.js";
 import { useValueChange } from "../../../hooks/useValueChange";
 import type {
@@ -280,9 +284,10 @@ function toNumericValue(raw: string | number): number | undefined {
  * That covers text that is not JSON *yet* — the mid-edit state this field's
  * whole draft/value split exists for — and text that parses but does not
  * survive being written back out: `JSON.parse("1e400")` yields `Infinity`,
- * which `JSON.stringify` writes as `null`. Both report no value, so the field
- * shows its error and `onValidityChange` blocks submission, rather than sending
- * a number the user never typed. Same guard, same reason, as
+ * which `JSON.stringify` writes as `null`, and a whole number past 2^53−1 is
+ * rounded to the nearest double. All three report no value, so the field shows
+ * its error and `onValidityChange` blocks submission, rather than sending a
+ * number the user never typed. Same guards, same reason, as
  * `parseJsonObjectDraft` and the raw-arguments editor above.
  */
 function parseJsonDraft(text: string): unknown {
@@ -295,7 +300,10 @@ function parseJsonDraft(text: string): unknown {
   } catch {
     return undefined;
   }
-  return isSerializableJson(parsed) ? parsed : undefined;
+  if (!isSerializableJson(parsed) || hasRoundedInteger(parsed)) {
+    return undefined;
+  }
+  return parsed;
 }
 
 /** Structural equality for the draft/value re-sync, via canonical JSON. */
@@ -609,6 +617,11 @@ function parseRawArgumentsDraft(
         "Numbers must be finite — a value like `1e400` overflows and would be sent as null",
     };
   }
+  // The quieter half of the same defect: a whole number past 2^53−1 parses to
+  // the nearest double, so the draft shows digits the wire will not carry.
+  if (hasRoundedInteger(parsed)) {
+    return { ok: false, error: ROUNDED_INTEGER_ERROR };
+  }
   return { ok: true, value: parsed };
 }
 
diff --git a/clients/web/src/lib/protocolReplay.test.ts b/clients/web/src/lib/protocolReplay.test.ts
index 35c80e3b..6e1d38cb 100644
--- a/clients/web/src/lib/protocolReplay.test.ts
+++ b/clients/web/src/lib/protocolReplay.test.ts
@@ -167,6 +167,38 @@ describe("reshapedReplayParam", () => {
     ).toMatch(/must be a JSON object/);
   });
 
+  // `getPrompt` runs `convertPromptArguments`, which JSON-stringifies anything
+  // that is not already a string — so `{count: 2}` goes out as `{count: "2"}`
+  // while the editor still shows the number. The spec types a prompt argument
+  // as a string, so this is the protocol, not a quirk to route around.
+  it("rejects a non-string prompt argument, which is stringified on the way out", () => {
+    expect(
+      reshapedReplayParam("prompts/get", {
+        name: "greet",
+        arguments: { count: 2, who: "ada" },
+      }),
+    ).toMatch(/`count` would be sent as JSON text/);
+  });
+
+  it("accepts string prompt arguments", () => {
+    expect(
+      reshapedReplayParam("prompts/get", {
+        name: "greet",
+        arguments: { count: "2", who: "ada" },
+      }),
+    ).toBeNull();
+  });
+
+  // `callTool` sends its arguments as given, so the same shape is fine there.
+  it("leaves a tool call's non-string arguments alone", () => {
+    expect(
+      reshapedReplayParam("tools/call", {
+        name: "add",
+        arguments: { a: 1, b: 2 },
+      }),
+    ).toBeNull();
+  });
+
   it("has nothing to say about methods that take no arguments", () => {
     expect(reshapedReplayParam("tools/list", { cursor: "a" })).toBeNull();
     expect(reshapedReplayParam("resources/read", { uri: "x://y" })).toBeNull();
diff --git a/clients/web/src/lib/protocolReplay.ts b/clients/web/src/lib/protocolReplay.ts
index 47fcff52..768909d2 100644
--- a/clients/web/src/lib/protocolReplay.ts
+++ b/clients/web/src/lib/protocolReplay.ts
@@ -138,8 +138,8 @@ export function replayableParams(
  * kept param is sent as written.
  *
  * {@link replayableParams} answers which keys survive; this answers whether the
- * surviving ones survive *intact*. Only `arguments` can fail that today, and it
- * fails two ways, both of which the Edit-and-replay editor has to catch before
+ * surviving ones survive *intact*. Only `arguments` can fail that, and it fails
+ * three ways, each of which the Edit-and-replay editor has to catch before
  * Send:
  *
  * - `arguments: null` is nullish, so `?? {}` replaces it — the editor shows
@@ -147,6 +147,13 @@ export function replayableParams(
  * - `arguments: [1,2]` or `arguments: 4` is not nullish, so the cast carries it
  *   through unchanged into a call whose type says it is a named-argument
  *   record. Nothing reshapes it, but nothing can send it either.
+ * - For **`prompts/get` only**, a non-string *value*. `getPrompt` runs
+ *   `convertPromptArguments`, which `JSON.stringify`s anything that is not
+ *   already a string — so `{"count": 2}` is sent as `{"count": "2"}` while the
+ *   editor still shows the number. This is not a quirk to route around: the
+ *   spec types `GetPromptRequest.params.arguments` as `Record<string, string>`,
+ *   so a string is the only thing a prompt argument can be. `tools/call` is
+ *   unaffected — `callTool` sends its arguments as given.
  *
  * `name` and `uri` are deliberately not checked here: the dispatch already
  * refuses a missing or non-string one with a reason the caller surfaces as a
@@ -162,10 +169,20 @@ export function reshapedReplayParam(
   const args = params.arguments;
   const isRecord =
     args !== null && typeof args === "object" && !Array.isArray(args);
-  if (isRecord) return null;
-  return args === null
-    ? "`arguments` is sent as `{}` when null — remove it, or give it an object"
-    : "`arguments` must be a JSON object (`{ … }`)";
+  if (!isRecord) {
+    return args === null
+      ? "`arguments` is sent as `{}` when null — remove it, or give it an object"
+      : "`arguments` must be a JSON object (`{ … }`)";
+  }
+  if (method === "prompts/get") {
+    const coerced = Object.entries(args as Record<string, unknown>)
+      .filter(([, value]) => typeof value !== "string")
+      .map(([key]) => `\`${key}\``);
+    if (coerced.length > 0) {
+      return `A prompt argument is always a string — ${coerced.join(", ")} would be sent as JSON text`;
+    }
+  }
+  return null;
 }
 
 export async function replayProtocolRequest(
diff --git a/clients/web/src/utils/jsonObjectDraft.test.ts b/clients/web/src/utils/jsonObjectDraft.test.ts
index 51e835a4..5e8be685 100644
--- a/clients/web/src/utils/jsonObjectDraft.test.ts
+++ b/clients/web/src/utils/jsonObjectDraft.test.ts
@@ -1,5 +1,51 @@
 import { describe, it, expect } from "vitest";
-import { parseJsonObjectDraft } from "./jsonObjectDraft";
+import { hasRoundedInteger, parseJsonObjectDraft } from "./jsonObjectDraft";
+
+// `isSerializableJson` catches a literal too large to represent (`1e400` →
+// `Infinity` → `null` on the wire). This is the quieter one: a whole number
+// past 2^53−1 parses to the nearest double, so the draft shows digits the wire
+// will not carry.
+describe("hasRoundedInteger", () => {
+  it("accepts whole numbers inside the safe range", () => {
+    expect(hasRoundedInteger({ id: Number.MAX_SAFE_INTEGER })).toBe(false);
+    expect(hasRoundedInteger({ id: -Number.MAX_SAFE_INTEGER })).toBe(false);
+    expect(hasRoundedInteger({ id: 0 })).toBe(false);
+  });
+
+  it("rejects a whole number past the safe range", () => {
+    // Typed as …93, parsed as …92.
+    expect(hasRoundedInteger(JSON.parse('{"id":9007199254740993}'))).toBe(true);
+  });
+
+  // A fractional or exponent-form value is a double by nature and is sent as
+  // the double it parsed to, so nothing is lost between typing and sending.
+  it("accepts a fractional value however large", () => {
+    expect(hasRoundedInteger({ ratio: 1.5 })).toBe(false);
+    expect(hasRoundedInteger({ ratio: 0.1 + 0.2 })).toBe(false);
+  });
+
+  // JS stringifies from 1e21 upward in exponent form, so such a value was typed
+  // that way and the parsed double is what was asked for. `parseJsonObjectDraft`
+  // has always accepted these.
+  it("accepts a value whose shortest form carries an exponent", () => {
+    expect(hasRoundedInteger({ n: 1e308 })).toBe(false);
+    expect(hasRoundedInteger({ n: 1e21 })).toBe(false);
+  });
+
+  it("looks inside arrays and nested objects", () => {
+    expect(hasRoundedInteger(JSON.parse('[{"a":[9007199254740993]}]'))).toBe(
+      true,
+    );
+    expect(hasRoundedInteger({ a: { b: [1, 2, "x", null, true] } })).toBe(
+      false,
+    );
+  });
+
+  it("has nothing to say about non-numeric values", () => {
+    expect(hasRoundedInteger("9007199254740993")).toBe(false);
+    expect(hasRoundedInteger(null)).toBe(false);
+  });
+});
 
 describe("parseJsonObjectDraft", () => {
   it("reads empty text as the empty object rather than an error", () => {
diff --git a/clients/web/src/utils/jsonObjectDraft.ts b/clients/web/src/utils/jsonObjectDraft.ts
index 731d6b8f..d69ac326 100644
--- a/clients/web/src/utils/jsonObjectDraft.ts
+++ b/clients/web/src/utils/jsonObjectDraft.ts
@@ -20,6 +20,55 @@ export function isJsonObject(value: unknown): value is StrictJsonObject {
   return value !== null && typeof value === "object" && !Array.isArray(value);
 }
 
+/**
+ * Whether `JSON.parse` dropped digits from a whole number written out in full.
+ *
+ * `isSerializableJson` catches a literal too large to *represent* — `1e400`
+ * parses to `Infinity` and writes back as `null`. This catches the quieter one:
+ * a whole number past 2^53−1 parses to the nearest double, so
+ * `{"id":9007199254740993}` becomes `…992` with no error anywhere. The draft
+ * still shows what was typed, which is what makes it a misreport rather than a
+ * formatting difference — the editor displays one value and the wire carries
+ * another. Snowflake-style ids are the realistic case.
+ *
+ * Two conditions, and the second is what keeps this from over-reaching:
+ *
+ * - **An unsafe whole number.** A fractional value is a double by nature and is
+ *   sent as the double it parsed to, so nothing is lost between typing and
+ *   sending.
+ * - **Whose shortest representation is written out in full.** JS switches to
+ *   exponent form at 1e21, so a value that stringifies as `1e+308` was typed in
+ *   exponent form, where the parsed double *is* the value the user asked for.
+ *   Without this, `{"n":1e308}` — a legitimate `_meta` value — would be
+ *   refused, which `parseJsonObjectDraft`'s own tests pin against.
+ *
+ * It still refuses the rare full-form integer past 2^53−1 that happens to be
+ * exactly representable (2^54, say), because nothing here can tell it apart
+ * from one that lost digits without the original literal. That errs toward
+ * refusing a sendable value rather than misreporting an unsendable one, which
+ * is the same trade `toNumericValue` makes for the schema form's number input.
+ */
+export function hasRoundedInteger(value: unknown): boolean {
+  if (typeof value === "number") {
+    return (
+      Number.isInteger(value) &&
+      !Number.isSafeInteger(value) &&
+      !String(value).includes("e")
+    );
+  }
+  if (Array.isArray(value)) return value.some(hasRoundedInteger);
+  if (value !== null && typeof value === "object") {
+    return Object.values(value as Record<string, unknown>).some(
+      hasRoundedInteger,
+    );
+  }
+  return false;
+}
+
+/** The message every draft parser uses for {@link hasRoundedInteger}. */
+export const ROUNDED_INTEGER_ERROR =
+  "Whole numbers must be within ±(2^53 − 1) — a longer one is rounded when parsed, so a different value would be sent";
+
 /**
  * Read a JSON-object editor's draft text.
  *
@@ -60,5 +109,8 @@ export function parseJsonObjectDraft(text: string): JsonObjectDraft {
         "Numbers must be finite — a value like `1e400` overflows and would be sent as null",
     };
   }
+  if (hasRoundedInteger(parsed)) {
+    return { ok: false, error: ROUNDED_INTEGER_ERROR };
+  }
   return { ok: true, value: parsed };
 }