diff --git a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
index 81cde850..ad49c770 100644
--- a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
+++ b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.test.tsx
@@ -136,6 +136,48 @@ describe("EditReplayModal", () => {
     ).toBeInTheDocument();
   });
 
+  // Seeding from the projection is only half of it: a key the dispatcher does
+  // not read can be *added* to the draft, and Send would then discard an edit
+  // the user was looking at.
+  it("disables Send for a key this method's dispatch would not read", async () => {
+    renderWithMantine(
+      <EditReplayModal {...baseProps} method="tools/list" params={undefined} />,
+    );
+    await setAceText('{"cursor":"abc","_meta":{"progressToken":1}}');
+    expect(screen.getByRole("button", { name: "Send" })).toBeDisabled();
+    expect(
+      screen.getByText(/`_meta` is not carried by tools\/list/),
+    ).toBeInTheDocument();
+  });
+
+  it("re-enables Send once the unsupported key is removed", async () => {
+    renderWithMantine(
+      <EditReplayModal {...baseProps} method="tools/list" params={undefined} />,
+    );
+    await setAceText('{"cursor":"abc","_meta":{}}');
+    expect(screen.getByRole("button", { name: "Send" })).toBeDisabled();
+
+    await setAceText('{"cursor":"abc"}');
+    expect(screen.getByRole("button", { name: "Send" })).not.toBeDisabled();
+    expect(screen.queryByText(/not carried by/)).toBeNull();
+  });
+
+  // The check is per method, not a fixed key list: `arguments` is read by a
+  // tool call and by nothing on a list request.
+  it("judges a key against the method it would be sent to", async () => {
+    const { rerender } = renderWithMantine(
+      <EditReplayModal {...baseProps} method="tools/call" />,
+    );
+    await setAceText('{"name":"echo","arguments":{"a":1}}');
+    expect(screen.getByRole("button", { name: "Send" })).not.toBeDisabled();
+
+    rerender(<EditReplayModal {...baseProps} method="tools/list" />);
+    expect(screen.getByRole("button", { name: "Send" })).toBeDisabled();
+    expect(
+      screen.getByText(/`name`, `arguments` are not carried by tools\/list/),
+    ).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 be1dd877..d8fcfb83 100644
--- a/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx
+++ b/clients/web/src/components/groups/EditReplayModal/EditReplayModal.tsx
@@ -2,6 +2,7 @@ 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 { replayableParams } from "../../../lib/protocolReplay";
 import { isSerializableJson } from "@inspector/core/json/jsonUtils.js";
 
 export interface EditReplayModalProps {
@@ -60,10 +61,12 @@ function formatDroppedKeys(keys: string[]): string {
  * genuinely carries none, and an empty editor wearing a red error would read as
  * broken. A non-object is reported separately from text that is not JSON at
  * all: JSON-RPC `params` must be a structured value, so `42` is a shape that
- * cannot be sent however finished it is. A number that overflows is a third
- * case, and the subtlest — see the note on that branch.
+ * cannot be sent however finished it is. A number that overflows, and a key
+ * this method's dispatch does not read, are the two subtle cases — see the
+ * notes on those branches.
  */
 function parseParamsDraft(
+  method: string,
   text: string,
 ):
   | { ok: true; value: Record<string, unknown> | null }
@@ -89,6 +92,20 @@ function parseParamsDraft(
         "Numbers must be finite — a value like `1e400` overflows and would be sent as null",
     };
   }
+  // 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
+  // built from, so the editor and the dispatcher cannot disagree about which
+  // keys survive.
+  const { dropped } = replayableParams(method, parsed);
+  if (dropped.length > 0) {
+    return {
+      ok: false,
+      error: `${formatDroppedKeys(dropped)} not carried by ${method} — remove ${
+        dropped.length === 1 ? "it" : "them"
+      } so this matches what is sent`,
+    };
+  }
   return { ok: true, value: parsed };
 }
 
@@ -107,7 +124,7 @@ function EditReplayForm({
   const [draft, setDraft] = useState(() =>
     params === undefined ? "" : JSON.stringify(params, null, 2),
   );
-  const parsed = parseParamsDraft(draft);
+  const parsed = parseParamsDraft(method, draft);
 
   return (
     <Stack gap="sm">