diff --git a/src/application/ClientConfigurationDocument.ts b/src/application/ClientConfigurationDocument.ts
index b32df3fd0..1b241a98b 100644
--- a/src/application/ClientConfigurationDocument.ts
+++ b/src/application/ClientConfigurationDocument.ts
@@ -1,9 +1,5 @@
 import { parse as parseToml, stringify as stringifyToml } from "smol-toml";
-import {
-  parse as parseYaml,
-  parseDocument as parseYamlDocument,
-  stringify as stringifyYaml,
-} from "yaml";
+import { stringify as stringifyYaml } from "yaml";
 import { isDeepStrictEqual } from "node:util";
 
 import {
@@ -12,6 +8,10 @@ import {
   editDshDocument,
   dshClientRegistrationEntry,
 } from "./DshCordisPatch.js";
+import {
+  editHermesConfigurationYaml,
+  parseHermesConfigurationYaml,
+} from "./HermesConfigurationYaml.js";
 import {
   applyEdits,
   findNodeAtLocation,
@@ -118,7 +118,10 @@ const parseDocument = (
   if (isEmptyClientConfigurationText(text)) return {};
   // Hermes stores MCP servers in `mcp_servers` inside a YAML document.
   if (format === "hermes")
-    return requireObject(parseYaml(text), "The configuration root");
+    return requireObject(
+      parseHermesConfigurationYaml(text),
+      "The configuration root",
+    );
   // ZCode loads nonempty native files with strict JSON.parse.
   if (format === "zcode")
     return requireObject(JSON.parse(text), "The configuration root");
@@ -1419,12 +1422,11 @@ const serializeHermesConfiguration = (
     typeof servers === "object" && servers !== null
       ? (servers as Record<string, unknown>)[PRODUCT_IDENTITY.mcpServerKey]
       : undefined;
-  const yamlDocument = parseYamlDocument(originalText);
-  if (entry === undefined)
-    yamlDocument.deleteIn([serversKey, PRODUCT_IDENTITY.mcpServerKey]);
-  else yamlDocument.setIn([serversKey, PRODUCT_IDENTITY.mcpServerKey], entry);
-  // Editing REA must not reflow unrelated user prompts or other YAML scalars.
-  return yamlDocument.toString({ lineWidth: 0 });
+  return editHermesConfigurationYaml(
+    originalText,
+    [serversKey, PRODUCT_IDENTITY.mcpServerKey],
+    entry,
+  );
 };
 
 /** Build the stdio entry shape expected by one client's configuration dialect. */
diff --git a/src/application/HermesConfigurationYaml.test.ts b/src/application/HermesConfigurationYaml.test.ts
new file mode 100644
index 000000000..41b74fcc9
--- /dev/null
+++ b/src/application/HermesConfigurationYaml.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  editHermesConfigurationYaml,
+  parseHermesConfigurationYaml,
+} from "./HermesConfigurationYaml.js";
+
+describe("Hermes configuration YAML", () => {
+  it("writes an edited number from its new value", () => {
+    expect(
+      editHermesConfigurationYaml("mode: 0600\nmask: 0x1F\n", ["mode"], 384),
+    ).toBe("mode: 384\nmask: 0x1F\n");
+  });
+
+  it("accepts keys Hermes reads as distinct and rejects a repeated key", () => {
+    expect(() =>
+      parseHermesConfigurationYaml("0600: octal\n600: decimal\n"),
+    ).not.toThrow();
+    expect(() => parseHermesConfigurationYaml('a: 1\n"a": 2\n')).toThrow();
+  });
+});
diff --git a/src/application/HermesConfigurationYaml.ts b/src/application/HermesConfigurationYaml.ts
new file mode 100644
index 000000000..8d245da37
--- /dev/null
+++ b/src/application/HermesConfigurationYaml.ts
@@ -0,0 +1,74 @@
+import {
+  isScalar,
+  parse,
+  parseDocument,
+  type ParsedNode,
+  type ScalarTag,
+  type Tags,
+} from "yaml";
+
+const NUMBER_TAGS = new Set([
+  "tag:yaml.org,2002:int",
+  "tag:yaml.org,2002:float",
+]);
+
+/**
+ * Write a parsed number with its source spelling while that spelling still
+ * reads as the node's value. Hermes reads config.yaml as YAML 1.1 with exact
+ * integers, so re-printing a JavaScript number changes what it reads: a
+ * Discord channel ID loses its low digits, `0600` (octal) becomes `600`, and
+ * `0.0000001` becomes `1e-7`, which YAML 1.1 does not define as a float.
+ */
+const keepNumberSource = (tag: Tags[number]): Tags[number] => {
+  if (typeof tag === "string" || tag.collection !== undefined) return tag;
+  const { stringify } = tag;
+  if (!NUMBER_TAGS.has(tag.tag) || stringify === undefined) return tag;
+  return {
+    ...tag,
+    stringify: (item, context, ...rest) => {
+      const { source } = item;
+      if (source === undefined) return stringify(item, context, ...rest);
+      const reader = context.doc.schema.tags.find(
+        (candidate): candidate is ScalarTag =>
+          candidate.default === true && candidate.test?.test(source) === true,
+      );
+      const read = reader?.resolve(
+        source,
+        () => undefined,
+        context.doc.options,
+      );
+      return Object.is(isScalar(read) ? read.value : read, item.value)
+        ? source
+        : stringify(item, context, ...rest);
+    },
+  };
+};
+
+const HERMES_YAML_OPTIONS = {
+  customTags: (tags: Tags) => tags.map(keepNumberSource),
+  // Hermes keeps integers exact, so IDs that round to one double, or `0600`
+  // and `600`, are distinct keys.
+  uniqueKeys: (a: ParsedNode, b: ParsedNode) =>
+    a === b ||
+    (isScalar(a) &&
+      isScalar(b) &&
+      a.value === b.value &&
+      a.source === b.source),
+};
+
+/** Parse a Hermes `config.yaml`. */
+export const parseHermesConfigurationYaml = (text: string): unknown =>
+  parse(text, HERMES_YAML_OPTIONS);
+
+/** Set or delete one value in a Hermes `config.yaml`, keeping the rest as written. */
+export const editHermesConfigurationYaml = (
+  text: string,
+  path: readonly string[],
+  value: unknown,
+): string => {
+  const document = parseDocument(text, HERMES_YAML_OPTIONS);
+  if (value === undefined) document.deleteIn(path);
+  else document.setIn(path, value);
+  // Editing REA must not reflow unrelated user prompts or other YAML scalars.
+  return document.toString({ lineWidth: 0 });
+};
diff --git a/tests/boundary/filesystem/hermesClientConfiguration.test.ts b/tests/boundary/filesystem/hermesClientConfiguration.test.ts
index 43a5f841c..218573d5c 100644
--- a/tests/boundary/filesystem/hermesClientConfiguration.test.ts
+++ b/tests/boundary/filesystem/hermesClientConfiguration.test.ts
@@ -237,6 +237,38 @@ mcp_servers:
     expect(await readFile(client.configPath, "utf8")).toBe(original);
   });
 
+  it("keeps unrelated numbers as written, including Discord IDs", async () => {
+    const { home, client } = await hermesClient();
+    // Hermes reads these exactly: two IDs one apart, an octal mode, a float.
+    const original = `discord:
+  free_response_channels:
+    - 1187654321098765432
+  channel_prompts:
+    1187654321098765433: Build questions.
+    1187654321098765434: Release questions.
+  mode: 0600
+  min_score: 0.0000001
+mcp_servers:
+  other:
+    command: other-server
+`;
+    await writeFile(client.configPath, original);
+    expect(
+      (await configureClientConfiguration(client, {}, command)).status,
+    ).toBe("configured");
+    const text = await readFile(client.configPath, "utf8");
+    expect(text.startsWith(original)).toBe(true);
+    expect(parseYaml(text, { intAsBigInt: true })).toMatchObject({
+      mcp_servers: {
+        rea: { command: command[0], args: command.slice(1), enabled: true },
+      },
+    });
+    expect((await systemUninstallHost(home).removeClient(client)).status).toBe(
+      "removed",
+    );
+    expect(await readFile(client.configPath, "utf8")).toBe(original);
+  });
+
   it("creates config.yaml when Hermes has no configuration yet", async () => {
     const { client } = await hermesClient();
     expect(