diff --git a/changelog.d/features/7872-m365-tone-model-variants.md b/changelog.d/features/7872-m365-tone-model-variants.md
new file mode 100644
index 0000000000..5492efae1d
--- /dev/null
+++ b/changelog.d/features/7872-m365-tone-model-variants.md
@@ -0,0 +1 @@
+- feat(providers): copilot-m365-web tone-selected model variants (#7872)
diff --git a/open-sse/config/providers/registry/copilot-m365-web/index.ts b/open-sse/config/providers/registry/copilot-m365-web/index.ts
index 98a288fa4b..c5baab9166 100644
--- a/open-sse/config/providers/registry/copilot-m365-web/index.ts
+++ b/open-sse/config/providers/registry/copilot-m365-web/index.ts
@@ -8,5 +8,14 @@ export const copilot_m365_webProvider: RegistryEntry = {
   baseUrl: "wss://substrate.office.com/m365Copilot/Chathub",
   authType: "apikey",
   authHeader: "cookie",
-  models: [{ id: "copilot-m365", name: "Microsoft 365 Copilot (BizChat)", toolCalling: false }],
+  models: [
+    { id: "copilot-m365", name: "Microsoft 365 Copilot (BizChat)", toolCalling: false },
+    { id: "copilot-m365-claude-opus", name: "Microsoft 365 Copilot — Claude Opus", toolCalling: false },
+    {
+      id: "copilot-m365-gpt-5-6-reasoning",
+      name: "Microsoft 365 Copilot — GPT 5.6 Reasoning",
+      toolCalling: false,
+    },
+    { id: "copilot-m365-gpt-5-5-chat", name: "Microsoft 365 Copilot — GPT 5.5 Chat", toolCalling: false },
+  ],
 };
diff --git a/open-sse/executors/copilot-m365-frames.ts b/open-sse/executors/copilot-m365-frames.ts
index 33074c069f..c15782e756 100644
--- a/open-sse/executors/copilot-m365-frames.ts
+++ b/open-sse/executors/copilot-m365-frames.ts
@@ -193,6 +193,29 @@ export function resolveChatInvocationOverrides(tier: string | undefined): {
   };
 }
 
+/**
+ * BizChat exposes several models selected by the `tone` field of the `type:4` chat
+ * invocation (#7872, values confirmed against a real enterprise tenant in #7850). Each
+ * tone-selected variant is registered as its own model id; the bare `copilot-m365` id is
+ * intentionally absent here so it keeps the tier default tone (`Magic` on enterprise, `""`
+ * otherwise) resolved by {@link resolveChatInvocationOverrides}.
+ */
+export const M365_MODEL_TONE_MAP: Readonly<Record<string, string>> = {
+  "copilot-m365-claude-opus": "Claude_Opus",
+  "copilot-m365-gpt-5-6-reasoning": "Gpt_5_6_Reasoning",
+  "copilot-m365-gpt-5-5-chat": "Gpt_5_5_Chat",
+};
+
+/**
+ * Resolve the `tone` for a requested model id, or `undefined` when the id is the bare
+ * `copilot-m365` / unknown — callers then fall back to the tier default tone. Model-driven
+ * tone takes precedence over the tier default (see the executor wiring).
+ */
+export function resolveToneForModel(model: string | undefined): string | undefined {
+  if (!model) return undefined;
+  return M365_MODEL_TONE_MAP[model];
+}
+
 /**
  * Build the `type:4` chat invocation frame body (not yet `\x1e`-terminated).
  * Mirrors the argument shape captured on the individual M365 path in #4042.
diff --git a/open-sse/executors/copilot-m365-web.ts b/open-sse/executors/copilot-m365-web.ts
index 36cd186eb5..7e85c43e84 100644
--- a/open-sse/executors/copilot-m365-web.ts
+++ b/open-sse/executors/copilot-m365-web.ts
@@ -20,6 +20,7 @@ import {
   keepaliveFrame,
   parseFrame,
   resolveChatInvocationOverrides,
+  resolveToneForModel,
   splitFrames,
 } from "./copilot-m365-frames.ts";
 
@@ -167,6 +168,9 @@ export class CopilotM365WebExecutor extends BaseExecutor {
             const sendChat = () => {
               ws?.send(keepaliveFrame());
               const overrides = resolveChatInvocationOverrides(input.tier);
+              // Model-driven tone (#7872) wins over the tier default; a bare/unknown id
+              // keeps the tier tone resolved above.
+              const tone = resolveToneForModel(input.model) ?? overrides.tone;
               ws?.send(
                 encodeFrame(
                   buildChatInvocation({
@@ -175,6 +179,7 @@ export class CopilotM365WebExecutor extends BaseExecutor {
                     sessionId,
                     isStartOfSession: true,
                     ...overrides,
+                    tone,
                   })
                 )
               );
diff --git a/tests/unit/m365-tone-model-variants.test.ts b/tests/unit/m365-tone-model-variants.test.ts
new file mode 100644
index 0000000000..741ecd73a6
--- /dev/null
+++ b/tests/unit/m365-tone-model-variants.test.ts
@@ -0,0 +1,58 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  M365_MODEL_TONE_MAP,
+  resolveToneForModel,
+  resolveChatInvocationOverrides,
+} from "../../open-sse/executors/copilot-m365-frames.ts";
+import { copilot_m365_webProvider } from "../../open-sse/config/providers/registry/copilot-m365-web/index.ts";
+
+// #7872 — tone-selected model variants for copilot-m365-web.
+// The wiring (model id → tone → invocation payload) is unit-tested here; whether a tone
+// actually selects that model upstream is a live enterprise-tenant check (release-drain).
+
+test("resolveToneForModel maps each variant id to its confirmed tone", () => {
+  assert.equal(resolveToneForModel("copilot-m365-claude-opus"), "Claude_Opus");
+  assert.equal(resolveToneForModel("copilot-m365-gpt-5-6-reasoning"), "Gpt_5_6_Reasoning");
+  assert.equal(resolveToneForModel("copilot-m365-gpt-5-5-chat"), "Gpt_5_5_Chat");
+});
+
+test("resolveToneForModel returns undefined for the bare id and unknown ids", () => {
+  // bare id must fall back to the tier default, not a hard-coded tone
+  assert.equal(resolveToneForModel("copilot-m365"), undefined);
+  assert.equal(resolveToneForModel("totally-unknown"), undefined);
+  assert.equal(resolveToneForModel(undefined), undefined);
+  assert.equal(resolveToneForModel(""), undefined);
+});
+
+test("model-driven tone overrides the tier default; bare id keeps the tier tone", () => {
+  const enterprise = resolveChatInvocationOverrides("enterprise");
+  const individual = resolveChatInvocationOverrides(undefined);
+
+  // enterprise tier default tone is Magic
+  assert.equal(enterprise.tone, "Magic");
+  assert.equal(individual.tone, "");
+
+  // precedence: resolveToneForModel(model) ?? overrides.tone  (mirrors the executor wiring)
+  const toneFor = (model: string | undefined, tierTone: string) =>
+    resolveToneForModel(model) ?? tierTone;
+
+  // a variant id wins over BOTH tier defaults
+  assert.equal(toneFor("copilot-m365-claude-opus", enterprise.tone), "Claude_Opus");
+  assert.equal(toneFor("copilot-m365-claude-opus", individual.tone), "Claude_Opus");
+
+  // the bare id keeps whatever the tier resolved
+  assert.equal(toneFor("copilot-m365", enterprise.tone), "Magic");
+  assert.equal(toneFor("copilot-m365", individual.tone), "");
+});
+
+test("registry exposes the bare id (first) plus every tone variant", () => {
+  const ids = copilot_m365_webProvider.models.map((m) => m.id);
+  assert.equal(ids[0], "copilot-m365", "bare Auto/default id must be first");
+  for (const variantId of Object.keys(M365_MODEL_TONE_MAP)) {
+    assert.ok(ids.includes(variantId), `registry missing variant ${variantId}`);
+  }
+  // no duplicate ids
+  assert.equal(ids.length, new Set(ids).size);
+});