diff --git a/open-sse/executors/copilot-m365-connection.ts b/open-sse/executors/copilot-m365-connection.ts
new file mode 100644
index 0000000000..b3c2a1507d
--- /dev/null
+++ b/open-sse/executors/copilot-m365-connection.ts
@@ -0,0 +1,116 @@
+/**
+ * Microsoft 365 Copilot (individual / Substrate BizChat) connection helpers.
+ *
+ * Pure URL / credential / prompt builders for the #4042 individual M365 path.
+ * Kept transport-free (no BaseExecutor import — only a type import) so they can
+ * be unit-tested without the executor's heavy runtime dependency chain. The
+ * access_token rides in the WS query string per the protocol, so any logging of
+ * the URL MUST go through redactWsUrl().
+ */
+
+import { randomUUID, randomBytes } from "node:crypto";
+import type { ProviderCredentials } from "./base.ts";
+
+type JsonRecord = Record<string, unknown>;
+
+/** Individual-tier defaults observed in @skyzea1's #4042 capture. */
+export const M365_INDIVIDUAL_DEFAULTS = {
+  host: "substrate.office.com",
+  source: "officeweb",
+  product: "Office",
+  agentHost: "Bizchat.FullScreen",
+  licenseType: "Starter",
+  agent: "web",
+  scenario: "OfficeWebPaidConsumerCopilot",
+} as const;
+
+export interface M365ConnectionParams {
+  host: string;
+  chathubPath: string; // "<user-oid>@<tenant-id>"
+  accessToken: string;
+}
+
+/** A new 32-hex chat session id (== XRoutingParameterSessionKey == clientrequestid). */
+export function newChatSessionId(): string {
+  return randomBytes(16).toString("hex");
+}
+
+/**
+ * Read the pasted credential bits. The individual access_token is opaque (JWE),
+ * so it is consumed verbatim. The Chathub path (`user@tenant`) is pasted
+ * alongside it because it is not derivable from the opaque token.
+ */
+export function resolveConnectionParams(
+  credentials: ProviderCredentials | undefined
+): M365ConnectionParams | { error: string } {
+  const psd = (credentials?.providerSpecificData ?? {}) as JsonRecord;
+  const accessToken =
+    (typeof credentials?.apiKey === "string" && credentials.apiKey) ||
+    (typeof psd.accessToken === "string" && psd.accessToken) ||
+    (typeof psd.access_token === "string" && psd.access_token) ||
+    "";
+  if (!accessToken) {
+    return { error: "Missing M365 Copilot access_token. Paste it as the provider credential." };
+  }
+  const chathubPath =
+    (typeof psd.chathubPath === "string" && psd.chathubPath) ||
+    (typeof psd.userTenant === "string" && psd.userTenant) ||
+    "";
+  if (!chathubPath || !chathubPath.includes("@")) {
+    return {
+      error:
+        "Missing M365 Chathub path. Paste the '<user-oid>@<tenant-id>' segment from the WebSocket URL.",
+    };
+  }
+  const host = (typeof psd.host === "string" && psd.host) || M365_INDIVIDUAL_DEFAULTS.host;
+  return { host, chathubPath, accessToken };
+}
+
+/**
+ * Build the BizChat WebSocket URL. The access_token rides in the query string
+ * (per the protocol), so callers must never log the returned URL verbatim — use
+ * redactWsUrl() for any logging.
+ */
+export function buildWsUrl(params: M365ConnectionParams): string {
+  const sessionKey = newChatSessionId();
+  const query = new URLSearchParams({
+    chatsessionid: sessionKey,
+    XRoutingParameterSessionKey: sessionKey,
+    clientrequestid: sessionKey,
+    "X-SessionId": randomUUID(),
+    ConversationId: randomUUID(),
+    access_token: params.accessToken,
+    source: M365_INDIVIDUAL_DEFAULTS.source,
+    product: M365_INDIVIDUAL_DEFAULTS.product,
+    agentHost: M365_INDIVIDUAL_DEFAULTS.agentHost,
+    licenseType: M365_INDIVIDUAL_DEFAULTS.licenseType,
+    isEdu: "false",
+    agent: M365_INDIVIDUAL_DEFAULTS.agent,
+    scenario: M365_INDIVIDUAL_DEFAULTS.scenario,
+  });
+  return `wss://${params.host}/m365Copilot/Chathub/${params.chathubPath}?${query.toString()}`;
+}
+
+/** Strip the access_token from a WS URL so it is safe to log. */
+export function redactWsUrl(wsUrl: string): string {
+  return wsUrl.replace(/access_token=[^&]*/i, "access_token=REDACTED");
+}
+
+/** Flatten OpenAI messages into a single prompt (system instructions prepended). */
+export function buildPrompt(body: JsonRecord | undefined): string {
+  const messages = (body?.messages as Array<JsonRecord>) || [];
+  const systemMsgs = messages.filter((m) => m.role === "system");
+  const userMsg = messages.filter((m) => m.role === "user").pop();
+  const userText =
+    typeof userMsg?.content === "string" ? userMsg.content : JSON.stringify(userMsg?.content ?? "");
+  let prompt = "";
+  if (systemMsgs.length > 0) {
+    const sysText = systemMsgs
+      .map((m) => (typeof m.content === "string" ? m.content : ""))
+      .filter(Boolean)
+      .join("\n");
+    if (sysText) prompt += `[System Instructions]\n${sysText}\n\n`;
+  }
+  prompt += userText;
+  return prompt;
+}
diff --git a/open-sse/executors/copilot-m365-frames.ts b/open-sse/executors/copilot-m365-frames.ts
new file mode 100644
index 0000000000..e236792dc2
--- /dev/null
+++ b/open-sse/executors/copilot-m365-frames.ts
@@ -0,0 +1,201 @@
+/**
+ * Microsoft 365 Copilot (BizChat / Substrate) SignalR-over-WebSocket framing.
+ *
+ * Pure, transport-free helpers that translate between the OpenAI chat shape and
+ * the Substrate BizChat SignalR JSON protocol observed on the individual M365
+ * path (`m365.cloud.microsoft/chat` → `wss://substrate.office.com/m365Copilot/
+ * Chathub/...`). Keeping these pure lets us unit-test the wire format against the
+ * real frame captures contributed in #4042 without opening a live socket — the
+ * live round-trip is the separate Rule #18 validation gate for the executor.
+ *
+ * Protocol (from @skyzea1's #4042 capture):
+ *   - JSON messages terminated with the SignalR record separator `\x1e`.
+ *   - Handshake: → {"protocol":"json","version":1}  ← {}  → {"type":6}
+ *   - Send: type:4 invocation to target "chat" with arguments[0] = { message, ... }
+ *   - Stream: type:1 target:"update" deltas (bot text at arguments[0].messages[].text,
+ *     accumulated — NOT incremental) → isLastUpdate:true → type:2 final → type:3 completion.
+ */
+
+/** SignalR record separator (0x1e) terminating every JSON frame. */
+export const RECORD_SEPARATOR = String.fromCharCode(0x1e);
+
+/** SignalR handshake request — the first frame the client must send. */
+export const HANDSHAKE_REQUEST = { protocol: "json", version: 1 } as const;
+
+/** SignalR keepalive ping frame. */
+export const KEEPALIVE_PING = { type: 6 } as const;
+
+/** Allowed message types observed in the individual M365 send frame. */
+export const ALLOWED_MESSAGE_TYPES = [
+  "Chat",
+  "Suggestion",
+  "InternalSearchQuery",
+  "Disengaged",
+  "InternalLoaderMessage",
+  "Progress",
+  "GeneratedCode",
+  "RenderCardRequest",
+  "AdsQuery",
+  "SemanticSerp",
+  "GenerateContentQuery",
+] as const;
+
+/** Append the record separator to a JSON-serializable frame. */
+export function encodeFrame(obj: unknown): string {
+  return JSON.stringify(obj) + RECORD_SEPARATOR;
+}
+
+/** Serialized handshake request frame. */
+export function handshakeFrame(): string {
+  return encodeFrame(HANDSHAKE_REQUEST);
+}
+
+/** Serialized keepalive ping frame. */
+export function keepaliveFrame(): string {
+  return encodeFrame(KEEPALIVE_PING);
+}
+
+/**
+ * Split a raw socket buffer into complete `\x1e`-terminated frames, returning any
+ * trailing partial frame as `rest` so it can be prepended to the next chunk.
+ */
+export function splitFrames(buffer: string): { frames: string[]; rest: string } {
+  const parts = buffer.split(RECORD_SEPARATOR);
+  // The last element is either "" (buffer ended on a separator) or a partial frame.
+  const rest = parts.pop() ?? "";
+  const frames = parts.filter((p) => p.length > 0);
+  return { frames, rest };
+}
+
+/** Safely JSON.parse a single frame body; returns null on malformed input. */
+export function parseFrame(frame: string): Record<string, unknown> | null {
+  const trimmed = frame.trim();
+  if (!trimmed) return null;
+  try {
+    const parsed = JSON.parse(trimmed);
+    return parsed && typeof parsed === "object" && !Array.isArray(parsed)
+      ? (parsed as Record<string, unknown>)
+      : null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * A SignalR handshake response is `{}` on success, or `{ error: "..." }` on
+ * failure. Returns the error string, or null when the handshake succeeded.
+ */
+export function handshakeError(frame: Record<string, unknown> | null): string | null {
+  if (!frame) return null;
+  const err = frame.error;
+  return typeof err === "string" && err.length > 0 ? err : null;
+}
+
+export interface ChatInvocationOptions {
+  text: string;
+  /** Per-connection trace id (hex), reused as clientCorrelationId/traceId. */
+  traceId: string;
+  /** Per-session id (GUID). */
+  sessionId: string;
+  /** Whether this is the first turn of the conversation. */
+  isStartOfSession?: boolean;
+  /** Tier-specific option flags; left empty by default (tuned during live validation). */
+  optionsSets?: string[];
+  tone?: string;
+}
+
+/**
+ * Build the `type:4` chat invocation frame body (not yet `\x1e`-terminated).
+ * Mirrors the argument shape captured on the individual M365 path in #4042.
+ */
+export function buildChatInvocation(opts: ChatInvocationOptions): Record<string, unknown> {
+  return {
+    type: 4,
+    target: "chat",
+    invocationId: "0",
+    arguments: [
+      {
+        source: "officeweb",
+        clientCorrelationId: opts.traceId,
+        sessionId: opts.sessionId,
+        optionsSets: opts.optionsSets ?? [],
+        streamingMode: "ConciseWithPadding",
+        spokenTextMode: "None",
+        options: {},
+        extraExtensionParameters: {},
+        allowedMessageTypes: [...ALLOWED_MESSAGE_TYPES],
+        sliceIds: [],
+        threadLevelGptId: {},
+        traceId: opts.traceId,
+        isStartOfSession: opts.isStartOfSession ?? true,
+        clientInfo: {},
+        message: {
+          author: "user",
+          inputMethod: "Keyboard",
+          text: opts.text,
+          messageType: "Chat",
+        },
+        plugins: [],
+        isSbsSupported: false,
+        tone: opts.tone ?? "",
+        renderReferencesBehindEOS: true,
+        disconnectBehavior: "",
+      },
+    ],
+  };
+}
+
+/** True when the frame is a SignalR invocation/streamItem (`type:1`) update. */
+export function isUpdateFrame(frame: Record<string, unknown> | null): boolean {
+  return !!frame && frame.type === 1 && frame.target === "update";
+}
+
+/** True when the frame is the SignalR completion (`type:3`) for the chat invocation. */
+export function isCompletionFrame(frame: Record<string, unknown> | null): boolean {
+  return !!frame && frame.type === 3;
+}
+
+/** True when an update frame is flagged as the last update of the turn. */
+export function isLastUpdate(frame: Record<string, unknown> | null): boolean {
+  if (!isUpdateFrame(frame)) return false;
+  const args = (frame as Record<string, unknown>).arguments;
+  const first = Array.isArray(args) ? (args[0] as Record<string, unknown> | undefined) : undefined;
+  return first?.isLastUpdate === true;
+}
+
+/**
+ * Extract the accumulated bot text from a `type:1` update frame, reading the last
+ * bot-authored message's `.text`. Returns null when the frame carries no bot text
+ * (Progress/Suggestion/ReferencesListComplete updates, throttling-only frames, etc.).
+ */
+export function extractBotText(frame: Record<string, unknown> | null): string | null {
+  if (!isUpdateFrame(frame)) return null;
+  const args = (frame as Record<string, unknown>).arguments;
+  const first = Array.isArray(args) ? (args[0] as Record<string, unknown> | undefined) : undefined;
+  const messages = first?.messages;
+  if (!Array.isArray(messages)) return null;
+  // Prefer the last bot-authored message with non-empty text.
+  for (let i = messages.length - 1; i >= 0; i--) {
+    const m = messages[i] as Record<string, unknown> | undefined;
+    if (!m) continue;
+    const author = m.author;
+    const text = m.text;
+    if ((author === "bot" || author === undefined) && typeof text === "string" && text.length > 0) {
+      return text;
+    }
+  }
+  return null;
+}
+
+/**
+ * BizChat update frames carry the FULL accumulated answer each time, not an
+ * incremental delta. Given the previously-emitted text and the new accumulated
+ * text, return the new suffix to stream. When the new text does not extend the
+ * previous (a replace/rewrite), the whole new text is returned so nothing is lost.
+ */
+export function incrementalDelta(previous: string, next: string): string {
+  if (!next) return "";
+  if (next === previous) return "";
+  if (next.startsWith(previous)) return next.slice(previous.length);
+  return next;
+}
diff --git a/tests/unit/m365-bizchat-frames-4042.test.ts b/tests/unit/m365-bizchat-frames-4042.test.ts
new file mode 100644
index 0000000000..b67323148f
--- /dev/null
+++ b/tests/unit/m365-bizchat-frames-4042.test.ts
@@ -0,0 +1,215 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// #4042 — Microsoft 365 Copilot (individual / Substrate BizChat) frame mapping.
+// Fixtures below are taken from @skyzea1's real, sanitized capture of an
+// `m365.cloud.microsoft/chat` round-trip (test prompt → one-word "pong" reply).
+// These pin the wire format so the executor's encode/decode cannot drift; the
+// live socket round-trip is the separate Rule #18 validation gate.
+
+import {
+  RECORD_SEPARATOR,
+  encodeFrame,
+  handshakeFrame,
+  keepaliveFrame,
+  splitFrames,
+  parseFrame,
+  handshakeError,
+  buildChatInvocation,
+  isUpdateFrame,
+  isCompletionFrame,
+  isLastUpdate,
+  extractBotText,
+  incrementalDelta,
+} from "../../open-sse/executors/copilot-m365-frames.ts";
+
+// ── Real captured frames (#4042) ──────────────────────────────────────────
+
+const HANDSHAKE_ACK = {}; // SignalR success ack
+const PROGRESS_UPDATE = {
+  type: 1,
+  target: "update",
+  arguments: [{ messages: [{ messageType: "Progress", author: "bot" }] }],
+};
+const BOT_UPDATE = {
+  type: 1,
+  target: "update",
+  arguments: [
+    {
+      messages: [
+        {
+          text: "pong",
+          author: "bot",
+          responseIdentifier: "Default",
+          messageId: "00000000-0000-0000-0000-000000000001",
+          requestId: "trace-id",
+          adaptiveCards: [
+            { type: "AdaptiveCard", version: "1.0", body: [{ type: "TextBlock", text: "pong" }] },
+          ],
+          sourceAttributions: [],
+          contentOrigin: "DeepLeo",
+        },
+      ],
+      nonce: "nonce",
+      requestId: "trace-id",
+    },
+  ],
+};
+const FINAL_UPDATE = {
+  type: 1,
+  target: "update",
+  arguments: [
+    {
+      messages: [
+        { text: "pong", author: "bot", sourceAttributions: [], references: {}, contentOrigin: "DeepLeo" },
+      ],
+      isLastUpdate: true,
+      requestId: "trace-id",
+    },
+  ],
+};
+const FINAL_ITEM = { type: 2, invocationId: "0", item: { messages: [], requestId: "trace-id" } };
+const COMPLETION = { type: 3, invocationId: "0" };
+
+// ── Framing ────────────────────────────────────────────────────────────────
+
+test("RECORD_SEPARATOR is the SignalR 0x1e control char", () => {
+  assert.equal(RECORD_SEPARATOR, String.fromCharCode(0x1e));
+  assert.equal(RECORD_SEPARATOR.charCodeAt(0), 0x1e);
+});
+
+test("handshakeFrame / keepaliveFrame emit the exact SignalR bytes", () => {
+  assert.equal(handshakeFrame(), `{"protocol":"json","version":1}` + RECORD_SEPARATOR);
+  assert.equal(keepaliveFrame(), `{"type":6}` + RECORD_SEPARATOR);
+});
+
+test("encodeFrame appends the record separator", () => {
+  assert.equal(encodeFrame({ a: 1 }), `{"a":1}` + RECORD_SEPARATOR);
+});
+
+test("splitFrames separates complete frames and keeps the trailing partial", () => {
+  const buffer = encodeFrame(HANDSHAKE_ACK) + encodeFrame(BOT_UPDATE) + `{"type":1,"par`;
+  const { frames, rest } = splitFrames(buffer);
+  assert.equal(frames.length, 2);
+  assert.deepEqual(parseFrame(frames[0]), {});
+  assert.equal(isUpdateFrame(parseFrame(frames[1])), true);
+  assert.equal(rest, `{"type":1,"par`);
+});
+
+test("splitFrames returns empty rest when the buffer ends on a separator", () => {
+  const { frames, rest } = splitFrames(encodeFrame(COMPLETION));
+  assert.equal(frames.length, 1);
+  assert.equal(rest, "");
+});
+
+// ── Handshake ────────────────────────────────────────────────────────────
+
+test("handshakeError is null on the {} ack and surfaces an error string", () => {
+  assert.equal(handshakeError(parseFrame(encodeFrame(HANDSHAKE_ACK).slice(0, -1))), null);
+  assert.equal(handshakeError({ error: "bad handshake" }), "bad handshake");
+});
+
+// ── Send (type:4) ──────────────────────────────────────────────────────────
+
+test("buildChatInvocation produces a type:4 chat invocation carrying the user text", () => {
+  const frame = buildChatInvocation({
+    text: "protocol capture test. Reply with one word: pong.",
+    traceId: "trace-id",
+    sessionId: "session-id",
+    isStartOfSession: true,
+  });
+  assert.equal(frame.type, 4);
+  assert.equal(frame.target, "chat");
+  assert.equal(frame.invocationId, "0");
+  const arg = (frame.arguments as Array<Record<string, unknown>>)[0];
+  assert.equal(arg.source, "officeweb");
+  assert.equal(arg.streamingMode, "ConciseWithPadding");
+  assert.equal(arg.traceId, "trace-id");
+  assert.equal(arg.clientCorrelationId, "trace-id");
+  assert.equal(arg.sessionId, "session-id");
+  assert.equal(arg.isStartOfSession, true);
+  assert.ok(Array.isArray(arg.allowedMessageTypes));
+  assert.ok((arg.allowedMessageTypes as string[]).includes("Chat"));
+  const message = arg.message as Record<string, unknown>;
+  assert.equal(message.author, "user");
+  assert.equal(message.inputMethod, "Keyboard");
+  assert.equal(message.messageType, "Chat");
+  assert.equal(message.text, "protocol capture test. Reply with one word: pong.");
+});
+
+test("buildChatInvocation serializes/round-trips through the framing", () => {
+  const frame = buildChatInvocation({ text: "hi", traceId: "t", sessionId: "s" });
+  const wire = encodeFrame(frame);
+  assert.ok(wire.endsWith(RECORD_SEPARATOR));
+  const { frames } = splitFrames(wire);
+  assert.deepEqual(parseFrame(frames[0]), frame);
+});
+
+// ── Response decode (type:1/2/3) ─────────────────────────────────────────
+
+test("isUpdateFrame / isCompletionFrame classify the captured frames", () => {
+  assert.equal(isUpdateFrame(BOT_UPDATE), true);
+  assert.equal(isUpdateFrame(FINAL_UPDATE), true);
+  assert.equal(isUpdateFrame(COMPLETION), false);
+  assert.equal(isUpdateFrame(FINAL_ITEM), false);
+  assert.equal(isCompletionFrame(COMPLETION), true);
+  assert.equal(isCompletionFrame(BOT_UPDATE), false);
+  assert.equal(isCompletionFrame(FINAL_ITEM), false); // type:2 is the final item, not completion
+});
+
+test("isLastUpdate only fires on the isLastUpdate:true update", () => {
+  assert.equal(isLastUpdate(BOT_UPDATE), false);
+  assert.equal(isLastUpdate(FINAL_UPDATE), true);
+  assert.equal(isLastUpdate(COMPLETION), false);
+});
+
+test("extractBotText reads the bot answer and ignores Progress frames", () => {
+  assert.equal(extractBotText(BOT_UPDATE), "pong");
+  assert.equal(extractBotText(FINAL_UPDATE), "pong");
+  assert.equal(extractBotText(PROGRESS_UPDATE), null);
+  assert.equal(extractBotText(COMPLETION), null);
+});
+
+// ── Accumulated → incremental delta ─────────────────────────────────────
+
+test("incrementalDelta emits only the new suffix of accumulated text", () => {
+  assert.equal(incrementalDelta("", "pong"), "pong");
+  assert.equal(incrementalDelta("pong", "pong"), "");
+  assert.equal(incrementalDelta("po", "pong"), "ng");
+  assert.equal(incrementalDelta("", ""), "");
+});
+
+test("incrementalDelta falls back to the full text on a non-extending replace", () => {
+  assert.equal(incrementalDelta("abc", "xyz"), "xyz");
+});
+
+// ── End-to-end decode of the captured stream ─────────────────────────────
+
+test("decoding the captured frame sequence reconstructs the bot answer once", () => {
+  const wire =
+    encodeFrame(HANDSHAKE_ACK) +
+    encodeFrame(PROGRESS_UPDATE) +
+    encodeFrame(BOT_UPDATE) +
+    encodeFrame(FINAL_UPDATE) +
+    encodeFrame(FINAL_ITEM) +
+    encodeFrame(COMPLETION);
+
+  const { frames } = splitFrames(wire);
+  let emitted = "";
+  let prev = "";
+  let completed = false;
+  for (const raw of frames) {
+    const frame = parseFrame(raw);
+    if (isUpdateFrame(frame)) {
+      const text = extractBotText(frame);
+      if (text != null) {
+        emitted += incrementalDelta(prev, text);
+        prev = text;
+      }
+    } else if (isCompletionFrame(frame)) {
+      completed = true;
+    }
+  }
+  assert.equal(emitted, "pong");
+  assert.equal(completed, true);
+});
diff --git a/tests/unit/m365-connection-4042.test.ts b/tests/unit/m365-connection-4042.test.ts
new file mode 100644
index 0000000000..af4890de62
--- /dev/null
+++ b/tests/unit/m365-connection-4042.test.ts
@@ -0,0 +1,109 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// #4042 — M365 Copilot (individual) connection helpers: credential resolution,
+// WS URL building, token redaction, and prompt flattening. Pure functions, no
+// live socket — the round-trip is the separate Rule #18 validation gate.
+
+import {
+  M365_INDIVIDUAL_DEFAULTS,
+  newChatSessionId,
+  resolveConnectionParams,
+  buildWsUrl,
+  redactWsUrl,
+  buildPrompt,
+} from "../../open-sse/executors/copilot-m365-connection.ts";
+
+// ── Credential resolution ────────────────────────────────────────────────
+
+test("resolveConnectionParams errors when the access_token is missing", () => {
+  const r = resolveConnectionParams(undefined);
+  assert.ok("error" in r);
+  assert.match((r as { error: string }).error, /access_token/i);
+});
+
+test("resolveConnectionParams errors when the Chathub path is missing", () => {
+  const r = resolveConnectionParams({ apiKey: "tok" });
+  assert.ok("error" in r);
+  assert.match((r as { error: string }).error, /Chathub path/i);
+});
+
+test("resolveConnectionParams reads token from apiKey and path from providerSpecificData", () => {
+  const r = resolveConnectionParams({
+    apiKey: "opaque-jwe-token",
+    providerSpecificData: { chathubPath: "user-oid@tenant-id" },
+  });
+  assert.ok(!("error" in r));
+  const p = r as { host: string; chathubPath: string; accessToken: string };
+  assert.equal(p.accessToken, "opaque-jwe-token");
+  assert.equal(p.chathubPath, "user-oid@tenant-id");
+  assert.equal(p.host, M365_INDIVIDUAL_DEFAULTS.host);
+});
+
+test("resolveConnectionParams accepts access_token in providerSpecificData and a custom host", () => {
+  const r = resolveConnectionParams({
+    providerSpecificData: {
+      access_token: "tok2",
+      userTenant: "u@t",
+      host: "substrate.svc.cloud.microsoft",
+    },
+  });
+  assert.ok(!("error" in r));
+  const p = r as { host: string; chathubPath: string; accessToken: string };
+  assert.equal(p.accessToken, "tok2");
+  assert.equal(p.chathubPath, "u@t");
+  assert.equal(p.host, "substrate.svc.cloud.microsoft");
+});
+
+// ── WS URL building ──────────────────────────────────────────────────────
+
+test("buildWsUrl targets the substrate Chathub with the individual-tier query", () => {
+  const url = buildWsUrl({ host: "substrate.office.com", chathubPath: "u@t", accessToken: "TOK" });
+  assert.ok(url.startsWith("wss://substrate.office.com/m365Copilot/Chathub/u@t?"));
+  const qs = new URLSearchParams(url.split("?")[1]);
+  assert.equal(qs.get("licenseType"), "Starter");
+  assert.equal(qs.get("agent"), "web");
+  assert.equal(qs.get("scenario"), "OfficeWebPaidConsumerCopilot");
+  assert.equal(qs.get("source"), "officeweb");
+  assert.equal(qs.get("access_token"), "TOK");
+  // chatsessionid == XRoutingParameterSessionKey == clientrequestid (same value)
+  const sid = qs.get("chatsessionid");
+  assert.ok(sid && /^[0-9a-f]{32}$/.test(sid));
+  assert.equal(qs.get("XRoutingParameterSessionKey"), sid);
+  assert.equal(qs.get("clientrequestid"), sid);
+});
+
+test("redactWsUrl strips the access_token so the URL is safe to log", () => {
+  const url = buildWsUrl({ host: "substrate.office.com", chathubPath: "u@t", accessToken: "SECRET" });
+  const redacted = redactWsUrl(url);
+  assert.ok(!redacted.includes("SECRET"), "token must not survive redaction");
+  assert.match(redacted, /access_token=REDACTED/);
+});
+
+test("newChatSessionId is 32 lowercase hex chars", () => {
+  const id = newChatSessionId();
+  assert.match(id, /^[0-9a-f]{32}$/);
+  assert.notEqual(newChatSessionId(), id);
+});
+
+// ── Prompt flattening ────────────────────────────────────────────────────
+
+test("buildPrompt returns the last user message", () => {
+  const prompt = buildPrompt({
+    messages: [
+      { role: "user", content: "first" },
+      { role: "user", content: "second" },
+    ],
+  });
+  assert.equal(prompt, "second");
+});
+
+test("buildPrompt prepends system instructions", () => {
+  const prompt = buildPrompt({
+    messages: [
+      { role: "system", content: "Be terse." },
+      { role: "user", content: "hi" },
+    ],
+  });
+  assert.match(prompt, /\[System Instructions\]\nBe terse\.\n\nhi$/);
+});