Merge pull request #2022 from modelcontextprotocol/v2/feat/2018-custom-authorization-params

feat: per-server custom OAuth authorization-request parameters

AGENTS.md

@@ -38,7 +38,14 @@ v2/main/
 │   │                                   #   issuerBinding.ts SEP-2352 callback-leg failure
 │   │                                   #   classification — separates a recoverable
 │   │                                   #   "lost authorization state" from a genuine
-│   │                                   #   cross-AS issuer mismatch — #1808)
+│   │                                   #   cross-AS issuer mismatch — #1808;
+│   │                                   #   authorizationParams.ts per-server custom
+│   │                                   #   authorization-request parameters — the
+│   │                                   #   reserved-key list plus the merge applied in
+│   │                                   #   providers.redirectToAuthorization, the only
+│   │                                   #   seam that sees the SDK-built authorize URL;
+│   │                                   #   authorization request only, never the token
+│   │                                   #   request — #2018)
 │   │   ├── browser/                    # Browser-side OAuth (sessionStorage, BrowserNavigation)
 │   │   ├── node/                       # Node-side OAuth (NodeOAuthStorage, OAuthCallbackServer,
 │   │   │                               #   runner-interactive-oauth loopback callback flow)

clients/web/src/App.tsx

@@ -63,6 +63,7 @@ import { ManagedRequestorTasksState } from "@inspector/core/mcp/state/managedReq
 import { ResourceSubscriptionsState } from "@inspector/core/mcp/state/resourceSubscriptionsState.js";
 import {
   cleanRoots,
+  oauthAuthorizationParamsFromSettings,
   serializeMcpConfig,
 } from "@inspector/core/mcp/serverList.js";
 import type { ClientConfig } from "@inspector/core/client/types.js";
@@ -2301,11 +2302,15 @@ function App() {
               .map((m) => [m.key, m.value]),
           )
         : undefined;
+      const serverAuthorizationParams = savedSettings
+        ? oauthAuthorizationParamsFromSettings(savedSettings)
+        : undefined;
       const oauthFromServer =
         savedSettings &&
         (savedSettings.oauthClientId ||
           savedSettings.oauthClientSecret ||
           savedSettings.oauthScopes ||
+          serverAuthorizationParams ||
           savedSettings.enterpriseManaged)
           ? {
               ...(savedSettings.oauthClientId && {
@@ -2317,6 +2322,9 @@ function App() {
               ...(savedSettings.oauthScopes && {
                 scope: savedSettings.oauthScopes,
               }),
+              ...(serverAuthorizationParams && {
+                authorizationParams: serverAuthorizationParams,
+              }),
               ...(savedSettings.enterpriseManaged && {
                 enterpriseManaged: true,
               }),

clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.test.tsx

@@ -700,10 +700,272 @@ describe("ServerSettingsForm", () => {
       clientId: "a",
       clientSecret: "",
       scopes: "",
+      authorizationParams: [],
       enterpriseManaged: false,
     });
   });
 
+  // #2018 — custom authorization-request parameters.
+  it("shows the empty hint for authorization parameters when none are set", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={emptySettings}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.getByText("Additional authorization parameters"),
+    ).toBeInTheDocument();
+    expect(
+      screen.getByText("No additional parameters configured"),
+    ).toBeInTheDocument();
+  });
+
+  it("adds an authorization-parameter row through onOAuthChange", async () => {
+    const user = userEvent.setup();
+    const onOAuthChange = vi.fn();
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        onOAuthChange={onOAuthChange}
+        settings={emptySettings}
+        expandedSections={["oauth"]}
+      />,
+    );
+    await user.click(screen.getByRole("button", { name: "+ Add Parameter" }));
+    expect(onOAuthChange).toHaveBeenCalledWith(
+      expect.objectContaining({
+        authorizationParams: [{ key: "", value: "" }],
+      }),
+    );
+  });
+
+  it("edits and removes an authorization-parameter row", async () => {
+    const user = userEvent.setup();
+    const onOAuthChange = vi.fn();
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        onOAuthChange={onOAuthChange}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "kc_idp_hint", value: "corp" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    const valueInput = screen.getByDisplayValue("corp");
+    await user.type(valueInput, "x");
+    expect(onOAuthChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        authorizationParams: [{ key: "kc_idp_hint", value: "corpx" }],
+      }),
+    );
+
+    onOAuthChange.mockClear();
+    const removeButton = screen
+      .getAllByRole("button")
+      .find((b) => b.textContent === "X");
+    await user.click(removeButton as HTMLElement);
+    expect(onOAuthChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({ authorizationParams: [] }),
+    );
+  });
+
+  it("clears an authorization-parameter key via its Clear button", async () => {
+    const user = userEvent.setup();
+    const onOAuthChange = vi.fn();
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        onOAuthChange={onOAuthChange}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "kc_idp_hint", value: "corp" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    await user.click(clearButtonFor(screen.getByDisplayValue("kc_idp_hint")));
+    expect(onOAuthChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        authorizationParams: [{ key: "", value: "corp" }],
+      }),
+    );
+
+    onOAuthChange.mockClear();
+    await user.click(clearButtonFor(screen.getByDisplayValue("corp")));
+    expect(onOAuthChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        authorizationParams: [{ key: "kc_idp_hint", value: "" }],
+      }),
+    );
+  });
+
+  it("rejects a reserved authorization-parameter key inline and with a warning", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "state", value: "spoofed" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.getByText(
+        '"state" is set by the authorization flow and cannot be overridden.',
+      ),
+    ).toBeInTheDocument();
+    expect(screen.getByText("Reserved parameters ignored")).toBeInTheDocument();
+    expect(screen.getByDisplayValue("state")).toHaveAttribute(
+      "aria-invalid",
+      "true",
+    );
+  });
+
+  // #2018 — the section heading and the placeholders are not programmatically
+  // associated with these inputs, so each control carries its own aria-label and
+  // the reserved-key reason is the input's `error` string (which Mantine wires
+  // to the input via aria-describedby) rather than free-standing text.
+  it("gives each authorization-parameter control an accessible name", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "kc_idp_hint", value: "corp" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.getByRole("textbox", {
+        name: "Authorization parameter name, kc_idp_hint",
+      }),
+    ).toHaveValue("kc_idp_hint");
+    expect(
+      screen.getByRole("textbox", {
+        name: "Authorization parameter value, kc_idp_hint",
+      }),
+    ).toHaveValue("corp");
+    expect(
+      screen.getByRole("button", {
+        name: "Remove authorization parameter kc_idp_hint",
+      }),
+    ).toBeInTheDocument();
+  });
+
+  it("falls back to a row number in the accessible name when the key is blank", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "", value: "" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.getByRole("textbox", {
+        name: "Authorization parameter name, row 1",
+      }),
+    ).toBeInTheDocument();
+  });
+
+  // The reason must reach assistive tech through the input, not just as text on
+  // the page: Mantine emits the `error` string with an id the input references.
+  it("associates the reserved-key reason with the key input", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "state", value: "spoofed" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    const keyInput = screen.getByRole("textbox", {
+      name: "Authorization parameter name, state",
+    });
+    const describedBy = keyInput.getAttribute("aria-describedby");
+    expect(describedBy).toBeTruthy();
+    const reason = describedBy
+      ?.split(" ")
+      .map((id) => document.getElementById(id)?.textContent ?? "")
+      .join(" ");
+    expect(reason).toContain(
+      '"state" is set by the authorization flow and cannot be overridden.',
+    );
+  });
+
+  // #2018 — the EMA leg authorizes against the enterprise IdP, a different
+  // authorization server, and deliberately sends none of these parameters. The
+  // description has to say so, or the form shows a configured value as active
+  // when it will not be sent.
+  it("warns that authorization parameters are unused under enterprise-managed auth", () => {
+    const { rerender } = renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{ ...emptySettings, enterpriseManaged: true }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.getByText(/Not sent while Enterprise-managed authorization is on/),
+    ).toBeInTheDocument();
+
+    rerender(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{ ...emptySettings, enterpriseManaged: false }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.queryByText(
+        /Not sent while Enterprise-managed authorization is on/,
+      ),
+    ).not.toBeInTheDocument();
+  });
+
+  it("does not flag a blank authorization-parameter row as reserved", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [{ key: "", value: "" }],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(
+      screen.queryByText("Reserved parameters ignored"),
+    ).not.toBeInTheDocument();
+  });
+
+  it("pluralizes the reserved-parameter warning for multiple keys", () => {
+    renderWithMantine(
+      <ServerSettingsForm
+        {...baseHandlers}
+        settings={{
+          ...emptySettings,
+          oauthAuthorizationParams: [
+            { key: "state", value: "x" },
+            { key: "scope", value: "y" },
+          ],
+        }}
+        expandedSections={["oauth"]}
+      />,
+    );
+    expect(screen.getByText(/state, scope are set/)).toBeInTheDocument();
+  });
+
   it("invokes onOAuthChange with the chosen insufficient-scope policy (SEP-2350)", async () => {
     const user = userEvent.setup();
     const onOAuthChange = vi.fn();
@@ -744,6 +1006,7 @@ describe("ServerSettingsForm", () => {
       clientId: "",
       clientSecret: "",
       scopes: "",
+      authorizationParams: [],
       enterpriseManaged: true,
     });
   });
@@ -848,6 +1111,7 @@ describe("ServerSettingsForm", () => {
       clientId: "",
       clientSecret: "z",
       scopes: "",
+      authorizationParams: [],
       enterpriseManaged: false,
     });
   });

clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx

@@ -1,6 +1,7 @@
 import {
   Accordion,
   ActionIcon,
+  Alert,
   Button,
   Checkbox,
   Flex,
@@ -26,6 +27,11 @@ import {
   MODERN_LOG_LEVELS,
 } from "@inspector/core/mcp/types.js";
 import { isOAuthCapableServerType } from "@inspector/core/mcp/config.js";
+import {
+  RESERVED_AUTHORIZATION_PARAMS,
+  authorizationParamKeyError,
+  isReservedAuthorizationParam,
+} from "@inspector/core/auth/authorizationParams.js";
 import { ADVERTISABLE_EXTENSIONS } from "@inspector/core/mcp/extensions.js";
 import type { Root } from "@modelcontextprotocol/client";
 
@@ -184,6 +190,25 @@ const EmptyHint = Text.withProps({
   fs: "italic",
 });
 
+// Matches the label/description pair Mantine renders for a single input, so the
+// authorization-parameter row list reads as one labelled field next to Scopes.
+const FieldLabel = Text.withProps({
+  size: "sm",
+  fw: 500,
+});
+
+const FieldDescription = Text.withProps({
+  size: "xs",
+  c: "dimmed",
+});
+
+// `align="flex-start"` keeps the key/value/remove controls top-aligned when the
+// key input grows an error message, so the row doesn't jump as it appears.
+const AuthorizationParamRow = Group.withProps({
+  grow: true,
+  align: "flex-start",
+});
+
 const ClearStoredOAuthButton = Button.withProps({
   variant: "light",
   color: "red",
@@ -245,6 +270,74 @@ function KeyValueRows({
   );
 }
 
+// Reserved-key rejection is inline per row (#2018): the reason is passed as the
+// key input's `error` **string**, so Mantine renders it in the input's own error
+// slot and wires the `aria-describedby` association — a bare boolean would mark
+// the field invalid without telling a screen reader why. The section-level Alert
+// below repeats it so the rejection stays visible when the offending row is
+// scrolled out of view.
+//
+// Each control carries a per-row `aria-label`: the section heading and the
+// placeholders are not programmatically associated with the inputs, so without
+// one an assistive technology cannot tell the key box from the value box, and
+// the remove button announces only "X".
+function AuthorizationParamRows({
+  params,
+  onChange,
+  onRemove,
+}: {
+  params: { key: string; value: string }[];
+  onChange: (index: number, key: string, value: string) => void;
+  onRemove: (index: number) => void;
+}) {
+  return (
+    <>
+      {params.map((param, index) => {
+        const rowLabel = param.key.trim() || `row ${index + 1}`;
+        return (
+          <AuthorizationParamRow key={index}>
+            <ClearableTextInput
+              aria-label={`Authorization parameter name, ${rowLabel}`}
+              placeholder="Parameter (e.g. kc_idp_hint)"
+              value={param.key}
+              error={authorizationParamKeyError(param.key)}
+              onChange={(e) =>
+                onChange(index, e.currentTarget.value, param.value)
+              }
+              rightSection={
+                param.key ? (
+                  <ClearButton
+                    onClick={() => onChange(index, "", param.value)}
+                  />
+                ) : null
+              }
+            />
+            <ClearableTextInput
+              aria-label={`Authorization parameter value, ${rowLabel}`}
+              placeholder="Value"
+              value={param.value}
+              onChange={(e) =>
+                onChange(index, param.key, e.currentTarget.value)
+              }
+              rightSection={
+                param.value ? (
+                  <ClearButton onClick={() => onChange(index, param.key, "")} />
+                ) : null
+              }
+            />
+            <RemoveIcon
+              aria-label={`Remove authorization parameter ${rowLabel}`}
+              onClick={() => onRemove(index)}
+            >
+              X
+            </RemoveIcon>
+          </AuthorizationParamRow>
+        );
+      })}
+    </>
+  );
+}
+
 function RootRows({
   roots,
   onChange,
@@ -374,16 +467,52 @@ export function ServerSettingsForm({
       onTimeoutChange(field, numValue);
     };
 
+  // #2018 — custom authorization-request parameters. The rows ride the single
+  // `onOAuthChange` callback with the rest of the Authorization section rather
+  // than growing three more props on this form.
+  const authorizationParams = settings.oauthAuthorizationParams ?? [];
+
   function currentOAuth(): OAuthSettings {
     return {
       clientId: settings.oauthClientId ?? "",
       clientSecret: settings.oauthClientSecret ?? "",
       scopes: settings.oauthScopes ?? "",
+      authorizationParams,
       enterpriseManaged: settings.enterpriseManaged ?? false,
       onInsufficientScope: settings.oauthOnInsufficientScope,
     };
   }
 
+  const rejectedParamKeys = authorizationParams
+    .map((p) => p.key.trim())
+    .filter((key) => isReservedAuthorizationParam(key));
+
+  function changeAuthorizationParams(
+    params: { key: string; value: string }[],
+  ): void {
+    onOAuthChange({ ...currentOAuth(), authorizationParams: params });
+  }
+
+  function handleAddAuthorizationParam(): void {
+    changeAuthorizationParams([...authorizationParams, { key: "", value: "" }]);
+  }
+
+  function handleRemoveAuthorizationParam(index: number): void {
+    changeAuthorizationParams(
+      authorizationParams.filter((_, i) => i !== index),
+    );
+  }
+
+  function handleAuthorizationParamChange(
+    index: number,
+    key: string,
+    value: string,
+  ): void {
+    changeAuthorizationParams(
+      authorizationParams.map((p, i) => (i === index ? { key, value } : p)),
+    );
+  }
+
   return (
     // Stays inline: Accordion is a compound, `multiple`-discriminated generic,
     // so `.withProps({ multiple: true, ... })` loses its JSX call signature
@@ -699,6 +828,41 @@ export function ServerSettingsForm({
                   ) : null
                 }
               />
+              <Stack gap="xs">
+                <FieldLabel>Additional authorization parameters</FieldLabel>
+                <FieldDescription>
+                  Extra query parameters appended to the authorization request
+                  only — never the token request. Use them for provider-specific
+                  hints such as kc_idp_hint, login_hint, prompt, acr_values, or
+                  audience.
+                  {settings.enterpriseManaged
+                    ? " Not sent while Enterprise-managed authorization is on: that flow authorizes against the enterprise IdP, a different authorization server."
+                    : ""}
+                </FieldDescription>
+                {authorizationParams.length === 0 ? (
+                  <EmptyHint>No additional parameters configured</EmptyHint>
+                ) : (
+                  <AuthorizationParamRows
+                    params={authorizationParams}
+                    onChange={handleAuthorizationParamChange}
+                    onRemove={handleRemoveAuthorizationParam}
+                  />
+                )}
+                {rejectedParamKeys.length > 0 ? (
+                  <Alert color="red" title="Reserved parameters ignored">
+                    {`${rejectedParamKeys.join(", ")} ${
+                      rejectedParamKeys.length === 1 ? "is" : "are"
+                    } set by the authorization flow and will not be sent. Overriding ${
+                      rejectedParamKeys.length === 1 ? "it" : "them"
+                    } would break PKCE, the CSRF state binding, or the resource indicator. Reserved: ${RESERVED_AUTHORIZATION_PARAMS.join(
+                      ", ",
+                    )}.`}
+                  </Alert>
+                ) : null}
+                <AddButton onClick={handleAddAuthorizationParam}>
+                  + Add Parameter
+                </AddButton>
+              </Stack>
               <Select
                 label="Insufficient-scope response"
                 description="On a 403 insufficient_scope challenge (SEP-2350): re-authorize with the accumulated scope union, or surface the error to you."

clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx

@@ -440,10 +440,35 @@ describe("ServerSettingsModal", () => {
       oauthClientId: "",
       oauthClientSecret: "",
       oauthScopes: "",
+      oauthAuthorizationParams: [],
       enterpriseManaged: true,
     });
   });
 
+  // #2018 — the authorization-parameter rows ride the same onOAuthChange
+  // callback, so the modal folds them onto the settings object.
+  it("persists an added authorization-parameter row onto the settings", async () => {
+    const user = userEvent.setup();
+    const onSettingsChange = vi.fn();
+    renderWithMantine(
+      <ServerSettingsModal
+        opened
+        settings={emptySettings}
+        serverType="streamable-http"
+        isStdio={false}
+        onClose={vi.fn()}
+        onSettingsChange={onSettingsChange}
+      />,
+    );
+    await user.click(screen.getByRole("button", { name: "OAuth Settings" }));
+    await user.click(screen.getByRole("button", { name: "+ Add Parameter" }));
+    expect(onSettingsChange).toHaveBeenCalledWith(
+      expect.objectContaining({
+        oauthAuthorizationParams: [{ key: "", value: "" }],
+      }),
+    );
+  });
+
   it("calls onSettingsChange with a blank row when adding a root", async () => {
     const user = userEvent.setup();
     const onSettingsChange = vi.fn();

clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.tsx

@@ -179,6 +179,10 @@ export function ServerSettingsModal({
       oauthClientId: oauth.clientId,
       oauthClientSecret: oauth.clientSecret,
       oauthScopes: oauth.scopes,
+      // #2018: kept as rows (blank ones included) so a half-typed parameter
+      // survives a re-render; the drop-blank/omit-empty filtering happens on the
+      // way to disk. An emptied list persists as `[]`, which writes nothing.
+      oauthAuthorizationParams: oauth.authorizationParams,
       enterpriseManaged: oauth.enterpriseManaged ? true : undefined,
       // SEP-2350: persist only the non-default ('throw') so unset servers keep
       // the SDK's `reauthorize` behavior without writing a spurious field.

clients/web/src/test/core/auth/authorizationParams.test.ts

@@ -0,0 +1,111 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import {
+  RESERVED_AUTHORIZATION_PARAMS,
+  applyAuthorizationParams,
+  authorizationParamKeyError,
+  isReservedAuthorizationParam,
+} from "@inspector/core/auth/authorizationParams.js";
+
+const AUTHORIZE_URL =
+  "https://as.example.com/authorize?client_id=abc&response_type=code&state=xyz";
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("isReservedAuthorizationParam", () => {
+  it.each(RESERVED_AUTHORIZATION_PARAMS)("reserves %s", (key) => {
+    expect(isReservedAuthorizationParam(key)).toBe(true);
+  });
+
+  it("ignores case and surrounding whitespace", () => {
+    expect(isReservedAuthorizationParam("  Client_ID ")).toBe(true);
+  });
+
+  it("allows a provider-specific parameter", () => {
+    expect(isReservedAuthorizationParam("kc_idp_hint")).toBe(false);
+  });
+});
+
+describe("authorizationParamKeyError", () => {
+  it("returns no error for a blank key (a half-edited row)", () => {
+    expect(authorizationParamKeyError("")).toBeUndefined();
+    expect(authorizationParamKeyError("   ")).toBeUndefined();
+  });
+
+  it("returns no error for an allowed key", () => {
+    expect(authorizationParamKeyError("login_hint")).toBeUndefined();
+  });
+
+  it("names the offending key for a reserved one", () => {
+    expect(authorizationParamKeyError(" state ")).toBe(
+      '"state" is set by the authorization flow and cannot be overridden.',
+    );
+  });
+});
+
+describe("applyAuthorizationParams", () => {
+  it("returns the same URL instance when no params are configured", () => {
+    const url = new URL(AUTHORIZE_URL);
+    expect(applyAuthorizationParams(url, undefined)).toBe(url);
+  });
+
+  it("returns the same URL instance when the params object is empty", () => {
+    const url = new URL(AUTHORIZE_URL);
+    expect(applyAuthorizationParams(url, {})).toBe(url);
+  });
+
+  it("appends custom params without mutating the input URL", () => {
+    const url = new URL(AUTHORIZE_URL);
+    const merged = applyAuthorizationParams(url, {
+      kc_idp_hint: "corp-idp",
+      login_hint: "user@example.com",
+    });
+
+    expect(merged).not.toBe(url);
+    expect(url.searchParams.get("kc_idp_hint")).toBeNull();
+    expect(merged.searchParams.get("kc_idp_hint")).toBe("corp-idp");
+    expect(merged.searchParams.get("login_hint")).toBe("user@example.com");
+    // The SDK-built parameters survive untouched.
+    expect(merged.searchParams.get("client_id")).toBe("abc");
+    expect(merged.searchParams.get("state")).toBe("xyz");
+  });
+
+  it("trims the key and preserves the value verbatim", () => {
+    const merged = applyAuthorizationParams(new URL(AUTHORIZE_URL), {
+      "  prompt  ": " login ",
+    });
+    expect(merged.searchParams.get("prompt")).toBe(" login ");
+  });
+
+  it("skips blank keys", () => {
+    const url = new URL(AUTHORIZE_URL);
+    const merged = applyAuthorizationParams(url, { "   ": "nope" });
+    expect(merged).toBe(url);
+  });
+
+  it("drops a reserved key with a warning rather than overriding it", () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    const merged = applyAuthorizationParams(new URL(AUTHORIZE_URL), {
+      state: "attacker-controlled",
+      kc_idp_hint: "corp-idp",
+    });
+
+    expect(merged.searchParams.get("state")).toBe("xyz");
+    expect(merged.searchParams.get("kc_idp_hint")).toBe("corp-idp");
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(warn.mock.calls[0]?.[0]).toContain("state");
+  });
+
+  it("returns the original URL when every configured key is reserved", () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    const url = new URL(AUTHORIZE_URL);
+    expect(
+      applyAuthorizationParams(url, {
+        Client_Id: "spoofed",
+        code_challenge: "spoofed",
+      }),
+    ).toBe(url);
+    expect(warn).toHaveBeenCalledTimes(2);
+  });
+});

clients/web/src/test/core/auth/ema/transportProvider.test.ts

@@ -1,6 +1,14 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import type { OAuthTokens } from "@modelcontextprotocol/client";
-import type { BaseOAuthClientProvider } from "@inspector/core/auth/providers.js";
+import {
+  BaseOAuthClientProvider,
+  CallbackNavigation,
+  MutableRedirectUrlProvider,
+} from "@inspector/core/auth/providers.js";
+import type { OAuthStorage } from "@inspector/core/auth/storage.js";
+import { OAuthStorageBase } from "@inspector/core/auth/oauth-storage.js";
+import { OAuthMemoryStore } from "@inspector/core/auth/store.js";
+import type { OAuthPersistBackend } from "@inspector/core/auth/oauth-persist.js";
 import type { EmaFlowConfig } from "@inspector/core/auth/ema/emaFlow.js";
 import { EmaTransportOAuthProvider } from "@inspector/core/auth/ema/transportProvider.js";
 import {
@@ -18,6 +26,20 @@ const startIdpMock = vi.mocked(startEmaIdpAuthorization);
 
 const SERVER_URL = "http://127.0.0.1:9999/mcp";
 
+/**
+ * A real `OAuthStorage` — in-memory state over a no-op persist backend — for the
+ * one test below that drives an actual `BaseOAuthClientProvider`. Building the
+ * genuine article costs two lines and avoids an `as unknown as` stub that would
+ * stop type-checking against the interface the provider is handed.
+ */
+function makeRealStorage(): OAuthStorage {
+  const backend: OAuthPersistBackend = {
+    read: async () => null,
+    write: async () => {},
+  };
+  return new OAuthStorageBase(new OAuthMemoryStore(), backend);
+}
+
 function jwtWithExp(expSec: number): string {
   const payload = btoa(JSON.stringify({ exp: expSec }))
     .replace(/\+/g, "-")
@@ -39,6 +61,7 @@ interface FakeInner {
   tokens: ReturnType<typeof vi.fn>;
   saveTokens: ReturnType<typeof vi.fn>;
   redirectToAuthorization: ReturnType<typeof vi.fn>;
+  redirectToExternalAuthorization: ReturnType<typeof vi.fn>;
   clearCapturedAuthUrl: ReturnType<typeof vi.fn>;
   saveCodeVerifier: ReturnType<typeof vi.fn>;
   codeVerifier: ReturnType<typeof vi.fn>;
@@ -55,6 +78,7 @@ function createInner(): FakeInner {
     tokens: vi.fn(),
     saveTokens: vi.fn(),
     redirectToAuthorization: vi.fn(),
+    redirectToExternalAuthorization: vi.fn(),
     clearCapturedAuthUrl: vi.fn(),
     saveCodeVerifier: vi.fn(),
     codeVerifier: vi.fn(() => "verifier-xyz"),
@@ -178,6 +202,43 @@ describe("EmaTransportOAuthProvider", () => {
 
     expect(startIdpMock).toHaveBeenCalledWith(emaConfig);
     expect(inner.clearCapturedAuthUrl).toHaveBeenCalledTimes(1);
-    expect(inner.redirectToAuthorization).toHaveBeenCalledWith(idpUrl);
+    expect(inner.redirectToExternalAuthorization).toHaveBeenCalledWith(idpUrl);
+    // The custom-parameter merge lives on `redirectToAuthorization`, so the EMA
+    // leg must never take that path.
+    expect(inner.redirectToAuthorization).not.toHaveBeenCalled();
+  });
+
+  // #2018 — the inner provider carries the per-server custom authorization
+  // parameters, which belong to the MCP server's authorization server. The EMA
+  // leg sends the user to a *different* authorization server (the enterprise
+  // IdP), so those parameters must not reach it. Driven against a real
+  // BaseOAuthClientProvider rather than the fake inner above, since the defect
+  // is precisely in what the real provider does with the URL it is handed.
+  it("does not leak custom authorization params onto the IdP authorize URL", async () => {
+    const navCallback = vi.fn();
+    const realInner = new BaseOAuthClientProvider(SERVER_URL, {
+      // A real OAuthStorage rather than a cast stub: the redirect path never
+      // reads it, and a genuine implementation keeps this test honest if the
+      // storage contract changes.
+      storage: makeRealStorage(),
+      redirectUrlProvider: new MutableRedirectUrlProvider(),
+      navigation: new CallbackNavigation(navCallback),
+      authorizationParams: { kc_idp_hint: "corp-idp", audience: "api://mcp" },
+    });
+    const emaProvider = new EmaTransportOAuthProvider(realInner, emaConfig);
+
+    const idpUrl = new URL("https://idp.test/authorize?state=abc");
+    startIdpMock.mockResolvedValue(idpUrl);
+
+    await emaProvider.redirectToAuthorization(
+      new URL("https://resource-as.test/authorize"),
+    );
+
+    const navigated = navCallback.mock.calls[0]?.[0] as URL;
+    expect(navigated.searchParams.get("kc_idp_hint")).toBeNull();
+    expect(navigated.searchParams.get("audience")).toBeNull();
+    expect(navigated.href).toBe(idpUrl.href);
+    // The captured URL (what the step-up UI shows) must agree.
+    expect(realInner.getCapturedAuthUrl()?.href).toBe(idpUrl.href);
   });
 });

clients/web/src/test/core/auth/providers.test.ts

@@ -352,6 +352,68 @@ describe("OAuthNavigation", () => {
       expect(navCallback).toHaveBeenCalledWith(url);
     });
 
+    // #2018 — the provider is the seam where per-server custom parameters reach
+    // the authorize URL, since the SDK builds that URL with no hook of its own.
+    function makeProviderWithParams(
+      storage: OAuthStorage,
+      authorizationParams: Record<string, string>,
+      navCallback = vi.fn(),
+    ): BaseOAuthClientProvider {
+      return new BaseOAuthClientProvider(SERVER, {
+        storage,
+        redirectUrlProvider: new MutableRedirectUrlProvider(),
+        navigation: new CallbackNavigation(navCallback),
+        authorizationParams,
+      });
+    }
+
+    it("redirectToAuthorization merges configured authorization params", () => {
+      const storage = makeStorage();
+      const navCallback = vi.fn();
+      const provider = makeProviderWithParams(
+        storage,
+        { kc_idp_hint: "corp-idp" },
+        navCallback,
+      );
+      const target = new EventTarget();
+      const handler = vi.fn();
+      target.addEventListener("oauthAuthorizationRequired", handler);
+      provider.setEventTarget(target);
+
+      const url = new URL("https://mcp.example.com/authorize?client_id=abc");
+      provider.redirectToAuthorization(url);
+
+      const navigated = navCallback.mock.calls[0]?.[0] as URL;
+      expect(navigated.searchParams.get("kc_idp_hint")).toBe("corp-idp");
+      expect(navigated.searchParams.get("client_id")).toBe("abc");
+      // The captured URL, the event detail, and the navigation all agree.
+      expect(provider.getCapturedAuthUrl()?.href).toBe(navigated.href);
+      const detail = (handler.mock.calls[0]?.[0] as CustomEvent<{ url: URL }>)
+        .detail;
+      expect(detail.url.href).toBe(navigated.href);
+      // The URL the SDK handed in is left untouched.
+      expect(url.searchParams.get("kc_idp_hint")).toBeNull();
+    });
+
+    it("redirectToAuthorization refuses a reserved authorization param", () => {
+      const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+      const storage = makeStorage();
+      const navCallback = vi.fn();
+      const provider = makeProviderWithParams(
+        storage,
+        { state: "spoofed" },
+        navCallback,
+      );
+
+      const url = new URL("https://mcp.example.com/authorize?state=real");
+      provider.redirectToAuthorization(url);
+
+      const navigated = navCallback.mock.calls[0]?.[0] as URL;
+      expect(navigated.searchParams.get("state")).toBe("real");
+      expect(warn).toHaveBeenCalled();
+      warn.mockRestore();
+    });
+
     it("delegates token and scope persistence to storage", async () => {
       const storage = makeStorage();
       const provider = makeProvider(storage);

clients/web/src/test/core/client/runner.test.ts

@@ -86,6 +86,44 @@ describe("runner client auth options", () => {
     expect(opts.oauth?.clientId).toBe("resource-client");
   });
 
+  // #2018 — the CLI/TUI leg picks the authorization parameters up from the same
+  // per-server settings the web leg reads, so all three clients agree.
+  it("buildRunnerClientAuthOptions forwards custom authorization params", () => {
+    const settings: InspectorServerSettings = {
+      oauthAuthorizationParams: [
+        { key: "kc_idp_hint", value: "corp" },
+        { key: "", value: "dropped" },
+      ],
+      requestTimeout: 0,
+      connectionTimeout: 0,
+      taskTtl: 60000,
+      maxFetchRequests: 10,
+      autoRefreshOnListChanged: false,
+      metadata: [],
+      headers: [],
+      env: [],
+      roots: [],
+    };
+    const opts = buildRunnerClientAuthOptions({}, settings);
+    expect(opts.oauth?.authorizationParams).toEqual({ kc_idp_hint: "corp" });
+  });
+
+  it("buildRunnerClientAuthOptions ignores all-blank authorization param rows", () => {
+    const settings: InspectorServerSettings = {
+      oauthAuthorizationParams: [{ key: "  ", value: "x" }],
+      requestTimeout: 0,
+      connectionTimeout: 0,
+      taskTtl: 60000,
+      maxFetchRequests: 10,
+      autoRefreshOnListChanged: false,
+      metadata: [],
+      headers: [],
+      env: [],
+      roots: [],
+    };
+    expect(buildRunnerClientAuthOptions({}, settings)).toEqual({});
+  });
+
   it("buildRunnerClientAuthOptions returns no oauth when nothing supplies it", () => {
     expect(buildRunnerClientAuthOptions({})).toEqual({});
   });

clients/web/src/test/core/mcp/oauthManager.test.ts

@@ -1574,6 +1574,53 @@ describe("OAuthManager", () => {
       const provider = await manager.createOAuthProviderForTransport();
       expect(provider.constructor.name).toBe("EmaTransportOAuthProvider");
     });
+
+    // #2018 — the manager→provider bridge for the custom authorization
+    // parameters. The provider test proves the merge and the runner test proves
+    // the options object, but neither crosses this seam: without this case,
+    // deleting `authorizationParams:` from `createOAuthProvider` would leave the
+    // feature dead with every other test still green.
+    it("forwards configured authorizationParams to the provider it builds", async () => {
+      const params = createMockParams();
+      const manager = new OAuthManager(params);
+      manager.setOAuthConfig({
+        authorizationParams: { kc_idp_hint: "corp-idp", prompt: "login" },
+      });
+
+      const provider = await manager.createOAuthProviderForTransport();
+      provider.redirectToAuthorization(
+        new URL("https://as.example.com/authorize?client_id=abc&state=xyz"),
+      );
+
+      const navigate = params.initialConfig.navigation
+        ?.navigateToAuthorization as ReturnType<typeof vi.fn>;
+      const navigated = navigate.mock.calls[0]?.[0] as URL;
+      expect(navigated.searchParams.get("kc_idp_hint")).toBe("corp-idp");
+      expect(navigated.searchParams.get("prompt")).toBe("login");
+      // The flow's own parameters are untouched.
+      expect(navigated.searchParams.get("state")).toBe("xyz");
+    });
+
+    it("drops a reserved key configured on the manager", async () => {
+      const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+      const params = createMockParams();
+      const manager = new OAuthManager(params);
+      manager.setOAuthConfig({
+        authorizationParams: { state: "spoofed", kc_idp_hint: "corp-idp" },
+      });
+
+      const provider = await manager.createOAuthProviderForTransport();
+      provider.redirectToAuthorization(
+        new URL("https://as.example.com/authorize?client_id=abc&state=xyz"),
+      );
+
+      const navigate = params.initialConfig.navigation
+        ?.navigateToAuthorization as ReturnType<typeof vi.fn>;
+      const navigated = navigate.mock.calls[0]?.[0] as URL;
+      expect(navigated.searchParams.get("state")).toBe("xyz");
+      expect(navigated.searchParams.get("kc_idp_hint")).toBe("corp-idp");
+      expect(warn).toHaveBeenCalled();
+    });
   });
 
   describe("getOAuthState (enterprise managed / scope)", () => {

clients/web/src/test/core/mcp/serverList.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import {
+  cleanAuthorizationParams,
   cleanRoots,
   DEFAULT_SEED_CONFIG,
   envPairsToRecord,
@@ -10,6 +11,7 @@ import {
   mcpConfigToServerEntries,
   mergeSecretsIntoStored,
   normalizeServerType,
+  oauthAuthorizationParamsFromSettings,
   serverEntriesToMcpConfig,
   serializeMcpConfig,
   storedFieldsToInspectorSettings,
@@ -924,6 +926,42 @@ describe("extractSecretsFromStored", () => {
     expect(stripped.oauth).toEqual({ enterpriseManaged: true });
   });
 
+  // The stripped entry now subtracts the secret instead of enumerating the
+  // fields to keep, so a field added to `oauth` later survives by construction.
+  // `onInsufficientScope` is the case the old allow-list had already lost.
+  it("preserves oauth.onInsufficientScope when lifting clientSecret to keychain", () => {
+    const stored: StoredMCPServer = {
+      type: "streamable-http",
+      url: "https://x.test",
+      oauth: {
+        clientId: "cid",
+        clientSecret: "shh",
+        onInsufficientScope: "throw",
+      },
+    };
+    const { stripped, secrets } = extractSecretsFromStored(stored);
+    expect(secrets).toEqual({ [SECRET_FIELD_OAUTH_CLIENT_SECRET]: "shh" });
+    expect(stripped.oauth).toEqual({
+      clientId: "cid",
+      onInsufficientScope: "throw",
+    });
+  });
+
+  it("preserves oauth.authorizationParams when lifting clientSecret to keychain", () => {
+    const stored: StoredMCPServer = {
+      type: "streamable-http",
+      url: "https://x.test",
+      oauth: {
+        clientSecret: "shh",
+        authorizationParams: { kc_idp_hint: "corp" },
+      },
+    };
+    const { stripped } = extractSecretsFromStored(stored);
+    expect(stripped.oauth).toEqual({
+      authorizationParams: { kc_idp_hint: "corp" },
+    });
+  });
+
   it("removes the oauth block entirely when clientSecret was its only property", () => {
     const stored: StoredMCPServer = {
       type: "streamable-http",
@@ -1188,6 +1226,68 @@ describe("oauthOnInsufficientScope (SEP-2350)", () => {
     });
     expect(stored.oauth?.onInsufficientScope).toBeUndefined();
   });
+
+  // #2018 — custom authorization-request parameters.
+  it("lifts oauth.authorizationParams into settings rows", () => {
+    const settings = storedFieldsToInspectorSettings({
+      oauth: { authorizationParams: { kc_idp_hint: "corp", prompt: "login" } },
+    });
+    expect(settings?.oauthAuthorizationParams).toEqual([
+      { key: "kc_idp_hint", value: "corp" },
+      { key: "prompt", value: "login" },
+    ]);
+  });
+
+  it("reads an empty authorizationParams object back as unset", () => {
+    const settings = storedFieldsToInspectorSettings({
+      oauth: { clientId: "cid", authorizationParams: {} },
+    });
+    expect(settings?.oauthAuthorizationParams).toBeUndefined();
+  });
+
+  it("persists authorizationParams under oauth, dropping blank-key rows", () => {
+    const stored = inspectorSettingsToStoredFields({
+      ...baseSettings,
+      oauthAuthorizationParams: [
+        { key: "kc_idp_hint", value: "corp" },
+        { key: "   ", value: "orphan" },
+      ],
+    });
+    expect(stored.oauth?.authorizationParams).toEqual({ kc_idp_hint: "corp" });
+  });
+
+  it("omits the oauth block when every authorization-param row is blank", () => {
+    const stored = inspectorSettingsToStoredFields({
+      ...baseSettings,
+      oauthAuthorizationParams: [{ key: "", value: "" }],
+    });
+    expect(stored).not.toHaveProperty("oauth");
+  });
+
+  it("omits authorizationParams when the rows are absent", () => {
+    const stored = inspectorSettingsToStoredFields({
+      ...baseSettings,
+      oauthClientId: "cid",
+    });
+    expect(stored.oauth?.authorizationParams).toBeUndefined();
+  });
+
+  it("derives the client option record, or undefined when nothing survives", () => {
+    expect(
+      oauthAuthorizationParamsFromSettings({
+        oauthAuthorizationParams: [
+          { key: "kc_idp_hint", value: "corp" },
+          { key: "", value: "x" },
+        ],
+      }),
+    ).toEqual({ kc_idp_hint: "corp" });
+    expect(
+      oauthAuthorizationParamsFromSettings({
+        oauthAuthorizationParams: [{ key: " ", value: "x" }],
+      }),
+    ).toBeUndefined();
+    expect(oauthAuthorizationParamsFromSettings({})).toBeUndefined();
+  });
 });
 
 describe("envRecordToPairs / envPairsToRecord", () => {
@@ -1316,3 +1416,94 @@ describe("stdio env / cwd mirroring", () => {
     });
   });
 });
+
+// #2018 — `StoredMCPServer.oauth.authorizationParams` is typed
+// `Record<string, string>`, but only the web client's `/api/servers` route
+// validates it; the CLI and TUI read `mcp.json` off disk. These cases drive the
+// shapes a hand-edited file can hold, which the type says are impossible.
+describe("cleanAuthorizationParams", () => {
+  let warnSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    warnSpy.mockRestore();
+  });
+
+  it("passes a well-formed record through", () => {
+    expect(
+      cleanAuthorizationParams({ kc_idp_hint: "corp", prompt: "login" }),
+    ).toEqual({ kc_idp_hint: "corp", prompt: "login" });
+    expect(warnSpy).not.toHaveBeenCalled();
+  });
+
+  it("returns undefined for an absent or empty record", () => {
+    expect(cleanAuthorizationParams(undefined)).toBeUndefined();
+    expect(cleanAuthorizationParams({})).toBeUndefined();
+  });
+
+  // A bare string enumerates as character-indexed pairs (0=o, 1=o, …), so
+  // without this guard the Inspector would send four invented parameters.
+  it("rejects a string with a warning instead of enumerating its characters", () => {
+    expect(
+      cleanAuthorizationParams("oops" as unknown as Record<string, string>),
+    ).toBeUndefined();
+    expect(warnSpy).toHaveBeenCalledTimes(1);
+  });
+
+  it("rejects an array with a warning", () => {
+    expect(
+      cleanAuthorizationParams(["a"] as unknown as Record<string, string>),
+    ).toBeUndefined();
+    expect(warnSpy.mock.calls[0]?.[1]).toBe("array");
+  });
+
+  it("rejects null with a warning", () => {
+    expect(
+      cleanAuthorizationParams(null as unknown as Record<string, string>),
+    ).toBeUndefined();
+    expect(warnSpy).toHaveBeenCalledTimes(1);
+  });
+
+  // `URLSearchParams.set` stringifies whatever it is given, so a number would
+  // otherwise reach the authorize URL as "5".
+  it("drops a non-string value with a warning and keeps the rest", () => {
+    expect(
+      cleanAuthorizationParams({
+        audience: 5,
+        kc_idp_hint: "corp",
+      } as unknown as Record<string, string>),
+    ).toEqual({ kc_idp_hint: "corp" });
+    expect(warnSpy).toHaveBeenCalledTimes(1);
+  });
+
+  it("returns undefined when every value was dropped", () => {
+    expect(
+      cleanAuthorizationParams({ audience: 5 } as unknown as Record<
+        string,
+        string
+      >),
+    ).toBeUndefined();
+  });
+
+  it("skips a blank key", () => {
+    expect(cleanAuthorizationParams({ "  ": "x", ok: "y" })).toEqual({
+      ok: "y",
+    });
+  });
+
+  it("is applied when lifting stored fields into settings", () => {
+    const settings = storedFieldsToInspectorSettings({
+      type: "streamable-http",
+      url: "https://x.test",
+      oauth: {
+        authorizationParams: { audience: 5, kc_idp_hint: "corp" },
+      },
+    } as unknown as StoredMCPServer);
+    expect(settings?.oauthAuthorizationParams).toEqual([
+      { key: "kc_idp_hint", value: "corp" },
+    ]);
+  });
+});

clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts

@@ -701,6 +701,58 @@ describe("server.ts supplemental coverage", () => {
         await stop(h);
       }
     });
+
+    // #2018 — authorizationParams must be a string record; a hand-edited file
+    // with anything else drops the whole `oauth` node rather than feeding
+    // non-string values to the authorize-URL merge.
+    it("drops oauth whose authorizationParams is not a string record", async () => {
+      const h = await start({
+        seedConfig: JSON.stringify({
+          mcpServers: {
+            srv: {
+              type: "streamable-http",
+              url: "https://x.test/mcp",
+              oauth: { authorizationParams: { kc_idp_hint: 5 } },
+            },
+          },
+        }),
+      });
+      try {
+        const res = await fetch(`${h.baseUrl}/api/servers`);
+        expect(res.status).toBe(200);
+        const body = (await res.json()) as {
+          mcpServers: Record<string, Record<string, unknown>>;
+        };
+        expect(body.mcpServers.srv).not.toHaveProperty("oauth");
+      } finally {
+        await stop(h);
+      }
+    });
+
+    it("keeps a well-formed authorizationParams record on read", async () => {
+      const h = await start({
+        seedConfig: JSON.stringify({
+          mcpServers: {
+            srv: {
+              type: "streamable-http",
+              url: "https://x.test/mcp",
+              oauth: { authorizationParams: { kc_idp_hint: "corp" } },
+            },
+          },
+        }),
+      });
+      try {
+        const res = await fetch(`${h.baseUrl}/api/servers`);
+        const body = (await res.json()) as {
+          mcpServers: Record<string, Record<string, unknown>>;
+        };
+        expect(body.mcpServers.srv?.oauth).toEqual({
+          authorizationParams: { kc_idp_hint: "corp" },
+        });
+      } finally {
+        await stop(h);
+      }
+    });
   });
 
   describe("plaintext-secret migration over a mixed config", () => {

clients/web/src/test/integration/mcp/remote/servers-route.test.ts

@@ -449,6 +449,53 @@ describe("/api/servers routes", () => {
       });
     });
 
+    // #2018 — the authorization-parameter rows travel as a pair array on the
+    // wire and land as a record under `oauth` on disk, mirroring `headers`.
+    it("persists custom authorization params under oauth on POST", async () => {
+      const res = await fetch(`${h.baseUrl}/api/servers`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          id: "authparams",
+          config: { type: "streamable-http", url: "https://x.test/mcp" },
+          settings: {
+            headers: [],
+            metadata: [],
+            connectionTimeout: 0,
+            requestTimeout: 0,
+            oauthAuthorizationParams: [
+              { key: "kc_idp_hint", value: "corp" },
+              { key: "", value: "dropped" },
+            ],
+          },
+        }),
+      });
+      expect(res.status).toBe(200);
+      const stored = readConfig(h.configPath).mcpServers.authparams;
+      expect(stored?.oauth).toEqual({
+        authorizationParams: { kc_idp_hint: "corp" },
+      });
+    });
+
+    it("rejects a malformed oauthAuthorizationParams payload", async () => {
+      const res = await fetch(`${h.baseUrl}/api/servers`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          id: "badauthparams",
+          config: { type: "streamable-http", url: "https://x.test/mcp" },
+          settings: {
+            headers: [],
+            metadata: [],
+            connectionTimeout: 0,
+            requestTimeout: 0,
+            oauthAuthorizationParams: { kc_idp_hint: "corp" },
+          },
+        }),
+      });
+      expect(res.status).toBe(400);
+    });
+
     it("updates Inspector-extension fields at the top level on PUT", async () => {
       writeFileSync(
         h.configPath,

core/auth/authorizationParams.ts

@@ -0,0 +1,105 @@
+/**
+ * Per-server custom authorization-request parameters (#2018).
+ *
+ * Real deployments gate IdP selection, prompt behavior, or audience on a query
+ * parameter the OAuth/OIDC core specs never standardized — Keycloak's
+ * `kc_idp_hint`, OIDC's `login_hint` / `prompt` / `acr_values`, Auth0's
+ * `audience`. The Inspector never builds the authorize URL itself (the SDK's
+ * `auth()` does, and exposes no hook for extra parameters), so the merge
+ * happens at the one seam that sees the finished URL:
+ * `BaseOAuthClientProvider.redirectToAuthorization`.
+ *
+ * The parameters apply to the **authorization request only** — never the token
+ * request, which the SDK builds separately and which this module is not wired
+ * into. They also apply only to the server's **own** authorization server:
+ * the enterprise-managed (EMA) leg redirects to a different one (the enterprise
+ * IdP) and goes through `BaseOAuthClientProvider.redirectToExternalAuthorization`,
+ * which deliberately skips the merge.
+ */
+
+/**
+ * Parameters the OAuth flow owns and a user may not override.
+ *
+ * Overriding any of these does not produce a useful debugging scenario — it
+ * breaks PKCE (`code_challenge`, `code_challenge_method`), CSRF binding
+ * (`state`), the callback leg (`redirect_uri`, `response_type`, `client_id`),
+ * or RFC 8707 resource indicators (`resource`), and the failure surfaces as an
+ * opaque authorization-server error rather than as an Inspector problem.
+ * `scope` is excluded too: it has its own field, and the SEP-2350 step-up union
+ * is computed from that field rather than from these parameters.
+ */
+export const RESERVED_AUTHORIZATION_PARAMS = [
+  "client_id",
+  "code_challenge",
+  "code_challenge_method",
+  "redirect_uri",
+  "resource",
+  "response_type",
+  "scope",
+  "state",
+] as const;
+
+export type ReservedAuthorizationParam =
+  (typeof RESERVED_AUTHORIZATION_PARAMS)[number];
+
+const RESERVED: ReadonlySet<string> = new Set(RESERVED_AUTHORIZATION_PARAMS);
+
+/**
+ * Whether `key` names a parameter the OAuth flow owns. Compared case- and
+ * whitespace-insensitively: query parameter names are case-sensitive on the
+ * wire, but a `Client_Id` typed into the form is plainly an attempt to set the
+ * reserved one, and letting it through would be the silent override this rule
+ * exists to prevent.
+ */
+export function isReservedAuthorizationParam(key: string): boolean {
+  return RESERVED.has(key.trim().toLowerCase());
+}
+
+/**
+ * Validation message for one authorization-parameter key, or `undefined` when
+ * the key is acceptable. A blank key is not an error — the form lets a user
+ * leave a freshly-added row empty mid-edit, and blank rows are dropped rather
+ * than rejected.
+ */
+export function authorizationParamKeyError(key: string): string | undefined {
+  const trimmed = key.trim();
+  if (trimmed === "") return undefined;
+  if (isReservedAuthorizationParam(trimmed)) {
+    return `"${trimmed}" is set by the authorization flow and cannot be overridden.`;
+  }
+  return undefined;
+}
+
+/**
+ * Merge custom parameters into a finished authorization URL.
+ *
+ * Blank keys are skipped (a half-edited form row) and reserved keys are dropped
+ * with a warning — never silently, and never overriding what the SDK put there.
+ * Returns the original URL instance when there is nothing to apply, and
+ * otherwise a copy, so the caller's URL is never mutated.
+ */
+export function applyAuthorizationParams(
+  authorizationUrl: URL,
+  params: Record<string, string> | undefined,
+): URL {
+  if (!params) return authorizationUrl;
+
+  const applicable = Object.entries(params).filter(([key]) => {
+    if (key.trim() === "") return false;
+    if (isReservedAuthorizationParam(key)) {
+      console.warn(
+        `[oauth] Ignoring reserved authorization parameter "${key.trim()}" — it is set by the authorization flow.`,
+      );
+      return false;
+    }
+    return true;
+  });
+
+  if (applicable.length === 0) return authorizationUrl;
+
+  const merged = new URL(authorizationUrl.href);
+  for (const [key, value] of applicable) {
+    merged.searchParams.set(key.trim(), value);
+  }
+  return merged;
+}

core/auth/ema/transportProvider.ts

@@ -84,7 +84,11 @@ export class EmaTransportOAuthProvider implements OAuthClientProvider {
   async redirectToAuthorization(_authorizationUrl: URL): Promise<void> {
     const idpAuthorizationUrl = await startEmaIdpAuthorization(this.emaConfig);
     this.inner.clearCapturedAuthUrl();
-    this.inner.redirectToAuthorization(idpAuthorizationUrl);
+    // The IdP is a different authorization server than the one the inner
+    // provider was configured for, so the per-server custom authorization
+    // parameters (#2018) must not be appended here — hence the "external"
+    // entry point rather than `redirectToAuthorization`.
+    this.inner.redirectToExternalAuthorization(idpAuthorizationUrl);
   }
 
   saveCodeVerifier(codeVerifier: string): void | Promise<void> {

core/auth/index.ts

@@ -27,6 +27,15 @@ export { ensureCimdClientRegistration } from "./cimd.js";
 export { mcpAuth, type McpAuthOptions, type McpAuthResult } from "./mcpAuth.js";
 export { computeScopeUnion, isStrictScopeSuperset } from "./scopes.js";
 
+// Custom authorization-request parameters (#2018)
+export {
+  RESERVED_AUTHORIZATION_PARAMS,
+  isReservedAuthorizationParam,
+  authorizationParamKeyError,
+  applyAuthorizationParams,
+} from "./authorizationParams.js";
+export type { ReservedAuthorizationParam } from "./authorizationParams.js";
+
 // Storage
 export type {
   OAuthStorage,

core/auth/providers.ts

@@ -11,6 +11,7 @@ import type {
 } from "@modelcontextprotocol/client";
 import type { OAuthStorage, SaveClientInformationOptions } from "./storage.js";
 import { generateOAuthState } from "./utils.js";
+import { applyAuthorizationParams } from "./authorizationParams.js";
 
 /**
  * Redirect URL provider. Returns the redirect URL for OAuth flows.
@@ -96,6 +97,14 @@ export type OAuthProviderConfig = {
   redirectUrlProvider: RedirectUrlProvider;
   navigation: OAuthNavigation;
   clientMetadataUrl?: string;
+  /**
+   * Per-server custom authorization-request parameters (#2018), merged into the
+   * finished authorize URL in {@link BaseOAuthClientProvider.redirectToAuthorization}.
+   * Reserved (protocol-critical) keys are dropped with a warning — see
+   * `authorizationParams.ts`. Authorization request only; the token request is
+   * unaffected.
+   */
+  authorizationParams?: Record<string, string>;
 };
 
 /**
@@ -115,6 +124,8 @@ export class BaseOAuthClientProvider implements OAuthClientProvider {
   protected redirectUrlProvider: RedirectUrlProvider;
   protected navigation: OAuthNavigation;
   public clientMetadataUrl?: string;
+  /** Custom authorization-request parameters (#2018). Authorize URL only. */
+  protected authorizationParams?: Record<string, string>;
 
   constructor(serverUrl: string, oauthConfig: OAuthProviderConfig) {
     this.serverUrl = serverUrl;
@@ -122,6 +133,7 @@ export class BaseOAuthClientProvider implements OAuthClientProvider {
     this.redirectUrlProvider = oauthConfig.redirectUrlProvider;
     this.navigation = oauthConfig.navigation;
     this.clientMetadataUrl = oauthConfig.clientMetadataUrl;
+    this.authorizationParams = oauthConfig.authorizationParams;
   }
 
   /**
@@ -272,6 +284,34 @@ export class BaseOAuthClientProvider implements OAuthClientProvider {
   }
 
   redirectToAuthorization(authorizationUrl: URL): void {
+    // #2018: the SDK builds the authorize URL and offers no hook for extra
+    // parameters, so the per-server ones are merged here — the one seam that
+    // sees the finished URL before navigation. Everything downstream (the
+    // captured URL used by the step-up modal, the event, the navigation) uses
+    // the merged URL so all three agree on what was actually requested.
+    this.performAuthorizationRedirect(
+      applyAuthorizationParams(authorizationUrl, this.authorizationParams),
+    );
+  }
+
+  /**
+   * Redirect to an authorize URL belonging to a **different** authorization
+   * server than the one this provider was configured for, so the per-server
+   * custom parameters (#2018) MUST NOT be applied.
+   *
+   * The only caller is the enterprise-managed (EMA) transport provider, which
+   * discards the resource AS URL and sends the user to the enterprise IdP
+   * instead. Parameters a user configured for their MCP server's authorization
+   * server — a `kc_idp_hint`, an `audience` — are meaningless and potentially
+   * flow-breaking on the IdP's, and appending them there would leak per-server
+   * config across an authorization-server boundary.
+   */
+  redirectToExternalAuthorization(authorizationUrl: URL): void {
+    this.performAuthorizationRedirect(authorizationUrl);
+  }
+
+  /** Capture, announce, and navigate to a finished authorize URL. */
+  private performAuthorizationRedirect(authorizationUrl: URL): void {
     // Capture URL for return value
     this.capturedAuthUrl = authorizationUrl;
 

core/client/runner.ts

@@ -11,6 +11,7 @@ import type {
   InspectorClientOptions,
   InspectorServerSettings,
 } from "../mcp/types.js";
+import { oauthAuthorizationParamsFromSettings } from "../mcp/serverList.js";
 import { loadClientConfig } from "./config.js";
 import type { ClientConfig } from "./types.js";
 import {
@@ -70,11 +71,16 @@ export function buildRunnerClientAuthOptions(
   const activeIdp = getActiveEnterpriseManagedAuthIdp(clientConfig);
   const activeCimdUrl = getActiveCimdClientMetadataUrl(clientConfig);
 
+  const serverAuthorizationParams = savedSettings
+    ? oauthAuthorizationParamsFromSettings(savedSettings)
+    : undefined;
+
   const oauthFromServer =
     savedSettings &&
     (savedSettings.oauthClientId ||
       savedSettings.oauthClientSecret ||
       savedSettings.oauthScopes ||
+      serverAuthorizationParams ||
       savedSettings.enterpriseManaged)
       ? {
           ...(savedSettings.oauthClientId && {
@@ -86,6 +92,9 @@ export function buildRunnerClientAuthOptions(
           ...(savedSettings.oauthScopes && {
             scope: savedSettings.oauthScopes,
           }),
+          ...(serverAuthorizationParams && {
+            authorizationParams: serverAuthorizationParams,
+          }),
           ...(savedSettings.enterpriseManaged && {
             enterpriseManaged: true,
           }),

core/mcp/inspectorClient.ts

@@ -5436,6 +5436,8 @@ export class InspectorClient extends InspectorClientEventTarget {
     clientSecret?: string;
     clientMetadataUrl?: string;
     scope?: string;
+    /** Custom authorization-request parameters (#2018). Authorize URL only. */
+    authorizationParams?: Record<string, string>;
   }): void {
     if (!this.oauthManager) {
       throw new Error(