diff --git a/AGENTS.md b/AGENTS.md
index 08944062d..4c00b29f4 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -39,7 +39,9 @@ inspector/
│ ├── json/ # JSON utilities and parameter/argument conversion
│ │ # (xMcpHeader.ts: SEP-2243 `x-mcp-header`
│ │ # annotation scan/validation + mirrored-param
-│ │ # derivation, used by the Tools tab — #1632)
+│ │ # derivation, used by the Tools tab — #1632;
+│ │ # plus `Mcp-Param-*` header building for the
+│ │ # wire, used by both `tools/call` paths — #1846)
│ ├── logging/ # Silent pino logger singleton
│ ├── mcp/ # InspectorClient runtime + state stores
│ │ # (modernTaskSchemas.ts: SEP-2663 modern Tasks
diff --git a/README.md b/README.md
index 058fb77ea..cf034ba32 100644
--- a/README.md
+++ b/README.md
@@ -113,7 +113,7 @@ The Vite alias `@modelcontextprotocol/inspector-test-server` (in `clients/web/vi
A streamable-HTTP server can also serve the **modern (2026-07-28) protocol era** via the SDK's `createMcpHandler` — set `transport.modern` in the JSON config (`true` for dual-era stateless serving, or `{ "legacy": "reject" }` for modern-only strict), or pass `modern` on the `ServerConfig` for an in-process `createTestServerHttp`. This is what lets an Inspector connection negotiating `protocolEra: "auto" | "modern"` reach the modern leg (populated `server/discover`, sessionless). See `test-servers/configs/modern-http.json`. `test-servers/configs/modern-mrtr-http.json` additionally serves the `mrtr_confirm` tool (preset `mrtr_confirm`, `createMrtrTool`) over the modern leg: its handler returns `inputRequired(...)` embedding a form elicitation, so invoking it produces a real MRTR round-trip (`input_required` → the client fulfils the embedded elicitation and retries with a new id → `complete`). The Inspector drives MRTR manually (`inputRequired: { autoFulfill: false }`), so the embedded elicitation pauses at the pending-request modal (tagged "input_required") for you to answer, then the retry completes — useful for eyeballing both that pending-request UX and the Protocol view's MRTR conversation grouping. `test-servers/configs/mrtr-showcase-http.json` bundles every MRTR preset in one modern server for manual testing: `mrtr_confirm` (single round), `mrtr_two_step` (two elicitation rounds via `requestState`), `mrtr_sample` (embedded sampling → the Sampling panel), `mrtr_roots` (embedded `roots/list`, auto-answered silently from configured roots — no modal), `mrtr_edge` (an `inputRequests`-only round then a `requestState`-only round), and `mrtr_loop` (never completes → the `MRTR_MAX_ROUNDS` bound trips). (The legacy `collect_elicitation` preset calls `server.elicitInput`, which errors on the 2026-07-28 leg — server→client requests aren't allowed there; MRTR is the modern replacement.)
-`test-servers/configs/modern-network-http.json` is the **Network-tab showcase** for the standardized HTTP headers and new error taxonomy (SEP-2243 / SEP-2575). It serves a `get_weather` tool whose `city` argument carries an `x-mcp-header: "City"` annotation (so a modern client mirrors it to `Mcp-Param-City`), plus four `trigger_*` tools that the modern leg's spec-error injector (`transport.modern.injectSpecErrors: true`) answers with a real HTTP status + JSON-RPC error body: `trigger_header_mismatch` → `400 / -32020`, `trigger_missing_capability` → `400 / -32021`, `trigger_unsupported_version` → `400 / -32022` (with `data.supported`), `trigger_method_not_found` → `404 / -32601`. Connect to it with **Protocol Era = Modern** and open the Network tab to see the mirrored `Mcp-*` headers highlighted, sentinel values decoded, and each error rendered distinctly. Note: `Mcp-Param-*` mirroring is **skipped by the SDK in the browser** (`detectProbeEnvironment() !== "browser"`), so calling `get_weather` from the **web** client omits `Mcp-Param-City` and the strict server answers `-32020` — the same tool is callable from the Node CLI/TUI, where mirroring is active.
+`test-servers/configs/modern-network-http.json` is the **Network-tab showcase** for the standardized HTTP headers and new error taxonomy (SEP-2243 / SEP-2575). It serves a `get_weather` tool whose `city` argument carries an `x-mcp-header: "City"` annotation (so a modern client mirrors it to `Mcp-Param-City`), plus four `trigger_*` tools that the modern leg's spec-error injector (`transport.modern.injectSpecErrors: true`) answers with a real HTTP status + JSON-RPC error body: `trigger_header_mismatch` → `400 / -32020`, `trigger_missing_capability` → `400 / -32021`, `trigger_unsupported_version` → `400 / -32022` (with `data.supported`), `trigger_method_not_found` → `404 / -32601`. Connect to it with **Protocol Era = Modern** and open the Network tab to see the mirrored `Mcp-*` headers highlighted, sentinel values decoded, and each error rendered distinctly. Note: the SDK only mirrors `Mcp-Param-*` inside `client.callTool()`, and skips it in the browser (`detectProbeEnvironment() !== "browser"`). The Inspector routes `tools/call` through `client.request()` to drive MRTR manually, so it builds the mirrored headers itself (#1846) — on **every** client, web included, since the web client's upstream request is issued by the Node backend rather than the browser. So `get_weather` is callable from web, CLI, and TUI alike, in both the plain and "Run as task" forms.
`test-servers/configs/xmcpheader-modern-http.json` is the **`x-mcp-header` Tools-tab showcase** (#1632). It serves `echo`, a `get_weather` tool with a **valid** `x-mcp-header: "City"` annotation on its `city` argument, an `invalid_header_tool` whose annotation uses the header name `"Bad Header"` (a space makes it an invalid RFC 9110 token, so the whole tool definition is invalid), and a `trigger_invalid_params` tool that the modern leg's spec-error injector (`transport.modern.injectSpecErrors: true`) answers with a real `-32602 Invalid params` JSON-RPC error whose message is not about a missing tool. Connect with **Protocol Era = Modern** and open the Tools tab: `get_weather`'s detail panel shows a **"Mirrored request headers (SEP-2243)"** section (`city → Mcp-Param-City`), and `invalid_header_tool` appears struck-through under an **"Excluded (SEP-2243)"** divider in the sidebar with the reason on hover (a conforming Streamable HTTP client MUST drop it from `tools/list`; the Inspector surfaces _why_). Under SDK v2 a `tools/call` that rejects with **`-32602`** now renders as a distinct error panel rather than an `isError` result — headed **"Unknown Tool"** when the message names a missing tool (reproduce by calling a tool the server dropped from its list), or **"Invalid Parameters"** for any other `-32602` (run `trigger_invalid_params`).
diff --git a/clients/web/src/components/groups/ToolDetailPanel/ToolDetailPanel.tsx b/clients/web/src/components/groups/ToolDetailPanel/ToolDetailPanel.tsx
index 64b0ef12a..a74c63a96 100644
--- a/clients/web/src/components/groups/ToolDetailPanel/ToolDetailPanel.tsx
+++ b/clients/web/src/components/groups/ToolDetailPanel/ToolDetailPanel.tsx
@@ -306,8 +306,8 @@ export function ToolDetailPanel({
))}
These argument values are mirrored into HTTP headers on the
- call. The SDK sends them only on a Node/proxy transport — the
- browser omits Mcp-Param-* headers.
+ call. In the web client the Mcp-Param-* headers are
+ applied by the Node backend that issues the upstream request.
)}
diff --git a/clients/web/src/test/core/xMcpHeader.test.ts b/clients/web/src/test/core/xMcpHeader.test.ts
index 0f043da05..07235c892 100644
--- a/clients/web/src/test/core/xMcpHeader.test.ts
+++ b/clients/web/src/test/core/xMcpHeader.test.ts
@@ -2,6 +2,8 @@ import { describe, it, expect } from "vitest";
import {
scanXMcpHeaderDeclarations,
getMirroredHeaderParams,
+ buildMcpParamHeaders,
+ mcpParamHeadersForTool,
MCP_PARAM_HEADER_PREFIX,
X_MCP_HEADER_KEY,
} from "@inspector/core/json/xMcpHeader.js";
@@ -279,3 +281,153 @@ describe("getMirroredHeaderParams", () => {
).toEqual([]);
});
});
+
+function declsFor(inputSchema: Tool["inputSchema"]) {
+ const scan = scanXMcpHeaderDeclarations(inputSchema);
+ if (!scan.valid) throw new Error(`expected valid scan: ${scan.reason}`);
+ return scan.declarations;
+}
+
+const P = MCP_PARAM_HEADER_PREFIX;
+
+function decodeSentinel(value: string): string {
+ const inner = value.slice("=?base64?".length, -"?=".length);
+ const bin = atob(inner);
+ const bytes = Uint8Array.from(bin, (ch) => ch.codePointAt(0) ?? 0);
+ return new TextDecoder().decode(bytes);
+}
+
+describe("buildMcpParamHeaders", () => {
+ const decls = declsFor({
+ type: "object",
+ properties: {
+ owner: { type: "string", [X_MCP_HEADER_KEY]: "owner" },
+ count: { type: "integer", [X_MCP_HEADER_KEY]: "Count" },
+ flag: { type: "boolean", [X_MCP_HEADER_KEY]: "Flag" },
+ ratio: { type: "number", [X_MCP_HEADER_KEY]: "Ratio" },
+ },
+ });
+
+ it("mirrors a string argument verbatim into Mcp-Param-{Name}", () => {
+ expect(buildMcpParamHeaders(decls, { owner: "octocat" })).toEqual({
+ [`${P}owner`]: "octocat",
+ });
+ });
+
+ it("stringifies boolean and numeric values per the spec", () => {
+ expect(
+ buildMcpParamHeaders(decls, {
+ count: 42,
+ flag: false,
+ ratio: 3.5,
+ }),
+ ).toEqual({
+ [`${P}Count`]: "42",
+ [`${P}Flag`]: "false",
+ [`${P}Ratio`]: "3.5",
+ });
+ expect(buildMcpParamHeaders(decls, { flag: true })).toEqual({
+ [`${P}Flag`]: "true",
+ });
+ });
+
+ it("omits declarations whose value is absent or null", () => {
+ expect(buildMcpParamHeaders(decls, { owner: null })).toEqual({});
+ expect(buildMcpParamHeaders(decls, {})).toEqual({});
+ });
+
+ it("omits non-primitive values rather than emitting malformed headers", () => {
+ expect(
+ buildMcpParamHeaders(decls, {
+ owner: { nested: true },
+ count: [1, 2],
+ }),
+ ).toEqual({});
+ });
+
+ it("omits non-finite numbers and unsafe integers", () => {
+ expect(buildMcpParamHeaders(decls, { ratio: Infinity })).toEqual({});
+ expect(buildMcpParamHeaders(decls, { ratio: NaN })).toEqual({});
+ expect(
+ buildMcpParamHeaders(decls, { count: Number.MAX_SAFE_INTEGER + 2 }),
+ ).toEqual({});
+ });
+
+ it("base64-wraps values that are not safe plain-ASCII field values", () => {
+ const out = buildMcpParamHeaders(decls, { owner: "münchen" });
+ const encoded = out[`${P}owner`];
+ expect(encoded.startsWith("=?base64?")).toBe(true);
+ expect(encoded.endsWith("?=")).toBe(true);
+ expect(decodeSentinel(encoded)).toBe("münchen");
+ });
+
+ it("base64-wraps empty, whitespace-padded, and sentinel-colliding values", () => {
+ const empty = buildMcpParamHeaders(decls, { owner: "" })[`${P}owner`];
+ expect(decodeSentinel(empty)).toBe("");
+ const padded = buildMcpParamHeaders(decls, { owner: " x " })[`${P}owner`];
+ expect(decodeSentinel(padded)).toBe(" x ");
+ const collide = "=?base64?zzz?=";
+ const wrapped = buildMcpParamHeaders(decls, { owner: collide })[
+ `${P}owner`
+ ];
+ expect(wrapped).not.toBe(collide);
+ expect(decodeSentinel(wrapped)).toBe(collide);
+ });
+
+ it("reads a nested property path", () => {
+ const nested = declsFor({
+ type: "object",
+ properties: {
+ filter: {
+ type: "object",
+ properties: {
+ city: { type: "string", [X_MCP_HEADER_KEY]: "City" },
+ },
+ },
+ },
+ });
+ expect(
+ buildMcpParamHeaders(nested, { filter: { city: "London" } }),
+ ).toEqual({ [`${P}City`]: "London" });
+ expect(buildMcpParamHeaders(nested, { filter: "not-an-object" })).toEqual(
+ {},
+ );
+ });
+});
+
+describe("mcpParamHeadersForTool", () => {
+ it("builds headers for a tool's valid annotations", () => {
+ expect(
+ mcpParamHeadersForTool(
+ tool({
+ type: "object",
+ properties: {
+ owner: { type: "string", [X_MCP_HEADER_KEY]: "owner" },
+ },
+ }),
+ { owner: "octocat" },
+ ),
+ ).toEqual({ [`${P}owner`]: "octocat" });
+ });
+
+ it("returns {} for a tool with no annotations", () => {
+ expect(
+ mcpParamHeadersForTool(
+ tool({ type: "object", properties: { a: { type: "string" } } }),
+ { a: "x" },
+ ),
+ ).toEqual({});
+ });
+
+ it("returns {} for a tool whose annotations are invalid", () => {
+ expect(
+ mcpParamHeadersForTool(
+ tool({
+ type: "object",
+ properties: { a: { type: "object", [X_MCP_HEADER_KEY]: "A" } },
+ }),
+ { a: "x" },
+ ),
+ ).toEqual({});
+ });
+});
diff --git a/clients/web/src/test/integration/mcp/inspectorClient-xmcpheader-mirroring.test.ts b/clients/web/src/test/integration/mcp/inspectorClient-xmcpheader-mirroring.test.ts
new file mode 100644
index 000000000..fadb29877
--- /dev/null
+++ b/clients/web/src/test/integration/mcp/inspectorClient-xmcpheader-mirroring.test.ts
@@ -0,0 +1,184 @@
+import { describe, it, expect, afterEach } from "vitest";
+import { InspectorClient } from "@inspector/core/mcp/inspectorClient.js";
+import { createTransportNode } from "@inspector/core/mcp/node/transport.js";
+import { eraToVersionNegotiation } from "@inspector/core/mcp/types.js";
+import {
+ createTestServerHttp,
+ type TestServerHttp,
+ createTestServerInfo,
+ createEchoTool,
+ createGetWeatherTool,
+} from "@modelcontextprotocol/inspector-test-server";
+import type { Tool } from "@modelcontextprotocol/client";
+
+/**
+ * SEP-2243 `x-mcp-header` → `Mcp-Param-*` mirroring on `tools/call` (#1846).
+ *
+ * The SDK only mirrors inside `client.callTool()` (and skips it in a browser
+ * environment); the Inspector routes `tools/call` through `client.request()`
+ * for manual MRTR driving (#1704), so it mirrors the headers itself. A strict
+ * modern server (e.g. GitHub's) rejects a call whose annotated argument isn't
+ * mirrored, so this must ride the wire. Here we spy on the transport `fetch` and
+ * assert the `tools/call` POST carries the mirrored header with the spec's
+ * value encoding.
+ */
+describe("x-mcp-header Mcp-Param-* mirroring on tools/call", () => {
+ let client: InspectorClient | null = null;
+ let server: TestServerHttp | null = null;
+
+ afterEach(async () => {
+ if (client) {
+ try {
+ await client.disconnect();
+ } catch {
+ // Ignore disconnect errors
+ }
+ client = null;
+ }
+ if (server) {
+ try {
+ await server.stop();
+ } catch {
+ // Ignore server stop errors
+ }
+ server = null;
+ }
+ });
+
+ /** Records the request headers of every `tools/call` POST the client sends. */
+ function makeSpyFetch(): {
+ fetch: typeof fetch;
+ toolCallHeaders: Headers[];
+ } {
+ const toolCallHeaders: Headers[] = [];
+ const spy: typeof fetch = async (input, init) => {
+ const body = init?.body;
+ if (typeof body === "string" && body.includes('"tools/call"')) {
+ try {
+ const parsed = JSON.parse(body) as { method?: string };
+ if (parsed.method === "tools/call") {
+ toolCallHeaders.push(new Headers(init?.headers));
+ }
+ } catch {
+ // Non-JSON body — ignore.
+ }
+ }
+ return fetch(input, init);
+ };
+ return { fetch: spy, toolCallHeaders };
+ }
+
+ async function connectModern(
+ url: string,
+ fetchFn: typeof fetch,
+ ): Promise {
+ const connected = new InspectorClient(
+ { type: "streamable-http", url },
+ {
+ environment: { transport: createTransportNode, fetch: fetchFn },
+ versionNegotiation: eraToVersionNegotiation("auto"),
+ },
+ );
+ await connected.connect();
+ client = connected;
+ return connected;
+ }
+
+ async function startWeatherServer(): Promise {
+ const started = createTestServerHttp({
+ serverInfo: createTestServerInfo("xmcpheader-test", "1.0.0"),
+ tools: [createEchoTool(), createGetWeatherTool()],
+ modern: {},
+ });
+ await started.start();
+ server = started;
+ return started;
+ }
+
+ async function weatherTool(c: InspectorClient): Promise {
+ const { tools } = await c.listTools();
+ const weather = tools.find((t) => t.name === "get_weather");
+ expect(weather).toBeDefined();
+ return weather!;
+ }
+
+ it("mirrors an annotated argument into the Mcp-Param-* request header", async () => {
+ const started = await startWeatherServer();
+ const spy = makeSpyFetch();
+ const connected = await connectModern(started.url, spy.fetch);
+ expect(connected.getProtocolEra()).toBe("modern");
+
+ const result = await connected.callTool(await weatherTool(connected), {
+ city: "London",
+ });
+
+ expect(result.success).toBe(true);
+ expect(spy.toolCallHeaders.length).toBeGreaterThan(0);
+ const sent = spy.toolCallHeaders.at(-1)!;
+ expect(sent.get("Mcp-Param-City")).toBe("London");
+ });
+
+ it("mirrors on the task-augmented tools/call too (callToolStream)", async () => {
+ // The "Run as task" path builds its own request options, so it needs the
+ // same mirroring — a strict modern server rejects the task-augmented
+ // `tools/call` with -32020 when the header is missing, exactly as it does
+ // the plain one.
+ const started = await startWeatherServer();
+ const spy = makeSpyFetch();
+ const connected = await connectModern(started.url, spy.fetch);
+
+ const result = await connected.callToolStream(
+ await weatherTool(connected),
+ { city: "London" },
+ undefined,
+ undefined,
+ { ttl: 60_000 },
+ );
+
+ expect(result.success).toBe(true);
+ expect(spy.toolCallHeaders.at(-1)!.get("Mcp-Param-City")).toBe("London");
+ });
+
+ it("sends no Mcp-Param-* header for a tool without annotations", async () => {
+ const started = await startWeatherServer();
+ const spy = makeSpyFetch();
+ const connected = await connectModern(started.url, spy.fetch);
+
+ const { tools } = await connected.listTools();
+ const echo = tools.find((t) => t.name === "echo")!;
+ await connected.callTool(echo, { message: "hi" });
+
+ const sent = spy.toolCallHeaders.at(-1)!;
+ let sawMcpParam = false;
+ sent.forEach((_v, k) => {
+ if (k.toLowerCase().startsWith("mcp-param-")) sawMcpParam = true;
+ });
+ expect(sawMcpParam).toBe(false);
+ });
+
+ it("does not mirror on a legacy connection", async () => {
+ const started = createTestServerHttp({
+ serverInfo: createTestServerInfo("xmcpheader-legacy", "1.0.0"),
+ tools: [createGetWeatherTool()],
+ modern: { legacy: "stateless" },
+ });
+ await started.start();
+ server = started;
+
+ const spy = makeSpyFetch();
+ const connected = new InspectorClient(
+ { type: "streamable-http", url: started.url },
+ {
+ environment: { transport: createTransportNode, fetch: spy.fetch },
+ versionNegotiation: eraToVersionNegotiation("legacy"),
+ },
+ );
+ await connected.connect();
+ client = connected;
+ expect(connected.getProtocolEra()).toBe("legacy");
+
+ await connected.callTool(await weatherTool(connected), { city: "London" });
+ const sent = spy.toolCallHeaders.at(-1)!;
+ expect(sent.get("Mcp-Param-City")).toBeNull();
+ });
+});
diff --git a/clients/web/src/test/integration/mcp/remote/remoteClientTransport-unit.test.ts b/clients/web/src/test/integration/mcp/remote/remoteClientTransport-unit.test.ts
index 05b8346d6..fddc10c9d 100644
--- a/clients/web/src/test/integration/mcp/remote/remoteClientTransport-unit.test.ts
+++ b/clients/web/src/test/integration/mcp/remote/remoteClientTransport-unit.test.ts
@@ -642,6 +642,58 @@ describe("RemoteClientTransport (focused branch coverage)", () => {
await t.close();
});
+ it("forwards per-send headers (SEP-2243 Mcp-Param-*) in the send body", async () => {
+ let sentBody: { headers?: Record } | undefined;
+ const encoder = new TextEncoder();
+ let sseController: ReadableStreamDefaultController | null =
+ null;
+ const pushSseMessage = (message: JSONRPCMessage) => {
+ const payload = JSON.stringify({ type: "message", data: message });
+ sseController?.enqueue(encoder.encode(`data: ${payload}\n\n`));
+ };
+ const fetchFn = vi.fn(
+ async (input: RequestInfo | URL, init?: RequestInit) => {
+ const url = typeof input === "string" ? input : input.toString();
+ if (url.includes("/connect")) return jsonResponse({ sessionId: "s" });
+ if (url.includes("/events")) {
+ return new Response(
+ new ReadableStream({
+ start(controller) {
+ sseController = controller;
+ controller.enqueue(encoder.encode(": keepalive\n\n"));
+ },
+ }),
+ { status: 200 },
+ );
+ }
+ if (url.includes("/send")) {
+ sentBody = JSON.parse(init!.body as string);
+ const requestId = (
+ sentBody as { message: { id?: string | number } }
+ ).message.id;
+ pushSseMessage({ jsonrpc: "2.0", id: requestId!, result: {} });
+ return jsonResponse({ ok: true });
+ }
+ return jsonResponse({ ok: true });
+ },
+ );
+ const t = new RemoteClientTransport(
+ {
+ baseUrl: "http://remote.test",
+ fetchFn: fetchFn as unknown as typeof fetch,
+ sseResponseTimeoutMs: 2000,
+ },
+ CONFIG,
+ );
+ await t.start();
+ await t.send(
+ { jsonrpc: "2.0", id: 7, method: "tools/call" },
+ { headers: { "Mcp-Param-City": "London" } },
+ );
+ expect(sentBody?.headers).toEqual({ "Mcp-Param-City": "London" });
+ await t.close();
+ });
+
it("throws Remote send failed with status on non-OK send", async () => {
const t = makeTransport({
events: () =>
diff --git a/clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts b/clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts
index 04e55ed20..613d4c6a7 100644
--- a/clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts
+++ b/clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts
@@ -22,6 +22,7 @@ import type pinoType from "pino";
import {
createRemoteApp,
requestIdForSendWait,
+ mcpParamHeadersOnly,
} from "@inspector/core/mcp/remote/node/server.js";
import {
InMemorySecretStore,
@@ -139,6 +140,43 @@ describe("server.ts supplemental coverage", () => {
});
});
+ describe("mcpParamHeadersOnly (SEP-2243 upstream header allowlist, #1846)", () => {
+ it("keeps only Mcp-Param-* headers (case-insensitive)", () => {
+ expect(
+ mcpParamHeadersOnly({
+ "Mcp-Param-City": "London",
+ "mcp-param-owner": "octocat",
+ }),
+ ).toEqual({ "Mcp-Param-City": "London", "mcp-param-owner": "octocat" });
+ });
+
+ it("drops non-Mcp-Param headers a client tries to inject", () => {
+ expect(
+ mcpParamHeadersOnly({
+ Authorization: "Bearer evil",
+ "X-Custom": "nope",
+ "Mcp-Param-City": "London",
+ }),
+ ).toEqual({ "Mcp-Param-City": "London" });
+ });
+
+ it("drops non-string values", () => {
+ expect(
+ mcpParamHeadersOnly({
+ "Mcp-Param-Bad": 5,
+ "Mcp-Param-City": "London",
+ }),
+ ).toEqual({ "Mcp-Param-City": "London" });
+ });
+
+ it("returns undefined when nothing survives the filter", () => {
+ expect(mcpParamHeadersOnly({ Authorization: "x" })).toBeUndefined();
+ expect(mcpParamHeadersOnly(undefined)).toBeUndefined();
+ expect(mcpParamHeadersOnly(null)).toBeUndefined();
+ expect(mcpParamHeadersOnly("not-an-object")).toBeUndefined();
+ });
+ });
+
describe("/api/log forwardLogEvent shapes", () => {
let h: Harness;
let records: Array<{ level: string; args: unknown[] }>;
diff --git a/clients/web/src/test/integration/mcp/remote/transport.test.ts b/clients/web/src/test/integration/mcp/remote/transport.test.ts
index 35f9ec70e..d66ae36a5 100644
--- a/clients/web/src/test/integration/mcp/remote/transport.test.ts
+++ b/clients/web/src/test/integration/mcp/remote/transport.test.ts
@@ -30,9 +30,13 @@ import {
createTestServerHttp,
getTestMcpServerCommand,
createEchoTool,
+ createGetWeatherTool,
createTestServerInfo,
} from "@modelcontextprotocol/inspector-test-server";
-import type { MCPServerConfig } from "@inspector/core/mcp/types.js";
+import {
+ eraToVersionNegotiation,
+ type MCPServerConfig,
+} from "@inspector/core/mcp/types.js";
interface StartRemoteServerOptions {
logger?: pino.Logger;
@@ -447,6 +451,70 @@ describe("Remote transport e2e", () => {
await client.disconnect();
}
});
+
+ it("end-to-end: SEP-2243 Mcp-Param-* mirroring reaches the upstream modern server (#1846)", async () => {
+ // A modern (2026-07-28) server whose `get_weather` tool annotates `city`
+ // with `x-mcp-header: "City"`. The SDK's modern handler validates the
+ // mirrored header and rejects the call with -32020 when it is missing, so
+ // a *successful* call proves the header rode browser → backend → upstream.
+ mcpHttpServer = createTestServerHttp({
+ serverInfo: createTestServerInfo(),
+ tools: [createGetWeatherTool()],
+ serverType: "streamable-http",
+ modern: {},
+ });
+ await mcpHttpServer.start();
+
+ const { baseUrl, server, authToken } = await startRemoteServer(0);
+ remoteServer = server;
+
+ const config: MCPServerConfig = {
+ type: "streamable-http",
+ url: mcpHttpServer.url,
+ };
+ const createTransport = createRemoteTransport({ baseUrl, authToken });
+ const client = new InspectorClient(config, {
+ environment: { transport: createTransport },
+ versionNegotiation: eraToVersionNegotiation("auto"),
+ });
+ const fetchRequestLogState = new FetchRequestLogState(client);
+
+ try {
+ await client.connect();
+ expect(client.getProtocolEra()).toBe("modern");
+
+ const { tools } = await client.listTools();
+ const weather = tools.find((t) => t.name === "get_weather");
+ expect(weather).toBeDefined();
+
+ const invocation = await client.callTool(weather!, { city: "London" });
+ // Success only happens if the strict modern server saw Mcp-Param-City.
+ expect(invocation.success).toBe(true);
+
+ // And the backend's upstream fetch tracking shows the mirrored header on
+ // the wire it sent to the modern server.
+ const toolCallPost = fetchRequestLogState
+ .getFetchRequests()
+ .filter((r) => r.method === "POST")
+ .find((r) => {
+ const lowered: Record = {};
+ for (const [k, v] of Object.entries(r.requestHeaders ?? {})) {
+ lowered[k.toLowerCase()] = v;
+ }
+ return lowered["mcp-param-city"] !== undefined;
+ });
+ expect(toolCallPost).toBeDefined();
+ const lowered: Record = {};
+ for (const [k, v] of Object.entries(
+ toolCallPost?.requestHeaders ?? {},
+ )) {
+ lowered[k.toLowerCase()] = v;
+ }
+ expect(lowered["mcp-param-city"]).toBe("London");
+ } finally {
+ await client.disconnect();
+ }
+ });
});
describe("authentication", () => {
diff --git a/core/json/xMcpHeader.ts b/core/json/xMcpHeader.ts
index 97000dd83..739eb6a2b 100644
--- a/core/json/xMcpHeader.ts
+++ b/core/json/xMcpHeader.ts
@@ -194,6 +194,125 @@ export function scanXMcpHeaderDeclarations(
: { valid: false, reason: fault };
}
+/**
+ * The `=?base64?…?=` sentinel wrapping a value that cannot be sent as a plain
+ * ASCII HTTP field value (SEP-2243 value-encoding rules).
+ */
+const BASE64_SENTINEL_PREFIX = "=?base64?";
+const BASE64_SENTINEL_SUFFIX = "?=";
+
+/**
+ * Convert a primitive argument value to its string form per the spec's
+ * type-conversion rules: strings pass through, booleans become lowercase
+ * `'true'`/`'false'`, integers/numbers become their decimal string. Non-finite
+ * numbers and integers outside the safe range are refused (returns `undefined`,
+ * meaning "do not emit a header for this value"). Anything non-primitive
+ * (object/array/null/undefined) also yields `undefined`.
+ */
+function mcpParamPrimitiveToString(value: unknown): string | undefined {
+ if (typeof value === "string") return value;
+ if (typeof value === "boolean") return value ? "true" : "false";
+ if (typeof value === "number") {
+ if (!Number.isFinite(value)) return undefined;
+ if (Number.isInteger(value) && !Number.isSafeInteger(value)) {
+ return undefined;
+ }
+ return String(value);
+ }
+ return undefined;
+}
+
+/**
+ * `true` when `s` cannot be safely represented as a plain ASCII HTTP field
+ * value (RFC 9110 §5.5): it is empty, contains a byte outside `0x20–0x7E`/`0x09`,
+ * has leading/trailing whitespace (which field parsing strips), or already
+ * matches the Base64 sentinel pattern (the spec's "to avoid ambiguity" rule).
+ */
+function needsBase64(s: string): boolean {
+ if (s.length === 0) return true;
+ if (
+ s.startsWith(BASE64_SENTINEL_PREFIX) &&
+ s.endsWith(BASE64_SENTINEL_SUFFIX)
+ ) {
+ return true;
+ }
+ if (s !== s.trim()) return true;
+ for (let i = 0; i < s.length; i++) {
+ // Non-null: `i` is always in bounds, so `codePointAt` returns a number.
+ const c = s.codePointAt(i)!;
+ if (c === 9 || (c >= 32 && c <= 126)) continue;
+ return true;
+ }
+ return false;
+}
+
+function utf8ToBase64(s: string): string {
+ const bytes = new TextEncoder().encode(s);
+ let bin = "";
+ for (const b of bytes) bin += String.fromCodePoint(b);
+ return btoa(bin);
+}
+
+/**
+ * Encode a string value as an HTTP field value per SEP-2243: a value that is
+ * already a safe plain-ASCII field value passes through unchanged; anything
+ * else is wrapped as `=?base64?{b64-of-utf8}?=`.
+ */
+function encodeMcpParamValue(value: string): string {
+ return needsBase64(value)
+ ? `${BASE64_SENTINEL_PREFIX}${utf8ToBase64(value)}${BASE64_SENTINEL_SUFFIX}`
+ : value;
+}
+
+function valueAtPath(root: unknown, path: string[]): unknown {
+ let node: unknown = root;
+ for (const key of path) {
+ if (node === null || typeof node !== "object") return undefined;
+ node = (node as Record)[key];
+ }
+ return node;
+}
+
+/**
+ * Build the `Mcp-Param-{Name}` headers for one `tools/call` from validated
+ * `x-mcp-header` declarations and the call's `arguments`. A declaration whose
+ * value is `null` or absent is omitted (the spec's "client MUST omit the header"
+ * rows); a value that is not a primitive of the declared kind is omitted rather
+ * than emitted malformed. Faithful port of the SDK's internal helper (not part
+ * of its public surface), so the headers match what a conforming client sends.
+ */
+export function buildMcpParamHeaders(
+ declarations: XMcpHeaderDeclaration[],
+ args: Record,
+): Record {
+ const out: Record = {};
+ for (const decl of declarations) {
+ const raw = valueAtPath(args, decl.path);
+ if (raw === undefined || raw === null) continue;
+ const stringValue = mcpParamPrimitiveToString(raw);
+ if (stringValue === undefined) continue;
+ out[`${MCP_PARAM_HEADER_PREFIX}${decl.headerName}`] =
+ encodeMcpParamValue(stringValue);
+ }
+ return out;
+}
+
+/**
+ * SEP-2243 `Mcp-Param-*` headers a `tools/call` must carry for a given tool and
+ * arguments. Returns `{}` when the tool declares no `x-mcp-header`, when its
+ * annotations are invalid (such a tool is excluded from `tools/list`), or when
+ * no declared argument has a mirrorable value. Callers attach the result to the
+ * `tools/call` request headers on a modern connection.
+ */
+export function mcpParamHeadersForTool(
+ tool: Tool,
+ args: Record,
+): Record {
+ const scan = scanXMcpHeaderDeclarations(tool.inputSchema);
+ if (!scan.valid || scan.declarations.length === 0) return {};
+ return buildMcpParamHeaders(scan.declarations, args);
+}
+
/** A tool the Inspector keeps, paired with its mirrored-header declarations. */
export interface MirroredHeaderParam {
/** Dot-joined property path (e.g. `region` or `filter.city`). */
diff --git a/core/mcp/inspectorClient.ts b/core/mcp/inspectorClient.ts
index 527219ad9..b53beb842 100644
--- a/core/mcp/inspectorClient.ts
+++ b/core/mcp/inspectorClient.ts
@@ -18,7 +18,10 @@ import type {
ResourceSubscriptionStreamState,
ExcludedTool,
} from "./types.js";
-import { scanXMcpHeaderDeclarations } from "../json/xMcpHeader.js";
+import {
+ scanXMcpHeaderDeclarations,
+ mcpParamHeadersForTool,
+} from "../json/xMcpHeader.js";
// Re-export so v1.5 tests that do `import { InspectorClientOptions } from
// "@inspector/core/mcp/inspectorClient.js"` keep resolving.
export type {
@@ -3304,6 +3307,52 @@ export class InspectorClient extends InspectorClientEventTarget {
}
}
+ /**
+ * Coerce the string-valued entries of a tool's arguments to the types its
+ * `inputSchema` declares (the Tools form hands everything over as text).
+ * Shared by the two `tools/call` entry points — {@link attemptToolCall} and
+ * {@link callToolStream} — so both put the SAME arguments on the wire.
+ */
+ private convertStringToolArgs(
+ tool: Tool,
+ args: Record,
+ ): Record {
+ const stringArgs: Record = {};
+ for (const [key, value] of Object.entries(args)) {
+ if (typeof value === "string") {
+ stringArgs[key] = value;
+ }
+ }
+ if (Object.keys(stringArgs).length === 0) return args;
+ return { ...args, ...convertToolParameters(tool, stringArgs) };
+ }
+
+ /**
+ * SEP-2243: mirror `x-mcp-header`-annotated arguments into `Mcp-Param-*`
+ * headers on a modern connection. The SDK only does this inside
+ * `client.callTool()` (and skips it in the browser), but we route
+ * `tools/call` through `client.request()` for manual MRTR driving (#1704), so
+ * we mirror ourselves. `Protocol.request` forwards `headers` (preserved
+ * across MRTR retry legs) to the transport, and the remote transport relays
+ * them to the backend's upstream send — issued server-side, where the browser
+ * skip doesn't apply. No-op on legacy/stdio (no annotations).
+ *
+ * Applied by BOTH `tools/call` entry points: a plain call
+ * ({@link attemptToolCall}) and a task-augmented one
+ * ({@link callToolStream}) — a strict modern server rejects either with
+ * `-32020` when the mirrored header is missing.
+ */
+ private applyMirroredParamHeaders(
+ tool: Tool,
+ convertedArgs: Record,
+ requestOptions: RequestOptions,
+ ): void {
+ if (this.protocolEra !== "modern") return;
+ const paramHeaders = mcpParamHeadersForTool(tool, convertedArgs);
+ if (Object.keys(paramHeaders).length === 0) return;
+ requestOptions.headers = { ...requestOptions.headers, ...paramHeaders };
+ }
+
/**
* Run a single tools/call attempt: convert args, issue the request, validate,
* and return a successful {@link ToolCallInvocation}. Throws on any error
@@ -3326,17 +3375,7 @@ export class InspectorClient extends InspectorClientEventTarget {
if (!client) {
throw new Error("Client is not connected");
}
- let convertedArgs: Record = args;
- const stringArgs: Record = {};
- for (const [key, value] of Object.entries(args)) {
- if (typeof value === "string") {
- stringArgs[key] = value;
- }
- }
- if (Object.keys(stringArgs).length > 0) {
- const convertedStringArgs = convertToolParameters(tool, stringArgs);
- convertedArgs = { ...args, ...convertedStringArgs };
- }
+ const convertedArgs = this.convertStringToolArgs(tool, args);
// Merge general metadata with tool-specific metadata; tool-specific wins.
const callMetadata: Record | undefined =
@@ -3367,6 +3406,7 @@ export class InspectorClient extends InspectorClientEventTarget {
metadata?.progressToken,
signal,
);
+ this.applyMirroredParamHeaders(tool, convertedArgs, requestOptions);
// Route through the MRTR driver (`requestWithInputRequired`) so a modern
// `input_required` result pauses at the pending-request UI and retries with
// the user's answer (#1704). Both eras use `client.request` with
@@ -3949,17 +3989,7 @@ export class InspectorClient extends InspectorClientEventTarget {
throw new Error("Client is not connected");
}
try {
- let convertedArgs: Record = args;
- const stringArgs: Record = {};
- for (const [key, value] of Object.entries(args)) {
- if (typeof value === "string") {
- stringArgs[key] = value;
- }
- }
- if (Object.keys(stringArgs).length > 0) {
- const convertedStringArgs = convertToolParameters(tool, stringArgs);
- convertedArgs = { ...args, ...convertedStringArgs };
- }
+ const convertedArgs = this.convertStringToolArgs(tool, args);
// Merge general metadata with tool-specific metadata; tool-specific wins.
const callMetadata: Record | undefined =
@@ -3999,6 +4029,9 @@ export class InspectorClient extends InspectorClientEventTarget {
// emit requestorTaskProgress) for task calls only, bypassing the toggle
// that governs every other call path.
const requestOptions = this.getRequestOptions(metadata?.progressToken);
+ // The task-augmented `tools/call` needs the same SEP-2243 mirroring as the
+ // plain one — a strict modern server rejects it with -32020 otherwise.
+ this.applyMirroredParamHeaders(tool, convertedArgs, requestOptions);
if (this.progress) {
const innerOnProgress = requestOptions.onprogress;
requestOptions.onprogress = (progress: Progress) => {
diff --git a/core/mcp/remote/createRemoteFetch.ts b/core/mcp/remote/createRemoteFetch.ts
index cfcec90d0..4dcbf6891 100644
--- a/core/mcp/remote/createRemoteFetch.ts
+++ b/core/mcp/remote/createRemoteFetch.ts
@@ -13,11 +13,12 @@
* on the browser path and reach the server through this proxy.
* - The custom `Mcp-Param-*` headers are gated on a non-browser environment in
* the SDK (`detectProbeEnvironment() !== "browser"`, avoiding a CORS
- * preflight), so they are NEVER built in the browser — the proxy would
- * forward them, but the SDK never creates them here. A tool with an
- * `x-mcp-header` annotation is therefore uncallable from the web client
- * against a strict modern server (it answers `-32020 HeaderMismatch`); such
- * tools work from the Node CLI/TUI, where mirroring is active.
+ * preflight), so the SDK never builds them here. The Inspector builds them
+ * itself instead (`mcpParamHeadersForTool`, #1846) — but they do NOT travel
+ * this path: they ride the transport's per-send `headers` through
+ * `/api/mcp/send`, where the Node backend applies them to the upstream
+ * request it issues. So an `x-mcp-header`-annotated tool IS callable from
+ * the web client against a strict modern server, same as from the CLI/TUI.
*/
export interface RemoteFetchOptions {
diff --git a/core/mcp/remote/node/server.ts b/core/mcp/remote/node/server.ts
index 4f5c2a2db..4bbc518a0 100644
--- a/core/mcp/remote/node/server.ts
+++ b/core/mcp/remote/node/server.ts
@@ -33,6 +33,7 @@ import type {
} from "../types.js";
import type { JSONRPCMessage } from "@modelcontextprotocol/client";
import { AuthChallengeError } from "../../../auth/challenge.js";
+import { MCP_PARAM_HEADER_PREFIX } from "../../../json/xMcpHeader.js";
import {
DEFAULT_MAX_FETCH_REQUESTS,
DEFAULT_TASK_TTL_MS,
@@ -362,6 +363,28 @@ export function requestIdForSendWait(
return undefined;
}
+/**
+ * Restrict client-supplied per-send headers to SEP-2243 `Mcp-Param-*` mirroring
+ * with string values. The sanctioned channel for arbitrary upstream headers is
+ * the server's configured `settings.headers`; this per-send channel must not let
+ * a client inject other headers (e.g. `Authorization`) onto the upstream fetch.
+ */
+export function mcpParamHeadersOnly(
+ headers: unknown,
+): Record | undefined {
+ if (headers === null || typeof headers !== "object") return undefined;
+ const out: Record = {};
+ for (const [key, value] of Object.entries(headers)) {
+ if (
+ typeof value === "string" &&
+ key.toLowerCase().startsWith(MCP_PARAM_HEADER_PREFIX.toLowerCase())
+ ) {
+ out[key] = value;
+ }
+ }
+ return Object.keys(out).length > 0 ? out : undefined;
+}
+
export function createRemoteApp(
options: RemoteServerOptions,
): CreateRemoteAppResult {
@@ -705,7 +728,7 @@ export function createRemoteApp(
return c.json({ error: "Invalid JSON body" }, 400);
}
- const { sessionId, message, relatedRequestId } = body;
+ const { sessionId, message, relatedRequestId, headers } = body;
if (!sessionId || !message) {
return c.json({ error: "Missing sessionId or message" }, 400);
}
@@ -730,8 +753,14 @@ export function createRemoteApp(
void responseWait.catch(() => {});
}
try {
+ // SEP-2243: apply the client's mirrored headers to the upstream request,
+ // restricted to the `Mcp-Param-` prefix so a client can't inject other
+ // upstream headers. The StreamableHTTP transport merges these onto the
+ // outbound fetch; other transports ignore unknown send options.
+ const paramHeaders = mcpParamHeadersOnly(headers);
await session.transport.send(message, {
relatedRequestId: relatedRequestId as string | number | undefined,
+ ...(paramHeaders && { headers: paramHeaders }),
});
if (responseWait) {
await responseWait;
diff --git a/core/mcp/remote/remoteClientTransport.ts b/core/mcp/remote/remoteClientTransport.ts
index 9f3ac4198..03c924611 100644
--- a/core/mcp/remote/remoteClientTransport.ts
+++ b/core/mcp/remote/remoteClientTransport.ts
@@ -704,6 +704,9 @@ export class RemoteClientTransport implements Transport {
...(options?.relatedRequestId != null && {
relatedRequestId: options.relatedRequestId,
}),
+ // Forward per-send `Mcp-Param-*` headers (SEP-2243) for the backend to
+ // apply to the upstream send; the browser can't set them cross-origin.
+ ...(options?.headers != null && { headers: options.headers }),
};
const res = await this.fetchFn(`${this.baseUrl}/api/mcp/send`, {
diff --git a/core/mcp/remote/types.ts b/core/mcp/remote/types.ts
index 2b9c0e584..23bb6aa98 100644
--- a/core/mcp/remote/types.ts
+++ b/core/mcp/remote/types.ts
@@ -120,6 +120,11 @@ export interface RemoteSendRequest {
message: JSONRPCMessage;
/** Optional, for associating response with request (e.g. streamable-http) */
relatedRequestId?: string | number;
+ /**
+ * Per-send `Mcp-Param-*` headers (SEP-2243 mirroring). The backend applies
+ * them to the upstream `transport.send`, filtered to the `Mcp-Param-` prefix.
+ */
+ headers?: Record;
}
export type RemoteEventType =