fix(core): mirror SEP-2243 x-mcp-header args to Mcp-Param-* on tools/call (#1847)

* fix(core): mirror SEP-2243 x-mcp-header args to Mcp-Param-* on tools/call (#1846)

A strict modern (2026-07-28) HTTP server (e.g. the GitHub MCP Server)
rejects a tools/call whose argument carries an `x-mcp-header` annotation
when the matching `Mcp-Param-*` request header is missing (-32020
"header mismatch"). The Inspector never sent that header.

The SDK only mirrors inside `client.callTool()` (and skips it entirely in
a browser environment), but the Inspector routes tools/call through
`client.request()` for manual MRTR driving (#1704), so the SDK never
mirrors for us — on any transport. On top of that, the remote (web)
transport dropped per-request headers before they reached the backend's
upstream fetch.

- core/json/xMcpHeader.ts: port the SDK's non-public `buildMcpParamHeaders`
  + value encoding (Base64 sentinel for non-ASCII/whitespace/empty values)
  and add `mcpParamHeadersForTool`.
- InspectorClient.attemptToolCall: on a modern connection, build the
  headers from the tool's declarations + call arguments and attach them to
  the tools/call request options. `Protocol.request` forwards `headers`
  to the transport and preserves them across MRTR retry legs, so the
  direct StreamableHTTP transport applies them. Fixes CLI/TUI.
- RemoteClientTransport.requestSend + /api/mcp/send + RemoteSendRequest:
  forward per-send headers and apply them to the backend's upstream
  transport.send. The browser can't set cross-origin headers, but the
  Node backend can — where the Inspector's real request is issued. Fixes web.

Verified end-to-end against the live GitHub MCP Server (modern HTTP).

Closes #1846

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 36dbeb09-12b0-4c19-af36-2573772a7dbd

* review: harden backend header allowlist, fix breadcrumbs, trim comments

Address Copilot review on #1847:
- server.ts: restrict client-supplied per-send headers to the `Mcp-Param-`
  prefix (string values only) via `mcpParamHeadersOnly` before applying them
  upstream, so the per-send channel can't inject arbitrary headers (e.g.
  Authorization). `settings.headers` remains the channel for configured
  upstream headers. Covered by new unit tests.
- Fix stale `#1810` breadcrumbs → `#1846` in the two new tests.
- Trim the verbose mirroring comments in inspectorClient/types/remote transport.
- Drop the redundant base64-on-the-wire integration test (encoding is unit-
  tested; wire delivery is proven by the verbatim test), remove two unneeded
  test casts, and cover the boolean `true` branch.

Not changed (declined with reason on the PR): buildMcpParamHeaders remains a
faithful port of the SDK's helper and does not re-validate values against the
declared JSON Schema type — arg types are enforced by schema validation on
both sides, and matching the SDK's conforming wire output is the module's goal.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 36dbeb09-12b0-4c19-af36-2573772a7dbd

* review: mirror Mcp-Param-* on the task-augmented tools/call too

Address the maintainer review on #1847:

- `callToolStream` (the "Run as task" path) built its own request options and
  never mirrored, so a task-augmented `tools/call` on an annotated tool was
  still rejected by a strict modern server with -32020. It now mirrors like the
  plain path.
- Extract the two blocks both `tools/call` entry points had duplicated —
  `convertStringToolArgs` (the string-arg coercion) and
  `applyMirroredParamHeaders` (the SEP-2243 mirroring) — so the paths can't
  drift again. That duplication is why the task path was missed.
- Regression test: `callToolStream` against the modern `get_weather` server
  asserts the POST carries `Mcp-Param-City` (verified failing without the fix).
- README: the modern-network showcase claimed mirroring is skipped in the
  browser so the web client gets -32020, and that CLI/TUI mirror. Both are now
  wrong — the Inspector mirrors itself, on every client. AGENTS: note that
  xMcpHeader.ts now also builds the wire headers.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkhhCHryVp5H3dqEnhwZ7t

* docs: correct the stale "browser omits Mcp-Param-*" copy

Both statements predate this PR and are now the opposite of true, per the
wire capture on #1847 (proxy-observed upstream headers, web leg included).

- ToolDetailPanel: the mirrored-headers note claimed the browser omits
  `Mcp-Param-*`. It now says the web client's headers are applied by the Node
  backend that issues the upstream request.
- createRemoteFetch: the header comment claimed an `x-mcp-header`-annotated
  tool is "uncallable from the web client against a strict modern server" —
  the exact wrong conclusion to reach while debugging a -32020. It now records
  that the Inspector builds the headers itself, and that they do NOT travel
  this `/api/fetch` proxy path: they ride the transport's per-send `headers`
  through `/api/mcp/send`.

Copy only — no test asserted the old note text (ToolDetailPanel.test.tsx
matches the section title only).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkhhCHryVp5H3dqEnhwZ7t

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: cliffhall <cliff@futurescale.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Copilot-Session: 36dbeb09-12b0-4c19-af36-2573772a7dbd

README.md

@@ -117,7 +117,7 @@ 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`).