* 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
5d91d4acd0