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fix(core): reject x-mcp-header on number-typed parameters #2762

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fix(core): reject x-mcp-header on number-typed parameters #2762

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7 changes: 7 additions & 0 deletions .changeset/x-mcp-header-reject-number.md
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@@ -0,0 +1,7 @@
---
'@modelcontextprotocol/core-internal': patch
'@modelcontextprotocol/client': patch
'@modelcontextprotocol/server': patch
---

`x-mcp-header` on a `number`-typed tool parameter is now rejected, matching the 2026-07-28 spec ("Parameters with type `number` are not permitted"; clients MUST exclude such tools from `tools/list`). Previously `number` was accepted only to satisfy an older conformance fixture that has since been corrected. `integer`, `string` and `boolean` are unaffected.
15 changes: 6 additions & 9 deletions packages/core-internal/src/shared/mcpParamHeaders.ts
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Expand Up @@ -58,16 +58,13 @@ export type XMcpHeaderScanResult = { valid: true; declarations: readonly XMcpHea
const RFC9110_TOKEN = /^[!#$%&'*+\-.^_`|~0-9A-Za-z]+$/;

/**
* JSON Schema `type` values the spec admits on an `x-mcp-header` property.
*
* The spec text names `integer`, `string`, `boolean` and explicitly excludes
* `number`. The published conformance referee at the pinned release ships its
* `http-custom-headers` scenario with two `type: "number"` `x-mcp-header`
* parameters and expects the client to mirror them, so `number` is accepted
* here so that the conformance gate passes; the discrepancy is tracked
* upstream. Everything else (`object`, `array`, `null`, absent) is rejected.
* JSON Schema `type` values the spec admits on an `x-mcp-header` property:
* `integer`, `string` and `boolean`. `number` is explicitly not permitted
* (2026-07-28 Streamable HTTP, "Schema Extension"), so a tool that annotates a
* `number`-typed property is rejected like any other invalid declaration.
* Everything else (`object`, `array`, `null`, absent) is rejected too.
*/
const PERMITTED_X_MCP_HEADER_TYPES: ReadonlySet<string> = new Set(['string', 'integer', 'boolean', 'number']);
const PERMITTED_X_MCP_HEADER_TYPES: ReadonlySet<string> = new Set(['string', 'integer', 'boolean']);

/**
* Scan a tool's JSON-serialized `inputSchema` for `x-mcp-header` declarations
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4 changes: 4 additions & 0 deletions packages/core-internal/test/shared/mcpParamHeaders.test.ts
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Expand Up @@ -144,6 +144,10 @@ describe('scanXMcpHeaderDeclarations — constraint table', () => {
expect(invalid({ type: 'object', properties: { a: { type: 'array', [X_MCP_HEADER_KEY]: 'Items' } } })).toMatch(/primitive/);
});

test('number-typed property is rejected (only integer, string, boolean are permitted)', () => {
expect(invalid({ type: 'object', properties: { a: { type: 'number', [X_MCP_HEADER_KEY]: 'Score' } } })).toMatch(/primitive/);
});

test('null-typed property is rejected', () => {
expect(invalid({ type: 'object', properties: { a: { type: 'null', [X_MCP_HEADER_KEY]: 'Nil' } } })).toMatch(/primitive/);
});
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