diff --git a/src/scenarios/client/http-custom-headers.test.ts b/src/scenarios/client/http-custom-headers.test.ts
index 8a68f22a..56bf8c8a 100644
--- a/src/scenarios/client/http-custom-headers.test.ts
+++ b/src/scenarios/client/http-custom-headers.test.ts
@@ -30,6 +30,18 @@ async function post(
   });
 }
 
+async function postJson(serverUrl: string, body: object): Promise<any> {
+  const res = await fetch(serverUrl, {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+      Accept: 'application/json, text/event-stream'
+    },
+    body: JSON.stringify(body)
+  });
+  return res.json();
+}
+
 function idsOf(checks: { id: string }[]): Set<string> {
   return new Set(checks.map((c) => c.id));
 }
@@ -198,4 +210,42 @@ describe('HttpInvalidToolHeadersScenario (SEP-2243) check IDs', () => {
       await scenario.stop();
     }
   });
+
+  it('FAILs primitive-only when the client calls the number-typed tool', async () => {
+    const scenario = new HttpInvalidToolHeadersScenario();
+    const { serverUrl } = await scenario.start(testScenarioContext());
+    try {
+      const listed = await postJson(serverUrl, {
+        jsonrpc: '2.0',
+        id: 1,
+        method: 'tools/list'
+      });
+      // SEP-2243 permits x-mcp-header only on integer/string/boolean, so the
+      // number-typed tool must be served for the client to reject it.
+      const numberTool = listed.result.tools.find(
+        (t: { name: string }) => t.name === 'invalid_number_header'
+      );
+      expect(numberTool?.inputSchema.properties.score).toEqual({
+        type: 'number',
+        'x-mcp-header': 'Score'
+      });
+
+      await post(serverUrl, {
+        jsonrpc: '2.0',
+        id: 2,
+        method: 'tools/call',
+        params: { name: 'invalid_number_header', arguments: { score: 1.5 } }
+      });
+      const checks = scenario.getChecks();
+      expect(
+        statusesFor(checks, 'sep-2243-x-mcp-header-primitive-only')
+      ).toContain('FAILURE');
+      // The other constraints were not violated.
+      expect(
+        statusesFor(checks, 'sep-2243-x-mcp-header-not-empty')
+      ).not.toContain('FAILURE');
+    } finally {
+      await scenario.stop();
+    }
+  });
 });
diff --git a/src/scenarios/client/http-custom-headers.ts b/src/scenarios/client/http-custom-headers.ts
index 8e64e6ab..4a73df37 100644
--- a/src/scenarios/client/http-custom-headers.ts
+++ b/src/scenarios/client/http-custom-headers.ts
@@ -68,6 +68,7 @@ const INVALID_TOOL_CONSTRAINT_IDS: Record<string, string> = {
   invalid_object_header: 'sep-2243-x-mcp-header-primitive-only',
   invalid_array_header: 'sep-2243-x-mcp-header-primitive-only',
   invalid_null_header: 'sep-2243-x-mcp-header-primitive-only',
+  invalid_number_header: 'sep-2243-x-mcp-header-primitive-only',
   invalid_duplicate_same_case: 'sep-2243-x-mcp-header-unique',
   invalid_duplicate_diff_case: 'sep-2243-x-mcp-header-unique',
   invalid_space_in_name: 'sep-2243-x-mcp-header-charset',
@@ -861,6 +862,23 @@ export class HttpInvalidToolHeadersScenario extends BaseHttpScenario {
             }
           },
 
+          // ── Invalid: x-mcp-header on number type ──
+          // `number` is a JSON Schema primitive but SEP-2243 excludes it from
+          // the permitted set: "Parameters with type `number` are not
+          // permitted." Only integer, string and boolean may be annotated.
+          {
+            name: 'invalid_number_header',
+            description:
+              'x-mcp-header MUST NOT be on number type (MUST be rejected)',
+            inputSchema: {
+              type: 'object',
+              properties: {
+                score: { type: 'number', 'x-mcp-header': 'Score' }
+              },
+              required: ['score']
+            }
+          },
+
           // ── Invalid: duplicate same-case x-mcp-header values ──
           {
             name: 'invalid_duplicate_same_case',
diff --git a/src/seps/sep-2243.yaml b/src/seps/sep-2243.yaml
index a39b6d2c..ddb06ffb 100644
--- a/src/seps/sep-2243.yaml
+++ b/src/seps/sep-2243.yaml
@@ -23,13 +23,13 @@ requirements:
     text: 'The x-mcp-header value MUST be case-insensitively unique within a single tool definition.'
     url: https://modelcontextprotocol.io/specification/draft/server/tools#custom-headers
   - check: sep-2243-x-mcp-header-primitive-only
-    text: 'x-mcp-header MUST only be applied to parameters with primitive types (number, string, or boolean).'
+    text: 'x-mcp-header MUST only be applied to parameters with primitive types (integer, string, boolean). Parameters with type `number` are not permitted.'
     url: https://modelcontextprotocol.io/specification/draft/server/tools#custom-headers
   - check: sep-2243-client-reject-invalid-tool
     text: 'Clients MUST reject tool definitions where any x-mcp-header value violates these constraints. Rejection means the client MUST exclude the invalid tool from the set of tools returned by tools/list.'
     url: https://modelcontextprotocol.io/specification/draft/server/tools#custom-headers
   - check: sep-2243-client-encode-values
-    text: 'Clients MUST encode parameter values before including them in HTTP headers: number values MUST be converted to their decimal string representation; boolean values MUST be converted to the lowercase strings "true" or "false".'
+    text: 'Clients MUST encode parameter values before including them in HTTP headers: integer values MUST be converted to their decimal string representation; boolean values MUST be converted to the lowercase strings "true" or "false".'
   - check: sep-2243-client-base64-unsafe
     text: 'When a value cannot be safely represented as plain ASCII (e.g., contains non-ASCII characters, control characters, or leading/trailing whitespace), clients MUST use Base64 encoding of the UTF-8 representation, wrapped as =?base64?{encoded}?=.'
   - check: sep-2243-server-decode-base64
diff --git a/src/seps/traceability.json b/src/seps/traceability.json
index bdf6c628..674f2e9c 100644
--- a/src/seps/traceability.json
+++ b/src/seps/traceability.json
@@ -209,7 +209,7 @@
         {
           "check": "sep-2243-x-mcp-header-primitive-only",
           "status": "tested",
-          "text": "x-mcp-header MUST only be applied to parameters with primitive types (number, string, or boolean).",
+          "text": "x-mcp-header MUST only be applied to parameters with primitive types (integer, string, boolean). Parameters with type `number` are not permitted.",
           "url": "https://modelcontextprotocol.io/specification/draft/server/tools#custom-headers"
         },
         {
@@ -221,7 +221,7 @@
         {
           "check": "sep-2243-client-encode-values",
           "status": "tested",
-          "text": "Clients MUST encode parameter values before including them in HTTP headers: number values MUST be converted to their decimal string representation; boolean values MUST be converted to the lowercase strings \"true\" or \"false\"."
+          "text": "Clients MUST encode parameter values before including them in HTTP headers: integer values MUST be converted to their decimal string representation; boolean values MUST be converted to the lowercase strings \"true\" or \"false\"."
         },
         {
           "check": "sep-2243-client-base64-unsafe",