diff --git a/mcp/streamable_headers.go b/mcp/streamable_headers.go
index 7133e26c..26772326 100644
--- a/mcp/streamable_headers.go
+++ b/mcp/streamable_headers.go
@@ -309,6 +309,12 @@ func filterValidTools(logger *slog.Logger, tools []*Tool) []*Tool {
 // are valid. Annotations may appear on properties at any nesting
 // depth within the inputSchema and must be unique across all of them.
 func validateParamHeaderAnnotations(tool *Tool) error {
+	var schema any
+	if err := remarshal(tool.InputSchema, &schema); err == nil {
+		if err := checkUnreachableParamHeaders(schema, "", true); err != nil {
+			return err
+		}
+	}
 	props := unmarshalSchemaProperties(tool.InputSchema)
 	if len(props) == 0 {
 		return nil
@@ -353,6 +359,61 @@ func validateParamHeadersIn(props map[string]headerSchemaProperty, prefix string
 	return nil
 }
 
+// Keywords whose values are subschemas. The spec only allows x-mcp-header on
+// a property reached from the root through "properties" alone, so an
+// annotation anywhere under one of these makes the tool invalid.
+var (
+	subschemaKeywords = []string{
+		"items", "prefixItems", "additionalItems", "contains",
+		"additionalProperties", "unevaluatedItems", "unevaluatedProperties", "propertyNames",
+		"allOf", "anyOf", "oneOf", "not", "if", "then", "else",
+	}
+	subschemaMapKeywords = []string{"patternProperties", "dependentSchemas", "$defs", "definitions"}
+)
+
+// checkUnreachableParamHeaders reports an x-mcp-header annotation that is not
+// statically reachable from the schema root through "properties" keys.
+func checkUnreachableParamHeaders(schema any, path string, reachable bool) error {
+	m, ok := schema.(map[string]any)
+	if !ok {
+		return nil
+	}
+	if _, ok := m["x-mcp-header"]; ok && !reachable {
+		return fmt.Errorf("x-mcp-header at %q is not reachable from the schema root through properties", path)
+	}
+	if props, ok := m["properties"].(map[string]any); ok {
+		for name, prop := range props {
+			if err := checkUnreachableParamHeaders(prop, path+"/properties/"+name, reachable); err != nil {
+				return err
+			}
+		}
+	}
+	for _, kw := range subschemaKeywords {
+		switch v := m[kw].(type) {
+		case map[string]any:
+			if err := checkUnreachableParamHeaders(v, path+"/"+kw, false); err != nil {
+				return err
+			}
+		case []any:
+			for i, s := range v {
+				if err := checkUnreachableParamHeaders(s, fmt.Sprintf("%s/%s/%d", path, kw, i), false); err != nil {
+					return err
+				}
+			}
+		}
+	}
+	for _, kw := range subschemaMapKeywords {
+		if defs, ok := m[kw].(map[string]any); ok {
+			for name, s := range defs {
+				if err := checkUnreachableParamHeaders(s, path+"/"+kw+"/"+name, false); err != nil {
+					return err
+				}
+			}
+		}
+	}
+	return nil
+}
+
 // validateHeaderName checks that a header name matches the HTTP field-name
 // token syntax (1*tchar).
 func validateHeaderName(name string) error {
diff --git a/mcp/streamable_headers_test.go b/mcp/streamable_headers_test.go
index d80c781f..4c9b3140 100644
--- a/mcp/streamable_headers_test.go
+++ b/mcp/streamable_headers_test.go
@@ -897,6 +897,164 @@ func TestValidateToolParamHeaders(t *testing.T) {
 			wantErr:    true,
 			wantErrSub: "primitive types",
 		},
+		{
+			name: "x-mcp-header under items is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"tags": map[string]any{
+							"type":  "array",
+							"items": map[string]any{"type": "string", "x-mcp-header": "Tag"},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header under oneOf is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"region": map[string]any{
+							"oneOf": []any{
+								map[string]any{"type": "string", "x-mcp-header": "Region"},
+								map[string]any{"type": "integer"},
+							},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header under anyOf is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"anyOf": []any{
+						map[string]any{
+							"properties": map[string]any{
+								"region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+							},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header under allOf is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"region": map[string]any{
+							"allOf": []any{
+								map[string]any{"type": "string", "x-mcp-header": "Region"},
+							},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header under not is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"region": map[string]any{
+							"type": "string",
+							"not":  map[string]any{"type": "string", "x-mcp-header": "Region"},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header under if/then/else is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"if": map[string]any{
+						"properties": map[string]any{"kind": map[string]any{"const": "a"}},
+					},
+					"then": map[string]any{
+						"properties": map[string]any{
+							"region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+						},
+					},
+					"else": map[string]any{
+						"properties": map[string]any{
+							"zone": map[string]any{"type": "string", "x-mcp-header": "Zone"},
+						},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header in $defs reached through $ref is rejected",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"$defs": map[string]any{
+						"Region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+					},
+					"properties": map[string]any{
+						"region": map[string]any{"$ref": "#/$defs/Region"},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "not reachable",
+		},
+		{
+			name: "x-mcp-header on nested properties next to unannotated subschemas is valid",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type":                 "object",
+					"additionalProperties": false,
+					"properties": map[string]any{
+						"config": map[string]any{
+							"type": "object",
+							"properties": map[string]any{
+								"region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+							},
+						},
+						"tags": map[string]any{
+							"type":  "array",
+							"items": map[string]any{"type": "string"},
+						},
+						"mode": map[string]any{
+							"oneOf": []any{
+								map[string]any{"const": "fast"},
+								map[string]any{"const": "slow"},
+							},
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {