diff --git a/core/src/main/kotlin/com/avsystem/justworks/core/parser/SpecParser.kt b/core/src/main/kotlin/com/avsystem/justworks/core/parser/SpecParser.kt
index c45053f..446555a 100644
--- a/core/src/main/kotlin/com/avsystem/justworks/core/parser/SpecParser.kt
+++ b/core/src/main/kotlin/com/avsystem/justworks/core/parser/SpecParser.kt
@@ -16,6 +16,7 @@ import com.avsystem.justworks.core.Warnings
 import com.avsystem.justworks.core.accumulate
 import com.avsystem.justworks.core.accumulateAndReturnNull
 import com.avsystem.justworks.core.ensureNotNullOrAccumulate
+import com.avsystem.justworks.core.ensureOrAccumulate
 import com.avsystem.justworks.core.model.ApiKeyLocation
 import com.avsystem.justworks.core.model.ApiSpec
 import com.avsystem.justworks.core.model.ContentType
@@ -186,11 +187,15 @@ object SpecParser {
             }
 
             val syntheticModels = collectModels(emptySet(), emptyList())
+            val schemas = schemaModels + syntheticModels
+
+            warnOnUnknownTypes(endpoints, schemas)
+
             return ApiSpec(
                 title = title,
                 version = info?.version ?: "0.0.0",
                 endpoints = endpoints,
-                schemas = schemaModels + syntheticModels,
+                schemas = schemas,
                 enums = enumModels,
                 securitySchemes = securitySchemes,
             )
@@ -506,6 +511,50 @@ object SpecParser {
                 )
             }
 
+    context(_: Warnings)
+    private fun warnOnUnknownTypes(endpoints: List<Endpoint>, schemas: List<SchemaModel>) {
+        for (schema in schemas) {
+            for (prop in schema.properties) {
+                ensureOrAccumulate(!containsUnknown(prop.type)) {
+                    Issue.Warning(
+                        "Schema '${schema.name}', property '${prop.name}': unresolvable type mapped to JsonElement",
+                    )
+                }
+            }
+            if (schema.underlyingType != null) {
+                ensureOrAccumulate(!containsUnknown(schema.underlyingType)) {
+                    Issue.Warning(
+                        "Schema '${schema.name}': underlying type contains unresolvable type mapped to JsonElement",
+                    )
+                }
+            }
+        }
+        for (endpoint in endpoints) {
+            val op = endpoint.operationId
+            for ((code, response) in endpoint.responses) {
+                ensureOrAccumulate(response.schema == null || !containsUnknown(response.schema)) {
+                    Issue.Warning("Endpoint '$op', response '$code': unresolvable type mapped to JsonElement")
+                }
+            }
+            ensureOrAccumulate(endpoint.requestBody == null || !containsUnknown(endpoint.requestBody.schema)) {
+                Issue.Warning("Endpoint '$op', request body: unresolvable type mapped to JsonElement")
+            }
+            for (param in endpoint.parameters) {
+                ensureOrAccumulate(!containsUnknown(param.schema)) {
+                    Issue.Warning("Endpoint '$op', parameter '${param.name}': unresolvable type mapped to JsonElement")
+                }
+            }
+        }
+    }
+
+    private fun containsUnknown(type: TypeRef): Boolean = when (type) {
+        TypeRef.Unknown -> true
+        is TypeRef.Array -> containsUnknown(type.items)
+        is TypeRef.Map -> containsUnknown(type.valueType)
+        is TypeRef.Inline -> type.properties.any { containsUnknown(it.type) }
+        is TypeRef.Primitive, is TypeRef.Reference -> false
+    }
+
     private fun generateOperationId(method: HttpMethod, path: String): String {
         val segments = path
             .split("/")
diff --git a/core/src/test/kotlin/com/avsystem/justworks/core/parser/SpecParserTest.kt b/core/src/test/kotlin/com/avsystem/justworks/core/parser/SpecParserTest.kt
index ad055d5..d651e3d 100644
--- a/core/src/test/kotlin/com/avsystem/justworks/core/parser/SpecParserTest.kt
+++ b/core/src/test/kotlin/com/avsystem/justworks/core/parser/SpecParserTest.kt
@@ -160,6 +160,49 @@ class SpecParserTest : SpecParserTestBase() {
         assertEquals("Pet", itemType.schemaName)
     }
 
+    // -- SPEC-01b: Warnings for unknown schemas --
+
+    @Test
+    fun `parse spec with unresolvable schema emits warning`() {
+        val result = SpecParser.parse(
+            """
+            openapi: 3.0.0
+            info:
+              title: Test
+              version: 1.0.0
+            paths: {}
+            components:
+              schemas:
+                Container:
+                  type: object
+                  properties:
+                    data:
+                      type: object
+            """.trimIndent().toTempFile(),
+        )
+        assertIs<ParseResult.Success<*>>(result)
+        val warningMessages = result.warnings.map { it.message }
+        assertTrue(
+            warningMessages.any {
+                it.contains("Container") && it.contains("data") && it.contains("JsonElement")
+            },
+            "Expected warning about unresolvable type, got: $warningMessages",
+        )
+    }
+
+    @Test
+    fun `parse spec without unknown schemas has no unknown-type warnings`() {
+        val result = SpecParser.parse(loadResource("petstore.yaml"))
+        assertIs<ParseResult.Success<*>>(result)
+        val unknownWarnings = result.warnings.filter {
+            it.message.contains("JsonElement")
+        }
+        assertTrue(
+            unknownWarnings.isEmpty(),
+            "Petstore should have no unknown-type warnings, got: $unknownWarnings",
+        )
+    }
+
     // -- SPEC-02: $ref resolution --
 
     @Test