diff --git a/wire-compiler/src/test/java/com/squareup/wire/schema/OptionsLinkingTest.kt b/wire-compiler/src/test/java/com/squareup/wire/schema/OptionsLinkingTest.kt
index 3fc4052783..4a2df8a160 100644
--- a/wire-compiler/src/test/java/com/squareup/wire/schema/OptionsLinkingTest.kt
+++ b/wire-compiler/src/test/java/com/squareup/wire/schema/OptionsLinkingTest.kt
@@ -18,12 +18,14 @@
 package com.squareup.wire.schema
 
 import assertk.assertThat
+import assertk.assertions.contains
 import assertk.assertions.isEqualTo
 import assertk.assertions.isNotNull
 import com.squareup.wire.testing.add
 import okio.Path
 import okio.Path.Companion.toPath
 import okio.fakefilesystem.FakeFileSystem
+import org.junit.Assert.fail
 import org.junit.Test
 
 class OptionsLinkingTest {
@@ -169,6 +171,62 @@ class OptionsLinkingTest {
     assertThat(typeRange.field("max")).isNotNull()
   }
 
+  @Test
+  fun rejectsInvalidOptionScalarLiterals() {
+    fs.add(
+      "source-path/a.proto",
+      """
+             |import "formatting_options.proto";
+             |
+             |message A {
+             |  option (message_options).enabled = "false; static { } //";
+             |  optional string s = 1 [
+             |    (formatting_options).max = "80; static { } //",
+             |    (formatting_options).casing = "LOWER_CASE; static { } //"
+             |  ];
+             |}
+      """.trimMargin(),
+    )
+    fs.add(
+      "source-path/formatting_options.proto",
+      """
+             |import "google/protobuf/descriptor.proto";
+             |
+             |message MessageOptions {
+             |  optional bool enabled = 1;
+             |}
+             |
+             |message FormattingOptions {
+             |  optional int32 max = 1;
+             |  optional StringCasing casing = 2;
+             |  optional string documentation = 3;
+             |}
+             |
+             |enum StringCasing {
+             |  LOWER_CASE = 1;
+             |}
+             |
+             |extend google.protobuf.MessageOptions {
+             |  optional MessageOptions message_options = 22001;
+             |}
+             |
+             |extend google.protobuf.FieldOptions {
+             |  optional FormattingOptions formatting_options = 22002;
+             |}
+      """.trimMargin(),
+    )
+
+    try {
+      loadAndLinkSchema()
+      fail()
+    } catch (expected: SchemaException) {
+      val message = expected.message!!
+      assertThat(message).contains("invalid option value \"false; static { } //\" for bool")
+      assertThat(message).contains("invalid option value \"80; static { } //\" for int32")
+      assertThat(message).contains("invalid option value \"LOWER_CASE; static { } //\" for StringCasing")
+    }
+  }
+
   @Test
   fun extensionTypesInExternalFile() {
     fs.add(
diff --git a/wire-java-generator/src/test/java/com/squareup/wire/java/JavaGeneratorTest.java b/wire-java-generator/src/test/java/com/squareup/wire/java/JavaGeneratorTest.java
index 39820078a9..78d5787a75 100644
--- a/wire-java-generator/src/test/java/com/squareup/wire/java/JavaGeneratorTest.java
+++ b/wire-java-generator/src/test/java/com/squareup/wire/java/JavaGeneratorTest.java
@@ -24,6 +24,7 @@
 import com.squareup.wire.schema.MessageType;
 import com.squareup.wire.schema.PruningRules;
 import com.squareup.wire.schema.Schema;
+import com.squareup.wire.schema.SchemaException;
 import java.io.IOException;
 import okio.Path;
 import org.junit.Test;
@@ -608,21 +609,20 @@ public void defaultValues() throws IOException {
 
   @Test
   public void defaultValuesMustNotBeOctal() throws IOException {
-    Schema schema =
-        new SchemaBuilder()
-            .add(
-                Path.get("message.proto"),
-                ""
-                    + "message Message {\n"
-                    + "  optional int32 a = 1 [default = 020 ];\n"
-                    + "  optional int64 b = 2 [default = 021 ];\n"
-                    + "}\n")
-            .build();
     try {
-      new JavaWithProfilesGenerator(schema).generateJava("Message");
+      new SchemaBuilder()
+          .add(
+              Path.get("message.proto"),
+              ""
+                  + "message Message {\n"
+                  + "  optional int32 a = 1 [default = 020 ];\n"
+                  + "  optional int64 b = 2 [default = 021 ];\n"
+                  + "}\n")
+          .build();
       fail();
-    } catch (IllegalStateException expected) {
-      assertThat(expected).hasMessageThat().contains("Octal literal unsupported: 020");
+    } catch (SchemaException expected) {
+      assertThat(expected).hasMessageThat().contains("invalid default value \"020\" for int32");
+      assertThat(expected).hasMessageThat().contains("invalid default value \"021\" for int64");
     }
   }
 
diff --git a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Field.kt b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Field.kt
index c353a904a2..62b0b383c6 100644
--- a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Field.kt
+++ b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Field.kt
@@ -162,6 +162,7 @@ data class Field(
       }
     }
     syntaxRules.validateDefaultValue(default != null, linker.errors)
+    validateDefaultValue(linker)
     if (type!!.isMap) {
       val valueType = linker.get(type!!.valueType!!)
       if (valueType is EnumType && valueType.constants[0].tag != 0) {
@@ -171,6 +172,15 @@ data class Field(
     linker.validateImportForType(location, type!!)
   }
 
+  private fun validateDefaultValue(linker: Linker) {
+    val default = default ?: return
+    val type = type!!
+
+    if (!isValidLiteral(linker, type, default)) {
+      linker.errors += "invalid default value \"$default\" for $type"
+    }
+  }
+
   fun retainAll(schema: Schema, markSet: MarkSet, enclosingType: ProtoType): Field? {
     // TODO(jwilson): perform this transformation in the Linker.
     val type = type ?: return null
diff --git a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/LiteralValidation.kt b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/LiteralValidation.kt
new file mode 100644
index 0000000000..a04c7199ad
--- /dev/null
+++ b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/LiteralValidation.kt
@@ -0,0 +1,134 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.wire.schema
+
+internal fun isValidLiteral(linker: Linker, type: ProtoType, value: String): Boolean = when (type) {
+  ProtoType.BOOL -> value == "true" || value == "false"
+  ProtoType.BYTES, ProtoType.STRING -> true
+  ProtoType.DOUBLE, ProtoType.FLOAT -> value.isValidFloatingPointDefault()
+  ProtoType.FIXED32, ProtoType.UINT32 -> value.isValidUnsignedIntegerDefault(UINT32_MAX)
+  ProtoType.FIXED64, ProtoType.UINT64 -> value.isValidUnsignedIntegerDefault(UINT64_MAX)
+  ProtoType.INT32, ProtoType.SFIXED32, ProtoType.SINT32 -> {
+    value.isValidSignedIntegerDefault(INT32_MIN, INT32_MAX)
+  }
+  ProtoType.INT64, ProtoType.SFIXED64, ProtoType.SINT64 -> {
+    value.isValidSignedIntegerDefault(INT64_MIN, INT64_MAX)
+  }
+  else -> {
+    val valueType = linker.get(type)
+    valueType is EnumType && valueType.constant(value) != null
+  }
+}
+
+private const val INT32_MAX = "2147483647"
+private const val INT32_MIN = "-2147483648"
+private const val UINT32_MAX = "4294967295"
+private const val INT64_MAX = "9223372036854775807"
+private const val INT64_MIN = "-9223372036854775808"
+private const val UINT64_MAX = "18446744073709551615"
+
+private val DECIMAL_INTEGER_REGEX = Regex("-?[0-9]+")
+private val FLOATING_POINT_REGEX = Regex("-?((([0-9]+)(\\.[0-9]*)?)|(\\.[0-9]+))([eE][+-]?[0-9]+)?")
+private val HEX_INTEGER_REGEX = Regex("-?0[xX][0-9a-fA-F]+")
+
+private fun String.isValidFloatingPointDefault(): Boolean {
+  if (this in listOf("inf", "-inf", "nan", "-nan")) return true
+  return FLOATING_POINT_REGEX.matches(this) && toDoubleOrNull() != null
+}
+
+private fun String.isValidSignedIntegerDefault(min: String, max: String): Boolean {
+  if (!isIntegerLiteral() || hasOctalPrefix()) return false
+  return compareIntegerLiterals(min, this) <= 0 && compareIntegerLiterals(this, max) <= 0
+}
+
+private fun String.isValidUnsignedIntegerDefault(max: String): Boolean {
+  if (!isIntegerLiteral() || hasOctalPrefix() || startsWith("-")) return false
+  return compareIntegerLiterals(this, max) <= 0
+}
+
+private fun String.isIntegerLiteral() = DECIMAL_INTEGER_REGEX.matches(this) || HEX_INTEGER_REGEX.matches(this)
+
+private fun String.hasOctalPrefix(): Boolean {
+  val digits = removePrefix("-")
+  return digits.length > 1 && digits[0] == '0' && digits[1] != 'x' && digits[1] != 'X'
+}
+
+private fun compareIntegerLiterals(a: String, b: String): Int {
+  val aNegative = a.startsWith("-")
+  val bNegative = b.startsWith("-")
+  if (aNegative != bNegative) return if (aNegative) -1 else 1
+
+  val magnitudeComparison = compareIntegerMagnitudes(
+    a.removePrefix("-"),
+    b.removePrefix("-"),
+  )
+  return if (aNegative) -magnitudeComparison else magnitudeComparison
+}
+
+private fun compareIntegerMagnitudes(a: String, b: String): Int {
+  val aDecimal = a.decimalMagnitude()
+  val bDecimal = b.decimalMagnitude()
+  if (aDecimal.length != bDecimal.length) return aDecimal.length.compareTo(bDecimal.length)
+  return aDecimal.compareTo(bDecimal)
+}
+
+private fun String.decimalMagnitude(): String {
+  val stripped = if (startsWith("0x") || startsWith("0X")) {
+    hexMagnitudeToDecimal(substring(2))
+  } else {
+    trimStart('0').ifEmpty { "0" }
+  }
+  return stripped.trimStart('0').ifEmpty { "0" }
+}
+
+private fun hexMagnitudeToDecimal(hex: String): String {
+  var decimal = "0"
+  hex.forEach { digit ->
+    decimal = decimal.multiplyDecimalBy(16)
+    decimal = decimal.addDecimal(digit.digitToInt(16))
+  }
+  return decimal
+}
+
+private fun String.multiplyDecimalBy(multiplier: Int): String {
+  var carry = 0
+  val result = StringBuilder(length + 2)
+  for (i in indices.reversed()) {
+    val value = (this[i] - '0') * multiplier + carry
+    result.append(value % 10)
+    carry = value / 10
+  }
+  while (carry > 0) {
+    result.append(carry % 10)
+    carry /= 10
+  }
+  return result.reverse().toString().trimStart('0').ifEmpty { "0" }
+}
+
+private fun String.addDecimal(addend: Int): String {
+  var carry = addend
+  val result = StringBuilder(length + 2)
+  for (i in indices.reversed()) {
+    val value = (this[i] - '0') + carry
+    result.append(value % 10)
+    carry = value / 10
+  }
+  while (carry > 0) {
+    result.append(carry % 10)
+    carry /= 10
+  }
+  return result.reverse().toString().trimStart('0').ifEmpty { "0" }
+}
diff --git a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Options.kt b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Options.kt
index 1d66fb3115..87311e2e43 100644
--- a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Options.kt
+++ b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Options.kt
@@ -220,6 +220,7 @@ class Options(
       }
 
       is String -> {
+        validateOptionValue(linker, context, value)
         return coerceValueForField(context, value, isRepeated)
       }
 
@@ -233,6 +234,12 @@ class Options(
     }
   }
 
+  private fun validateOptionValue(linker: Linker, context: ProtoType, value: String) {
+    if (!isValidLiteral(linker, context, value)) {
+      linker.errors += "invalid option value \"$value\" for $context"
+    }
+  }
+
   private fun coerceValueForField(context: ProtoType, value: Any, isRepeated: Boolean): Any = when {
     isRepeated || context.isMap -> value as? List<*> ?: listOf(value)
     value is List<*> -> value.single()!!
diff --git a/wire-schema/src/jvmTest/kotlin/com/squareup/wire/schema/SchemaTest.kt b/wire-schema/src/jvmTest/kotlin/com/squareup/wire/schema/SchemaTest.kt
index df84028371..4a0265cd24 100644
--- a/wire-schema/src/jvmTest/kotlin/com/squareup/wire/schema/SchemaTest.kt
+++ b/wire-schema/src/jvmTest/kotlin/com/squareup/wire/schema/SchemaTest.kt
@@ -2162,6 +2162,68 @@ class SchemaTest {
     }
   }
 
+  @Test
+  fun proto2AllowsValidDefaultValues() {
+    buildSchema {
+      add(
+        "defaults.proto".toPath(),
+        """
+        |syntax = "proto2";
+        |
+        |message Defaults {
+        |  optional bool bool_field = 1 [default = false];
+        |  optional int32 int32_field = 2 [default = -0x80000000];
+        |  optional uint32 uint32_field = 3 [default = 4294967295];
+        |  optional int64 int64_field = 4 [default = -9223372036854775808];
+        |  optional uint64 uint64_field = 5 [default = 18446744073709551615];
+        |  optional float float_field = 6 [default = inf];
+        |  optional double double_field = 7 [default = -1.23e45];
+        |  optional string string_field = 8 [default = "source syntax is just data here"];
+        |  optional bytes bytes_field = 9 [default = "abc"];
+        |  optional Choice enum_field = 10 [default = TWO];
+        |
+        |  enum Choice {
+        |    ONE = 0;
+        |    TWO = 1;
+        |  }
+        |}
+        """.trimMargin(),
+      )
+    }
+  }
+
+  @Test
+  fun proto2RejectsInvalidDefaultValues() {
+    try {
+      buildSchema {
+        add(
+          "defaults.proto".toPath(),
+          """
+        |syntax = "proto2";
+        |
+        |message Defaults {
+        |  optional bool bool_field = 1 [default = "false; static { } //"];
+        |  optional float float_field = 2 [default = "0.0f; init { } //"];
+        |  optional double double_field = 3 [default = "0.0d; static { } //"];
+        |  optional Choice enum_field = 4 [default = "ONE; static { } //"];
+        |
+        |  enum Choice {
+        |    ONE = 0;
+        |  }
+        |}
+          """.trimMargin(),
+        )
+      }
+      fail()
+    } catch (expected: SchemaException) {
+      val message = expected.message!!
+      assertThat(message).contains("invalid default value \"false; static { } //\" for bool")
+      assertThat(message).contains("invalid default value \"0.0f; init { } //\" for float")
+      assertThat(message).contains("invalid default value \"0.0d; static { } //\" for double")
+      assertThat(message).contains("invalid default value \"ONE; static { } //\" for Defaults.Choice")
+    }
+  }
+
   @Test
   fun repeatedNumericScalarsShouldBePackedByDefaultForProto3() {
     val schema = buildSchema {