fix(anthropic): support for claude mythos and fable models (#3474)

Author: ssddOnTop

crates/forge_app/src/dto/anthropic/response.rs

@@ -34,6 +34,8 @@ impl From<Model> for forge_domain::Model {
             || value.id.contains("claude-sonnet")
             || value.id.contains("claude-opus")
             || value.id.contains("claude-haiku")
+            || value.id.contains("claude-mythos")
+            || value.id.contains("claude-fable")
         {
             vec![
                 forge_domain::InputModality::Text,
@@ -77,6 +79,11 @@ impl From<Model> for forge_domain::Model {
 ///   header
 /// - Legacy models may have different context lengths
 fn get_context_length(model_id: &str) -> Option<u64> {
+    // Claude Mythos / Fable models (1M context)
+    if model_id.starts_with("claude-mythos") || model_id.starts_with("claude-fable") {
+        return Some(1_000_000);
+    }
+
     // Current models (200K context)
     if model_id.starts_with("claude-sonnet-4-5-")
         || model_id.starts_with("claude-haiku-4-5-")

crates/forge_app/src/transformers/model_specific_reasoning.rs

@@ -31,6 +31,8 @@ impl ModelSpecificReasoning {
             || id.contains("48-opus")
             || id.contains("opus-4-7")
             || id.contains("47-opus")
+            || id.contains("mythos")
+            || id.contains("fable")
         {
             // Opus 4.8 shares Opus 4.7's API contract: adaptive thinking only
             // (legacy `budget_tokens` returns 400) and non-default sampling

crates/forge_repo/src/provider/anthropic.rs

@@ -101,7 +101,9 @@ fn interleaved_thinking_required(model: Option<&ModelId>) -> bool {
     !(id.contains("opus-4-8")
         || id.contains("opus-4-7")
         || id.contains("opus-4-6")
-        || id.contains("sonnet-4-6"))
+        || id.contains("sonnet-4-6")
+        || id.contains("mythos")
+        || id.contains("fable"))
 }
 
 impl<T: HttpInfra> Anthropic<T> {

crates/forge_repo/src/provider/provider.json

@@ -634,6 +634,16 @@
     "response_type": "Anthropic",
     "url": "https://api.anthropic.com/v1/messages",
     "models": [
+      {
+        "id": "claude-fable-5",
+        "name": "Claude Fable 5",
+        "description": "Most capable widely released model for demanding reasoning and agentic work",
+        "context_length": 1000000,
+        "tools_supported": true,
+        "supports_parallel_tool_calls": true,
+        "supports_reasoning": true,
+        "input_modalities": ["text", "image"]
+      },
       {
         "id": "claude-opus-4-8",
         "name": "Claude Opus 4.8",