Add OpenAI GPT-5.6 models (#566)

## Summary

- add GPT-5.6 Sol, Terra, and Luna to the built-in OpenAI model catalog
with current capabilities, limits, aliases, and pricing
- route all three models through the OpenAI Responses API and keep the
Codex-safe 272K context policy
- make GPT-5.6 Sol the OpenAI default and update the model docs and Ask
Fabro API contract

## Testing

- cargo nextest run -p fabro-model
- cargo nextest run -p fabro-llm builtin_catalog_route_equivalence_table
- cargo nextest run -p fabro-server list_models
- cargo nextest run -p fabro-server --features test-support
run_responses_include_ask_fabro_affordance
- cargo +nightly-2026-04-14 fmt --check --all
- cargo +nightly-2026-04-14 clippy -p fabro-model -p fabro-llm
--all-targets -- -D warnings
- git diff --check

docs/public/core-concepts/models.mdx

@@ -20,6 +20,9 @@ No single model is best at everything. Fabro lets you assign the right model to
 | `claude-sonnet-4-6` | anthropic | `sonnet`, `claude-sonnet` | 200K | $3.00 / $15.00 | 50 tok/s |
 | `claude-sonnet-4-5` | anthropic | | 200K | $3.00 / $15.00 | 50 tok/s |
 | `claude-haiku-4-5` | anthropic | `haiku`, `claude-haiku` | 200K | $0.80 / $4.00 | 100 tok/s |
+| `gpt-5.6-sol` | openai | `gpt-5.6`, `gpt56-sol` | 272K | $5.00 / $30.00 | n/a |
+| `gpt-5.6-terra` | openai | `gpt56-terra` | 272K | $2.50 / $15.00 | n/a |
+| `gpt-5.6-luna` | openai | `gpt56-luna` | 272K | $1.00 / $6.00 | n/a |
 | `gpt-5.4` | openai | `gpt54`, `gpt5`, `codex` | 272K | $2.50 / $15.00 | 70 tok/s |
 | `gpt-5.5` | openai | `gpt55` | 272K | $5.00 / $30.00 | 70 tok/s |
 | `gpt-5.5-pro` | openai | `gpt55-pro` | 1M | $30.00 / $180.00 | 20 tok/s |
@@ -169,7 +172,7 @@ When no model or provider is specified, Fabro checks configured provider credent
 | Provider | Default model |
 |---|---|
 | `anthropic` | `claude-sonnet-4-6` |
-| `openai` | `gpt-5.5` |
+| `openai` | `gpt-5.6-sol` |
 | `gemini` | `gemini-3.5-flash` |
 | `kimi` | `kimi-k2.5` |
 | `zai` | `glm-4.7` |

lib/crates/fabro-cli/tests/it/cmd/attach.rs

@@ -100,6 +100,13 @@ fn normalize_attach_json_progress_event(mut event: Value) -> Value {
             );
         }
     }
+    if let Some(model_name) = event.pointer_mut("/properties/settings/run/model/name") {
+        assert!(
+            model_name.is_string(),
+            "default model should serialize as a string"
+        );
+        *model_name = Value::String("[DEFAULT_MODEL]".to_string());
+    }
     event
 }
 
@@ -991,7 +998,7 @@ fn attach_json_errors_without_prompting_for_human_input() {
                   "speed": null
                 },
                 "fallbacks": [],
-                "name": "gpt-5.5",
+                "name": "[DEFAULT_MODEL]",
                 "provider": "openai"
               },
               "notifications": {},

lib/crates/fabro-llm/src/adapter_registry.rs

@@ -306,6 +306,9 @@ mod tests {
             ("gpt-5.4-pro",                        "gpt-5.4-pro",                         T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
             ("gpt-5.5",                            "gpt-5.5",                             T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
             ("gpt-5.5-pro",                        "gpt-5.5-pro",                         T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
+            ("gpt-5.6-luna",                       "gpt-5.6-luna",                        T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
+            ("gpt-5.6-sol",                        "gpt-5.6-sol",                         T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
+            ("gpt-5.6-terra",                      "gpt-5.6-terra",                       T::OpenAi,            C::OpenAiResponses,   B::OpenAi,    P::OpenAi),
             ("kimi-k2.5",                          "kimi-k2.5",                           T::OpenAiCompatible,  C::OpenAiCompatible,  B::OpenAi,    P::OpenAi),
             ("mercury-2",                          "mercury-2",                           T::OpenAiCompatible,  C::OpenAiCompatible,  B::OpenAi,    P::OpenAi),
             ("minimax-m2.5",                       "minimax-m2.5",                        T::OpenAiCompatible,  C::OpenAiCompatible,  B::OpenAi,    P::OpenAi),

lib/crates/fabro-model/src/catalog.rs

@@ -2137,7 +2137,8 @@ enabled = true
         let m = Catalog::builtin()
             .default_for_provider(&ProviderId::openai())
             .unwrap();
-        assert_eq!(m.id, "gpt-5.5");
+        assert_eq!(m.provider, ProviderId::openai());
+        assert!(m.default);
 
         let m = Catalog::builtin()
             .default_for_provider(&ProviderId::gemini())
@@ -2910,12 +2911,10 @@ probe = false
         ))
         .expect("sparse built-in model override should build");
 
+        let openai = ProviderId::openai();
         assert_eq!(
-            catalog
-                .probe_for_provider(&ProviderId::openai())
-                .unwrap()
-                .id,
-            "gpt-5.5"
+            catalog.probe_for_provider(&openai).unwrap().id,
+            catalog.default_for_provider(&openai).unwrap().id
         );
     }
 
@@ -3319,12 +3318,10 @@ small_default = false
         ))
         .expect("sparse built-in model override should build");
 
+        let openai = ProviderId::openai();
         assert_eq!(
-            catalog
-                .small_default_for_provider(&ProviderId::openai())
-                .unwrap()
-                .id,
-            "gpt-5.5"
+            catalog.small_default_for_provider(&openai).unwrap().id,
+            catalog.default_for_provider(&openai).unwrap().id
         );
     }
 
@@ -4447,6 +4444,9 @@ sampling_params = false
             "gpt-5.4",
             "gpt-5.4-mini",
             "gpt-5.5",
+            "gpt-5.6-luna",
+            "gpt-5.6-sol",
+            "gpt-5.6-terra",
         ] {
             assert_eq!(
                 catalog.get(model).unwrap().context_window(),

lib/crates/fabro-model/src/catalog/providers/openai.toml

@@ -8,6 +8,82 @@ priority = 90
 [providers.openai.auth]
 credentials = ["env:OPENAI_API_KEY", "vault:OPENAI_API_KEY", "vault:OPENAI_CODEX"]
 
+[models."gpt-5.6-sol"]
+provider = "openai"
+api_id = "gpt-5.6-sol"
+display_name = "GPT-5.6 Sol"
+family = "gpt-5"
+training = "2026-02-16"
+knowledge_cutoff = "February 16, 2026"
+default = true
+aliases = ["gpt56-sol", "gpt-56-sol", "gpt-5.6", "gpt56", "gpt-56"]
+
+[models."gpt-5.6-sol".limits]
+context_window = 272000
+max_output = 128000
+
+[models."gpt-5.6-sol".features]
+tools = true
+vision = true
+reasoning = true
+reasoning_effort = "levels"
+prompt_cache = true
+
+[models."gpt-5.6-sol".costs]
+input_cost_per_mtok = 5.0
+output_cost_per_mtok = 30.0
+cache_input_cost_per_mtok = 0.5
+
+[models."gpt-5.6-terra"]
+provider = "openai"
+api_id = "gpt-5.6-terra"
+display_name = "GPT-5.6 Terra"
+family = "gpt-5"
+training = "2026-02-16"
+knowledge_cutoff = "February 16, 2026"
+aliases = ["gpt56-terra", "gpt-56-terra"]
+
+[models."gpt-5.6-terra".limits]
+context_window = 272000
+max_output = 128000
+
+[models."gpt-5.6-terra".features]
+tools = true
+vision = true
+reasoning = true
+reasoning_effort = "levels"
+prompt_cache = true
+
+[models."gpt-5.6-terra".costs]
+input_cost_per_mtok = 2.5
+output_cost_per_mtok = 15.0
+cache_input_cost_per_mtok = 0.25
+
+[models."gpt-5.6-luna"]
+provider = "openai"
+api_id = "gpt-5.6-luna"
+display_name = "GPT-5.6 Luna"
+family = "gpt-5"
+training = "2026-02-16"
+knowledge_cutoff = "February 16, 2026"
+aliases = ["gpt56-luna", "gpt-56-luna"]
+
+[models."gpt-5.6-luna".limits]
+context_window = 272000
+max_output = 128000
+
+[models."gpt-5.6-luna".features]
+tools = true
+vision = true
+reasoning = true
+reasoning_effort = "levels"
+prompt_cache = true
+
+[models."gpt-5.6-luna".costs]
+input_cost_per_mtok = 1.0
+output_cost_per_mtok = 6.0
+cache_input_cost_per_mtok = 0.1
+
 [models."gpt-5.4"]
 provider = "openai"
 api_id = "gpt-5.4"
@@ -40,7 +116,6 @@ display_name = "GPT-5.5"
 family = "gpt-5"
 training = "2025-12-01"
 knowledge_cutoff = "December 2025"
-default = true
 estimated_output_tps = 70
 aliases = ["gpt55", "gpt-55"]
 

lib/crates/fabro-server/tests/it/api/runs.rs

@@ -1,5 +1,6 @@
 use axum::body::Body;
 use axum::http::{Request, StatusCode};
+use fabro_model::{Catalog, ProviderId};
 use fabro_types::settings::run::EnvironmentProvider;
 use tower::ServiceExt;
 
@@ -160,7 +161,13 @@ _version = 1
         created["ask_fabro"]["unavailable_reason"],
         "sandbox_not_ready"
     );
-    assert_eq!(created["ask_fabro"]["default_model"], "gpt-5.5");
+    let default_openai_model = Catalog::builtin()
+        .default_for_provider(&ProviderId::openai())
+        .expect("the built-in OpenAI provider should have a default model");
+    assert_eq!(
+        created["ask_fabro"]["default_model"].as_str(),
+        Some(default_openai_model.id())
+    );
 
     let get_request = Request::builder()
         .method("GET")