Expose structured thinking without polluting normal assistant output

Extended thinking needed to travel end-to-end through the API,
runtime, and CLI so the client can request a thinking budget,
preserve streamed reasoning blocks, and present them in a
collapsed text-first form. The implementation keeps thinking
strictly opt-in, adds a session-local toggle, and reuses the
existing flag/slash-command/reporting surfaces instead of
introducing a new UI layer.

Constraint: Existing non-thinking text/tool flows had to remain backward compatible by default
Constraint: Terminal UX needed a lightweight collapsed representation rather than an interactive TUI widget
Rejected: Heuristic CLI-only parsing of reasoning text | brittle against structured stream payloads
Rejected: Expanded raw thinking output by default | too noisy for normal assistant responses
Confidence: medium
Scope-risk: moderate
Reversibility: clean
Directive: Keep thinking blocks structurally separate from answer text unless the upstream API contract changes
Tested: cargo fmt --all; cargo clippy --workspace --all-targets -- -D warnings; cargo test -q
Not-tested: Live upstream thinking payloads against the production API contract

rust/crates/runtime/src/session.rs

@@ -19,6 +19,10 @@ pub enum ContentBlock {
     Text {
         text: String,
     },
+    Thinking {
+        text: String,
+        signature: Option<String>,
+    },
     ToolUse {
         id: String,
         name: String,
@@ -257,6 +261,19 @@ impl ContentBlock {
                 object.insert("type".to_string(), JsonValue::String("text".to_string()));
                 object.insert("text".to_string(), JsonValue::String(text.clone()));
             }
+            Self::Thinking { text, signature } => {
+                object.insert(
+                    "type".to_string(),
+                    JsonValue::String("thinking".to_string()),
+                );
+                object.insert("text".to_string(), JsonValue::String(text.clone()));
+                if let Some(signature) = signature {
+                    object.insert(
+                        "signature".to_string(),
+                        JsonValue::String(signature.clone()),
+                    );
+                }
+            }
             Self::ToolUse { id, name, input } => {
                 object.insert(
                     "type".to_string(),
@@ -303,6 +320,13 @@ impl ContentBlock {
             "text" => Ok(Self::Text {
                 text: required_string(object, "text")?,
             }),
+            "thinking" => Ok(Self::Thinking {
+                text: required_string(object, "text")?,
+                signature: object
+                    .get("signature")
+                    .and_then(JsonValue::as_str)
+                    .map(ToOwned::to_owned),
+            }),
             "tool_use" => Ok(Self::ToolUse {
                 id: required_string(object, "id")?,
                 name: required_string(object, "name")?,