Accept reasoning-style content blocks in the Rust API parser

The Rust API layer rejected thinking-enabled responses because it only recognized text and tool_use content blocks. This commit extends the response and SSE parser types to accept reasoning-style content blocks and deltas, with regression coverage for both non-streaming and streaming responses.

Constraint: Keep parsing compatible with existing text and tool-use message flows
Rejected: Deserialize unknown content blocks into an untyped catch-all | would weaken protocol coverage and test precision
Confidence: high
Scope-risk: narrow
Directive: Keep new protocol variants covered at the API boundary so downstream code can make explicit choices about preservation vs. ignoring
Tested: cargo test -p api thinking -- --nocapture
Not-tested: Live API traffic from a real thinking-enabled model

rust/crates/api/src/sse.rs

@@ -216,4 +216,64 @@ mod tests {
             ))
         );
     }
+
+    #[test]
+    fn parses_thinking_content_block_start() {
+        let frame = concat!(
+            "event: content_block_start\n",
+            "data: {\"type\":\"content_block_start\",\"index\":0,\"content_block\":{\"type\":\"thinking\",\"thinking\":\"\",\"signature\":null}}\n\n"
+        );
+
+        let event = parse_frame(frame).expect("frame should parse");
+        assert_eq!(
+            event,
+            Some(StreamEvent::ContentBlockStart(
+                crate::types::ContentBlockStartEvent {
+                    index: 0,
+                    content_block: OutputContentBlock::Thinking {
+                        thinking: String::new(),
+                        signature: None,
+                    },
+                },
+            ))
+        );
+    }
+
+    #[test]
+    fn parses_thinking_related_deltas() {
+        let thinking = concat!(
+            "event: content_block_delta\n",
+            "data: {\"type\":\"content_block_delta\",\"index\":0,\"delta\":{\"type\":\"thinking_delta\",\"thinking\":\"step 1\"}}\n\n"
+        );
+        let signature = concat!(
+            "event: content_block_delta\n",
+            "data: {\"type\":\"content_block_delta\",\"index\":0,\"delta\":{\"type\":\"signature_delta\",\"signature\":\"sig_123\"}}\n\n"
+        );
+
+        let thinking_event = parse_frame(thinking).expect("thinking delta should parse");
+        let signature_event = parse_frame(signature).expect("signature delta should parse");
+
+        assert_eq!(
+            thinking_event,
+            Some(StreamEvent::ContentBlockDelta(
+                crate::types::ContentBlockDeltaEvent {
+                    index: 0,
+                    delta: ContentBlockDelta::ThinkingDelta {
+                        thinking: "step 1".to_string(),
+                    },
+                }
+            ))
+        );
+        assert_eq!(
+            signature_event,
+            Some(StreamEvent::ContentBlockDelta(
+                crate::types::ContentBlockDeltaEvent {
+                    index: 0,
+                    delta: ContentBlockDelta::SignatureDelta {
+                        signature: "sig_123".to_string(),
+                    },
+                }
+            ))
+        );
+    }
 }

rust/crates/api/src/types.rs

@@ -135,6 +135,15 @@ pub enum OutputContentBlock {
         name: String,
         input: Value,
     },
+    Thinking {
+        #[serde(default)]
+        thinking: String,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        signature: Option<String>,
+    },
+    RedactedThinking {
+        data: Value,
+    },
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -190,6 +199,8 @@ pub struct ContentBlockDeltaEvent {
 pub enum ContentBlockDelta {
     TextDelta { text: String },
     InputJsonDelta { partial_json: String },
+    ThinkingDelta { thinking: String },
+    SignatureDelta { signature: String },
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]