fix: require reasoning block before injecting context for Claude models

- Add hasReasoningInCurrentAssistantTurn to verify Claude has emitted its
  required thinking block before we inject context
- Add isIgnoredUserMessage to skip UI-only messages when checking turn state
- Update getLastUserMessage to skip ignored messages for accurate model detection
- Split GitHub Copilot and Anthropic injection guards into separate checks

Author: Tarquinen

lib/messages/inject.ts

@@ -7,6 +7,8 @@ import {
     extractParameterKey,
     buildToolIdList,
     createSyntheticAssistantMessageWithToolPart,
+    isIgnoredUserMessage,
+    hasReasoningInCurrentAssistantTurn,
 } from "./utils"
 import { getFilePathFromParameters, isProtectedFilePath } from "../protected-file-patterns"
 import { getLastUserMessage } from "../shared-utils"
@@ -144,9 +146,17 @@ export const insertPruneToolContext = (
         providerID === "github-copilot" || providerID === "github-copilot-enterprise"
     const isAnthropic = modelID.includes("claude")
 
-    if (isGitHubCopilot || isAnthropic) {
+    if (isGitHubCopilot) {
         const lastMessage = messages[messages.length - 1]
-        if (lastMessage?.info?.role === "user") {
+        if (lastMessage?.info?.role === "user" && !isIgnoredUserMessage(lastMessage)) {
+            return
+        }
+    }
+
+    // Anthropic extended thinking models require a thinking block at the start of its turn
+    // This can probably be improved further to only trigger for the appropriate thinking settings
+    if (isAnthropic) {
+        if (!hasReasoningInCurrentAssistantTurn(messages)) {
             return
         }
     }

lib/messages/utils.ts

@@ -193,3 +193,37 @@ export function buildToolIdList(
     }
     return toolIds
 }
+
+export const isIgnoredUserMessage = (message: WithParts): boolean => {
+    if (!message.parts || message.parts.length === 0) {
+        return true
+    }
+
+    for (const part of message.parts) {
+        if (!(part as any).ignored) {
+            return false
+        }
+    }
+
+    return true
+}
+
+export const hasReasoningInCurrentAssistantTurn = (messages: WithParts[]): boolean => {
+    for (let i = messages.length - 1; i >= 0; i--) {
+        const message = messages[i]
+        if (message.info?.role === "user") {
+            if (isIgnoredUserMessage(message)) {
+                continue
+            }
+            return false
+        }
+        if (message.info?.role === "assistant" && message.parts) {
+            for (const part of message.parts) {
+                if (part.type === "reasoning") {
+                    return true
+                }
+            }
+        }
+    }
+    return false
+}

lib/shared-utils.ts

@@ -1,4 +1,5 @@
 import { SessionState, WithParts } from "./state"
+import { isIgnoredUserMessage } from "./messages/utils"
 
 export const isMessageCompacted = (state: SessionState, msg: WithParts): boolean => {
     return msg.info.time.created < state.lastCompaction
@@ -7,7 +8,7 @@ export const isMessageCompacted = (state: SessionState, msg: WithParts): boolean
 export const getLastUserMessage = (messages: WithParts[]): WithParts | null => {
     for (let i = messages.length - 1; i >= 0; i--) {
         const msg = messages[i]
-        if (msg.info.role === "user") {
+        if (msg.info.role === "user" && !isIgnoredUserMessage(msg)) {
             return msg
         }
     }