fix: Fixes race conditions and missing error handling that cause subagents to hang indefinitely when errors occur.

packages/opencode/src/session/prompt.ts

@@ -82,6 +82,15 @@ export namespace SessionPrompt {
     },
     async (current) => {
       for (const item of Object.values(current)) {
+        // Reject all pending callbacks to prevent callers from hanging
+        const cancelError = new DOMException("Session cancelled", "AbortError")
+        for (const callback of item.callbacks) {
+          try {
+            callback.reject(cancelError)
+          } catch (e) {
+            log.error("failed to reject callback during dispose", { error: e })
+          }
+        }
         item.abort.abort()
       }
     },
@@ -265,6 +274,15 @@ export namespace SessionPrompt {
       SessionStatus.set(sessionID, { type: "idle" })
       return
     }
+    // Reject all pending callbacks to prevent callers from hanging
+    const cancelError = new DOMException("Session cancelled", "AbortError")
+    for (const callback of match.callbacks) {
+      try {
+        callback.reject(cancelError)
+      } catch (e) {
+        log.error("failed to reject callback during cancel", { error: e, sessionID })
+      }
+    }
     match.abort.abort()
     delete s[sessionID]
     SessionStatus.set(sessionID, { type: "idle" })
@@ -281,8 +299,14 @@ export namespace SessionPrompt {
     const abort = resume_existing ? resume(sessionID) : start(sessionID)
     if (!abort) {
       return new Promise<MessageV2.WithParts>((resolve, reject) => {
-        const callbacks = state()[sessionID].callbacks
-        callbacks.push({ resolve, reject })
+        const current = state()
+        const sessionState = current[sessionID]
+        // Check if session state exists to prevent hanging on race conditions
+        if (!sessionState) {
+          reject(new DOMException("Session state not found", "AbortError"))
+          return
+        }
+        sessionState.callbacks.push({ resolve, reject })
       })
     }
 

packages/opencode/src/tool/task.ts

@@ -127,22 +127,28 @@ export const TaskTool = Tool.define("task", async (ctx) => {
       using _ = defer(() => ctx.abort.removeEventListener("abort", cancel))
       const promptParts = await SessionPrompt.resolvePromptParts(params.prompt)
 
-      const result = await SessionPrompt.prompt({
-        messageID,
-        sessionID: session.id,
-        model: {
-          modelID: model.modelID,
-          providerID: model.providerID,
-        },
-        agent: agent.name,
-        tools: {
-          todowrite: false,
-          todoread: false,
-          ...(hasTaskPermission ? {} : { task: false }),
-          ...Object.fromEntries((config.experimental?.primary_tools ?? []).map((t) => [t, false])),
-        },
-        parts: promptParts,
-      })
+      let result: MessageV2.WithParts
+      try {
+        result = await SessionPrompt.prompt({
+          messageID,
+          sessionID: session.id,
+          model: {
+            modelID: model.modelID,
+            providerID: model.providerID,
+          },
+          agent: agent.name,
+          tools: {
+            todowrite: false,
+            todoread: false,
+            ...(hasTaskPermission ? {} : { task: false }),
+            ...Object.fromEntries((config.experimental?.primary_tools ?? []).map((t) => [t, false])),
+          },
+          parts: promptParts,
+        })
+      } catch (error) {
+        // Ensure proper error propagation to parent agent
+        throw new Error(`Subagent execution failed: ${error instanceof Error ? error.message : String(error)}`)
+      }
 
       const text = result.parts.findLast((x) => x.type === "text")?.text ?? ""