fix: handle stream interruption for OpenAI-compatible providers

- Add SSE timeout, connection reset, abort, and stream truncation
  patterns to retryable() matching for automatic retry
- Classify 'SSE read timed out' as APIError(isRetryable: true)
- Emit error event in flush() when stream ends without finish_reason
  while output is still active, instead of silently accepting truncation

Closes #20466

Author: jwcrystal

packages/opencode/src/provider/provider.ts

@@ -845,7 +845,7 @@ const ProviderCapabilities = Schema.Struct({
   input: ProviderModalities,
   output: ProviderModalities,
   interleaved: ProviderInterleaved,
-  systemMessage: Schema.Union([Schema.Literal("single"), Schema.Literal("multiple")]).optional(),
+  systemMessage: Schema.optional(Schema.Union([Schema.Literal("single"), Schema.Literal("multiple")])),
 })
 
 const ProviderCacheCost = Schema.Struct({

packages/opencode/src/provider/sdk/copilot/chat/openai-compatible-chat-language-model.ts

@@ -645,6 +645,15 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
           },
 
           flush(controller) {
+            // If the stream ended without a finish_reason and output is still
+            // active, the stream was truncated — emit an error to trigger retry
+            const hasActiveOutput = isActiveReasoning || isActiveText || toolCalls.some((tc) => !tc.hasFinished)
+            if (finishReason.unified === "other" && finishReason.raw === undefined && hasActiveOutput) {
+              controller.enqueue({
+                type: "error",
+                error: new Error("Stream ended unexpectedly — no finish reason received while output was still active"),
+              })
+            }
             if (isActiveReasoning) {
               controller.enqueue({
                 type: "reasoning-end",

packages/opencode/src/session/message-v2.ts

@@ -1028,6 +1028,15 @@ export function fromError(
         },
         { cause: e },
       ).toObject()
+    case e instanceof Error && e.message === "SSE read timed out":
+      return new APIError(
+        {
+          message: "Stream read timed out — no data received within the chunk timeout window",
+          isRetryable: true,
+          metadata: { code: "SSE_TIMEOUT", message: e.message },
+        },
+        { cause: e },
+      ).toObject()
     case e instanceof Error:
       return new NamedError.Unknown({ message: errorMessage(e) }, { cause: e }).toObject()
     default:

packages/opencode/src/session/retry.ts

@@ -63,14 +63,18 @@ export function retryable(error: Err) {
     return error.data.message.includes("Overloaded") ? "Provider is overloaded" : error.data.message
   }
 
-  // Check for rate limit patterns in plain text error messages
+  // Check for stream interruption and rate limit patterns in plain text error messages
   const msg = error.data?.message
   if (typeof msg === "string") {
     const lower = msg.toLowerCase()
     if (
       lower.includes("rate increased too quickly") ||
       lower.includes("rate limit") ||
-      lower.includes("too many requests")
+      lower.includes("too many requests") ||
+      lower.includes("sse read timed out") ||
+      lower.includes("connection reset") ||
+      lower.includes("aborted") ||
+      lower.includes("stream ended unexpectedly")
     ) {
       return msg
     }