fix(provider): complete DeepSeek reasoning_content round-trip for multi-turn conversations

Fixes the two-layer bug where reasoning_content is dropped on conversation
replay for DeepSeek thinking mode and OpenRouter-routed DeepSeek models.

Three changes:

1. provider.ts: Auto-enable interleaved for reasoning models
   - When model.reasoning is true but interleaved is not explicitly set,
     default to { field: "reasoning_content" } instead of false
   - This triggers the interleaved transform that extracts reasoning
     and passes it via providerOptions

2. transform.ts: Use dynamic SDK key in interleaved transform
   - Replace hardcoded "openaiCompatible" with sdkKey(model.api.npm)
   - Fixes OpenRouter provider which expects "openrouter" key, not
     "openaiCompatible" (prevents key mismatch in providerOptions)

3. transform.ts: Inject reasoning_content for ALL assistant messages
   - New fallback transform fires when capabilities.reasoning is true
   - Sets reasoning_content: "" in providerOptions for every assistant
     message, including historical messages stored before reasoning mode
     was enabled (no reasoning part to extract from)
   - Also expands DeepSeek detection to check model.id in addition to
     model.api.id, covering OpenRouter-routed DeepSeek models

Closes #24104
Related: #24203 (OpenRouter users still affected by PR #24218 alone)
Supersedes partial fix from PR #24146 (merged but incomplete)

Author: fenk

packages/opencode/src/provider/provider.ts

@@ -1177,7 +1177,7 @@ const layer: Layer.Layer<
                     model.modalities?.output?.includes("video") ?? existingModel?.capabilities.output.video ?? false,
                   pdf: model.modalities?.output?.includes("pdf") ?? existingModel?.capabilities.output.pdf ?? false,
                 },
-                interleaved: model.interleaved ?? existingModel?.capabilities.interleaved ?? false,
+                interleaved: model.interleaved ?? existingModel?.capabilities.interleaved ?? (model.reasoning ? { field: "reasoning_content" } : false),
               },
               cost: {
                 input: model?.cost?.input ?? existingModel?.cost?.input ?? 0,

packages/opencode/src/provider/transform.ts

@@ -176,7 +176,8 @@ function normalizeMessages(
   }
 
   // Deepseek requires all assistant messages to have reasoning on them
-  if (model.api.id.includes("deepseek")) {
+  // Check both API ID and model ID to cover OpenRouter-routed DeepSeek models
+  if (model.api.id.includes("deepseek") || model.id.includes("deepseek")) {
     msgs = msgs.map((msg) => {
       if (msg.role !== "assistant") return msg
       if (Array.isArray(msg.content)) {
@@ -195,6 +196,7 @@ function normalizeMessages(
 
   if (typeof model.capabilities.interleaved === "object" && model.capabilities.interleaved.field) {
     const field = model.capabilities.interleaved.field
+    const sdk = sdkKey(model.api.npm) ?? "openaiCompatible"
     return msgs.map((msg) => {
       if (msg.role === "assistant" && Array.isArray(msg.content)) {
         const reasoningParts = msg.content.filter((part: any) => part.type === "reasoning")
@@ -211,8 +213,8 @@ function normalizeMessages(
           content: filteredContent,
           providerOptions: {
             ...msg.providerOptions,
-            openaiCompatible: {
-              ...msg.providerOptions?.openaiCompatible,
+            [sdk]: {
+              ...msg.providerOptions?.[sdk],
               [field]: reasoningText,
             },
           },
@@ -223,6 +225,36 @@ function normalizeMessages(
     })
   }
 
+  // When reasoning is active but interleaved is not configured, still inject empty reasoning_content
+  // for ALL assistant messages. This covers historical messages from DB that were stored before
+  // reasoning mode was enabled — they have no reasoning part to extract but DeepSeek's API still
+  // requires reasoning_content on every assistant turn in thinking mode.
+  if (model.capabilities.reasoning) {
+    msgs = msgs.map((msg) => {
+      if (msg.role !== "assistant") return msg
+      if (Array.isArray(msg.content)) {
+        const sdk = sdkKey(model.api.npm) ?? "openaiCompatible"
+        return {
+          ...msg,
+          providerOptions: {
+            ...msg.providerOptions,
+            [sdk]: {
+              ...msg.providerOptions?.[sdk],
+              reasoning_content: "",
+            },
+          },
+        }
+      }
+      if (typeof msg.content === "string") {
+        return {
+          ...msg,
+          content: [{ type: "text" as const, text: msg.content }, { type: "reasoning" as const, text: "" }],
+        }
+      }
+      return msg
+    })
+  }
+
   return msgs
 }