From a78cb5f36f1315c5f4079588351227b4546fd529 Mon Sep 17 00:00:00 2001
From: skeem <215233343+skeeminator@users.noreply.github.com>
Date: Sun, 26 Apr 2026 23:48:15 -0700
Subject: [PATCH] fix(openaiShim): unconditionally set reasoning_content for
 DeepSeek V4 thinking mode
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DeepSeek V4 (and similar reasoning providers) require the reasoning_content
field to be present on every assistant message when thinking mode is
enabled — even as an empty string. The previous guard only set it when
thinking text was non-empty (typeof === 'string' && trim().length > 0),
causing 400 errors on multi-turn tool-call rounds.

Three sites fixed in convertMessages():
- Assistant messages with thinking blocks: now always set reasoning_content
  (fallback to empty string when thinking text is absent)
- Coalescing synthetic interrupt message: added reasoning_content: ''
- String-content fallback branch: added reasoning_content: '' when
  preserveReasoningContent is true
---
 src/services/api/openaiShim.ts | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/services/api/openaiShim.ts b/src/services/api/openaiShim.ts
index 015c0e2e4..e258f1186 100644
--- a/src/services/api/openaiShim.ts
+++ b/src/services/api/openaiShim.ts
@@ -539,9 +539,11 @@ function convertMessages(
         // (harmless) or strict-reject unknown fields (harmful).
         if (preserveReasoningContent) {
           const thinkingText = (thinkingBlock as { thinking?: string } | undefined)?.thinking
-          if (typeof thinkingText === 'string' && thinkingText.trim().length > 0) {
-            assistantMsg.reasoning_content = thinkingText
-          }
+          // DeepSeek V4 (and similar reasoning providers) require reasoning_content
+          // to be present on every assistant message, even as empty string, when
+          // thinking mode is enabled. The field's presence satisfies the API contract;
+          // omitting it causes 400: "reasoning_content must be passed back to the API".
+          assistantMsg.reasoning_content = typeof thinkingText === 'string' ? thinkingText : ''
         }
 
         if (toolUses.length > 0) {
@@ -635,6 +637,12 @@ function convertMessages(
           })(),
         }
 
+        // Reasoning providers require reasoning_content on every assistant
+        // message when thinking mode is active (empty string is accepted).
+        if (preserveReasoningContent) {
+          assistantMsg.reasoning_content = ''
+        }
+
         if (assistantMsg.content) {
           result.push(assistantMsg)
         }
@@ -659,6 +667,7 @@ function convertMessages(
       coalesced.push({
         role: 'assistant',
         content: '[Tool execution interrupted by user]',
+        reasoning_content: '',
       })
     }