[Bug]: SSE reasoning stream deltas (delta.reasoning / thought) accumulate duplicate state objects in SQLite causing context explosion across turns

Description

During multi-turn testing with reasoning models (such as ox-alpha and models outputting streaming thought/reasoning tokens), we observed severe prompt token inflation across consecutive turns. After investigating the local session storage, we found that every streaming reasoning SSE chunk (delta.reasoning / delta.thought) was being persisted as an individual { type: "reasoning", state: ... } object inside session_message.content[] instead of being concatenated / folded into a single reasoning content block.

(Attribution: Discovered during multi-turn testing with ox-alpha/reasoning models; investigated and code-isolated together with my AI pairing assistant).

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Empirical Evidence & Token Metrics

1. SQLite Inspection (session_message)

Inspecting the SQLite database for a single streaming assistant message containing reasoning tokens revealed over 375 discrete reasoning objects within content:

// session_message.content (excerpt)
[
  { "type": "reasoning", "state": { "text": "Let" } },
  { "type": "reasoning", "state": { "text": " me" } },
  { "type": "reasoning", "state": { "text": " check" } },
  ... // 375+ entries (~1.09 MB of redundant JSON structure per single response)
]

2. Context Inflation Across Turns

When subsequent turns re-serialize the full conversation history to upstream providers, the repeated JSON envelope overhead and duplicated objects inflate prompt context rapidly:

Turn Count	Prompt Tokens (Without Folding)	Prompt Tokens (With In-Place Folding)	Context Bloat Factor
Turn 1	~2.5k	~2.5k	1.0x
Turn 5	~118k	~14k	~8.4x
Turn 10	300k+ (Context Explosion / OOM)	~28k	~10.7x

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Root Cause Analysis

In the streaming reducer / message processor, incoming SSE delta chunks containing delta.reasoning or delta.thought push a new object onto the message content[] array on each chunk arrival, rather than identifying the existing reasoning block and appending chunk text in-place:

// Problematic behavior:
// Every chunk creates a new content array element
content.push({ type: "reasoning", state: { text: chunk.delta.reasoning } });

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Proposed Fix

1. In-Place Stream Accumulator / Reducer

When processing incoming delta chunks, fold into the active reasoning block or append to the existing block:

function reduceReasoningDelta(content: ContentPart[], deltaText: string): ContentPart[] {
  const lastPart = content[content.length - 1];

  if (lastPart && lastPart.type === "reasoning") {
    // Fold in-place
    lastPart.state = {
      ...lastPart.state,
      text: (lastPart.state?.text ?? "") + deltaText,
    };
  } else {
    // Initialize single reasoning part
    content.push({
      type: "reasoning",
      state: { text: deltaText },
    });
  }

  return content;
}

2. Request Scrubbing Before Dispatch

When serializing conversation history for upstream LLM completion requests, ensure consecutive/fragmented reasoning blocks are compacted into single text blocks or filtered according to provider capabilities:

function compactMessageContent(content: ContentPart[]): ContentPart[] {
  return content.reduce<ContentPart[]>((acc, current) => {
    const prev = acc[acc.length - 1];
    if (prev && prev.type === "reasoning" && current.type === "reasoning") {
      prev.state = {
        ...prev.state,
        text: (prev.state?.text ?? "") + (current.state?.text ?? ""),
      };
      return acc;
    }
    acc.push(current);
    return acc;
  }, []);
}

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Additional Context

• OpenCode Version: 1.18.x / latest
• OS: Linux / WSL2 / macOS
• Willingness to contribute: Happy to open a clean PR with unit tests for the stream reducer and message normalizer if maintainers would like!