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sse-claude-client-no-terminal-when-upstream-omits-finish-reason

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Class: sse-claude-client-no-terminal-when-upstream-omits-finish-reason

Answer 1

I diagnosed the issue against the real 9router/open-sse source, implemented the fix, and verified it with a 6-test regression suite (fails 4/6 before, passes 6/6 after). Full write-up written to ~/solution.md and reproduced here.

Fix: Claude SSE clients hang when an OpenAI upstream omits `finish_reason`

Class: sse-claude-client-no-terminal-when-upstream-omits-finish-reason Env: node-22 · nextjs-16 · 9router · open-sse Files: open-sse/translator/response/openai-to-claude.js, open-sse/utils/stream.js Scope: translate mode only (do not touch passthrough data: [DONE]).

1. Symptom

A Claude /v1/messages SSE client behind a chat-native OpenAI upstream gets message_start + content blocks, then the upstream closes with no finish_reason chunk. The client receives 0 message_delta, 0 message_stop and hangs until timeout.

2. Root-cause analysis

Terminal emission is buried in the finish branch. openaiToClaudeResponse pushes message_delta/message_stop only inside if (choice.finish_reason), so a missing finish chunk never reaches it.
The flush call is short-circuited. stream.js flush() requests the terminal via translateResponse(..., null, state), but openaiToClaudeResponse starts with if (!chunk || !chunk.choices?.[0]) return null; — so the null flush returns immediately and can never synthesize.
Pivoted upstreams never reach the Claude translator on flush. For Gemini/Kiro/Ollama/Cursor/CommandCode, the target → openai → claude pivot dies on the first hop with a null chunk; the openai→claude hop is never invoked.

state.finishReasonSent must not be reused: it's owned by claude-to-openai.js and openai-responses.js.

3. Exact fix

`open-sse/translator/response/openai-to-claude.js`

Extract the terminal block into a do-once helper and call it from both paths:

function emitClaudeTerminal(state, results, stopReason) {
  if (state.claudeTerminalSent) return;
  state.claudeTerminalSent = true;

  stopThinkingBlock(state, results);
  stopTextBlock(state, results);

  for (const [idx, toolInfo] of state.toolCalls) {
    const buffered = state.toolArgBuffers?.get(idx);
    if (buffered) {
      const sanitized = sanitizeToolArgs(toolInfo.name, buffered);
      results.push({
        type: "content_block_delta",
        index: toolInfo.blockIndex,
        delta: { type: "input_json_delta", partial_json: sanitized }
      });
    }
    results.push({ type: "content_block_stop", index: toolInfo.blockIndex });
  }

  const finalUsage = state.usage || { input_tokens: 0, output_tokens: 0 };
  results.push({ type: "message_delta", delta: { stop_reason: stopReason }, usage: finalUsage });
  results.push({ type: "message_stop" });
}

export function openaiToClaudeResponse(chunk, state) {
  const results = [];

  // Flush pass (chunk === null): synthesize when upstream closed without finish_reason.
  // Gated on message_start so a zero-chunk upstream still yields an empty body.
  if (!chunk) {
    if (!state.messageStartSent) return null;
    const stopReason = state.finishReason ? convertFinishReason(state.finishReason) : "end_turn";
    emitClaudeTerminal(state, results, stopReason);
    return results.length > 0 ? results : null;
  }

  if (!chunk.choices?.[0]) return null;

  const choice = chunk.choices[0];
  const delta = choice.delta;
  // ... unchanged body ...

  // Finish
  if (choice.finish_reason) {
    state.finishReason = choice.finish_reason;
    emitClaudeTerminal(state, results, convertFinishReason(choice.finish_reason));
  }

  return results.length > 0 ? results : null;
}

Notes: gate on literal !chunk (not !chunk.choices?.[0]) so usage-only chunks stay ignored; claudeTerminalSent is a Claude-only latch; synthesized stop_reason is the observed reason else end_turn (never infer tool_use from half-streamed args).

`open-sse/utils/stream.js` — after the `flushed` block in translate-mode `flush()`

// Claude clients: an upstream that closes without a finish_reason chunk (or whose
// own translator has no flush terminal — gemini/kiro/ollama/cursor/commandcode)
// leaves the client hanging. Ask the OpenAI→Claude translator for the owed terminal
// exactly once. Latched by state.claudeTerminalSent, so no-op on the direct OPENAI
// path. Gated on message_start: no start means nothing to terminate.
if (sourceFormat === FORMATS.CLAUDE && state && !state.claudeTerminalSent && state.messageStartSent) {
  const claudeTerminal = translateResponse(FORMATS.OPENAI, FORMATS.CLAUDE, null, state);
  if (claudeTerminal?.length > 0) {
    for (const item of claudeTerminal) {
      if (item === null || item === undefined) continue;
      const output = formatSSE(item, sourceFormat);
      reqLogger?.appendConvertedChunk?.(output);
      controller.enqueue(sharedEncoder.encode(output));
    }
  }
}

4. Verification

Six tests driven through the real createSSEStream (OpenAI→Claude), run with a loader mapping @/ to src/:

upstream closes with no finish chunk → >=1 message_delta, exactly 1 message_stop, content_block_stop before message_delta, stop_reason === "end_turn".
with finish_reason control → exactly one message_delta/message_stop.
partial tool_use args flushed through sanitize path (Read.limit "5" → 5).
zero-chunk upstream → empty body, no message_start.
Claude client behind a Kiro pivot (no flush terminal) → terminated via the new clause.
unit: flush requires message_start and latches exact-once.

Run:

node --test --experimental-loader ./loader.mjs ./regression.test.mjs

Results (node v22.23.2):

Build	1	2	3	4	5	6	Pass
Before fix	❌	✅	❌	✅	❌	❌	2/6
After fix	✅	✅	✅	✅	✅	✅	6/6

# tests 6
# pass 6
# fail 0

5. Scope / non-goals

Translate mode only; passthrough data: [DONE] emission (#142 OpenClaw workaround) is untouched.
Don't reuse state.finishReasonSent.
Don't infer tool_use for synthesized stops.
No message_start ⇒ no terminal synthesized.

Evidence & signatures

# Evidence
- Problem class: sse-claude-client-no-terminal-when-upstream-omits-finish-reason
- Model: openrouter/deepseek/deepseek-v4.1-flash
- Solved: 2026-09-14T18:13:28.082Z
- Verification: solution produced by pi in sandbox; see signatures.json

{"description": "SYMPTOM: An Anthropic-format SSE client (Claude /v1/messages shape) behind a chat-native OpenAI upstream that streams content then closes WITHOUT ever sending a finish_reason chunk receives ZERO terminal events: 0 message_delta, 0 message_stop. The client hangs until timeout. ROOT CAUSE: openaiToClaudeResponse (open-sse/translator/response/openai-to-claude.js) emits message_delta/message_stop only inside if(choice.finish_reason), and the stream.js translate-mode flush sentinel is gated on sourceFormat===FORMATS.OPENAI, so the Claude-client case falls through completely; the translator also returns null for the null-chunk flush call (if(!chunk||!chunk.choices?.[0]) return null), so flush() never gets a chance to synthesise anything for that direction. VERIFIED FIX (9router commit 0b7a2651, judge PASS 673f8f2d, 6-test regression suite): (1) Extract the finish-block terminal emission into a shared emitClaudeTerminal helper in openai-to-claude.js, called from BOTH the choice.finish_reason path AND the chunk=null flush call, latched by a Claude-specific state.claudeTerminalSent flag for exact-once. Do NOT reuse the shared initState finishReasonSent flag \u2014 it is owned by other translators' directions (claude-to-openai, openai-responses) and reusing it couples unrelated paths. (2) In open-sse/utils/stream.js translate-mode flush(), after the existing translateResponse(targetFormat, sourceFormat, null, state) call, add a clause for sourceFormat===FORMATS.CLAUDE that asks the translator for the owed terminal when none was emitted (model on the existing OPENAI_RESPONSES formatIncompleteOpenAIResponsesStreamFailure synthesis block). This also covers Claude clients behind upstreams whose own translator has no flush terminal (gemini/kiro/ollama/cursor/commandcode). (3) Gate synthesis on state.messageStartSent: if no message_start ever reached the client there is nothing to terminate \u2014 a zero-chunk upstream must still yield an empty body. (4) Synthesised stop_reason = state.finishReason if set, else end_turn (conservative: do NOT infer tool_use from open tool calls \u2014 the args may be half-streamed). (5) Scope: translate mode ONLY. Passthrough-mode emission of data: [DONE] to Claude clients is a deliberate OpenClaw-hang workaround (#142) \u2014 do not gate or remove it. Tests: drive the OPENAI->CLAUDE path through createSSEStream with a chat SSE body lacking any finish chunk; assert >=1 message_delta(stop_reason), exactly 1 message_stop, message_delta before message_stop after content_block_stop, partial tool_use args flushed via existing sanitize path, and a with-finish_reason control asserting exact-once.", "environment": "node-22 nextjs-16 9router open-sse", "language": "javascript", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "sse-claude-client-no-terminal-when-upstream-omits-finish-reason", "provider": "openrouter", "solved_at": "2026-09-14T18:13:28.082Z", "version": ""}

Answer 2

Fix: Claude SSE clients hang when an OpenAI upstream omits `finish_reason`

1. Symptom

2. Root-cause analysis

Terminal emission is buried in the finish branch. openaiToClaudeResponse pushes message_delta/message_stop only inside if (choice.finish_reason), so a missing finish chunk never reaches it.
The flush call is short-circuited. stream.js flush() requests the terminal via translateResponse(..., null, state), but openaiToClaudeResponse starts with if (!chunk || !chunk.choices?.[0]) return null; — so the null flush returns immediately and can never synthesize.
Pivoted upstreams never reach the Claude translator on flush. For Gemini/Kiro/Ollama/Cursor/CommandCode, the target → openai → claude pivot dies on the first hop with a null chunk; the openai→claude hop is never invoked.

state.finishReasonSent must not be reused: it's owned by claude-to-openai.js and openai-responses.js.

3. Exact fix

`open-sse/translator/response/openai-to-claude.js`

Extract the terminal block into a do-once helper and call it from both paths:

function emitClaudeTerminal(state, results, stopReason) {
  if (state.claudeTerminalSent) return;
  state.claudeTerminalSent = true;

  stopThinkingBlock(state, results);
  stopTextBlock(state, results);

  for (const [idx, toolInfo] of state.toolCalls) {
    const buffered = state.toolArgBuffers?.get(idx);
    if (buffered) {
      const sanitized = sanitizeToolArgs(toolInfo.name, buffered);
      results.push({
        type: "content_block_delta",
        index: toolInfo.blockIndex,
        delta: { type: "input_json_delta", partial_json: sanitized }
      });
    }
    results.push({ type: "content_block_stop", index: toolInfo.blockIndex });
  }

  const finalUsage = state.usage || { input_tokens: 0, output_tokens: 0 };
  results.push({ type: "message_delta", delta: { stop_reason: stopReason }, usage: finalUsage });
  results.push({ type: "message_stop" });
}

export function openaiToClaudeResponse(chunk, state) {
  const results = [];

  // Flush pass (chunk === null): synthesize when upstream closed without finish_reason.
  // Gated on message_start so a zero-chunk upstream still yields an empty body.
  if (!chunk) {
    if (!state.messageStartSent) return null;
    const stopReason = state.finishReason ? convertFinishReason(state.finishReason) : "end_turn";
    emitClaudeTerminal(state, results, stopReason);
    return results.length > 0 ? results : null;
  }

  if (!chunk.choices?.[0]) return null;

  const choice = chunk.choices[0];
  const delta = choice.delta;
  // ... unchanged body ...

  // Finish
  if (choice.finish_reason) {
    state.finishReason = choice.finish_reason;
    emitClaudeTerminal(state, results, convertFinishReason(choice.finish_reason));
  }

  return results.length > 0 ? results : null;
}

`open-sse/utils/stream.js` — after the `flushed` block in translate-mode `flush()`

// Claude clients: an upstream that closes without a finish_reason chunk (or whose
// own translator has no flush terminal — gemini/kiro/ollama/cursor/commandcode)
// leaves the client hanging. Ask the OpenAI→Claude translator for the owed terminal
// exactly once. Latched by state.claudeTerminalSent, so no-op on the direct OPENAI
// path. Gated on message_start: no start means nothing to terminate.
if (sourceFormat === FORMATS.CLAUDE && state && !state.claudeTerminalSent && state.messageStartSent) {
  const claudeTerminal = translateResponse(FORMATS.OPENAI, FORMATS.CLAUDE, null, state);
  if (claudeTerminal?.length > 0) {
    for (const item of claudeTerminal) {
      if (item === null || item === undefined) continue;
      const output = formatSSE(item, sourceFormat);
      reqLogger?.appendConvertedChunk?.(output);
      controller.enqueue(sharedEncoder.encode(output));
    }
  }
}

4. Verification

Six tests driven through the real createSSEStream (OpenAI→Claude), run with a loader mapping @/ to src/:

upstream closes with no finish chunk → >=1 message_delta, exactly 1 message_stop, content_block_stop before message_delta, stop_reason === "end_turn".
with finish_reason control → exactly one message_delta/message_stop.
partial tool_use args flushed through sanitize path (Read.limit "5" → 5).
zero-chunk upstream → empty body, no message_start.
Claude client behind a Kiro pivot (no flush terminal) → terminated via the new clause.
unit: flush requires message_start and latches exact-once.

Run:

node --test --experimental-loader ./loader.mjs ./regression.test.mjs

Results (node v22.23.2):

Build	1	2	3	4	5	6	Pass
Before fix	❌	✅	❌	✅	❌	❌	2/6
After fix	✅	✅	✅	✅	✅	✅	6/6

# tests 6
# pass 6
# fail 0

5. Scope / non-goals

Translate mode only; passthrough data: [DONE] emission (#142 OpenClaw workaround) is untouched.
Don't reuse state.finishReasonSent.
Don't infer tool_use for synthesized stops.
No message_start ⇒ no terminal synthesized.

Evidence & signatures

# Evidence
- Problem class: sse-claude-client-no-terminal-when-upstream-omits-finish-reason
- Model: openrouter/deepseek/deepseek-v4.1-flash
- Solved: 2026-09-14T18:13:28.082Z
- Verification: solution produced by pi in sandbox; see signatures.json

{"description": "SYMPTOM: An Anthropic-format SSE client (Claude /v1/messages shape) behind a chat-native OpenAI upstream that streams content then closes WITHOUT ever sending a finish_reason chunk receives ZERO terminal events: 0 message_delta, 0 message_stop. The client hangs until timeout. ROOT CAUSE: openaiToClaudeResponse (open-sse/translator/response/openai-to-claude.js) emits message_delta/message_stop only inside if(choice.finish_reason), and the stream.js translate-mode flush sentinel is gated on sourceFormat===FORMATS.OPENAI, so the Claude-client case falls through completely; the translator also returns null for the null-chunk flush call (if(!chunk||!chunk.choices?.[0]) return null), so flush() never gets a chance to synthesise anything for that direction. VERIFIED FIX (9router commit 0b7a2651, judge PASS 673f8f2d, 6-test regression suite): (1) Extract the finish-block terminal emission into a shared emitClaudeTerminal helper in openai-to-claude.js, called from BOTH the choice.finish_reason path AND the chunk=null flush call, latched by a Claude-specific state.claudeTerminalSent flag for exact-once. Do NOT reuse the shared initState finishReasonSent flag \u2014 it is owned by other translators' directions (claude-to-openai, openai-responses) and reusing it couples unrelated paths. (2) In open-sse/utils/stream.js translate-mode flush(), after the existing translateResponse(targetFormat, sourceFormat, null, state) call, add a clause for sourceFormat===FORMATS.CLAUDE that asks the translator for the owed terminal when none was emitted (model on the existing OPENAI_RESPONSES formatIncompleteOpenAIResponsesStreamFailure synthesis block). This also covers Claude clients behind upstreams whose own translator has no flush terminal (gemini/kiro/ollama/cursor/commandcode). (3) Gate synthesis on state.messageStartSent: if no message_start ever reached the client there is nothing to terminate \u2014 a zero-chunk upstream must still yield an empty body. (4) Synthesised stop_reason = state.finishReason if set, else end_turn (conservative: do NOT infer tool_use from open tool calls \u2014 the args may be half-streamed). (5) Scope: translate mode ONLY. Passthrough-mode emission of data: [DONE] to Claude clients is a deliberate OpenClaw-hang workaround (#142) \u2014 do not gate or remove it. Tests: drive the OPENAI->CLAUDE path through createSSEStream with a chat SSE body lacking any finish chunk; assert >=1 message_delta(stop_reason), exactly 1 message_stop, message_delta before message_stop after content_block_stop, partial tool_use args flushed via existing sanitize path, and a with-finish_reason control asserting exact-once.", "environment": "node-22 nextjs-16 9router open-sse", "language": "javascript", "model": "openrouter/deepseek/deepseek-v4.1-flash", "problem_class": "sse-claude-client-no-terminal-when-upstream-omits-finish-reason", "provider": "openrouter", "solved_at": "2026-09-14T18:13:28.082Z", "version": ""}

Generated from the verified corpus · MIT licensedBack to the catalog

sse-claude-client-no-terminal-when-upstream-omits-finish-reason

Answer 1

Fix: Claude SSE clients hang when an OpenAI upstream omits finish_reason

1. Symptom

2. Root-cause analysis

3. Exact fix

open-sse/translator/response/openai-to-claude.js

open-sse/utils/stream.js — after the flushed block in translate-mode flush()

4. Verification

5. Scope / non-goals

Evidence & signatures

Answer 2

Fix: Claude SSE clients hang when an OpenAI upstream omits finish_reason

1. Symptom

2. Root-cause analysis

3. Exact fix

open-sse/translator/response/openai-to-claude.js

open-sse/utils/stream.js — after the flushed block in translate-mode flush()

4. Verification

5. Scope / non-goals

Evidence & signatures

Fix: Claude SSE clients hang when an OpenAI upstream omits `finish_reason`

`open-sse/translator/response/openai-to-claude.js`

`open-sse/utils/stream.js` — after the `flushed` block in translate-mode `flush()`

Fix: Claude SSE clients hang when an OpenAI upstream omits `finish_reason`

`open-sse/translator/response/openai-to-claude.js`

`open-sse/utils/stream.js` — after the `flushed` block in translate-mode `flush()`