Gemini 2.5 Flash: thinking tokens silently truncated JSON output
After switching to Gemini, every analysis came back as `GEMINI_PARSE_FAILED: Unexpected end-of-input` at the same column. Cause: Gemini 2.5 Flash burns 'thinking' tokens before emitting visible output, eating most of `maxOutputTokens: 800` and truncating the JSON mid-string. Fix: `thinkingConfig: { thinkingBudget: 0 }` and bumped output cap to 4096.
The first wave of Gemini calls all failed the same way:
Gemini response parsing failed:
Unexpected end-of-input: was expecting closing quote for a string value
at [Source: ...; line: 3, column: 51]Every time, mid-string. That ruled out malformed JSON in the prompt template — the model was reaching maxOutputTokens mid-emission and stopping cold.
What I missed: gemini-2.5-flash is a reasoning model. It spends an internal "thinking" budget before emitting visible text, and that budget counts against maxOutputTokens. With maxOutputTokens: 800, the thinking burned a few hundred and the visible JSON had no room.
Two changes:
"generationConfig", Map.of(
"responseMimeType", "application/json",
"maxOutputTokens", 4096,
"temperature", 0.2,
"thinkingConfig", Map.of("thinkingBudget", 0) // ← disable reasoning
)thinkingBudget: 0 disables reasoning entirely. For our narrow structured-output task (translate + extract vocab + generate scenario), reasoning is overkill anyway — and bizarre to silently cap the visible response.
I also added a short raw-response preview to the parse-failed log so the next time this kind of issue shows up, the cause is one log line away:
String preview = rawResponse.replace("\n", " ").substring(0, Math.min(300, rawResponse.length()));
log.warn("Gemini response parsing failed: {} | raw preview: {}", ex.getMessage(), preview);End-to-end latency dropped too: 11.7s → 6.7s, because the thinking pass no longer happens.
Pattern: reasoning-mode models silently steal output tokens. For narrow structured-output tasks (translation, classification, extraction), disable thinking. For open-ended generation, keep it.
Commit: ba90e00 — [feat] i18n + 직독직해 + Output quizzes + Decks + Playlist + project log