# How to Translate an SRT Subtitle File with AI

> Translate subtitles without breaking timestamps — a copy-paste Python script, the prompt that keeps line counts, and the checks that catch drift.

- Canonical: https://guides-ai.pages.dev/guides/translate-subtitles-with-ai/
- Plate 13.06 · Topic: Everyday tasks (https://guides-ai.pages.dev/topics/tasks/)
- Published: 06 Sept 2026 · 4 min read
- Source site: guides-ai — https://guides-ai.pages.dev/

The mistake everyone makes first is pasting a whole `.srt` into a chat box. The model translates the timestamps too — shifting a digit here, merging two cues there — and the file no longer plays.

The fix is structural: **never let the model see the timing lines.** Parse the file yourself, send only the dialogue, rebuild from your own untouched indices and timestamps.

## 1. Install

```bash
pip install anthropic
export ANTHROPIC_API_KEY=sk-ant-...
```

Installs the SDK and puts your key where it is picked up. On Windows PowerShell: `$env:ANTHROPIC_API_KEY = "sk-ant-..."`.

## 2. The script

```python
import re, pathlib
from anthropic import Anthropic

SRC, DST, LANG, BATCH = "movie.srt", "movie.de.srt", "German", 40

raw = pathlib.Path(SRC).read_text(encoding="utf-8-sig").strip()
cues = []
for block in re.split(r"\n\s*\n", raw):
    lines = block.splitlines()
    if len(lines) >= 3:
        cues.append((lines[0], lines[1], " ".join(lines[2:])))   # index, timing, text

client = Anthropic()
SYSTEM = (
    f"You translate film subtitles into {LANG}. The input is numbered lines. "
    f"Return exactly one translated line per input line, with the same numbering and "
    f"nothing else: no notes, no blank lines, no merged or split lines. "
    f"Keep proper names, and keep HTML tags such as <i> exactly where they are."
)

out = []
for i in range(0, len(cues), BATCH):
    part = cues[i:i + BATCH]
    payload = "\n".join(f"{n}. {c[2]}" for n, c in enumerate(part, 1))
    msg = client.messages.create(
        model="claude-opus-5", max_tokens=16000, system=SYSTEM,
        messages=[{"role": "user", "content": payload}],
    )
    reply = "".join(b.text for b in msg.content if b.type == "text")
    got = [re.sub(r"^\d+\.\s*", "", ln) for ln in reply.strip().splitlines() if ln.strip()]
    assert len(got) == len(part), f"cue {i}: got {len(got)} lines, expected {len(part)}"
    out += [(c[0], c[1], t) for c, t in zip(part, got)]
    print(f"{len(out)}/{len(cues)}")

pathlib.Path(DST).write_text(
    "\n\n".join(f"{i}\n{t}\n{x}" for i, t, x in out) + "\n", encoding="utf-8")
```

Sends dialogue 40 cues at a time, keeps your original indices and timing strings, and **asserts** the line count on every batch so a drifting response fails loudly instead of corrupting the file. Multi-line cues are joined with a space — players re-wrap anyway, and one-line-in/one-line-out is what makes that check reliable. The reply is read by filtering content blocks on `type == "text"`, not by grabbing `content[0]`.

Batching is not optional: 40 cues per call give the model surrounding dialogue, so pronouns and formality stay consistent.

## 3. For a short file, prompt directly

```text
Below are numbered subtitle lines. Translate them into German.
Return exactly one line per input line with the same numbering, nothing else.
Keep names and <i> tags. Keep each line short enough to read in the time a
subtitle allows — shorten rather than explain.

1. ...
```

Works in any chat UI — count the returned lines before rebuilding the file.

## 4. Check it

```bash
grep -c ' --> ' movie.srt movie.de.srt
```

Both files must report the same number of timing lines. Then open the translation in a player and skip to the last cue — if it is in sync there, the whole file is. Finally, scan for lines much longer than the original: nobody can read those in time.

Prefer local? Swap the client for [the Ollama API](/guides/call-ollama-api/) with a multilingual model, keeping the batching and the assert. Quality on idiom drops.

---

Next: [make the subtitles first with Whisper](/guides/whisper-transcribe-audio-locally/) or [learn the Claude API basics](/guides/claude-api-first-request/).