fix: forks overflow the model's window and then stay stuck on "Prompt is too long"

[saucam]

What happened

The interop-testing (fork) session failed on its very first message with "Prompt is too long", and then on every message after that.

When you fork a session, codeoid copies the parent's conversation into the new session's first prompt. It decides how much fits by assuming 3.5 characters per token. This conversation was mostly code and tool output, which comes out closer to 2.25. So a history sized to fit a 1M-token window (2.36M characters) was really about 1.09M tokens, and the request was rejected.

The session then couldn't recover. Each later message resumed the same backing Claude session, which still held the oversized prompt. /rotate, the usual way out, refused to run because the session had "only 2 turns".

What changes

• The carried-over history is sized realistically. The estimate is now 2 characters per token. On a 1M window that caps the history at about 1.4M characters, leaving room for the system prompt and tools.
• An overflowing fork retries itself. If the first turn after a fork or backend switch is rejected as too long, the session starts a fresh backing session, rebuilds the history at half the size, and re-runs the user's message as the same turn, without posting it twice. It tries up to 3 times and posts a note each time. This is safe because nothing runs on a rejected request. If all attempts fail, the error says to use /rotate.
• /rotate works on a stuck session. It skips its minimum-turn check when the last turn overflowed or the session is in an error state. This also holds after a daemon restart, so existing stuck sessions can be rotated once this is deployed.
• The sizing is configurable. New session settings, also shown under advanced in the settings UI:

Key	Env var	Default
seedBudgetChars	CODEOID_SEED_BUDGET_CHARS	0 (worked out from the window)
seedCharsPerToken	CODEOID_SEED_CHARS_PER_TOKEN	2
seedWindowFraction	CODEOID_SEED_WINDOW_FRACTION	0.7

CODEOID_SEED_BUDGET_CHARS already existed as a .env-only setting. It is now a config key too, and the env var still takes precedence.

Testing

• New src/tests/session-seed-overflow.test.ts (9 tests): the retry with halving, giving up after 3 attempts, /rotate after an overflow (including after a restore), /rotate still refused on a healthy young session, no retry when no history was carried over, and the config settings.
• Typecheck and lint are clean. Full suite on top of current main: 3127 passed, 0 failed.

For agents

• src/daemon/providers/context-windows.ts: SEED_CHARS_PER_TOKEN changed from 3.5 to 2. seedBudgetChars() takes SeedBudgetOptions (capChars, charsPerToken, windowFraction). Precedence: CODEOID_SEED_BUDGET_CHARS at call time, then opts.capChars, then the computed value. Adds isPromptTooLongError().
• src/daemon/session.ts:
  • #seedProviderFromHistory(retry?) records #pendingSeed (historyLength, budgetChars, attempts), but only for transcript seeds a warm backend actually stashed (outcome.truncation is set).
  • #sendInner stamps prompt, sender and turn (id/text/at) on the pending seed when the first fresh turn starts.
  • In the turn_done handler: an error that is prompt-too-long (from errorMessage, or this turn's committed assistant text) with a pending seed and attempts < SEED_OVERFLOW_MAX_RETRIES sets #seedOverflowRetry and skips the error message and #lastTurnError. The finally in #consumeEvents then calls #scheduleSeedOverflowRetry().
  • The retry is chained on #sendChain. It tears the provider down, calls resetToNewSession and persists the new backing id, truncates the accumulator to historyLength, and re-seeds at half the budget. It then restores #currentTurnId to the original turn (already indexed) and calls runTurn with the original prompt.
  • A successful turn clears #pendingSeed.
  • manualRotate bypasses min-turns when #promptTooLong is set or status is error. restoreScrollback sets #promptTooLong when the last assistant row is the overflow.
• src/config.ts: adds session.seedBudgetChars / seedCharsPerToken / seedWindowFraction to the schema, the type and ENV_OVERRIDES.
• src/daemon/settings/manifest.ts: the env-only CODEOID_SEED_BUDGET_CHARS field is replaced by three config-backed fields.

🤖 Generated with Claude Code

[saucam]

pr-shepherd: CI is green at 4138401 (daemon-check, web-check, Socket Security x2 all SUCCESS). Branch is current with main. No auto-fixes were needed, nothing pushed. Ready for human review.