feat(combos): universal context handoff for cross-model conversation continuity

[herjarsa]

Summary

When a combo routes between different models (e.g., Claude Sonnet -> GPT-4o -> Kimi K2), each model switch causes context loss. This PR implements Universal Context Handoff - a structured summary system that preserves conversation continuity across model switches.

Changes

New Module: open-sse/services/contextHandoff.ts (+355 lines)

• UniversalHandoffConfig interface (combo-level + global settings)
• resolveUniversalHandoffConfig() - layered config resolution
• buildUniversalHandoffSystemMessage() - builds <context_handoff> XML
• injectUniversalHandoffBody() - injects handoff into request body
• shouldGenerateUniversalHandoff() - determines when to generate
• generateUniversalHandoffAsync() - generates summary via current model

DB Migrations

• 062: Add last_model column to context_handoffs table
• 063: Create session_model_history table for model tracking

Integration: open-sse/services/combo.ts (+59 lines)

• Tracks last model per session via DB
• Detects model switches during combo routing
• Generates and injects handoff before dispatching to new model

Tests: tests/unit/universal-handoff.test.ts (+210 lines)

• 19 unit tests: 19/19 passing

Related Issue

Closes #2652

[kilo-code-bot]

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[gemini-code-assist]

Code Review

This pull request implements a 'Universal Handoff' mechanism to maintain conversation context when switching between different LLM models or providers. Key changes include model usage tracking in SQLite, asynchronous summary generation, and automated injection of handoff context into prompts. Feedback highlights several critical logic errors: the body variable is incorrectly reassigned within a loop leading to context bloat, model switch detection fails because usage is recorded before the check, and database queries lack the necessary precision and deterministic ordering for reliable history retrieval. There is also a language inconsistency in a system message template.

[gemini-code-assist]

Reassigning the body variable inside the loop causes it to accumulate multiple handoff messages if the request falls back through multiple models that trigger the injection condition. This can lead to bloated requests and potential context confusion for the LLM. It also affects the handoffSourceMessages used for summary generation later in the function, potentially including the handoff instructions in the source text for the next summary.

Suggested change

	// Universal handoff: inject existing handoff if model changed
	if (
	universalHandoffConfig.enabled &&
	relayOptions?.sessionId &&
	!(body as Record<string, unknown>)?.[SKIP_UNIVERSAL_HANDOFF_FLAG]
	) {
	const lastModel = getLastSessionModel(relayOptions.sessionId, combo.name);
	if (lastModel && lastModel !== modelStr) {
	const existingHandoff = getHandoff(relayOptions.sessionId, combo.name);
	body = injectUniversalHandoffBody(
	body,
	lastModel,
	modelStr,
	`Model routing: ${lastModel} → ${modelStr}`,
	existingHandoff
	);
	}
	}
	const result = await handleSingleModelWrapped(body, modelStr, {
	let bodyForModel = body;
	// Universal handoff: inject existing handoff if model changed
	if (
	universalHandoffConfig.enabled &&
	relayOptions?.sessionId &&
	!(body as Record<string, unknown>)?.[SKIP_UNIVERSAL_HANDOFF_FLAG]
	) {
	const lastModel = getLastSessionModel(relayOptions.sessionId, combo.name);
	if (lastModel && lastModel !== modelStr) {
	const existingHandoff = getHandoff(relayOptions.sessionId, combo.name);
	bodyForModel = injectUniversalHandoffBody(
	body,
	lastModel,
	modelStr,
	`Model routing: ${lastModel} → ${modelStr}`,
	existingHandoff
	);
	}
	}
	const result = await handleSingleModelWrapped(bodyForModel, modelStr, {

[gemini-code-assist]

getLastSessionModel is called after recordSessionModelUsage. Since recordSessionModelUsage is a synchronous database operation that inserts the current modelStr, getLastSessionModel will return the model that was just recorded. Consequently, prevModel !== modelStr will always be false, and maybeGenerateUniversalHandoff will never be triggered.

Suggested change

	recordSessionModelUsage(
	relayOptions.sessionId,
	combo.name,
	modelStr,
	provider,
	target.connectionId
	);
	const prevModel = getLastSessionModel(relayOptions.sessionId, combo.name);
	if (prevModel && prevModel !== modelStr) {
	const prevModel = getLastSessionModel(relayOptions.sessionId, combo.name);
	recordSessionModelUsage(
	relayOptions.sessionId,
	combo.name,
	modelStr,
	provider,
	target.connectionId
	);
	if (prevModel && prevModel !== modelStr) {

[gemini-code-assist]

This note is in Spanish, while the rest of the handoff message and the project's default language appear to be English. Furthermore, it claims that a summary follows ("A continuación se resume..."), but in this specific code path (!payload || !payload.summary), no summary is actually provided in the XML. This is misleading for the LLM.

Suggested change

	<note>A continuación se resume toda la conversacion para continuar sin perder el hilo.</note>
	<note>Conversation transferred from ${escapedPrev} to ${escapedCurr}. A detailed summary will be available in the next turn.</note>

[gemini-code-assist]

SQLite's datetime('now') has only 1-second precision. In high-concurrency scenarios or fast model fallbacks, multiple entries might share the same timestamp, making the ORDER BY used_at DESC query in getLastSessionModel non-deterministic.

  used_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%f', 'now'))

[gemini-code-assist]

To ensure deterministic results when multiple model usage records have the same timestamp, the query should also order by the primary key id.

Suggested change

	ORDER BY used_at DESC
	ORDER BY used_at DESC, id DESC

[diegosouzapw]

Hi @herjarsa! Thank you so much for this contribution.

We've resolved the migration index conflict with the existing 062 migration by shifting the new migrations to 063 and 064 (and updating the sentinels). We also resolved the explicit any linter error in combo.ts by exporting and using MessageLike instead of any[] to respect the strict linter budget.

The PR is now merged into our staging branch release/v3.8.3 and will be included in the upcoming release. Awesome work!

[herjarsa]

Addressed all code review feedback:

✅ Fixes Applied

Issue	Fix
🔴 Body mutation in loop	Changed body = injectUniversalHandoffBody() to const requestBody + requestBody ?? body fallback. Handoff no longer leaks across loop iterations.
🔴 Model switch detection broken	Moved getLastSessionModel() BEFORE recordSessionModelUsage(). Now correctly detects model changes instead of always seeing the same model.
⚪ Language inconsistency	Spanish → English in handoff template.

Verification

• ✅ TypeScript diagnostics: clean
• ✅ Tests: 19/19 passing
• ✅ Branch force-pushed