Problem

SessionConfig has 16+ properties, most of which are internal tuning knobs that nobody changes. This makes the config surface confusing for developers and agents.

Current properties (16)

Actually useful to configure (~5):

• Timeout settings (3 separate: SidecarLlmTimeoutSeconds, TurnLlmTimeoutSeconds, ToolExecutionTimeoutSeconds) — could be 1 base timeout with others derived
• IdleTimeout — passivation timing
• MemoryObserverIdleSeconds — observer timing
• MaxToolCallsPerTurn — safety rail

Feature flags that should be removed once stable (2):

• MemorySidecarsEnabled
• DeterministicRetrievalEnabled

Internal tuning nobody changes (8):

• CompactionThreshold, KeepRecentToolResults, KeepRecentMessages, SnapshotInterval, TitleGenerationInterval, MaxInlineToolResultChars, DiscoveredToolRetentionTurns, DiscoveredToolMaxCount

Not configurable — runtime-derived (4):

• ModelId, ContextWindowTokens, InputModalities, OutputModalities

Proposed approach

1. Consolidate 3 timeout settings into 1 base timeout with derived values
2. Hardcode internal tuning as constants (still overridable via config for testing)
3. Remove feature flags once memory and retrieval are stable
4. Separate runtime-derived model properties from user config
5. Update JSON schema to document the remaining ~5 settings with additionalProperties: false

Related

• Session schema is currently additionalProperties: true (no validation)
• CLAUDE.md Configuration Schema Sync Rule requires schema updates when adding properties