Stream Reconstruction

Stream reconstruction is the bridge layer that translates heterogeneous provider stream deltas into a uniform sequence of OpenAI-compatible ResponseStreamEvent objects. Without it, every upstream provider would emit chunks in its own shape, and downstream consumers (SSE encoders, trace loggers, session persistence) would need provider-specific logic. The reconstruction layer gives GodeX a single, predictable event model regardless of which LLM provider is handling the request.

At a Glance

Concern	Component	Key File
State machine phases	ResponseStreamStateMachine	response-stream-state-machine.ts:37
Delta-to-event mapping	mapProviderDeltasToEvents	stream-reconstructor.ts:17
Delta validation	validateDelta + helpers	stream-reconstructor.ts:69
Delta type contract	ProviderStreamDelta	stream-delta.ts:28
Deferred terminal events	deferFinish / deferTerminal	response-stream-state-machine.ts:263

Stream State Machine Phases

The ResponseStreamPhase enum defines five phases that every stream passes through:

Phase	Description
IDLE	Initial state; no events emitted yet
IN_PROGRESS	Stream is actively receiving deltas
COMPLETED	Stream finished normally
INCOMPLETE	Stream hit a length or context-window limit
FAILED	Stream terminated due to an error

The transition logic lives in response-stream-state-machine.ts:787, which maps the provider finish reason to the correct terminal phase.

Block Management

While in the IN_PROGRESS phase, the state machine manages four categories of output blocks:

Block Type	Fields	Tracked By
Text	itemId, outputIndex, contentIndex, text	activeText
Refusal	same shape as Text	activeRefusal
Reasoning	itemId, outputIndex, contentIndex, text	activeReasoning
Tool Call	callId, providerName, arguments, customInput	activeToolCalls Map

Each block is lazily created on its first delta and closed when the stream reaches a terminal phase via closeActiveBlocks (response-stream-state-machine.ts:428).

Delta Validation Pipeline

mapProviderDeltasToEvents (stream-reconstructor.ts:17) is the core loop. Before any state machine transition, every raw delta passes through validateDelta which enforces the ProviderStreamDelta contract (stream-delta.ts:28).

Validated Field	Rules
text	Optional string
reasoning	Optional string
refusal	Optional string
toolCall	Optional object; index must be non-negative integer; id/type/name/arguments must be strings
usage	Object with required input_tokens, output_tokens, total_tokens (finite numbers)
finishReason	String, null, or undefined
error	Object with required message string and optional code string

Unrecognized fields cause a BridgeError with code BRIDGE_STREAM_INVALID_TRANSITION (stream-reconstructor.ts:120).

Deferred Terminal Events

When deferTerminal is true (as it is in the streaming pipeline), the state machine's deferFinish method stores the finish reason without transitioning to a terminal phase (response-stream-state-machine.ts:263). This allows downstream transformers -- notably the output contract validation transformer -- to inspect and potentially rewrite the terminal event before it reaches the client.

The ProviderStreamEventBridge.flush method (stream-pipeline.ts:123) calls machine.finish(machine.deferredFinishReason) when the upstream stream closes, ensuring the terminal event is always emitted.

Tool Call Reconstruction

Tool call blocks are tracked by streamIndex in a Map<number, ToolCallBlock> (response-stream-state-machine.ts:94). When a tool call block is closed, the state machine:

1. Verifies both callId and providerName are present (otherwise throws BRIDGE_STREAM_INCOMPLETE_TOOL_CALL)
2. Checks the ToolIdentityMap to determine if the tool is a custom tool
3. Calls restoreToolCall from call-restorer.ts:16 to map the provider function call back to the correct Responses API type (function_call, local_shell_call, shell_call, apply_patch_call, or custom_tool_call)

Error Normalization

Provider errors are normalized through normalizeError (response-stream-state-machine.ts:753), which maps the provider error code against a known set of ResponseErrorCode values. Unknown codes fall back to server_error.

Cross-References

• Streaming Pipeline -- orchestrates the transform chain that feeds into this state machine
• Output Contracts -- uses deferred terminal events for JSON validation
• Tool Planning -- produces the ToolIdentityMap consumed during tool call reconstruction
• Sync Pipeline -- non-streaming counterpart that reconstructs a complete ResponseObject

References

• response-stream-state-machine.ts:37 -- ResponseStreamPhase enum definition
• response-stream-state-machine.ts:86 -- ResponseStreamStateMachine class
• stream-reconstructor.ts:17 -- mapProviderDeltasToEvents function
• stream-reconstructor.ts:69 -- validateDelta function
• stream-delta.ts:28 -- ProviderStreamDelta interface
• response-stream-state-machine.ts:263 -- deferFinish method
• response-stream-state-machine.ts:428 -- closeActiveBlocks method