Model context boundary

modelcontext.Registry is the only application-facing boundary for values that are shortened before an LLM call and restored afterwards. The whole codec lives in this one package:

File	Role
registry.go	Registry facade — the only type request lifecycles use
handle_table.go	handleTable[M], the generic bidirectional primitive behind every handle space
sources.go	source handles (cN/dN/bN/wN) and the tool-argument codec
citations.go	protocol prompts, <ref/> → <kb/>/<web/> expansion, citation stream expander
model_output.go	compact source-centric renderings of tool results
resources.go	durable-resource handles (res://NNNN)
stream.go	streamHold suffix-hold primitive and every streaming decoder
tool_policy.go	the single policy layer: per-tool key contracts + sourceKeySpaces dispatch
handles.go	exported HandleTable for invocation-local spaces (iN, ref-N, c000)

Identity rules

• Durable application identities: UUIDs, Wiki slugs, URLs, and resource://... references.
• Request-local model handles: cN, dN, bN, wN, and res://NNNN.
• Tool-private request handles: iN for Wiki issues.
• Ingest-call handles: cNNN and ref-N, allocated with HandleTable.
• Temporary handles are decoded before tool execution, persistence, or UI delivery. They are never accepted as durable authorization or routing data.

Request lifecycle

1. Create one Registry for the complete request or Agent execution.
2. Register structured source identities as they enter the context.
3. Call EncodeMessages immediately before the model call.
4. Decode tool calls through DecodeToolCalls before parsing or execution.
5. Decode complete responses with DecodeResponse, or streaming text with one StreamDecoder per response channel.

Unknown handle-shaped values in declared tool fields are rejected before tool execution. The same per-tool policy is applied when historical assistant tool calls and tool results are replayed, so handles neither disappear nor drift to new numbers across Agent rounds.

The registry owns codec ordering. Resource references are encoded before source IDs so a Wiki slug such as summary/<knowledge-id> cannot be corrupted into summary/d1.

Observability

Langfuse generation observations contain the exact encoded payload sent to and returned by the model. Agent tool spans contain both model_arguments (the model-emitted handles) and resolved_arguments (the durable values actually executed), plus argument_resolution and any unresolved handles. Sensitive tools report only argument keys and resolution counts.

Langfuse is an observer of this boundary; it must not implement another handle mapping.

Built-in tool policies

Canonical ID-bearing arguments for knowledge, graph, data, web, and Wiki tools are decoded centrally through a tool-name plus JSON-field allowlist. The sourceKeySpaces table is the single source of truth for which keys are ID-bearing and which handle space they register into; per-tool sourceIDKeys contracts gate which of those keys each tool may use. database_query.sql and data_analysis.sql have an explicit policy because source handles can be embedded inside quoted SQL values; unquoted SQL aliases and arbitrary prose are never rewritten. Wiki issue IDs use the same lifecycle via an iN handle space.

Dynamic MCP tools are intentionally opaque in both arguments and results. Their schemas and identity semantics are controlled by the MCP server, so the application must not guess that an arbitrary id, knowledge_id, or url field is durable or rewrite it. Durable resource handles inside MCP arguments still use the normal res://NNNN codec. A future MCP ID mapping must be an explicit server/tool annotation and should plug into this registry rather than create a parallel mapper.

Wiki routing

WikiRouteResolver is intentionally separate. It stores server-side slug-to-knowledge-base provenance gathered during Wiki search/read operations, and can only return knowledge bases already present in the Agent's authorized Wiki scope. It is routing state, not a model handle table. Reads scan every legal Wiki KB so cached provenance cannot hide a duplicate slug; mutations require one unique owner. New pages use link provenance, a single Wiki scope, or a single source-document owner and reject ambiguous routing instead of asking the model for a durable KB ID.

Document/tag-constrained Wiki and graph requests are filtered by the same server-owned SearchTargets provenance. Uncited/global Wiki pages and graph results outside that subset fail closed; only an explicit whole-KB target authorizes whole-KB content.