core

Imported from microsoft/agent-framework (python/packages/core/AGENTS.md). Install upstream with npx skills add microsoft/agent-framework --skill core. Copyright stays with the author.

Core Package (agent-framework-core)

The foundation package containing all core abstractions, types, and built-in OpenAI/Azure OpenAI support.

Module Structure

agent_framework/
├── __init__.py          # Lazy runtime public API exports
├── __init__.pyi         # Public API typing surface for lazy root exports
├── security.py          # Public security primitives, middleware, and tools
├── _agents.py           # Agent implementations
├── _clients.py          # Chat client base classes and protocols
├── _types.py            # Core types (Message, ChatResponse, Content, etc.)
├── _tools.py            # Tool definitions and function invocation
├── _vectors.py          # Vector store models, CRUD/search abstractions, and protocols
├── _middleware.py       # Middleware system for request/response interception
├── _sessions.py         # AgentSession and context provider abstractions
├── _filesystem.py       # Private filesystem safety helpers (atomic writes, link detection, storage-key derivation)
├── _skills.py           # Agent Skills system (models, executors, provider)
├── _mcp.py              # Model Context Protocol support
├── _telemetry.py        # User-Agent identity and internal feature-usage mask
├── _workflows/          # Workflow orchestration (sequential, concurrent, handoff, etc.)
├── openai/              # Built-in OpenAI client
├── azure/               # Lazy-loading entry point for Azure integrations
└── <provider>/          # Other lazy-loading provider folders

Core Classes

Root Public API (__init__.py / __init__.pyi)

• agent_framework.__init__ uses lazy module-level __getattr__ for most public exports to keep cold import agent_framework lightweight.
• Keep _LAZY_MODULE_EXPORTS, _LAZY_EXPORTS, the explicit runtime __all__, and __init__.pyi synchronized whenever adding, removing, or moving a root public export.
• Runtime __all__ is still required for from agent_framework import *; the .pyi file is for type checkers and editors and does not replace runtime exports.
• Public deprecation behavior for a lazy export belongs in the owning module. The root package should delegate via the normal lazy export map instead of carrying one-off branches.

Settings (_settings.py)

• SecretString is a non-str wrapper: string conversion, representation, formatting, and concatenation display a mask. String-only APIs such as str.join() and JSON encoding reject it unless callers explicitly convert it. Use get_secret_value() when passing credentials to provider SDKs; str(secret) returns the mask.
• load_settings accepts plain string overrides for SecretString fields and wraps them, as it does for environment and .env values. Existing SecretString overrides are preserved. Invalid supplied numeric and boolean environment or .env values raise ValueError identifying the field and source instead of falling back to raw strings.

Agents (_agents.py)

• SupportsAgentRun - Protocol defining the agent interface
• BaseAgent - Abstract base class for agents
• Agent - Main agent class wrapping a chat client with tools, instructions, and middleware
• RawAgent.open() / close() (inherited by Agent) - Explicitly enter the client's and configured MCP tools' async contexts, then release them along with lazily connected MCP tools. async with agent delegates to these methods; a partially failed open() closes resources already entered.
• Providers that store service-owned continuation handles in AgentSession.state declare their keys through service_session_state_keys. as_tool(propagate_session=True) combines declarations from the parent and child agents and their clients to isolate those handles in both directions while ordinary application-owned state propagates. The automatic function loop places parent declarations on FunctionInvocationContext; direct or custom-loop invocation must set parent_service_session_state_keys explicitly for a non-empty shared session and otherwise fails closed.

Chat Clients (_clients.py)

• SupportsChatGetResponse - Protocol for chat client implementations
• BaseChatClient - Abstract base class with middleware support; subclasses implement _inner_get_response() and _inner_get_streaming_response()

Types (_types.py)

• Message - Represents a chat message with role, content, and metadata
• ChatResponse - Response from a chat client containing messages and usage
• ChatResponseUpdate - Streaming response update
• AgentResponse / AgentResponseUpdate - Agent-level response wrappers
• Content - Base class for message content (text, function calls, images, etc.)
• Content identity and metadata - function_call.id is Agent Framework occurrence identity; provider item IDs belong in the provider namespace. function_call.hosted controls execution ownership, while server_name is the descriptive hosted-server boundary and never implies hosted=True. Provider metadata uses one-level additional_properties namespaces and streamed mergeable content uses merge_mode; response updates can use the same field to replace all contents of one logical message; see spec 005.
• Computer use - ComputerSafetyCheck and the Content.from_computer_tool_call / from_computer_tool_result constructors are experimental under COMPUTER_USE; the rest of Content retains its existing stage. Computer results can omit screenshots in core; OpenAI-based connectors require them when converting to Responses items. Completed call/result pairs remain in the transcript but are not user-input requests.
• ChatOptions - TypedDict for chat request options

Tools (_tools.py)

• ToolProtocol - Protocol for tool definitions
• FunctionTool - Wraps Python functions as tools with JSON schema generation
• @tool decorator - Converts functions to tools
• use_function_invocation() - Decorator to add automatic function calling to chat clients
• _normalize_tool_description_format / _format_tool_parameters - Private configuration normalizer and structured parameter formatter shared by Hyperlight and Monty descriptions. They validate and detach compact/JSON settings, return detached parameter data, and fall back to full JSON Schema when compact data cannot preserve constraints. They do not change FunctionTool.parameters() or render runtime-specific text.

Vector stores

The vector store API is experimental under the shared VECTOR_STORES feature ID.

• @vectorstoremodel - Declares key, data, and vector fields on dataclasses, Pydantic models, and plain classes
• register_vectorstoremodel - Registers one definition and msgspec-backed codec pair per model type
• VectorStoreField - Frozen core key/data/vector metadata; common index and distance values remain open to provider-defined strings, key fields can be store-generated, and copied provider_annotations remains mutable for connector-specific configuration
• Filter / FilterGroup - Mutable data-only filter inputs shared by local and remote vector stores; collection operations bound structural traversal before copying and pass an independent snapshot to connectors. Parameter detection includes collection members and mapping keys; string operators require string operands after resolution
• Param - Native typed search-tool parameter reference embedded in filter values or paging options
  • Defaults and supplied mutable values are copied per filter invocation, including for definition-less search tools
  • Filter parameters may opt into null omission with a nullable type and explicit default=None, such as Param("text", str | None, default=None, omit_if_none=True); absent/null arguments remove that leaf, while remaining AND/OR children still apply. Empty groups (including NOT with an omitted child) are removed recursively; removing the whole tree means no filter. Paging parameters do not support null omission.
• BaseVectorCollection - Base class for collection lifecycle and msgspec-backed record CRUD operations; upserts generate embeddings by default, retrieval excludes vectors by default, and filtered retrieval is an alternate mode to key lookup. Agent-facing CRUD tools use its key_json_schema, key_from_json, and key_to_json hooks so connectors can preserve native key identity at JSON boundaries.
• Embedding generation selection - generate_vectors=True regenerates every vector field, False preserves all values, and a list or tuple of logical vector field names generates only those fields so connectors can combine local, precomputed, and provider-side vectorization
• Vector payloads - Shared dense query/generated vectors accept numeric sequences and binary bytes; connectors declare supported element/representation types. Sparse vectors remain provider-native through codecs or search(values=...), not a core sparse type.
• Vector dimensions - Final dense sequence lengths are checked after optional generation for the whole write batch before connector conversion or writes, and for the selected query field before search dispatch. In-memory queries also check their normalized numeric sequence, including array-like inputs and empty collections. These are length checks, not element validation; null vectors, source text, binary payloads, and non-sequence provider-native representations remain connector-owned.
• BaseVectorStore - Base class for stores that create collection clients
• BaseVectorSearch - Base class for vector and keyword-hybrid search; core validates portable requests and deserializes results without interpreting thresholds or re-filtering returned scores. Connectors own scoring, filter execution, score thresholds (including provider-defined/default metrics), and paging. Use native backend execution where available, otherwise an explicit connector-local fallback or reject unsupported options
• Embedding request options - upsert accepts either flat embeddings_options for all generated vector fields or embeddings_options_by_field keyed by logical field name, never both. search and create_vector_search_tool accept flat embeddings_options for local query generation. Core supplies declared field dimensions and rejects conflicting values before embedding; search ignores these options when a precomputed vector is supplied. create_upsert_tool and VectorCollectionContextProvider forward the same operation-specific settings to their generated tools.
• create_vector_search_tool - Creates an agent tool from any SupportsVectorSearch implementation
• create_upsert_tool / create_get_tool / create_delete_tool - Create agent tools for collection CRUD; upsert and delete require approval by default, while get does not. Auto-generated keys are omitted from upsert input only when the record is a dictionary or the typed model declares a key default. Connector partial-write errors propagate because the collection contract cannot report unknown committed subsets.
• VectorStoreHistoryProvider - Stores full scoped conversation history in a provider-owned collection; optional embeddings enable session-scoped history search and optional compaction affects only loaded context. Embedding-enabled history requires an explicit collection name; physical retention, large-history paging, and concurrent clear semantics remain backing-store guarantees.
• VectorCollectionContextProvider - Adds instructions and configurable CRUD/search tools for a caller-owned collection. Callers explicitly provide a best-effort logical scope filter (or None); it is not a security boundary. Independently configured additional search tools retain their own filters.
• InMemoryCollection / InMemoryStore - Dependency-free, process-local development and test implementation; cosine scoring scales finite inputs, all metrics reject non-finite scores, and unsupported distance functions fail before record scanning. Hamming scores/thresholds use the fraction of unequal dimensions, not a count. Scoring, filtering, and thresholds run locally before paging; DEFAULT means cosine distance and uses a maximum distance threshold. Shared serialization normalizes stored data; codecs and connector overrides remain trusted Python code
• SupportsVectorUpsert / SupportsVectorSearch - Structural protocols for vector store capabilities

Middleware (_middleware.py)

• AgentMiddleware - Intercepts agent run() calls
• ChatMiddleware - Intercepts chat client get_response() calls
• FunctionMiddleware - Intercepts function/tool invocations
• AgentContext / ChatContext / FunctionInvocationContext - Context objects passed through middleware. A tool can declare a FunctionInvocationContext parameter to receive it; context.tools is the live, mutable tools list for the run, and context.add_tools(...) / context.remove_tools(...) enable progressive tool exposure (changes apply on the next function-calling iteration). Chat middleware that constructs provider-local replacement messages must call context.record_message_replacement(...) for every replacement before downstream compaction so complete summaries can reconcile to caller-owned messages without persisting the replacements themselves.
• MessageInjectionMiddleware - Session-scoped chat middleware that lets tools or other code enqueue messages for the next model call in the current AgentSession; it drains queued messages into the next call and loops only when no function calls need to be handled by the function invocation layer.

Sessions (_sessions.py)

• AgentSession - Manages conversation state and session metadata
• SessionStore - Experimental in-memory opaque session_id -> AgentSession snapshot store; reads return independent copies
• FileSessionStore - Experimental msgspec file-backed session snapshot store with atomic last-writer-wins updates; JSON is the default, serialization_format="msgpack" enables binary MessagePack, opaque keys are encoded to portable filenames, and only syntactically malformed snapshots are quarantined (schema, version, and state-decoder failures preserve the original file)
• Storage-key derivation (_filesystem._storage_key_segment) - Every component that maps a caller-controlled identifier (session id, owner id, memory scope) onto a storage location uses this one private helper: FileSessionStore / FileHistoryProvider (~session-), TodoFileStore (~todo-), MemoryFileStore (~memory-), and FileMemoryProvider (~scope-). Its contract is injectivity — two byte-distinct identifiers must not produce the same segment, because the segment is an isolation boundary. Identifiers that are already a safe single segment (lowercase ASCII alnum plus ._-, no leading ., no trailing ./space, not a Windows reserved stem) are used verbatim so on-disk layouts stay readable; everything else is encoded as lowercase base32 under the component prefix, with a sha256- digest segment past a length cap (_MAX_ENCODED_STORAGE_KEY_SEGMENT_LENGTH). Only that digest branch weakens the contract to collision resistance. The charset is deliberately narrow so the filesystem cannot fold two distinct segments together: non-ASCII values are encoded because macOS APFS/HFS+ fold NFC vs NFD, and uppercase values are encoded (never lowercased, which would collide with a genuine lowercase identifier) because NTFS and APFS are case-insensitive by default — base32's a-z2-7 alphabet is itself case-stable. Never pass such an identifier through a path normalizer (e.g. _normalize_relative_path) to derive storage: that mapping is lossy, and id / id/ / id\ would collide. New components must reuse this helper with their own ~-prefixed namespace.
• register_state_type - Registers custom AgentSession.state classes with stable, process-wide type IDs and optional mapping codecs. Provider modules own registration for their custom state types and should use package-qualified IDs. Implicit Pydantic registration remains temporarily with DeprecationWarning, but module-level registration is needed to guarantee cold-start restoration.
• ServiceSessionId - Mapping alias for structured service-owned continuation handles used in AgentSession.service_session_id
• SessionContext - Context object for session-scoped data during agent runs. extend_messages(...) can attach ordered, deduplicated origin_session_ids attribution when a provider injects content from other sessions.
• ContextProvider - Base class for context providers (RAG, memory systems)
• HistoryProvider - Base class for conversation history storage
• InMemoryHistoryProvider - Built-in session-state history provider for local runs
• FileHistoryProvider - Experimental append-only file-backed history provider; msgspec JSON Lines is the default and serialization_format="msgpack" uses length-prefixed binary MessagePack records. Custom dumps/loads remain as deprecated JSON-only compatibility hooks and emit DeprecationWarning when supplied.
• Mixed computer/function workflow history - The default HistoryProvider.after_run defers completed local function results for loadable providers that store inputs until the computer reply arrives, so history records them once in call order. Custom after_run implementations must handle this themselves.

Skills (_skills.py)

• Skill frontmatter parsing - Local and MCP archive skill loaders use PyYAML's SafeLoader node composition, not dictionary construction, so duplicate mapping entries remain available for validation and no YAML object constructors run. Recognized root names must be lowercase and unique after YAML decoding. Scalar fields remain text (including numeric/boolean scalar spellings); null root values remain unset. Decoded keys and retained scalar values cannot contain Unicode surrogate code points, which cannot be encoded as UTF-8. Invalid optional metadata mappings or entries warn and are skipped; case-sensitive duplicate metadata keys keep the first valid value. Invalid YAML syntax or invalid recognized root fields reject the skill. Quoting, escapes, multiline folding/chomping, and line-ending normalization follow PyYAML. YAML merge keys (<<) are not expanded: they are invalid at the root and skipped with warnings in metadata.

• Skill - Abstract base for a skill definition bundling instructions (content) with frontmatter metadata, resources, and scripts. Concrete subclasses (InlineSkill, FileSkill, ClassSkill) accept a frontmatter=SkillFrontmatter(...) argument carrying the spec fields. Adding new spec fields is done in one place — on SkillFrontmatter — keeping the subclass constructors stable.

• SkillFrontmatter - L1 discovery metadata for a skill (name, description, license, compatibility, allowed_tools, metadata). All fields are mutable plain attributes; the constructor validates name, description, and compatibility against the spec but post-construction assignments are not re-validated. Spec fields are reachable on every skill via skill.frontmatter.

• SkillResource - Named supplementary content attached to a skill; holds either static content or a dynamic function (sync or async). Exactly one must be provided.

• SkillScript - An executable script attached to a skill; inline scripts run in-process, while file-based scripts are delegated to a runner. Inline callbacks can declare a FunctionInvocationContext parameter (hidden from the schema) to access host values separately through ctx.kwargs. Custom run() overrides opt in with the same annotation; unannotated overrides retain runtime keyword forwarding.

• SkillScriptRunner - Protocol for file-based script execution. Any callable matching (skill, script, args) -> Any satisfies it. Code-defined scripts do not use a runner. Runners can opt into context injection with an additional keyword-bindable FunctionInvocationContext parameter that has a default value.

• SkillScriptArgumentParser - Public type alias for an optional callable (raw args: dict | list[str] | str | None) -> dict | None that converts the raw args value before an InlineSkillScript runs (applied before the inline list-args guard). It is an opt-in customization hook (port of .NET PR #6498) that lets callers support backends sending tool-call arguments in a non-conforming shape (e.g. vLLM JSON strings). The output is constrained to a dict (named keyword arguments) or None, because inline scripts bind arguments by keyword name. Supply it via the argument_parser= constructor arg on InlineSkillScript, InlineSkill (default for scripts added via @skill.script), or ClassSkill (default for scripts discovered via @ClassSkill.script). When None (the default), the raw value is used unchanged. File-based scripts are unaffected (their runner owns arg handling).

• SkillsProvider - Context provider (extends ContextProvider) that discovers file-based skills from SKILL.md files and/or accepts code-defined Skill instances. Follows progressive disclosure: advertise → load → read resources / run scripts. By default all three tools it exposes (load_skill, read_skill_resource, run_skill_script) are registered with approval_mode="always_require", so every skill operation needs approval. To run unattended, pass one of the static auto-approval rules to ToolApprovalMiddleware (via auto_approval_rules): SkillsProvider.read_only_tools_auto_approval_rule approves only the read-only tools (load_skill, read_skill_resource) while still prompting for run_skill_script, and SkillsProvider.all_tools_auto_approval_rule approves every skill tool including script execution. Both rules reject any call carrying a server_label so they stay scoped to this provider's local tools and never auto-approve a same-named hosted tool. Alternatively, for trusted skills, the constructor / from_paths kwargs disable_load_skill_approval, disable_read_skill_resource_approval, and disable_run_skill_script_approval (all default False) opt individual tools out of approval entirely by registering them with approval_mode="never_require" (the auto-approval rules only apply to tools that still require approval). The tool names are also exposed as class constants (LOAD_SKILL_TOOL_NAME, READ_SKILL_RESOURCE_TOOL_NAME, RUN_SKILL_SCRIPT_TOOL_NAME).

• FileSkillsSource - SkillsSource that discovers file-based skills by scanning configured root paths for SKILL.md. The configured root paths define the trust boundary and are used as given (a root may itself be a symlink); everything discovered below a root is link-checked and fails closed. _discover_skill_directories rejects any entry that is a symbolic link, junction, or other reparse point (via the shared agent_framework._filesystem.is_link_or_reparse_point helper) before descending into it, and rejects a directory whose SKILL.md is itself such a link — otherwise a link planted under a root would be adopted as the skill root, and since every later guard treats the skill root as the boundary and only inspects segments below it, the link itself would never be inspected. Resource and script discovery apply the same rule per path segment via _has_link_or_reparse_point_in_path; source-discovered resources and scripts retain a _SkillPathScope pairing their configured root with their skill directory, and repeat containment, regular-file, and link checks immediately before reading or execution — scanning every segment from the configured root down to the file, so a skill directory (or any directory between it and the root) swapped for a link after discovery is rejected too. An OSError while inspecting an entry is treated as unsafe (skip / reject), never as "safe".

• MCPSkillsSource - SkillsSource that discovers Agent Skills served over MCP by reading the well-known skill://index.json (SEP-2640). Index entries are dispatched by their type (case-insensitive): skill-md entries become one MCPSkill each (its SKILL.md body and sibling resources are fetched on demand via resources/read), and archive entries are downloaded as a single ZIP blob and unpacked entirely in memory (via the private _ArchiveEntryLoader) into a FileSkill whose SKILL.md body drives it and whose sibling files (matching the resource extensions, within the search depth) become in-memory InlineSkillResource resources. Nothing is written to disk — there are no temporary directories to create, own, or prune (this is a deliberate divergence from .NET, which extracts ZIP archives to disk; it removes the temp-dir leak and the dangerous prune-of-unowned-subdirs footgun). Entries whose type has no handler (e.g. mcp-resource-template) are skipped. MCP-delivered scripts are never runnable: the loader emits no SkillScripts, so a bundled script can at most surface as a readable resource (and only if it matches the resource extensions — .py is not a default resource extension). The archive SKILL.md frontmatter name must match the advertised index-entry name or the skill is skipped. ZIP extraction is hardened: a .. path-traversal ("zip-slip") member name raises via _normalize_archive_member_name and aborts the whole skill (like the file-count/size limits), and file-count / uncompressed-size (_read_member_with_limit) / download-size limits are enforced. Archive behavior is configured with archive_* constructor kwargs (archive_resource_extensions, archive_resource_search_depth, archive_max_file_count, archive_max_size_bytes, archive_max_uncompressed_size_bytes) — Python uses plain kwargs, not a *Options object as in .NET. A non-"resource not found" error while downloading an archive propagates (so a failed CachingSkillsSource refresh does not overwrite a cached list with a partial result). Unlike .NET's AgentMcpSkillsSourceOptions.RefreshInterval, this source has no built-in refresh interval; wrap it in CachingSkillsSource(..., refresh_interval=...) for caching/refresh. This is a port of .NET PR #6631; the FileSkillsSource script_extensions/resource_extensions kwargs default to the built-in tuples and treat None as "use defaults" and an empty tuple as "discover none" (an empty tuple previously fell back to defaults). FoundryToolbox.as_skills_provider() forwards matching archive_* kwargs to this source.

• SkillsSource decorators - Skill sources are composable: SkillsSource is the abstract base, with concrete sources (InMemorySkillsSource, FileSkillsSource, MCPSkillsSource) and decorators that wrap an inner source — AggregatingSkillsSource (concatenate several sources), FilteringSkillsSource (predicate filter), DeduplicatingSkillsSource (first-wins by name), and CachingSkillsSource (cache the inner source's skills list). DelegatingSkillsSource is the abstract base for decorators. get_skills takes a SkillsSourceContext: every source/decorator implements async def get_skills(self, context: SkillsSourceContext) -> list[Skill] and forwards context to inner sources. SkillsSourceContext (frozen) carries the invoking agent (SupportsAgentRun) and optional session (AgentSession | None); SkillsProvider builds it from before_run's agent/session and passes it into the pipeline. FilteringSkillsSource's predicate is context-aware: Callable[[Skill, SkillsSourceContext], bool] (port of .NET #6797). Default caching is applied only to the built-in, context-independent leaf sources: for the Skill / sequence-of-skills / from_paths constructors, SkillsProvider builds DeduplicatingSkillsSource(CachingSkillsSource(<file|in-memory leaf>)) so expensive filesystem/network discovery runs once. A caller-supplied SkillsSource is used as-is — never auto-wrapped in caching or deduplication — because auto-caching a context-aware caller source in a single shared bucket would replay the first invocation's skills for later SkillsSourceContexts and leak skills across agents/tenants (matches .NET, whose custom-source constructor also adds no caching/dedup). Callers who want caching on a custom pipeline compose CachingSkillsSource(inner, cache_isolation_key_selector=...) themselves. disable_caching=True only affects the built-in leaf caching (it has no effect on a caller-supplied source, which is never cached). CachingSkillsSource shares a single in-flight fetch across concurrent callers (per cache key) and does not update its cache on a failed fetch, so the next call retries (an initial failure leaves the cache empty; a refresh failure keeps the previously cached list). By default all callers share one cache bucket; pass cache_isolation_key_selector=Callable[[SkillsSourceContext], str | None] to cache separately per key (e.g. per agent name) for context-aware inner sources — the key should be low-cardinality and stable, and returning None (or leaving the selector None) uses the shared bucket. By default a cached list never expires; pass refresh_interval=timedelta(...) (port of .NET CachingAgentSkillsSourceOptions.RefreshInterval) to treat a cached list as stale once it is older than the interval so the next call re-queries the inner source (useful when an inner source such as MCPSkillsSource changes over the process lifetime; a zero/negative interval makes every result immediately stale, and a failed refresh keeps the prior list and retries). Freshness is measured with a monotonic clock (time.monotonic()). SkillsProvider.__init__ / from_paths expose a cache_refresh_interval kwarg that is threaded into the built-in CachingSkillsSource (it has no effect on a caller-supplied source or when disable_caching=True). MCPSkillsSource and MCPSkill accept exactly one of client (a fixed, already-entered high-level Client or deprecated caller-owned ClientSession) or the compatibility-named session_provider (Callable[[], Client | ClientSession], resolved on every fetch); providing both/neither raises ValueError. The source borrows the supplied connection and never enters or closes it. Use session_provider when an owner replaces its connection over time, so cached MCPSkills keep fetching through the current normalized connection without reading MCPTool.session.

• MCP archive digests - MCPSkillsSource verifies each non-null archive digest against the decoded ZIP bytes after the download-size guard and before extraction. The accepted form is sha256: followed by 64 lowercase hexadecimal characters. Omitted/null digests remain supported; malformed values (including non-string JSON), unsupported algorithms, and mismatches warn and skip only that archive. A successful CachingSkillsSource refresh therefore replaces its list without rejected archives; transport errors still propagate and leave the stored cache unchanged. Verification does not apply to lazy skill-md entries or their supporting resources. A digest only proves consistency with the index, not that the server or skill content is trustworthy.

Model Context Protocol (_mcp.py)

• Client lifecycle design - ADR 0045 defines the ownership boundary. Both framework-created and caller-supplied _ClientMCPConnection instances expose and route operations through a high-level mcp.Client; their session remains None, and is_framework_owned determines whether Agent Framework may manage the Client lifecycle. Caller-supplied _SessionMCPConnection instances expose and call their ClientSession directly as a deprecated low-level compatibility path. Consequently, MCPTool.session is meaningful only for that low-level path.
• MCPTool connection state - The private _MCPConnection Protocol is the normalized tools/prompts/catalog surface. _ClientMCPConnection and _SessionMCPConnection each own one lifecycle responsibility; None means disconnected, and the Client implementation is the single update point for SDK cache policy.
• MCPStdioTool / MCPStreamableHTTPTool - Supported transport-specific subclasses. MCPWebsocketTool remains only as a deprecated compatibility symbol because MCP v2 removed WebSocket transport; it cannot create a connection.
• Argument allowlist (_prepare_call_kwargs) - Before each tools/call, kwargs are filtered to an allowlist built from the tool's declared parameters (inputSchema.properties) plus any user-configured extras. The declared half comes from the server's advertised schema, and runtime kwargs (FunctionInvocationContext.kwargs, seeded from function_invocation_kwargs) are merged with the model-supplied arguments upstream in _call_tool_with_runtime_kwargs, so provenance is gone by the time the filter runs. A runtime kwarg is therefore forwarded whenever the server declares a property of that name, without the model mentioning it — the server, not the caller, decides which runtime kwarg names it receives. A tool that declares no usable properties (including schemas with additionalProperties: true) forwards only the configured extras. _MCP_FRAMEWORK_DENYLIST is a narrow safety net covering only non-serializable framework objects a server declares in its schema (those are dropped); it does not generalize to arbitrary caller-chosen names, and explicit extras always win. The reserved _meta key is never forwarded as an argument; trusted caller/runtime _meta is validated as MCP request metadata, model-supplied _meta is discarded in generated MCP functions, and metadata precedence is caller/runtime < OpenTelemetry < tools/list metadata.
• allowed_tools (constructor arg on all MCPTool subclasses) - Restricts exposed MCP tools by raw remote MCP tool identity. Prefixed local names remain accepted only when the raw remote name already matches its normalized form; normalized/local aliases do not authorize a different raw remote name. Configured allow/approval names must identify at most one raw remote name across loaded tools and prompts: a raw name that overlaps another tool's prefixed alias raises ToolExecutionException. Discovery validates all pages before publishing new functions; a failed reload retains the previous functions and metadata. The allowlist is also revalidated when exposing functions so runtime changes cannot select an ambiguous name. If multiple raw remote tool names map to the same local function name, tool loading raises ToolExecutionException instead of first-one-wins shadowing.
• Progressive MCP disclosure (use_progressive_disclosure, always_load) - When enabled on any MCPTool subclass, the initial model-facing surface is loader tools (list_mcp_tools / load_tool / unload_tool, prefixed by tool_name_prefix when configured) plus allowed tools selected by always_load and tools loaded earlier on the same MCPTool instance. list_mcp_tools only reports tools that pass allowed_tools; filtered tools are not listed or loadable. Loader tool names are reserved in progressive mode: remote MCP tools whose local generated name collides with a loader name are omitted from the initial/listed surface, and explicit load_tool calls return a model-visible message pointing callers to tool_name_prefix or excluding the colliding tool. load_tool accepts one tool name or a list of tool names and uses FunctionInvocationContext.add_tools(...) so the selected generated MCP FunctionTools become available on the next function-calling iteration while keeping existing approval mode, argument filtering, header-provider runtime kwargs, result parsing, and OTel. unload_tool accepts one dynamically loaded tool name or a list of names and removes them from the live tool list and persisted progressive surface, but it does not remove tools configured in always_load. Invalid always_load entries are ignored like unmatched allowed_tools entries.
• Tool discovery refresh - Validate the complete current tool snapshot alongside retained non-tool functions, not stale tool wrappers. Successful refreshes atomically replace the previous tool set and metadata, including empty snapshots, while preserving prompts, custom functions, and unchanged tool wrappers (including caller customizations). Removed or replaced tool identities lose their persisted progressive-loaded state; failed refreshes preserve the previous state.
• additional_tool_argument_names (constructor arg on all MCPTool subclasses) - Opt extra argument names back into the allowlist. Accepts a Sequence[str] (applied to every tool) or a Mapping[str, Sequence[str]] keyed by remote tool name, where the reserved key "*" denotes global extras. It is configured only in user code at construction; there is no per-call/runtime override, so a model-issued tool call cannot change which names pass through — but note this constrains the model, not the server, which still widens the effective allowlist through its schema. To use a server that accepts additionalProperties: true, list the extra names here and then either (1) manually extend that tool's inputSchema (via the .functions list after connecting) so the model is prompted to supply them, or (2) supply the values yourself via function_invocation_kwargs. If a normal forwarded argument name is supplied by both the model and function_invocation_kwargs, the model-supplied value wins; _meta is the exception and only trusted runtime/caller metadata is used.
• MCP HTTP header request scoping - static_headers supplies fixed, origin-scoped headers without serializing concurrent calls. Generated tool and prompt calls give header_provider only host runtime kwargs, separately from model arguments; direct call_tool calls give it caller kwargs. Model-over-runtime precedence is confined to outbound tool arguments. Fixed and dynamic headers form the session's complete effective identity (case-insensitive names, case-sensitive values), with dynamic values overriding fixed values of the same name. Agent runs reconcile a connected tool before copying its functions; generated tool and prompt calls perform the same check at invocation. Connected run preparation defers when a provider needs runtime values supplied by invocation middleware; invocation-time resolution remains strict. A different identity waits for in-flight discovery, reconnects framework-created sessions without loading, then refreshes session-derived discovery under the same lock. Because a caller-supplied session's established identity is unknown and the wrapper cannot reconnect it, dynamic header resolution on that wrapper is rejected even while disconnected. Ambient requests retain the bound set. With a shared http_client, processing stays scoped to the originating MCPStreamableHTTPTool, and every injected header is stripped from cross-origin redirects. The session exit stack removes its request hook after transport shutdown, including failed initialization or discovery, and closes framework-created HTTP clients. Failed discovery resets the connection and discovery flags and rolls back partial function/metadata additions. Framework-created sessions are discarded; constructor-supplied sessions remain caller-owned and reusable across cleanup, close, and reset.
• MCP lifecycle caller cancellation - The lifecycle owner skips cancelled queued connect requests and waits for the caller to acknowledge successful setup. If the caller cancels before accepting a newly established connection, the owner tears it down before processing the next request. Cancelling a close waiter does not interrupt teardown, and cancelling a redundant connect does not discard a previously established session.
• Streamable HTTP cookies - Framework-created clients reject response-cookie persistence with or without a header_provider; explicit Cookie headers from static_headers or a provider remain supported. Caller-provided clients retain their cookie behavior and ownership. Applications requiring cookie persistence must scope clients and MCP sessions to one authenticated principal. Cookie rejection does not isolate MCP protocol sessions or other server-side state.
• function_invocation_kwargs and MCP servers - That dict is shared across every tool in the run, including every attached MCPTool, and any name in it reaches a server that declares a matching inputSchema property. header_provider does not mitigate this — it reads the kwargs without consuming them. To keep a credential out of tool arguments, source it outside function_invocation_kwargs: read a ContextVar inside the provider (this still allows a different value per request), configure a custom http_client, or use env for MCPStdioTool.
• Sampling guardrails (sampling_callback) - Passing client= advertises SamplingCapability so the server can send sampling/createMessage. Because remote servers are untrusted (confused-deputy risk), the default sampling_callback is deny-by-default and applies, in order: a per-session rate limit (sampling_max_requests, default _DEFAULT_SAMPLING_MAX_REQUESTS), an approval gate (sampling_approval_callback), and a maxTokens cap (sampling_max_tokens, default _DEFAULT_SAMPLING_MAX_TOKENS). The approval callback (constructor arg on all subclasses; exported type alias SamplingApprovalCallback) receives the raw CreateMessageRequestParams, may be sync or async, and must return truthy to approve. When it is None (the default) every sampling request is denied; pass lambda params: True to restore legacy auto-approve as an explicit opt-in. Requests and denials are logged at WARNING (content is not logged). The per-session counter resets in _reset_session_state.
• MCP response caching - High-level Clients use SDK response caching for tool/prompt catalogs and resource reads regardless of whether Agent Framework or the caller owns the Client, honoring modern server ttlMs / cacheScope hints. Legacy peers default to zero TTL, caller-supplied ClientSession connections stay uncached, and reconnect or effective header-identity changes replace a framework-owned Client's per-Client cache. Agent Framework does not expose shared-store or cache-mode configuration.
• MCP Tasks extension - MCP Python SDK 2.2 does not implement the 2026 io.modelcontextprotocol/tasks runtime. Tools that advertise execution.taskSupport == "required" are skipped with a warning; optional and task-free tools continue through ordinary tools/call. Restoration of the experimental Tasks API and removed sample is tracked in microsoft/agent-framework#8245.

File Access Harness (_harness/_file_access.py)

• AgentFileStore - Abstract async store backing the file-access harness. Implementations expose write, read, delete, list_children, file_exists, search, and create_directory over forward-slash relative paths. list_children returns the direct children (files and subdirectories, subdirectories first) as FileStoreEntry instances; search accepts a keyword-only recursive flag (default False) and, when recursive=True, walks all descendants and returns file_name values relative to the search directory. The line-numbering contract lives on the base class: split_lines publishes the \n-only keepends split that every line_number addresses, scan_content is the numbering primitive both shipped stores report through, and search is now concrete — it asks the overridable find_matching_files hook which files to consider (superset semantics; a backend with a native index overrides it and prunes server-side) and then reads and numbers them itself. A store may still override search outright, but then it owns numbering: it must report line_number as a 1-based coordinate into split_lines of the content read returns. Nothing checks that at run time, so a store that numbers differently makes a later edit land on the wrong line silently. The same applies to ReDoS: the grep pattern is model-supplied, so search compiles it through the regex module against a single monotonic deadline that bounds the whole scan (CPython's re holds the GIL for an entire match, so asyncio.wait_for around it bounds nothing). A store overriding search with bare re silently opts itself back out of that guarantee.
• InMemoryAgentFileStore - Dict-backed store suitable for tests and lightweight scenarios.
• FileSystemAgentFileStore - Disk-backed store rooted under a configurable directory. Enforces relative-path normalization, root containment, and rejects symlink/reparse-point segments to prevent escape.
• FileSearchResult / FileSearchMatch - SerializationMixin DTOs returned by search, carrying the matching file name, a context snippet, and the matching lines with 1-based line numbers. Implementers should report each matching line verbatim, including its own terminator, so it can be reused as a file_access_replace_lines new_line; the pattern itself is matched against the line with its whole terminator removed, so ^/$ anchor to the line's text on a CRLF file as they already did on an LF one. A custom store populates these DTOs from its own search; the verbatim text is a recommendation, but the line number is not — it must address split_lines.
• FileStoreEntry - SerializationMixin DTO returned by list_children, carrying an entry name and type ("file" or "directory").
• FileAccessProvider - ContextProvider that adds shared file-access tools (file_access_write, file_access_read, file_access_read_lines, file_access_delete, file_access_ls, file_access_grep, file_access_replace, file_access_replace_lines) plus default usage instructions to each invocation. file_access_ls enumerates direct children (both files and subdirectories) as {name, type} entries with an optional glob_pattern, so the agent can walk the tree level by level; file_access_grep searches recursively from an optional base directory and returns relative file_name paths, scoped via an fnmatch glob_pattern (where * crosses /, e.g. *.md, reports/*). file_access_replace substitutes old_string with new_string (failing if not found, or if multiple matches and replace_all is false); file_access_replace_lines replaces whole 1-based lines with literal text (each new_line includes its own trailing newline; an empty new_line deletes the line, including its line break). file_access_read_lines returns a 1-based inclusive line range, one line per row as <line_number>\t<line>; end_line may be omitted to read to the end of the file, and an end_line past the last line clamps to it. Everything after the tab is verbatim, including the line's own terminator (which therefore doubles as the row separator), so a row's text can be fed straight back as a file_access_replace_lines new_line without losing a \r\n. Its line numbering comes from the same _split_lines_keepends split as file_access_replace_lines and as the stores in this package, so with one of those a number reported by grep addresses the same line in all three tools, including the trailing empty line of a newline-terminated file; grep itself runs through AgentFileStore.search, which must number by the same split but does not inherit it, so a store overriding search owns its numbering and nothing verifies it at run time. All tools are registered with approval_mode="always_require" by default, so every file operation needs host approval. Pass disable_write_tools=True to advertise only the read-only tools. To run unattended you can disable approval at the source with disable_readonly_tool_approval=True (read, read_lines, ls, grep) and/or disable_write_tool_approval=True (write, delete, replace, replace_lines), which register the affected tools with approval_mode="never_require"; alternatively, keep approval on and pass one of the static auto-approval rules to ToolApprovalMiddleware (via auto_approval_rules): FileAccessProvider.read_only_tools_auto_approval_rule approves only the read-only tools (read, read_lines, ls, grep), while FileAccessProvider.all_tools_auto_approval_rule approves every file-access tool including the write tools. Both rules reject any call carrying a server_label so they stay scoped to this provider's local tools and never auto-approve a same-named hosted tool. The tool names are also exposed as class constants (WRITE_TOOL_NAME, READ_TOOL_NAME, READ_LINES_TOOL_NAME, DELETE_TOOL_NAME, LS_TOOL_NAME, GREP_TOOL_NAME, REPLACE_TOOL_NAME, REPLACE_LINES_TOOL_NAME). Unlike MemoryContextProvider, the store is intentionally shared across sessions and agents; pass session_scoped=True (with an optional explicit scope) to confine tool operations to a working folder derived from the session id or scope via the shared _storage_key_segment derivation (the provider fails closed when neither is available).

File Memory Harness (_harness/_file_memory.py)

• FileMemoryProvider - ContextProvider that gives an agent a session-scoped, file-based memory backed by the same AgentFileStore abstraction. Adds tools (file_memory_write, file_memory_read, file_memory_delete, file_memory_ls, file_memory_grep, file_memory_replace, file_memory_replace_lines) plus default usage instructions. Port of the .NET FileMemoryProvider.
• Scoping - Memories are isolated per session by default: each session writes under a working folder derived from context.session_id. Pass an explicit scope (e.g. a user id) to group memories across sessions, mirroring FoundryMemoryProvider's scope arg. The scope (or session id) is an opaque namespace key, not a path: it is mapped to exactly one folder via the shared agent_framework._filesystem._storage_key_segment derivation, which is injective (except for a collision-resistant digest fallback past a length cap) — two byte-distinct values do not share a working folder, so a caller authorized for one identifier cannot reach another's memories. A multi-segment value such as "tenants/alice" therefore becomes a single encoded folder rather than a nested directory. The provider fails closed (ValueError) when neither a scope nor a session_id is available, because an empty working folder would be the shared store root. Applications should still canonicalize and authorize externally supplied identifiers themselves; the storage mapping is the storage-layer guarantee, not a substitute for that check.
• Descriptions & index - file_memory_write accepts an optional description, stored in a companion <stem>_description.md sidecar. After each write/delete the provider rebuilds a capped (50-entry) memories.md index, and before_run injects that index as a user context message so the model knows what memories exist. Sidecars and the index are internal files hidden from file_memory_ls/file_memory_grep and rejected as write targets.
• DEFAULT_FILE_MEMORY_SOURCE_ID / DEFAULT_FILE_MEMORY_INSTRUCTIONS - Public defaults for the provider's source id and instruction banner.
• Harness wiring - create_harness_agent includes the FileMemoryProvider by default; the FileAccessProvider is opt-in and added only when a file_access_store is supplied (no implicit {cwd}/working store is created). Disable file memory via disable_file_memory; override its backing store via file_memory_store. When no file-memory store is supplied, the default is FileSystemAgentFileStore rooted at {cwd}/agent-file-memory. create_harness_agent also wires in MessageInjectionMiddleware by default (mirroring the .NET harness's UseMessageInjection); it is always on with no opt-out because it is a no-op when no messages are queued for the session.
• Experimental-feature gating - create_harness_agent itself is released (no longer @experimental), but a few features it can wire in remain experimental or pre-release: background agents (background_agents), file access (file_access_store), looping (loop_should_continue), and the shell tooling (shell_executor, from the pre-release agent-framework-tools package). Enabling any of them emits a single ExperimentalWarning (naming the responsible parameter). The three experimental harness providers share one HARNESS dedup key so the downstream experimental provider does not warn a second time; the shell tooling uses a separate dedup key (it is not a HARNESS-decorated feature).

Tool Approval Harness (_harness/_tool_approval.py)

• ToolApprovalMiddleware - Opt-in agent middleware that coordinates session-backed approval rules, heuristic auto_approval_rules, queued approval requests, collected approval responses, and streaming/non-streaming approval prompts. Heuristic callbacks receive the underlying function_call content.
• ToolApprovalRule / ToolApprovalState - Serializable state models for standing approvals and queued approval flow. ToolApprovalRule.arguments is None means a tool-wide rule; an empty dict {} means an exact no-argument call for create_always_approve_tool_with_arguments_response.
• create_always_approve_tool_response / create_always_approve_tool_with_arguments_response - Helpers that return normal function_approval_response content with additional_properties metadata consumed by ToolApprovalMiddleware. Standing rules for hosted tools include the server_label boundary, so same-named tools on different hosted servers do not share approvals.
• Mixed tool-call batches use a default .NET-style bypass in the function invocation loop: when a session is available, approval requests for known non-approval-required tools are treated as already approved, hidden, stored in session state keyed to the visible approval request ids from that batch, and reinjected only when that visible approval flow resumes.
• Approval resume is an immutable response boundary: the function invocation layer normalizes a private copy of caller messages, returns approved and rejected terminal results in the resumed response (and stream) before any final assistant message, and does not mutate the caller's approval Message or the earlier approval-request response.
• Approval/result correlation is occurrence-aware. Provider/service correlation stays in function_call.call_id, while new locally actionable calls carry one stable Agent Framework occurrence identity in function_call.id. New local approval request ids use that occurrence id; hosted provider-issued approval ids remain unchanged. Legacy stored pending calls without function_call.id retain exact request-id binding for one warned compatibility resume. A call_id may be reused after a completed round, so approval normalization matches ordered call occurrences and consumes approved results per occurrence rather than using one global result per call_id. All contents produced by one execution remain one result group and are consumed together, including multiple user-input requests.
• A local (non-hosted) function_approval_response authorizes execution only when it binds to an approval request recorded in an authoritative AgentSession. Runs without one drop inbound local approval responses with a warning and execute nothing, so callers must pass the session that issued the request back on the resuming run. An approval request that merely appears in the caller-supplied history is not proof the framework asked for approval. Hosted provider-issued approvals still pass through untouched, and a response already settled by a terminal result is replayed history rather than an authorization, so replaying a completed transcript keeps working without a session. That settled exemption is an allow-list evaluated per response object, not per approval id, because several responses can share one approval id and only the first is eligible to execute. Filtering runs before stateless mixed-batch completeness is enforced, so a dropped response is never counted as an answer. Set the disable_approval_response_binding function invocation configuration option to restore the previous unbound behavior.
• Approval resume keeps terminal function_result contents in tool-role messages and follow-up user-input requests in assistant-role messages, including mixed sibling batches.
• Function-call budget accounting counts one unit per executed result group, not per emitted function_result, so executions that pause for user input still consume max_function_calls.
• Once an invocation limit disables local tools, locally actionable calls and local approval requests are removed from streaming and final output. Provider-executed informational call/result pairs, hosted approval requests, and metadata-only stream updates remain visible.
• Declaration-only streamed calls emit their arguments only from the provider stream. The function layer sends a metadata-only follow-up (arguments=None) so id and user_input_request survive final aggregation without duplicating arguments.
• function_approval_request and function_approval_response are control-plane contents. History providers may retain them in their backing store for audit. The base HistoryProvider.before_run filters resolved wrappers from later model replay, but preserves unresolved requests/responses until a terminal result or follow-up request closes the occurrence. On unrelated turns the function layer omits the complete pending call batch from model input while retaining it for a later approval response. Providers configured with load_messages=False do not replay history, so this filter is intentionally not invoked.
• Reasoning content or opaque reasoning metadata bound to a function call is part of the same logical group as the call and terminal result. Function-loop replay and compaction must preserve that group atomically; adapters should fail before a stateless request when required reasoning cannot be reconstructed.
• Compaction scans assistant function calls and assistant- or tool-role terminal results in transcript/content order, pairing reused call_id values by ordered unambiguous occurrence. A result links only when exactly one preceding declaration remains unmatched; genuinely ambiguous duplicate declarations stay separate.

Agent Loop (_harness/_loop.py)

• AgentLoopMiddleware - AgentMiddleware that re-runs an agent in a loop by calling call_next() repeatedly (the pipeline re-reads context.messages each time). One configurable class covers two patterns: a required user should_continue predicate (sync or async, the first positional/keyword arg), and a chat-client judge built via the .with_judge(...) factory (a second chat client decides whether the original request was answered; loops while it is not, using a `Ju

Truncated - read the full file at https://github.com/microsoft/agent-framework/blob/44fe223da7ea6853eefb7053e25c2c213a2e5c15/python/packages/core/AGENTS.md.