Writing Friday Python Agents

Writing Friday Python Agents

Friday agents are single-file Python modules that the platform spawns as native subprocesses and communicates with via NATS. The SDK is a normal Python package installed into your environment. There are no compile-time or WASM steps.

The mental model: your Python code runs as a normal process. All I/O that crosses network boundaries or needs credentials routes through host capabilities the platform provides — LLM generation, HTTP requests, MCP tools, and progress streaming. You declare what you need in the @agent decorator, and the platform wires it up at execution time via AgentContext.

Every Agent Looks Like This

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from friday_agent_sdk import agent, ok, AgentContext, run

@agent(
    id="my-agent",
    version="1.0.0",
    description="What this agent does — the planner reads this to decide when to invoke it",
)
def execute(prompt: str, ctx: AgentContext):
    # Your logic here
    return ok({"result": "data"})

if __name__ == "__main__":
    run()

Three things are non-negotiable:

1. The @agent decorator with at least id, version, description.
2. run() in __main__ — this is the NATS entry point. Without it, the process spawns and immediately exits. run() checks FRIDAY_VALIDATE_ID (registration) or FRIDAY_SESSION_ID (execution), connects to NATS, and handles the protocol.
3. Return ok() or err() — never return raw dicts/strings.

Capabilities Quick Reference

All capabilities live on AgentContext and are always initialized. They may be stubs in test contexts, but never None.

ctx.llm — LLM Generation

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# Text generation
response = ctx.llm.generate(
    messages=[{"role": "user", "content": "Summarize this document"}],
    model="anthropic:claude-sonnet-4-6",  # fully qualified
)
print(response.text)  # str

# Structured output — returns response.object (dict), response.text is None
response = ctx.llm.generate_object(
    messages=[{"role": "user", "content": "Extract key facts"}],
    schema={"type": "object", "properties": {"facts": {"type": "array", "items": {"type": "string"}}}},
    model="anthropic:claude-haiku-4-5",
)
print(response.object)  # dict matching schema

Pass a fully qualified model like anthropic:claude-sonnet-4-6, or set defaults in the llm decorator param and omit the per-call model.

Optional params: max_tokens, temperature, provider_options (dict).

ctx.http — Outbound HTTP

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response = ctx.http.fetch(
    "https://api.example.com/data",
    method="POST",
    headers={"Authorization": f"Bearer {ctx.env['API_KEY']}"},
    body='{"query": "test"}',
    timeout_ms=30000,
)
data = response.json()  # convenience method
# response.status (int), response.headers (dict), response.body (str)

The host handles TLS, audit logging, and rate limits. 5MB response body limit. Note: HTTP status errors (4xx, 5xx) don't raise — check response.status manually.

ctx.tools — MCP Tools

Declare MCP servers in the decorator, then call tools at runtime:

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@agent(
    id="my-agent", version="1.0.0", description="...",
    mcp={
        "github": {
            "transport": {
                "type": "stdio",
                "command": "npx",
                "args": ["-y", "@modelcontextprotocol/server-github"],
                "env": {"GITHUB_TOKEN": "{{env.GITHUB_TOKEN}}"}
            }
        }
    },
)
def execute(prompt: str, ctx: AgentContext):
    available = ctx.tools.list()  # list[ToolDefinition] — name, description, input_schema
    result = ctx.tools.call("create_issue", {"title": "Bug", "repo": "owner/repo"})

{{env.VARIABLE}} in MCP config references agent environment variables. Currently only stdio transport is supported.

Do not bypass ctx.tools. Python agents must not call local MCP HTTP endpoints such as http://localhost:8002/mcp, hardcode bearer tokens, or guess provider-specific tool names. Use ctx.tools.list() to inspect the runtime tool surface and ctx.tools.call(name, args) to invoke it. Host-side tool calls are credentialed, audited, and recorded in session history; direct HTTP calls are invisible to Friday and commonly fail with unknown-tool or invalid-token errors.

If ctx.tools.call raises ToolCallError("Unknown tool ..."), list tools and fix the workspace/agent config rather than retrying a guessed name.

Platform-injected tools

The host may inject additional tools into ctx.tools beyond what you declared in mcp={...} — workflows like human-input elicitations, memory writes, artifact handling. These are platform features, not SDK features; their names, arg shapes, and semantics belong to whichever platform you're running on. Call ctx.tools.list() at runtime to see what's actually available, and consult the platform's own docs for which tools to call. The SDK contract is just ctx.tools.call(name, args) -> dict and ToolCallError on failure.

ctx.stream — Progress Events

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ctx.stream.progress("Analyzing document...", tool_name="analysis")
ctx.stream.intent("Switching to code review phase")
ctx.stream.emit("custom-event", {"key": "value"})  # raw event

Emit progress before expensive operations so the UI shows what's happening.

Structured Input Handling

A type: user agent receives input through one of two channels. Pick by how the job is wired, not by preference:

ctx.input does NOT carry the signal payload — a job triggered by a signal with no upstream producer has an empty ctx.input. If you guessed a key for ctx.input.get(...) and got nothing back, you almost certainly wanted the signal payload — read prompt with parse_input instead.

ctx.input — Upstream step output

When the action declares inputFrom, use ctx.input rather than scraping the prompt. Producers may compact bulky outputs into summary + artifact refs; downstream agents dereference those refs through host capabilities instead of asking the producer to inline large payloads.

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# Compact value (whatever the upstream step put in its outputTo doc)
payload = ctx.input.get("emails-result")

# Or, when the doc carries artifact refs, hydrate the JSON contents
payload = ctx.input.artifact_json("emails-result")
emails = payload.get("emails", [])
return ok({"count": len(emails)})

Useful methods (all take an optional doc-id; omit it to operate on the full raw input):

• ctx.input.get(name, default=None) — compact input payload for an inputFrom document.
• ctx.input.require(name) — same, but raises ValueError if missing.
• ctx.input.artifact_refs(name) — artifact refs attached to the input.
• ctx.input.artifact_json(name) — fetch via get_artifact and parse JSON contents.

prompt still exists for natural-language task instructions and backwards compatibility, but parse_input(prompt) is not the right abstraction for multi-step outputTo/inputFrom handoffs.

parse_input — Signal-payload extraction

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from friday_agent_sdk import parse_input

# Raw dict extraction
data = parse_input(prompt)  # returns dict

# Typed extraction with a dataclass schema
@dataclass
class TaskConfig:
    url: str
    max_retries: int = 3

config = parse_input(prompt, TaskConfig)  # returns TaskConfig instance

Extraction strategy (3-level fallback):

1. Balanced-brace JSON scan (handles nested objects)
2. Code-fenced ```json blocks
3. Entire prompt as JSON

Unknown keys are automatically filtered when using a dataclass schema.

parse_operation — Discriminated dispatch

For agents that handle multiple operation types:

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from friday_agent_sdk import parse_operation
from dataclasses import dataclass

@dataclass
class ViewConfig:
    operation: str
    item_id: str

@dataclass
class SearchConfig:
    operation: str
    query: str
    max_results: int = 50

OPERATIONS = {
    "view": ViewConfig,
    "search": SearchConfig,
}

config = parse_operation(prompt, OPERATIONS)  # dispatches on "operation" field

match config.operation:
    case "view": return handle_view(config, ctx)
    case "search": return handle_search(config, ctx)

Result Types

Always return ok() or err(). Never return raw values.

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from friday_agent_sdk import ok, err

# Simple success
return ok({"status": "done", "count": 42})

# Error
return err("Jira API returned 403: insufficient permissions")

ok() accepts any JSON-serializable data: dicts, lists, strings, dataclass instances (auto-converted via dataclasses.asdict).

The Execution Model — What You Cannot Do (And Why)

Your agent runs as a normal Python subprocess, so the standard library and most installed packages work. The constraint is not a sandbox — it's that I/O should route through host capabilities so the platform manages credentials, rate limits, and audit logging centrally.

Recommended — use host capabilities for I/O:

• ctx.http.fetch() instead of requests/httpx — host manages TLS, logging, limits
• ctx.llm.generate() instead of anthropic/openai — host manages API keys, routing
• ctx.tools.list() + ctx.tools.call() instead of direct API/MCP HTTP calls — MCP servers run centrally and calls remain observable
• ctx.stream.progress() instead of print() — UI integration, not stdout
• ctx.env instead of os.environ — only declared variables are injected

Available — Python standard library and installed packages:

• json, re, base64, urllib.parse, dataclasses
• collections, itertools, functools, typing
• math, datetime, uuid, hashlib
• pydantic if installed, numpy if installed, etc.

When in doubt: if the operation touches a network boundary or needs credentials, use the host capability. If it's pure data manipulation, standard library or installed packages are fine.

Getting Agents Into a Workspace

The SDK's job ends at the file: a registered @agent function plus run() in __main__. How that file becomes a usable agent on the host is a platform concern (Friday Studio's CLI, an in-tree dev daemon, a deployment pipeline) — see your platform's docs for the registration command, and do not have the agent code shell out to a daemon to register itself.

Once the platform knows about it, reference it in workspace.yml:

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agents:
  - id: my-agent
    type: user

The id matches the id= value from the @agent decorator. The host resolves type: user to the registered Python agent and routes invocations into your execute function over NATS.

The agent process is spawned per invocation by the host; it sets FRIDAY_VALIDATE_ID (registration) or FRIDAY_SESSION_ID (execution) plus NATS_URL, and run() handles the rest. Your code should not open its own NATS connection.

Casing Convention

This is a subtle but real source of bugs:

• Decorator kwargs: snake_case (Pythonic) — display_name, use_workspace_skills
• Dict values inside decorator: camelCase (matches host Zod schemas) — linkRef, displayName inside environment dicts
• MCP config keys: camelCase in transport config
• Result data: your choice, but snake_case is conventional for Python agents

The bridge layer (_bridge.py) handles converting decorator metadata to camelCase for the host automatically. You don't need to worry about the boundary — just follow the convention above.

References

For deeper dives, read these reference files:

• references/api.md — Complete API reference: every decorator param, every context field, every method signature and return type
• references/examples.md — Annotated example agents from simple (echo) to complex (Jira multi-operation, GitHub PR operations)
• references/constraints.md — Full list of constraints, casing rules, build pipeline details, and common mistakes with fixes