MCP: consume `func --json` output and forward structured errors via CallToolResult.StructuredContent

[lkingland]

Summary

Once func supports a --json flag (#3769), the MCP server should invoke func with --json everywhere and forward the structured payload to MCP clients via CallToolResult.StructuredContent — replacing all output parsing / error scraping currently done (or attempted) on the MCP side.

Depends on

• Add --json output mode to every func subcommand, including errors #3769 — --json output mode on every func subcommand (prerequisite). This issue is implementable only after that lands.

Motivation

The MCP server currently treats func as a text-emitting black box: it captures stdout/stderr and either presents the blob to the agent verbatim or tries to classify it heuristically. Both shapes are dead ends:

• Verbatim text gives agents no machine-readable handle to dispatch on.
• Heuristic classification (see feat(mcp): add structured error categories for agentic error handling #3753) requires substring matching against output the producer can change at any time. Brittle, incomplete, and only deferred the problem.

Once func --json exists, the right MCP wrapper is roughly:

result, err := executor.Run("func", "build", "--json", ...)
// parse result.Stdout as the envelope from #3769
// on success: stick payload in StructuredContent
// on error: stick error object in StructuredContent, set IsError=true

Required changes

• Invoke func with --json in every MCP tool handler (build, deploy, delete, create, list, describe, invoke, all config_*, healthcheck, etc.). Currently inconsistent — only some handlers attempt any structure at all.
• Populate CallToolResult.StructuredContent with the parsed JSON, not TextContent. MCP already has a machine-readable channel; use it.
• Surface the full error taxonomy to agents: READONLY_ERROR, TIMEOUT_ERROR, and any other categories func emits — don't drop them on the floor.
• Forward the retryable / transient boolean from the structured error. This is the actual dispatch signal agents need ("try again" vs "give up").
• Keep TextContent populated with a short human-readable summary for fallback / log display, but the canonical channel is StructuredContent.
• Remove the in-MCP categorization heuristics entirely — func is the source of truth.

Related

• MCP: Implement Structured Error Categories for Agent Decision-making #3750 — original ask that motivated this. Should be closed-as-superseded once both this issue and Add --json output mode to every func subcommand, including errors #3769 are scoped.
• feat(mcp): add structured error categories for agentic error handling #3753 — first attempt, rejected because the categorization belonged in func rather than MCP.

Non-goals

• Defining the error taxonomy itself — that's Add --json output mode to every func subcommand, including errors #3769's job.
• Backwards compatibility with the current text-blob shape (the MCP surface is young; we'd rather get the shape right than freeze the current one).

[lkingland]

The end boss is actually #3771. If pkg/mcp calls pkg/functions directly (as the CLI does) there's no func --json subprocess output to consume — typed errors and structured values flow naturally through Go calls. This is still an important first-step, and the --json structured interaction important for agents choosing to use the CLI rather than MCP, and scripts/automation.

[Ankitsinghsisodya]

/assign