Fix .md endpoint serving "[object Object]" for externalSource-backed pages

[pjb157]

Problem

curl https://docs.doubleword.ai/inference-api/async-agents.md (also pr-review-bot.md, cli-examples.md) returns HTTP 200 with the literal body [object Object]. These are Sanity docPages whose real content lives in the top-level externalSource field (raw GitHub README URLs). Per AGENTS.md, AI agents rely on the .md endpoints, so this is user-facing breakage.

Root cause

Two bugs in src/app/api/markdown/[product]/[...slug]/route.ts:

1. The route's GROQ query only selected linkedPost->{body, externalSource} — never the docPage's own externalSource — so the handler never fetched the external README and fell through to doc.body.
2. doc.body for these pages is a placeholder Portable Text block array, not a markdown string. Passing the array to new NextResponse(...) stringifies it (the fetch spec's ToString conversion), producing [object Object].

The HTML page route (src/app/[product]/[...slug]/page.tsx) already resolves content as externalSource > linkedPost > body and coerces Portable Text — the markdown route had drifted from it.

Fix

• Select externalSource in the markdown route's query and resolve content in the same order as the page route: externalSource → linkedPost.externalSource → linkedPost.body → body.
• Mirror the page route's HTML guard: if externalSource returns text/html (pages that use it only to power the "View source" link), fall back to the Sanity body instead of serving HTML.
• Extract coerceMarkdownContent (Portable Text → markdown) from MarkdownRenderer.tsx into shared src/lib/portable-text.ts and use it in the route, so the endpoint can never emit a stringified object regardless of body shape.
• New: optional SANITY_API_HOST env override on the Sanity client (no-op unless set, documented in AGENTS.md) so a local mock Sanity can back offline dev/verification in sandboxed environments that can't reach *.sanity.io.

Tests

• route.test.ts: externalSource page serves fetched markdown; failed external fetch falls back to coerced Portable Text (asserts no [object Object]); HTML externalSource falls back to body; plain string body + image rewriting unchanged; linkedPost.externalSource path unchanged; 404 for unknown docs; .md suffix stripping.
• portable-text.test.ts: coercion contract (strings pass through, blocks render, non-strings never stringify to [object Object]).

Verification

Ran npm run dev against a local mock Sanity API (via SANITY_API_HOST) returning the async-agents doc (Portable Text placeholder body + real externalSource URL) and a normal string-body page:

• Before (original route): curl localhost:3000/inference-api/async-agents.md → HTTP 200, body [object Object] — reproduces the production symptom exactly.
• After: same curl → HTTP 200, 12,371 bytes of the actual README fetched from raw.githubusercontent.com/doublewordai/use-cases/.../async-agents/README.md ("# Async Agents: Deep Research in a Day for $0.34 …").
• Normal Sanity-body page (quickstart.md) serves its markdown string unchanged before and after.

vitest run (51 tests, only pre-existing search.test.ts > stripMarkdown failure, which also fails on a clean checkout of main), tsc --noEmit clean, eslint introduces no new findings. The Vercel preview build for this PR passed (deployment Ready), which covers npm run build (not runnable in this sandbox — no Sanity network access).

Preview spot-check (sandbox can't reach *.vercel.app, so please click — each should be markdown, not [object Object]):

• async-agents.md
• pr-review-bot.md
• cli-examples.md
• normal Sanity-body page: batch-inference.md

https://claude.ai/code/session_017nS4EWobkNNbu1jCGZ92LL

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Project	Deployment	Actions	Updated (UTC)
documentation	Ready	Preview, Comment	Jun 11, 2026 2:16pm

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Pull request overview

Fixes the .md markdown API route so Sanity docPages backed by externalSource return actual markdown (instead of stringifying Portable Text to "[object Object]"), and keeps markdown/body resolution consistent with the HTML doc page route.

Changes:

• Update the markdown route GROQ query + resolver order to support docPage.externalSource, with an HTML-response guard for external fetches.
• Extract Portable Text → markdown coercion into a shared helper and use it in both the renderer and the markdown API route.
• Add targeted Vitest coverage for the markdown API route and the Portable Text coercion contract; document SANITY_API_HOST and add an optional client override for offline/local mocking.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/sanity/lib/client.ts	Adds optional SANITY_API_HOST override for pointing the Sanity client at a stand-in API host.
src/lib/portable-text.ts	New shared coerceMarkdownContent helper to safely convert Portable Text blocks to markdown strings.
src/lib/portable-text.test.ts	Unit tests for coercion behavior (strings passthrough, blocks rendered, no "[object Object]").
src/components/MarkdownRenderer.tsx	Switches renderer to use shared coercion helper (removes duplicated implementation).
src/app/api/markdown/[product]/[...slug]/route.ts	Fixes .md endpoint content resolution to handle externalSource and Portable Text bodies.
src/app/api/markdown/[product]/[...slug]/route.test.ts	Adds route-level tests for externalSource fetch, fallbacks, HTML guard, image rewriting, 404, and .md stripping.
AGENTS.md	Documents SANITY_API_HOST for offline dev/verification workflows.

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