fix(server): tell agents how to nest fenced code inside code blocks

[pc-style]

What Changed

One sentence added to the shared runtime instructions that every provider adapter appends (Codex, Claude, Cursor, Grok, OpenCode, Antigravity): a code block that itself contains triple backticks must be fenced with four backticks or ~~~. No parser or renderer change.

Why

When an agent replies with a code block containing a fenced block (a prompt to paste, a README snippet), the chat renders it broken: the inner closing fence ends the outer block and the rest spills out as prose. The renderer is right, that is standard CommonMark. The agent just never knew it needed a longer fence. Diverging from CommonMark in the renderer would break copy and paste round trips with GitHub and every other markdown surface, so the fix is to tell the agent.

UI Changes

The change is in what the agent emits, so the visible effect is in how replies render. Both captures below use the same ordinary prompt in a fresh thread with Codex (GPT-5.6), nothing about fences in the prompt:

Write a README.md for a tiny Python CLI called todo that can add and list tasks. Show me the whole file so I can copy it. Do not create or change any files.

Before, server on main: the reply opened with a triple-backtick markdown fence. The first inner bash fence closes it, the rest of the README renders as prose, and a stray empty text block is left at the end.

After, server on this branch: the reply opened with a four-backtick fence and renders as one block with all six inner fences intact.

Streaming the after reply, sped up:

after-readme-streaming-fast2.mp4

Validation:

• vp test run apps/server/src/provider/RuntimeInstructions.test.ts passes (6 tests, including a new one for the guidance)
• Raw replies checked in the state database: before starts with ```markdown, after starts with markdown ````

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots
• I included a video for the streaming render

[macroscopeapp]

Approvability

Verdict: Approved at f4a9b2b

Macroscope's review found this PR approvable — The change adds a focused sentence to shared agent instructions so nested fenced code renders correctly, plus a test verifying the guidance. It affects response formatting across existing providers but introduces no new workflow, schema, deployment, security, or static-analysis behavior.

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[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6c58362 and f4a9b2b.

📒 Files selected for processing (2)

• apps/server/src/provider/RuntimeInstructions.test.ts
• apps/server/src/provider/RuntimeInstructions.ts

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📝 Walkthrough

Walkthrough

buildRuntimeInstructions now includes CommonMark guidance for nesting fenced code blocks. A test verifies that the output recommends a longer fence.

Changes

Runtime instruction guidance

Layer / File(s)	Summary
Nested fence guidance apps/server/src/provider/RuntimeInstructions.ts, apps/server/src/provider/RuntimeInstructions.test.ts	The runtime instructions explain that code blocks containing triple backticks must use four backticks or a ~~~ fence. The test verifies this guidance.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to f4a9b

Runtime instructions now explain safe nested CommonMark fencing, reducing malformed code-block rendering without changing parser or renderer behavior. The change is ready to merge.

Suggested reviewers: juliusmarminge

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: informing agents how to nest fenced code blocks.
Description check	✅ Passed	The description covers what changed, why it changed, UI evidence, validation results, and the required checklist. It is focused and complete.

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