Oslo Vibe Coding

Session 05 / AI-native engineering

Spec-Driven
Development
in the AI age.

When implementation is abundant, a clear specification becomes the shared contract.

No one codes alone.

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The shift

When implementation gets cheap, ambiguity gets expensive.

Less time writing codeMore need for clear intent

Implementation moved downstream.

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Choose the mode

Vibe coding and SDD are two gears.

ExploreFast, disposable, reversible
CommitShared, durable, consequential

Shift gears as the cost of being wrong rises.

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One app / two processes

The difference is where decisions happen.

Vibe coding

Decide inside the loop

  1. 01IdeaRegistration app
  2. 02PromptDescribe it
  3. 03GenerateGet code
  4. 04InspectFind gaps
  5. 05Re-promptChange direction
  6. 06ShipWhen it feels ready

Spec-driven

Decide before the build

  1. 01IntentOutcome + users
  2. 02SpecifyRules + limits
  3. 03ApproveReview decisions
  4. 04PlanDesign + tasks
  5. 05ImplementGenerate code
  6. 06VerifyProve behavior

Same app. Different location for engineering decisions.

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What a spec is

A spec is a shared contract for five decisions.

Example / Community event registration

  1. Who has the problem?Attendees and event organizers
  2. What outcome do they need?Fill seats without spreadsheet chaos
  3. What must the system do?Confirm 100 people, then start a waitlist
  4. What must it never do?Overbook or accept duplicate email addresses
  5. How will we know it works?Every acceptance scenario passes

Clear enough to build. Specific enough to verify.

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The artifact chain

Decisions become artifacts before code.

01ConstitutionPrinciples
02SpecifyWhat + why
03ClarifyUnknowns
04PlanHow
05TasksSequence
06ImplementCode
07ValidateEvidence

GitHub Spec Kit, simplified.

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Live demo

Watch ambiguity collapse.

“Build an event registration app.”
CapacityStop at 100 confirmed guests
OverflowStart a first-in waitlist
IdentityBlock duplicate email addresses
FeedbackShow status immediately

The spec removes decisions from the coding loop.

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Make it testable

Acceptance criteria turn intent into evidence.

Given

100 people are confirmed

When

A new person registers

Then

They join the waitlist

If “done” is observable, the agent can verify it.

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How teams use it

Keep the agent inside reviewable checkpoints.

Intent
Design
Build
Proof
Human
Define outcome
Approve plan
Review changes
Accept result
Agent
Draft spec
Propose plan
Implement tasks
Run checks
Evidence
Open questions
Architecture
Diffs
Tests + review

Humans decide. Agents accelerate. Evidence closes the loop.

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Tool landscape

Three tools. Three different bets.

ToolCore betBest fit
KiroSpecs live inside the development environmentGuided feature work
Spec KitAn open artifact workflow can work across agentsTeams that want portability
TesslThe specification can become the sourceHigher automation experiments

The workflow matters more than the logo.

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Maturity

How strongly should the spec govern code?

Level 1

Spec-first

Write the spec before implementation.

Level 2

Spec-anchored

Keep specification and code aligned as both evolve.

Level 3

Spec-as-source

Humans change the spec; the system maintains the code.

Most teams are still learning Level 1 and Level 2.

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What the evidence says

Promising evidence. Not settled evidence.

73%

Fewer security defects reported

A machine-readable security constitution constrained AI-generated banking services.

One case study / preprint
128

Agent runs evaluated

Repository discovery hooks reduced context blindness and improved benchmark results.

Early evaluation / preprint

Use the findings as signals, not guarantees.

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What SDD does not fix

A spec can still create false confidence.

Failure 01

Wrong intent

A detailed specification can precisely describe the wrong product.

Failure 02

Stale context

The document drifts away from the code, users, or constraints.

Failure 03

No verification

Generated code looks complete without proving the behavior.

Ground it. Version it. Test it. Review it.

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The decision rule

Match the spec to the cost of being wrong.

Low consequence

Prompt

Tiny, reversible experiment with no important data.

Shared feature

Lightweight spec

Users, data, several decisions, or team handoffs.

High consequence

Detailed spec + gates

Payments, privacy, security, or irreversible actions.

More consequence means more explicit agreement.

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Before the next agent run

Write five lines.
Then write code.

ProblemUserOutcomeConstraintsDone

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