# How does a shipped game (Valve: Left 4 Dead, Portal, Dota) decide which contextual NPC line to play, in terms a newcomer can reproduce?

- id: art_01M5MV08H0NKBYPH3WNQECGA79
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T01, T18 · tags: none
- updated: 2026-10-23T16:55:13.145Z
- url: https://veyrul.com/articles/art_01M5MV08H0NKBYPH3WNQECGA79

The Valve designer-programmer who built the system describes it in his own GDC 2012 slides: when a character wants to speak, the game builds a flat dictionary of facts (the query), every rule in a writer-editable database is tested (a rule is a list of criteria that must all be true), the matching rules are scored by how many criteria they contain, and one of the top-scoring rules is chosen at random. General rules act as fall-backs for specific ones. The deck is a first-person talk, not a measured study; the Source SDK 2013 code confirms the same select-best-then-random-tie pattern in an earlier (Half-Life 2 lineage) version.

The Valve designer-programmer who built the system describes it in his own GDC 2012 slides: when a character wants to speak, the game builds a flat dictionary of facts (the query), every rule in a writer-editable database is tested (a rule is a list of criteria that must all be true), the matching rules are scored by how many criteria they contain, and one of the top-scoring rules is chosen at random. General rules act as fall-backs for specific ones. The deck is a first-person talk, not a measured study; the Source SDK 2013 code confirms the same select-best-then-random-tie pattern in an earlier (Half-Life 2 lineage) version.

Curator qualifications: Excerpted evidence located (two excerpt-less rows); slides' criteria-count scoring vs SDK weighted sum (equivalent only with default weights) kept. Rights: Valve SDK licence scope per C3R-0275/FB43 (reimplement the idea; do not copy SDK code outside its scope).

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://cdn.akamai.steamstatic.com/apps/valve/2012/GDC2012_Ruskin_Elan_DynamicDialog.pdf",
      "note": "candidate evidence excerpts"
    },
    {
      "url": "https://github.com/ValveSoftware/source-sdk-2013/blob/be57e3d01ac7d7aa6a2cacbfef187c0b46616db0/src/game/server/AI_ResponseSystem.cpp",
      "note": "candidate evidence excerpts"
    },
    {
      "url": "https://web.archive.org/web/20141108091121id_/http://www.gdcvault.com/play/1015528/AI-driven-Dynamic-Dialog-through",
      "note": "candidate evidence excerpts"
    },
    {
      "url": "https://emshort.blog/2012/03/16/gdc-2012-talk-on-dynamic-dialogue/",
      "note": "candidate evidence excerpts"
    },
    {
      "url": "https://web.archive.org/web/20140110062400id_/http://assemblyrequired.crashworks.org/2012/03/13/ai-driven-dynamic-dialog-at-gdc-2012/",
      "note": "candidate evidence excerpts"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] The Valve designer-programmer who built the system describes it in his own GDC 2012 slides: when a character wants to speak, the game builds a flat dictionary of facts (the query), every rule in a writer-editable database is tested (a rule is a list of criteria that must all be true), the matching rules are scored by how many criteria they contain, and one of the top-scoring rules is chosen at random. General rules act as fall-backs for specific ones. The deck is a first-person talk, not a measured study; the Source SDK 2013 code confirms the same select-best-then-random-tie pattern in an earlier (Half-Life 2 lineage) version. · locator: candidate evidence excerpts · source: https://cdn.akamai.steamstatic.com/apps/valve/2012/GDC2012_Ruskin_Elan_DynamicDialog.pdf

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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