# Claude Fable 5.1 on iac-bench 2026-10

> Source: <https://www.massdriver.cloud/blogs/iac-bench-2026-10-claude-fable-5-1>
> Published: 2026-11-04 00:00:00+00:00

November changelog: Runtimes & OIDC cloud credentials, plus imports with Claude Code & opencode

[Read more](https://www.massdriver.cloud/blogs/november-2026-changelog)

How Claude Fable 5.1 did on iac-bench 2026-10: an overall composite of 86.9, scores per fixture, strengths, weaknesses and the patterns in its OpenTofu modules.

All ten models side by side, with a sortable leaderboard, picks by use case and the cost of each: [LLM rankings](https://www.massdriver.cloud/llm-rankings).

| Developer | Anthropic | 
| API identifier | `claude-fable-5-1` | 
| Access | Anthropic API | 
| Weights | Closed | 
| List price (USD / M tokens, in / out) | 10.00 / 50.00 (OpenRouter, 2026-10-23) | 
| Evaluated | 2026-10-22 (generation) · 2026-10-23 (judge) | 
| Attempts | 9 (3 fixtures × 3 samples) | 
| **Overall composite** | **86.9** (95% CI 83.7–90.2) | 
| Rank | 2 of 10 (95% rank interval 1–5) | 
| Mechanical only / judge only | 85.0 (rank 2) / 91.2 (rank 5) | 
| Failed gates · insecure attempts | 0/9 · 0/9 | 
| Estimated cost, all 9 attempts | $13.97 | 

Claude Fable 5.1 places second, close behind Claude Opus 5.5. All 9 modules planned, and none had a security finding. Together with Claude Opus 5.5, it shows the deepest service expertise in the field on Cloud SQL and App Service (median 8/9 and 5/5 expertise items).

Its cost is the main counterweight: it was the most expensive model on every fixture, at nearly four times the per-attempt cost of Claude Opus 5.5. Its interfaces are also consistently larger than the benchmark's budget for each audience.

| Fixture | Composite median (range) | Rubric median | Expertise | Judge median | ✗ gate | ⚠ security | Median inputs | Median output tokens | Median time | Median cost | 
|---|---|---|---|---|---|---|---|---|---|---|
| G001 · S3 | 85.6 (82.6–90.2) | 0.826 | 6/7 | 92.1 | 0/3 | 0/3 | 12 | 12,112 | 302 s | $0.935 | 
| G002 · Cloud SQL | 85.6 (79.0–90.0) | 0.832 | 8/9 | 94.7 | 0/3 | 0/3 | 30 | 14,402 | 196 s | $1.229 | 
| G003 · App Service | 89.5 (88.5–94.9) | 0.911 | 5/5 | 86.8 | 0/3 | 0/3 | 28 | 27,308 | 566 s | $2.157 | 

Rubric items not at full credit, counted over attempts that planned: G001: `A5` 3/3, `B1` 3/3, `H12` 2/3, `H13` 3/3, `H15` 1/3; G002: `A8` 3/3, `B1` 3/3, `H4` 3/3, `H5` 3/3, `H10` 1/3, `H11` 3/3; G003: `A7` 3/3. Definitions are in [Scoring criteria](https://www.massdriver.cloud/blogs/iac-bench-2026-10-scoring-criteria).

`environment` input sets the
network (`apps-<env>`), the HA type and backup retention.` terraform_data` guard on App Service that holds five capacity rules.`A5`, 11–13 inputs against 8.` A8`, 26–37 inputs against 25. One attempt scored zero.` A7`, 27–29 inputs against 20.` intelligent_tiering_after_days`.`>=`, or `>= 5.30, < 7.0`, in
6 of 9 attempts (`B1`).` H11` lost in 3/3). One
attempt adds a `deletion_policy` input, but its default is `DELETE`. One attempt also
writes a break-glass password into state (`H10`).` H13`, 3/3).` observability.tf`, `scaling.tf`,
`slot.tf`), but still lets `main.tf` grow past 200 lines in 4/9 attempts.`for_each` for multiplicity and `count` only as a switch.
`G002/claude-fable-5-1-e1/main.tf` lines 11–21

```
  # Flags that are the same on every instance and cannot be overridden by callers.
  enforced_database_flags = {
    "cloudsql.iam_authentication" = "on"
    "log_connections"             = "on"
    "log_disconnections"          = "on"
    "log_checkpoints"             = "on"
    "log_lock_waits"              = "on"
    "log_temp_files"              = "0"
  }

  database_flags = merge(var.database_flags, local.enforced_database_flags)
```

The same pattern as the leader, with a broader enforced audit set: connections, disconnections, checkpoints, lock waits and temporary files are always logged.

How Claude Haiku 5.5 did on iac-bench 2026-10: an overall composite of 68.0, scores per fixture, strengths, weaknesses and the patterns in its OpenTofu modules.

How Claude Opus 5.5 did on iac-bench 2026-10: an overall composite of 88.9, scores per fixture, strengths, weaknesses and the patterns in its OpenTofu modules.

How Claude Sonnet 5.5 did on iac-bench 2026-10: an overall composite of 86.1, scores per fixture, strengths, weaknesses and the patterns in its OpenTofu modules.

## Also reported by

- massdriver.cloud: Claude Fable 5.2 on iac-bench 2026-10 <https://www.massdriver.cloud/blogs/iac-bench-2026-10-claude-fable-5-2>