April rewrote the floor: open weights crossed the closed frontier, and the safety-gated frontier became a procurement criterion.

Registry movement

The Anthropic line gained two flagship releases plus one gated preview in a single month. DeepSeek shipped V4 in two scales (Pro and Flash) plus a same-month R2 reasoning successor — a cadence no other vendor matched in April.

Added

claude-opus-4-6, qwen-3-6-plus, claude-mythos, glm-5-1, claude-opus-4-7, qwen-3-6-35b-a3b, kimi-k2-6, gpt-5-5, deepseek-v4-pro, deepseek-v4-flash, deepseek-r2

Updated

claude-opus-4-5, kimi-k2-5, gpt-5-4, qwen-3-5

Anthropic · 2026-04-16 · Frontier · Agentic

Claude Opus 4.7

Adaptive-thinking flagship with ultra-long context and full agentic stack (computer-use + MCP + tool use).

Reset the closed-frontier reasoning ceiling. The agentic surface area (computer-use, MCP, tool-use) makes Opus 4.7 the default reference for closed-frontier procurement when adaptive thinking and long-horizon agent runs are required. Pair with Sonnet 4.6 for the cost-tiered deployment.

Model registry ID

claude-opus-4-7

Sources anthropic.com release notes, model card

OpenAI · 2026-04-23 · Frontier · Reasoning

GPT-5.5

Frontier reasoning flagship with multimodal and ultra-long-context as defaults.

OpenAI's response to the V4 / Opus 4.7 surge. Sets the closed-frontier reasoning bar that the open-frontier challengers (V4 Pro, K2.6, GLM-5.1) measure against on SWE-Bench Pro and GPQA. The 5.x family now spans Instant, Thinking, Pro, mini, and nano tiers — useful for matching workload class to model class.

Model registry ID

gpt-5-5

Sources openai.com blog, GPT-5.5 system card

Anthropic · 2026-04-07 · Specialist · Agentic

Claude Mythos Preview

Frontier-class capability withheld from public access on cyber-capability grounds.

The first major model gated by its own developer for safety reasons after independent (UK AISI) red-teaming confirmed autonomous offensive capability. Vendor-risk frameworks now need a capability-gate criterion alongside availability SLAs — diligence on 'what could the next release do that the current one cannot?' is no longer optional.

Model registry ID

claude-mythos

Sources anthropic.com, red.anthropic.com, aisi.gov.uk

Anthropic · 2026-04-02 · Frontier · Reasoning

Claude Opus 4.6

Predecessor to 4.7; held the SWE-Bench Pro closed-frontier line at 57.3 mid-month.

Useful as the April 02 baseline against which open-weights eventually moved past mid-month. Procurement teams running 4.5 should plan upgrade to 4.7 directly; 4.6 will likely move to maintenance status before mid-May.

Model registry ID

claude-opus-4-6

Sources anthropic.com release notes

DeepSeek AI · 2026-04-24 · Open Frontier · Moe

DeepSeek-V4 Pro

1.6T MoE, 1M context, MIT license, frontier-class on coding.

The headline release of April. First open-weights model with frontier-class SWE-Bench Pro performance under a permissive license. Procurement teams blocked on closed-source data residency or licensing constraints now have an open frontier alternative for coding workloads. The on-prem floor moved up; the merchant-vs-self-host fork is now real.

Model registry ID

deepseek-v4-pro

Sources huggingface.co/deepseek, deepseek.com

DeepSeek AI · 2026-04-24 · Open Frontier · Moe

DeepSeek-V4 Flash

Cost-tiered V4 sibling for high-throughput inference on smaller fabric.

Open-weights answer to GPT-4o-mini and Claude Sonnet at the inference-cost tier. Useful for batch coding agents and high-volume serving where Pro is overkill. Same MIT license, same 1M context, lower active-parameter cost per token.

Model registry ID

deepseek-v4-flash

Sources huggingface.co/deepseek model card

Moonshot AI · 2026-04-20 · Open Frontier · Moe

Kimi K2.6

58.6 on SWE-Bench Pro — the first open-weights model to top the closed leaders on coding.

The benchmark moment. K2.6's coding score sits above GPT-5.4 (57.7) and Claude Opus 4.6 (57.3) under an open-weights license. For coding-heavy procurement reads, K2.6 is the new default open-frontier baseline; closed-source premium needs new justification beyond raw score.

Model registry ID

kimi-k2-6

Sources moonshot.cn, Artificial Analysis SWE-Bench Pro

Z.AI (Zhipu) · 2026-04-08 · Open Frontier · Agentic

GLM-5.1

58.4 on SWE-Bench Pro with multi-agent-native architecture and ultra-long context.

Second open-weights model to clear the closed-frontier coding bar this month. The multi-agent-native design point makes GLM-5.1 the open-source reference for agent-of-agents deployments — relevant when coordination overhead matters more than single-shot completion.

Model registry ID

glm-5-1

Sources z.ai, GLM-5.1 model card

Alibaba · 2026-04-16 · Open Frontier · Moe

Qwen3.6-35B-A3B

Open-weights MoE successor to Qwen3.5; agentic + multimodal at deployable scale.

The most deployable of April's open releases on commodity inference fabric. 35B total / 3B active makes Qwen3.6-A3B the open multimodal pick when V4 Pro is too large and K2.6 is single-modality. Pairs naturally with Qwen3.6-Plus on the closed-source side for hybrid deployments.

Model registry ID

qwen-3-6-35b-a3b

Sources qwenlm.github.io, model card

DeepSeek AI · 2026-04 · Reasoning · Reasoning

DeepSeek-R2

Open-weights reasoning successor to R1, frontier-class long-context.

Same vendor shipping V4 (general) and R2 (reasoning) inside one calendar month is the cadence story. R2 closes the open-vs-closed reasoning gap that R1 opened in 2025 — open-weights reasoning is now a separate procurement category, not a niche.

Model registry ID

deepseek-r2

Sources deepseek.com, huggingface.co/deepseek

Reasoning becomes the default mode, not a separate model.

April's flagships ship reasoning behavior as a default rather than a separate o-series-style fork. Adaptive thinking, where the model decides how much to deliberate, is now the closed-frontier expectation; explicit reasoning toggles look dated by month-end. Procurement consequence: the reasoning-vs-non-reasoning split is collapsing — assume reasoning is on, budget tokens accordingly.

Examples

Claude Opus 4.7 (adaptive thinking), GPT-5.5, DeepSeek-R2, GLM-5.1

Sources Vendor model cards (Anthropic, OpenAI, DeepSeek, Z.AI)

Million-token context as baseline.

Every flagship released in April clears the 1M-token threshold. Long-context is no longer a differentiator at the frontier; it is the floor. Workloads that previously required RAG architectures may now be expressible as direct in-context loads — re-evaluate the retrieval layer when re-baselining for V4 / K2.6 / Opus 4.7.

Examples

DeepSeek-V4 Pro (1M), Kimi K2.6 (1M+), Claude Opus 4.7 (ultra-long), GPT-5.5 (ultra-long)

Sources Model cards; HuggingFace technical reports

Agentic surface area as a first-class capability.

Agentic isn't a benchmark category anymore; it is a model property declared on the card. Computer-use, MCP support, and multi-agent coordination ship with the flagship rather than as a downstream wrapper. Architectural read: the boundary between 'model' and 'agent runtime' is dissolving — procurement should evaluate the full agent surface, not just the LM weights.

Examples

Claude Opus 4.7 (computer-use, MCP, tool-use), GLM-5.1 (multi-agent-native), Qwen3.6-Plus (agentic), Kimi K2.6 (agentic-ready)

Sources Vendor model cards; Model Context Protocol announcements

Capability gating as a market signal.

Anthropic withholding Mythos after UK AISI red-team findings is the first time a major lab gated a frontier-class release on its own initiative for cyber-capability reasons. The signal: capability evaluation now happens before public release, and 'gated' is a status that procurement teams need to track in vendor risk frameworks alongside 'available' and 'deprecated.'

Examples

Claude Mythos Preview (withheld)

Sources anthropic.com, red.anthropic.com, aisi.gov.uk

SWE-Bench Pro (coding)

Open weights overtook closed for the first time. Two open-weights models clear both closed leaders.

Kimi K2.6

58.6

GLM-5.1

58.4

GPT-5.4

57.7

Claude Opus 4.6

57.3

Sources Artificial Analysis SWE-Bench Pro leaderboard, vendor model cards

Long-context retrieval (1M+ tokens, vendor-reported)

Million-token context lands at the frontier with sub-2% retrieval-error rates across multiple vendors.

DeepSeek-V4 Pro

1M ctx, 1.4% err

Kimi K2.6

1M ctx, 1.7% err

Claude Opus 4.7

ultra-long, vendor-reported

GPT-5.5

ultra-long, vendor-reported

Sources Vendor technical reports; HuggingFace evaluations

Agentic tool-use (vendor + third-party)

Closed flagships still lead agentic tool-use; open frontier is one tier behind but closing.

Claude Opus 4.7

Closed leader

GPT-5.5

Closed challenger

GLM-5.1

Open leader (multi-agent-native)

DeepSeek-V4 Pro

Open challenger

Sources Vendor agent benchmarks; community evaluations

2026-04-21 · Anthropic

Mythos withheld from public access pending capability-gate review.

Sets a precedent for vendor-initiated gating on cyber-capability grounds. Procurement frameworks now need a 'gated' status alongside 'available' / 'deprecated.' Adds a new diligence question: what is the next release the vendor evaluated and chose not to ship?

Sources anthropic.com, red.anthropic.com, aisi.gov.uk

2026-04-24 · DeepSeek AI

Two-config V4 launch (Pro + Flash) plus same-month R2 release under MIT.

Cadence and licensing both shifted. A monthly cycle with permissive licensing across coding, reasoning, and inference-tier variants is the new open-frontier benchmark for vendor velocity. Closed-source vendors with quarterly cadence have a velocity gap, not just a capability gap.

Sources deepseek.com release notes; huggingface.co model cards

2026-04 · OpenAI

GPT-4 family deprecation timeline tightened; 5.x consolidated to five tiers (Instant, Thinking, Pro, mini, nano).

Pricing migration for GPT-4-era workloads accelerates through Q2. Customers should plan migration to 5.x within 60 days; the tier consolidation simplifies model selection but compresses the price-vs-capability slope on the low end.

Sources openai.com platform changelog

2026-04 · Multiple (open-weights serving)

Inference price cuts of 25-40% across major open-weights serving providers following V4 / K2.6 launches.

Open-weights inference economics improved materially this month. Re-baseline cost per token for batch agentic workloads against the new floor; the merchant-vs-self-host break-even moved.

Sources Provider pricing pages; community comparison threads

May 1-7

Closed-frontier response to V4 / K2.6 coding scores.

Anthropic and OpenAI both have credible reasons to push a coding-tuned point release to recover the SWE-Bench Pro lead. Watch for Opus 4.7-Codex or GPT-5.5-Codex within two weeks.

May 1-14

Hyperscaler Q1 prints (MSFT / GOOG / META / AMZN).

AI capex commentary will set the inference-pricing trajectory for the rest of Q2. Tracked in The AI Stack Weekly's capital-flow lens; relevant here as the upstream signal for serving costs.

May 5-20

Mythos status update from Anthropic + AISI.

If gating becomes permanent or extended, 'gated' becomes a durable status class. If gating lifts under conditions, a capability-gate playbook emerges — either way, the procurement read changes.

May 10-25

Open-source reasoning successor cluster.

DeepSeek-R2 set the bar; Qwen, Z.AI, and Moonshot all have R2-class candidates plausibly within 30 days. Watch for the second open-weights reasoning model to clear o-series parity.

May 15-31

First gated-capability standardization proposals.

Mythos creates pressure for a cross-vendor gating taxonomy (capability levels, evaluation protocols, release criteria). Watch for AISI / METR / OpenAI / Anthropic joint statements.

• Inaugural issue. Cadence: monthly recap covering April 2026 in one read; weekly cadence begins in May.
• Tree delta sourced from content/llm-tree/models.yaml April 2026 release rows.
• Benchmark scores cite Artificial Analysis leaderboards and vendor model cards as of publish date.
• Scorecard reflects April 2026 close; will refresh weekly as new releases land.