fix(runs): say why the backend refused a run instead of that it did

[camielvs]

An agent built a pipeline, validated it with zero issues, submitted it, and got
back Failed to create pipeline run. It retried the identical request, failed
identically, went looking for the spec to submit another way, hit a second bug
doing that, and concluded after four minutes:

This appears to be a browser pipeline serialization/submission bug rather than
a problem with the DAG.

It wasn't. The backend had said exactly what was wrong, and we threw it away:

if (!response.ok) {
  throw new Error("Failed to create pipeline run");
}

The response body behind that sentence:

{"detail": [{
  "type": "string_type",
  "loc": ["body", "root_task", "componentRef", "spec", "inputs", 0, "default"],
  "msg": "Input should be a valid string",
  "input": true
}]}

InputSpec.default is string | null on the backend. A number or a boolean
there is refused with a 422 — the DAG is fine, one field has the wrong type, and
that is a one-call fix for whoever is holding the pipeline.

What changed

requestFailureMessage(response, fallback) reads what the server said and adds
it to what we were doing:

Failed to create pipeline run (422 Unprocessable Entity): root_task.componentRef.spec.inputs.0.default: Input should be a valid string (got true)

It handles a FastAPI detail array (field path, message, and the offending
value), a plain-string detail, a message field, a non-JSON body, and an
empty one — which still reports the status rather than nothing. Several rejected
fields are listed up to three, and a large rejected value is truncated so one
runaway string cannot bury the rest.

Only createPipelineRun is wired up here: it is the call that bit, and the one
an agent drives. The other callers in this service already go through
fetchWithErrorHandling.

Testing

Full suite green — 318 files, 3235 passed. The formatter's tests are built from
the real response body above, captured from the local backend.

Verified end to end against that backend: submitting a spec with a numeric input
default through createPipelineRun now throws a message naming
inputs.0.default, the rule it broke, and the value it was given.

🤖 Generated with Claude Code

[github-actions]

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A preview build has been created at: 09-23-fix_say_why_the_backend_refused_a_run/36fde75

[camielvs]

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• fix(runs): say why the backend refused a run instead of that it did #2794 : 2 dependent PRs (#2796 , #2798 ) 👈 (View in Graphite)
• feat(tangent): let the agent name the project, session and pipelines #2792
• feat(projects): fix cancelling, track shared projects, flag unreachable pipelines #2790
• feat(tangent): keep an agent's edits in frame on the canvas #2789
• feat(tangent): bring Tangent onto the dashboard #2787
• feat(tangent): show a project's runs on the tangent page #2786
• fix(projects): show an agent's run on the project it belongs to #2785
• fix(agent): submit a spec that came from the editor's store #2784
• chore: increase max turns in remote agent #2779
• fix(tangent): stop the tab strips scrolling up and down #2777
• feat(projects): make the agents' instructions a project document #2776
• feat(tangent): list the pipelines the backend holds #2775
• feat(projects): show a project's documents on both pages #2774
• fix(tangent): project window parity — share, new-session links, agent-created pipelines, and a folded-away project window #2773
• refactor(projects): describe a project's resources one way #2765
• feat(projects): delete and share a project from either page #2764
• feat: open a new project ready to work in #2763
• feat: let the project page and Tangent reach each other #2762
• fix: keep a long project name inside the tile it belongs to #2761
• feat: say so when Tangent cannot be reached #2760
• feat: open a project where the work happens #2759
• feat: run a pipeline inside a project #2754
• feat: add pipelines held in this browser to a project #2745
• feat: project page #2738
• feat: create a project from a tile in the grid #2735 : 2 other dependent PRs (#2736 , #2737 )
• feat: projects dashboard #2734
• chore: tangent - annotate runs submission to projectid #2758
• fix: tangent - proper debug sequence system prompt #2757
• fix: tangent - quick run button from mini content window #2744
• fix: tangent - remote agent send delta immediately #2743
• fix: tangent - mint proper authorization with tangent #2742
• feat: tangent - debug in tangent sequence #2741
• chore: add autolayout tool #2740
• feat: tangent - resource management via document entity #2733
• feat: tangent - interactive chips in tangent chat #2732
• chore: save id stack to replay ids on reloads #2731
• refactor: tangent - migrate tangetproectcontext onto mobx #2730
• feat: tangent - keep track of open tabs while switching sessions inside project #2729
• feat: tangent - remote agent for runview and editor #2728
• refactor: tangent - structured workarea target #2727
• feat: tangent - embedded pipeline run view #2726
• feat: tangent - embedded pipeline editor #2725
• feat: tangent - dynamic workarea remote tools #2724
• feat: tangent - resources management in project view #2723
• feat: tangent project view page #2722
• feat: workspaces and projects service #2720
• chore: api client for workspaces and projects #2719
• master

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